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[
"Is there a theoretical solution to the acceleration problem of getting a human from earth to Andromeda in an hour (traveler time)?"
] |
[
false
] |
In theory if you accelerate fast enough towards the speed of light, you could travel any distance in a short amount of time from the traveler's perspective. The problem is that if you're accelerating at 1G, the trip to our closest galaxy will take around 50 years. Are there any theoretical solutions to this problem?
|
[
"Really soft chairs?",
"No, seriously, the best examples of very high accelerations for humans is being hit by a train or a drop from a plane on to concrete. Imagine the effect of being hit by a high speed train many, many times a second. Figure out what you can do to a human body to make it survive that and you have your anwser. Note that any kind of protection is cheating. Has to be skin to train impact. "
] |
[
"Note that any kind of protection is cheating.",
"Not necessarily, protection is exactly what we are looking for in this case. Assuming we had a craft capable of arbitrary acceleration there are many things we could do to enable humans to survive extreme acceleration.",
"For example a prolonged 3g acceleration vertically along a standing or sitting human will kill even those trained to withstand extreme g-forces. Three minutes or so would be lethal. However if we lay a person down and exert the acceleration horizontally even a layperson can withstand extreme g-forces; untrained people should be able to withstand up to 12g's for extended periods of time.",
"Another interesting method is to put our passengers into big water tanks (presumably with oxygen masks and tethers to prevent them rattling around). Water is largely incompressible so the deformation due to acceleration should be negligible, and much of the stress that would otherwise be placed on the human body would be instead transferred to the container housing the water and distributed evenly. Things only get better if we can fill the air voids in the human body with an oxygenated liquid such as perfluorocarbon as might be used in deep sea diving. Unfortunately perfluorocarbon isn't suitable as it is nearly twice as dense as water, but something to fill the same role might be possible to develop.",
"Immersion in liquid while still keeping air in the lungs tops out in effectiveness around 20g's but I can't really find any data on the top limit when the lungs are filled with liquid. Portions of the human body are still different densities than water and will therefore respond to acceleration but with a combination of these techniques I am confident the human body can be made to withstand some extreme accelerations."
] |
[
"I meant physical protection on the body (like amour or something). What that does is increase the impact time from the train and thereby decreases the number of g's you experience. This clearly wouldn't work in a spacecraft where the g's are constant.",
"But yeah, there's certainly things you can do to help the body. I doubt down to 1 hour of transport. I'd think that's many 1000s of g's, although I haven't down the math."
] |
[
"Is there a Universal Center?"
] |
[
false
] |
For the most part, star systems, star clusters, galaxies, and galactic groups are bound by some super dense place at the center of said systems. I guess this isn't always the case, but as a general rule in the cosmos, things collapse spherically. Obviously the universe is doing just the opposite of collapsing but anyway. My point is clear enough I hope. My question of course: is there any research suggesting, or proving that the known cosmos follows along with this rule?
|
[
"No, ",
"on a large scale, the universe looks same everywhere",
". ",
"If you are referring to the expansion of universe, the expansion isn't from any central point. It is the metric itself which is expanding, i.e. the \"scale\" of things. All distances increase at the same rate."
] |
[
"But if there is an edge, where there is no matter beyond that point, surely there must be a point halfway between there and the opposite \"edge\" which is the middle?",
"Well, yes. But there is no evidence of any edge. In our current models the universe is infinite. "
] |
[
"As I understand it there are 3 remaining possibilities for the shape of our universe, but none of the remaining possibilities has an edge (\"boundary\").",
"The possibility that is most consistent with observation is that our universe is perfectly flat and infinite in all directions. You could travel as far as you want and never reach an end. If this is the case, then there is an infinite amount of matter with roughly the same density everywhere (on the largest scales).",
"It is also possible that our universe has \"positive curvature\" which means it resembles the ",
" of a sphere (but only the surface, not the inside). If you were an ant on the surface of a sphere, you could travel forever and never reach a boundary, but you would eventually end up where you started from. The larger the radius (which means smaller curvature), the farther you have to travel to wrap around. It is possible that our universe does have a little bit of positive curvature, and has a very large radius (larger than our observable universe). If that's the case, then the amount of matter in the universe is finite, but there is still no edge or boundary.",
"The last possibility is that the universe may have negative curvature overall, which gives it a \"",
"hyperbolic",
"\" geometry. It is less intuitive, but like a flat universe, a hyperbolic universe is also infinite in extent and has no boundary.",
"My understanding is that, to date, observations in cosmology of ruled out all other possible topologies for our universe, except for these three. Our measurements of the global curvature of our observable universe put it within the margin of error of \"flat.\" We aren't sure if it's actually flat but it is effectively flat and practically infinite; we may never be able to answer the question through measurements alone since we are confined to our observable universe.",
"Also, note that ",
" of these topologies have a \"center\" point. To be a central point, it would have to have equal numbers of points in any direction. For a flat, infinite universe, this is true for ",
" point, so every point could be equally considered as \"the center.\" The same is true for a hyperbolic universe. Now, for a spherical universe, you may say \"there is actually a center,\" but actually we are only talking about the ",
" of the sphere. The center of the sphere is not actually a point somewhere on the surface, and every point on the surface still has the same number of points in every direction that's part of the surface, so there is still no central point.",
"Expansion of the universe, then, is like increasing the scale of the sphere, or the (flat) grid, or the (hyperbolic) saddle-shaped grid. Not like an \"explosion\" from some point ",
" space, but rather an \"inflation\" that happens to every point in space simultaneously.",
"Hope that helps."
] |
[
"Biochemically, how do anabolic steroids assist muscle growth, and what are the differences between different types of steroids"
] |
[
false
] |
[deleted]
|
[
"This information is wrong. hGH is a peptide hormone (a protein) not a steroid. Steroids are lipids (basically fat molecules). Also hGH act on cell surface receptors. Steroids act on intracellular DNA transcription factors. "
] |
[
"The difference lies in their chemical structure and how they interact with receptors, I don't know the specific differences though. For example human growth hormone (hGH) binds to certain receptors found on the surface of cells which then induces growth in that cell. Steroids all do this by promoting proliferation of cells and proteins they secrete. "
] |
[
"My fault, of course hGH is a peptide, and sorry about the misinformation. All steroids are lipids meaning they can enter the cell (hGH can't) and thereafter bind to different nucleus receptors which when activated changes transcription in the cell, meaning changing the protein setup in the cell."
] |
[
"What is happening physiologically when you have a “knot” in a muscle?"
] |
[
false
] |
What is happening physiologically when you have a “knot” in a muscle? By knot I am referring to a tight or particularly sore area in a muscle belly. When palpated it can feel like a small lump or tense area. They tend to go away with stretching, and or some pressure to the area.
|
[
"Nobody knows, actually. There have been numerous studies where a single patient or small set of patients with knots are blind-tested or otherwise examined by various professionals who claim to be able to diagnose them and basically none of them agree. Similarly there is not a good theoretical basis for guesses as to what they are. ",
"The precise etiology of MPS is not clear yet, and further research is needed to determine the root cause.",
"For reference:",
"https://pubmed.ncbi.nlm.nih.gov/33936911/"
] |
[
"\"We aren't sure.\" Is certainly the most accurate answer.",
"The mechanism that I get the most mileage out of with my patients (Physical Therapist), is that the act of sarcomere contraction (the smallest unit of muscle fiber) requires actin and myosin to attach to active sites on each other and \"power stroke\" to shorten. Energy (stored as ATP) is used to release those actin and myosin bonds, and they reattach further down stream to get ready to contract again.",
"If those active sites were to remain bonded to each other, or otherwise get stuck, it could theoretically cause a buildup on tension or \"knot\". Enough knotting could form a 'trigger point' (which might not be a real term, not everyone agrees), or an area of such high tension there is minimal blood flow. Lack of blood flow would cause hypoxia and other sorts of restriction that is likely to be interpreted by the brain as tenderness and pain with palpation.",
"Keep in mind that might be the wrong explanation, but is the model that makes the most sense based on what I've read or been taught."
] |
[
"Ya, this is the most reasonable answer. Whether “knots” even exist depends a lot on definitions. What they are biologically is certainly not known, and a LOT of people basing treatments off of knots are BS. ",
"A longer read, but as a bonus it keeps things in layman’s terms so thought it would be worth a post: ",
"https://www.painscience.com/articles/trigger-point-doubts.php"
] |
[
"Why do warm-blooded animals have different internal body temperatures?"
] |
[
false
] |
It seems like most (I'm scared to say all) proteins found in almost all life work best at higher temperatures (Ex. The optimal temperature for ATP hydrolysis is 60°C (140°F) ). I'm sure anyone able to answer the first question would probably be able to answer the question, Just for kicks, some interesting numbers on internal body temperatures. Human - 37°C (98.6°F) Chicken - 41.8°C (107°F) Cow - 38°C (101°F) Emperor Penguin - 39°C (102°F)
|
[
"Of all the organisms on the planet, only mammals and birds display this phenomenon.",
"Well... kind of. The dichotomy between endothermy and ectothermy is mostly artificial and perhaps even an artifact of inadequate information. Insects, for example various species of honeybee, will use their wing muscles to raise their temperature to boost their metabolism. When we do a similar action to keep our bodies warm, we call it shivering. Other classic \"ectotherms\" past a certain size exhibit gigantothermy. Now you might argue that gigantothermy is just a consequence of surface/volume ratio, and you'd be right. But the body temperature of classic endotherms is also a function of surface area, volume, and metabolic rate as well.",
"Additionally, many endotherms regularly use circadian torpor, where they reach body temps near ambient. This further blurs the endo/ecto line. It's all a gradient.",
"One theory regarding the evolution of endothermy in birds and mammals is that it was a defense against fungal pathogens.",
"I'm skeptical of this claim. Higher mammals and birds are much more resistant to fungal pathogens because of the adaptive immune system. In fact, when humans lose their adaptive immune systems (mostly T cells) due to HIV or drug treatments, fungal pathogens are a serious threat. Amniotes (reptiles and mammals [recall that birds are actually reptiles]) have a more complex immune system than other animals. Lymphoid organs and adaptive immunity only appear in the advanced chordates. It's also worth noting that many fungi grow ",
" at temperatures at or beyond the core temperatures of many classic endotherms (labs culture fungi in the high 30s-low 40s centigrade)."
] |
[
"ATP hydrolysis isn't the only reaction that takes place. Each chemical reaction has its own set of enzymes and substrates; these enzymes may be more or less thermostable than others. The DNA polymerase from ",
", used in PCR, is functional at 72°C, even after being heated to 95°C. Many proteins or cellular structures are damaged at these temperatures.",
"Temperature can also shift biochemical reactions, in a fashion that may not be favorable to an organism. In C3 plants, higher temperature leads to more ",
"photorespiration",
" by increasing the oxygenase activity of RuBisCO. C4 and CAM plants have ways around this.",
"There are ways that organisms can protect themselves from higher temperatures by changing the concentrations of things like trehelose or producing heatshock proteins. There is a plant, ",
" which survives in geothermal soils of Yellowstone where the rhizosphere temperature may reach upwards of 65°C. It is able to do this because of a fungal endophyte, ",
", which contains a virus that is thought to modify the regulation of fungal trehelose production and heatshock proteins. No endophyte or a virus-free endophyte and the plant does not survive.",
"Endothermy has an incredible metabolic cost to it. Of all the organisms on the planet, only mammals and birds display this phenomenon. One theory regarding the evolution of endothermy in birds and mammals is that it was a defense against fungal pathogens. There are many many fungal pathogens of plants and terrestrial arthropods, while only a handful of fungi can cause disease in vertebrate endotherms.",
"Hope that answers your question and sorry if I started rambling too much about fungi, plants, and microbes. I've always found the topic of thermal tolerance to be fascinating."
] |
[
"Yeah, I agree with those points. I guess I was thinking of dimorphic fungi that tend to infect their animal hosts as a yeast and live a filamentous lifestyle in the soil. There are many other fungi which are opportunist pathogens and primarily cause problems for immune compromised individuals.",
"Fungi grow better in higher temperatures to a point. I researched fungal endophytes in the Sonoran Desert for three years and many of those isolates just absolutely refused to grow above 35 C. However, there were a few that just loved the higher temperatures."
] |
[
"Does the expansion of spacetime happen at every scale of the universe?"
] |
[
false
] |
Does the same type of expansion happen at an atomic scale as it does in space? As far as I understand it right now, gravity counteracts that expansion. Is that true? If so, does that mean that in the early life of our universe, there was no gravity?
|
[
"In the presence of matter, no. Einstein's field equations are non-linear. They include contributions from both matter/energy and dark energy. So at small scales (matter dominated) there is no expansion; at large scales (dark energy dominated) there is expansion. This may seem contradictory, but in GR superposition doesn't hold as the equations are non-linear. "
] |
[
"If by early you mean mere moments after the Big Bang, then yes. The four fundamental forces we know of today arose out of a single, all encompassing force. Furthermore, gravity - when compared to the other forces at least - is extraordinarily weak, and our laws of gravity tend to break down at the atomic scale. A working theory of quantum gravity is a current goal of many scientists.",
"The force holding atoms together is known as the strong nuclear force, and at the distance scale of relevance to nuclei, it is dominant. There are a few theories about the fate of the universe, but at least one posits that eventually the expansion will occur at a sufficient rate to pull even nuclei apart, until the universe is a sea of infinitely separated particles that never interact again."
] |
[
"Two very minor clarifications concerning the above. It's important to note that the statement,",
"The four fundamental forces we know of today arose out of a single, all encompassing force",
"is speculative. There are theoretical and aesthetic reasons supporting unification of all the fundamental forces but there is no observable evidence of it as of now. Also, the strong nuclear force is responsible for binding ",
", not atoms. The term \"atom\" is usually taken to refer to the system of interacting electrons around a central nucleus and not the nucleus alone. "
] |
[
"Does aluminum foil help things bake?"
] |
[
false
] |
I've always used aluminum foil when baking things (particularly when making pizza bagels, which I'm enjoying right now), and I've always lined the bottom with foil. While I've always assumed it was just to get any mess on the foil, and not the pan, so that it's an easy clean up, I was thinking about an article I read about how heat has an assosciated infrared radiation (but I can't recall the specific term used for this), so this led me to think that the foil would reflect some of the 'heat rays' and help cook my food. Does anyone have any insight into this? I hope that I'm not completely wrong in thinking this. I'm a budding physics major and this stuff fascinates me.
|
[
"Excellent question (I love these cooking questions). Aluminum is good for cooking because it has really good heat conductivity (transfer) and a low specific heat (doesn't 'hold' much energy). So, if you heat up one part of the foil the rest of the foil will heat up pretty quickly, too. Alternatively, if you take the foil out of the oven when you're done cooking, you can usually touch it within seconds without burning yourself because it transferred all its heat to the surrounding air pretty quickly.",
"Aluminum foil is also used to wrap foods to keep the moisture in, since it's impermeable to water, and clean up messes (like you mentioned).",
"I'm not sure about the infrared radiation you talked about, but it is reflective and likely bounces off radiation like a mirror. Interestingly, if you've seen those new '",
"crisping sleeves",
"' used in microwaves - like in Hot Pockets, they sometimes use a ",
"metal",
" (like aluminum flakes) which absorb electromagnetic radiation (microwaves) and convert them to infrared heat to brown your food.",
"COOL SCIENCE."
] |
[
"There is also a group of people that say the shiny surface of the aluminum foil reflects heat better than the rough side of aluminum foil (there is a shiny and rough side due to the processing of the foil, they roll two aluminum foil sheets at once that are pressed together and that gives a rough surface at the interface). Because of the difference in reflecting heat, this group of people might argue that depending on how you wrap your potato, it might cook differently. The shiny side outward will reflect more radiant heat, and the rough surface will absorb more.",
"But I have a scientifically oriented friend who told me that this effect is negligible when cooking foods, and that it doesn't matter.",
"Do you know the right answer?"
] |
[
"The engineer in me would tell your friend to punch a long nail through the center of the potato to cook it faster and forget about the foil ;D"
] |
[
"How do stars with a CNO cycle run out of fuel?"
] |
[
false
] |
I used to think fusion cycles simply evolved from basic hydrogen mergers up until iron, at which point a star begins to collapse. Now that I know about the I want to know how any star that fuels in this manner actually runs out of fuel. Is it just a matter of proton-proton fusion slowly whittling the fuel supply down?
|
[
"Question: why would they not run out of fuel by this method?",
"Edit: that is to say, what about the CNO process would make the star not run out of fuel, opposed to any other process? The star still radiates energy, and that energy must come from somewhere. "
] |
[
"The effective net result is the same for both the p-p and CNO cycles. 4 protons react and form 1 helium + energy. In the p-p chain, the protons fuse with one another. In the CNO cycle, the protons fuse with C, N, or O, and at various stages the energy, helium, and other products are given off. So in both cases, the hydrogen is being consumed."
] |
[
"Ah, now I see it more clearly. I should have skipped the equations and gone straight for the diagram. I see now the input of hydrogen at the various stages (which would naturally deplete the supply). Sorry for the foolish question."
] |
[
"Is there a significant correlation between how well you chew your food and how much of the nutrients are absorbed by your body?"
] |
[
false
] |
I assume there is difference, but how significant is it? It makes sense that the things that (certain seeds, corn, etc) are obviously better broken up more by your teeth, but for everything else, is it simply a matter of the rest of your digestive system having to do the work that your mouth did not?
|
[
"I assume there is some small difference too, but it is probably much smaller than you would assume. Your body is quite good at taking it's time and carefully extracting the maximum nutritional value from food that it can.",
"It might vary with the type of food you are eating - for example, we all know how corn comes back around to visit again. So I imagine your hypothesis would hold more true were you to eat a pound of corn than a pound of apples.",
"Science cannot really answer your question, however, because we have no way of measuring this. What comes out the other end is made up partially of undigested foodstuff, but much of that is stuff that your body can't digest in the first place, and wouldn't absorb anyways - what we call fiber. Fiber is types of vegetable starches that are undigestable by your body, such as cellulose. Additionally, the majority of stool is made up of bacteria that breed within your own gut.",
"So, while we can measure the amount of organic material in stool (really by just measuring the stool, since it is all organic material unless you have been swallowing ball barrings or something), there really is no good way to separate it into organic things that could have potentially been digested and absorbed, had you just chewed better, and other organic material such as starch and bacteria.",
"Edit - At least no good way that I am aware of. If anyone can shed more light on this via something I am not aware of, I am thoroughly interested to hear it as well.",
"Edit 2 - read below down to Scientist_shmientist's answer. He actually provides a study that shows a negative correlation. And I remember reading another study along these same lines a few years ago."
] |
[
"\"Nature shall castigate those who don't masticate.\" ",
"-",
"Horace Fletcher",
", The Great Masticator",
"But yes, on a more serious note: ",
"More Mastication Cuts Calorie Intake by 12 Percent",
"."
] |
[
"Like AnatomyGuy said, if there is any difference it's insignificant. The main purpose of mastication is just to get your food into a bolus to pass easily through the esophagus- extra chewing is not going to chemically alter nutrients, and the GI system physically churns the food anyways via peristalsis."
] |
[
"How do artificial neural networks work?"
] |
[
false
] |
I've looked up a bunch of things about how ANN work but can't seem to get it so if someone could explain it for me, I would appreciate it.
|
[
"Here is a mathless intuition: ",
"Fundamentally, a neural network is a way of relating an input to an output. You can think of it as a function or a mapping.",
"Here is an example of a very simple network: a smoke detector: there is one sensor (input node) that detects smoke. If there is smoke (there is input / the node is active), the alarm goes off (output). If there is no smoke, the alarm doesn't go off.",
"Often we are interested in much more complicated inputs that may have many features rather than simply presence or absence of smoke. If we are doing some sort of image classification, our inputs might be all of the pixels in an image. So if you've got a 16×16 pixel image, you've got 256 total pixels or 256 features or 256 little sensors that are detecting the presence/ absence of light just like we had one sensor detecting smoke. For now, let's just assume that the pixels can be on/off, black or white.",
"What the network is \"learning\" is the relationship between all of these pixels and what output it should five. If we are doing digit recognition, for example our output might be one of 10 digits (0-9) represented by 10 on/off switches, one for each digit (9 would be off and one would be on). Now we need some way of connecting our 256 pixel nodes to these 10 outputs. Let's pretend each input is connected to every output, but the strengths of the connections (weights) aren't 1 (or -1). In the smoke detector example, if there was smoke detected, the alarm would always go off. For digits, just because one pixel is black doesn't mean that the image is necessarily that of an 8. People write 8s differently, some more slanty, some more loopy, so what might be a black pixel in one image of an 8 might be white in another. To represent this, we might say that if a pixel is black, 80% of the time it might mean an 8, but not 100%; sometimes the pixel might be white and still be an 8. That very same pixel might also be a good indicator for 0s, after all 0 and 8 look pretty similar. So a single input may have strong connections to multiple outputs.",
"Now we can start to see part of the problem: a single pixel can't tell us for sure whether a particular digit is present and it can't differentiate between all of the digits. ",
"We can solve this by looking at not just one pixel, but at many. You can think of it like assessing the stats of an athlete to determine whether they are good or not - it's not enough to just look at 1 number since that might not be sufficient to discriminate between all of the good and bad athletes. Maybe you need to look at both batting average and number of home runs, for example.",
"So while a single pixel can't tell us what digit we're looking at, many pixels might be able to. For example, an 8 has black pixels at the top, bottom and in the middle while a 0 has black pixels only at the top and bottom (discounting the sides). ",
"That's the basic idea of neural networks up until the late 60's. Then, Minsky and others pointed out a big problem: such simple networks can't handle XOR and other kinds of problems. That's where hidden layers, backpropagation and several other tricks started to develop.",
"Sidenote: there is also a graphical interpretation and a regression interpretation that can be helpful and I am happy to go into if anyone is interested.",
"Addendum: I never really described how all of those inputs are used to arrive at a choice of output. Recall that all of the inputs are connected to all of the outputs in our example and that the connections don't necessarily have a weight of 1. We're going to essentially be checking which output has the most evidence in its favor. Think of the weights as evidence - a weight of 1 means really strong evidence in favor; a weight of 0 means it is uninformative (regardless of whether the input is on or off / black or white, it has the exact same effect on the output: nothing); a weight of -1 means very strong evidence against. Looking at all of the pixels, we can combine their evidence for each of the 10 outputs and see which has the most.",
"There is an outstanding question of how the weights get determined in the first place, and this is the learning part, which I can address separately if anyone is interested."
