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[
"Gravity and jerk"
] |
[
false
] |
Suppose I hold a ball above the ground. When I'm holding it, it has an acceleration of zero. When I let it go, how much jerk does the ball experience? I imagine it depends greatly on the air, and the way I drop it. Surely it doesn't instantaneously experience 1g of acceleration?
|
[
"'Jerk' is the fairly standard term for the time derivative of acceleration."
] |
[
"There will be a short period of time when the ball is scraping along your fingers, because they are still touching the ball, but not applying enough pressure to hold it in place. This time is tiny, so the jerk on the ball is enormous."
] |
[
"This is correct. There is no infinite jerk. Just really high."
] |
[
"With all the recent posts about atmospheric pollution clearance secondary to wide spread quarantining, is this having any effect on climate changes and temperatures? Or is it too small a time frame to see any real changes?"
] |
[
false
] | null |
[
" The current, temporary reduction in emissions (even if sustained for many more months beyond the present length) will basically do nothing and this would only have some longer lasting influence if it led to a real, and sustained change in behavior/emissions post COVID-19 (but even then, it would take decades to centuries for a change to be reflected in temperatures).",
" First off, it is worth starting with the observations. Emissions of a variety of compounds have been temporarily reduced by the various stay-at-home / quarantine orders reducing travel and manufacturing, etc, e.g ",
"NO2",
" and ",
"CO2",
", the latter being the more (directly) relevant for the question of anthropogenic climate change. ",
"A reduction in CO2 emissions seems like it should have a direct impact, but there are two categories of things to consider. The first is a set of short term considerations, i.e. for the duration of COVID-19 quarantines / slowdowns in emissions. For example, there are clear temporary reductions in emissions from fewer cars on the road or airplanes in the sky, but we're also all at home, using energy at a greater rate than we otherwise would, in likely less efficient ways (e.g. 100 people running their AC at home during the middle of the day, vs 100 people in an office with one large AC unit running during the middle of the day). There are some suggestions that this increase in ",
"home energy usage might cut into the emissions reduction in the end",
". Additionally, when considering a reduction in emissions, not all emissions are going to have the same influence. So while a reduction in CO2 is contributing less greenhouse gases to the atmosphere seems like it could have a cooling effect (more on that in a minute), reduction in other emissions may actually have a warming effect. Specifically, things like ",
"aerosols tend to block sunlight",
", so reductions in this type of pollution paradoxically would tend to increase the temperature.",
"Ultimately, everything in the preceding paragraphs are focused on changes that are happening for the duration of the COVID-19 related reduction in emissions. The real question is, ",
" The answer, unfortunately, is no and the reason is largely because of the residence time of CO2 in the atmosphere. Basically, reducing emissions for 1 year does not directly influence the trend of climate change because (1) you still have the background concentration of CO2 you had before and (2) the timescale to remove CO2 from the atmosphere through functioning of the ",
"carbon cycle",
" is MUCH longer than a year. As an analogy, imagine you've been slowly filling a bucket with water for 2 hours (or you've been pumping carbon into the atmosphere for 200 years). The bucket has a hole at the bottom that lets water trickle out at a very slow rate, which is actually dependent on the amount of water in the bucket (the rate by which carbon is taken back up and sequestered is slow compared to the rate at which we add it and the ability for it to be taken back up decreases as the concentration in the atmosphere decreases). If you turn off the hose for a minute (you reduce emissions due to a global pandemic for a year), the level of water is going to decrease a tiny bit but most of the bucket is going to still be full (all the CO2 causing warming is still there) and you will almost instantaneously refill the bucket to your pre-pause level once you turn the faucet back on. To demonstrate this in a more rigorous way, we can look at models of what would happen if we stopped emitting greenhouse gases permanently. What we see is that even after a complete and permanent cessation of emissions, warming persists for hundreds of years (e.g. ",
"Solomon et al 2010",
") and in detail, it will likely take ",
"1000s of years",
" for the concentration of CO2 in the atmosphere to return to pre-industrial levels after a complete and permanent cessation of emissions. That means that the only way this experience has a net-positive benefit for the global warming story would be if this experience led to permanent reduction in emissions (e.g. we all realize don't have to travel by car and plane as much, etc) and even then, it would be decades to centuries before we saw a reduction in temperatures as a result."
] |
[
"Note that \"pollution\" usually refers to emissions that have adverse health effects to humans and other organisms. Carbon dioxide, the main driver of the climate change, is pretty much neutral at atmospheric concentration. It also disperses quickly to be in a roughly homogenous concentration globally. The usual \"pollutants\", being heavier particles, stay quite local. Which is why the air in the countryside is \"cleaner\" than in the city centre a mere hour driving away.",
"My point: clean air experienced by the population is not the same, even if related, as less carbon emissions."
] |
[
"Thanks for having a short and a long answer"
] |
[
"Is what we call \"the observable universe\" the same as a Hubble volume?"
] |
[
false
] |
As I understand it, since the expansion of the universe is accelerating, there is a point at which objects in space begin to travel away from us faster than the speed of light, thus making the light that those objects emit impossible for us to see since it can never reach us. A Hubble volume is everything we can see in space from our perspective. A.k.a all the light emitting objects that aren't traveling away from us faster than the speed of light. So, if my understanding is correct, what is the difference (if any) between a Hubble volume and "the observable universe"?
|
[
"The Hubble volume is the region in which objects are receeding from us at a rate less than the speed of light.",
"Objects outside of the Hubble volume are receeding faster than the speed of light, but we can still observe them. The photons from those objects leave and travel to us - they don't care how fast the object was moving. The speed of light is constant in all reference frames - you can't think of it \"classically\".",
"So, the observable universe is much, much larger than the Hubble volume."
] |
[
"Could it be that entropy is somehow decreasing and time is somehow reversing direction beyond the hubble volume?",
"It seems strange to say that the universe could be expanding faster than the speed of light. ",
"Are the implications any different if we say that the rate of time (rate of entropy increase) is a local phenomenon which shifts out of phase?",
"Is it possible that rate of change in Entropy (time) could be in balance on the whole but our observable universe is in a \"positive entropy pocket\""
] |
[
"Could it be that entropy is somehow decreasing and time is somehow reversing direction beyond the hubble volume?",
"No.",
"It seems strange to say that the universe could be expanding faster than the speed of light. ",
"Information is not being transmitted faster than the speed of light, so the laws of physics are not violated.",
"Are the implications any different if we say that the rate of time (rate of entropy increase) is a local phenomenon which shifts out of phase?",
"I'm not sure what you mean by this. The entropy of the universe is always increasing, but it can be vastly different on small scales.",
"Is it possible that rate of change in Entropy (time) could be in balance on the whole but our observable universe is in a \"positive entropy pocket\"",
"See above response."
] |
[
"What could future earth look like?"
] |
[
false
] | null |
[
"Such hypothetical / speculative / open-ended questions are better suited for our sister-sub ",
"/r/asksciencediscussion",
". Please post there instead."
] |
[
"Thank you. I'll be sure to do that. Should I delete this post?"
] |
[
"Already removed."
] |
[
"Which car will win when 2 cars with same engine and configuration but in different gear collide?"
] |
[
false
] |
[deleted]
|
[
"Whichever car starts in the lower gear will have more speed. Whichever is in a lower gear when they collide will have more power, because it will not be able to shift up as quickly. I'd say the car that starts in first will be in a higher gear when they collide, but will have more speed. Unless the car that started in the higher gear has continuous throttle throughout the collision, I would say the car starting in 1st would \"win\"."
] |
[
"If they are at the same rpm but different gears, the higher gear will be going faster, therefore more momentum."
] |
[
"Thanks for the reply. Although I wasn't clear enough, I meant two cars remain in same gear and same throttle/engine power. As they are consuming same energy they should be equals when they collide. And nothing out of two can win. But that doesn't feel right."
] |
[
"Can the Earth ever become tidally locked to the Sun so one side is always facing inwards (like the Moon)?"
] |
[
false
] |
The titles pretty much explains it all. I understand how the moon is locked to the Earth but wondering if this could ever happen on a scale as large as the Earth/Sun. Bonus question: What would happen to the moon if the Earth was locked to the sun?
|
[
"The earth used to spin much faster millions of years ago and has been gradually losing rotational energy. Not sure of the exact details but it is caused by the interaction between the moon and the earth through ocean tides. A torque is set up which accelerates the moon and decelerates the earth. Here's the ",
"wiki",
"Anyway the earth will eventually be tidally locked to the moon so that 1 day = 1 month.",
"Because of the relative distance, the sun has a much smaller tidal effect on the earth so the process of becoming tidally locked to the sun will take a lot longer, but will occur by the same process as with the earth to the moon.",
"Edit: Someone has mentioned that the earth can't become tidally locked to the sun once it becomes tidally locked to the moon. I admit I don't understand the topic enough to understand if or why that would happen."
] |
[
"Anyway the earth will eventually be tidally locked to the moon so that 1 day = 1 month.",
"1 earth day = 1 lunar month?",
"Oh, and fun fact:"
] |
[
"Oh wow imagine that humidity."
] |
[
"Are there any examples of cross species breeding in the wild?"
] |
[
false
] | null |
[
"Grizzly-polar bear hybrids have ",
"been confirmed in the wild",
"."
] |
[
"Coyotes and wolfs form ",
"coywolf"
] |
[
"They are different species. Ursus maritimus and Ursus arctos horribilis (Ursus being the genus).",
"Maybe you're thinking of grizzlies, kodiaks and alaskan brown bears which are all subspecies of brown bear."
] |
[
"Does a graviton \"shadow\" exist?"
] |
[
false
] |
For instance, if the Moon is "hidden" behind the Earth from the Sun, should there exist a graviton shadow? Does the Earth intercept gravitons that would have otherwise interacted with the Moon (which would, I presume, have at least some effect on the moon's orbit)? If this does in fact occur, are there laws that balance out this issue? If gravitons are "virtual particles", does that somewhat account for this? Edit: Clarity
|
[
"Well, we haven't discovered gravitons (yet?). We also don't have a quantum theory of gravity.",
"If you're asking whether the gravitational pull on the moon from the sun becomes weaker when the Earth goes between them, the answer would be no. Such a thing would have an observable effect on the orbit of the moon (which you are correct on), but we haven't observed it. The superposition principle in classical gravity seems to hold."
] |
[
"Well, not really. If there were an analogous force-carrier particle for gravity, the normal gravitational attraction between objects would be mediated by virtual gravitons. Real gravitons would pop up when you're discussing stuff like gravitational waves, disturbances in the gravitational field. This is analogous to how photons act for the electromagnetic force.",
"I suppose that one could block real gravitons (gravitational waves) with matter, but I'm not an expert on this subject so I can't really comment further."
] |
[
"I guess this is equivalent to say that gravitons are not absorbed by matter (while photons are). Is this correct?"
] |
[
"Is a circle with a radius of zero still a circle?"
] |
[
false
] |
The question on a chem report: when graphing the relationship between diameter and circumference of a circle, does the y-axis pass through (0,0)? On one hand, yes, because when r = zero then the circumference is also zero. But that got me thinking... is a circle with a radius of zero still even a circle? Is this just a matter of a strangely worded question? Science, help!
|
[
"It is a point. The question is whether a single point still counts as a circle.",
"One way to state the definition of a circle is that it's the set of all points that are a distance ",
" from a given center point. If you set r=0, then your circle is the set of all points that are 0 distance from the center. That set only contains one point, but it's still a set, and it still meets the definition.",
"There are ways to phrase the definition, for instance ones that involve ",
"conic sections",
", but they all wind up meaning the same thing. And as far as I can tell, a zero-size circle fits all of them."
] |
[
"It's just a point then"
] |
[
"I'm in higher mathematics and I consider it a circle. It's just an unsatisfying circle."
] |
[
"AskScience: What are the best science documentaries?"
] |
[
false
] | null |
[
"For biology and nature, you cannot beat the David Attenborough series.",
"I recommend Blue Planet, Planet Earth, and his \"Life\" series:\n- Life on Earth\n- The Living Planet\n- The Trials of Life\n- Life in the Freezer\n- The Private Life of Plants\n- The Life of Birds\n- The Life of Mammals\n- Life in the Undergrowth\n- Life in Cold Blood\n- First Life",
"They are without a doubt the greatest nature documentaries in history, by one of the greatest narrators."
] |
[
"Outdated, sure, but I watched it for an appreciation of the mentality involved. To watch someone so brilliant talk about something so passionately can teach you plenty, and I doubt that will ever be outdated."
] |
[
"I like the Horizon series from the BBC, it covers a lot of specific subjects from different disciplines of science and is very well produced."
] |
[
"Are there any other species of ape that could learn to talk?"
] |
[
false
] |
I know that some gorillas have been taught limited sign language. Are there any specific reasons why they wouldn't be able to actually talk (brain development, vocal cords, tongue etc)? Also, are there any other apes that might be able to communicate with us?
|
[
"While the animals can learn to USE the sign language most of them cannot really understand it. There is a lot of debate on this matter. Some say it is as simple as association like your dog shaking hands for treats. While others (usually the trainers) believe that the animals are very funny and interesting to talk to. I recall one example of a \"famous\" chimp study who had been taught sign language. When asked a question like \"what do you like to do?\" She responded something like \"purple nipple big.\" And any question asked of her got a similarly random set of words as an answer, while the trainer would \"translate\" it into some kind of story because of their confirmation bias. But there have also been examples of possible understanding of language. There was once a scientist who was going to visit a chimp he had taught sign language to at a zoo. He claimed that the chimp immediately recognized him and said \"help caged(or trapped).\" So whether or not they can communicate effectively needs more and better research though I would lean towards no, they can't comprehend language as we use it. Dolphins on the other hand have been able to understand words we teach them arranged in ways never before heard (and then \"discuss\" the sentence between themselves before acting on it) which implies the possibility of infinite generatively (one of the traits required to be classified as capable of language). But dolphins do not have vocal chords or hands to converse with. "
] |
[
"Besides differences in their respective vocal (and manual) apparatus(es), there are cognitive differences between humans and other primates that prevent the latter from learning language like ours. What exactly this is is a matter of great debate. A popular answer (the mainstream one, I'd say), espoused ",
"here",
", is that non-human animals lack the capacity for a recursive system that generates and maps syntactic objects to conceptual-intentional and sensory-motor systems.",
"EDIT: Elaborated the Hauser et al proposal a little."
] |
[
"so they physically cannot speak? "
] |
[
"Is there such a thing as true randomness?"
] |
[
false
] |
Is entropy random?
|
[
"So if the smallest unit is random is everything that occurs random?"
] |
[
"Quantum processes are inherently random."
] |
[
"To my understanding, yes. Radioactive decay, for example. ",
"In resistors, heat produces a very small amount of voltage. This fluctuates extremely randomly. Fed into an opamp it can provide entropy for encryption. "
] |
[
"What did the SapceX Falcon 9 rocket launch look the way it did?"
] |
[
false
] |
Why did it look like some type of cloud, is that just vapor trails or something else? (I also don’t really know what flair I should add so I just put the one that makes the most sense)
|
[
"A lot of folks noticed that the plume looked a lot like a contrail at first, then ballooned outwards later.",
"As the rocket reached higher and higher altitude, there's less ambient air pressure to push against the rocket exhaust coming out of the engine, so the plume is able to expand much farther - this is what gives the plume its characteristic balloon shape.",
" Since a lot of folks are asking what the \"bright dot\" was on the inside of the balloon structure:",
"That was the separation of the reusable primary stage. ",
"In this high-res video",
" you can actually see the primary stage end (when the thrust goes dark), the secondary stage ignite (when the thrust goes bright again), and then the bright dot of the separated first stage lagging behind and dropping a bit. Note that it doesn't just drop like a rock, since it's also on a ballistic trajectory - it takes some time to lag behind and start falling. If you look closely, you can also see some spiral waves coming out from it, presumably because it's tumbling around while thrusting a bit to control its eventual descent.",
"Eventually that first stage will land and be used again. ",
"Here's a schematic",
" of how all of the above actually works."
] |
[
"You saw liftoff, stage one separation, stage two light off and leave great plume, sunlight reflection and then the fairing separation which many camera persons captured greatly trailing the stage two separation. Great view of Sunlight plume. Probably the best view people has ever seen of a rocket launch. You only see this when you launch at a late hour just after sunset or in the morning just before dawn. Try imagining where the sun is in the video and then you get why its only lighting up high flying objects or exhaust plumes in this scenario. When the earth rotates another hour, the sunlight would be blocked completely be earth and we would see only the fire from exhaust not the smoke light up like in the video. When rockets launch at daytime, the smoke plume gets illuminated just as in this video. But against an already bright sky it does not look super bright or glowing."
] |
[
"No. Rocket launch times are driven by what orbit the payload is going to. This was a set of communication satellites going to ",
" low earth orbit (Thanks, ",
"u/kendrome",
" !) and the launch window for it just so happened to be right.",
"While you could theoretically move a launch time wherever you like, it would cost a lot more fuel to do so and would generally be wasteful in both time and money."
] |
[
"How do the cells of complex organisms know which direction to grow in so that they get the desired shape on a macro level?"
] |
[
false
] |
I was thinking about this while looking at a plant. I was wondering from where in the plant do the cells multiply to make it grow. But more interestingly, how do they know to multiply in a line stretching upwards and outwards along the stem rather than into a random blob of cells. Then I started wondering the same about animals too; how do organs know to get into that shape?
|
[
"As the other commenter said, there's an entire field devoted to answering questions like this, and there's ",
" that we still don't understand, but it's still possible to make some general statements.",
"So first of all - and sorry if this is review for you - every somatic cell (as opposed to gametes, i.e. reproductive cells) in a complex organism has essentially the same set of genes. The shape and function of a cell depends on which genes are expressed (mostly producing proteins, though some genes' produce mRNA with a direct effect on the cell), and to what extent. Gene expression is controlled by an unfathomably complex network of molecular interactions, but the basic premise is that signals from outside the cell set of a cascade of intracellular signals that, through the action of various proteins called ",
", genes get \"turned on\" or off. Cells have ways of sensing temperature, pressure (touch), particular molecules, and voltage. They also have internal regulatory cycles (the circadian rhythm that controls your sleep is an example of this) that can, for instance, turn on genes during certain times of day, or during a particular phase of development.",
"The point is, the molecular machinery within cells is so diverse that they can sense and respond to practically any relevant information in their environment. The result is that a cell \"knows\" where it is in the context of a larger organism, and it behaves accordingly. The state of a cell is sort of like the state of a computer, except that instead of information being stored as the voltage over millions of transistors, it's stored as the physical arrangement of the genome and the concentrations of different chemicals inside the cells.",
"Let's put that in context now: You can actually take pretty much any plant material and, by exposing it to two types of plant hormones called auxin and cytokinin, turn it into a \"blob of cells\" called callus. (Unlike animal cells, plant cells can go \"backwards\" in development, essentially becoming less specialized and regaining the ability to become a different type of cell.) In this state, every cell is exposed to homogeneous conditions, and in order to become a plant, the cells must \"figure out\" where they are. The most relevant information for a plant is which way is up and which way is down, so cells express proteins that look for the clearest indicators of direction: light and gravity. The detection of light causes cells to start producing auxin and to pump it from cell to cell in the opposite direction of where the light is coming from (i.e. downwards) as well as to generally start behaving like a shoot, while cells that don't perceive any light figure out which way the gravitational force is pointing and pump cytokinin in the opposite direction. Now the plant has established auxin and cytokinin gradients which can be used as information that tell cells where they are and what they should be doing. These hormones (and others) have ",
" of functions, but just for example, a high influx of auxin and a low influx of cytokinin into a plant cell is basically telling it, \"You're close to the top of the shoot; don't make any branches\".",
"The more time goes on, the more information is gathered and the more complexity emerges through a recursive process. The establishment of one cell type in a particular area provides a basis for adjacent cells to define themselves based not only on environmental signals, but also on the information that they're next to a particular type of cell (this is how root epidermal cells decide whether to produce hairs or not). When you look at the finished product, everything seems hopelessly complicated, and the answer to every question of \"How does ",
" cell know what to do?\" seems to kick the can down the road by saying \"Well, ",
" cell told it what to do\"; but in the end, the idea is that a few environmental signals at the beginning of the development provide a starting point that allows an organism to gather more and more information that informs its further development.",
"Please ask follow-up questions if anything is still unclear! I left a lot out, and this still got rather long."
] |
[
"Answering this question is an entire field of science:",
" developmental biology.",
" A full explanation would fill an entire textbook, but if you're curious you can start reading one. I recommend ",
"this one",
" (also available on Libgen)."