] |
[
"Artificial neural networks are a type of general purpose function approximator. You know through algebra (I assume you've got at least an algebra-level experience in math for this answer) that if you have a function y = f(x), given any x, you can map that to y by putting the value of x into the equation. You can also, if you have data that has exactly the right number of degrees of freedom, take a set of Xs and Ys and solve for the function f, but you have to have exactly the right number of data points (i.e. you can fit any set of N data points with a polynomial solution with N degrees of freedom).",
"What if you have a set of points that outnumbers the function, and you know that the function shouldn't turn out to be a super twisted polynomial, but rather that the points are noisy and the function is a straight line? You can do a linear regression - you know, drawing a line right through the points such that you minimize the residuals, the error between the points and the line. This is a statistical version of fitting the polynomial, but there is a drawback: you have to know ahead of time what the desired number of degrees of freedom are. You can fit a line, or a parabola, or any function you want, really, but you have to select the function you want to model the data.",
"That's where a ",
" function approximator comes in. You are solving for the function f with no idea what shape it might take. Artificial neural networks tend to be a fairly good solution to the problem, \"given a lot of training data, find the pattern or function that relates the output values to the input values\".",
"As to how they work, it's fairly simple, really: each neuron is a transfer function, that is, a simple function that maps its inputs to its outputs. Often a function called the sigmoid function is used for this purpose, because it's mostly linear, but has nonlinearities towards the ends, which allows the network to better model nonlinear functions. Each neuron is connected to others with an edge, and this edge has a weight that scales the output of the neuron: the \"strength of the connection\" between neurons.",
"You pass data into the network and look at the output. You compare that to the known output in the training case, and this gives you an error. You divide that error amongst all of the incoming edges to the output node that provided the output value according to the weights of the edges (you're saying here that each contributor to the output error, each edge is proportionally responsible for any incorrectness). This value is then passed ",
" through the transfer function to figure out how much error was present in the ",
" to the transfer function. This is then divided amongst all of the incoming edges to that node that provided the value according to their weights... you see you just chain the error backwards amongst all of the nodes.",
"When you get to the end, then, you have a value for how wrong every individual edge was in creating the error for that training case. You use this to adjust the weights for the edges. This is called \"backpropagation\": you're propagating back the errors from the training case to correct the edge weights.",
"Then move on to the next training case and repeat. Hundreds or thousands of training cases later, if there is a function f that can be approximated with that neural network, if everything is coded correctly, the neural network would then approximate the function, such that a new input can predict what the output value should be.",
"As you're reading up on them, you'll see that there is talk about ",
" the neural network. This is equivalent to your linear regression function (mentioned before) is starting to look more and more like the N polynomial function: it's becoming a perfect fit for the data; too perfect, in fact, so that it's not becoming a mimic of an underlying pattern of the data but rather ",
" the data. That's why most neural networks use a validation set: you keep training the neural network with a training set, but then you pass in some data you've kept out of the training set to see if the neural network would have done a good job with that data. When the errors in the validation set start increasing without going back down, you know the neural network is starting to ",
" the training set, and not simply find a function that also works with the validation set."
] |
[
"Yes absolutely. That would correspond to something like a grayscale image in the picture example or to any continuous or graded feature more generally. ",
"In the case of binary inputs, you can think of it as representing whether a feature is present or absent. Other than for black and white images, this can be useful if our features are encoding properties of something like: has 2 legs, does not have 4 legs, has feathers, has wings, has a head, weighs more than 50 lbs, does not fly, etc. That neural network might be taking as input properties of animals and outputting animal species as predicted from the presence or absence of those properties.",
"This animal example also highlights why the \"evidence\" or informativeness is captured by the weights/ connections -- not flying (absence of a feature) was extremely informative, just as much as having feathers, say. Both of these might have strong connections (positive and negative! -1 to 1) to various outputs. But \"has a head\" is a totally useless feature since all animals have heads; it likely has a connection weight of 0 since it's totally useless in helping identify anything.",
"Edit: incidentally, you can also think of positive weights as \"excitatory connections\" and negative weights as \"inhibitory\" ones if you want to emphasize the neural analogy."
] |
[
"Why do people wake up when someone gets in close proximity?"
] |
[
false
] | null |
[
"Your sensory systems are still functioning and processing input, including the breathing of someone else, their body heat, changes in amount of light falling on the body, etc."
] |
[
"We're most sensitive to sudden changes (even when awake). So if there is continuous loud music playing, for example, you might be able to fall asleep / sleep through it. The evolutionary story is that things that change in the environment are exactly the sort of things we should be detecting: if something is different, something is happening / could be dangerous. If things are constant, we don't need to continuously devote resources to processing the same thing over and over so we habituate / tune it out. "
] |
[
"These are the same people that will sleep through a riot but wake if I'm 2 inches from their face. I understand that events affect people differently. I jump awake at any change in light while it's very difficult to wake me with noise. I have found this to be pretty much universal (also, I lean in extremely silently and no breathing). Thank you for your answere. I was also curious if this was an evolutionary advancement of sort or was learned at a young age for some purpose."
] |
[
"What type of tree is this"
] |
[
false
] | null |
[
"That is the leaf of the ",
"Gingko biloba",
", which is actually the only existing species in the entire genus Gingko! The modern Gingko tree has fossil relatives going back all the way to the Permian era, ~270 million years ago."
] |
[
"Gingko biloba is not only the only species of the genus, but the only species of an entire order, Ginkgoales---\n",
"http://www.ucmp.berkeley.edu/seedplants/ginkgoales/ginkgo.html"
] |
[
"Ginkgo",
"http://en.wikipedia.org/wiki/Ginkgo_biloba"
] |
[
"What exactly is this property that things have called 'momentum'?"
] |
[
false
] |
When I started studying classical mechanics, momentum was . Later I learned that Newton's second law was actually expressed as force being proportional to the the rate of change of momentum, which mathematically I'm happy with if you accept as momentum = . This is especially true when I also learned that term impulse is the change in momentum and again thinking in classical terms I can see that something that is massive and travelling fast has greater energy than something less massive or slower - hence impulse is larger or smaller in collisions. However when I read about the quantum world momentum seems to be a much more basic attribute that either mass or velocity. So: edit. looking at the the classical energy equation - this is an integration of , This makes me think that perhaps momentum is perhaps a component of energy? Is this just nonsense?
|
[
"Momentum is the quantity that is conserved in systems that are invariant under spatial translations.",
"For a nonrelativistic, massive particle, this comes out to approximately equal m",
"."
] |
[
"As trite as this definition sounds, this is the most fundamental definition there is for momentum. To expand on this a bit, ",
"Noether's theorem",
" is a very deep theorem in mathematical physics which states that every symmetry in a system has an associated conservation law. ",
"More simply, if you do an experiment and then do the exact same experiment some time later, you should expect the same results, because the dynamical equations governing the system behave the same if you substitute ",
" We call this time translational symmetry, and the associated conserved physical quantity for this symmetry is energy. ",
"Similarly, a physical system should behave the same in two different locations: this is spatial translational symmetry and the associated conserved quantity is linear momentum. There are many different conserved quantities in nature, and each one is associated with a fundamental symmetry in physics."
] |
[
"It’s true in general relativity as well. There is a Killing vector corresponding to each direction of spacetime that the system has translational invariance in."
] |
[
"[Chemistry] how does a medicine, like Mucinex DM, have a cough suppressant and an expectorant not cancel each other out?"
] |
[
false
] |
Taking it right now (few days) and not coughing so how is the expectorant supposed to work. How do they not cancel each other out as they are opposites?
|
[
"They aren't opposites. A cough suppressant inhibits your cough reflex, whereas an expectorant loosens up the secretions in your respiratory system. ",
"The first one makes you less prone to coughing, and the second one makes the stuff that's stuff in your airways easier to cough up.",
"In other words, you cough less, but when you do cough, stuff comes up more easily. "
] |
[
"It's been a long time since my respiratory pharm class in med school but if I'm not mistaken it's because those two properties are due to two different active ingredients, thus they compliment each other instead of canceling each other out. One is guaifenesin that helps loosen mucus and dextromethorphan (related to opioids, contra in children under 6 if I'm not mistaken) which had antitussive effect. "
] |
[
"Adding on, ",
"dextramorphan",
" was developed to replace codeine as a cough suppressant. Codeine and opioids can act as a cough suppressant due to their CNS activity, however due to serious side effects, like dependence, toxicity, and respiratory depression, they usually are not prescribed for this purpose, but still are in some cases in rural areas and areas outside the US. ",
"EDIT: Updated after corrections by Coomb and HashbrownPotato"
] |
[
"Does the Human Brain have a Limit on the Amount of Information that can be Learned and Stored?"
] |
[
false
] | null |
[
"Oh, thank you. I didn't know that sub-reddit existed. Is there a difference between the two?"
] |
[
"A good home for this question would be ",
"/r/AskScienceDiscussion",
"."
] |
[
"The other is a sister subreddit of AskScience that we run. It's good for more open ended questions like this memory one."
] |
[
"Could you anchor a big cable to the moon and let it dangle into Earth's atmosphere?"
] |
[
false
] |
[deleted]
|
[
"There is no material strong enough for a cable going from the Moon all the way to Earth. Space elevators to geostationary orbit (and a bit beyond) might work in the future.",
"If we ignore material strength then xkcd covered it: ",
"https://what-if.xkcd.com/157/"
] |
[
"You wouldn't have to go to the moon. GEO space elevators have been in the books for a while now",
"You mentioned the weight of the cable, it is one of the main hurdles in the design. We don't have a material that is quite right for this application. We need low weight/high tensile strength material",
"I have a feasibility study from 2013 that goes into details, pm me if you are interested in knowing more about the subject"
] |
[
"This is the answer, a cable hanging from geostationary orbit would just hover over the ground, 0 lateral speed. But mind you need an equivalent weight beyond geostationary to balance the hanging line.",
"The one hanging from the moon is a whip of destruction cutting the atmosphere at nearly 1400 kmh, does not sound so great..."
] |
[
"Can Musou black / Vanta Black paint defeat police laser and radar speed guns?"
] |
[
false
] |
I recently watched a YouTube video where they painted a car in Musou black paint. These paints are known as the blackest paint in existence since they can they only reflect ~0.0005% of incoming radiation (including light, microwaves and radio waves). Considering this, it got me wondering if a vehicle painted these colors would be significantly more difficult for police speed guns to register a reflection (laser light or radar) and report an accurate speed? Or would the license plate and windows reflect enough to get a reading?
|
[
"For the sake of argument, I'll assume that the speed guns in question emit a single pulse and look for a reflection. I would expect actual operation to involve multiple pulses and some behind-the-scenes algorithms to detect a vehicle's speed. Let's establish some parameters to start.",
"Musou Black published some ",
"specifications",
". If you look at the reflectance plots, you'll see that the reflectance of Musou Black increases sharply once we're out of the visible range. However, the plot only shows reflectance values of up to 2%, so we don't really know how well Musou Black performs at the wavelengths of interest. Linearly extrapolating, the paint might fuzz a 950 nm lidar, but a vehicle would likely still reflect significantly at the license plate and headlights. Higher wavelengths as with a radar-based speed gun would produce a much more significant reflection. ",
"In summary, coating your car with Musou Black might make it slightly more difficult for it to register on a speed gun, under certain circumstances ... but I wouldn't recommend relying on that."
] |
[
"Yes maybe effective against lasers but definitely not microwave radar.",
"BTW radar speed guns generally use CW (continuous wave, not pulsed) waveforms, just a triangle wave, mixed straight back down to DC using the tx signal, with the frequency difference being the range rate (then use the cosine of the radar unit to the road direction to get the vehicle speed).",
"The frequency difference is going to be in the audio range (kHz) so most of the circuitry will be low frequency hence cheap."
] |
[
"You could, but anybody could detect your car emitting tons of RF signals which, if it is strong enough (which should be if you want to drown out the signal from the radar) would be illegal as hell and you would be caught in a day or two (or earlier, depending on how strong your jammer is and how much you annoy the devices and radio amateurs around you to get your reported)"
] |
[
"(Biochemistry) Most naturally occurring fatty acids have even numbered carbon chains. Why is that?"
] |
[
false
] | null |
[
"http://en.wikipedia.org/wiki/Acetyl-CoA",
"http://en.wikipedia.org/wiki/File:Saturated_Fatty_Acid_Synthesis.png",
"Most fatty acid synthesis is done using this coenzyme.",
"You can see on the end there is a two carbon chain. This two carbon chain is the building block used to make fatty acids. Since every multiple of two is even most naturally occurring fatty acids are even."
] |
[
"Thank you (:"
] |
[
"The oxidation of even numbered fatty acids requires less ATP than the oxidation of odd numbered fatty acids.",
"Example:",
"Starting with a 16 carbon fatty acid. Each cycle reduces it by 2 carbons. Eventually you get to 4 carbons, which is cleaved again producing all cleaved products as acetyl CoA.",
"For odd # fatty acids, starting with a 15 carbon fatty acid. Again it's reduced each cycle by 2 carbons until you reach a 3 carbon fatty acid (propionyl CoA). Instead of cleaving this into 2 acetyl CoA, the cell converts propionyl CoA into Succinyl CoA. This mechanism requires the extra input of ATP. Succinyl CoA can be used in the citric acid cycle.",
"So overall, most fatty acids are even numbered carbon chains because it requires less energy to oxidize it. I'm sure you can see why the body would choose to synthesize even #'d fatty acids rather than odd.",
"Are you taking biochem 2 as well? I'm studying this chapter at the moment."
] |
[
"at what speed would you have to travel west at, in order for the sun to appear to stand still in the sky?"
] |
[
false
] |
[deleted]
|
[
"lets see:\ncircumference at the equator is about 25000 miles, you'd have to run that in 24 hrs, so about 1,041 mph."
] |
[
"In order to solve this, we can think of the Sun as a stationary point with the Earth rotating such that you are in sunlight if you are on the half of Earth facing the sun. So, if the Sun is going to appear stationary, you must be moving at the speed of the Earth's rotation for where you are, in the opposite direction of its rotation. So, let's calculate it. ",
"The radius of the Earth is 3963 miles (assuming a spherical Earth, which is close enough). It completes one revolution in 24 hours. The Earth is rotating such that a point on the equator is a point that travels the furthest (since a point on the North Pole isn't moving at all, for instance). So, we want to find the speed of a point on the equator as our baseline. Well, the circumference of a circle, radius 3963 mi is 24,900 miles. (How handy!) So, right at the equator, you'd have to be moving at 24900 mi/ 24 hours = 1037 mph. ",
"But we talked about how the spin of the Earth is different speeds at different places (fastest at the equator, slowest at the poles). It turns out you can find the speed anywhere by taking the speed at the equator and multiplying it by the cos of your latitude. So, at any latitude, the answer is:",
"** speed = 1037*cos(latitude) miles per hour **"
] |
[
"This speed would be reduced the further north or south you go. Once you are beyond the arctic circle in summer, or antarctic circle in winter, you would not need to be moving at all for the sun to always be in the sky. "
] |
[
"When we see electricity arc or a lightning strike are we really seeing the \"color\" of those flowing electrons?"
] |
[
false
] |
Electrons are too small to be observed directly using light microscopes (or electron microscopes), so are we seeing electron density itself or is a reaction with air producing the visual?
|
[
"If my past chemistry knowledge serves me correctly, I believe that the light you see from lightning is caused by the electricity exciting electrons in atoms in the air. These excited electrons move up to a higher orbital than usual, and when they fall back down, they release a photon, which is what you see. This is the same science that applies to neon signs."
] |
[
"You are seing the color that air makes when the electrons drop from an excited state. The color is dependent on the gas composition not the electrons. If lighting where to happen in a neon gas then the lightning bolt would be orange. "
] |
[
"In addition to the light coming from excited gas, as other commenters have pointed out, in hot plasmas a lot of the light also comes from blackbody radiation; it's just so hot that it glows bright white. If you looked at the spectrum of a lightning bolt, you'd see a smooth curve corresponding to the temperature of the bolt, and then some peaks in that curve corresponding to the elements being excited along the path."
] |
[
"Does your brain \"compute\" physics or does it \"go from experience\"?"
] |
[
false
] |
Let's say i'm trying to throw a ball at a target. When I take aim, does my brain try to calculate the physics "behind the scenes" or is it more like "the last few times I threw a ball this heavy this hard with this wind, it'll land about there"?
|
[
"This is a current hot topic in psychology. For some recent reviews, see ",
"Ullman et al. (2017)",
" (<- pdf!) and ",
"Kubricht et al. (2017)",
" (<- pdf!) which separately argue that we do physics-engine-like simulations that instantiate physical laws. These groups argue that we \"compute\" physics as you say, but not explicitly (i.e. consciously). They often describe the opposite view as heuristics-based, but I'm not sure if it's necessary to talk about it that way; instead, you could probably articulate an alternative perceptual-learning-based explanation, but I'm not aware of a recent paper that argues this position. I do recommend this classic Scientific American article on the topic: ",
"McCloskey (1983)",
" (<- pdf!)"
] |
[
"“Physics” in general is complicated, and taken altogether is probably a little bit of both. The element I learned about a little while back was depth perception. Even the first time babies are put on a part clear / part not clear platform called a “visual cliff,” babies are resistant to crawl over it. These researchers believed that this finding shows that depth perception is innate and doesn’t come from experience.",
"https://en.m.wikipedia.org/wiki/Visual_cliff",
"The other side of the coin is that nobody is born knowing how to juggle/ride a bike which are advanced applications of coordination and depth perception. So probably both."
] |
[
"Catching and throwing things is a learned skill, so experience is needed. It is a “muscle memory”, meaning that the thinking brain has little to do with it. Professional athletes often make a mental image of the result they want to achieve, rather than the mechanics of how they will achieve it. Thinking too hard will ruin the result. So whatever happens, happens in the unconscious brain.",
"Here is an interesting study of how a baseball outfielder learns to catch a fly ball. \n",
"https://www.livescience.com/3445-baseball-players-catch-fly-balls.html"
] |
[
"As the earth orbits the sun, does it speed up and slow down? Is there a \"slingshot effect\"?"
] |
[
false
] |
Also, does "rounding" the "peak" (perihelion?) affect the rotation of the earth in any way?
|
[
"Yes, it speeds up and slows down. Planets follow the second law of Kepler (A line joining a planet and the Sun sweeps out equal areas during equal intervals of time) so that means they have to move faster in the perihelion. I don't know what you mean with \"slingshot effect\", that term is used for gravity assists where a body is accelerated and released from a gravity field, and Earth is not scaping and will never be.",
"The only effect the orbit has in the rotation is the little influence of the tidal forces of the Sun (the closest part of the Earth is more attracted to the Sun than the farthest one, resulting in a little force which tends to slow down the rotation)."
] |
[
"I think by \"slingshot\" OP is referring to Earth's closest approach to the Sun as opposed to it's most distant. But, as you point out, Earth doesn't escape the effect of the Sun's gravity so the term \"slingshot\" doesn't apply.",
"What is the percentage in speed change and during which times of year does this happen?"
] |
[
"Earth's orbit is almost circular, so the speed change is very small. Taking this numbers (",
"http://curious.astro.cornell.edu/question.php?number=614",
") the Earth is 4% slower in the aphelion than in the perihelion.",
"Right now we get to the perihelion at January 3rd and to the aphelion at July 3rd. Even if it looks like it, it doesn't play a role in earth's seasons."
] |
[
"If time is affected by gravity then how can we measure light years accurately?"
] |
[
false
] |
I'm not very knowledgable of and haven't studied physics or astronomy, so this is something I can't get my head around. Time is affected by gravity, right? So something with a strong gravitation pull (a planet?) is going to have slightly faster "time" than the "time" that is further away from the gravitational pull. And we measure light years by the amount of time it takes for light to travel from A to B. But if A and B have different gravitational strengths surrounding them, then how can we measure the exact amount of travel time? For some reason I feel like I'm missing something here. Also isn't light meant to be affected by gravity also? Thanks
|
[
"Oh I see now, you mean that the distance error is 20 orders of magnitude higher than time dilation error, I thought that you meant something entirely different."
] |
[
"Oh I see now, you mean that the distance error is 20 orders of magnitude higher than time dilation error, I thought that you meant something entirely different."
] |
[
"20 orders of magnitude? If we are that imprecise how do estimates even mean anything? "
] |
[
"What is the mechanism that causes particle decay?"
] |
[
false
] |
As the title.
|
[
"Particles decays because there exists a state of the energy of this particular system which is lower, hence more stable. It is the reason why particle decays, because they want to reach this minimum energy state. This decay can take many forms afterwards depending on the kind of particle which is decaying. We say that the decay is mediated by a given force, which changes according to the kind of decay : beta decay is mediated by weak interaction for instance. ",
"If you take the example of the neutron, its rest mass is slighty bigger than the proton rest mass, hence it will decay left on its own after with a period of 15 min according to the equation : n -> p + e- + antineutrino. "
] |
[
"Please don't use words like \"want\" or \"like\" when talking about elemental particles. There's no difference in transition probability between a decay and a capture reaction, it's just that it's much less likely to have the (usually) multiple products at the same place. A proton can turn into a neutron through electron capture, it's just unikely to find an electron with enough excess energy to create a neutron inside the nucleus."
] |
[
"Particles want to be in their lowest energy state. Think about a ball on a slope. The ball will roll until it reaches a flat surface. The ball will always roll downward spontaneously and never upward spontaneously. This is the same concept as particle decay. The particle wants to be at the bottom of the hill. In this case it is not rolling down an actual hill but rather decreasing in energy. There are different reasons that particles want to roll down that are more involved, but the idea is that some configurations are just lower in energy than others.",
"Decay is a spontaneous reaction that occurs when a particle can reach a lower energy state by emitting a particle such as an alpha, beta+, beta-, gamma photon, neutrino, or neutron. ",
"The universe likes stability!"
] |
[
"Flight from Australia to South America?"
] |
[
false
] | null |
[
"Hello,",
"We don't allow this kind of question here, but you could try ",
"/r/answers",
"."
] |
[
"Maybe I wasn't explicit enough. I'm trying to find out what kind of tools are needed to measure a plane moving around the globe. Thought that might fall into science?"
] |
[
"Well what do you mean by \"measure the plane moving\"?"
] |
[
"Can you get radiation poisoning by having contact with or being near someone who's been irradiated?"
] |
[
false
] | null |
[
"Not ",
"\"poisoning\"",
", which requires a massive acute dose, but under a few specific circumstances people could get a small dose which has a tiny chance of causing cancer.",
"The key is to distinguish ",
", which in this context is fast particles which do their damage almost instantaneously and then are no longer hazardous, and ",
", which are substances which can emit radiation after some delay.",
"For example, in radiotherapy most treatments are with external beams of radiation, and these cause an absolutely negligible amount of radioactivity within the patient. ",
"Some treatments involve implantation of radioactive sources in the body or injections with radiopharmaceuticals, and these will indeed cause people around the patient to be irradiated. However there are regulations as to how radioactive a person can be before they are released, and this is to protect the people around them. They may have to take precautions for a few days when released such as sleeping alone and avoiding direct contact with children. These precautions are to reduce the risk to others down to minimal levels. But even if they ignored these precautions they would still fall far short of causing actual radiation poisoning.",
"As for non-medical irradiations, that would depend on the exact context. But it's difficult to imagine someone being so contaminated with radioactive material that they could cause radiation poisoning in someone else, but perhaps it would be possible in some type of nuclear accident or terrorism event."
] |
[
"But that is putting a radionuclide into their body. It is not "
] |
[
"Low-energy won't, but above several MeV it can cause photonuclear reactions and activate material.",
"This is a negligible amount for this discussion though."
] |
[
"Does basic rain exist?"
] |
[
false
] |
Like acid rain, but with a pH higher than 7?
|
[
"Not on this planet. Due to the presence of carbon dioxide, and occasionally sulphur dioxide and nitrogen oxides as well, in the atmosphere, rain has the pH of about 5.8 on the Earth.",
"In other planets and satellites, it is a possibility due to the presence of ammonia. However, from here on, our working definition of pH/acidity/basicity really starts not mattering."
] |
[
"I had a read of the full text, and I better make some minor corrections. In the article the highest recording for rain pH was 7.6, although the vast majority were still below 7.0. (The paper's definition of \"alkaline\" is anything above pH 5.5.) High pH rain is usually recorded in areas with high quarry mining activity (calcite dust), but in this case the calcite came from the Sahara desert, which is natural."
] |
[
"There's ",
"this article",
" from a 2000 edition of the journal Science of the Total Environment which reports identifying alkaline rain in Turkey. They don't include the pH in the abstract (and the article is behind a paywall), but they say that 58% of their samples were alkaline. Do they just mean \"more alkaline than other rain, but still not above pH 7\"?"
] |
[
"Magnetic Gas"
] |
[
false
] |
Can gasses be magnetic? If so, where in any given cloud of the substance would the north and south poles be? How would it react to other ferrous objects interacting with it?
|
[
"It's because magnetism in materials comes from the alignment and ordering of adjacent atoms. When the atoms are all moving about randomly they can't be ordered."
] |
[
"MIT has observed magnetic properties in Cyrogenic Lithium Gas. ",
"http://www.popsci.com/scitech/article/2009-09/scientists-create-first-ever-magnetic-gas",
"This would be the only known magnetic gas. ",
"Someone can explain why gases don't make for good magnets, id assume there is too much chaos and motion with atoms of gases for anything to become magnetic "
] |
[
"They can in the presence of an external field, can't they?",
"A gas can never be ferromagnetic, paramagnetism is possible (and common) though."
] |
[
"At the challenger deep, would my body gets crushed to a thin film?"
] |
[
false
] |
I understand that the pressure at those depths is immense. But I'm not sure if it'll just crush my air cavities like lungs or if it'd completely crush meat and bones to a thin paste/film.
|
[
"Undersea pressure is uniform in all directions. Its not like being crushed under a weight on land. The result is that things are compressed, not squashed. Pressure difference across a boundary is what causes deformation, like the air in your lungs.",
"Water is ",
" incompressible, but at 1100 times atmospheric pressure it will compress a bit, like 5%. So, in addition to collapsing lungs and any other voids like sinuses, there would be an additional compression factor.",
"I have a styrofoam cup (8 oz.) that went to the bottom of Challenger Deep, although I'm sorry I can't get to it right now. It is smaller than the one in ",
"this image",
" that went to 4,500m, but not by a lot."
] |
[
"In a gas (which styro",
" is full of), pressure and volume are inversely proportional (all else equal), so doubling the pressure cuts the volume in half. So if you're normal at 15 psi, v=.5 at 30 psi, .25@60 psi, .125@120psi, etc. etc.",
"That first 15 psi gets you a .5 reduction, while the next 90 psi only gets you a .375 reduction. The returns continue to diminish more and more. Every 10 meters of depth underwater adds another atmosphere of pressure, so at 1000' (305 meters) underwater gas is already being compressed to 3.3% of its original volume. You'll have to go down another 1000' to even get past 1.7%.",
"Naturally it's a lot more complicated when it comes to actual materials and not just ideal gas models, but it shows very well why we can't free dive that deeply. At 100 meters underwater your lungs are trying to shrink to 10% of their normal size, and those tissues are being crushed against each other in ways they never otherwise would be. The fact that people can free dive to that depth, much less ",
" that depth, is incredible."
] |
[
"I have one that only went down 1000ft or so and it's about the size of a shot glass. It doesn't take much depth for the compression to begin."
] |
[
"How are neutron beams guided?"
] |
[
false
] |
Charged particle beams (protons, electrons) are guided with an electric and/or magnetic field, but neutrons are uncharged.
|
[
"Apart from extremely low-energy neutron beams which can be guided just with gravity and/or their magnetic dipole moments, they're simply not guided at all.",
"You start with some source of neutrons, and you just collimate down to the beam direction and size you want. ",
"Here's",
" an example from a spallation source at Los Alamos in the USA.",
"So you're not changing the direction of individual particles, you're just removing the particles that aren't already going where you want them to."
] |
[
"Neutrons can be described by their de Broglie wavelength and their behavior can be predicted using neutron optics theory. This is similar to light optics with scattering and reflection, so materials have characteristic neutron refractive indexes, which can be derived from the atom number density and the coherent scattering length of the material. So there are neutron reflecting materials, like nickel, from which you can make a neutron mirror guide. This ",
"presentation",
" from the NCNR at NIST shows the calculations."
] |
[
"Cold neutrons can be reflected using \"mirrors\" under certain angles and can be steered. Ultracold neutrons reflect under all angles on some materials (e.g. nickel) and fill a volume made of such a material like an ideal gas."
] |
[
"Can you make a magnetic liquid by dissolving iron shavings in water?"
] |
[
false
] |
[deleted]
|
[
"Well, for one, iron fillings wont dissolve. They're much too large."
] |
[
"NaCl behaves differently when solvated by water than would elemental Iron. Primarily, you wouldn't see the same ionic solvation, that is the Iron atoms would not break apart. Same thing as trying to dissolve an iron nail in water, the nail won't dissolve. The best you could achieve is a suspension."
] |
[
"There are substances called ",
"ferrofluids",
" that are mixtures of extremely fine magnetic solids interspersed in some kind of fluid (which is sometimes water).",
"However, iron shavings will not dissolve. Also, the iron (if it is just iron) would likely rust in the water, seeing as how finely dispersed it is (large surface area to volume ratio) in a solvent that is corrosive to it. The actual magnetic iron (magnetite) is already an iron oxide, so it doesn't rust very much.",
"If you could grind the iron filings down to a nanoscale size, maybe you might be able to produce a ferrofluid, maybe not.",
"Here's a ",
"link ",
" to a PopSci article that talks about making your own ferrofluid; although they use an oily liquid (kerosene) to colloid the ferromagnetic powder."
] |
[
"Why can bats still navigate with echolocation when in a large swarm?"
] |
[
false
] |
Bats use echolocation but when there are ~1 000 000 Bats in the cave, there must be an enormous interference right? How can the bats still orientate?
|
[
"There are two things going on here, orthogonal signals and contextual processing. ",
"In signal processing theory there is something called a matched filter which actively looks for a pattern that was transmitted to reduce noise, radar and sonar both do this. Orthogonal signals are signals that can be distinguished from each other easily.",
"An example of where this is useful is imagine you and your friend bob are standing at different distances from a wall, You are closer to the wall than bob. Bob yells \"ECHO!\" then you yell it a short time later. You will hear \"ECHO\" a total of four times (assuming you aren't yelling when another one passes). They are listed below but not in order of when you hear it.",
"1) when bob's yell goes by you the first time",
"2) when you yell it",
"3) Your echo off the wall",
"4) bob's echo off the wall",
"So the question is if you know the speed of sound can you locate the location of the wall assuming you can't tell the difference between your voice and bob's? The answer is no because you can't tell which echo is yours, which is bobs, and which is bob's yell just passing you before the echo.",
"The solution to this is for you and bob to yell different things such as \"blue\" and \"giraffe\". If you yell \"blue\" then you know to listen for \"blue\" rather than \"giraffe\". Now you can contextually determine the distance to the wall from the two way travel time.",
"This gets more complicated when you have lots more people operating in the same area since you run out of \"words\". The words in the case of signal processing are complex patterns of amplitude, frequency, and phase and require more complicated math to determine the best orthogonality which limits how many you can have.",
"However you have another tool at your disposal: context. You can discriminate signals based on other factors such as direction, amplitude, and time. In the case of you and bob shouting \"ECHO\" you can reasonably distinguish which one is when bob yells it before it hits the wall due amplitude and direction, from this you can decide which one is your echo and which one is bobs and decide how far away the wall is.",
"The mathematics of this become insanely complicated and its something that radar technology is only scratching the surface of these days. Bats are truly amazing creatures since they have evolved this ability all on their own.",
"Further reading:",
"http://en.wikipedia.org/wiki/Matched_filter",
"http://en.wikipedia.org/wiki/Bat#Echolocation"
] |
[
"Is it not possible that bats use simple \"swarm\" behavior (like birds) when in large groups, allowing the bats at the front (who have less going on, sonically) to guide the entire group?"
] |
[
"Bats have no method of vision beyond the sonar so they still need to use fancy signal processing to triangulate the position of other organisms. And regardless of their position in the swarm they will still be experiencing noise from all the other organisms in the area. Individual bats would likely need to either use their own sonar or utilize signals from other organisms to detect when they are getting close to walls or prey while swarming. The specific details of how they coordinate transmissions is very difficult to determine due to the nature of natural systems.",
"Edit: People have not actually figured out how to solve this problem in radar and sonar systems due to the number of variables. Because of this I suspect that biologists don't actually have the particulars of the system worked out yet. I worked on a project to do this artificially with man made radar systems so I spent some time trying to research how natural systems worked. The project never gained the funding it needed unfortunately. The literature on specifics of the signal processing is sparse and mostly very recent."
] |
[
"Why do some flowers, mushrooms, and plants prefer areas with less light to grow? Shouldn't all plants want more energy from the sun?"
] |
[
false
] | null |
[
"Firstly, mushrooms are parts of fungi and do NOT photosynthise. The mushroom is a fruiting body used for reproduction. ",
"As for plants, sunlight is a double edged sword. It provides the energy to do photosynthesis, but sunlight is bad for plants the same way it can give us sunburn-- the high energy photons damage cells and components and cause injury.",
"Plants thay prefer low light environments probably evolved to tolerate limited light. As such, they would have less pressure to evolve mechanisms to handle harsher, more direct light that other plants would."
] |
[
"Biology PhD student here. ",
"In general there are 2 types of leaves (regarding to sunlight) shadow leaves and sun leaves. Shadow leaves being thinner and having less palisade mesophyll. Palisade mesophyll is a layer of columnar cells lined up to each other and it's found on the upper side of the leaf.",
"Sun leaves do have thick palisade mesophyll, which filters out a lot of radiation and thus decreases the effective yield compared to the amount of light filtered out. It does this because the same radiation is 1. Ionizing the DNA so better keep it away from the precious energy producing chloroplasts and 2. Thicker leaves means more gradual heating and thus it saves water (which is quite important when you are in the scorching sun)",
"Shadows leaves lack this sun-protection but because they filter less light out they are producing high yields in low light. The extra water loss is not nearly as bad because the sun is not directly on them.",
"Some plants produce both, depending on the light conditions the leave grow up in. Top branches producing sun leaves and lower branches producing shadow leaves. Beech for example do this. Other plants only produce one of the 2 and thus will fare better in their respective condition."
] |
[
"evolution removes traits that hurt the species through natural selection. plants that evolved to inhabit typically dark areas like forest floors did so by losing the trait of needing lots of sunlight for energy. along the way they become intolerant of too much sun. the fern is an example. ferns that needed a lot of light died because they weren't getting nutrients, ferns that didn't survived. the ones that didn't need a lot of light also couldn't tolerate too much light and would become burned if left in direct sunlight all day every day. this trait was never selected out since it was not harming the species. "
] |
[
"Can someone really get knocked out by having a pressure point hit?"
] |
[
false
] |
Like the points shown in this video.
|
[
"He's not asking if it's safe for his friend to knock his pressure points to cure his insomnia. Medical advice is not the same thing as asking a question of physiology. "
] |
[
"Ignore tag, not highly relevant.",
"I am suspicious. Obviously, anything that blocks the cartoid artery has the potential to knock someone out. (Using a simple sleeper hold where you put your arm around the front of the neck and grip on an arm to the side of the head, then put pressure. This technique will produce unconsciousness in 10-30 seconds.) You could probably hit someone in the neck hard enough to cause unconsciousness or death. ",
"source",
"Of course any shots to the head have the potential to end in unconscious and sometimes shots that look relatively minor ",
"for example",
" can lead to knock outs.",
"Interestingly, if you hit the chest with a blunt object within a certain (very small) time window, you can cause death. It's incredibly unlikely a human could deliver a blow with this timing. ",
"Great source",
"Causing pain with a 'pressure point' is much more likely. Liver shots are a known quick end to a boxing match ",
"source",
", ",
"example",
"There is exactly one paper which confirms that peripheral hits like shown in the video, might cause brief periods of unresponsiveness. However, I can't access this paper, there has been no follow up, and no mechanism is suggested.",
"I'd say, it's possible these points exist, there has been little or no formal study, and hitting a specific point during a fight would be rather difficult. (Even if these do exist, and you can some how hit a space the size of a quarter on someone, why not just hit the temple, the eye, the throat, the floating rib, the chin, the liver, the sternum, the ear with an open hand, the back of the knee, the instep, or the groin.)",
"EDIT: Also worth noting many of these 'pressure points' are informed by accupuncture, several studies have demonstrated that it doesn't really matter where the needles go in actupuncture to deliver a proper effect. Also, the video you posted are points to help you sleep, not knock you out. These might work by releasing endorphins which would in fact help you sleep. (though again, the point probably doesn't matter, a good hard long rub on any painful point will probably do the trick.)"
] |
[
"That's ridiculous for its own reasons, however I see no reason to censor askscience because somebody somewhere might hurt someone with the knowledge they learn here. Where do you even draw the line? "
] |
[
"If you get all your daily nutritional value in one meal and don't consume anything for the rest of the day, is it bad for you? If so, why?"
] |
[
false
] |
Can your body not sufficiently use all of that in one sitting? Will it store most of it instead? What exactly happens?
|
[
"Some of the good stuff can't be stored by your body, and it's more about having a constant adequate supply over time than it is about getting your daily/weekly/annual amounts.",
"Imagine taking all your vitamins for the year on the 1st January then not consuming any more for the rest of the year. You're not going to do so well. The same is true on a day and even hourly scale, but there's a point at which it gets absurd, and 'daily' is about it."
] |
[
"Your body can only absorb so much of certain nutrients in one sitting, so if you are eating all you need at once, much of that is just going to travel through your GI tract and right out of you."
] |
[
"How would it physically affect your body though? Would you gain/lose weight with either method as oppose to the other?"
] |
[
"How do hospitals go about using radioactive isotopes for medical testing if the isotopes have a short half-life?"
] |
[
false
] |
[deleted]
|
[
"While not an expert by far I have had some exposure to radioactive isotope imaging basics.",
"Isotopes are chosen for their particular half-lifes. And most do get delivered made directly from fission sources, from memory isotopes like iodine-123.",
"However I know of at least one instance of an isotope being used that has a short half-life and is like you say from a decay product. Technetium-99 only has a half life of 6 hours and is used for medical imaging. It is provided by a parent isotope molybdenum-99 which has a half-life of 66 hours. ",
"The technetium is separated via chromatography. This method is called a technetium cow or moly cow. ",
"http://en.wikipedia.org/wiki/Moly_cow"
] |
[
"Most are produced either on sight or rapidly delivered from medical particle accelerators, mostly cyclotrons. ",
"Canadian hospital Youtube video!",
" These devices were originally developed for nuclear science, but these devices are mostly industrialized now. "
] |
[
"Your last sentence essentially answers the question. What I think you fail to grasp is that the radioactivity is their useful property. The EM radiation from the decay is measured and sometimes imaged, to provide the desired information. Depending on the delivery mechanism (I.E. What type of substrate it is combined with) they can target different organs or systems. There is no need to separate them from the decayed isotopes, that is just one of the costs of using this type of material. "
] |
[
"Have we ever found \"Indiana Jones\" like traps while exploring sites?"
] |
[
false
] | null |
[
"Sort of. There's records indicating the first Emporer Qin of China had elaborate traps. The only traps encountered are high levels of mercury, supposedly from artificial rivers of mercury meant to ensure immortality for the late emperor. ",
"Here's an article on it.",
"I recommend asking in ",
"/r/AskHistorians",
" "
] |
[
"http://www.reddit.com/r/AskHistorians/comments/spezo/did_ancient_temples_and_tombs_and_such_ever_have/",
"I asked the same thing a while back. "
] |
[
"But surely for that to be considered a trap it has to be designed with the intention of stopping intruders? The mercury is dangerous but that's not why it's there. "
] |
[
"2 questions for the geneticists"
] |
[
false
] |
Could FISH be considered optical mapping and could whole genome shotgun be considered massively parallel sequencing?
|
[
"I think FISH ",
" be considered optical mapping, though in general optical mapping is just done with a chromosomal stain and smashed chromosomes. I wouldn't consider old-school shotgun sequencing to be massively parallel sequencing though. I think the term is used more for the current Illumina, 454, and SOLID systems.",
"Relevant credentials: Sequenced 40 genomes. "
] |
[
"So everyone else knows what's going on here:",
"I'm not qualified to talk about shotgun sequencing and massively parallel sequencing, maybe someone else can chime in. ",
"additional info- ",
"-",
" basically means \"in the situation,\" and it's considered an intermediate between ",
" (a living sample) and ",
" (in a test tube). It's the equivalent of studying a picture of a person instead of a video. ",
"-Nucleotides are the building blocks of DNA and RNA, just as amino acids are the building blocks of proteins. ",
"-Hybridization is the term when two complementary strands of nucleic acids join together through hydrogen bonding. So Fluorescent In Situ Hybridization means glowing thing attaching in the situation by sticking two strands of nucleic acids together. FISH. "
] |
[
"Ok thanks, it was more of a 'where is the cut off point for these terms' as in, when does something become 'massively' parallel and is optical specific or and optical method, but from what you say it is more of a technique than a family of techniques. Thanks :P"
] |
[
"Relative Spin"
] |
[
false
] |
Is spin relative to a frame of reference as is normal motion? Example: I'm in a spinning spaceship and I experience something that mimics gravity. This implies that the ship is not only spinning but that I can compute how fast it is spinning by the G forces I'm experiencing. What am I spinning in relation to? How come I couldn't just say that everything is revolving around me if all motion has to be in context of a frame of reference? Obviously that can't be the case or I wouldn't be stuck to the inside of the spaceship due to its spinning.
|
[
"Einsteins Theory of General relativity states that acceleration is the same as Gravity. That means that if you are indeed spinning in the spaceship and cannot see outside, you would not be able to do any experiment that says you are not being pulled down by gravity.\nSo, you are in a non-inertial reference frame (there are forces acting on you), and you are at rest within that frame.\nIf you could see outside, it would seem as if everything else was accelerating around you, and you were stationary while it was spinning around you. All you would feel is the pull of \"gravity\""
] |
[
"Actually what you are saying is not what happens but is exactly what I'm asking about in relation to why there \"seems\" to be a preferred state of rest which should not be the case according to relativity. In your example, regardless if I have windows that can see out, I can tell that my spaceship is spinning by that fact that I'm pinned to the outer walls of my ship no matter what direction I go. In this case it would be like I was surrounded by gravity."
] |
[
"You are incorrect.",
"What you are refering to is described in this ",
"page",
". But what OP is describing would be if the elevetor was spinning along one of its axes.",
"The ",
"coriolis_effect",
" is one of the things you can observe in an experiment.",
"And there are other properties, you can observe, such as the lack of acceleration along the spin axis, the reduction in apparent gravity as you get closer to the spin axis, etc. "
] |
[
"Is there an \"uncanny valley\" for sound like there is for sight? Does the sound of very nearly replicated voices, or oddly filtered voices, elicit the same sense of revulsion as does the sight of very nearly-human robots?"
] |
[
false
] |
I guess it's all in the title. When we see things that look human but are ever so slightly off, like some bad robots or zombies etc, there is a characteristic revulsion that people seem to feel, attributed to a variety of neurological and psychological responses which I don't fully understand. Does the same effect exist for sound? Obviously heavily synthesized sound (e.g. Daft Punk vocals) don't trigger this, but as roboticists get better at synthesizing speech do we expect this to become a major engineering hurdle?
|
[
"This paper",
" offers some speculation about the \"audio uncanny valley.\" The author gives the following recommendations about how to design uncanny sound:",
"Certain amplitude envelopes applied to sound affect perceptions of urgency.",
"Frequency might have an effect on the unpleasantness of sound and this might lead to negative affect.",
"Familiar or iconic sounds can be defamiliarized and this can lead to perceptions of uncanniness.",
"Uncertainty about the location of a sound source, its cause or its meaning in the virtual world increases the fear emotion.",
"An aural resolution that is lower than a high quality, human-like visual resolution might lead to the uncanny.",
"An exaggerated articulation of the mouth whilst speaking might lead to the uncanny.",
"A lack of synchronization between lips and voice for photo-realistic virtual characters leads to a perception of the uncanny.",
"However, the author emphasizes above all the \"framing context,\" in which the uncanniness of the sound depends on the context in which it is heard (e.g. a horror video game vs. a humorous video game)."
] |
[
"Possibly not related to this but you might want to check out the coconut effect. Which is where early media used false sounds as recording the real effect was to difficult but now recording has got better people have become conditioned to associate certain false sounds and find it hard to accept the correct audio. Its called the coconut effect as coconuts were often used to replicate the sound of horse hooves."
] |
[
"Desynchronization is a big one - even just a could hundred milliseconds of desync will make someone speaking seem off."
] |
[
"Do Black holes and anti-matter Black holes have different characteristics, and if so what are those characteristics?"
] |
[
false
] |
I know they both consist of having an Event horizon and a singularity, but other then that, what makes them different?
|
[
"No, there is no distinction between a black hole formed from matter or from anti-matter. General relativity tells us that the properties of a black hole are determined by its mass, electric charge, and angular momentum. How those properties arise is irrelevant."
] |
[
"They're saying that it doesn't matter (heh) what type of matter went into the black hole. If you must draw the distinction, the collision you're talking about would look the same as if two \"regular\" black holes merged (because it ",
" the same). LIGO observed one of those last year."
] |
[
"Ok thanks for the info, I apologise for asking and I know what I'm about to describe has never been observed but would would happen if a matter and anti-matter black hole where to collide?",
"Would we get what is supposed to happen with matter Black holes I.E huge tides of gravitational waves and they form as 1? Or is the answer unknown? "
] |
[
"What state of matter is fire?"
] |
[
false
] | null |
[
" is most accurately described as a ",
" - an event - and therefore is not a state of matter. While there are components involved that are in the solid, liquid, gaseous, or plasma states, fire is more than just the sum of the components. It is also a chemical reaction - that is, these components are undergoing combustion reaction, and releasing heat in the form of atomic or molecular emission, and black body radiation.",
"The analogy I've always used is this: what is a ",
"? Well it's not just a bunch of people on a track with a starter's pistol. It's an ",
". The people are ",
" something, and there is sound, light, and all that stuff going on. So asking \"what state of matter is fire\" is akin to asking \"what type of building is a race?\""
] |
[
"Would it be correct to call fire just, light? An EM wave."
] |
[
"But not any light source is a fire.",
"Edit: Come on guys, don't downvote for asking questions!"
] |
[
"Why is time squared in D=1/2gt^2?"
] |
[
false
] |
Why is time squared in D=1/2gt^2? I think I can sort of understand why it's 1/2 cuz the trajectory path is one half of a parabola (though that's just what I thought of, I could be totally wrong there lol) but I don't really understand why t^2.
|
[
"In general, the equation is:",
"d = v0*t + 1/2*a*t",
"and if you're starting from rest and falling under the force of only gravity, yet it does simplify to ",
"d = 1/2*g*t",
"So, why the (1/2) and why the t",
"?",
"First, let's look at the t",
" term. The ",
"g",
" is an acceleration due to gravity. Acceleration tells you how fast velocity is changing. So it has units of (m/s)/s or m/s",
" (velocity divided by time). So, the longer you apply an acceleration, the faster you go. And velocity tells you how quickly your position is changing, that's why it has units of distance divided by time. So, the time is doing double duty- the longer you accelerate the faster you're going, and the faster you're going the further you go. So, you get the t",
".",
"Or perhaps put more simply: ",
"d = v*t and v = a*t so (missing the 1/2) d = a*t*t = a*t",
"So, perhaps the above equations tell you where the one half comes from. When we say d = v*t what velocity do we use? We have to use the average velocity. So, if you're starting from rest, your starting velocity is 0. After ",
"t",
" seconds, you're velocity is a*t, so your average velocity is (a*t - 0)/2 = 1/2*a*t",
"So, now you can say d = (average velocity)*t and so d = 1/2*a*t*t = 1/2*a*t"
] |
[
"Not really different than what was said in the other post, but simpler:",
"The distanced traveled by an object is the object's average velocity times the time it is in motion.",
"For an object that started from rest and is accelerating at a constant rate, such as a dropped object, the average velocity is half of the final velocity.",
"So d = 1/2 v_f * t.",
"For this same object, the final velocity is a function of the acceleration. ",
"So v_f = a*t.",
"Sub that in and you get:",
"d = 1/2 * (a*t) *t = 1/2at",
"And in the case of a falling object a = g."