] |
[
"Let's put that in context now: You can actually take pretty much any plant material and, by exposing it to two types of plant hormones called auxin and cytokinin, turn it into a \"blob of cells\" called callus. (Unlike animal cells, plant cells can go \"backwards\" in development, essentially becoming less specialized and regaining the ability to become a different type of cell.) In this state, every cell is exposed to homogeneous conditions, and in order to become a plant, the cells must \"figure out\" where they are. The most relevant information for a plant is which way is up and which way is down, so cells express proteins that look for the clearest indicators of direction: light and gravity. The detection of light causes cells to start producing auxin and to pump it from cell to cell in the opposite direction of where the light is coming from (i.e. downwards) as well as to generally start behaving like a shoot, while cells that don't perceive any light figure out which way the gravitational force is pointing and pump cytokinin in the opposite direction. Now the plant has established auxin and cytokinin gradients which can be used as information that tell cells where they are and what they should be doing. These hormones (and others) have tons of functions, but just for example, a high influx of auxin and a low influx of cytokinin into a plant cell is basically telling it, \"You're close to the top of the shoot; don't make any branches\".",
"Wow! That was so interesting! Thank you so much for your detailed answer!"
] |
[
"Do antibodies get used up when preventing an infection?"
] |
[
false
] |
Suppose I have COVID-19 neutralizing antibodies from a vaccine. I get exposed to SARS-CoV-2, but neutralizing antibodies prevent the virus from reaching my cells. Does that lower my antibody levels, leaving me more vulnerable on next exposure? Would antibody levels drop for other reasons than this? Can the distinction (if this depletion does occur) be measured?
|
[
"Yes, antibodies get used up while preventing an infection. ",
"Antibodies also get used up when ",
" preventing an infection, and the difference in rates is pretty small. ",
"Antibodies normally only have a half-life of 2-3 weeks. The reason antibodies seem to remain roughly constant is that they’re being continuously made, at roughly the same rate they turn over. They’re constantly being made by plasma cells, which are renewed by memory B cells (I am simplifying wildly here). ",
"If there’s an infection, antibodies do get used up as they bind to pathogens, which might get destroyed by white blood cells or simply blown out your nose, covered with antibodies. So the rate of destruction of antibodies during an infection increases. ",
"But B cells also respond to the presence of the pathogen by making more plasma cells and more antibodies. A single plasma cell can make thousands of antibody molecules every second, and there may be millions of plasma cells during an infection; overall, infection leads to a dramatic increase in antibody concentration even though some are being used up while attacking the pathogen."
] |
[
"It’s normal, it’s ",
", for B cells to reduce in number after an antigen is no longer present. You’re exposed to literally millions of antigens throughout your life; if your B cells never reset their number after responding, you’d just be a thick gelatinous ooze of B cells by the time you were 20. ",
"The whole “waning immunity” for COVID has been wildly exaggerated. Most people had great long-standing immunity against the virus, with little or no waning. Some people - especially the elderly and otherwise immunocompromised - did show waning, but people under around 60 with no complications showed solid, long-lasting protection. Delta to a small extent, and omicron to a great extent, changed that, but there’s really nothing unusual, or surprisingly rapid, about vaccine immunity waning for COVID."
] |
[
"I think there's some confusion here about antibodies. Antibodies are proteins that have a shelf life, just like any other circulating protein in your body. All proteins get degraded by proteasomes at some point. What's important is your cell-mediated immunity and your memory cells. These are the factories that produce the antibodies. You don't really care so much that your antibodies are used up in an immune response (that's what they're designed to do). You care that your memory cells \"remember\" the foreign antigen so that when you're re-challenged, you can mass produce more antibodies."
] |
[
"How do we know a T-Rex sight was based on movement?"
] |
[
false
] |
A very specific thing to know about a species that's been dead for millions of years.
|
[
"Super late, but it is extremely unlikely that T-rex's visual acuity was based on movement. That was complete fiction from Crighton (I still love his book, though). If it were reality, the Tyrannosaurus would not have been near the predator it is reputed to be. ",
"In reality, the Tyrannosaurus Rex probably had insanely good vision given its massive ocular cavities. Some paleontologists an paleobiologists suggest the T-Rex may have been able to spot prey and/or carrion from several ",
" away using binocular vision similar to that of a hawk, eagle, or owl. ",
"Using their modern ancestors (primarily birds of prey) scientists have estimated that T-rex's vision may be upwards of ",
" better than a human beings. ",
"The T-rex was ",
" apex predator of the late Cretaceous period."
] |
[
"Crighton didn't completely pull it out of his ass. There was a study done at the time which compared the brain of a T-Rex (made from a plaster cast of the inside of a T-Tex skull) to the brain of a frog, and found similarities in the visual cortex which they interpreted to mean that the T-Rex could only track movement, much like a modern frog will sit on top of a pile of dead insects and ignore them looking for a living, moving insect.",
"That study has since been discredited, but at the time of filming it was the latest discovery on T-Rex.",
"If Jurassic Park had been made a few years later we might have seen feathered T-Rexes."
] |
[
"It is not actually. The myth was first made in Micharl Crichton's Jurassic Park. And made known in the film adaptation. An apex predator having a massive visual flaw like that would likely result in the extinction of said species."
] |
[
"How does the body obtain Vitamin D from Sunlight?"
] |
[
false
] | null |
[
"Vitamin D3 (cholecalciferol) is produced in the skin in a ",
"photochemical reaction",
" driven by UV light. Photons in this range add to and break the C9-C10 bond in the B ring of 7-dehydrocholesterol, shaded in pink in ",
"this figure",
".* ",
"Two additional hydroxylation steps then occur",
": C25 is hydroxylated in the liver (by one of a variety of hydroxylases, the most important of which appears to be CYP2R1) and C1 is hydroxylated (principally by CYP27B1) in the kidney. The final product is 1,25-dihydroxycholecalciferol, a biologically active hormone that regulates calcium uptake in the intestine and calcium homeostasis in bone and the kidney.",
"*Image adapted from Lehninger ",
", 5th ed., Figure 10-20."
] |
[
"This is true. Mushrooms are rich in ergosterol, which can be converted to ergocalciferols (pro-vitamin D2) by UV B light. Upon consumption, these are similarly dihydroxylated to yield the active hormone (1,25-dihydroxycholecalciferol)."
] |
[
"Thank you so much! Then I’ll keep sunbathing my mushrooms!"
] |
[
"Do plant enzymes play any role/benefit for the human body? Or, because plant-enzymes are created by the plants for their own metabolic processes, their role in human digestion is nil?"
] |
[
false
] |
Do plant enzymes aid human digestion, or help us absorb more nutrients aside from the role of the enzymes our body currently produces? EDIT: I did find by Stephen Barrett from . Here are some of his key points:
|
[
"You're asking if when we eat plants, the enzymes in the plant cells help us absorb more nutrients? ",
"No, the enzymes are all denatured and digested in the harsh environment of the stomach, anyway."
] |
[
"Enzymes are proteins that speed up a reaction. When you eat a plant and it reaches your stomach gastric juices chop up the enzymes or proteins into bits called amino acids which are the building blocks of all proteins. Your body can use these to make its own proteins that it needs. This is especially important for the amino acids that the body cannot synthesize (8 out of the 22 amino acids). "
] |
[
"TL;DR Plant enzymes are not significant in normal, healthy digestive tracts. Some enzymes, however, such as papain and bromelain may aid in digestion with individuals who have abnormal digestive tract issues. Abnormal digestive tract issues can arise from taking antibiotics, proton-pump inhibitors, or other antacids. ",
" Plant enzymes indirectly affect the human body.",
" Yes, plant enzymes are broken down into amino acids, which are used to create new proteins. However, plant proteins tend to have a lower density of essential amino acids than animal proteins.",
" Specific plant enzymes directly and uniquely affect the human body.",
" The human digestive system is an ecosystem of bacteria, human cellular activity, and the nutrients that enter it.",
" The means for direct cellular interactions with larger substances are receptor Proteins.",
" ",
"Resting pH for the stomach is ~4-5, active digestion pH is ~1-2.",
" For enzymes to work, they must stay within their tolerance ranges of pH and temperature.",
" ",
"The digestive system has many regions where enzymes are employed.",
" ",
" Are some plant enzymes, or polypeptides capable of significantly interacting with bacteria, human cells, or the other nutrients while in the digestive system without being denatured, or prior to being denatured?",
" One enzyme capable of interacting with proteins to aid in digestion is lactase (produced by mammals), that significantly breaks down lactose for people who lack sufficient production of lactase.",
" Prions (produced by animals) are bad proteins that are thought to be transmitted through ingestion that cause good proteins in the host to go bad.",
" Keratin (produced by animals) is an indigestible protein.",
" ",
"Papain is a plant enzyme that breaks down animal proteins.",
" Since some enzymes survive digestion long enough to directly affect humans, it is possible that other enzymes could as well. However, these enzymes are the exception to the rule, so there is a limited likelihood that any specific enzyme would directly affect humans.",
" Enzymes are one type of catalyst. Enzymes are large, ",
"the smallest of which have ~1,000 atoms in them.",
" Keep in mind that there are many other ways for substances to interact with an organism. Other chemicals than enzymes that are found in plants are beneficial to people, such as vitamin C."
] |
[
"How are astronomers able to determine the spin of a black hole?"
] |
[
false
] |
I saw a post about a black hole 300 million l/y away spinning at 50% the speed of light, but how are they able to determine that? As far as I know we are barely able to see Sagittarius A*. Edit: Veritasium actually recently, check it out, it's a really nice video.
|
[
"First ask yourself this: How does something with no surface spin with an angular velocity measured in units of linear velocity?",
"When you realize the answer, you will understand where that number came from: <spoiler>bad reporting</spoiler>.",
"OK, but seriously, it's actually the estimated velocity of material in the accretion disk around a black hole that emitted a huge quasi-periodic pulse of X-rays. By looking at the frequencies of the periodicity, you can figure out how often the material is swirling around the black hole. Then, you use some reasonable guesses about where the material would be and how big the black hole is to figure out what valid orbits would be for a Kerr spacetime geometry.",
"The paper isn't super-readable, but you can take a look at it here: ",
"https://arxiv.org/abs/1810.10713"
] |
[
"That was very jargony, and I love it. But for the breakdown for non-astrophysics types: you can use the laws of motion (particularly angular momentum and the law of conservation thereof) to determine the speed of the disk of stuff the black hole is feeding on and reverse engineer the math to give you the rotation speed of the thing that disk is orbiting. Was their math right? No idea, as I haven’t looked into it. "
] |
[
"Also note: we can’t see Sag A* because of all the crap in the way (molecular clouds and silicate dust and such). But we can figure out its rotation speed but knowing the orbital velocities of the stars and such going around it and the mass within its gravitational influence "
] |
[
"Why wasn't ocean acidification a problem millions of years ago when the dinosaurs were experiencing extremely high levels of CO2?"
] |
[
false
] |
My friend and I were talking about the effects of CO2 and he brought up a good point that CO2 has been really high in the past, but the ecosystems didn't collapse like we think they might from the recent spike in climate change. He is pretty anti-climate change, and I usually lean toward the side of most scientists. I just wasn't able to give him a good answer for his argument. My thinking was something like the rate of change in CO2 levels is much higher than it was millions of year ago and maybe the life forms had more time to adapt back then, but I'm not sure.
|
[
"High atmospheric concentrations of CO2 that rise rapidly (like they are now) have most definitely been a problem for oceanic organisms in the geologic past. Specifically, ocean acidification caused by this phenomena is considered a contributing cause in the Permian-Triassic, the largest extinction event in Earth history, (e.g. ",
"Clarkson et al, 2015",
"), Triassic-Jurassic (e.g. ",
"Greene et al, 2012",
"), and the Cretaceous-Tertiary extinctions (e.g. ",
"Alegret et al, 2012",
"). More generally, it's been argued that ocean acidification is one of the most potent mechanisms for causing mass extinctions in the oceans (e.g. ",
"Veron, 2008",
"). Another contributor with more of a background in ocean chemistry, like say, ",
"/u/Chlorophilia",
", might be able to provide more details."
] |
[
"Records of past climate change are very useful for understanding how the climate system works (and interactions with the biosphere). However, caution is needed when using past climates as analogues for today and this question highlights why. To cut a long story short, ocean chemistry was very different during the Mesozoic which meant that prolific carbonate production was possible despite the high CO2 concentrations. ",
"It is completely possible to have productive carbonate systems in a high-CO2 world. Carbonate systems were productive throughout much of the Mesozoic where CO2 concentrations were significantly higher than today, e.g. ",
"Pomar & Hallock (2008)",
" as a good review. ",
"The vulnerability of marine calcifiers to dissolution (i.e. the primary reason why we're concerned about ocean acidification) is often thought about in the context of pH, or the acidity of the ocean, which is directly related to the atmospheric CO2 concentration. However, in reality, the critical parameter is ",
" pH but rather than carbonate ",
", which is effectively a measure of how stable carbonate (specifically calcite and aragonite in the case of most marine calcifiers) is in seawater. If the saturation state is below 1, carbonate is unstable and will dissolve. Conversely, if the saturation state is above 1, carbonate is physically stable - but most marine calcifiers require a carbonate saturation state that is ",
"substantially higher",
".",
"The carbonate saturation state is approximately proportional to the product [Ca",
"][CO3",
"], i.e. the concentrations of calcium and carbonate ions in seawater. [CO3",
"] is ",
"strongly dependent on pH",
" - as the ocean pH drops, the concentration of carbonate ions also drops and therefore so does the carbonate saturation state. This is the mechanism by which ocean acidification damages carbonate systems. ",
"During the Mesozoic, due to the higher atmospheric CO2 concentration and the resulting lower ocean pH, [CO2",
"] was also lower. ",
", [Ca",
"] was significantly higher. The reasons for this are complex and not fully understood but is generally thought to be due to changes in rates of oceanic crust production, e.g. ",
"Hardie (1996)",
". However, the fact that [Ca",
"] was higher is well constrained and the result is that, despite the lower ocean pH, the background carbonate saturation state during the Mesozoic was actually similar to today, e.g. ",
"Tyrell and Zeebe (2004)",
". So higher CO2 was not an impediment to carbonate production during the Mesozoic, and in fact may have even contributed to their productivity, e.g. ",
"Iglesias-Rodriguez et al. (2008)",
".",
"So there is no contradiction to productive carbonate systems during times of high CO2 per se, ",
". The issue is when you rapidly change change one of the levers of the carbonate saturation state (which in practice means ocean acidification, because [Ca",
"] generally only changes over millions of years). As ",
"/u/CrustalTrudger",
" explained, there are many cases in the past where rapid injections of CO2 (where the sudden drop in [CO3",
"] overwhelms the higher background [Ca",
"]) results in a significant drop in the carbonate saturation state and hence catastrophic consequences for marine calcifiers. A similar thing is occurring today, except even more rapidly than any geological analogue we are aware of."
] |
[
"Also particularly relevant for today is the Paleocene-Eocene Thermal Maximum. Just as mammals were gaining a strong foothold 55 million years ago, the planet saw a very sudden, very strong atmospheric carbon spike in less than 10k years, quickly followed by a +7 degree temperature excursion. (Coincidentally, that's the same change in global temperature between the last glacial period and now). ",
"There is reasonable evidence that oceans did see acidification back then, as well as very strong evidence of widespread ecosystem disruption. From the intro of ",
"Penman, et al, 2014",
":",
"Planktic foraminifers show transient population changes and ecosystem disruption over the PETM, including the temporary disappearance of species abundant before the event, and the evolution of “excursion taxa” [Kelly et al., 1996; Raffi and De Bernardi, 2008; Raffi et al., 2009]. In addition, changes in shell morphology may reflect rapid acidification during the CIE [carbon isotope excursion] followed by elevated saturation during the “overshoot” phase of the recovery [Kelly et al., 1996, 2010]."
] |
[
"If Energy cannot be created, and the Universe IS expanding, will the energy eventually become so dispersed enough that it is essentially useless?"
] |
[
false
] |
I've read about conservation of energy, and the laws of thermodynamics, and it raises the question for me that if the universe really is expanding and energy cannot be created, will the energy eventually be dispersed enough to be useless?
|
[
"Probably. In an infinitely expanding universe (which we think we have), you will eventually reach a point of maximum entropy where the energy is evenly distributed, and essentially nothing happens. This is called the \"heat death\" of the universe, and would occur in more than 10",
" (10 to the power of 100, for you mobile users) years (the biggest black holes are expected to last at least this long, and we wouldn't reach maximum entropy until they're all gone) ",
"However, we don't necessarily know what will happen. We could be wrong about the infinite expansion of the universe, we're not accounting for the effects of other possible universes outside of our universe acting on ours, not accounting for the ability of sentient species to figure out reversal of entropy & whatnot (10",
" years is a long time to figure something like that out), etc. ",
"Source: mostly what I've read in the past, supplemented by wikipedia (see ",
"http://en.wikipedia.org/wiki/Heat_death_of_the_universe",
" and the various pages it links to)",
"edit: added (10 to the power of 100) to clear up any confusion about 10",
" showing up wrong for mobile users. "
] |
[
"I'm glad you pointed out that this assumes our current physical models are in fact perfectly correct. As a layman, who is simply interested in this, there would appear to be a lot of unknowns predicting this far out, so we really don't know. "
] |
[
"My physics professor said that most laypeople see the field as unknowable predictions of irrelevant things to an unmeasurable degree of certainty. ",
"Of course, it isn't true. Predicting the heat death of the Universe is merely an interesting extrapolation of current models being tested. Theoretical physics will only become more and more relevant to our daily lives as time marches on."
] |
[
"Why isn't spiciness a basic taste?"
] |
[
false
] |
Per this and the guy explaining about wine and food pairing, spiciness is apparently not a basic taste but something called "umami" is. How did these come about?
|
[
"It qualifies as a sensation",
". As for why it's not a basic taste -- I don't know. That's not my department."
] |
[
"Are you saying that the criteria for being a \"taste\" is that it activates taste receptors alone",
"No, that's not what I'm saying. Many \"tastes\" are not basic tastes, rather, they are additional stimuli (smell, pain receptors, carbonic acid, etc...) that change the ",
" of the basic tastes. They alter, on some level, the subjective perception of them. In the case of spiciness, though, yes, it's not classified as a taste in and of itself. It's a reaction between capsaicin and the trigeminal nerve. From that point it changes ",
" of the tastes. ",
"Important, what is classified as the ",
"basic tastes",
" are really only terms used by tasting experts. It is hard to distinguish, properly, the differences between some of these except in the most extreme of cases (e.g., quinine for bitter, citric acid for sour). For example, the terms sour, acidic, and bitter are used ",
"incorrectly",
" and often interchangeably in a number of cultures. However, these basic tastes are fairly established as how to perceive the taste of items. This is often why in many circles people are asked to use other words (e.g., Earthy, chocolately, burnt) so that an analog can be drawn between what people ",
" and what basic tastes are ",
".",
"There are defined criteria of what things called ",
"basic tastes",
", but interaction between items that stimulate the perception of these things, as well as additional items, change subjective taste. For example, Pepsi and Coke and other colas have a nearly disgusting level of sugar. Most people find flat colas to be unpleasant because the amount of sweetness in these are on the high end of a U-shaped curve. The reason we don't find them disgustingly unpleasant (in most cases) is because carbonic acid from CO2 release tricks how we perceive the sugar. "
] |
[
"Warning: Threadjack ",
"How on earth isn't fat a basic taste? I can clearly taste the difference"
] |
[
"We know its possible for pets to become bored, but what about depressed?"
] |
[
false
] |
[deleted]
|
[
"Can you take her to the vet to see if she just does not feel good?"
] |
[
"She's become sedentary, doesn't listen, and often hangs out under tables and away from people.",
"Take her to the vet, those might be signs that she is in pain or has some sort of medical problem."
] |
[
"Hm. I would hope she hasn't been hurt. She isn't limping or showing any physical characteristics of pain, but I'll take her anyways. Thank you!"
] |
[
"Why are you not allowed to cross your legs when getting your blood pressure taken?"
] |
[
false
] |
How does it skew the reading and why?
|
[
"The mechanical compression from crossing your legs moves the blood from your leg veins toward your chest. This provides more blood to be pumped from your heart, resulting in an elevated blood pressure reading.",
"For the physiology-inclined –",
"The pooling of the blood towards the chest results in an increase in stroke volume (SV), which translates to a rise in cardiac output (CO). This rise in cardiac output results in the rise of blood pressure (BP). ",
"Cardiac output (CO) = stroke volume (SV) x heart rate (HR)",
"Blood Pressure (BP) = cardiac output (CO) x total peripheral resistance (TPR) ",
"Reference: ",
"https://www.ncbi.nlm.nih.gov/pubmed/18300852"
] |
[
"So hypothetically, if someone is losing blood pressure rapidly you could save their lives by crossing their legs?"