] |
[
"g is acceleration, so to get the speed you need to multiply g and time.",
"To get the distance D you need to multiply the speed and time.",
"As you can see, you need to multiply g and time twice to go from acceleration to distance. The reason there's a 1/2 factor is because the speed is changing at a steady rate. At the instant you drop an object, its speed is zero. After 1s, its speed is g, so its effective speed during that period is just half of g. You can apply the same logic for any other period of time."
] |
[
"How come, when we rub our eyes hard enough we see those weird colors and patterns?"
] |
[
false
] | null |
[
"Last time someone asked this one of the answers said it had to do with physical stimulus to the optical nerves, and the nerves sending the information the only way they know how. I'll see if I can find it hold on",
"Edit: ",
"/u/kgluds",
" with the answer, link to comment: ",
"https://www.reddit.com/r/askscience/comments/29d26x/what_is_the_circle_that_sometimes_forms_when/cik0num",
"Edit 2: or for the lazy,",
"\"These are called phosophenes. This happens when you touch or rub your closed eyes, or even squeeze them tightly closed, because the pressure stimulates your retina and makes your brain perceive light. Your retina has tiny cells in it that are used to collect light. When it takes in a light stimulus, the sensation crosses over to the other side of the eyeball which is why you see the light form opposite of the side of the eye that pressure was added. Phosophenes are different from hallucinations because they are caused by a physical pressure to the eyeball that creates the sensation of \"seeing light\" whereas hallucinations are generated solely in the occipital lobe.\"",
"Edit 3: I am not an opthalmologist, I just remembered someone asked a similar question. I know nothing of Opthalmology aside from that iritis sucks a whole hell of a lot."
] |
[
"Those are vitreous floaters. They are actually inside your eye. They are basically pieces of collagen. "
] |
[
"What about when you get a head rush, you know stand up too fast or feel odd and you get like pixels n fractal weirdness going on for a moment whether you close you eyes or don't- is that basically the same thing?"
] |
[
"What does it mean when a series converges? Laymans terms."
] |
[
false
] |
I know that if you use it in the context of area under a curve that when it is convergent, it has a finite area. When it is divergent then it has an infinite area. Thanks guys.
|
[
"Consider 1/2+1/4+1/8+1/16... each term is smaller than the last and each additional term brings the series closer to 2. This is a convergent series, each additional term brings it closer to converging on a finite value.",
"Consider 1/2+1/3+1/4+1/5+1/6+1/7... this series does not converge. It reaches infinity with an infinite number of terms, or an arbitrarily large number with a very large number of terms."
] |
[
"Yes it is hard to conceptualize but there is a very simple proof that the sum of 1/n will grow without bounds as n gets large. Look at this series:",
"A = 1/2 + 1/4 + 1/4 + 1/8 + 1/8 + 1/8 + 1/8 + ...",
"Which is just a laborious way of writing ",
"1/2 + 1/2 + 1/2 + ...",
"It is obviously divergent. Now compare that to the series 1/n:",
"B = 1/2 + 1/3 + 1/4 + 1/5 + ...",
"B is greater than A for n = 3, 4, ... for every term. Because of this, 1/n must also be divergent. ",
"Trying to visualize it as a curve on a graph, slowly creeping to infinity, is usually just confusing."
] |
[
"Your first impression may be that if you add up an infinite number of anything, that their sum must be infinite. However, this isn't necessarily the case. ",
"The most common example is the series 1/n",
", where n is integers. Writing out the first couple of terms you get 1/1 + 1/2 + 1/4 + 1/16 +... You can try it, keep typing as many of those as you want into your calculator, and you'll see that while the sum is (of course) always increasing, no matter how many you punch in, it will never get larger than 1.7 (in fact, it goes to \"pi squared over six\"). ",
"If this is still confusing, imagine an even simpler series: 0.9 + 0.09 + 0.009 + 0.0009... you can see that each term of this series will get you a little closer to 1, but you'll never be larger than 1. So from these examples you can start to see how series convergence works. You are adding up an infinite number of things, but if the series gets \"smaller\" quickly enough, the sum can be contained. "
] |
[
"Is Montessori's \"Sensitive Phases/Periods\" an accepted concept? Alternatively: Is it possible children never catch up something they didn't learn at the right time?"
] |
[
false
] |
This came up in a conversation with a coworker about the Corona lockdowns. She claimed that children would have life long deficits in speaking, mimicking, etc. because when they don't learn such things at the correct time, it's almost impossible to catch up to the usual path. I doubted that because I thought those skills are rather being improved thw whole life, but had no experience with children and related science. I only found out that goes back to Montessori when I googled it - and then I realized I ONLY find Montessori schools or forums on that topic but no studies or else. Are there scientific studies/theories/experiments that support or deny that claim?
|
[
"A starting point might be Piaget's theory of cognitive development and the various studies which find that past a certain age, the brain is not capable of forming some structures (i.e. grammar).",
"However, and saying this as a parent of children who went to a Montessori kindergarten/school (and were taken away from there), the implementations are seldom based on actual science.",
"Montessori herself was aiming her methods at poor children (from, supposedly, environments with a certain level of sensory deprivation). It has, however, evolved into a posh concept for parents with money.",
"It competes with other education systems like the Waldorf/Steiner schools, which, on the contrary, are proudly anti-intellectual and keep children from learning to read/write for a longer time.",
"I have found that most of these systems are essentially esoteric, many are propounded by people incapable of understanding basic principles of science let alone applying them, and the end goals are usually: (1) get money from gullible parents (2) disseminate some fringe, sectarian system of \"knowledge\" or (3) act out of blissful ignorance.",
"As a parent I have observed that some of the things which \"our\" Montessori system was telling us about, like the \"sensitive period\" of children for order at about age 2, might have been true, but, the brain is so amazingly flexible, that I'm pretty convinced they will grow up ok even without strict adherence to this system, which, for the most part, appears to be made up."
] |
[
"Are there scientific studies/theories/experiments that support or deny that claim?",
"In linguistics and speech pathology we definitely have the concept of 'critical periods'. That should provide you with a good springboard to go do further research. The 'critical period hypothesis'",
"And yes, it's widely accepted. Certain language skills need to be developed early or else further speech and language will forever be lagging behind. ",
"2nd language learning also has some 'critical periods'.",
"It's important to note that some of them are softer or blurrier lines than others. Child and language development is complex. But that doesn't make it any less real."
] |
[
"As others have pointed out, humans do have sensitive periods during which neurological and sensory growth is tied to increased receptivity to environmental information and skill development.",
"Some areas that you can research are:",
"Of course, these are general sensitive periods, and they may be able to be supported through scaffolding other intensive interventions if a sensitive period is missed. But trying to re-train a non-verbal, non-motor skill is almost impossible after a developmental stage is passed.",
"The thing to keep in mind about the CORONA lockdown and any deficits that might occur, is that they will occur in a generational cohort, and so the effects may not be as stark as if an individual is affected alone.",
"For example, media has resulted in significant changes to personality development for many people. Because it is almost global, it has not had as profound an impact on select individuals.",
"Guru, B. P., Aabid Nabi, and Raja Raslana. \"Role of television in child development.\" Research Journal of Humanities and Social Sciences 4.2 (2013): 264-270."
] |
[
"How does Narcolepsy work, and how is it diagnosed?"
] |
[
false
] |
I just want to know is it really the uncontrollable urge to fall asleep (out of their control and must fall asleep). Plus how do they diagnose it?
|
[
"Narcolepsy is a disease associated with a defect for a neurotransmitter called orexin (synonym hypocretin). Orexin, secreted from neurons of the hypothalamus (interbrain), plays a small role in appetite regulation (stimulates the feeling of hunger)... but its main purpose is its modulating effect on the sleep centers of the brain stem. Basically, the orexin system responds to different homeostatic (\"household\") stimuli of the central nervous system, like temperature, nutrition, water balance among others. orexin via the orexin receptors stimulates the so called ARAS (ascending reticular activating system) in the brain stem (in the reticular formation) and other neurological structures associated with the ARAS. The ARAS itself plays an essential role in the regulation of wakefulness and sleep: among other things, it most importantly stimulates the thalamus (the \"gate to consciousness\") to be more reactive towards sensitive input from the body periphery and motor output from the cerebral cortex. The logical chain of reaction is thus that orexin stimulates the ARAS, while the ARAS stimulates wakefulness and inhibits sleepiness. Functionally, orexin is important to link homeostatic needs with the state of awakeness (e.g. \"need to be more wakeful because hungry and looking out for food\"). In narcolepsy, because of the disturbance of either orexin secretion or orexin receptor function, ergo the functional loss of the system, wakefulness is a lot more sensitive towards disruption, and narcoleptics end up being way more susceptible towards simply losing consciousness and/or muscle control. (please note that there are other neurotransmitter systems also governing over the ARAS, so the loss of function of orexin does not lead to absolute unconsciousness, but it's a mikado kind of situation where even one failing system ends up causing severe dysfunctions) What exactly causes this disturbance is not fully known, animal experiments and epidemiological studies have led us to believe that a genetical defect in the gene for the orexin receptor may be involved. Autoimmune disorders which attack the nerve cells excreting orexin are also in discussion."
] |
[
"Wow, thank you for that explanation. It makes a little more sense."
] |
[
"We do not allow medical advice or anecdotes on ",
"/r/AskScience",
". Do not ask users to post personal medical information here."
] |
[
"Has there or can there ever be an animal that is allergic to itself?"
] |
[
false
] |
[deleted]
|
[
"There are a wide range of what are known as '",
"autoimmune diseases",
"' . These are cases in which the immune system attacks a tissue or substance that is a normal part of the body."
] |
[
"Quite right. I assumed that the OP intended 'allergic' in the colloquial sense."
] |
[
"Quite right. I assumed that the OP intended 'allergic' in the colloquial sense."
] |
[
"Can other animals be allergic to us?"
] |
[
false
] |
We all know that people can be allergic to cats and dogs but is the opposite true? Can our pets be allergic us? If so, is this just in mammals or across all/most species?
|
[
"Yes! You can get your pet tested for human dander and even get them allergy immunotherapy shots for it. ",
"Info here: ",
"https://wagwalking.com/condition/human-dander-allergies",
"Here is the veterinary catalog for Greer which makes allergen extracts. Human dander is on page 20 (item E18): ",
"https://www.stagrveterinaryallergy.com/wp-content/uploads/2019/12/Stallergenes-Greer-Veterinary-Product-and-Services-Catalog.pdf",
"Source: work in the allergy immunotherapy market."
] |
[
"Yes. Cats and Dogs can be allergic to humans. For the same reasons humans can be allergic to them ",
"\"Dander\" or dry shed skin.",
" Dogs and Cats can also be allergic to one another. ",
"To my knowledge most if not all animals can have allergies and against most if not all shed skin it should be theoretically possible for any animal to allergic to another."
] |
[
"Allergic dogs' most basic symptom is being itchy.",
"In lay terms - their skin barrier doesn't function correctly, leaving gaps for opportunistic bacterial infections, exacerbated by the act of scratching. Their body overreacts to normal elements of their environment with an out-of-control inflammatory response.",
"So, skin and ear infections are common.",
"Source: Veterinary Technician with 2 years experience in dermatology practice"
] |
[
"Why does a single gas molecule which is hot rise above another one which is cool?"
] |
[
false
] |
I know that they say an area of hot air becomes less dense, and so it rises above colder areas of air. However, this seems to be treating areas of gas molecules as though they are a coherent aggregation, like a solid. To explain my question, let me refer to an example: a plank of wood floats because the molecules constituting it are less densely packed, and so water is held down more strongly, 'pushing up' the wood relative to the water (at least, that's my understanding). But what if there was a single constituent molecule from that wood existing in the water? Would it float? It is neither densely nor sparsely aggregated, existing all by itself. So, back to my original question: aren't the individual molecules that make up a heated area of air independent, like a single molecule from that wooden plank? And if so, why do they act like an aggregated 'body' with those molecules around them, just because they are at the same temperature? Thanks. Hope this made sense to someone.
|
[
"Metrics such as temperature describe the behavior of a system that is made up of components. These types of properties are termed emergent properties because they are derived from the behavior of the system as a whole and are not observable if you were to look only at the components.",
"So, for your specific question, individual molecules do not have temperature. They are not \"hot\" or \"cool\" exactly, although they do have energy that is zipping them around their surroundings. Molecules with more energy will move faster and collide with other matter more frequently and with more force. It is only when you begin to look at a system of molecules that ideas such as temperature start to be meaningful. In that sense, a group of molecules with a certain amount of energy will correspond to a certain temperature. If these molecules are \"hotter\" than other molecules, then they will be moving about much more rapidly and they will be less dense than the latter group of molecules. Properties such as temperature and density are emergent from the system of molecules interacting with each other and interacting with their surroundings."
] |
[
"an area of hot air becomes less dense, and so it rises above colder areas of air. ",
"First, you need to look at it the other way around - hot air doesn't rise, cold air sinks. As it sinks, it forces the hotter air upwards.",
"Now, think of a mess (and I do mean 'mess' for the imagery, not 'mass') of cold air, with the molecules fairly still and fairly dense. Then, something heats up a bit of it near the bottom - what's going to happen?",
"The molecules of hot air will bounce around a lot more than the cold, and sometimes they're going to bounce up. When they do, the less active cold air is more likely to fall into the gap than to move in another direction, and now there's nowhere for that hot air molecule to go because it will only bounce off the cold air molecule if it bounces downward again. (Transferring some heat in the process, but we can ignore that for the purposes of this explanation)",
"Multiply this by unimaginable numbers of interactions, and you end up with a column of hot air rising while all the cold air around it rushes in to fill the gap at the bottom."
] |
[
"But what if there was a single constituent molecule from that wood existing in the water? Would it float? It is neither densely nor sparsely aggregated, existing all by itself.",
"My reaction to this is no not really - a single molecule would have dynamical behavior that isn't familiar like the bouyant force example you gave. I have no idea how to describe what that situation ",
" be like - but I'm positive it would be invalid to treat it like a whole plank of wood floating."
] |
[
"What limits the speed of healing?"
] |
[
false
] |
I was thinking about this earlier... it seems reasonable to assume that a living thing that was able to heal faster than its competition would have a definite Darwinian advantage, but it seems as though most animals have sort of plateaued at about the same place. For example if I get a small cut it will probably take a couple of days to heal, and it would probably take a cat or a cow or a platypus about the same amount of time. There's no animal I (that I know of) that would heal in seconds, for example. So what prevents this? If I had to guess I'd say that fast healing would either use up energy too quickly to be viable, or that rapid cell multiplication like that would cause something like cancer or premature aging. But I am no scientician!
|
[
"In a way it does happen fast! The most important part of fixing an injury would be to make sure you don't get any infections from it, and we do mount responses to that end in minutes (or maybe an hour or two) by clotting the wound, recruiting LOTS of immune cells and making sure that infectious agents don't enter and you don't loose blood.",
"But full healing is mechanistically very complex: you need to first clear all the cells that died, then make new cells in the same design as before, all the while making sure you don't loose any blood or get any infections. I'd say our bodies do a pretty amazing feat already, even the theoretically best possible levels!"
] |
[
"growth factors are released locally when you are wounded and are highly regulated. you need them for healing, but they are released in tiny bursts. every time they are released, they almost immediately stop being released. the reason for this is that releasing growth factors without \"rest\" (bad term, eh..) is one of the things that can cause cancer.",
"other than this limit, the speed of cellular division. if it divides faster, you end up getting more errors, and that is bad."
] |
[
"How does hyperbaric treatment accelerate healing? or is this just a gimmick professional athletes fall victim to?"
] |
[
"How does the light during \"daytime\" on other bodies in our solar system compare with the light on Earth?"
] |
[
false
] |
So if I were to take a space ship right now to the surfaces of the following: I'm ignoring things like atmosphere and just wondering how much sunlight makes it to those distances. How would it compare to times of day here? I was wondering whether we would be able to see if we were on the surface of places like this. It seems like a weird concept that even during the day in some places in our solar system it might be as dark as night here.
|
[
"Light from the sun travels away from it in a sphere, which means that the intensity of the light at a certain distance changes with the inverse of the square of that distance. For example, an observer at twice the distance from the sun as the earth is would experience 1/4 the intensity of light we experience on earth. Note: the comparison with earth is easiest when distances are measured in AU, since earth is approximately 1 AU from the sun at all times.",
"Mercury experiences approximately six times the intensity of light earth experiences.",
"At its greatest distance from the sun, Pluto receives only 4*10",
" the intensity of light we do on earth.",
"Voyager 1 is getting only 6*10",
" the amount of light we get on earth."
] |
[
"Although Sunlight intensity decreases quite rapidly with distance (1/r",
" so twice as far away from the Sun, the sunlight is a quarter as bright), the Sun is so incredibly bright that even as far out as Pluto day time will be noticably much brighter than night time. (Phil Plait did the maths for Pluto and the Sun from Pluto is around 150 to 450 times as bright as the full Moon is from Earth).",
"At the distance of Voyager, around 120 AU (Sun-Earth distance is 1 AU), sunlight will still end up around 27 times brighter than the full Moon from Earth and still much, much brighter than any other star.",
"Using similar calculations, at a distance of 632 AU, the Sun would be about as bright as the full Moon is from Earth. That's about 0.01 light years away, and the Moon is still 60,000 times brighter than the brightest star in our sky, Sirius!",
"Sun from Pluto calculation: ",
"http://blogs.discovermagazine.com/badastronomy/2012/03/15/bafact-math-how-bright-is-the-sun-from-pluto/#.VFKE-3tCjVI"
] |
[
"I'm ignoring things like atmosphere and just wondering how much sunlight makes it to those distances. ",
"This isn't a good idea, and I'll tell you why in just a moment.",
"How would it compare to times of day here? ",
"And that's the point, there is no such thing as a universal daylight brightness here on earth, because of the atmosphere. The absorbtion of the atmosphere is so big, that we only receive half of the light than an astronaut in a high orbit around earth. And I'm not even talking about extreme conditions like living near the poles, sunset and sunrise or having an cloudy sky. ",
"However, this isn't a big impact for anyone involved, because the human eye is working roughly at a logarithmic scale. Traveling to another place on earth or simply having clouds at the sky can really be a bigger change than traveling to another planet. ",
"It seems like a weird concept that even during the day in some places in our solar system it might be as dark as night here. ",
"This is only the case on the surface of dwarf planets far, far out. On the planets the sun might appear dimmer, but it's still as bright as to be considered 'day'. The major difference are the appearance of the sky and the size of the sun. On a moon of Saturn, the brightness is about the same as a cloudy day on earth. However it isn't scattered, because there are no clouds, but pointed from a mini sun. ",
"A good comparison would be a single but extremely bright floodlight in a stadium which illuminates the field to daylight level, but when you look up you'll see a black sky. Same goes for any other planet, with decreasing size and brightness (yet enough to make it 'day', even on Neptune.",
"The further you move out, the dimmer the sun becomes, but even on Pluto at its farthest distance, the sun is more than 150 times brighter than the full moon. This is about the brightness right after sunset. You could easily walk around or read, except for freezing your toes and fingers. ",
"In order to have night at day you still have to move much further. Sedna and (308933) 2006 SQ372 are great candidates for having night at day. At Sedna's greatest distance, the sun has the brightness of a gibbeous moon, and for (308933) 2006 SQ372 it would only appear as bright as a crescent moon. You would still have very faint shadows tho. ",
"The voyager probes are somewhere between Pluto and Sedna, so it's still 'day' on them. Mercury is a whole different story. When closest to the sun, it receives 18 times more light than somebody on the surface of the earth (or 9 times more than in orbit). Although the surface is darker than asphalt, it would appear like an extremely bright rocky desert.When looking above, you'll only see the instant blinding flash of the sun in a pitch black sky. The sun appears more than 3 times larger than the full moon and burns directly off your retina without wearing protection goggles. ",
"I'll add a list of the sun's apparent magnitudes for comparison here. Each magnitude means 2,512 times difference. Lower values are increased magnitudes, so -1 is 2,512 times brighter than 0 for example.",
"Mercury:",
"perihelion: -29,3 Mag\naphelion: -28,4 Mag ",
"Venus: -27,44 Mag (almost circular orbit, so only a minuscule difference between aphelion and perihelion) ",
"Earth/Moon: -26,74 Mag ",
"Mars:",
"perihelion: -26,04 Mag\naphelion: -25,63 Mag ",
"Ceres: ",
"perihelion: -24,71 Mag\naphelion: -24,38 Mag ",
"Metis (Jupiter): -23,16 Mag",
"Full Jupiter Magnitude from Metis: -20,8 Mag\nSize comparison: Sun: 0,15° Jupiter: 58,06°(!) ",
"Enceladus (Saturn): -21,85 Mag\nFull Saturn: -17,5 Mag ",
"Miranda (Uranus): -20,33 Mag\nFull Uranus: -15,96 Mag ",
"Triton (Neptune): -19,35 Mag\nFull Neptune: -12,62 Mag (as bright as the earth's moon, but roughly 15 times larger) ",
"Pluto:",
"perihelion: -19,38 Mag\naphelion: -18,28 Mag ",
"Voyager 1: -16,18 Mag\nVoyager 2: -16,61 Mag ",
"(308933) 2006 SQ372 (object with the furthest aphelion currently known):",
"perihelion: -19,83 Mag\naphelion: about -10,5 Mag ",
"I hope that helps. Sorry for bad spelling and/or bad english."
] |
[
"How can trauma to the spinal cord cause quadriplegia while leaving all other bodily processes requisite for continued life unaffected?"
] |
[
false
] | null |
[
"without getting too technical zebra-stampede has the right idea. Now allow me to get a little bit technical. ",
"Your heart basically beats on its own. The brain modulates the rate and force of that beating via sensory afferents which are integrated in a part of the brain called the ",
"nucleus solitary tract",
" (The NTS). The NTS receives information on blood pressure via baroreceptors in the aortic arch and ",
"carotid sinus",
". These signals mediate the ",
"baroreceptor reflex",
" (modulation of heart rate and force of contraction in response to body needs). Afferent nerves in the carotid sinus travel via the glossopharyngeal nerve, cranial nerve IX while afferents from the aortic arch travel via the ",
"vagus nerve",
" (cranial nerve X). ",
"As these are cranial nerves, they are not within the spinal cord, and not affected (generally) by focal spinal trauma. That being said, the vagus nerve provides the parasympathetic drive to the heart. The sympathetic nervous system may exhibit control over the heart hormonally in the form of circulating epinephrine (colloquially known as adrenaline), or by inhibiting activity in vagal efferents at higher levels. Alternatively, modulation may be conveyed more directly via cardiac splanchnic nerves which CAN be damaged by spinal injury. As stated previously, the heart can largely beat without the brain telling it to do so, although patients with spinal trauma may show cardiovascular abnormalities or pathologies, and this may put them at further risk. These abnormalities are usually limited to the first two weeks after spinal trauma ",
"(source)",
"Respiration is a little bit different. There are many parts of the brain which control respiratory pattern, rate, and rhythym. Notable among them is the pre-botzinger complex (preBotC) if you were curious. The brain then sends the impulse to breathe primarily via the ",
"phrenic nerve.",
". Unlike the nerves discussed previously, the phrenic nerve does branch from roots of the cervical spinal cord in your neck, specifically nerves exiting below vertebrae C3, C4, and C5, with C4 providing the largest contribution.",
"Now, to be quadriplegic, you by definition have no use of any of your limbs, so we will assume the damage is at the lowest point possible which still prevents all upper limb function. The nerves of the upper limb are collectively termed the ",
"brachial plexus",
" which receives input from C5, C6, C7, C8, and T1 which eventually give rise to named nerves (musculo-cutaneous, axillary, radial, ulnar, and median). Clearly, based on C5 contributing to both the brachial plexus and the phrenic nerve, there is a bit of respiratory activity which is mediated by a nerve that has ceased functioning in quadriplegics. However, as stated, The most important part of the phrenic nerve derives from C4. That being said, damage to C5 can result in problems with respiration, although not always ",
"(source)",
". Many patients who initially may require a ventilator may learn to breathe without the use of C5. ",
"Fantastic question, and I hope my answer wasn't too long or technical for you. I've gone on long enough, and I have exams to study for :) I'll let some other brave soul field the GI and splanchnic areas :)"
] |
[
"In my anatomy lab we used a rhyme \"C3-5 keep the diaphragm alive\" to remind us of what levels make up the phrenic nerve"
] |
[
"Without doing too much research, based on my physiology and anatomy classes I'd say because your spinal cord houses your peripheral nerves, but the brain and brain stem are controlling everything else. So presuming those parts aren't damaged you can continue with your other processes. You might not be able to move your arms, but your brain also stimulates a lot of chemical and hormonal pathways that don't rely on nerves to do your essential functions. "
] |
[
"Why was Aristotle wrong?"