] |
[
"Crossing legs theoretically would increase venous return to the heart, providing more blood to critical organs in the chest and head. But there are so many variables needing to be considered and addressed as to why blood pressure is falling rapidly - trauma? medication-induced? etc. As to whether this could hypothetically save lives in the real world - unfortunately I cannot answer (could be medical advice)..."
] |
[
"How does a nuclear reactor increase power output without crossing into prompt criticality?"
] |
[
false
] |
I think my question stems from a misunderstanding of the balancing act nuclear reactors play. My understanding is that prompt neutrons comprise of almost all neutrons produced in a reactor (>99%), while delayed neutrons provide the rest (<1%). Due to how quickly prompt neutrons are created (on the order of a few nano seconds), reaching prompt criticality results in a dangerous power excursion too fast to control for. Therefore, we keep the reactor just under prompt criticality and have the delayed neutrons provide the rest to reach criticality and therefore a sustained chain reaction (prompt sub-critical and delayed critical). So, let's say a reactor is currently in this balancing act and has reached a sustained/controlled reaction and is producing 1000MW. The electricity demand has gone up and now the power plant wants to increase power to 1500MW. How is this possible without tipping over the already very fine balancing act into prompt criticality? Put another way, how can a reactor have a dynamic power output when increasing or decreasing power by a significant amount means either a power excursion or fizzling out?
|
[
"Between subcriticality and prompt criticality, there's another option called ",
", where the prompt neutrons alone are not enough to sustain the chain reaction, but the combined prompt and delayed neutrons are.",
"Since the delayed neutrons are emitted over a much longer timescale than the prompt neutrons, the average timescale of the prompt and delayed neutrons is long. Therefore, a delayed-supercritical, prompt-subcritical reactor will experience exponential growth in the neutron population, but over an easily controllable timescale.",
"You can use that to slowly increase the power from 1 GW to 1.5 GW, and then bring the system back to criticality to return to steady-state. You just need to be able to very precisely control the reactivity, and stay within the range between delayed critical and prompt critical."
] |
[
"The margin between overall criticality (Keff for all neutrons = 1) and prompt criticality (Keff for only prompt neutrons = 1) is small from a overall sense, but in the margins that nuclear reactors operate on that difference is relatively large compared to the magnitude of adjustments made by other reactor control methods (rod movement and water temperature). The difference between criticality and prompt criticality is roughly 75 units of reactivity (δk) which would correspond to at least 6inches of instantaneous rod withdrawal or instantaneous temperature excursion of at least 30F, which reactor safety systems would be built to protect against and emergency shutdown if such conditions became possible. If the system is self-stabilizing any change in reactor power will also cause a change in plant conditions that adds reactivity in the opposite direction (power goes up, water heats up, expands and acts as a worse moderator, lowering power), preventing power runaway upwards or downwards. I can't speak for all reactor designs, but for pressurized light water reactors this should hold true."
] |
[
"This is the bit I'm confused with. Reading your paragraph, it sounds like the criticality itself (the balancing point between an exponential increase or decrease in power) can be increased? I don't understand how, with a set arrangement of fuel rods, there can be an adjustable criticality. To my knowledge, no fuel is moved or added in a reactor during operation. Everything else that is moved (e.g. control rods and water flow) are just to control the neutron population and maintain the balance.",
"Reactors have a number of ways to adjust the criticality of a reactor. The one you've probably heard of are control rods. When you move a control rod further into or farther out of the reactor you change the criticality of the reactor. Additionally not all control rods are equal. Some control rods are designed to add fine control of these reactor. For example, instead of being made out of a strong absorber, the made be made out of a passive material that is invisible to neutrons. So motions of this control rod will displace a small amount of water, which has a small impact on the criticality enabling fine control.",
"Besides control rods, modern reactor also control the reactivity by adding absorbing mater into the water flowing through the reactor. By controlling the concentration of the absorbing material, the operators can make small adjustments to the criticality of the reactor.",
"The only resolution I can think of is if the neutron population required to maintain 1.5GW is the same as 1GW. Therefore, you're just increasing the flux temporarily (delayed supercritical) until you get a higher core temperature, and then you bring it back to the same criticality/balance, with the same neutron population as before.",
"This is not correct. The criticality is not a measure of the neutron flux, but instead it measures the rate of change of the neutron flux. ",
"The thing that I think you're missing is that the critically only weekly depends on the neutron flux. Instead, the critically is largely determined by the geometry of the reactor, and the material composition of the different components. For example, when you move control rods you are changing the geometry. When you add boron to the water, you are changing the material composition."
] |
[
"How does 911 call your local police station?"
] |
[
false
] |
Since all police stations in the U.S.A. have the same number, how does it always direct you to your local station?
|
[
"911 calls on your PSTN provider are routed to a \"public safety answering point\", a call center responsible for routing calls to police, firefighting, ambulance services. The way PSAPs are set up differs from country to country/state, but they are generally statically set up depending on location & capability and do not frequently change. Whereas for your regular phone calls routing happens based on the destination address, for emergency services routing happens based on your source address. Since PSTN does not transmit address information, the lookup is done via a so-called Automatic Location Identification (ALI) or Automatic Number Identification (ANI) databases. For every new account created, the customer's given address is stored in the Master Street Address Guide, which they use. The interconnection happens between the PSAPs and switching office through at least two dedicated, redundant, at least ",
"DS0",
" capable trunks (E0 in Europe).",
"The process for VoIP and mobile calls is similar, but with some differences:",
"here",
"For those that are interested in public safety services in mobile telecoms, I would suggest the excellent and surprisingly complete book ",
"LTE for Public Safety",
". It's surprisingly up-to-date for a 2015 book, it even covers the recent transitions to i3 in the US."
] |
[
"They also often get ANI/Ali data and approximate location from the cell phone carrier. (I help design PSAPS, Public Safety Answering Points) "
] |
[
"They also often get ANI/Ali data and approximate location from the cell phone carrier. (I help design PSAPS, Public Safety Answering Points) "
] |
[
"Why do some places only have 2 tidal changes per day as opposed to 4?"
] |
[
false
] |
A friend of mine stays in thailand at the moment, and told me, that high and low tide only occured once per day. I was very doubtful and suspected false observation (tidal change during night time or something like that), since i know from holidays at the sea that two high tides appear each day. But she was insisting her observation is correct, so I checked and apperently she is right: Koh Chang, one high tide per day: Kiel, Germany (a place I´m familiar with): 2 high tides a day: Can someone explain this difference to me?
|
[
"Tides are a dynamic phenomenon. Imagine a big, wide bowl partially filled with water. Now imagine tipping the bowl slightly back and forth (meaning: tip it very slightly to the right, then tip it very slightly to the left, and so on). This is analogous to tidal forces, you're changing where the \"flat\" level of the water is in relation to the bowl. You can think of the high and low spots on each side where the water level ends up as being \"tides\". If you move the bowl very slowly, the water will stay fairly flat all the time, and the tides will be very simple and predictable if you tilt the bowl the same amount every time. But once you start to tilt the bowl faster then the water starts to slosh. If you had a complicated bowl shape, this would add further complications.",
"This is basically what happens with Earth's tides. The tidal forces from the Sun and Moon drive the tides, but they do not entirely dictate them. How the water \"sloshes\" and interacts with the local coastline effects how high and low the tides will reach."
] |
[
"Tides are complicated. ",
"Here is a map of the 12 hour cycle",
". White lines are places of equal time of high tides. The naive \"two bulges move around the globe\" would be a parallel set of vertical lines. As you can see that is not even a good approximation. Some places have their high tide when the Moon is at the horizon, some places don't have this 12 hour component at all.",
"In addition to the 12 hour component there is also a 24 hour component, and a few others. In places with a weak 12 hour component the others can be important. Ko Chang is in such a place, and in general it is in a place with complicated tide patterns."
] |
[
"For another perspective: If the Earth had no continents, just a water planet with uniform depth, the variations in surface elevation due to tides would be a matter of inches. What the continents do is to make it ",
" in complex patterns.",
"Another way to look at it: Ever do your own oil changes without a lift? You put a pan under the engine, drain the oil, then have to pull it out from under the car. First time you do it, you'll probably pull it too fast and get oil all over the driveway!"
] |
[
"If visible light is on the same spectrum as radio and other waves, what causes things to be \"bright\", and can radio waves be \"bright\"?"
] |
[
false
] | null |
[
"Like any sine wave, two characteristics can be measured from any light source- the Wavelength and Amplitude.",
"Wavelength determines where on the EM spectrum the radiation lies- from infrared to visible light to x- and gamma rays. On the visible spectrum, this value determines color. On a wave, this represents the distance between peaks.",
"Amplitude is the height of each peak of our wave, and that determines brightness for the visible spectrum. Other types of light could have a high amplitude, and it would be OK to describe them as bright, but we couldn't see the light with our eyes. ",
"This amplitude is actually a measurement of how many individual photons are being received, as each photon has a fixed amplitude. The photons in bright light aren't more energetic than the ones in dim light, there are just more of them."
] |
[
"That makes so much sense. It's difficult (for me) to instinctively think of radio waves, x-rays, gamma rays as fundamentally the same as visible light, even though I know it intellectually, but your explanation really helped it to click!",
"I also never knew that amplitude of a wave was based on number of photons.",
"You expanded my mind today - thank you!"
] |
[
"Well, not quite. The amplitude of the wave is based on the number of photons and energy of the photons (wavelength). ",
"The intensity (power per area) of light can be found from the electric field strength (amplitude) by I = cnεE",
" /2 . The important part being the E",
" dependency, or the amplitude dependency. The speed of light, the refractive index, and vacuum permittivity just being factors. ",
"So if you had a fixed wavelength, more amplitude would indeed be more photons. But if you had say half the wavelength (twice the energy per photon) you still have sqrt(2) more amplitude with the same number of photons. More energy per photon means more intensity (energy/time/area) which means a stronger electric field (more amplitude) without increasing the number of photons. So for example, if you had radio waves of a set intensity/amplitude, to get the same amplitude with visible light you'd need a hell of a lot less photons. A 1000 W microwave would emit a lot more photons than a 1000 W light bulb, even if the intensity and therefore amplitude of both is the same. "
] |
[
"Why can a blue whale only dive 350ft, but a sperm whale can dive 8,200ft?"
] |
[
false
] | null |
[
"shamelessly stolen from wikipedia:",
"The sperm whale has adapted to cope with drastic pressure changes when diving. The flexible ribcage allows lung collapse, reducing nitrogen intake, and metabolism can decrease to conserve oxygen. Myoglobin, which stores oxygen in muscle tissue, is much more abundant than in terrestrial animals.",
"As to why they evolved as such, I guess it's because they feed on squid rather than krill, why tend to hang out further from the surface."
] |
[
"Link to post - Sperm Whale"
] |
[
"Probably evolution and habitus. Blue whales are suspension feeders; they feed on mostly surface dwelling zooplankton and the fish that feed on those. These will tend, as plankton do, to dwell nearer the surface. Thus, blue whales tend to inhabit the upper levels of the water column, and selective events have not had much to work on to counteract this habitus. Humans seem to be a selective pressure on size, such that smaller blue whales may be the norm as they breach adulthood, but that's an additional effect.",
"The short of it is: selection favors surface localized suspension feeding. Absence of selection pressure to dive this favors adaptations to stay near the surface. The opposite is true for sperm whales."
] |
[
"A question about the uncertainty principle?"
] |
[
false
] | null |
[
"Yes."
] |
[
"So that's why we say quantum objects Don't have specific position and momentum? the uncertainty comes from physically possible state not the individual particles?"
] |
[
"Correct."
] |
[
"What is a good explanation for the outcome of the following ball and ramp experiment? (assume no friction)"
] |
[
false
] |
My physics class is having a serious debate about the Galileo ball experiment. This is the experiment where there are two ramps with the same horizontal length but not the same length from start to finish because ramp B has a dip in it. Two identical balls are dropped from both ramps and the ball released from the dipped ramp ends up reaching the end of the ramp in less time. However, my classmates are angry because they do not believe that this is so. They believe that in a friction less world, the ball from ramp A and the ball from ramp B would reach the end at the same time. Ian, the main opposition to the idea that the dipped ramp is faster, says that if the dip is parabolic then maybe B would reach the end faster, however, if the dip has a stretch of flat surface then ramp B is slower. He says the shape of the ramp is fundamental to the experiment. Any explanations? Here is an example of the experiment's set up. But he is still not satisfied. Again, he says that the shape of the dip is fundamental in the answer. Can you please explain?
|
[
"I propose an alternative thought experiment to illustrate the solution:",
"You take your two ramps. One has a dip,like in your link, but does not actually come up again.",
"Now both balls start at a certain speed. When they reach the dip, both have the same X-position and the same speed. The dip, accelerates the ball on \"dip-ramp\" while it goes down, so once both ramps are straight again, the \"lower\" ball is faster than the upper ball and would be ahead of the upper ball.",
"In this case, the dip wins.",
"Now let's add the little ramp back up again. At the base of the the ramp, the lower ball is still faster than the upper ball and has gained quite some head start (depending on the length of the straight part) and is decelerated while it goes up. At the top, it has reached the same speed as the former \"upper ball\" but nothing has changed with respect to the fact that it is still ahead. ",
"Now the \"dip\" ball is still ahead and at the same speed. No way it cannot win...."
] |
[
"Am I right in thinking that in a frictionless world the balls wouldn't actually roll, they'd just slide? Because with rolling balls, rotational momentum is an important part of the system, so neglecting friction isn't just a simplifying assumption, I think it dramatically affects the behaviour of the system."
] |
[
"wow, thank you, this makes sense!"
] |
[
"Is there a reason to why the sugar is less sweet in the past 5 years?"
] |
[
false
] |
I don't know if this is a local thing, but I've asked to many people around here and they have the same perception: the sugar is less sweet. It takes more sugar to sweet a coffee, for example.
|
[
"It was ",
"correct",
" the first time."
] |
[
"Well maybe you are all just eating more sweet stuff, so you are used to it.",
"Since sugar is afaIk Saccharose, there isn't much you can change about it"
] |
[
"You mean sucrose. "
] |
[
"Coworker asked me about the legitimacy this DNA study on Homo sapiens cognatus \"bigfoot\". Article says 3 \"bigfoot\" genmoes sequences in a 5-year study"
] |
[
false
] |
The article piqued my interest but i have no familiarity with the website or the journals through which this article was published. i read over the article but have no strong background with bioinformatics and have been out of the sciences for a few years now. Anyone have any background or followed this type of study?
|
[
"The first thing I did was click through to the paper at denovajournal.com. Pretty flashy site for a scientific journal. Not much there except for the 'bigfoot' study. So then I googled \"DeNovo Scientific Publishing.com\", the parent org, supposedly. Again, not much there except for this one 'study'.",
"I leave you to draw your own conclusions."
] |
[
"You don't need a scientist to tell you the website for the journal is fishy. They have only one paper. They appear to have created the whole thing just to publish this one paper."
] |
[
"I don't have a background with bioinformatics, but as an anthropologist I think Bigfoot can be comfortably dismissed as non-existent. As such, I'd suggest these findings (that the hair is not human or animal) are much more probably incorrect or have misidentified some other sort of fibrous material. "
] |
[
"Why we can make plasma with a microwave if it uses non-ionizing frequencies?"
] |
[
false
] |
I've seen people putting a grape cut in half in a microwave and making plasma that i know is ionized gas, how could be possible if microwave uses 2.4GHz frquencies that are non-ionizing?
|
[
"Ionizing radiation is not necessary to generate a plasma. For example, you can use ",
"electron cyclotron resonance",
".",
"Even at relatively low temperatures, a gas will have some small degree of ionization, and some free electrons around. If you subject the gas to a static magnetic field, and shoot microwaves of the right frequency at it, the free electrons in the gas will be resonantly excited, heating the gas, and producing more ionization.",
"So non-ionizing radiation is dumping energy into free electrons in a resonant way, and those electrons are knocking more electrons out of nearby atoms."
] |
[
"Ions in a microwave or in a gas discharge lamp are not made from photon-electron collisions. You are correct that the photons required to do so would be in the x-ray range. Instead plasma is made by accelerating naturally occurring ions or electrons in an electric field, giving them enough kinetic energy to ionize other molecules. Think of it like a cascade where one electron gains enough energy to create a second free electron. Two electrons become four, and so on."
] |
[
"Thank you, so those free electrons could hit other atoms, also not of the same matter they come from, and ionize other things?\nIf that's true it could mean that every type of radiation, including light, in presence of air is not likey to but could cause cancer.\n(To everyone reading that: please don't take my assumptions as true, i'm only asking)"
] |
[
"Why do we get sick so fast after eating food, if it takes a couple hours to go through the digestive system?"
] |
[
false
] |
I remember seeing this question, but I am unable to find it.
|
[
"What do you mean by fast? It takes at least two hours for something like bacteria to get to a place where it can cause food poisoning symptoms. Although if it was a bacteria that releases a toxin (like S. aureus), the toxin may already be in the food or may be released earlier in digestion. This could cause a faster response.",
"To piggyback on your question, something I've never understood about myself - I have irritable bowel syndrome (nothing to do with pathogens) which sometimes acts up less than 30 minutes after eating. Can anyone explain this?"
] |
[
"Think about the scale of things. You can see the food that you're ingesting, meaning your body has lots of work to do in order for that food to be able to processed. First, the saliva breaks down carbohydrates. Food gets pushed into the stomach where acid breaks down proteins into component parts, as well as mechanical grinding offered by the stomach itself. This right here might take 5 hours in and of itself, depending on how heavy your meal was. Anyways, down into your small intestines the food stuffs goes where bile salts neutralize the acid from your stomach and further digests, especially fats; this is where they're emulsified, or put into small water soluble droplets so it can be absorbed by the intestines. In order for nutrients to be absorbed by the small intestines, it must be finely ground and processed into fractional pieces. Otherwise, the finger-like projections called villi will miss out on the feeding frenzy.",
"All this just to break down macromolecules. A bacterium is already small enough and can actually bypass every single one of these steps (if it can make it that far. Most bacteria that you ingest don't survive or are neutralized by the innate immune system, which is a whole other topic of discussion). ",
"I hope this answers your question. Feel free to post follow-ups",
"Edit: tl;dr - bacteria and other pathogens are much smaller in size and can travel through the entire digestive process much quicker. "
] |
[
"So can I have a TL;DR version of the answer?"
] |
[
"What is the effect of X-Ray radiation on gold?"
] |
[
false
] |
Hi I was wondering what is the effect of high energy ionizing radiation on gold. Does something 1000 grey of X-Ray irradiation cause gold to become radioactive? and if so what is the half life etc and what are the implications for health
|
[
"It depends on the energy of the x rays. If they are below a few MeV then there will be no activation of the gold: it will just heat up slightly. Above a threshold (don't have exact number on me, maybe 8 MeV) the x rays can start knocking neutrons off some of the nuclei, making them into radioactive isotopes. The neutrons can be captured by other nuclei, making them radioactive as well.",
"It would take quite a lot of radiation to make this significant. Even in radiotherapy linacs (which have this issue with the lead shielding) it is not a big deal. Just wait a few hours and most of the atoms have decayed."
] |
[
"the x rays can start knocking neutrons off some of the nuclei",
"If their energy is that high wouldn't they be gamma rays instead?"
] |
[
"The definition of x-rays vs gamma rays varies from field to field. In medical physics and nuclear physics, there is no separation between the two based on energy. Instead, it depends on where they came from. Gamma rays come from nuclear decay or annihilation, and everything else is x rays. So just two different names for the same thing."
] |
[
"What equations are used to demonstrate that an object moving at >c is also moving \"backwards\" in time?"
] |
[
false
] | null |
[
"The short answer is that you use the Lorentz transformations, which tell you how to translate from one frame of reference to another.",
"The precise statement that emerges is this: If you have an object that travels faster than the speed of light in one reference frame, there exist other reference frames -- which, relative to the first reference frame, are traveling ",
" than the speed of light -- in which that object will be traveling backwards in time.",
"It is critical that the two reference frames are not moving at a speed greater than ",
" relative to each other, as this ensures that the Lorentz transformations are completely well-defined mathematically."
] |
[
"I believe it's from when you do a ",
"Lorentz transformation",
" you get a imaginary Lorentz factor (ɣ) if velocity v > c.",
"ɣ = 1/sqr(1-( v^2 / c^2 ))\n"
] |
[
"Thank you for your reply! Looks like I'll be studying up on Lorentz transformations."