] |
[
false
] |
He argued that objects fall to the ground in proportion to their weight. Is the force of gravity not dependent on the mass between 2 objects? Wouldn't two objects of varying mass accelerate at different speeds on a micro scale? Is gravity (in this case) just negligible because of the mass difference between 2 rocks and our planet? Should we redo Galileo's experiment now that we have more accurate testing technology?
|
[
"Force = mass x acceleration",
"acceleration = Force / mass",
"Since, as you alluded to, the force of gravity on an object depends on it's mass, as the mass scales up, so does the force (in a linear relationship). This means that for objects held at the same height, they will accelerate towards the Earth's center of mass at the same speed.",
"Galileo's experiments ",
" been redone, many times, including on the moon (where the atmosphere is not an issue). A feather falls at the same \"speed\" as a lead ball. "
] |
[
"The force of gravity does increase with a rocks mass but so does the rocks inertia increase with mass, these two effects cancel out and result in acceleration that only depends on the planets mass.",
"You can see this by equating newtons 2nd law and his gravitational law",
"ma = GmM/r",
"where little m is the mass of our rock, big M the mass of the planet and r the distance between them. You can cancel out little m on both sides showing that the acceleration of the rock is independent of m."
] |
[
"Well there's a difference when you talk about an objects gravitational acceleration and the amount of gravitaional force between two objects. Gravitational acceleration on earth is the same for all objects because it only depends on the mass and radius of the earth which don't really change. The mass of the object falling towards earth is irrelevant. Now if you want to calculate the magnitude of gravitational force between two objects, THEN the masses of both DO matter, as does the distance between the centers of mass of the objects. If you'd like, I can show you the equations used to calculate each respectively! :)"
] |
[
"Can someone explain in depth the Receptor docking process?"
] |
[
false
] |
[deleted]
|
[
"Wow, something I know!",
"I am a PhD student working with LGICs and ligand binding specifically (trying to show a novel acetylcholine binding orientation in a set of genes that are different from standard nAChRs). ",
"Anyway, if you don't mind I will refer to the 5HT3 receptor as it is a ligand gated ion channel, which I am more familiar with than GPCRs.",
"How much of the compound directly binds to the receptor",
"This depends, we tend not to refer to binding in terms of how much serotonin is released compared to binding, but rather in terms of channel activation. But more directly, the amount of compound that binds depends on the amount released from the presynaptic neuron, how many receptors are expressed at the surface of the post synaptic neuron, the presence of other agonists / competitive antagonists, the efficacy / concentration of reuptake etc. Simply put, 'some' will bind, 'some' will be collect from the synaptic cleft, 'a little' will spill over and escape the synaptic cleft. 'some' will just get broken down/metabolized.",
"Why is the presence of a primary amine necessary?",
"This is actually exactly what I am looking at, especially comparing to acetylcholine which has a quaternary amine. The amino contributes to the formation of a pi-cationic interaction which is required for activation of the channel. This interaction is formed by the interaction of a cation and the negative quadrupole of an aromatic amino acid residue. In the case of the 5HT3 channel, this residue is Trytophan183 in binding loop B. Now, two things affect the strength of this pi-cationic interaction, 1 being the nature of the amine group (primary forms a stronger interaction than quaternary), and the precise location of the amine group.",
"Why does saturating it with carbon atoms affect the agonistic properties?",
"This I am less sure about. If you have a specific example about what was modified and how I can better explain. How ever if the carbons are reducing the cationic properties of the amine group, then the pi-cationic interaction is weakened substantially. In an experiment done by Pless ",
" 2008, they created unnatural amino acids (and corresponding tRNA) in the human glycine receptor. What they did was fluorinated the phenylalanine residue (serves the same function to the Trp183 I mentioned before) and replaced the wild type with it on the receptor. Each additional fluorine they add pulled electrons from the benzene ring, resulting in a weaker (but non-zero) pi-cationic interaction. They also added a cyano group (CN) which accomplishes the same thing. With each addition we saw an increase in the effective concentration of glycine to activate 50% of the channels (EC50), with a very high increase for a 3,4,5-flourylphenylalanine (I can't remember the exact name).",
"So you will still see partial agonism if you modify the amino acids of the receptor, just like you see partial agonism of 5-HT receptors with compounds different from serotonin.",
"Just ask if you would like to know more / if I was unclear about something."
] |
[
"I will read it later, I am doing revision at the moment, but you misinterpreted my first question (poor wording on my behalf).",
"What I meant is of the entire chemical makeup of a single neurotransmitter, how many of the atoms will actually interact with the receptor. ",
"Take Serotonin: Does the Amine, Indole and Indole ring substitutions all \"connect\" to the receptor is it more like the amine connects while the indoles nitrogen presence mediates the process and the backend substitutions are related purely to MAO degradation?"
] |
[
"Much of the 5HT3 stuff has been generated from homology modelling with specific experimental analysis (SCAM, standard amino acid mutagenesis, etc).",
"But for starters the binding pocket (as mentioned by XIllusions) is fairly well studied. I would suspect a majority of the core of serotonin doesn't directly interact with amino acids, as agonists will fit into what is known as the aromatic box. The aromatic box is 4 or 5 aromatic residues contributed by 2 separate subunits binding loops and will literally surround the core of the ligand (at least with acetylcholine, and my channels are extremely similar to Mod-1 which is another serotonin receptor). The hydroxyl group most likely will form a hydrogen bond with a residue (likely asparagine on loop E of the complimentary subunit). The key is pi-cationic interaction between the amine group and the tryptophan in loop B (primary subunit). ",
"There will be a number of other residues interacting with serotonin, so I will explain why it is difficult to solve this issue without crystalizing the protein (which is a monumental pain in the ass for transmembrane proteins). The major thing is distinguishing between what affects direct ligand binding and what affects the following changes in conformation. We can only really measure changes with electrophysiology which is a measurement of the changes in current/voltage of the membrane. So any modification we make will result in an increase/decrease in the ability of the channel to conduct ions (which could be a structural change or ligand binding change).",
"That being said these neurotransmitters are pretty specific in the level of response. Partial agonists for 5HT3 receptors do exist obviously. But it is likely the indole plays a role in binding."
] |
[
"What is the minimum number of mating pairs to sustain a human population?"
] |
[
false
] | null |
[
"Technically the only thing you need to create a new population of any sexual species is one male and one female, or a female which is pregnant with a male fetus.",
"Inbreeding is only detrimental if you have deleterious alleles. Not everybody processes such alleles, or \"bad genes\" (to a certain extent). Of course, the smaller the breeding population the greater chance you have of this occurring. In nature, such events are known to have occurred or are occurring: founder effects or bottleneck effects are two situations where the breeding population is reduced by a significant amount. ",
"Inbreeding",
" has two major problems: ",
"The accumulation of deleterious alleles",
"Reduced ability to adapt to change, be that to environmental change or some disease. ",
"In most cases, for the deleterious allele to be expressed they must be combined with someone else with the same deleterious allele. This is why genetic disorders / diseases are so rare. Moreover, for it to really effect the population it must somehow negatively impact that individuals likelihood of reproducing. Having a genetic anomaly for 6 fingers instead of 5 is hardly a handicap unless it is combined with other genetic issues. You also need to have the genetic disorder appear before reproduction, have a genetic predisposition to alzheimer's is irrelevant because it (usually) expresses itself later in life, after reproduction has occurred. Those \"deleterious\" genes are past on. However, in the eyes of natural selection they have no effect (they are neutral).",
"So of that minimum viable population - how many of people posses those deleterious alleles? Basically, what are the chances that someone is born with both copies of the deleterious allele? Of those how many negatively effect survival/reproduction?",
"I guess what I am saying is that there is a misconception that inbreeding = increase in genetic disorders in all cases. I mean, the Galapagos Finches were likely founded by only a few breeding pairs, yet they have become several healthy diverse species. If you have a genetically diverse pair, perhaps only one is needed. If you want to guarantee low rates of genetic disorders then obviously a larger population is needed, and as other have said this may be achievable with as few as 1000 breeding adults. By adult, we mean people who are sexually capable of reproducing, in some cases this could be as young as 10."
] |
[
"Why pairs?"
] |
[
"That figure is known as the ",
"minimum viable population",
". ",
"If you believe in the population bottleneck caused by the ",
"Toba supervolcano eruption",
", the human population survived with 1,000-10,000 breeding pairs. ",
"The number I see most often is about 5,000 adults. I wish i had more time to look for scholarly articles but I don't. I'd imagine that the number would go down quite a bit if geneticists had control over who mated to avoid an inbreeding depression."
] |
[
"Any Evidence regarding the pyramids in Egypt?"
] |
[
false
] | null |
[
"What are you asking exactly?"
] |
[
"Do you know any documentaries about the pyramids that don't involve woo"
] |
[
"Not the right sub for such questions. Perhaps try a history sub"
] |
[
"Why are there no three-legged animals?"
] |
[
false
] |
As we all know, there are two-legged animals, and four-legged animals. I was wondering why there aren't any three-legged animals. And yet there are five-legged animals, like sea stars. I understand that bilateral symmetry is important. But then why would a subset of animals develop five legs, and not three legs. Why aren't there any three-legged animals?
|
[
"I don't know that we can say that three legs are less efficient than four or two. A three legged dog isn't a dog that evolved to run with three legs, it is a dog that evolved to run with four legs and through a developmental or physical accident, it has lost one. To put this another way, consider three wheel vehicles. Three wheel vehicles are not built by simply removing a wheel from a four wheel vehicle, but the body plan is redesigned around a single front wheel or rear wheel. It is possible that an engineer could design a three limbed robot that is more efficient (or as efficient) as a four limbed robot.",
"How many \"legs\" does a dolphin have? Two pectoral limbs, but what about the tail? Couldn't that be considered a third limb? In that sense, the dolphin is an animal that evolved to move with three limbs, suggesting that there are cases in which three limbs is more efficient than two or four.",
"Monkeys with a prehensile tail, couldn't that functionally be a fifth limb? Prehensile tails are also used for locomation (balance and grabbing tree branches) and are also used to hold food.",
"Instead of looking at this as 3 versus 4 or 3 versus 2, look at it as 1 versus 2. What is more useful, one limb, or two limbs? Looking at the evolution of segmented animials, particularly insects, may be informative. Insects generally have two, four or no limbs per segment (this is also true of higher animals). One way insect body plans have changed is simply by adding or removing segments: the centipede, for example, has many segments with one pair of limbs per segment. Fruit flys have fewer segments; two segments have legs, another has the wings.",
"Having an even number of limbs per segment may be a quirk or accident of evolution (see, genetic drift), or it could be that one limb per segment was less useful than two in early organisms. (Two limbs may have been pure luck and if the dice had rolled another way, things would be different.) Once the body plan of two limbs per segment was established, significant evolutionary force would be necessary to change it. Two limbs per segment was maintained throughout evolution because there was no pressure for three limbs, unless we count the case of the dolphin and similar sea mammals -- these were tetrapods that lost their hind legs and evolved a tail (anatomically limbless segments) that serves a locomotive function.",
"In summary, I suggest that early in evolution it is likely that two limbs per segment served an adaptive purpose and over the course of evolution, there generally weren't significant forces to overcome the cost of an altered body plan. Exceptions are noted sea mammals, such as dolphins, and animals with prehensile tails, where evolution favored anatomy and physiology for an odd number of limbs."
] |
[
"Trilobozoa had tri-radial symmetry. But they did not use it for movement. So, I doubt inefficieny was the reason behind their extinction. I just used it as an example to state taht trilateral/tri-radial symmetry exists in nature.",
"But you are right, when it comes to animals, movement is extremely critical for survival and 3 limbs are not at all efficient.",
"From a symmetry perspective it is pretty rare as well. Banana is such a rare example, that has trilateral symmetry."
] |
[
"Odd numbered legs: good for sitting level on many surfaces, bad for locomotion."
] |
[
"Do animals with high body fat, like seals or bears, suffer higher rates of heart disease than other animals?"
] |
[
false
] | null |
[
"They have large amounts of subcutaneous fat to help insulate them from the cold. That does not mean they have more fat around their organs which is what adds to heart disease in humans."
] |
[
"And if they did I have a feeling it would be one of those things where it doesn't make a difference in their mortality as they would likely be killed by other causes before it becomes an issue."
] |
[
"This is pretty much the answer to a huge amount of species comparison questions that ask 'why does x species get y disease when humans do?'.",
"There are exceptions, elephants having multiple P53 gene copies is suspected to provide cancer resistance for example, but often the case is that wild animals normally do not have large enough lifespans to suffer diseases associated with human ageing. "
] |
[
"Why does multiple pregnancy cause pre-eclampsia?"
] |
[
false
] |
Also, a related condition, what exactly is the vanishing twin syndrome?
|
[
"According to UpToDate: \"The pathophysiology of preeclampsia likely involves both maternal and fetal/placental factors. Abnormalities in the development of the placental vasculature early in pregnancy, weeks to months before development of clinical manifestations of the disease, are well-documented. These abnormalities can result in placental underperfusion, and possibly hypoxia and ischemia. Observational data support the hypothesis that placental underperfusion, hypoxia, and/or ischemia may lead to release of circulating antiangiogenic factors and other substances that can cause widespread maternal systemic endothelial dysfunction [which causes the symptoms of pre-E]\"",
"Being pregnant with more than one baby increases your chances of having these \"fetal factors\" and multiple placentas increase the chance of having placental dysfunction.\nSome people describe pre-E as a \"reaction\" to the placenta or pregnancy itself. Multiple placentas or multiple fetuses means multiple chances to have a reaction."
] |
[
"Because it has an immune component. It has been hypothesized that it is actually an autoimmune disease. ",
"You don't have immune reactions the first time you are exposed.",
"Preeclampsia is associated with antibodies to a certain receptor that helps regulate blood pressure (angiotensin II type 1 receptor. Call it AT1R). These antibodies act to actually activate this receptor, which causes inflammation and increased blood pressure.",
"Your body shouldn't make antibodies against itself. There are mechanisms (collectively called \"tolerance\") that prevent this. You can have loss of tolerance, though, under certain extreme circumstances.",
"The human placenta has a LOT of AT1R. There is no tissue that expresses more of it. During childbirth, the placenta goes through lots of hypoxia (low oxygen), ischemia (low blood blow), inflammation, necrosis (cells dying), etc. These are all things that can promote loss of tolerance. ",
"So now you have a person that got \"immunized\" to AT1R during their first pregnancy. Second pregnancy rolls around and that antibody finds this rich concentration of targets forming (the placenta has lots of AT1R, remember). In the \"mind\" of the immune system, this is an infection. So more antibody is made. Which causes more inflammation and high blood pressure....preeclampsia. "
] |
[
"Thank you for this explanation! However, I was under the impression that preeclampsia/eclampsia can occur on the first pregnancy, not just the second one? One of the risk factors we learned for eclampsia was nulliparity.",
"In med school, we were only given a lecture about the clinical aspect of preeclampsia/eclampsia instead of our usual pathophysiology lecture and clinical lecture because our professor claimed no one really knew the exact pathophysiologic mechanism of it. ",
"He also showed us a photo he took of an empty portrait frame in a hospital somewhere (Chicago, maybe?) that the hospital has labeled as the next Nobel Prize in Medicine winner for whoever discovers the mechanism of eclampsia in the future. Good luck to everyone out there studying preeclampsia/eclampsia!"
] |
[
"Why is Water Most Dense at 4° C?"
] |
[
false
] |
[deleted]
|
[
"Hydrogen bonding. As water freezes, the strong bonds between hydrogens and oxygens of neighboring molecules cause water to \"spring back\" into a crystal lattice upon solidification.",
"Edit: spelling"
] |
[
"As water decreases in temperature, the speed of the molecules decreases. Below 4 degrees C, the molecular movement is slow enough for the formation of the lattice structure due to Van der Waal forces (hydrogen bonding, basically)."
] |
[
"What do you mean by most dense? I remember Brain Cox saying water on the surface of Titan would be as dense as steel. Is this question too vague or am I over analyzing this?"
] |
[
"Why are so many people allergic to peanuts?"
] |
[
false
] |
Peanut allergies seem to be incredibly prevalent. Why are so many people allergic to peanuts and not other foods?
|
[
"So parents are withholding allergens because they think their children could be allergic, but withholding them for so long is exactly what makes them allergic?"
] |
[
"It could also be an issue of ",
"exposure",
". In Israel the popular peanut-flavored snack, Bamba, is consumed from very young ages. Because of this, scientists have seen extremely low rates of peanut allergies throughout the Israeli population. "
] |
[
"Edit: As others have pointed out, parents choosing to withhold common allergens has been due to infant feeding consensus guidelines and advice provided by trusted medical professionals, such as their family GP. ",
"The Conversation makes a great point in their article",
":",
"The problem is, there have been so many changes to guidelines over the last few decades that parents are no longer sure what to believe.",
"There's speculation it has to do with parents choosing to withhold common allergens until too late. As far as I'm aware, there's no published work investigating what percentage of parents are making these choices, and their temporal trends, however there has been a meta analysis looking at the effect of timing of allergenic food introduction",
" which is supported by a randomised trial investigating peanut introduction in the first year of life vs complete avoidance.",
"Timing of Allergenic Food Introduction to the Infant Diet and Risk of Allergic or Autoimmune Disease: A Systematic Review and Meta-analysis.",
"Randomized trial of peanut consumption in infants at risk for peanut allergy."
] |
[
"Are capsaicin analogues like olvanil, arvanil or phenylacetylrinvanil also spicy in taste?"
] |
[
false
] |
These compounds are much stronger TRPV1 agonists than capsaicin, but according to various sources "lack pungency". If they indeed aren't spicy, how does it work? Paper about phenylacetylrinvanil:
|
[
"Nociception is the process whereby the nervous system makes the brain/mind aware of physiological pain.",
"A TRPV1 (pronounced TRIP-vee one) is one of the links in the nociception network. More specifically, a TRPV1 is an ion receptor channel located at the tip of the sensory neurons which relay pain signals to the brain.",
"Application of RTX to a TRPV1 stuns the ion receptor and props the receptor channel open. This allows calcium and sodium ions to flood into and overload the TRPV1 channel. This process results in the death, or at least the prolonged desensitization, of the sensory neuron. The nociception network is disrupted, and the brain/mind is no longer aware of the pain....",
"Unlike opioids, which operate in the brain, RTX operates downstream in the nociception network, so widespread adoption of RTX therapy may lessen the need for opioids.",
"RTX is extremely powerful. It’s also very selective. It is highly specific to TRPV1. “So you gain selectivity because it only acts on TRPV1, only on a certain class of fibers which only transmit pain,” says Tony Yaksh, a researcher at the University of California San Diego who has studied RTX. “Therefore you can selectively knock out pain without knocking out, say, the ability to feel a light touch or the ability to walk.”",
"The selectivity of RTX therapy has already earned it the title of “the Molecular Scalpel” for chronic pain relief. RTX knocks out pain receptors without affecting the motor, proprioceptive, and other somatosensory functions that are essential for performing the activities of daily living and maintaining quality of life.",
"RTX’s destruction of TRPV1 ion receptors isn’t necessarily permanent, but it can be quite a while before the receptors regain their sensitivity. This lesson was learned by researchers who have used RTX to abate pain in dogs. “It is profoundly effective, and the relief lasts much longer than I expected, maybe a median of 5 months before the dog’s owners asked for another injection,” says Michael Iadarola, who is studying RTX at the National Institutes of Health. “The animals went from a constant limp to freely running around. One dog was free of pain for 18 months before its owner noticed that the pain had returned.” ",
"https://www.nuvospineandsports.com/blog/one-of-the-hottest-chemicals-on-earth-very-promising-for-pain-relief"
] |
[
"That doesn't really answer the question, I might do some experiments myself."
] |
[
"So olvanil is still potent enough (64 times less than RTX) to kill the neuron before any pain is felt? That's why it's referred to as not pungent? Frankly this article doesn't seem to be of good quality."
] |
[
"Some of the pyramids in Egypt are over four thousand years old. If America was abandoned for a similar amount of time, what would be left standing?"
] |
[
false
] | null |
[
"While an avid reader myself, you might want to check out the television show \"Life After People\" ",
"http://www.history.com/shows/life-after-people",
"Each episode starts with the premise that every human on Earth instantly disappears without warning or leaving a body. It then follows what would happen to items without people to tend to them. Topics for episodes include pets and livestock, buildings and infrastructure, the Declaration of Independence, etc. Each show starts in the immediate aftermath of the disappearance and moves forward by days, months, years and sometimes centuries."
] |
[
"to add to that, this show depicts that the longest lasting structure in the united states would be ",
"Hoover Dam",
" and that it would out last all other structures in america.",
"edit:\nhere is the clip from the actual show he is referring to. Life After People claims that the hoover dam would last for 10,000 years until collapse. \n",
"http://www.youtube.com/watch?v=GVC1cy0yooc"
] |
[
"Some years ago there was a ",
"US government project",
" to solve exactly this problem, specifically to warn future generations away from nuclear waste deposit sites:",
"Full PDF.",
" 19.5 MB"
] |
[
"Do mice and birds have different colour vision?"
] |
[
false
] |
We recently took in a female ginger Manx cat (quite unusual). She’s a great mouser but doesn’t catch many birds, which is ideal. I’ve seen on Reddit a simulation of how tiger’s prey are typically colourblind to its stripes, allowing the orange to blend in with green grass and leaves. Is it possible that birds can easily see our ginger cat but rodents cannot due to differences in their colour vision?
|
[
"There's a huge difference in bird and mouse vision. Mice have two cones, and are basically red-green colorblind. They are also naturally nearsighted. ",
"Birds on the other hand have ",
" cones, which is actually more than humans have (we have 3). Furthermore their \"red\" and \"green\" cones are spaced further than in humans. As a result, birds can not only see distinguish colors better than we can, they can see UV too. And birds have good distance vision.",
"So yes, a bird is going to have an easier time spotting your cat. Also they can fly, and flying gives birds a distinct advantage in escaping predators. ",
"Mice aren't entirely helpless though, they do have a good sense of smell and hearing. ",
"Still, birds are in general less vulnerable to predation. One way this shows up is in lifespan. A long-lived mouse in captivity might make it 2 years. A long-lived house sparrow might make it two decades. There's no advantage to a mouse to have adaptations for living a long time because they are inevitably going to get eaten by something fairly quickly, but a sparrow can make use of adaptations to live a long time because if they make it to a couple years old they have a decent chance of avoiding predators for years longer. Birds (and bats) have longer lifespans than similarly sized ground-bound mammals."