] |
[
"How is it that different breeds of Canis lupus familiaris (domesticated dog) can develop to be so different in specific individual populations around the world, yet not be subject to speciation?"
] |
[
false
] |
Distinct breeds exist that vary in size, shape, and other physical qualities greatly, and have evolved that way in specific populations across the globe. Does the way humans selectively bred the species have something to do with it?
|
[
"Dogs have likely only existed as a separate subspecies of Canis lupus for ~ 40,000 yrs at max, which is too short a time for speciation to occur from wolves, much less different breeds of dog, even with geographical or other isolation. Most dogs are not \"purebreds\", anyway, so there is little chance for genetic drift. ",
"Additionally, most breeds the way we think of a dog breed are relatively recent (the last 50 to 200yrs). Breeds are 100% artificial categories, they are not subject to natural selection and the premises of natural selection do not apply. Dogs have the potential for huge amount of phenotypic variation and few selective pressures which accounts for the wide array of behavior and appearances. The end result is that genetically similar individuals could end up looking hugely different. "
] |
[
"It doesn't take much genetic difference to lead to dramatically different appearances, traits, and behaviors. This is part of the power of the genetic makeup of modern animals, it's a kind of \"meta-trait\" of modern organisms. Animals that didn't have the arrangements of genes and physiology which allowed for rapid adaptability were seemingly at a competitive disadvantage in the wild. There are still some examples of such creatures but modern mammals seem to be more successful overall.",
"Dogs are far from the only example. Domesticated horses have increased in size dramatically over the first versions, over the course of only a few thousand years. Domesticated farm animals as well, from goats to sheep to pigs to chickens to cows, etc.",
"To switch kingdoms entirely, there is also a lot of variability within otherwise singular plant species. For example, the common winter squash cucurbita pepo is made up of different cultivars including acorn squash, crookneck squash, scallop squash, zucchini, and pumpkin. Similarly, cabbage, collard greens, kale, brussels sprouts, broccoli, cauliflower, et al are all different cultivars of the same species: brassica oleracea.",
"Selective breeding is absolutely a big part of it, that vastly accelerates the process of selection compared to simple environmental pressure. Especially since in the absence of humans maintaining separate breeds intentionally they would generally just all mix together into \"mutts\" and lose most of their distinctive traits (like extreme size and coat differences)."
] |
[
"Because the variation is overwhelmingly accounted for by changes in allele frequencies, not by novel mutations. While it's novel mutations, and more specifically incompatible mutations that cause species divergence.",
"This might be confusing because usual explanations of the way evolution works are more or less only applicable to viruses and simplest bacteria, and even then not always (horizontal transfer etc).",
"We've got sexual reproduction precisely because it allows for lots of orders of magnitude faster adaptation that could possibly be provided by mutations. If you have literally trillions of viral RNA copies reproducing in your body every minute, then yeah, random mutations are a force to be reckoned with.",
"If you have a couple of hundreds of foxes bred for friendliness and successfully get ",
"friendly foxes",
" after mere tens of generations, that can't possibly be caused by mutations, the necessary rate of mutation would've totally destroyed the rest of the genome.",
"It's caused by higher animals having evolved to be very good at evolving, with a bunch of levels there. Not just varying allele frequencies can meaningfully contribute to the \"friendliness\" trait in every generation, smoothly increasing it, but it's probably a specific mechanism regulating neoteny in a wide range that allows for that, due to canines' evolutionary history being full of times when there was enough food for the pack to grow to large numbers (and it was good to be able to not destroy themselves), and then other times where they had to make do with small packs.",
"The Central Dogma of molecular biology is sort of reversed when we are talking about evolution: first, better phenotypes are selected by virtue of having better alleles, then new alleles are created by crossing-over, then novel mutations help with producing the same phenotype more efficiently, all on increasing time scales."
] |
[
"Is there any truth to native americans being less tolrant to alcohol?"
] |
[
false
] |
Stemming from a discussion in , I always took it for granted that natives were less tolerant to alcohol and a smaller amount had a much greater effect on them than say caucasians. Admittedly my workding was a little rough and people took it as a racist comment. So, is there any truth to this ethnic low tolerance?
|
[
"Yes, it's related to delayed breakdown of acetaldehyde caused by a genetic abnormality, however it is not ",
" by race; only more common in those of certain heritages. ",
"http://journals.psychiatryonline.org/article.aspx?Volume=160&page=41&journalID=13"
] |
[
"Our acknowledgment of race is certainly a social construction.",
"However, the reason that people of similar heritage look similar, or have similar predispositions to diseases, etc. is because they come from, what is known as, the same \"breeding population\"- meaning population who live in the same geographic area and mate amongst themselves over generations.",
"So, for instance, people of a particular Native American heritage tend to be the products of a small \"breeding population\" and this perpetuates certain genes, including that which causes them trouble in metabolizing alcohol.",
"I am but a mere social scientist, so my explanation may not be up to ",
"r/askscience",
" standards, but that's basically why, even though race is a social construct, people of the same \"racial\" background have similar genes.",
"Edited for clarity."
] |
[
"genetic abnormality",
"Sounds weird, it's a really common allele. 'Genetic abnormality' sounds way more ... large and less frequent. Like chromosomes fusing or something."
] |
[
"A scientific explanation for the enjoyment of unsavory pornography?"
] |
[
false
] | null |
[
"I can't delve into this too much right now, but just as a supplement to what others may give you as an answer: there is an interesting section regarding this in the book ",
"The Brain that Changes Itself",
". This is an awesome book in general if you are interested in the neuroplastic nature of the brain explained in laymans terms. "
] |
[
"I was under the impression that yourbrainonporn had been discredited as pseudo-science nonsense..."
] |
[
"Not an answer, but a clarification which I think is worthwhile to point out - These are very common kinks, i.e. fetishes for specific things, which are often expressed in a few different ways. But they're very generic, hit up FetLife and most everybody shares these core tastes. So rather than frame the question in presuming a raw attraction to the \"unsavory\", it may more so be a matter of \"why do I/we have these specific fetishes?\"",
"It might be worth perusing the wiki articles on ",
"sexual fetishes",
" and ",
"BDSM psychology",
" - the TL;DR seems to be that it's still a matter needing much investigation."
] |
[
"Why does the pH scale have 7 as neutral instead of 0?"
] |
[
false
] |
I understand that pH is determined using the negative logarithm, but wouldn't it be much easier if they stuck a "-7" at the end of the formula?
|
[
"Neutral in water is pH 7 is only \"neutral\" because there is an equal concentration of hydroxide ions. pH is really a shorthand for the ",
" or ",
"solvent-H",
" protonated cations",
". The point of pH 7 being \"neutral\" is dependent on the fact that the ",
" medium is water. In water, the concentration of protons (H",
" ), or rather ",
"hydronium ions",
" (H3O",
" ) and hydroxide ions (OH",
" ) is dependent on the temperature, hence the precise \"neutral\" pH is dependent on temperature and pressure, as ",
"the Kw of water depends on these",
". You can see from ",
"this graph of water's Kw as a function of temperature",
" that the \"neutral pH\" of water is 7 only at 25 °C. Here's the ",
"temperature dependence of pH for pure water",
":",
"Kw = [OH-] × [H3O+] = (mol H3O+ / L H2O ) × (mol OH- / L H2O)\n",
"Note that none of these are more or less acidic than any of the others, as all of them have ",
". It just describes the ",
" of hydronium ions.",
"Additionally, ",
"pH can be measured for non-aqueous systems",
", however the values are not directly comparable. But it would not make sense to subtract 7 from other pH scales, as the value is not universal for other solvents, nor for water, if you take temperature into account."
] |
[
"One small picky thing... remember, at room temperature ",
"Kw = [OH",
" ][H3O",
" ] = 1.0x10",
"and ",
"pH = -log([H3O",
" ]) ",
"Since the two concentrations are equal at neutral pH, ",
"[OH",
" ] = [H3O",
" ] = 1.0x10",
"and ",
"pH = -log(1.0x10",
" = 7.0",
"I agree with most of what you said, though your table shows the Kw values, not hydronium ion concentrations as you have labeled. "
] |
[
"pH is the negative log of the hydronium ion concentration (H+). Pure water happens to have a hydronium (and hydroxide) ion concentration of 1*10",
" moles/liter, which is really interesting... You may ask, how can PURE water (H20) have any ions in it???",
"The answer is cool: water is so polar, it actually splits itself apart. ",
"pH is instituted on a log scale for simplicity, but it's important to remember that the difference of 1 pH unit is a ten fold increase in hydronium ion concentration. "
] |
[
"Black hole question - What is a naked singularity?"
] |
[
false
] |
What is a naked singularity? I’ve heard it’s something like a black hole that’s evaporated completely from Hawking radiation, this is obviously trillions of years in the future, but where the only thing left is the singularity in the middle.? Is this right? And can you explain a little more?
|
[
"A naked singularity is generally a singularity that is not hidden behind an event horizon.",
"A black hole, on the other hand, is region of spacetime in which there ",
" a singularity and it is hidden behind an event horizon. So in these cases, it's not possible, for instance, to observe a particle actually fall into the singularity and it's not possible to observe an object (e.g., a star) collapse into a singularity.",
"A naked singularity ",
" allow such observations, but no naked singularity has been observed in nature. The only known naked singularity is the big bang, but this is a singularity in the absolute past of all observers, and so it is again not possible to observe a particle \"fall into\" this singularity.",
"In principle, a naked singularity ",
" exist in that there are solutions to the Einstein field equations which consist of naked singularities. But these solutions are all unphysical. For instance, one such solution is that of a rotating black hole for which the spin parameter is exactly equal to 1. This would be a black hole whose mass consists entirely of rotational kinetic energy, that is, a black hole with maximal angular momentum (J = Mc)."
] |
[
"These are unphysical solutions. ",
" of the black hole's mass is due to rotational energy. So it's as if a massless, spinning star collapsed into a black hole. This is not physically possible.",
"Another such unphysical solution is one in which the mass M is less than the electric charge Q (with suitable constants to make these quantities directly comparable). This is like a black hole that formed from a star with negative mass but net non-zero electric charge. Again, not physically possible."
] |
[
"To be clear, we're reasonably sure that naked singularities do not and cannot exist. They are the result of stretching the equations of General Relativity beyond the regime in which they provide accurate predictions, by plugging in values that are probably unphysical. This has nothing to do with black hole evaporation; if a black hole evaporated then its singularity would evaporate too and you would just be left with a bunch of radiation in otherwise empty space.",
"You can turn a black hole into a naked singularity two ways: make it spin very fast (Kerr solution) or give it a large electric charge (Reissner–Nordström solution). Both of these create a second event horizon inside the black hole where things falling in slow back down below the speed of light and spacetime becomes sort-of normal again before reaching the singularity. This inner horizon gets bigger if you add more angular momentum and/or charge and eventually intersects the outer horizon, at which point the horizons vanish and the singularity is exposed to the rest of the universe. This is believed not to be possible though, because it would require the black hole to have more energy from rotation/charge than from its own mass! We're pretty sure this can't happen."
] |
[
"If the neutrinos that were possibly going faster than the speed of light were travelling backwards through time, wouldn't they be seen as antineutrinos?"
] |
[
false
] |
[deleted]
|
[
"You seem to have a decent grasp on what FTL means according to special relativity. Most likely if this result holds we can't just naively interpret special relativity that way. If this result is correct (a huge if) it's much less likely that these neutrinos actually go back in time and violate causality than that they're going less than a maximum speed but through extra dimensions or something else more subtle. Going back in time is equivalent to violating cause and effect, and that's a major no-go in physics."
] |
[
"The notion that a particle traveling forward in time is equivalent to its antiparticle traveling backward in time is one that depends on special relativity, as is the notion that traveling faster than light means going backwards in time. If this neutrino result turns out to be true, chances are that such a naïve interpretation of special relativity will be unwarranted."
] |
[
"From my very limited knowledge about the alleged speeds reached at CERN, (I read a lot, no specific knowledge or science education) I tried my best to explain to a friend (who has less knowledge again) that in this instance the neutrino was supposed to be travelling FTL and would in fact be going backwards in time. He reckoned that surely the neutrino was heading forward in time. ",
"I tried to explain the tacyhon pistol duel and the grandfather paradox to him and about time dialation, I was barking up the wrong tree and now we're both confused. ",
"Does the \"back in time\" depend on which point in space you are observing the shots from. I mean, if I was guy A shooting rather than guy B shooting?",
"Can you think of an easier way to explain this to a lay person? I'd be very grateful. As we both have it arse-ways now and its very hard to find a simple enough explanation anywhere to do with this. A link to a good source would be great even. ",
"EDIT - I do realise that this duel / \"faster than light\" stuff is just a thought experiment, as it stands, i'm not assuming in reality that these things are fact.",
"Thanks mate."
] |
[
"In Grade and High School, I always heard about how Jupiter was a \"failed\" Star. Can it (under the right circumstances) eventually become a Star? If so, what process would it go through?"
] |
[
false
] | null |
[
"When people talk about \"failed stars\" they're referring to these fascinating little objects called ",
"brown dwarfs",
". Brown dwarfs are what happen when a star forms with so little mass - less than about 80 times the mass of Jupiter - that it doesn't have enough pressure from the inward pull of gravity to even ignite hydrogen fusion at the core. The fusion of hydrogen into helium is what keeps normal (or main sequence) stars like our Sun burning happily and producing light. Brown dwarfs only create light from a bit of deuterium burning (if they're more massive than about 13 Jupiters) and lithium burning (more than 65 Jupiter masses), and then whatever energy they have left over from gravitational contraction. This makes them incredibly dim and incredibly hard to spot, and it's only in the last 17 years that we've managed to find these brown dwarfs.",
"Hydrogen fusion, by the way, is also what causes stars to go through a life cycle: when a star like the Sun runs out of hydrogen to turn into helium, it expands into a red giant and gets by for a while on fusing heavier elements, and when those run out, it expels its outer layers and leaves behind its core as a white dwarf. More massive stars will mark this phase with a dramatic supernova explosion, leaving behind a white dwarf, a neutron star, or even a black hole. Brown dwarfs, however, never run out of hydrogen and so never go through the normal stellar lifecycle. Instead they just move through space, slowly flickering out like an ember of the cosmos.",
"There's a lot of debate about where to draw the line between brown dwarfs and planets. You might say that brown dwarfs are on their own and planets orbit stars, but that doesn't cover it: plenty of brown dwarfs orbit stars too (my first ever scientific claim to obscurity is that I found one such system), and now we've even found free-floating planets that managed to get away from their host stars.",
"Planets and stars are believed to form differently. Stars form when a cloud of gas collapses under its own gravity. Brown dwarfs form the same way, they just never reach the final step of hydrogen fusion. Planets, on the other hand, form from the gas and dust from that cloud which ",
" make it into the star. This gas and dust ends up forming a disk around the star, and pockets of matter within that disk clump together to form planets. So on a moral level, I think the real difference between a star and a brown dwarf is formation - and Jupiter wouldn't be a brown dwarf (or a \"failed star\") because it never formed the way a star did.",
"The problem is that it's usually next to impossible to distinguish, using a telescope, how one of these objects formed. The International Astronomical Union (of Pluto demotion infamy) uses the cutoff for deuterium burning - 13 Jupiter masses - as the dividing line between brown dwarfs and planets, because it's not too difficult to observe, and because deuterium is the last molecule you can burn. Something below that cutoff is really making no light of its own, except by leftover energy from its birth. There are brown dwarfs which fall below this limit - the lightest one I know of is only ",
"three times more massive than Jupiter",
", about a thousandth the mass of the Sun, a real extreme in star formation.",
"(Out of my current field of expertise - as my tags say - but I did a project on brown dwarfs a few years ago and got a paper or two out of it, plus ",
" of speaking experience to popular audiences.)"
] |
[
"Odd question: ",
"When the sun starts to burn out (assuming we're quite technologically advance), could we \"throw\" Jupiter in there to buy ourselves some time? How much time would it buy us?"
] |
[
"Wow, thanks for the info. Never heard of a Brown Dwarf either."
] |
[
"Is there a theoretical maximum for a how reflective a surface can be?"
] |
[
false
] |
I remember reading a while ago that household mirrors absorb around 10% of light that hits them. Is there specialised mirrors that are more efficient? If so, what are they used for?
|
[
"The theoretical maximum for reflection is 100%.",
"In theory, any ideal metal is 100% reflective. In practice, mirrors, coated with metallic layer on the back of a transparent glass, does not reflect 100% because even metal can absorb the incoming light slightly. Gold is a very good natural reflector with less than 2% (recalling this from my head really).",
"In practice, the often used way to effectively reflect light is to take advantage of the wave nature of light and reflect light via interference. These \"specialised mirrors\", or photonic mirrors, use layers of transparent mediums, often two alternating materials stacking on top of each other, that have thickness less than the wavelength of the light. If the thickness is correct, 100% of the light can be reflected due to interference. Of course, you can also adjust the thickness to decrease the reflection to 0%.",
"These specialised mirrors and lenses are already in use today. The most often ones I can think of is the eyeglass. Coated with so-called anti-reflection coatings, the eyeglass has layers of transparent mediums coated on top to cause constructive inteference, so the reflection can be decreased."
] |
[
"You couldn't really use this to make a mirror,",
"Sure you can! They're called reflective prisms, and they're commonly used in binoculars."
] |
[
"/u/isison",
" is close, though in the optical coating industry these are usually referred to as dielectric mirrors, not photonic mirrors. The simplest type is commonly called a \"quarter wave stack\". By stacking quarter wavelength [optical] thicknesses of material, you can achieve arbitrarily good reflection (as close to 100% as you want) at a specific wavelength (and its harmonics). However, you can be limited by material properties, such as adhesion of your materials to each other or to their substrate. I work in lasers and we use these mirrors to define the optical cavity of the laser. Use of anything other than dielectrics to produce the mirrors is discouraged for high power density lasers because any absorption (which metals do in small amounts) tends to result in catastrophic device failure."
] |
[
"Does free space have a Poisson ratio?"
] |
[
false
] |
I've been wondering since reading about the discovery of gravitational waves and how they 'compress and stretch' the earth. Does the compression of space-time along some axis correspond to an expansion about others? And like the title says, is there a Poisson ratio for space-time or free space?
|
[
"What a bizzarre way to put it... anyways when a gravitational wave passes, it only affects the two directions orthogonal to its motion. Moreover at any given time one is stretched and one is compressed so that the volume of a given region of space is kept constant. Since GWs are very weak, that means the relative compression in one direction is equal to the relative expansion in the other.",
"Quick proof (if you care):",
"You model weak fields in general relativity roughly this way: you consider a metric which is mostly that of flat space plus a minuscule correction. For example the spacial part of the metric, which is a 3x3 symmetric matrix, is",
"g = 1 + h",
"Where 1 is the identity matrix, which is the normal metric in euclidean space, and h is a very small traceless symmetric matrix. h is a dynamical field which is the thing that contains the gravitational waves.",
"The volume of an infinitesimal piece of space is, by the well-known formula, sqrt(det g) dx dy dz. Moreover I recall the expansion of the determinant",
"det (1+h) = 1 + Tr(h) + O(h",
")",
"And so if h is traceless, and it is, the volume is unchanged."
] |
[
"Young's Modulus is a material property like density, color, melting point, etc. Space time is not a material so you're talking about completely different things."
] |
[
"Young's modulus describes how hard it is to deform something. General Relativity describes gravity as the deformation of spacetime by mass. So the Young's modulus of spacetime would describe how hard it is to deform spacetime. It is a perfectly sensible notion, at least at some level. My question is whether it can be applied sensibly to spacetime. ",
"The amount of deformation can be calculated by the shape of the geodesics, i.e. gravitational lensing. So perhaps my question becomes how much force a mass exerts on spacetime. That may not have a definite answer, or it might."
] |
[
"I'm unable to spit out mucous no matter what I use(details as to why inside). What can I use to break down the phlegm so that I can swallow it instead?"
] |
[
false
] | null |
[
"What is the problem with swallowing it back down? It is coming from your respiratory system and swallowed down your digestive system... swallowing it doesnt \"send it back where it came from\"."
] |
[
"I think you misunderstood my post. I am asking for a way to swallow the mucus as opposed to spitting it out."
] |
[
"Right.... why don't you just swallow it?"
] |
[
"Black holes affect light; thus light is pulled by gravity. Does light exert its own pull in return?"
] |
[
false
] | null |
[
"Yep, light definitely does gravitate - everything does! In Einstein's gravity it's not just mass that gravitates, but mass and ",
" - and everything has energy.",
"When it comes to a black hole, the gravity of the light it pulls in is pretty much always negligible by comparison. But there are instances where the gravitational pull of light is significant. The most important of these is the ",
" of the early Universe, for about the first 80,000 years after the Big Bang, when light was actually denser than normal matter and so was the dominant gravitational influence in the entire Universe. As a result, the Universe expanded at a different rate than later on, when matter was dominant."