] |
[
"The spectral response functions (i.e. how sensitive is each cone to each wavelength) for some animals' eyes can be seen in ",
"this figure.",
"Source: ",
"https://doi.org/10.3389/fpls.2020.618203",
" (Fig 7)"
] |
[
"Compared to those animals, humans seem unusually bad at ultraviolet. The large overlap between the red and green receptors also seems pretty suboptimal.",
"I guess ultraviolet sensitivity is extra beneficial for small animals, since diffraction limits an eye's resolution (in radians) to the ratio of the wavelength and the pupil diameter. Being sensitive to short wavelengths helps offset the loss in resolution from having a small eye."
] |
[
"Because every cell division can lead to mutation, I suppose some mutations happen as a fetus grows. How many different genomes does a new-born baby have?"
] |
[
false
] |
[deleted]
|
[
"The machinery that replicates your DNA, DNA Polymerase, makes one mistake out of every billion base pairs that it creates. This is after a variety of proofreading proteins have done their job. You have 6 billion base pairs, so expect 6 mistakes every time a cell divides. ",
"Are the cells of my left arm more closely related to each other than to the rest of my body?",
"Depends on what you call related. In terms of their genome, they are just as close as all other somatic cells. In terms of which proteins they are expressing and what they are doing at a biochemical level, your arms are almost identical."
] |
[
"The somatic cells in your body have the same exact DNA sequence (almost), but not gamete cells (which are reproductive cells - sperm or egg).",
"As mentioned by skyskimmer12, DNA replication is prone to errors, but cells have mechanisms for DNA repair to ensure faithful replication. So even when mistakes occur (eg. wrong basepair is inserted), the cell recognizes this and fixes it. ",
"Sometimes, the mutation in DNA can result in a perfectly functional protein, or even the same exact protein sequence as the non-mutated DNA (especially when the mutation occurs at the third base pair of a codon). Since the DNA repair system is adapted to ensure no loss of function, it may skip the repair in these cases. This accounts for the very small nucleotide differences between somatic cells.",
"As a result, somatic cells have an identical DNA sequence (save for errors that still produce a functional protein)."
] |
[
"The rate of mutation depends on species and even genome region. Also, some mutations lead to cell death and are not propagated. I think this would be a difficult question to answer correctly, but information on rates of mutation can be found ",
"here",
"."
] |
[
"Can any carbon-based molecule undergo combustion?"
] |
[
false
] | null |
[
"Carbon dioxide can't."
] |
[
"Normally we think of combustion as a reaction involving some molecule and oxygen.",
"Since the carbon atom in CO2 is fully oxidized, it's an example of a carbon based molecule that won't be able to undergo combustion."
] |
[
"Ah, yes, I suppose that would be the obvious exception. CO can though?"
] |
[
"What causes electrons to move (and also eventually slow) in a current?"
] |
[
false
] |
I know that they interact with electric fields of conductors, but what gives them the initial push? And how do they eventually stop?
|
[
"At a finite temperature, free electrons are always moving due to pure thermal motion. This doesn't give rise to any current because the electrons are moving in random directions, and their net motion cancels to zero.",
"When an electric field exists across the material, it creates regions of high and low electric potential. The electrons move in the direction that minimizes their potential energy. This is known as the Coulomb force and causes electrons to accelerate. Since there is now a net direction of electron flow, the current is now positive.",
"When the current is zero, this either means that no charge exists (which is trivial) or that every part of the conductor/resistor is at the same electric potential. The electrons now have no preferred direction, so the net current is zero but the electrons don't actually stop moving. They still have thermal motion."
] |
[
"This is a simple model but fails to explain why some crystals are insulators and others conductors or semi-conductors. Why do the electrons only move in some crystal structures? A more sophisticated model revolves around the \"band structure\" of a crystal.",
"Electrons in a crystal can be treated as plane-waves propagating through the structure. Due to the periodicity of a crystal, only discrete wave-vectors can exist and these wave-vectors, k, have discrete energies when the Schrodinger equation is solved.",
"So the electrons can only be in discrete states and only one can exist in each state (or two if they are spin-degenerate) and if they are plotted (E vs k) along high symmetry axes of your crystal you get a bandstructure like the following:",
"http://upload.wikimedia.org/wikipedia/commons/0/04/Band_structure_Si_schematic.svg",
"There are a finite number of electrons that fill the states starting with the lowest energy. If the highest energy electron falls in the white region of the above plot, the crystal will conduct when an E field is applied. If the highest energy electron completely fills the white region leaving the grey gap between the next available state, the crystal will be an insulator or semiconductor depending on the size of the grey gap.",
"Its a very brief summary that I will happily expand upon if OP wishes."
] |
[
"There's a nice paper on this ",
"by the authors of M&I undergrad physics text.",
"Electric currents within circuits are driven by \"static electricity,\" by surface charges and electrostatic fields.",
"When a switch is closed, the imbalance of surface charges will redistribute across the metal wires at about the speed of light. Once the charges stop sloshing around, the static e-fields from surface charges then applies a constant Gauss-law force to the movable electrons within the wire. The electrons then slowly move through the wires (electric currents are sloooowww, go google Drift Velocity of electrons in metal.)",
"Electrons stopping? In the classical physics analogy, a group of electrons inside a metal will have very little inertia, and also experiences enormous friction. If no e-fields are causing a current, or if the e-fields are suddenly removed, the current comes to a halt within tiny fractions of a microsecond. This time can be greatly increased by including a large inductance in the circuit (a hunk of iron with a long wire wrapped around.)"
] |
[
"Why isn't Ireland more forested?"
] |
[
false
] |
The pictures I see of Ireland are mostly vast expanses of flat greenery. Why isn't it more forested? Or, are the pictures just playing to our expectations? If the answer is that there are conditions that favor flat greenery, are there other places in the world with the same conditions and same results?
|
[
"This article",
" discusses the history of forests in Ireland, and their decline (and the current efforts to increase the amount of forest). Much like other areas of Western Europe, Ireland previously had much more forest cover, much of which was removed by human activity. This was firstly to increase the amount of land available for farming, and then later wood became an important material for industrial uses (much of which was exported to England, which had suffered from the same deforestation)."
] |
[
"While that article is correct and that human activity has had massive impact in destroying much in the way of forest everywhere, it also doesn't cover an important aspect. ",
"What it doesn't cover is the fact that a large quantity of photos people take of rural Ireland tend to be in the west where the Burren is, a karst region where trees struggle to grow and in fact a lot of things struggle to grow because it's very exposed, limited amounts of soil, vast amounts of rock and what soil there is there is very liable to erosion. Even outside of this area it can be very rocky and therefore makes it hard for trees to get a foot hold. I've seen mention of many places being a few centimetres of soil sits on top of a thick layer of limestone rock.",
"The region is one of the key selling points used by the tourist boards in Ireland and it's a very popular tourist attraction with a mix of ancient monuments, natural features and other tourist attractions all a short distance away and it's therefore very popular for photographs due to it's unusual appearance.",
"http://en.wikipedia.org/wiki/The_Burren"
] |
[
"Whilst that may make Ireland look disproportionately ",
", I don't think Clare and Galway are too unrepresentative of the amount of ",
". Ireland has the second smallest amount of forested area in Europe.",
"With the exception, perhaps, of County Wicklow, a photo of a random piece of Irish countyside wouldn't have more than a couple of mature trees. The grass may be substantially richer though."
] |
[
"Why is aluminum (with internal inert coating) a greater material preference than copper for a reusable water container?"
] |
[
false
] |
I'm wondering why copper is not very common in reusable water containers- like the ones that SIGG makes... is it just due to cost? Would purchasing a copper water container (and only using it with water) be less desirable? Wouldn't the "antibacterial" characteristics in copper make it a good choice?
|
[
"Copper is over 3 times more dense than aluminum, much softer and does not stand up to the elements as well. The aluminum oxide layer that forms on any exposed aluminum is unnoticeable and almost completely inert while copper oxide is dark brown and only form more slowly. Copper can also form verdigris (copper carbonate) which is that green stuff you often see on old statues or other old copper pieces, plus a couple other undesirable tarnishings when exposed to different gasses.",
"Copper can also be poisonous if consumed in excess, such as if leached from a copper vessel with an acidic liquid. People who make water bottles don't want to get in trouble once you start putting soda in there and give yourself copper poisoning.",
"So yes, a copper water bottle may look cool and have some antibacterial properties, but it would be much heavier, not last as long, and could potentially be dangerous if used incorrectly. It would also be way more expensive.",
"If you did coat the interior of the container with an inert polymer (which sigg might do with their aluminum containers) then the toxicity problems might be removed, but you then don't gain any benefit from antibacterial copper either. At that point it would just be a battle between looks, price, durability, and weight."
] |
[
"http://en.wikipedia.org/wiki/Aluminium#Etymology",
"The British discoverer of this element initially referred to it as alumia, but eventually settled on aluminum. Others thought aluminium sounded better, and for most of the English speaking world, this spelling stuck. However, one of the American pioneers of production methods for this element marketed it as aluminum, which is why America uses that spelling."
] |
[
"http://en.wikipedia.org/wiki/Aluminium#Etymology",
"The British discoverer of this element initially referred to it as alumia, but eventually settled on aluminum. Others thought aluminium sounded better, and for most of the English speaking world, this spelling stuck. However, one of the American pioneers of production methods for this element marketed it as aluminum, which is why America uses that spelling."
] |
[
"Does Mars get Meteor Showers?"
] |
[
false
] |
Knowing that meteor showers are debris of comets/asteroids compressing atmospheric gasses to high densities/temperatures that appear as bright lights in the sky, and also that Mars has ~1% of the Earths atmosphere, would you see meteor showers from Mars? Or would they just plummet through to the surface relatively unscathed? OR other?
|
[
"I disagree with your conclusions. The small meteors in Earth's meteor showers, typically smaller than your average grain of sand, begin to glow at ",
"about 100 km altitude",
", where the atmospheric pressure is far less than 1 Pa (",
"around 33 mPa",
"). Mars' surface pressure ranges from 100-1000 Pa, depending on the terrain and time of year. There is no reason why small meteors (such as those that cause Earth's meteor showers) striking the Martian atmosphere would not glow and burn up the same as they do in Earth's atmosphere. They just will do it at a lower altitude, which would likely make them appear ",
" than those on Earth, just due to them being closer to the surface.",
"Additionally, Mars has a higher orbital velocity than Earth, so depending on the direction the meteors are traveling one would expect that they could burn even more spectacularly."
] |
[
"Atmospheric pressure is 101,000 Pa. No one made a typo but you.",
"EDIT: Why is your field of view smaller?"
] |
[
"Probably not. Mars has an effective surface pressure ",
" 1/80 of that on Earth. So if you consider that the average barometric pressure on Earth's surface is 1000 millibars, then Mars' atmospheric pressure would be ",
" 6 millibars. Furthermore, this means the atmospheric friction would also be significantly reduced (meaning the objects would reach the surface pretty much unscathed). ",
"On Mars, you can have a dust devil equivalent of that on Earth, with winds equal to ~200 miles per hour or higher (which is strong here on Earth). HOWEVER, you have to take this wind speed, and divide by a factor of 80, which means you get, roughly, a ~2 mph wind, just enough to feel on your face.",
"So, to answer your question, no. Mars doesn't have meteor showers like on Earth. It gets meteors that jettison straight to the surface from space. ",
" Thanks ",
"/u/criss_",
" for the correction on barometric pressure on Mars. Specifically,"
] |
[
"Why do we make calculations regarding radioactive decay in base e instead of base 1/2?"
] |
[
false
] |
The equations in base 0.5 seems so much more straightforward: N0 = Nt (0.5) Instead of in the base e where lambda have to be introduced as L = ln2/h And then plugged into the equation: N0 = Nt (e)
|
[
"You can do either. Most of us that do these kinds of calculations regularly can fluently switch between the two, so it doesn’t matter.",
"If you look at databases for nuclear and particle physics information, nuclear physicists tend to work in terms of half-lives (favoring base 1/2), while particle physicists tend to work in terms of mean lifetimes (favoring base e).",
"Even though I’m from a nuclear physics background, I’ll say that it’s convenient that d(e",
")/dx = e",
". This is not true for (1/2)",
"."
] |
[
"The half-life is ln(2) times the mean lifetime."
] |
[
"Thank you so much for the clarification, it was very clear. Just one point i could use some clearing up: what is the difference, or correlation, between half-life and mean lifetime?"
] |
[
"Where does the residue from deodorants go, and what effects does it have on our bodies?"
] |
[
false
] | null |
[
"\"This is about 2.5% of the aluminium typically absorbed by the gut from food over the same time period.\"",
"I'm going to continue on w/o sweaty pits."
] |
[
"\"This is about 2.5% of the aluminium typically absorbed by the gut from food over the same time period.\"",
"I'm going to continue on w/o sweaty pits."
] |
[
"The aluminium in deodorant is not aluminium oxide. Aluminium oxide is an insoluble ceramic, whereas the aluminium in deodorant is dissociated. Safety information for aluminium oxide is not relevant, as it is incredibly inert and biocompatible. Dissolved aluminium is not."
] |
[
"How does a computer know when one second has passed?"
] |
[
false
] |
Correct me if this is the wrong subreddit. Apologies if so.
|
[
"Three mechanisms.",
"First, every CPU has a thing called \"clock signal\", which is basically just a rapidly oscillating electrical signal. On the one hand, the rate at which the clock oscillates is known, so it can be used for timing; on the other hand, it can be off by several percent even in normal operation without any adverse effects, so it can't be used for precise timing.",
"Second, there is a dedicated circuit called \"real-time clock\" (RTC) whose entire purpose in life is to accurately keep track of passage of time, even when the computer is off (which is why it has its own battery). Every modern computer uses one of those for accurate timing. However, an RTC can only keep track of relative time (passage of time), not of absolute time (time of day).",
"For absolute time, computers rely on an external reference; typically, they synchronize their time over the internet, but if the computer has cellular capability, it can also synchronize time using cellular signal."
] |
[
"For the interested:",
"A real time clock circuit works in a very similar way to counting CPU clock oscillations. The main differences are that RTCs will use a more accurate oscillator (usually quarts based running at a constant clock rate) and they will sometimes compensate for the variables which change clock rates. The biggest cause for clock speed variance is temperature"
] |
[
"Without a peripheral, it doesn't. Not really. It can assume that a given number of CPU cycles is roughly a second, but this isn't really accurate, as the clock speed of a CPU can vary quite a bit, intentionally or unintentionally. For some purposes this is acceptable, so simply couldn't ting the appropriate number of cycles is adequate.",
"The peripheral that computers use to keep track of time is called a real-time clock. It works pretty much the same way as a quartz watch, but instead of displaying the time, it reports the time over a standard digital interface.",
"Even this clock will skew over time, but the skew is very small over short periods of time, so it's acceptable for checking things like how long a second is."
] |
[
"Siphonophores like the Portuguese Man o War are actually colonies of organisms. How do these colonies reproduce?"
] |
[
false
] |
If I understand my biology correctly—and please correct me if I don't—creatures like the Portuguese Man o War are not actually single organisms; rather, they are colonies of many smaller and highly specialized organisms. Here's my question(s): how do they reproduce, if they are a bunch of organisms? And how do the organisms form colonies with such regularity—that is, why do all Portuguese Man o Wars look the same? As an aside, what's the semantic difference between a bunch of organisms that have to live together in order to survive and a bunch of cells in other organisms?
|
[
"That's really informative, but now I'm even more confused. What makes these collective \"organisms\" different from our various organs and systems if they all come from an egg and can't survive separately?"
] |
[
"That's really informative, but now I'm even more confused. What makes these collective \"organisms\" different from our various organs and systems if they all come from an egg and can't survive separately?"
] |
[
"They share the same DNA, differentiate, and perform functions for the communal health, exactly like what our cells do. ",
"Why would they be classified as a community? I have no idea and am just as confused as you are. "
] |
[
"What happens if you put a spring in a centrifuge?"
] |
[
false
] |
This might be a stupid question, but what happens if you put a spring in a centrifuge? When I compress a spring with my hand, for example, the spring compresses uniformly as I increase the force. Would a spring in a centrifuge compress uniformly? I'm asking because I have an intuition that the centrifugal force is weaker near the center of the rotational axis since the path of rotation is smaller.
|
[
"Your intuition is correct, the centrifugal force is proportional to the distance from the axis of rotation.",
"This sounds like an interesting mechanics problem. If you have a spring (constant k) with one end fixed, and the other rotating with some constant angular velocity ω, and a mass m on the other end, there are two relevant forces in the co-rotating reference frame: the inward spring force, and the outward centrifugal force.",
"There is some equilibrium distance where the two cancel each other out. If the spring by itself has equilibrium distance r",
", the centrifugal force will change the equilibrium position to",
"kr",
"/(k - mω",
"), assuming I haven't made an error.",
"The effective potential energy for this system is k(r-r",
")",
"/2 - mω",
"r",
"/2, so the equilibrium can be stable or unstable depending on the relative values of the constants m, k, and ω."
] |
[
"There's no need to think about the equivalence principle or even different reference frames to see that the stability of the equilibrium can switch depending on the parameters.",
"It may be helpful to point out that the centrifugal term in the effective potential is actually just part of the kinetic energy (the angular part).",
"The second derivative of the effective potential at the equilibrium position is (k - mω",
"). If this quantity is positive, the equilibrium is stable, and if it's negative, the equilibrium is unstable."
] |
[
"Does it make sense that the equilibrium would be stable or unstable depending on certain parameters? There should be no difference between an accelerated reference frame and a gravitational one. This would imply that a regular old spring with a mass compressing it from above would have stable and unstable equilibria depending on maybe k, ω, and g. What would an unstable equilibrium look like?",
"EDIT: maybe the scale of the earth prevents us from commonly observing unstable equilibria, and near the surface is stable. "
] |
[
"Our bodies, our cells. Or: How much difference is there between the human body and a colony of ants?"
] |
[
false
] |
Many years ago, when I was in high-school biology class, I asked my teacher in several different ways whether or not human cells are sentient. I asked several different ways because at first I thought his inability to come up with an answer was because I was being unclear. Ultimately I realized that he had no idea, and perhaps never even thought about it. (He was a new teacher, and he was better known as a wrestling coach than as a science teacher.) I still think about this question, though, and lately I've been wondering about it in a different way. If our bodies are collections of cells--each cell a living thing--then in what ways are we like or unlike a colony of ants? (Let's ignore the fact, for a moment, that individual ants are made up of cells, just like we are.) In an ant colony, each individual is a specialist. Their anatomy is even different based on their specialization. And they seem evolved to be completely dependent on their collective existence as a colony. A single, isolated ant will not survive for long. Like the ants, the cells in our bodies share common traits, but their anatomy varies depending on their specialization. An isolated cell does not survive for long. But, like the ants, the great variety of specialized cells comes from a single source--a cell that is the result of sexual reproduction. Has anyone studied this topic? I'd love to hear any insights about how we are like or unlike social groups of organisms, or how the cells that make up our bodies are like or unlike single-celled organisms.
|
[
"We are actually very much like ants when it comes to cellular moving. Your immune cells move around the body based 100% on chemical gradients -- they move towards the source of the chemica signals, called chemokines. Just like ants follow chemical signals their friends leave in order to find the trail to the food. ",
"The difference is that your cells don't line up in a row and march to the site of infection. They're more like hanging out in the bloodstream and then go \"Oh shit, is that some CCL20? I'ma leave the bloodstream and stay here for a while.\""
] |
[
"This is an interesting question, but perhaps you are approaching it from the wrong angle. Perhaps you should ask: in terms of evolution, why do we have cooperative (everything from bacterial plaques to true multicellularism to colonies and civilization) behaviour? Part of the answer is simply that it works, but that is not very satisfying. So maybe we should look at the units of evolution, what can be considered an organism? ",
"You are essentially a colony of clones, individual cells/organs are specialized for different tasks but all have the same genome and so helping the germ line reproduce itself is in the interest of all cells (Cancer is an interesting case of some of your cells getting selfish and demanding sovereignty, but that's a dead end as they die with you). Think of a colony of ants or bees similarly, ",
", the relatedness of workers to the queen is very high thus it is in the interest of their genes for the queen to have more babies, a more successful colony and produce more queens to found other colonies. You may notice that this is starting to blur the line of what is an organism.",
"Specialization, then, is a consequence of group behaviour.",
"EDIT: Apparently I have the ploidy of the ants backwards, fixed kind of. Regardless always think in terms of evolution."
] |
[
"Leave the memes in another subreddit. Humor is fine, but make sure it adds to the discussion."
] |
[
"Question about gravity within our solar system"
] |
[
false
] |
How is the gravity from the sun strong enough to keep the giant planet of Jupiter in orbit, but not suck tiny little Earth right into itself?
|
[
"Earth moves faster than Jupiter and gravity decreases with distance."
] |
[
"Aside from effects of momentum the gravitational force between two objects is dependent on the masses of both objects and the distance between them. The only reason either planet dosn't fall into the sun is because the force applied from the orbit is canceling the gravitational force.",
"F= G x (m1 x m2)/r"
] |
[
"Does Jupiters mass make any difference? I didn't think so. If I waved my wand and turned Jupiter into a pea, I imagine the orbit would change due to a shift in the Sun/pea center of mass, but wouldn't it still obit at approximately the same radius if going the same speed? I imagine gravitational \"force\" goes way down, but so does the \"force\" needed to keep it in orbit. Am I an idiot?"
] |
[
"How long does it take for soil to lithify?"
] |
[
false
] |
Can it happen within a few thousand years, or does it always take millions?
|
[
"It really depends on a number of things. It usually takes several thousand to tens of thousands of years. ",
"The main variables are the availability of water, the supply of the dissolved \"cement\" species and whether the rate of chemical activity which allows precipitation of that cement. In some very specific environments where cement supply and use is optimal, cementation of loose sediment can be very rapid. One such environment is the shallow subsurface beneath carbonate beaches, where carbonate sand can become fully lithified within a few decades, producing \"beachrock\". There are numerous examples of beachrock containing soft-drink bottles or tires. "
] |
[
"Thank you for the reply, the info about the beach rock is fascinating "
] |
[
"A pleasure!",
"One last anecdote: there is an outcrop of beachrock in South-Africa (wish I had a link to the reference) next to an old weapons factory, which had hand grenades embedded in it. Truly exciting rocks, from which I would strive very hard to keep away the avid little hammers of first year undergrads..."
] |
[
"Can breast tissue \"migrate\"?"
] |
[
false
] |
I have been reading many of the posts in as well as many of the links provided therein. There is repeated mention of "breast tissue migration" described as breast tissue that has moved away from its original place to the sides and even onto one's back. In some of the posts/links, it is said that this "migration" is the result of years in an ill fitting bra. I googled "breast tissue migration" (even hit the scholar button) and can find no evidence that tissue can migrate away from the breast. Removing the word "breast" from the search resulted in a bit of information about zebrafish, nematode larvae and parasitic helminths (whatever those are) none of which I am interested in. Can fat tissue do that?
|
[
"http://www.venusianglow.com/2009/09/get-rid-of-armpit-rolls-in-5-seconds.html",
"This is the only \"explanation\" I found for it, and I'm... skeptical, to say the least. I'd like to hear some legitimate physiological backing up of this idea if it is indeed true. ",
"edit: Some more googling, and every single reference I see (either on the bra wiki or ",
"/r/abrathatfits",
" or other websites) seems to reference that single blog and/or another forum post. That seems to be the 'original source' for most of this. "
] |
[
"While AskScience isn't the place for jokes, a cursory observation would indicate that taking that as proof would also indicate that males who don't wear bras can have breast tissue migrate to their backs. "
] |
[
"Anatomically, breast tissue can often extend into the axilla (armpit). It is common enough to appear in some anatomy atlases. My guess is that what the women are seeing is the age-dependent accumulation of fat in those places, combined with the sagging as time stretches the ligaments that hold the tissue in place. ",
"There are also fat pads in the back that get larger with age. "
] |
[
"If we can manufacture mirrors to reflect upside down, can we manufacture them to show a \"reversed reflection\"?"