] |
[
"There are also massive bosons like the W and Z bosons, and they obviously interact by gravitational force.",
"There is no reason why the spin state (i.e.fermion or boson) should play a role when interaction gravitationally."
] |
[
"Oh whoops, my bad. Forgetting what I learnt years ago. "
] |
[
"When we eat, why is there such a long delay between being full and feeling full?"
] |
[
false
] | null |
[
"One of the key physiological regulators in appetite (note: I say ",
" rather than neurological) is the peptide hormone ",
". Ghrelin is released by the stomach into the bloodstream and is a noted appetite stimulant. When the stomach is full ghrelin release is inhibited (switching off the appetite, as it were) but this takes time to have an effect; there is still ghrelin circulating in the body and it takes time to be cleared from the system.",
"Interestingly, people who have had stomach banding or stapling often have reduced ghrelin production, resulting in a lower appetite."
] |
[
"Ghrelin is not the only peptide responsible for appetite and is certainly not the only one responsible satiety signals. There is also CCK, NPY, leptin, cannabinoids, and to some extent cortisol, which act on multiple brain areas such as he hypothalamus and parts of the brain stem, the nucleus of the solitary tract and area postrema. The process of satiety is mediated by a number of different factors that range from stomach stretching, stomach contents emptying into the duodenum (first part of the small intestine), hedonic aspects of taste as well as others.n"
] |
[
"This is only part of the answer. The stomach's stretch mechanism results in the inhibition of ",
" (a peptide that stimulates the appetite). As circulating ghrelin levels fall, so does the desire to eat. This does take a little time though as ghrelin needs to be cleared from the circulation (it's a hormone that gets secreted into the blood). People who have had stomach banding or stapling often have reduced ghrelin production, resulting in a lower appetite."
] |
[
"Is the cardinality of the set of points in a number line the same as the cardinality of the set of points of a plane?"
] |
[
false
] | null |
[
"Technically they just show there's at least as many points on a line.",
"And obviously, there are at least as many points in a plane as on a line. So by the Schröder–Bernstein theorem, they have the same cardinality."
] |
[
"Yep. See ",
"space filling curves",
" for examples of mappings between an interval and multidimensional spaces."
] |
[
"Yes. The easiest way to prove this explicitly, IMO, is to make use of the ",
"Schröder–Bernstein theorem",
", which says that if A and B are sets and f:A->B and g:B->A are both injective functions, then there exists a bijection between A and B. This way, you can avoid finding an explicit bijection between R and R",
", which, while doable, gets a little messy.",
"It's easy enough to build an injective function from R to R",
": take f(x)=(x,0), for example. To find your injection g from R",
" to R, one thing you can do is interleave the decimal expansions of the inputs to g to get a single real number. So we might set, for instance, g(0.12, 0.34)=0.1324. There are some finicky logistical issues to deal with, like what to do with negative numbers, or the fact that 0.999...=1, but I'll leave those as an exercise to the reader. I'll just say that it is possible to construct an injection using this kind of a trick from R",
" to R. And since there's an injection from R to R",
", and a different one from R",
" to R, the Schröder–Bernstein theorem gives us the existence of a bijection, which tells us that the two sets have the same cardinality."
] |
[
"Can we use Nuclear Magnetic Resonance to image modern semiconductor?"
] |
[
false
] |
Just like we can get brain scans/images with an MRI machine,Is it possible to get scans/images of a modern semiconductor microchip using an MRI machine Can the technique of Nuclear Magnetic Resonance be in some way used to image silicon structures?
|
[
"You can do MRI of semiconductors (and nanometer-sized objects), although not using a typical MRI setup...",
"170nm nuclear magnetic resonance imaging using magnetic resonance force microscopy",
".",
"We demonstrate one-dimensional nuclear magnetic resonance imaging of the semiconductor GaAs with 170 nm slice separation and resolve two regions of reduced nuclear spin polarization density separated by only 500 nm.",
"Nanometer-scale magnetic resonance imaging",
".",
"Magnetic resonance force microscopy (MRFM) images the three-dimensional spatial distribution of resonant spins by mechanical force detection.....Applications of MRFM include three-dimensional nanometer-scale mapping of dopant distributions in semiconductors, studies of magnetism of thin films, and spin diffusion physics.",
"There's also a lot of literature on the spectroscopy side of the house, of course. "
] |
[
"Thanks a lot!! this is what i need to get started.",
"Can you please elaborate on your last statement \"There's also a lot of literature on the spectroscopy side of the house, of course\""
] |
[
"You're welcome. ",
"Can you please elaborate on your last statement \"There's also a lot of literature on the spectroscopy side of the house, of course\"",
"Exactly what I said - there's been a lot of NMR spectroscopy (not imaging!) on semiconductors over the years. For example, optically pumped/detected NMR methods have ben used for quite some time now (see, for example, ",
"here",
", ",
"here",
", and ",
"here",
" to start).",
"I'm only passingly familiar with this particular area of NMR applications, but I'm sure a good ",
"Google Scholar",
" search will be informative. "
] |
[
"How sure are we that Proxima Centauri is indeed gravitationally bound to the Alpha Centauri binary pair?"
] |
[
false
] |
So therefore what are the odds that it is not?
|
[
"This study",
" concluded that there's only about a 4x10",
" (0.000004%) chance that Proxima Centauri is not gravitationally bound to Alpha Centauri AB."
] |
[
"Good question! ",
"This paper",
" published in 2017 sought to answer this question by looking at the dynamics of the system directly. While in their abstract they say \"we show that Proxima and Alpha Cen are gravitationally bound with a high degree of confidence\" it is somewhat difficult to give an absolute probability that this is true, however we can get a good sense of what this probability is by looking at some figures.",
"Figure 1 of the paper (",
"PDF linked here",
") shows the most likely orbit as the black line along with 100 random other orbits from a statistical ensemble they generated that represent the most likely orbits. The important thing to notice is that none of them are hyperbolic or parabolic, or otherwise unbound. Since they plotted 100 random orbits, this means that Alpha Centauri and Proxima Centauri are bound at least at the 99% level (3 sigma level). If by looking at their entire ensemble of simulated orbits (of which they might have millions) and we compared the ratio of the bound orbits to unbound orbits, we could put a definitive number on the probability that it is bound. Judging by the orbits in their figure, it is probably a crazy high probability (like, many 9s after the decimal point)."
] |
[
"Why are you guessing at a figure based on simulated orbits, when they already derive one from the relative velocity?",
"The observed velocity is therefore lower than the unbound velocity limit by −5.4σ, corresponding to a theoretical probability of 4×10",
" that the stars are not gravitationally bound."
] |
[
"Why do electrons pair up in superconductors?"
] |
[
false
] |
My limited knowledge keeps telling me that it doesn't make sense, opposite charges repel.
|
[
"Something that has been bothering me for a while about this subject: I've heard it explained that the reason Cooper Pairs have no resistance is that a Cooper Pair is a boson, so it follows Bose-Einstein statistics instead of the Fermi-Dirac statistics of a single electron. Why does this work? It seems there must be something a little more fundamental going on than simply, \"oh hey, if we pair them then we get a boson, so they can be in lower energy states.\" Why wouldn't they just pair up all the time to eliminate Pauli exclusion, thus lowering the energy of the system?"
] |
[
"Something that has been bothering me for a while about this subject: I've heard it explained that the reason Cooper Pairs have no resistance is that a Cooper Pair is a boson, so it follows Bose-Einstein statistics instead of the Fermi-Dirac statistics of a single electron. Why does this work? It seems there must be something a little more fundamental going on than simply, \"oh hey, if we pair them then we get a boson, so they can be in lower energy states.\" Why wouldn't they just pair up all the time to eliminate Pauli exclusion, thus lowering the energy of the system?"
] |
[
"Cooper showed, before the BCS paper, that if you have a non zero constant attractive interaction between electrons they will form bound state. In free space, of course they will repel, but put them in a system where this happens and they will always form a bound state. I do not believe he even mentions statistics in his original paper."
] |
[
"How do we know how old cave drawings are?"
] |
[
false
] |
A recent post showed cave painting approximately 30,000 years old. How do we know this?
|
[
"Cebus is correct in that we use other radiometric dating techniques to get \"absolute dates\" for some artifacts, but we almost always start with \"cross dating\" using other artifacts or ecofacts of known age. In the case of cave paintings we can also use chromatography to get the elemental composition of the pigments, then cross-date them to anything else we know to have used the same pigment. ",
"I'll edit this post later with a clearer answer and supply some links to a few of my favorite papers on dating techniques."
] |
[
"thanks!"
] |
[
"Either you date the rock the painting is on or you date the layer of rock above and below using ",
"radiometric dating",
"."
] |
[
"Why is the periodic table called the periodic table?"
] |
[
false
] |
[deleted]
|
[
"The idea behind the periodic table is that if you list all of the elements in the order of the number of protons they have, there are repeating patterns in their behavior. These repeating patterns are called \"periods\", because they are periodic. The periodic table is what happens when you lay out the elements in order of the number of protons they have, but add spacing and lines in order to get each column of elements to share important properties.",
"For example, all of the elements in the rightmost column of the table are explosively reactive, whereas the elements in the leftmost column are gases which rarely react with anything. If you look at other columns, there are other patterns. For example, Copper, Silver, and Gold share a column. The more chemistry you get into, the more the patterns in these columns become relevant.",
"Edit: I flipped left and right above"
] |
[
"Mostly true (you flipped the right and left on the stable noble gasses that are on the far right and the reactive alkalis on the left)."
] |
[
"The history is that John Newland noticed a periodicity of the chemical properties of chemical elements when arranged in the order of atomic weight. He called this the law of octaves in 1864. Gregor Mendeleev in Russia then started arranging the elements in a table in the 1870s, which, while different from the modern table, was pretty much the starting point of the modern table. He arranged elements by oxidation state. He predicted elements which were not known in his table, but were later discovered."
] |
[
"Can someone explain inverse-square law to me. I need to use it to work out the cutting measurements to build a pendulum wave."
] |
[
false
] |
It may be that you use inverse-square law to find the length of each pendulum. I am going to be building something .
|
[
"I'll allow it."
] |
[
"Oh ok. I was confused because usually inverse-square law refers to something else.",
"He lines it out here: ",
"the length you are looking for is the length of the longest pendulum multiplied by the square of the ratio of the time periods squared. ",
"So when you figure out what you want the swing times to be, just plug it into that equation. For example, if the first pendulum is 10 cm and you want the second one to have a period 1/3 shorter, the length is (2/3)",
" x 10 cm=4.4 cm."
] |
[
"When a pendulum's swing is quite free",
"It's always a marvel to me",
"The each tick and each tock",
"Of the grandfather clock",
"2 pi root L over g",
"Note that this is the small angle approximation."
] |
[
"Why do distant mountains fade to blue?"
] |
[
false
] |
I recently saw a post from called "Welcome to America." One of the pictures showed a mountain range becoming a light blue as the mountain range became more distant. In a middle school art class, I remember being told this occurred, but I wasn't told why this happened.
|
[
"The phenomenon which makes mountains (and everything else) further away from us fade to blue is actually Mie scattering. Mie scattering is similar to Rayleigh scattering, except the particle size is different. With Rayleigh scattering, the particles are much smaller than the wavelength of the light (gas molecules, for instance). This causes higher energy wavelengths to be scattered. This is why the sky appears blue. With Mie scattering, the longer wavelengths are affected. This means that the blue light reflected from mountains far away from us will be scattered less than red light, leading to the blue haze. The particles responsible for Mie scattering include dust and water droplets.",
"Tying into the relationship to trees: Trees produce water vapor (and in some cases oils, like eucalyptus). These particles increase the amount of Mie scattering, enhancing the blue haze."
] |
[
"I think, and I may be mistaken, that it actually isn't Rayleigh scattering here. Or if it is, there's an enhancement. I feel like I remember reading something about the water vapor given off by trees and such that makes the atmosphere bluer in a shorter distance than standard atmospheric conditions.",
"Edit: here's ",
"an article",
" that mentions it indirectly as a \"blue haze\" from aerosols emitted from trees.",
"Edit 2: from ",
"Eucalyptus",
": On warm days vaporised Eucalyptus oil rises above the bush to create the characteristic distant blue haze of the Australian landscape."
] |
[
"So if the larger particles responsible for Mie scattering weren't present, would the blue haze be less or even nonexistent? I realize that it's not probable for these larger particles to be absent over a large distance, such as a mountain range. I guess it's more of a hypothetical."
] |
[
"Why does a sink get to a stable level after a while of water flowing?"
] |
[
false
] |
I tried for literal 20 minutes to draw a diagram on pen and paper with the forces I've considered and the ones I've dismissed, but nothing really worth scanning. This is what I'm thinking. 0) We have a sink of unspecified shape, volume V0, contact area at the top A0 and drain hole area A1. 1) In the empty sink, there's only atmospheric pressure and the air pressure from within the drain pipes, which are more or less at equilibrium. When there's water flow, the atmospheric pressure becomes a negligible factor, as the room "tries" to equalize air pressure within its boundaries. 2) The open sink draws out water at a constant c1*m /dt, where dt is some time increment. I'm assuming there's no surface tension. 3) The drain is able to suck out water as a proportion of the differentials in pressure between the water volume V1 = L*V0 pushing down and the pipe system air pressure Z pushing up. Now, when I look at this system I think two things are possible: either the pressure of V1 m of water (P1 = G V1/A0 = k_w * V1/A0) is > Z and water never rises out of an empty sink, or it's < Z and it raises until it's = Z. The diff. equation for V1 without surface tension, turbulence and effects on system pressure Z turns out to be linear, which gives an equilibrium but is unsatisfactory because I can't measure Z to get a sink level for other sinks in the same pipe system and doesn't explain the onset of turbulent movement inside the stable sink volume (what, is there an increase in velocity?) I know this is pretty open-ended. I actually had a question (why does it get to a stable level) before I started to write down the question, but now I have more. I was just ... watching the foam from the shaving soap and was flooded with all these questions. (I'm deceptively childish for my age...)
|
[
"The basic concept is that friction is directly proportional to velocity squared. The higher the water level, the higher the velocity in the pipe, the higher the friction. As soon as the friction gets high enough to restrict the flow, you will arrive at a stable water level. "
] |
[
"Yes, the two forces that are balancing are the pressure due to the water level, and the resistance in the pipe. If you have pipe with no gunk built up, you probably won't have any resistance and the water will pass freely without water building up in the sink.",
"There is no restriction on the 'other side' of the pipe, so there will be no pressure force acting against the flow of water. Most sewer pipes are designed for open channel flow (not completely full), so in most cases there is never a pressure buildup. If there is a pressure buildup, it will be due to a clog, which then again the driving force is friction."
] |
[
"Wait. So ",
", rather than the air pressure from within the pipe system, is the counteracting force here? Air pressure from inside the tubes is negligible?"
] |
[
"How much do we really know about the likelihood of future earthquakes?"
] |
[
false
] |
I got an ad for earthquake insurance and it was super expensive. 3x my regular homeowners insurance and a deductable around 80k. Does the insurance company know something I don't? I also hear a lot about "overdue" earthquakes but how accurate is our knowledge on the subject? Can anyone say that an earthquake will happen around area X with percentage chance Y over the next Z years?
|
[
"If you want to see some of the nuts and bolts behind the probability used by structural engineers, the USGS has a pretty good site. Go here ",
"http://earthquake.usgs.gov/hazards/apps/gis/",
" and enter your location to find out what your probability of exceeding a certain PGA will be over a given time period.",
"Structural Engineers use this PGA - or peak ground acceleration (the maximum ground acceleration felt at a location expressed in % of gravity) as the first step in seismic design. The largest PGA ever recorded was 2.7g for the 2011 Tohoku quake in Japan. ",
"From there it gets way more complicated, but that is the basics. ",
"Source: Structural Engineering Graduate Student"
] |
[
"Haha \"overdue\" earthquakes. That's not how probability works. ",
"Event frequency is classified on a 1/x scale (1/2, 1/20, 1/1000), in this case measured in years. Generally speaking the severity of earthquake, as with cyclones, tidal waves, etc. increases inversely with the probability; severity from 1/1000 yr event is orders of magnitudes greater than a 1/2yr event. ",
"However. ",
"Just because there could be a 1/2 chance (50%) of an earthquake occurring, does not mean it will happen every two years... just like there is no expectation is you flip a coin ten times, you will get 5 heads and 5 tails. For more on this, ",
"\"Monte Carlo/Gambler's fallacy\"",
", after the roulette ball landed in black 26 times in a row. ",
"Over a long enough time frame (could be hundreds of years), the event frequency of 1/2 will generally hold true - 10 years of nothing, then 8 quakes in a year. Making things more confusing is the fact that, geologically speaking, the 8 quakes could all be linked to the same faulting rock... ",
"As such, the claims of \"overdue\" earthquake is just that... a claim. ",
"With regards to your insurance. ",
"Christchurch (NZ)",
" was recently SMASHED by an earthquake - half the city centre collapsed. This was due to the severity of the earthquake (on Richter scale) was much greater than the engineering design had allowed for - simply overwhelmed the structural capability in many places. (note I have massively simplified for this thread). This does have ramifications for the surrounding area, especially w.r.t. structural design. Those boys are getting smashed with review, and I expect that to translate to Australia in the next few years. ",
"The way I read your advert, one of the following could apply:",
"Your house might not be structurally sound w.r.t. earthquake design. If you are in a earthquake prone area, certain work is required to ensure your house can't collapse like a house of cards. SPOILER ALERT: if your house is built in line with the local governance Building Code and relevant Standard (ISO/AS/BSI), then nothing to worry about. If not, then it is generally something you can do to an existing structure. ",
"Insurance companies are leeches. They can raise premiums in one area where they do not expect claims, in order to increase income on a big outlay - like a flood in another area. Not exactly kosher, but they set their own prices. ",
"It's a cold calling ploy. Shop around and you should get an idea of local prices. NO-ONE can pinpoint a future earthquake location or severity to within a few city blocks. Its a current impossibility. To a local region yes, but city blocks no. ",
"Hope this helps. (Source: civil engineer)",
"Edit: a word. \nEdit2: hyperlinks. "
] |
[
"So... I didn't actually answer your second part very well. ",
"Yes, especially in urban or high-interest locations, geologists and risk analysts can assign a probability and a likely severity to future earthquakes. ",
"This is generally static, and will be the same risk profile for EVERY insurance company in the local area. "
] |
[
"What makes it rain harder? Why do some clouds cause flash floods while others hardly drizzle?"
] |
[
false
] | null |
[
"This is a really good question and I'm not sure I've ever seen it asked in this sub before! The answer is pretty complicated and multi-faceted but I think I can get it down in the 20 minutes while I finish my morning coffee.",
"Clouds can form in a few different ways, but the most common and most important for precipitation (the general term for things falling from clouds: rain, snow, hail, ",
"frogs",
", etc.) is rising air: an ",
"updraft",
". As air rises, the pressure of its surroundings is lower, and so it expands to match that lower pressure in an attempt to reach an equilibrium (as nature is wont to do). Expanding comes with ",
"a side effect of cooling",
": just like the air coming out of a can of compressed air ",
"gets very cold",
" as the pressure goes down, air that is expanding as it rises cools off. If this air has enough water vapor in it, and the air keeps rising, eventually this cooling will reach the so-called ",
"dew point",
": for a given temperature and amount of water vapor, this means that the humidity will be 100%.",
"So now we've made a bubble of air with 100% humidity. How do we make a cloud? To start, you might have heard that the humidity can never get above 100%. Well, ",
"it's a bit more complicated than that",
". In fact, if the air was super-pure, the humidity could keep climbing almost indefinitely. But if there are solid particles floating around in that air, water vapor will tend to condense on them as soon as the humidity gets even slightly above 100%. And even outside of human pollution, air is never super-pure, and is full of these so-called ",
"cloud condensation nuclei",
": there are particles of dust, smoke, sea salt, pollen, and even bacteria that water will condense on to start forming cloud particles. So as our bubble of air continues to rise and cool, more and more water vapor will start condensing into tiny cloud particles, making ",
"a towering cloud that grows and grows",
", so long as the air keeps rising.",
"So now we've made a cloud. How do we make rain? Well we could just keep growing these liquid cloud particles over time: after all, as long as the humidity is above 100%, more and more water vapor will continue to condense onto these drops, making them bigger. In addition, as these droplets wander around, they can bump into other droplets, causing them to grow by a process known as ",
"collision and coalescence",
". But while this does seem like a natural way to eventually get raindrops, this is a very, ",
" slow process in practice. It does lead to rain in certain situations: when there's very little air movement, and the cloud is very low or ",
"even on the ground",
", we can get very small raindrops that are commonly known as ",
"drizzle",
". And in tropical areas that have ",
" of water vapor, we can even get pretty heavy rain from this process, though this is pretty rare outside of the tropics and it still takes hours or even days for large rain drops to form.",
"So how do we make rain without waiting around for a few days? After all, we've all likely seen a clear summer sky quickly turn to thunderstorms in an hour or less. Well, the short answer is, we've got to make snow first. Water has a lot of weird properties that they don't necessarily teach you in middle school science class, and one of them is that there's a really big range of temperatures where you can have both liquid and solid water coexisting: roughly between 0 and -40˚C (32˚F to -40˚F....-40 is the best temperature because conversions are easy!). We don't usually see this in everyday life because in order to get liquid water in this range you either need it to be ",
"very pure",
" or very small: and cloud droplets tend to fit both criteria! Another weird property of water is that the humidity measured vs liquid water is ",
" than humidity measured vs solid ice. This concept is pretty hard to grasp so don't worry about trying to wrap your head around it (even college-level meteorology students struggle with this: trust me, I was one of them) but basically this means that if you have 100% humidity and a bunch of liquid cloud droplets, and suddenly a wild ice particle appears, the humidity is actually 110% compared to that piece of ice! And so all that vapor is going to very quickly deposit as ice, forming a snowflake in almost no time at all. If the air is still rising in this environment, it can keep these snowflakes from falling right away, keeping them in this high-humidity region so they keep growing and growing. These snowflakes can then start colliding with each other ",
" with other cloud particles, which will instantly freeze on them, accelerating the growth and making our neat, harmless snowflake into ",
"a scary ice-ball of death",
". ",
"But wait, you might say, I thought we were just talking about heavy summer thunderstorms? You know, the type where it's summer with rain, and not winter with snow? I'm glad you asked, because it all comes back to that initial bit about rising air. There are a few different ways we can get air to rise in a way that will eventually give us a cloud, but the easiest and most common way is by heating the ground. As the ground warms up, the air near it warms up too. And as described by our friend ",
"Mr. Ideal Gaslaw",
", air gets less dense as it warms up, which causes it to rise due to buoyant forces. Even though the air cools as it rises, if the conditions of the atmosphere are right it might still be warmer than the surrounding air, which means it will continue to rise. If air rises high enough, and fast enough, even on the warmest summer day it will eventually cool below freezing. And being summer helps us even more: the summer sun is much better at heating the ground (and the air near it) than other times of the year, so this air can get hotter, quicker, and therefore rise faster and farther. And these warmer days mean that hail stones can melt by the time they reach the ground. Really, if you're ever caught in a rain storm so heavy it feels like you're being pelted by water balloons, it's almost always due to hail that melted on the way down.",
"Bam, 67 minutes, only a little more than three times as long as I planned; I'm getting better at this! Now to get back to some wonderful Monday-morning work emails..."
] |
[
"Thanks for the explanation, you were great !\nHope the coffee didn’t get too cold ;)"
] |
[
"Amazing answer!"
] |
[
"How are dams able to be constructed against the constant push of flowing water?"
] |
[
false
] |
Secondly, how exactly does a dam hold back a constantly accumulating amount of water without simply overflowing? I know they drain some water through spillways, but it seems like the vast majority of water is still trapped above the dam. Where does it all go?
|
[
"Usually the river is redirected around the site where permanent dam construction is taking place. A temporary dam may be used to assist the redirection.",
"A hydroelectric dam routes some of the water through the turbines within the dam to create electricity. Excess water exits through spillways.",
"When the lake upstream of the dam has reached the target level, the dam engineers allow water to flow downstream, else the damn would overflow. So at some point in time, the water exiting through spillways (and turbines) is equal in volumetric flow rate to the amount of water entering the lake that has formed above the dam."
] |
[
"http://www.hydroquebec.com/learning/hydroelectricite/construction-refection.html",
"I live in Quebec and our home power company has all you need to know with a nice animation."
] |
[
"How on Earth do they build the temporary dam?"
] |
[
"A friend has a degenerative condition in her retinas; blindness is inevitable within a decade. Is there hope?"
] |
[
false
] | null |
[
"Right, I wasn't really looking for a diagnosis or case-specific information, rather just first-hand feedback from people who are working in the relevant fields, and what they anticipate as options for treatment for people with degenerative retinal disorders in the coming decade. "
] |
[
"Right, I wasn't really looking for a diagnosis or case-specific information, rather just first-hand feedback from people who are working in the relevant fields, and what they anticipate as options for treatment for people with degenerative retinal disorders in the coming decade. "
] |
[
"I understand, but it's a line we toe and when a specific person is mentioned we can't release the question. ",
"A ",
"quick search",
" on AskScience shows multiple recent questions on stem cells, most of which haven't garnered much of a response. ",
"This question",
" from a month ago is probably very similar to yours. ",
"We normally try to avoid repeat questions, but if you'd like to reframe your question in a similar way, I can approve it. ",
"I can't guarantee any responses, but more targeted questions generally fare better than \"tell me about topic X\". You could ask, for example, how these successful preliminary trials translate to future treatments of retinal diseases, or whether stem cells have more potential to treat some retinal diseases and not others and why, etc. (I'm just brainstorming here - you'd have to check on these). "
] |
[
"Water powered cars: Is this possible? Is there a catch?"
] |
[
false
] |
Hey, I saw , and I've seen other videos like it. Is this possible? Often I hear that more energy goes into it than can be taken out, does that mean that this car has a battery or something? How does stuff like this work?
|
[
"If it seems too good to be true, it probably is. The only sensible way to run a car on water is to split the water into hydrogen and oxygen (costs energy) then burn the hydrogen and oxygen back together to make water (runs your car). You could have the oxygen in the water react with something else to make energy, but then that something else would be the fuel and not the water.",
"The catch is that either water isn't the only \"fuel\" or you need to get the energy from somewhere to split the water into hydrogen and oxygen, so water isn't the fuel.",
"tl;dr water isn't fuel"
] |
[
"Apparently that Japanese company (Genepax) ",
"shutdown their website two years ago",
", and no information about what their supposed \"chemical reaction\" can be found. ",
"If their car really worked, it certainly did not use water as the energy source, it was probably using compound like potassium hydride (which releases hydrogen when you add water, thus the actual fuel is potassium hydride ). "
] |
[
"Is this possible?",
"No. I think that this can be, for all reasonable intents and purposes, completely ruled out.",
"At room temperature and pressure, or even anything close to room temperature and pressure, H2O is simply the most stable, and therefore low energy, chemical configuration of hydrogen and oxygen that there is. One cannot extract a meaningful quantity of energy from a tank of H2O, unless the water is at a higher temperature than the rest of the system. ",
"A lot of power generators ",
" water to turn heat energy into mechanical energy. One can create heat by burning an actual fuel source - which for fossil fuels actually takes hydrocarbons and releases energy by turning them into water and carbon dioxide. Then one can use the heat to cause water to boil, and set a turbine into motion with the steam. This process is actually near to the maximum efficiency one can get converting heat energy into usable mechanical or electrical energy."
] |
[
"What happened during Earth's \"Dark Age,\" or the first 500 million years after it formed?"
] |
[
false
] | null |
[
"Unfortunately, its quite difficult to tell exactly what was happening. The problem is, the earths tectonic plates are constantly undergoing subduction and deposition. Put simply, the entire surface of the earth back then has since been pushed underground, melted, and reformed. There are literally no rocks on the entire planet from back then. In fact, geologists have to look at moon rocks to get data because they are pretty sure the conditions would be similar. Hope this helps!"
] |
[
"There have certainly been tectonic processes (and of course weathering and erosion) operating for a long time between 4 billion+ years ago and now, though it sounds like your initial sentence assumes this was happening in the actual Hadean, sort of erasing the early Earth as it went along. It’s not so much that tectonics were resurfacing crust as far back as that, because the Earth was not born with active plate tectonics. Exactly how and when a somewhat complete plate tectonics system began operating (ie. with recycling via subduction) remains a large open question within geoscience, but the general consensus is that this was some time in the early to mid Archean. ",
"Also with the kind of recycling you describe, there is a tendency to preserve continental crust (especially the more stable cratons), which is why we have a select few rocks at the surface today which were formed more than 4 billion years ago. For the oldest parts of the continents, the fact that subduction doesn’t affect them means that any reworking of the original material has been done either through weathering and erosion, or through burial and metamorphism. Moreover though, when the Earth first formed there was no continental crust, because such crust is a result of chemical fractionation through tectonic processes (which ultimately requires several rounds of partial melting of the primitive crust). ",
"The reason why we have no record of the Earth’s first tens of millions of years at least, is because it took time to cool from its molten state. The Moon-forming impact would then have almost certainly created another global lava ocean (though probably didn’t melt the entire planet all the way through), which would have replayed the same issue with regards to preservation. ",
"As I mentioned before there ",
" rocks (and more often reworked mineral grains) which we have reliably dated as more than 4 billion years old; this is why it’s important to remember that the Hadean Eon is an informal designation. That is to say, although it runs from Earth’s formation ~4.6 billion years ago to 4 billion years ago, and although it was originally proposed as a time from which no geological records were preserved, this is now an outdated definition. As is the nature of geological and fossil records, they are only ever going to get pushed back further and further in time (though obviously there is a hard limit somewhere, so that progress will involve incrementally shorter pushes back in time after a while and ultimately we won’t find anything older at some point). So although Hadean used to mean the eon when everything was molten and no records survive, it’s end is only kept as the same absolute date by convention now, rather than having any formal requirement to be defined as devoid of any records."
] |
[
"Okay, I understand, but look to the root of the question. OP just wanted to know what was going down on our planet for the first 500 million years. I also understand that their are minerals (zircon in particular) from that time period (I might add, I'm not aware of a true rock, being comprised of many minerals, that has survived that long. So if you have any publications or references to such a discovery i'd love to look into that). You said it yourself best, we don't exactly know when tectonics first came about. We don't even know if there was a celestial impact that created the earth (although this is the current best theory). There is some record of the mid to late half of the time period in question, but no real answers. I mean jeez, there is still the debate on catastrophism vs. uniformitarianism. Yeah you can say, \"the earth was molten, still hot from the impact that created it, in addition to, radioactive elements are yet to sink to the core.\" But that has to be understood as conjecture, not the answer (at least until we better understand the formation of the earth) One thing that can be said with relative certainty, is that tectonic forces have caused nearly all of the geological record of early earth to vanish. That is why I said that, I was only trying to avoid giving any information that was controversial. If, however, you feel that research on early earth has surpassed conjecture and have any cool references, i'd absolutely love to take a look if you wouldn't mind sending a link!"
] |
[
"How does SSL prevent the initial key from being stolen?"
] |
[
false
] |
Hi there, I've been looking for this answer on the web and here, but haven't found anything yet. The best I've seen so far is this article on encryption. In step one of how SSL works, it says "The web server sends its public key with its certificate." (article ) And from there on the browser and web server can use the key to encrypt/decrypt. My question is, how does the initial key hand off prevent an attacker from catching the key such as in a MIM attack? There always has to be a way for the browser/server to send an initial key, but all I can think of is a never-ending loop of encryption on top of encryption for the keys.
|
[
"The article is extremely bad in omitting crucial assumptions, on top of the actual method.",
"Let's have a look at the two variations that answer your question. I've found the sequences in ",
"this illustration",
" to be easiest to digest. Note they are referring to TLS 1.3. We are going to start with TLS 1.2, find your favourite illustration of the three round-trip handshake to keep track of - again I suggest ",
"this",
". The server, in the ServerHello message, will select the highest TLS version it supports and a cipher suite from the list that the client indicated support for with ClientHello. The ServerHello message contains the server's certificate chain.",
"Here are the two answers to your question for TLS 1.2. There are two client key exchange methods described in the TLS v1.2 spec. They are RSA and Diffie-Hellman.",
"If RSA is used, the client generates a 48 byte \"pre-master secret\" (edit: after receiving ServerHello). The client encrypts it with the public key of the server certificate (assuming it's validated and valid) and sends it. Only the \"real\" server will have the private key to decrypt the pre-master secret and generate the same master secret [1] as the client. From that point on, the two parties can exchange encrypted data. If the server does not have the correct private key, the incoming data will be 'garbage'. [2]",
"If Diffie-Helman key exchange is used, there is no need to encrypt the generated parameters ([3]), so they are sent \"plain\" in the ClientKeyExchange message:",
"Since both parties can now compute a shared master secret, they can use it to symmetrically encrypt the rest of the traffic.",
"Forward to TLS 1.3. In TLS 1.3 the choice has been made to use Diffie-Helman key exchange exclusively, not only because of its superior properties offered (like perfect forward secrecy) but also because it enables a key improvement. The improvement is the handshake can now occur in a single round-trip rather than three:",
"from which point on all data is encrypted.",
"[1] The pre-master secret is combined with some pseudo-random functions to generate the master secret",
"[2] You hinted at another question saying how is the server data encrypted: this is the only way. All of this takes place before any (e.g. HTTP) data exchange between the client and the server.",
"[3] You should read on a more comprehensive description of how Diffie-Helman works. The short description is that it allows the server and client to agree on intermediate values from which they can easily produce the master key, while the method relies on a computationally hard problem (e.g. discrete logarithm) for the attacker to solve (easy to verify, hard to compute)."
] |
[
"I think your question is really about: how can 2 people share a secret when anyone could be watching?",
"Some algorithms are very easy to go in 1 direction, but hard to invert. For example:",
"\"Factor the number: 899 into it's prime components\" would take you a few seconds, but the results are 29 and 31.",
"Now if instead I posed the inverse problem and said: \"What is 29*31?\" you could immediately say 899.",
"Public key exchange involves algorithms that are easy to compute one way but difficult to invert. So the server and client will do easy calculations like 29*31, while any MITM will be faced with the inverse problem: factor 899 problem.",
"The most accessible of these algorithms for encryption is probably ",
"RSA",
") which has a key that involves 3 numbers: \"e\", \"d\", and \"N\". Of these 3 numbers, the entire world knows \"e\" and \"N\" (these make the public key) but only the server knows \"d\" (this is the private key).",
"A client connecting to the server, that wants to send the message \"a\" (which is just a number in binary) will compute \"c = a^e (mod N)\" where (mod N) means you take the remainder after division by N. And transmit that to the server.",
"Computing \"c\" if you know \"a\", \"e\", and \"N\" is a very fast calculation, but the inverse, computing \"a\" if you know \"c\", \"e\", and \"N\" is a very hard calculation. So someone listening in will see you send \"c\" to the server, but has no idea of what your original \"a\" was.",
"The reason the server keeps \"d\" secret, is because the number \"d\" (the private key) is special, and satisfies the equation \"a = c^d (mod N)\" which we said was a fast calculation. This means the server can decrypt your message back into \"a\", and do so quickly.",
"The last, most important thing, is that figuring out \"d\" is very hard unless you know how \"N\" was chosen. When \"N\" is generated it is very easy to compute \"d\", but if you didn't pick \"N\" yourself, figuring out \"d\" amounts to factoring \"N\", but \"N\" is a giant number (Reddit for example has an N that is ~10",
").",
"So after your exchange, you and the server both know \"a\" but any MITM will only know \"c\", \"e\", \"N\" from which computing either \"a\" or \"d\" would let them see your message, but those calculations are very hard."
] |
[
"The client encrypts it with the public key of the server certificate (assuming it's validated and valid) and sends it.",
"This answers OP's question, doesn't it? A user's web browser / operating system can already be sure that nobody's impersonating the web server, making it easy to prevent trivial MITM attacks. Of course, this doesn't solve the trust problem, it just moves it over to the certificate authorities."
] |
[
"Would keeping large bottles of ice in the freezer save electricity? How much?"
] |
[
false
] |
I heard that keeping solids in the freezer like ice saves electricity because it doesn't escape when you open the freezer door like air does, but I found post on AskScience and I'm not as sure anymore. And even if it does save electricity, is it a significant amount?
|
[
"Seems like it would keep the temperature more constant by increasing the heat capacity of the stuff stored inside, but unless it was taking up a very large volume I think you would still get the same amount of air exchanged and thus total heat being absorbed. So, probably not."
] |
[
"The less air a freezer has to chill every time you open the door, the less energy it'll take to run. Having stuff in the freezer excludes air so there's less air that can escape and less warm air that can get in. ",
"How much energy it saves depends on how big your freezer is, but rounding a lot, a full-sized freezer is about 8 cubic feet (.2 cubic meters), which is about a quarter of a kg of dry air. With the heat capacity of dry air being about 1 kJ/kg*K, and a 10 deg C temperature difference between your house and freezer, this works out to be about 2.5 kJ to cool down the entire freezer full of dry air. In kW-hours that's 7e-4, which works out to be about $0.00008 worth of electricity, at 11.20 cents/kW-hour. For the whole freezer. You could stand there with the door open once every day for a decade and it'll only cost you about 4 cents in electricity to cool the air back down. Note: This is assuming the freezer is 100% efficient in converting electrical energy to cooling energy, but even if it was 1% efficient, the answer would simply be multiplied by 100.",
"So the answer is yes, but don't bother to do it for the energy savings. "
] |
[
"So, If you have a small personal cooler and you put in the freezer, let the air inside the cooler chill to freezing then sit it out, with an open lid at room temperature, how long would the air inside the cooler stay cool?",
"Now, imagine that same cooler with filled with blocks of ice. Doesn't it seem logical that the interior space of that cooler will remain cold for a much longer period of time when exposed to room temperature?",
"Sorta like a battery stores energy, one might say that the denser matter as ice \"stores\" cold. Ok, well not really, that's completely not scientific, but you could sorta think of it that way."
] |
[
"Are galaxies that are moving faster than light due to cosmic expansion going backwards in time?"
] |
[
false
] | null |
[
"I'm going to deduce that the spirit of the question is something like \"does this qualify as FTL travel, and if so, doesn't that mean these galaxies can travel through time?\" This is a good question!",
"The restriction that things cannot travel faster than the speed of light is a ",
" one. That is, locally (essentially \"near me\"; see ",
"here",
" for a discussion of what locality means), nothing with mass can be traveling faster than the speed of light. This restriction is also a global one in the special case that the universe is flat and static (flat in the Euclidean sense, and static in the sense that the geometry does not change in time). Such a spacetime is geometrically described by the ",
"Minkowski metric",
", which is the spacetime of special relativity.",
"Our universe locally looks like Minkowski spacetime, which is why things cannot locally appear to be traveling faster than the speed of light. However, the ",
" geometry of our universe appears to be described by the ",
"FLRW metric",
". Such a metric is not static. It is not necessarily spatially flat either, ",
"though based on the various evidence, our universe appears to be spatially flat",
". Thus, over large scales, the local restrictions do not apply because of the global geometry and global behavior of the universe. As such, from the reference frame of Earth, distant galaxies can appear to be traveling faster than the speed of light. This doesn't violate any local physics however, because those galaxies are locally behaving in a causal way.",
"I hope this helps!"
] |
[
"Slightly less technical response: the real restriction is on ",
" propagating faster than light. So if I can send a message from point A to point B faster than light can travel, then there's the possibility to wreak real havoc with cause and effect. But the expansion of the Universe can't be used to send information. Everything recedes from everything else. You can't use a distant galaxy to communicate a faster-than-light signal, because you'd first have to get the message to that galaxy, and that you can only do at speeds below the speed of light."
] |
[
"Well, I'm not sure that qualifies as an explanation so much as a statement of fact. My goal was to get at why an expanding universe implies that FTL travel is not occurring."
] |
[
"Why does my bathroom door randomly close out of nowhere? Every day the door is cracked a bit and then out of nowhere it'll SLAM closed. Any ideas?"
] |
[
false
] | null |
[
"Hi Theethanlong thank you for submitting to ",
"/r/Askscience",
".",
" Please add flair to your post. ",
"Your post will be removed permanently if flair is not added within one hour. You can flair this post by replying to this message with your flair choice. It must be an exact match to one of the following flair categories and contain no other text:",
"'Computing', 'Economics', 'Human Body', 'Engineering', 'Planetary Sci.', 'Archaeology', 'Neuroscience', 'Biology', 'Chemistry', 'Medicine', 'Linguistics', 'Mathematics', 'Astronomy', 'Psychology', 'Paleontology', 'Political Science', 'Social Science', 'Earth Sciences', 'Anthropology', 'Physics'",
"Your post is not yet visible on the forum and is awaiting review from the moderator team. Your question may be denied for the following reasons, ",
"/r/AskScienceDiscussion",
"There are more restrictions on what kind of questions are suitable for ",
"/r/AskScience",
", the above are just some of the most common. While you wait, check out the forum \n",
" on asking questions as well as our ",
". Please wait several hours before messaging us if there is an issue, moderator mail concerning recent submissions will be ignored.",
" ",
" "
] |
[
"There is a vent to the outside (usually a fan in the ceiling) in most bathrooms. If the door is open a small amount and the air pressure changes quickly (from other parts of the house, or wind outside), it'll suck or blow through the crack between the door and the frame and slam it shut (or blow it open wider, depending on the direction of pressure change). "
] |
[
"I figured it out hahah. Thanks though!"