] |
[
false
] |
[deleted]
|
[
"It seems OP was asking about a single mirror system:",
"is it possible for ",
" mirror to be able to flip you over the vertical axis",
"Any ",
" mirror system will not flip an image, it will reflect it. Mathematically, a reflection and flip (rotation) are not equivalent but you can get a flip from two reflections (which is exactly what happens in the corner cuber example you mention)."
] |
[
"I'd like to answer this in two steps: First by addressing the 'funhouse mirror' concept, and then secondly by addressing what it means for an image to be 'flipped L/R but not U/D'",
"When you look at certain mirrors (I'm going to use fun house from here on out) you appear upside down in the image you see.",
"This is because you're looking at a ",
" mirror. Curved mirrors distort the way light travels and the orientation and magnification of the image depends on the object distance to the mirror, the distance of your eye to the mirror, as well as the direction the mirror is curved.",
"Instead of flipping you over the horizontal axis, is it possible for a mirror to be able to flip you over the vertical axis so that the pocket on your physical body's right is on the left side of the mirror's body?",
"The short answer to this question is no (",
": for plano mirrors only), but it's not because of any weird law of physics but rather the fact that this is a matter of ",
" not of ",
". ",
"If you stand in front of a (",
": plano) mirror and hold up your left hand you notice the reflection is holding up his right hand. But how do we determine that it's his right hand he's holding up? Mentally, we ",
" our body to be in the same orientation as the reflection and say \"his right hand is up\". However, mirror's don't rotate images, they ",
" them. So, mathematically, the mirror has ",
" the image, whereas mentally, you have ",
" the image. The discrepancy comes up due to the mathematical difference between these two transformations.",
"So the issue of flipping L/R is not something the ",
" does, it's something that ",
" do."
] |
[
"is it possible for a mirror to be able to flip you over the vertical axis",
"Short answer: yes it is.",
"You did a great job explaining why people might think a mirror flips left and right, but that wasn't the question. If a curved funhouse mirror can make you look upside down, then rotate that mirror 90 degrees and it will flip left and right. Or as I said in my other post, two verticle mirrors joined at 90 degrees will flip left and right. Basically two sides of a ",
"corner reflector",
" will work."
] |
[
"When things fall into a black hole, why does it not built up enough kinetic energy to come back to the original position?"
] |
[
false
] |
Classic physics demonstration of pendulum shows that when an object is falling, it will built up enough kinetic energy to come back to its original position. Classical orbital mechanics also confirm this as bodies in elliptical orbit always come back to the original position and velocity (in ideal cases atleast). But, if the object falls into the Schwarzschild radius of the black hole, the escape velocity of the object will be greater than the light speed, therefore it's impossible for it to escape the black hole. Now the question is, how is those two phenomena consistent with each other? Shouldn't the object falling into Schwarzschild radius build up enough kinetic energy to climb back up the potential energy wall? Wouldn't the fact that nothing escapes the black hole imply some sort of energy transfer out of the object? Ignoring the likely collisions with the things around the black hole, what could cause this loss of energy from the object? Where does the energy go?
|
[
"Orbital mechanics work differently in GR than they do in Newtonian gravity. As you noted, it is in fact possible for objects to fall into a body and not be able to escape in GR, whereas this is generally not possible in classical orbital mechanics for point particles (extended objects can of course collide). Such \"plunge orbits\" also do not have to perfectly radial; it is possible for two point particles to not be headed straight for each other and still collide in GR. ",
"Mathematically, this is because the form of the effective potential in GR is different than the one you get from a pure inverse square law form of gravity. In the Schwarzschild metric, another inverse cube term pops up in the effective orbital potential, and this term is what allows the non-radial plunge orbits. Also, it means that ",
" orbits precess; truly closed orbits are simply not possible, ",
" you do not return to quite the same orbital distance as where you started, due to gravitational wave emission. For an orbit like the Earth's around the Sun, these effects are fairly negligible (it will take something absurd like 10",
" years before the Earth's orbit decays from gravitational wave emission), but around compact massive objects such as neutron stars and black holes, they can be very important.",
"To answer the questions at the end of your post, we need to look at the conservation laws. Energy is conserved locally, and so is momentum. This means that the black hole will receive a kick from absorbing a particle. Normally, black holes accrete matter either spherically (in free floating black holes, the accretion should average out to a sphere) or from a disk, so they don't end up gaining any linear momentum. The associated energy can go to a few places. It can go towards the mass of the black hole, it can be radiated away gravitationally, or, if the particle is made of normal matter, it can radiate it away as light, depending on the circumstances."
] |
[
"Classic physics demonstration of pendulum shows that when an object is falling, it will built up enough kinetic energy to come back to its original position.",
"Actually, isn't the classic physics demonstration that it ",
" build up enough kinetic energy? That's why physics teachers holding bowling balls up to their nose, to let them swing away and back, and they don't end up with broken noses.",
"Even if we assume that a black hole is \"just a pendulum\" for something to start at the event horizon at 0 velocity, fall in and make it back out to the opposite side of the event horizon, it would have to convert potential energy to kinetic energy and back to potential energy at perfect 100% efficiency."
] |
[
"There are a couple of ways to look at it. One way is that the potential well for say, a pendulum has finite depth. A black hole is has a singularity, the potential well has infinite depth so you will never cross it and go back up."
] |
[
"Why do words begin to sound alien when repeated enough times?"
] |
[
false
] | null |
[
"This is called ",
"Semantic Satiation",
". ",
"The analogy I like is this: just like when you stare into a light you get a blind spot, repeating the same word over and over numbs out the neurons in your cortex that make sense of the word leaving you with that alien sounding feeling. "
] |
[
"Non mobile link: ",
"http://en.wikipedia.org/wiki/Semantic_satiation"
] |
[
"Words are \"saved\" in the mind in a cluster of information called a lexical entry. The lexical entry consists of the semantic information about the word -- its meaning and related concepts -- and the phonological information -- the sounds in the word and the mouth movements required to pronounce it.",
"When you repeat a word over and over, you are attending to the phonological information. The semantic information becomes decoupled from the lexical node temporarily, because that's where you are directing your attention.",
"Obviously simplifying and glossing over a bunch of stuff here, as language processing is extremely complex and not fully understood, but this may help you picture what is happening."
] |
[
"Is there any evidence that pricing a product at, say, 9.99, results in a higher volume of sales than pricing the same product at 10.00?"
] |
[
false
] |
Other than the obvious, "It's a penny cheaper," I was wondering if the received wisdom regarding this aspect of consumer psychology has ever been tested and vindicated. Or is it simply a truism that has stuck around out of tradition?
|
[
"It's a little hard to believe but it's a thing",
"Lay article: ",
"https://www.livescience.com/33045-why-do-most-prices-end-in-99-cents-.html",
"White paper 1",
" & ",
"paper 2 - PDF warning",
"EDIT – heart-felt and obligatory thanks for the awards!"
] |
[
"The theory is called \"Psychological Pricing\", and it yes, it's a real behavioral phenomena, but there are a number of theories that seek to explain it.",
"https://en.m.wikipedia.org/wiki/Psychological_pricing",
"The most commonly cited explanations are not that people are happy to save a penny, but rather that they use mental shortcuts to evaluate value which this type of pricing exploits. \"Left-digit anchoring\" or \"dropped significant digits\" are two of those theories, which suggest that in a $1.99 vs $2.00 comparison, consumers may interpret the price as being closer to $1 than to $2 by focusing on the first digit (the $1), and/or ignore the $-.99 that follows."
] |
[
"It's more than just 99 cents. It works with 97 and 96 cents hanging off the end of numbers as well. ",
"The phenomenon is well studied in behavioral economics."
] |
[
"Clouds (in the sky): a) why are they vapour? It's -45C up there, they should be ice! and b) why do they stick together? What force is making them do that?"
] |
[
false
] | null |
[
"They aren't vapour. They are actually small water droplets condenced on dust, charged particles, ion's etc.. They condence because of the cold up there. You usually cannot see vapur, and the steam rising from pot is visible for the same reason, condensation as water vapour cools down."
] |
[
"Clouds need a few things to be in place to form: water vapor (invisible water gas, from evaporation), a dip in temperature, and something for the water to condense on. As mentioned earlier, clouds are actually tiny drops of water, formed as water condenses onto tiny bits of dust or other particulates. ",
"As for your island thing: a couple of things might be happening. As wind blows water vapor over he ocean, the air over the ocean is too warm for the water to condense into a cloud. As it blows over the island, the wind \"ramps\" up higher by just enough to meet some cold air and condense. After the wind blows over he island, the cloudy air moves back down where it's too warm to be a cloud, it disappears again. Alternately, the island might be he only source of dust in the area, so as humid air passes over it , the WV is like \"oh dang finally something to condense onto!\", forming clouds in the immediate area. "
] |
[
"If it's cold enough to form ice, there is typically ice present in clouds. Clouds that appear \"wispy\" contain ice."
] |
[
"If new elements are created by fusing old ones together (I'm aware that this is greatly simplified). Are there infinitely many undiscovered elements?"
] |
[
false
] |
Are there infinitely many undiscovered elements waiting to be created (regardless of the difficulty of creating said element) or is there some sort of invisible wall that we might hit? Thank you.
|
[
"This gets brought up fairly often in these kinds of threads, but I'm of the opinion that it's not really useful to call a neutron star a \"nucleus\". Yes, it's a bound system of many nucleons. ",
" it's bound by gravity, completely different than the nuclear force, which saturates at a few femtometers (neutron stars have length scales of kilometers).",
"Also as a technical note, neutron stars cannot be arbitrarily big. At the TOV limit, they collapse into black holes (or your namesake, quark stars)."
] |
[
"No, probably not. The IUPAC sets the criteria for the discovery of a new element, and one of them is a minimum lifetime of around 10",
" seconds. That's roughly how long a nucleus needs to live for in order to form an electron cloud and be considered a chemical element.",
"You can have nuclei with much shorter lifetimes than that, but that's what the chemists decided is the limit for a chemical element.",
"We can produce heavier and heavier nuclei, but the nuclear force saturates at some finite radius, so I'd find it pretty hard to believe that you can have bound nuclei of arbitrarily high mass number.",
"As you go higher and higher in A, there should eventually be a region where the system is so unstable that ground state lifetimes start to dip below 10",
" seconds, even near the extrapolated valley of stability.",
"So these nuclei would not be able to form atoms, and would not meet the IUPAC definition of an \"element\"."
] |
[
"The ",
" of stability is different than the ",
" of stability. The valley of stability is just all stable-ish nuclides currently on the nuclear chart.",
"The island of stability is the hypothetical \"stable-ish-ish\" grouping of extremely heavy nuclei near shell closures.",
"It's possible that they have lifetimes of milliseconds, seconds, or even longer. Not much is known, and the theories which we have to predict things about them have to be extrapolated over huge ranges, so there's a lot of variability in what they say. As an experimentalist, I'd say it's up in the air. I also don't work with extremely heavy systems, so take that with a double grain of salt."
] |
[
"What's the mathematical proof that demonstrates that two parallel lines meet at infinity?"
] |
[
false
] | null |
[
"You need a setting where the \"Line at Infinity\" is actually a thing. If we use coordinates, then the ordinary plane is the collection of all points (x,y), where x and y can be any real numbers. In this setting, a line is the solution set of an equation of the form Ax+By=C, where A,B and C are real numbers, and at least on of A or B is nonzero. Two lines, Ax+By=C and Dx+Ey=F are parallel if AE=DB. We can clearly see that if AE=DB and C and F are not equal, then there are no simultaneous solutions to these equations and so the lines do not intersect. ",
"This is kinda inconvenient because we have theorems like \"All distinct pairs of lines intersect exactly once, unless they are parallel, which don't intersect\". Almost all pairs of lines have this nice property, and there are very few exceptions, but the existence of exceptions means that this is not a wholly reliable property that pairs of lines have. We essentially have two options, either accept that this is not always true and that there are these messy pairs of lines that don't intersect. Or, we can conclude that all pairs of lines ",
" intersect, it's just that the plane itself is too small to contain the place where parallel lines intersect. We'll investigate this latter case, as it cleans up our understanding of lines and is flexible enough to apply to other situations.",
"The way we'll do this is find a simple way that kinda does the trick, but is unsatisfying. We'll then use this as inspiration to make a somewhat usually unmotivated construction that allows us to do these things more naturally.",
"To do this, we need to extend the plane. But we don't want to extend it too much. For instance, one way to extend the plane is to go into 3D space, and we can then say that the line Ax+By=C is actually just the plane Ax+By=Cz in 3D coordinates (x,y,z) where the plane z=1 corresponds to the original plane. In this case, if the lines Ax+By=Cz and Dx+Ey=Fz are parallel, then they will actually intersect in the plane z=0 along the line Ax+By=0 in that plane. You can interpret this as follows: In if we have two lines Ax+By=C and Dx+Ey=F, then the corresponding planes Ax+By=Cz and Dx+Ey=Fz will intersect along a line in 3D. This line always intersects (0,0,0) and, in the case when they are not parallel, some point in the plane (x,y,1). This is because this line is oblique to this plane. As the lines get more and more parallel, the point of intersection, (x,y,1), goes out further and further from the origin and the 3D line of intersection begins to flatten. When Ax+By=C is parallel to Dx+Ey=F, then their 3D intersection line is parallel to the plane (x,y,1), and the point of intersection on this plane has been pushed out \"off\" the plane. This extension is fine, it offers some intuition. It works. Instead of the theorem \"All non-parallel lines intersect once\", we get the new theorem \"All lines correspond to planes that intersect along a line\". We've gotten rid of the non-parallel condition, but at the cost of the singular intersection point. Moreover, when we extend this theorem to higher dimensions, we still have things like parallel planes to deal with. If we extend the \"All non-parallel lines intersect once\" to higher dimensions, we get the theorem \"All non-parallel planes intersect along a line\" or \"All non-parallel 3D spaces in 4D space intersect along a plane\" etc. In all of these cases, the intersection set has dimension 1 lower than the object in question, which we lose when we use this higher dimension model. Moreover, it's a fairly unsatisfying setup and seems pretty ad hoc. We want a more universal way to extend flat, Euclidean space so that can take care of all of these cases, in a way that is ",
" to this space, that doesn't lose the 1-lower dimension, and doesn't force us to continually work in higher dimensions.",
"To resolve this, we can take inspiration to what we did above. After all, we ",
" manage to get parallel lines to \"intersect\". The key was the third coordinate z, as we could set it to zero and the line of intersection would be Ax+By=0. But we want this to be a single point. We note that, in this case, the solution set in 3-dimensions to Ax+By=0 and z=0 are the points (Ar,-Br,0) where r is any real number or, more pointedly, (Ar,-Br,0*r). That is, all of these points are the same up to scaling by r. What we want is for this \"plane\" of intersection to actually be a line, so we're going to do something very drastic and just say that this line is really only ",
" point. ",
"We're going to use three coordinates (x:y:z) to describe our space, but instead of having each triple of number correspond to their own point, we're going to say that two triples (x:y:z) and (u:v:w) correspond to the same point if they are the same point up to scaling by some value r. That is, if u=rx and v=ry and w=rz. For instance, we have (1:2:3)=(3:6:9), they're the same point because they are the same up to a scaling factor of 3. This set of points is called the ",
"Real Projective Plane",
".",
"Now, just what does this new projective plane look like? Note that if z is nonzero, then we can multiply the point (x:y:z) by the scaling factor of 1/z to get the point (x/z:y/z:1) and, moreover, the only way that two points (X,Y:1) and (U:V:1) are equal is if (X,Y)=(U,V) in the normal, traditional sense. That means that the ordinary, traditional plane lives inside the projective plane and is equal to the set of \"projective coordinates\" (x:y:z) where z is nonzero. This accounts for almost all points in the projective plane. So what about the points of the form (A:B:0)? Well, we can see where they live by \"tracking\" the point (A:B:Z) as we send Z to zero. This means that we'll be seeing where the point (A/Z,B/Z) goes on the plane as we send Z to zero. When we do this, the point will travel along the line Bx=Ay in the plane out towards infinity. This means that we can thing of the points (x:y:0) as the points \"at infinity\" or, as we usually call it, the \"line at infinity\".",
"Some may have noted something off here. When we track the point (A/Z,B/Z) as Z goes to zero, it's fine that it goes along the line Bx=Ay, but there are ",
" directions we can go to infinity with. If Z goes to zero from the positive numbers, it will travel along Bx=Ay in one direction and if it goes to zero from the negative numbers, then it will travel along Bx=Ay in the opposite direction. This process simultaneously sends (A/Z,B/Z) to completely opposite infinity points. But, for this to work, we need (A:B:0) to be a single point. And this is what we do. If we imagine the entire euclidean plane surrounded by a circle \"at infinity\", then to make the real projective plane, we'll need to take points on this circle that are opposite each other and glue them together into a single point. In this context, for instance, opposite infinities are the same. This is why visualizations of the real projective plane",
"look weird",
". ",
"This is good, this real projective plane is, essentially, the ordinary real plane but with a circle weirdly wrapped around it. It's and extension, but not too much of an extension. Note that each point on this line at infinity corresponds to a ",
". The point (A:B:0) corresponds to the slope of the line Bx=Ay. Intuitively, then, parallel lines should intersect at the point at infinity corresponding to their shared slope. A line on the real projective line is the set of all projective coordinates (x:y:z) that solve an equation of the form Ax+By=Cz and the solution to Ax+By=0 ( ie (-B:A:0)) corresponds to where it intersects the line at infinity. We can check and we find that non-parallel lines still only intersect at one point. But now, if Ax+By=Cz and Dx+Ey=Fz are parallel (which means that there is some nonzero real r so that D=Ar and E=Br), then they don't intersect off the line at infinity and, moreover, they intersect the line at infinity at the point corresponding to their shared slope: (-B:A:0) = (-Br:Ar:0) = (-E:D:0).",
"This is much more satisfying. We've only included the new points that we actually need. These new points allow us to say that \"All pairs of distinct lines intersect exactly once\". It makes more natural many other theorems in classical geometry, eg ",
"Pascal's Theorem",
". Furthermore, there are natural generalizations to this in higher dimensions that make everything nice and analogous to this. Moreover, the interpretation of their intersection makes sense of being \"at infinity\" while also having the natural understanding of they intersect \"at\" their slope, which is the same.",
"We can look at other objects in the projective plane, in particular we can look at conics. In the projective plane, we can conclude that all non-degenerate conics are of the same type. That is, there is nothing to distinguish between ellipses, parabolas and hyperbolas intrinsically, and the only way to classify them is how they interact with the line at infinity. An ellipse/circle is a conic that does not intersect the line at infinity. A parabola is a conic that intersects the line at infinity once, ie is tangent to it. And a hyperbola is a conic that intersects the line at infinity twice. The hyperbola intersects the line at infinity at the two slopes corresponding to the its two asymptotes, so we can think of their asymptotes as being the \"points at infinity\". In fact, if you trace along a hyperbola and when you get to an asymptote, you cycle around you'll trace both sides of the hyperbola nicely."
] |
[
"\"Two parallel lines meet an infinity\" is a fast and loose sentence that doesn't make much sense in most rigorous mathematical frameworks.",
"There are several such frameworks we can use. Most common are the Euclidean geometry (the one that is normally studied in high school), the projective geometry, the elliptic a.k.a. Riemannian geometry, and the hyperbolic a.k.a. Boliai-Lobachevskian geometry ",
"In ",
"Euclidean geometry",
", the parallel lines simply do not meet, and there is no such place as infinity.",
"In ",
"elliptic geometry",
", there are again no parallel lines. In this framework, lines behave much like great circles on a sphere. This system behaves rather differently from Euclidean system though, excluding an arbitrary line does not bring the rest anywhere close to the Euclidean system. More on this below in the projective geometry section.",
"In ",
"hyperbolic geometry",
", there are parallel lines. In fact there are more parallel lines than in Euclidean geometry. Recall that in Euclidean geometry, for any line ",
" and any point ",
", there is exactly one line that passes through ",
" and is parallel to ",
". In hyperbolic geometry, there are many such lines. Again though, there is no such place as infinity in this framework.",
"This leaves us with ",
"projective geometry",
". In this system, there are no parallel lines per se. Any two lines intersect in a point that is not in any way special or different from any other point. This is stated directly in the axioms of projective geometry. Here's a much simplified list:",
"1. For every two points there is exactly one line that passes through both of these points.\n2. For every two lines there is exactly one point that lies on both of these lines.\n",
"As you can see, Axiom 1 is no different from that of Euclidean geometry, whereas Axiom 2 is different (in Euclidean geometry, there is ",
" one point, rather than ",
" one point).",
"Now in this system any line ",
" can be arbitrarily declared \"the line at infinity\". A pair of lines that intersect at a point on ",
" can then be called \"parallel lines\". It should be noted that this choice of \"line at infinity\" is completely arbitrary. The remarkable thing is that when one excludes such arbitrarily chosen line, the rest behave much like in Euclidean geometry. This is perhaps the closest that we can get to the statement in question. There is no proof that needs to be given though. In this system, parallel lines meet at the line at infinity by definition. ",
"The other answer approaches this from a different angle, by extending the Euclidean plane with a special line called \"the line at infinity\". This other approach tends to understate the fact that in the resulting system the line at infinity is by no means special. I like the more direct approach of building the projective plane from scratch without involving any coordinates."
] |
[
"I've sometimes wondered what would happen if you do the opposite thing. The projective plane is what happens when you say \"just as any two points have exactly one line connecting them, so any two lines have exactly one point at which they intersect.\" But you could go the other way, and say, \"just as there exist pairs of lines which don't intersect, so there exist pairs of points without a line connecting them,\" and duality returns, just in a weird way.",
"This is smaller than the Euclidean plane, with some lines taken out. My favorite way to make it is by taking out the origin but leaving all other points, making it topologically a capless cylinder. I have no idea what, if anything, of use could be done with this, but I like thinking about stuff like that. Is there any case where a plane vaguely like this has been explored for some reason?"
] |
[
"How do ISS astronauts prevent pathogen transfer?"
] |
[
false
] |
I was watching a .gif of a water balloon being popped on the ISS and the force of the burst sent some of the water flying, yet still suspended in the air. It struck me that a sneeze would work the same way, with the mucous stuck in the air just waiting to spread disease to anyone who's floating between cabins. How does the ISS team counter pathogen transfer? Does bacteria even survive in zero gravity long enough to become a biological landmine?
|
[
"NASA uses a ",
"pre-launch quarantine",
" to ensure that no one in space is sick in the first place. In addition all food and water is sanitized before it is stored. "
] |
[
"The lesson NASA learned from the flight of Apollo 7 was to isolate the crew from external contact with non-quarantined personnel 10 days before launch. ",
"Wally Schirra's head cold on Apollo 7",
" jeopardized the mission and caused the astronauts to reject safety protocols by riding through re-entry without their helmets, out of concern the crew's clogged sinuses would cause their eardrums to burst."