] |
[
"Why do we never forget how to ride a bicycle?"
] |
[
false
] |
People say this all the time when someone is about to do something for the first time in a while. Is it true that we never forget how to ride a bike, and why?
|
[
"I don't know anything about human learning, but I have read about bicycles. Once you get it above a certain speed, a well-designed bike is stable in its upright position. It doesn't want to fall over; it practically rides itself. Unfortunately, there isn't a simple explanation for why bikes are stable. The easy answers you'll sometimes hear have been contradicted by experiment: see ",
"this recent article in ",
". It's the first reference cited by ",
"Wikipedia:Bicycle and motorcycle dynamics",
".",
"(Given the physics, I would go on to speculate that it takes surprisingly little skill to ride a bike in the first place. The important thing is to have enough confidence to power it up to a speed at which its own stability takes over.)"
] |
[
"Did you really just disagree with him that he did not forget to ride a bike? There will always be exceptions. Don't be ridiculous. "
] |
[
"I have to disagree.",
"COME ON, MAN"
] |
[
"In a discrete universe, what is the force exerted on an particle?"
] |
[
false
] |
Here is the situation, imagine a discrete universe. Two particles are in space. Particle A has a mass of 3kg and velocity of 5 meters a second in a straight line. Particle B has an unknown mass and velocity. Particle A and B collide. Since this is a discrete universe, the smallest unit of time is 1 second, so the collision exists for only 1 second. What is the force exerted by particle A on B? Since Force = Mass * Acceleration. Do I simply assume the acceleration is whatever acceleration it takes to drop particle A to a velocity of 0 in the minimum time unit? [Edit]Sorry just realised the title is very ambiguous :/ it should say "In a discrete universe, what is the force exerted on a particle during collision with another particle.[/Edit]
|
[
"Well wouldn't dp/dt be Δp/Δt ?",
"It depends on what you take to be an axiom. ",
"Do you accept ",
"Hamilton's principle of least action",
"? ",
"My math is rusty on this, when you apply this principle to a discrete world, does it come out with a discrete equation of motion? "
] |
[
"Yes it would. ",
"I think the least action principal is a bit too complicated for this problem. I think it's an exercise in Newtonian dynamics."
] |
[
"Exactly, all you need is to go from continuous exchange of momentum to discrete exchange. ",
"Although it really isn't easier in this case you could also do this via action principles etc. In that case the action integral would become a sum, and indeed the equation of motion would change from a differential equation to a difference equation."
] |
[
"If I take a two dimensional object and rotate it through the third dimension, it appears that the object has become a mirror image of itself to a two dimensional observer. Is there an equivalent to reflect a 3D object via rotation in the fourth dimension?"
] |
[
false
] | null |
[
"Yes. In every 3d rotation, there is a fixed plane (the rotation plane) and a fixed line, the axis of rotation. This decomposition in rotation is required for galaxys, solar systems and atoms exist.",
"In 4d we don't have this asymmetric decomposition. Instead, every 4d rotation has two fixed planes. Solar systems and atoms can't exist in 4d because that fixed line in 3D becomes a fixed plane. We can try find a 4d rotation that produces a 3d reflection keeping this decomposition in mind.",
"So let's try to reflect an object across the YZ plane and through the X axis. Let's have W be a fourth orthogonal axis. In a reflection, the plane of reflection is fixed, so that means that the YZ plane is one of our planes of rotation. But nothing happens to points on the reflection plane, so in 4d this has to be a zero degree rotation. But we still have the XW plane to work with. Since an object is moved to the other side of the origin, this suggests that we do a rotation through the angle pi (a 180 degree rotation) in the XW plane. It turns out that when you look at where a 3d object lands after a 180 degree rotation through the XW plane, while having a zero degree rotation about the YZ plane, you do get a reflection.",
"In fact, any N-dimensional reflection can be seen as a N+1 dimensional rotation."
] |
[
"Right. It's even weirder than that though, as every molecule of their body would be an enantiomer of their pre-4d-rotation self!",
"https://en.m.wikipedia.org/wiki/Enantiomer"
] |
[
"I'm struggling to visualize this. Can you describe what it would look like if an object on my desk went through this?"
] |
[
"Is the sun at the same focus for all elliptical orbits?"
] |
[
false
] |
[deleted]
|
[
"Since an ellipse is symmetrical, there's no way to tell the difference between the two foci. If I rotate an ellipse 180 degrees, the \"left\" focus becomes the \"right\" one, but nothing has changed."
] |
[
"I think you misunderstood the question. If you draw two ellipses with a single shared focus there is a difference. After pinning one down the angle between the two semi-major axes can be anything at all. I think you can rightly call it \"the same focus\" if the angle is less than 90 degrees",
"Doesn't the direction of a semi-major axis change with time? I would say that a definition of \"the same focus\" that flip-flops depending on how long you watch the system is not a good one. "
] |
[
"Though precession might cause the same problem",
"It sounds like apsidal precession is exactly what ",
"/u/Seraph062",
" is talking about.",
"For nonexperts: while the orientation of the long axis of the ellipse remains fixed in a simple textbook orbit, the gravitational pull from other planets and general relativity(!) cause the long axis to rotate over time. This is called \"apsidal precession\", and for Earth's orbit, it takes about 112,000 years.",
"Totally agree with ",
"/u/Seraph062",
" that even ",
"/u/Para199x",
" 's loose definition of \"the same focus\" doesn't work because the orbits' orientations change over time.",
"https://en.wikipedia.org/wiki/Milankovitch_cycles#Apsidal_precession"
] |
[
"AskScience Open House [meta]"
] |
[
false
] |
The time is ripe to look back and see how things are going for AskScience, and to look forward and see how we want things to go in the future. Here's your opportunity to voice your opinions on things going on in AskScience, things affecting AskScience, and things that AskScience affects. Please bring up anything you want - we're here to listen. We're interested in hearing what have to say. In the comments, we'll also share our own opinions, we'll explain what our current policies are with regards to any issues, our motivations for them, and how they are implemented. Meanwhile, we hope to learn more about how all this is perceived by our readers and the panelists. The purpose is just as a community health checkup, and to hopefully spawn some ideas for how we can serve our community better. Thanks for contributing! p.s. One concern I would like to nip in the bud is our overactive spam filter. It creates a lot of extra work for us, and we don't have control over it, and we don't like it any more than you do. The best thing for you to do is to check /new when making a post, and then let us know right away that the spam monster got it (provide a link!). Thanks! p.p.s. Oh yes, here are the .
|
[
"Brain Doc already said this, but I'd like to reiterate it. ",
" We do not control this. If your post doesn't show up, it's not because we're science Hitler. Message us with the link and we can fix it. Don't go on a tirade because the automatic bot automatically botted you.",
"For those of you who are new to askscience, please note our sacred rule: ",
" Don't make shit up, don't say \"wow what an interesting question!,\" just wait for somebody who does know to show up. You can, of course, ask followup questions.",
"Other than that, keep being awesome."
] |
[
"People always thank the mods, but remember that AskScience would be nothing without the community at large! So, thank you!"
] |
[
"I just want to say thank you for putting your time into one of the, if not the, most helpful and interesting subreddits :)"
] |
[
"Why doesn't fast food spoil, and should I care?"
] |
[
false
] |
Hi, I saw yet another "look, these hamburgers from McDonald's that I bought 2 years ago and left on the counter haven't spoiled or degraded!" post making the rounds on facebook. Basically, it's something like: The obvious subtext is "if nothing else will eat it, neither should you." Being a skeptical kind of guy, I am wondering if somebody can shed some light on this; if the unspoken statements (the food didn't spoil, things that don't spoil are bad for you, etc.) are true. Obviously I don't think fast food is , but being high in fat and cholesterol etc. is different than being inedible. 1) Is it true that it didn't spoil or is it that those particular items spoiled in a way that doesn't look obvious? I am guessing the latter. 2) Is it exceptional for something edible to not spoil? e.g. a pile of salt doesn't get eaten by anything either. A dark, unsweetened chocolate bar will probably not get eaten by anything, and although the oils inside may go rancid, it (I think, anyway) probably will look exactly the same after a few years. 3) Would it have gotten eaten by stuff, but it's just that all the things that would eat it are de facto prevented from eating it because the insides of houses don't have meat-eating wasps, or rats, and the dog and cat were prevented from eating it, etc.? 4) If in fact these items did not "spoil", what is it about them that makes it so? I haven't done the experiment myself, but it is vaguely plausible that taking fresh ground beef, making a patty, and cooking my own burger super well done, would result in something that would not look obviously spoiled after a year as well as long as I kept dogs/cats/rats away from it for the first month. The bun, however - afaik bread molds fairly quickly. Why doesn't their bun mold? 5) Finally, if it is the case that something was super unnatural and added to the food to make it not spoil (the preservative to make the bun not mold), does that actually matter to my health if I eat it? What is the mechanism that makes mold and bacteria grow on a starch/bread but is still safe for me to eat?
|
[
"The problem is the cited experiment is bad science.",
"The fast food in the example doesn't \"spoil\" because it dehydrates. If you do the same test but put the McDonalds in a warm, moist, dark place then it rots just as well as anything else.",
"Here's an at-home experiment that actually tries to control several variables."
] |
[
"this same science also works for a variety of non fast food products"
] |
[
"Yeah, people who post this stuff clearly haven't thought a bit about it. When you consider that the fries and burgers are ",
" at a ",
", there will be no to almost none bacteria or other microorganisms. If put in a sealed environment, it will not decompose because there isn't anything to decompose it. If you supercook your patty and carefully manipulate it in order to avoid contamination and put it in a sealed jar, it won't decompose just like the McDonald's hamburgers.",
"Of course, given a lot of time, it will start to look pretty bad because there is no such thing as \"completely clean\" and some decomposition reactions do not require microorganisms, like the oxidation of oils, that requires only oxygen (and the oil). You don't really need any advanced education, just high school chemistry and biology to critically analize stuff like this, but I guess that expecting this from facebook is just too much.",
"Take honey, for example. It is regarded as a healthy product (when consumed in moderation). I haven't tested it, but I'm pretty sure it doesn't get spoiled, because it is just so high in sugar that nothing will ever live there! Any simple organisms will get their water removed by osmosis in a second. As a side note, it is common to apply sugar paste to horses that are wounded to heal the cuts, as it prevents infections.",
"I won't enter in details about food preservatives, but I'm pretty sure that eating them isn't extremely harmful, as long as you don't ingest a lot (like anything, really!).",
"In fact, if you think about it, hamburgers not getting spoilt are a sign that they were made in a clean and hygienic environment. If you buy a hamburger from a suspicious place and it gets spoilt in just a few days, you can be pretty sure the chef had a cold and coughed on the patty."
] |
[
"Why doesn't your body heal cavities in your teeth?"
] |
[
false
] |
I was just wondering why ones body can't heal cavities in teeth.
|
[
"Dr. Weston Price",
" (who founded the American Dental Association's research section) is best known for having documented the oral health of indigenous populations, and compared/contrasted these populations based on what they ate. The result is his book, ",
"\"Nutrition and Physical Degeneration.\"",
" He was able to look at these peoples before, during, and immediately after the introduction of \"Western\" foods, containing lots of starch and sugar. It really is an amazing book.",
"Anyway- he found that, with the right diet (and vitamins- primarily vitamin D), teeth can heal. See ",
"figure 97.",
"Note that populations such as the Inuit would often chew their teeth flat, right down to the pulp- apparently without pain. (This was also observed with Arizona Native Americans that ate \"sand-food,\" a root parasite.)",
"Careful examination of archaeological specimens shows that dental caries appear upon introduction of carbohydrates- which are found in abundance in vegetable matter, but not in animal matter. Of the societies that ate primarily meat and fish (the Inuit, the Inupiat, the Maasai, the traditional Icelandic, the Plains Indians, many Pacific Islanders, the Maori, Aborigines, etc.), the dentition is ",
" with very few caries. Those populations that were introduced to Western foods (flour, canned fruits, jellies, jams, chocolate, etc.) were found to have their oral health deteriorate quickly, sometimes within years. (See \"When the Eskimo Comes to Town,\" and pretty much everything in Price's book.)",
"So, it would seem clear that, under the correct circumstances, human teeth ",
" heal themselves. However, the reliance upon a high-carbohydrate diet (probably in conjunction with insufficient exposure to sunlight, thus reducing vitamin D), insufficient vitamin A, and Price's ",
"Activator X",
" are all working against this in modern living."
] |
[
"Very interesting! Thanks for the links and the future reading material!"
] |
[
"Sucrose --> lactic acid.",
" Presumably other sugars and carbohydrates will work as well; lactic acid is a metabolic endpoint for a bunch of organisms.",
"Fat, interestingly, opposes this sort of thing; a combination of low sugars and relatively high-fat diets are probably why skulls recovered from pre-agricultural civilizations show lots of wear, but few caries (cavities). Similarly, it is well-documented (having occurred in the 1950s) how the introduction of sugar (in the form of canned fruits, jams, etc.) decimated the oral health of the Inuit. Once the pathways into the Arctic allowed construction for the Distant Early Warning Line, the foods accompanying Westerners were accepted by the Eskimo (a pejorative term- Inuit or Inupiat, depending upon the tribe, is generally better accepted). ",
"When the Eskimo Comes to Town",
" documents this very nicely."
] |
[
"Is there any reaction in the physical world caused by something else that is truly 'instant'?"
] |
[
false
] |
[deleted]
|
[
"Physics student here. The only thing that comes to mind that can be described as truly instantaneous would be the phenomenon of quantum entanglement, in which the quantum states of two (or more) objects depend on each other. Therefore, if two particles are entangled, and one is measured to have \"up\" spin, the other entangled particle will always have \"down\" spin, so it would not be incorrect to say that measuring the state of the first particle affects the second particle's state instantly. ",
"Wikipedia ",
"here",
" , quantum mechanics is super interesting, mind bending stuff. "
] |
[
"As you mention, it is convenient within some context to consider certain ohenomena to be happening instantaneously - but this is always an approximation rather than 'reality'. In general, when building a qiantitative model it is best to do what is most convenient; in this way the useability of your model will be increased.",
"We know from the energy-time uncertainty principle that the smaller a time interval the harder it is to distinguish between two events (assuming that 'events' are characterised by some change of state), the only exception being when there is no change at all.",
"You can also consider the situation in terms of the underlying mathematics. If you have some function F that describes the state of the system and its time derivative dF/dt , what happens when you have a truly instantaneous change? Answer: the derivative diverges at a single point; this discontinuity is in general problematic, to it is best to assume that there exists a mechanism that can explain what occurs during small time intervals.",
"It is also worth mentioning that in relativity time (and space) intervals are not invariant (they are ",
"Lorentz covariant",
"). Remember that light, for example, travels along lines of zero time; so if you wish to imagine the 'point of view' of a light ray, all of space in the direction of motion is sqeezed into a point and all of time passes in an 'instant'.",
"There are more formal points that can be raised, for example if you have discrete time, then the smallest interval possible is the discrete time unit; this has practical significance, as, for example, most simulations use discretised time steps.",
"But in general, 'instantaneous' changes are normally introduced so that the finer temporal structure, whatever it may be, doesn't have to enter into a given model.",
"I will not talk about quantum measurement and entanglement, as I believe these phenomena have to do with QM fundamentals and are not fully understood."
] |
[
"I don't have flair, but I know for a fact that it is possible to entangle any number of particles (at least in principle). The entanglement of the three particles, for example, is the cornerstone of the ",
"GZHM experiments",
" which aim to show extremely stark contrast between quantum mechanics and classical hidden variables theories."
] |
[
"Can a black hole ever reverse itself; or can a singularity become normal matter again?"
] |
[
false
] |
Can a black hole ever lose enough energy through energy emission (Hawking radiation or the like) that it become unable to maintain the gravitational force needed to hold itself together? As always, forgive my username in this subreddit.
|
[
"No. Once it's a black hole, it remains a black hole. Hawking radiation ",
" its transformation into \"regular\" matter. Over time it evaporates away to nothing, in the process releasing all of these various \"normal\" particles. "
] |
[
"It's a hot-cold thing. The mass (energy) of a black hole is proportional to the radius of the black hole's event horizon. The entropy is proportional to the surface area of the event horizon, which is in turn, proportional to the ",
" of the radius. So since temperature is internal energy over entropy, for larger radii, the temperature of the black hole is colder. Even right now, most black holes are colder than the CMB radiation (~2.745 kelvin), so black holes take in more energy than they emit through radiation now (in general). ",
"In the future, as the universe continues to expand and cool, the temperature of space (functionally the CMB radiation) continues to drop. Eventually it becomes colder than the black hole, at which point the black hole emits more Hawking radiation than it absorbs from background. This is generally true for all black holes (I'm unaware of any exceptions)"
] |
[
"When a particle is emitted through Hawking radiation it is emitted with sufficient momentum to escape the black hole. Otherwise, it would, as you note, fall back into the hole and not really be \"radiation\" ",
". So imagine you're a short distance outside of the event horizon and you gain (through a collision or a rocket or whatever) sufficient momentum to escape the black hole. You will then be free to do so. That's what's happening for these Hawking radiation particles"
] |
[
"How did natural selection cause the ability to fly?"
] |
[
false
] |
I don't have a hard time envisioning all the intermediate beneficial traits that took us from tree swinging to sentience but how did certain simple animals find their way from simple locomotion to something so complex and relatively scarce as flight?
|
[
"Flight is not scarce. Quite a lot of insects fly, bats fly, most species of birds fly, pterosaurs flew. Then there are gliders like flying squirrels and flying snakes and lizards.",
"There's a lot of conjecture about how flying evolved. Here's a ",
"link about vertebrate flight",
".",
"edit: Article about ",
"insect flight evolution",
".",
"Usually people figure flight evolved from animals that benefited from proto-wings for gliding, or benefited from proto-wings when leaping after prey or away from predators."
] |
[
"\"relatively\" there are many more crawlers and swimmers than flyers I would think.",
"Yes, but the point is that there are no less than four independent evolutionary origins of flight in the history of life: mammals, birds, pterosaurs, and insects. Further, it happened rather differently each time. Bats have massively expanded the mammalian hand in order to do it, birds turned the whole damn arm into a wing, pterosaur wings were basically just one big long finger with lots of webbing attached to it, and insects apparently evolved wings from an entirely different set of appendages, and we're not even sure what it was.",
"So the point is that it doesn't necessarily look like it's all that difficult to evolve flight, and there are at least four different ways to do it, so I'd be surprised if there aren't more.",
"You're right that it's not obvious how flight might have evolved. The intermediate steps seem a little obscured in some cases. However, gliding has evolved more times than flying is, and so it's not difficult to image that flying organisms could evolve from gliding organisms by modifying appendages to help glide farther, until eventually they were capable of flight."
] |
[
"Instead of looking at flying as the end of a chain devoted towards flight, look at it as gliding with style. Creatures could potentiality start in trees, eating insects. Eventually they get more and more accustomed to living in trees till its pretty much suicide to be on the ground for any amount of time. So a few random mutations later, there's a group of creatures that have skin attached to their arms that allows them to glide to the next tree. These creatures survive longer, and reproduce more as they don't need to be on the ground as often anymore. Eventually there's pressure on them to be able to fan out more, and the males to have their own tree to attract a mate. Some random mutations later, males that can glide further and avoid being on trees that groups of males are fighting over get more mates, and have more children. Soon females start selecting for males that have better flight characteristics, as that allows them to select better trees without fighting as much, and their offspring do the same, till they evolved proper wings.",
"Now I realize this isn't 100% accurate, but it does give an idea of why flight would be selected for."
] |
[
"how can we know the half-life of isotopes with extremely long halflives"
] |
[
false
] |
how can we know the half-life isotopes like tellurium-128(wikipedia says its about 2.2 10 about a septillion years) when the universe isnt even that old.
|
[
"If you have a large enough collection of slowly decaying atoms, some will be decaying at any given time and you can measure the activity of this decay and extrapolate to find the half-life. However, in the case of Tl-128, it looks like the number is based on ratios of daughter products found in old rocks, according to ",
"this abstract",
". I don't know too much about geological techniques."