] |
[
"I'm not a biologist, so perhaps somone can confirm this. I'm not currently aware of any functions in pathogens that require the use of gravity to function. Other than that, standard immunizations are kept up to date for the astronauts. I also do beleive that there is a purifier attatched to the air filtration system that helps eliminate commonly known pathogens so they do not stay suspended in the air for very long. "
] |
[
"When an anti-electron annihilates does it have to contact an electron or would a neutron (or any non-antimatter particle) also cause the annihilation? This is something I've always wondered."
] |
[
false
] |
[deleted]
|
[
"If you shoot antiparticles into a black hole, the black hole will still gain mass. Antiparticles do not have negative mass."
] |
[
"If you shoot antiparticles into a black hole, the black hole will still gain mass. Antiparticles do not have negative mass."
] |
[
"An antineutron will annihilate with a neutron. But a neutron will not annihilate with a positron."
] |
[
"When jumping into a body of water, is there a way to calculate the distance into the water someone will go based on the height they jump from?"
] |
[
false
] |
I notice that when jumping from say 1 metre, we might go 30cm down above head height into the water. Where as jumping from 5 metres, obviously this increases. But it seems to increase slowly after that, for example a 10 metre jump might not take you much deeper than a 5 metre one. Of course this depends on how you land in the water, but the main two areas of interest would be landing in a pin drop, and landing in a curve to decrease the depth you go (sometimes done by Olympic divers I think). I'd be interested to know if there's a way of calculating both of these, assuming a standard landing position and a standard weight for the diver.
|
[
"You can get a reasonable approximation if you take a 2 stage approach. First create an equation of motion for the air section of the motion containing gravity and drag:\np= density\nv=velocity\nA = surface area\nC = drag coefficent",
"ma = 0.5pv",
" CA -mg ",
"you can then integrate this with respect to time to give the velocity at the air/water boundary assuming you know how long the object has been freefalling. ",
"You then construct the same equation again but add an extra term for buoyancy and using the values of water for the drag coefficient and density and you velocity at impact with the water. ",
"ma = 0.5pv",
" CA -mg +buoyancy",
"You can then integrate this twice to give the depth at time t. ",
"Now this is all actually very difficult to do and is usually done via numerical time integration such as the Euler method. Typically this will give an estimate that is accurate enough for the purposes of the model. ",
"One further note is that I have ignored effects from water tension.",
"Moving on to your question, we can see that the faster you move the larger the drag component of the total force. In fact this scales are the v",
" so even small changes in velocity have large consequences in penetration depth. ",
"The shape of the diver then comes into account in the curve section. When you add a \"curve\" you are actually allowing the water to flow over you as if you are a wing. This will add further terms to the force equation, however from empirically watching diverse I would conclude that this extra term has an effect of reducing the effect of the drag. ",
"EDIT: Explained this a bit better"
] |
[
"Ill try and clear up some of the answer then. The first section of the answer is about finding out your impact velocity. This can actually be difficult to calculate depending on the fall height as the density of the air changes with height. ",
"Here is a graph",
" that I produced in my degree to demonstrate the falling of ",
"Felix Baumgatner",
". This graph shows us that at 2500m above the ground he was travelling at ~75m/s. Lets assume this is the velocity he impacted the floor and move to the second part.",
"This involves using the exact same force equation plus buoyancy. This is due to water and air both being modelled as fluids, allowing us to use the same set of equations (technically we should account for air buoyancy but this will be negligible unless you are a helium filled balloon). So when we did the previous force equation we started at x =0 v=0, i.e stationary. For the water equation we will start at the impact velocity of the person, so x=0 v=75m/s. If we produce another graph we will find that at some depth x-final the drag and buoyancy will be equal to gravity, however this is not always the deepest part of the graph, for the person to stop he must experience a negative force to slow him down thus when his velocity is zero for the ",
" time is the deepest due to the dive. x-stop-1st. This ends up with an oscillation type motion until the momentum is converted to heat"
] |
[
"Thanks! I don't fully understand the answer but some more research and hopefully I'll produce some numbers for this. "
] |
[
"Can chemists (or physicists?) 'predict' what stable elements should or could exist but have yet to be discovered?"
] |
[
false
] |
Stable elements or even just mostly stable ones. Iron for example has 26 electrons and protons and 30 neutrons while Uranium has has 92 electrons and protons and around 144(ish) neutrons. So could one simply predict elements which could exist based on known laws of physics & what is known about atomic structure?
|
[
"This already has happened. When Mendelev put together his Periodic Table, he noted \"gaps\" in it. He was able to predict the nature of the missing elements. They have, since then, been discovered and his predictions found to be correct. (Germanium, Gallium, and Scandium)\n",
"http://en.wikipedia.org/wiki/Mendelev"
] |
[
"It's difficult to tell, so we're not really sure. We're almost certain we've discovered every remaining naturally occurring element, but we may be able to produce more synthetic ones.",
"There's something call the ",
"island of stability",
" which is a hypothetical group of radioactive isotopes for very heavy elements (around 120 or so) that is theorized to be far more stable than most others.",
"We can keep placing heavier elements into the periodic table, but we can't really know how effective it will be at predicting their properties."
] |
[
"Yes, but only in approximations. As you try to look at larger and larger nuclei there are more parts so it gets very complicated. Nuculear structure (even among the well known stable elements) is still a very difficult problem and something that theorists and experimentalists are still working on. ",
"The theory of nuclei is described by Quantum Chromo Dynamics. The theory is really good execpt it is only possible to calculate things at really high energies, but a bound nucleus is a low energy thing so no one knows how to do the calculation. So instead you can make models that are appoximations but work really well with nuclear physics. For example pretending a nucleus is protons and neutrons bound by exchanging pions (in reality we know it is just a bunch of quarks exchanging gluons, and not really made of protons and neutrons but that approximation works really really well for most cases.)",
"Heavy nuclei nuclear physics is not my field at all, so hopfully someone can give you a better answer, but yes we can make predictions about what elements could exist but it is a hard problem. Some people have theories an ",
"Island of stability",
" for much heaver elements than we have seen so far. "
] |
[
"What would happen if two faults on opposite sides of a tectonic plate shifted simultaneously?"
] |
[
false
] |
I was reading and wondered what would happen if the and the had events occur simultaneously? I don't know that these faults are exactly on opposite sides of plates, but hypothetically, if an earthquake happened in both of the areas at the same time.
|
[
"First, the Cascadia subduction zone is on the western edge of the North American plate, but the New Madrid seismic zone is an intra-continental feature, i.e. it is in the middle of the North American plate. The eastern edge of the North American plate is the ",
"Mid-Atlantic Ridge",
". ",
"Going with the spirit of the question though, the short answer is nothing different than if the two earthquakes happened at different times. We can look at a ",
"simulated scenario for a magnitude 9 event on the Cascadia subduction zone",
" and see that the area that experiences shaking, while certainly large on a societal scale, is small compared to the entire plate. This is because seismic waves dissipate as they travel outward. An earthquake releases a set amount of energy (this amount of energy is what the magnitude is measuring) that is released as seismic waves. As those waves expand out, roughly as a half sphere from the earthquake location on a fault plane, the amount of energy gets spread out so the intensity of shaking decreases. Thus, by the time you're a few 100 km's away, let along on the other side of a plate, the earthquake waves are detectable by a seismometer, but certainly not felt. So, if you had two simultaneous earthquakes on either side of the North American plate, the only effect might be the weird behavior of a seismometer that happened to be positioned such that the arrival time of both earthquakes were exactly the same.",
"Obviously from a societal perspective, two large events simultaneoulsy in the same country would have lots of effects in terms of resource deployment, etc, but I think you were more asking about the geology aspects of things."
] |
[
"Thank you for answering! Yes I wondered if (and not knowing the proper terms and details I'll explain best I can) two events happened on opposite sides, would that effect the land in the middle. I imagined the plains becoming squished up against the Rockies in a ripple, you know? ",
"I think things like that are interesting and I'm always wanting to learn. I appreciate your taking the time to give such a thoughtful answer. "
] |
[
"Even in the largest earthquake events, the amount of surface deformation perpendicular to the fault drops off pretty quickly to the point where it is zero within 10s (to maybe 100s, but at that scale you're talking about perhaps fractions of a millimeter) of kilometers. So you would not expect any effect in the center of the country."
] |
[
"Where did all the lightning bugs go?"
] |
[
false
] |
The recent controversy about bee population decline being linked to pesticides got me wondering about another insect. I live in North Texas and around 15 years ago when I was really young (6ish), I remember going out during the night and catching lightning bugs. These things were everywhere. Not only do I remember my front yard having more than I could count but when I went to friend's houses I saw the same thing. Now I don't see them anymore when I visit the area, which I regularly do. Is this related in anyway to the bee situation? If not can someone explain to me why this has happened? Is it just this region?
|
[
"The two main factors that researchers believe to be responsible are destruction/disruption of their natural habitat, and an increase in light pollution. However, there are additional factors that are most likely more prevalent in other areas, such as pesticide usage, environmental pollution (both air and water), and even ecological changes that produce higher numbers of natural predators/lower numbers of natural prey. \nMost fireflies prefer warm, humid areas such as forests or marshes. The paving over of fields and forests, as well as increases in boat traffic can drive away fireflies or discourage them from breeding.",
"Light pollution can affect mating habits because very specific flash patterns are used to attract a possible mate. Fireflies can be confused by car headlights and an increase in overall brightness at night, thus deterring breeding.",
"Although both of those main factors are valid reasons, my money is most likely on subtle ecological shifts, which can be explained mostly by human interferences, such as pollution or food chain changes (increases or decreases in other species). It would not at all surprise me if you'd be correct in connecting their fates with that of bees due to pesticides.",
"It really is sad, I do remember having them as a big part of spring and summer growing up."
] |
[
"Fireflies are actually beetles (Coleoptera) and not part of any fly Order. I always thought that was neat. Carry on..."
] |
[
"http://articles.chicagotribune.com/2007-08-01/features/0707300227_1_photuris-fireflies-light-meal",
"http://www.thaibugs.com/?page_id=707",
"sorry it's not much in terms of links to studies, but they do make references to recent research."
] |
[
"Why do electrochemical cells (batteries) make a specific voltages?"
] |
[
false
] |
A guess I am asking why does one cell of Zinc-Carbon produce 1.5 volts? This voltage seems the same for both large and small cells. Unless I am wrong a battery cell the size of the earth would still produce 1.5 volts. Why do battery cells not produce more or less voltage?
|
[
"Understand it by looking at the electrical potential of a ",
" first, which comprises an ",
" and an ",
" and a reaction involving transfer of electrons (an electrochemical cell is two half cells).",
"Any redox reaction (transfer of electrons) will have a ",
". For example ",
"Zn(OH)4",
" + 2 e",
" ---> Zn(s) + 4 OH",
"Has an electricalpotential, V=-1.99Volts at room temp/pressure with a 1mol solution (or E",
" , but I'll use V for ease of typing). You don't need to work these out, you can look them up on a ",
"table in a chemistry text",
". The value of zero is defined by the ",
" and all other electrochemical reaction potentials are relative to that.",
"If the voltage of the reaction is positive then the reaction is ",
". If it is negative then the reaction is ",
". The voltage of the cell (two half cells connected by a salt bridge) is the sum of the two voltages ",
"ie. V(cell) = V(reaction at cathode) + V(reaction at anode)",
"If you want a higher voltage then you make the cell with a higher concentration of the solution (remember the standard potential is for a 1mol/litre solution).",
"PS: does this version of Markdown do subscript? Anyone know?"
] |
[
"You can achieve different voltages by changing the concentration of the dissolved species; in this case, particularly Zn ions. 1.5 V corresponds to ",
"standard conditions",
" (1 mol/l concentration). Since the concentration (or rather, activity) enters the ",
"Nernst equation",
" in a logarithmic manner, a higher voltage is not easily achieved. Also, the solubility of ions poses an upper limit to their concentration. In a real-life battery, you want to stay away from the maximum possible concentration, since e.g. temperature drop can lead to salt precipitation which, in turn, might lead to mechanical damage."
] |
[
"The voltage question has been answered quite well, so I'll cover the other two aspects of batteries, current and capacity. For capacity, let's talk about the standard 1.5v alkaline cells. AA. AAA, C, D, 9V (which actually use 6 AAAA in series). They all use the same chemical reaction to produce voltage, but the bigger batteries contain more reactant materials, and can produce the 1.5V for a longer period, at a higher current. In a battery, more current is produced by having greater surface area of the electrodes. ",
"Take a standard lead-acid battery, it produces 2V per cell. A car or truck battery contains 6 of these in series to give 12V. The truck battery has much larger plates in the cells, which will provide a greater current for starting the big Diesel engines, but the voltage is the same. "
] |
[
"Could someone please help with the uncertainty principle?"
] |
[
false
] |
and me and a flatmate were having a discussion and neither of us could answer why, when you can determine with greater accuracy the position, of a photon of light, you increase the uncertainty in the velocity. I understand the equation and that to keep within the limits when one increases, the other must decrease; but what, if anything, actually causes the photon to change course, or is it a case of "it just does"? I know at the end of the video it points out that if you consider light as a wave, all that is happening is that it is diffracting: so is light in this situation always considered as a wave, or is there differing explanations for each?
|
[
"I explain it to my students using this analogy:",
"Picture two cameras, with two shutter speeds. Both are used to take a picture of you running through the frame in a circle. The fast camera will tell us exactly where you were when the camera clicked-but no sense of what your speed was.",
"The slow camera tells us more about how fast you were going, but it is too blurry to tell exactly where you are."
] |
[
"I think it's best to look at how the uncertainty emerges. Any wave can be described as the sum of sine waves of different frequencies. In quantum mechanics the wave itself corresponds to position and the various frequencies of the sine waves that make that wave correspond to momentum. Let's start with a simple case: if the wave is just a simple sine wave, then there is only one frequency so you know the momentum exactly. But that sine wave is spread out over all the real numbers, so you have no idea what the position is. On the other hand, if you want to make a wave that is a pulse at a certain point, you would need to add up an infinite number of sine waves, each of different frequencies. So you would have a wave with an exact position, but you would have an infinite number of frequencies and, by extension, an infinite number of possible velocities. In general, every distribution has a corresponding distribution of frequencies, and the more you reduce the uncertainty of one, the more you increase the uncertainty of the other. From this you can see that the exact position and momentum of a particle simply can't be known simultaneously. "
] |
[
"I used a similar analogy a while ago, and the consensus was that it is insufficient to explain the Uncertainty Principle, also that all we do when giving analogies is clarifying the problem - but they do not really relate to the Uncertainty Principle itself.",
"My analogy was: If you want to know the speed of an object, you have to detect it at two consecutive positions. You then know the precise average speed between these two points, but you don't know the precise speed of any arbitrary point between these two points. When you keep pushing the detectors closer together between experiments, you can make a more and more precise statement about exactly ",
" the object had the average speed you measured/calculated, but the time between the two points becomes so short that the speed statement you derive becomes less and less precise. Until both detectors are eventually in the same place, allowing you to make a ",
" statement about the location, but you have no idea any more how fast the object was.",
"One of the objections was that the UP does not relate to a problem of measurement but to intrinsic properties of reality.",
"So, we're only presenting analogies, but they do not really relate to the UP. Are we really helping OP?"
] |
[
"How is the same species of moose native to both Eurasia and North America?"
] |
[
false
] |
With the Columbian Exchange, species like horses, pigs, maize, tomatoes, potatoes and others became available on both continents. Moose are native to both continents, but are also the same species, which seems odd to have not diverged in however many millions of years. I would be very surprised if early European explorers successfully put moose onto ships to bring them to another continent, without even beginning to discuss why they would try that. At least horses are useful and domesticated.
|
[
"Many animals that range into cold climates were able to disperse between continents when sea levels were lower during ice ages. A great many large mammals in the northern part of the northern hemisphere did this (including humans). although many have since gone extinct in one hemisphere or the other."
] |
[
"There was a land bridge between what is now Russia and Alaska called Beringia. It’s how humans migrated to the Americas around 16,000 years ago. Evidence suggests Moose came later, towards the end of the most recent period when the land bridge was exposed, as recently as 11,000 years ago. We have fossil and other evidence of moose from that time, so well before European rediscovery of the Americas within the last 600 years.",
"So a very short time in terms of evolutionary changes, not the millions of years you would need for speciation for large mammals."
] |
[
"According the Wikipedia \"moose\" article they migrated from Eurasia to North America in the late Pleistocene, less than a million years ago, in the Bering Sea area. So probably over land when sea levels were lower."
] |
[
"Is it possible to farm electricity using lightning rods or similar techniques?"
] |
[
false
] | null |
[
"It is pretty impractical. When farming electricity you want a steady constant source as much as possible so you don't need to store it which results in efficiency loss, not a huge violent (and random) blast 75 times a year."
] |
[
"In theory, yes. Practically we don't have reliable or safe means of handling and storing the incredible burst of energy contained in a lightning strike."
] |
[
"No, it is neither possible nor practical to farm electricity from lightning. ",
"There are electrical issues (converting lightning into usable electricity), location issues (lightning is not frequent enough in any known location), and total output (all lightning only produces 1250 kA at 250 kV).",
"There are many other natural sources of electricity that work so much better (solar, wind, tidal, geothermal) than lightning."
] |
[
"Is it possible to tell the shape of a piece of metal from the sound it makes when stuck?"
] |
[
false
] |
More wondering if that information is transmitted in the sound, and if with infinite precision, could it be done. Obviously this would be nearly impossible by hand.
|
[
"You're asking one of the great fun questions from the physics of the 20th century!",
"It turns out that the answer is yes, ",
"at least to a set of related shapes if not a single exact shape",
". The limitation is that there are groups of objects that are ",
" to one another, which is to say that their resonant frequencies are all the same, so they can produce the same sounds. If you know other constraints on the shape (i.e., roughly what it looks like) you may be able to eliminate all but one member of an isospectral set from sound alone as well. So the very general answer is \"no\", though it may still be \"yes\" for many practical purposes.",
"That said, it may also be possible to distinguish some pairs isospectral solids from one another, as the resonances may be excited to different degrees for the same excitation source between the two objects (that is to say that the eigenvectors associated with the same eigenvalues are not necessarily the same themselves, and may differ by some set of linear transformations between the shapes). So if you know the shapes are oriented in the same way, striking them at a series of locations that are the same on both objects may produce different sounds, allowing you to distinguish ",
" sets of isospectral objects by sound alone."
] |
[
"To an extent. The shape of the metal will determine the frequencies it resonates at (the overtones, in musical terms), which can be heard and recorded.",
"But to go from the spectrum of sound produced back to the shape of the object is an ",
". Inverse problems are generally hard. Without some additional constraints or knowledge about the object, I'm not sure you could determine its shape.",
"A practical application is the tuning of instruments such as bells and steelpans. A tuner will listen, and nowadays measure electronically, the overtunes the instrument produces, and will then change the shape of the bell or pan in order to change those overtones to home in on the desired tone. It's as much of an art as a science but I think it's close to what you're asking about."
] |
[
"It is indeed possible. Jet engine manufacturers use a principle like this, but in a different way.",
"Called a “ping test” or an “impact test”, this tests whether a pipe installed on an engine (like an oil pipe, air pipe, or fuel pipe) will survive the harsh vibratory environment of multiple flight cycles. The pipe is struck with a calibrated force, and the impact vibratory response of the pipe assembly is recorded and analyzed. This is a test where the impact of the pipe network’s shape on life is analyzed by the vibration response.",
"I can imagine doing this in (somewhat) reverse — using the vibratory response, analyze the network configuration (or shape). I don’t think we have the theoretical knowledge yet to figure it out for ANY configuration out there, but I can imagine if you were to calibrate this for a few distinct shapes, you can easily distinguish between them by vibration response alone. You might be able to detect out of tolerance conditions for machined parts in this manner if you control the experiment right. (Woah, I don’t think that’s been considered before. Might be patentable)"
] |
[
"Claim: \"Fourth-generation nuclear power plants will use current nuclear waste products as fuel.\" Is that true?"
] |
[
false
] |
In this Ted talk Stewart Brand makes the claim that fourth-generation nuclear power plants will use the nuclear waste from our current powerplants as fuel Is there any accuracy to that? is this one of those "in 20 years… XYZ will happen!" And then 20 years later somehow that thing is still 20 years in the future? or is it realistically happening right now?
|
[
"Some of them. There are a few different kinds of plants planned as the 4th generation. One of which is a fast spectrum reactor. These plants use the emissions from fuel which the other plants cannot. This means they are able to use fuel that has already been through an different plant. The spent fuel from these fast spectrum plants only needs to be stored for ~500 years before losing its radioactivity as apposed to the indefinite storage on current fuel. "
] |
[
"Safety is nearly a non issue. If you were to ask how many people have been killed by nuclear reactors I would bet they would guess very high. Actual number? 32. Plants are so safe it is laughable the injustice they are done. ",
"Then look at the Gen4 reactors and it becomes an even bigger joke. Pebble bed, gas cooled, and G4 single phase water reactors essentially remove even the rare risks associated with Gen2/3 plants.",
"The hurdles are political and nothing more. Proliferation is an issue, but one I feel could be easily handled."
] |
[
"It depends a bit on the type of IV-gen reactor, but there's nothing wrong with the actual principle. ",
"Fast-neutron reactors",
" can do this, and potentially convert long-lived nuclear waste into much shorter-lived isotopes. (Nuclear reprocessing of fuel, which also exists, can reduce the amount of waste as well, although it doesn't reduce its radioactivity)",
"There's a decent number of existing experimental Gen-IV reactors, and ones under construction. There's a sizable amount of money being put into it, and a good number of nations interested in developing the technology. But it's definitely the case that they need to do quite a bit more work and overcome a number of engineering challenges before they'll have commercial reactors that are safe and reliable. But it's definitely closer to that than fusion reactors are. How long it'll take depends very much on the amount of resources that get spent on it, as the challenges are not thought to be unsurmountable. ",
"There's also a set of rather non-scientific challenges with it, such as whether it'll be economical. Then there are political issues, with the inherent controversy surrounding nuclear technology. There are also non-proliferation issues involved, as some of these reactor types could potentially be used to produce fissile material for nuclear weapons, which is much less of a problem with conventional light-water reactors."
] |
[
"How big can black hole get ? Is there a limit ?"
] |
[
false
] | null |
[
"If by \"big\" you mean spatial extension, there is really no way to answer this question.",
"The black hole itself is a singularity, meaning it has no volume whatsoever. Thus, the size of black holes is mostly measured by the radius of its event horizon (or ",
"). ",
"The size of the Schwarzschild radius is proportional to the mass of the black hole. Since the mass of a black hole has no upper limit, the \"size\" of a black hole could, theoretically, be arbitrarily large."
] |
[
"Let's start with the trivial answer: a black hole's event horizon cannot be larger than the universe, so there's an upper limit right there. Next, since matter in the universe seems to be clumped in galaxies, and since the universe is expanding, and since the distances between galaxies is so large that in the future even neighboring galaxies will be receding from each other faster than light, that seems to put the upper limit on the mass of black holes at about the equivalent of the mass of a galaxy or several, so there's another upper limit which doesn't appear to be breakable with our current understanding of the universe.",
"As for a theoretical limit, since black holes dissipation rate is inversely proportional to their event horizon radius, there shouldn't be any theoretical limit, barring some unknown physics that's yet to be discovered."
] |
[
"It's a weird situation when that is mentioned in that context. The space in between distant galaxies can expand at such a rate that they are moving away from eachother faster than the speed of light ",
", but that is a different thing entirely from the objects themselves having a speed greater than light. There is no limit on the expansion of the fabric of space. There is a limit on the speed an object can travel."
] |
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