] |
[
"https://hps.org/publicinformation/ate/q8270.html",
"In the case of 238U and some other long-lived radionuclides, one approach that has been used is to separate a pure sample of the radionuclide in a known chemical form, weigh the sample, and then measure the activity, A (disintegration rate). The half-life is then determined from the fundamental definition of activity as the product of the radionuclide decay constant, ?, and the number of radioactive atoms present, N.",
"It is also possible to make theoretical estimations of some radionuclide half-lives using a quantum-mechanical approach described as the Geiger-Nuttal Law, which provides a means for estimating the decay constant associated with alpha decay. "
] |
[
"The approach used, as far as I can tell, is to measure the ratio of Tl-128 to it's daughter elements in old geological samples of a known age determined through other geochronological means such as U-Pb, and infer the rate of decay from that. ",
"For this to work, you need extremely stable mineral material, such as zircon crystals, which have the highest possible chance of behaving as closed systems and retaining all of the residual Tl-128 and daughter elements."
] |
[
"What happens if a mutation creates a codon that doesn't translate into an amino acid or stop codon?"
] |
[
false
] | null |
[
"From my understanding as a biology student, every possible combination of three bases codes for either an amino acid or a stop codon. Take a look at ",
"this chart",
". It shows what every possible combination codes for."
] |
[
"Research over the past few years has suggested that these non-coding regions play a role in gene expression and regulation. "
] |
[
"This is true. The four ",
"Nitrogenous Bases",
" that make up DNA arranged in any order code for an amino acid, or a stop codon. This includes the start codon AUG (see above chart), which codes for methionine.",
"Let me know if this makes sense!"
] |
[
"Is it even feasible to terraform mars without a magnetic field?"
] |
[
false
] |
I hear a lot about terraforming mars and just watched a video about how it would be easier to do it with the moon. But they seem to be leaving out one glaring problem as far as I know. You need a magnetic field so solar winds don't blow the atmosphere away. Without that I don't know why these discussions even exist.
|
[
" If you’re just joining us, read ",
"this",
" comment within this thread for a comprehensive answer.",
"This is a common question, and a common one to which ",
"/u/astromike23",
" provides a comprehensive answer. If they want to join in and provide more context, they're welcome, but I'll also spare them the effort and point out that ultimately, this is a common misconception. In detail intrinsic magnetic fields are not as crucial to the preservation of planetary atmospheres as is commonly assumed. This is well explained in ",
"Gunnell, et al., 2018",
". With reference to the Gunnell paper and borrowed from one of ",
"/u/astromike23",
" answers on this: ",
"The basic premise is that terrestrial planets with magnetic fields lose their atmospheres faster than those without magnetic fields. While magnetic fields do block the solar wind, they also create a polar wind: open field lines near the planet's poles give atmospheric ions in the ionosphere a free ride out to space. Earth loses many tons of oxygen every day due to the polar wind, but thankfully our planet's mass is large enough to prevent too much escape. Until you get to Jupiter-strength magnetic fields that have very few open field lines, the polar wind will generally produce more atmospheric loss than the solar wind.",
"Additionally, if you look at the loss rate and estimated history of the loss of Martian atmosphere (e.g., the recent review by ",
"Jakosky, 2021",
"), it's important to remember that Mars lost its atmosphere over 100 of millions to billion(s) of years. So, hypothetically assuming we had the ability to rapidly (even if by rapid we meant a few hundred years) add an atmosphere to Mars, it would take an extremely long time for it to escape."
] |
[
"So, hypothetically assuming we had the ability to rapidly (even if by rapid we meant a few hundred years) add an atmosphere to Mars, it would take an extremely long time for it to escape.",
"Yep. Having the tech to add an atmosphere should also make it trivial to ",
" one, even if its loss rate were much higher than it actually is."
] |
[
"It’s kind of like saying “Why build a power grid if light bulbs all eventually burn out?”"
] |
[
"What would happen to a salt-water fish if you threw it into freshwater?"
] |
[
false
] |
Alternatively, can you season a freshwater fish by throwing it into saltwater to die? Ignoring ethical issues and all.
|
[
"Most fish species would die from osmotic shock, however evolution has equipped some to survive this transition. Salmon transit from fresh to salt and back again, and eels from salt to fresh, However many more species, if gradually equilibrated, can live quite well in the other environment. Guppies come to mind as a fresh-water species that can be adapted to seawater.",
"In nature there are many partly salty waters, called brackish, such as the central African lakes that support their own half-way ecology."
] |
[
"The key word here is \"osmotic\".",
"In the case of a freshwater fish in salt water, water would essentially leak out of the fish's tissues until the osmotic charge (or salt concentration as a rough estimate) is equivalent on both sides of the membrane.",
"So the fish receives a one-two punch of losing a lot of water and suffering from a sudden drastic chemical imbalance."
] |
[
"What exactly causes the shock, and why is it so deadly?"
] |
[
"What are some of the newer theories on the nature of consciousness?"
] |
[
false
] |
Any names or links to recent or ongoing studies would be much appreciated.
|
[
"In spite of Postmodern_Pat sentiments, there is a healthy, active and growing literature regarding the phenomenon of consciousness studied from neural perspectives.",
"There is Baars' ",
"Global Workspace",
" model, for instance, as wells as Edelman's concept of ",
"primary consciousness",
", which would go on to evolve into the the ",
"Dynamic Core Hypothesis",
". Here's ",
"a relatively accessible, short paper",
" co-written by both Edelman and Baar's that attempts to reconcile the two.",
"These, however, are only theories of the specific neurobiological processes underlying conscious experience. ",
"Another theory",
" has been put forward by Tononi hypothesizing the specific ",
" aspect of those processes which underlies conscious experience."
] |
[
"Another interesting area of research is Dissociative Identity Disorder (DID), where some people can have more than one consciousness or personality existing in the same brain at the same time. Up until recently, there was little (if any) hard evidence to prove that it even existed, so the very idea of it was controversial. Now, though...",
"A 2012 study used brain scans to show that DID cannot be faked or \"instructed\" into existence for people who do not already have DID, even if they are very \"fantasy-prone\". (",
"Plain English article",
" / ",
"Actual research paper",
")",
"A 2013 study looked at different types of alternate personalities using fMRI, classifying them as \"emotional parts\" and \"apparently normal parts\". They found that the emotional parts had an extremely high subliminal (pre-conscious) reaction to faces, compared to the apparently normal parts which had slightly below-normal reactions - and again, both groups were different from fantasy-prone controls. (",
"Research paper",
")",
"These don't exactly answer the ultimate question of what consciousness is, but they do hint that the answer might not be a simple one."
] |
[
"I'll save you the time. With regard to the 'so-called' hard problem, nothing of any value has come along recently (or perhaps ever). These 'thories' people are citing really just recast the question in fancy new terms. For example, consciousness arises from integrated networks (Koch). Well, ok, but ",
"?"
] |
[
"uncertain re the uncertainty principle"
] |
[
false
] |
Okay, so I'm a determinist...I don't believe in free-will and I think everything has a causal relationship. Everything I've seen leads me to believe as much. However, I've read that the Heisenberg Uncertainty Principle has "disproven" determinism. Naturally, I went to for clarification...and it didn't help. Okay, so I'm reading this and it seems to say that you can't measure X and Y at the same exact time. Like if you're studying a baseball thrown by a pitcher, you can either measure its velocity, or its specific location in space...but measuring one would make it impossible to measure the other at the same exact time with 100% accuracy. That's fine...I can sort of wrap my head around that, but I don't see how that disproves determinism...I just see that as a limitation of our ability to observe certain things due to the way we experience and perceive reality. Oh...okay, so I was wrong. Perhaps it will be clarified then. . what system? I read this and again it seems like it's an issue with our to observe...not something inherently indeterminate in our universe. But the article explicitly states that this is the case...? So if it's not our ability to observe...then what the hell is it?
|
[
"This is a very subtle point, one that I only really got a solid grip on somewhere between my 2nd and 3rd quantum classes, so don't feel bad.",
"First, an explanation of what we're dealing with. In quantum mechanics, as I'm sure you've heard, things are described by wavefunctions. Now, a wavefunction is this ephemeral entity that exists in a many-dimensional mathematical object called a Hilbert space. It has many projections, onto things like position, momentum, energy, and time. As a concrete 2-d analogue, think of a finite diagonal line in Cartesian coordinates. The shadow that it makes on the horizontal axis from a light shining from above is its projection onto that axis; likewise onto the vertical axis due to a light shining from the right. ",
"So we have this system, let's say it's an electron, traveling through the room. Its wavefunction is well-defined, and if we had a wavefunction-measurer, we could theoretically know it exactly. But the wavefunction is really just a mathematical convenience to make our description of quantum mechanics easier; the only properties that are real (and thus measurable) are things like position, momentum, etc., so we want to measure one of those babies.",
"Let's imagine a ",
"Gaussian",
" wavepacket. What this means is that projected onto position space, the shape of the wavefunction as a function of coordinate (we're talking about the coordinate parallel to the electron's direction of travel, the other two aren't relevant) is a Gaussian. It's also a mathematically convenient choice because it turns out that the same will be true for the projection onto momentum space. We'll hit the \"pause\" button on our electron and think about what's happening for a moment.",
"He's sitting there, but he's not really a particle. The square of his wavefunction (for Gaussians, this is conveniently another Gaussian) in any given \"space\", whether it's position or momentum or something else, gives you a probability density function. Focusing on the more intuitive position for a moment, you can think of that in two different ways: right now, it's really the probability density of the electron. It's not that 95% of the time he's within 2 sigma of the peak, because the electron is never localized: it's that 95% of the electron density is within 2 sigma of the peak. The momentum-space wavefunction is a bit harder to wrap your head around - its square is the probability density of the electron having certain momenta. So the electron mostly has a momentum value near the center, but there's some probability density further away, where it has significantly more or less momentum.",
"The difficulty arises because of the relationships between these observable coordinates in the Hilbert space. It turns out that that position and momentum are related in such a way that the position wavefunction and the momentum wavefunction are Fourier transforms of each other. I implicitly mentioned this earlier - a gaussian's fourier transform is a gaussian. Don't worry about how to do Fourier transforms; the property you need to know about them is the following.",
"It turns out (more mathematical facts) that given an infinite set of functions that are orthogonal (the integral of the product of any two over the domain in question is 0), you can construct an arbitrary function. This is the idea behind Taylor series: given some function, you can use an expansion in polynomials to get an arbitrarily close approximation, arbitrarily far from the expansion point (although it might take a whole lot of terms to get there). A fourier expansion does the exact same thing, but with sine waves - the infinite series comes from the fact that you take sin(x), sin(2x), sin(3x), etc. The \"wiggling\" becomes increasingly \"fast\". Just one more note to get back to business: in what we're talking about, the discrete 1, 2, 3 becomes continuous, so you work with sin(kx), varying k between 0 and infinity, and k becomes our new coordinate, with the value at each k determined, you could say, by the amount of wiggliness in the original function at the frequency k. ",
"Ok, back to quantum mechanics. We have our happy little electron with its gaussian position and momentum projections, and we want to measure it. But we can't measure probabilities - as the wiki quote says, in order to get an accurate reading of, say, the position, the position wavefunction needs to be compressed to only lie within the margin of error of the measuring device. In doing this, though, you're changing the underlying wavefunction and thus the momentum wavefunction as well. And it's a property of fourier transforms that \"narrower\" functions have \"wider\" transforms - that is, the more localized a particle becomes in one observable, the less localized it necessarily becomes in the conjugate observable. This is not a problem with our measurement systems; it states that in order to measure, this must happen. ",
"The reason this is a problem for determinism is because we measure things all the time (nothing to do with a human seeing it, but just the easiest example to talk about), and when you do this localization thing to measure a quantity, you don't get something fuzzy with the same maximum point and width as the original wavepacket; you get a random value picked from somewhere where the wavepacket has a nonzero value, weighted according to its square. These truly random occurrences are why you can't take a closed system, even given all the computing power you want, and predict its future state."
] |
[
"I'd like to try to explain the fourier transform with a bit less mathematics here. The essential point I'm trying to explain: ",
"And it's a property of fourier transforms that \"narrower\" functions have \"wider\" transforms",
"The most familiar example of a fourier transform is \"equalizer\"-like visualizations for music. They take a bit of sound, which is basically a function of time, and transform it to a function of frequency. If you have sound of just one frequency (like a single 'note' in 8-bit music), the sound is represented by a perfect sine wave, or equivalently, a single spike on the equalizer graph. Usually, music is a collection of frequencies so there will be more than one spike, even bands of frequencies showing up on the equalizer graph.",
"We have already seen one extreme: a narrow spike in frequency is a (infinite) sine wave in time. The opposite: a narrow spike of sound in time (for example, a gunshot), will show up as a wide range of frequencies.",
"When you apply the fourier transform to position and momentum instead of time and frequency, the same thing holds. So when you have forced the position to a very narrow probability density (by measuring it at a certain position with a certain amount of accuracy) it's probability density in momentum is spread out over a very broad range of momenta. ",
"EDIT: The essential answer to your question is then the passage I've quoted below:",
"when you do this localization thing to measure a quantity, you don't get something fuzzy with the same maximum point and width as the original wavepacket; you get a random value picked from somewhere where the wavepacket has a nonzero value, weighted according to its square."
] |
[
"Just to add a bit of a more experimental way of thinking about this problem. To physically measure the particle (i.e. determine its position) you have to interact with it. It's not like a baseball where you can look and say \"well it's over there!\" With an electron if you hit it with a photon (so you can see it) the electron is going to start to move because of the change in momentum of the recoiling particle. Thus once you measure the electron's position, the electron is suddenly moving in a different manner than it was before (yes this is a bit of a semi-classical picture since if we don't measure it it doesn't have a well defined momentum anyway, it's for illustration). ",
"The key is that you MUST interact with the system in order to measure, and that's a way of thinking about the uncertainty principle without the mathematics. It's the idea that measuring something has an effect that is non-negligible on quantum system, so there are things that you can't measure above a certain accuracy at the same time since measuring one disturbs the other."
] |
[
"Is there a maximum frequency sound that air can support? What happens beyond that point?"
] |
[
false
] | null |
[
"In air, molecules go 68 nm between collisions on average. The speed of sound is about 330 ms",
" Dividing gives 4.8 GHz. That's the absolute limit though; you're not going to see practical transmission anywhere near that high."
] |
[
"The maximum frequency is given by the space between particles. If the speaker was pulsing back and forth smaller than the distance between the particles in front of it, no phonons (waves) would be created. A more dense material (water or metal) will allow for higher frequencies. ",
"At some point you'll be putting in so much energy that your material will boil off just from the vibration. 4.8 gigahertz as a classical example (a speaker) is impossible to produce, we literally couldn't comprehend a speaker moving that quickly.",
"This is why we use crystal oscillators to produce frequencies in the gigahertz range, and signals up to ",
"1 terrahertz",
" are possible in nanotechnology. At 480 kilohertz you can melt pretty much anything using induction. This is quantum mechanics and has nothing to do with phonons. ",
"Like many of you, when I think of frequencies higher than a few kilohertz, I immediately switch to electromagnetic radiation. However, this is not what OPs question is regarding. The question is specifically about phonons. "
] |
[
"In air, molecules go 68 nm between collisions on average.",
"But that's in air ",
". If you would excite the air with high-frequency pressure waves, this may no longer hold. (The same goes for the speed of sound).",
"A few years ago I read a couple of books about sound propagation and if memory serves, in one of them the author gave an upper-bound in the terahertz range -- which surprised me at the time. I am sorry that I can't get anymore specific than that."
] |
[
"Does bronze have any anti biological properties?"
] |
[
false
] |
I have heard that for some reason bronze can kill bacteria/viruses but have never heard of this material being used in hospitals, be it for doorknobs, bedframes or whatever else. Is there any truth in this?
|
[
"Copper has antimicrobial properties",
", so copper door handles, etc., may be useful in certain settings, though I've not personally seen it used that way in hospitals, for whatever reason.",
"Bronze is an alloy made of mostly copper. I've no idea how much the inclusion of other elements affects bronze's antimicrobial properties."
] |
[
"From your interesting link: A 1983 report documenting the beneficial effects of using brass and bronze on doorknobs to prevent the spread of microbes in a hospitals remained largely unnoticed (18)."
] |
[
"There is certainly reason to believe copper has anti-microbial effects on the basis of in vitro testing. However, data examining the ability of copper-containing materials to reduce contact transmission are rather sparse, but there is a nice open access article reviewing what has been published: ",
"doi: 10.3390/antibiotics10030286.",
"Other than relatively little data to confirm an effect on disease transmission, cost is likely to be the main issue. Copper alloys are going to cost much more than other materials currently in use (plastic, which may be disposable, or stainless steel). There is also the potential for over-exposure to copper, which is now recognized to be a key contributor to liver disease in Indian childhood cirrhosis. This was first appreciated in rural India, where milk and water were transported in copper pots and dispensed using brass or copper utensils. ",
"https://doi.org/10.1093/ajcn/67.5.1074S",
" ",
"It has subsequently been observed in other countries."
] |
[
"Where do 2048 bit long prime numbers come from? (e.g. as used in RSA)"
] |
[
false
] |
[deleted]
|
[
"To find a prime of legth 2048, simply generate a random odd 2048-bit integer and test whether it is prime by your primality test of choice, e.g. the Miller-Rabin test. If the test fails, generate another random integer, and repeat until you succeed. ",
"A slightly different approach is this: begin by choosing a random odd integer x, and test whether it is prime. If it isn't, try if x+2 is prime. If this too fails, try x+4, and so on, incrementing by two until you hit a prime number. A downside of this is that some primes are now more likely to be generated than others, but so far no one has been able to exploit this for an attack against RSA.",
"These approaches are fairly efficient, because prime numbers are actually quite common. By the prime number theorem, among the numbers from 1 to N, there are about N/ln(N) primes. In particular, a random 2048-bit number has a chance about 1 in 2048*ln(2) of being prime, which is about 1 in 1420. You can expect to hit a prime after a few thousand random trials at most.",
"For cryptographic applications, it is actually crucial to use a randomly generated prime, and having a strong random generator for the task. Any deterministic prime-generating algorithm, or even a random algorithm with a poor source of randomness, would allow an attacker to generate the same primes that you have generated, and crack your RSA key, or, more plausibly, some other legitimate RSA user might inadvertently generate the same prime number that you have generated, which again would allow an attacker to crack both your key and their key. ",
"This is a very real threat",
".",
"For more details, see Chapter 4 of ",
"Handbook of Applied Cryptography",
"."
] |
[
"To find a prime of legth 2048, simply generate a random odd 2048-bit integer and test whether it is prime by your primality test of choice, e.g. the Miller-Rabin test. If the test fails, generate another random integer, and repeat until you succeed.",
"This is true but it's important to note that the Miller-Rabin test is probabilistic. It cannot tell you a number is prime with absolute certainty. So, if the randomly generated possible prime passes the first round, you redo the test with a different parameter. Again and again."
] |
[
"This method wouldn't really help you generate a 2048-bit prime number, since you effectively need to generate every prime number less than the target magnitude."
] |
[
"How is pH calculated exactly? Wikipedia seems to state that -log[H+] is only an approximation."
] |
[
false
] | null |
[
"The Wikipedia article discusses the definition of pH more thoroughly a bit further down in its current incarnation. Namely, it involves the consideration of ",
"activity",
" and not just concentration.",
"You can also check out the IUPAC Gold Book ",
"entry for pH",
" if you'd prefer a non-Wikipedia source. "
] |
[
"The pH of a solution is calculated approximately by ",
" pH=-log[H] (1)\n",
"where [H] is the concentration of hydrogen ions in moles/liter. If you know the molar concentration then this calculation is a decent approximation.\nAs mentioned, this only a rough approximation. A better way of estimating pH is by taking into account the total ionic strength of the solution and the activity of the solution. The ionic strength is defined as the electronic effects of all ions present in solution, whether or not they contribute to the pH. The total ionic strength of a solution is given by the equation",
" μ= 1/2 ∑〖Z^2 [X_Z ] 〗 (2)\n",
"where μ is the total ionic strength, Z is the charge of a given ion, and X_z is the concentration of ion Z. If you perform this calculation for all ionic species present you get the total ionic strength. This is done because by using equation (1) alone it does not account for all other species, which do effect the ionicity of the pH contributors. ",
"Next, activity is used over concentration (has the same units of mol/L) because it incorporates the ionic strength and a few other factors. Activity is given by the equation",
" -log(γ)=(0.509Z^2 √μ)/(1+〖3.28α〗 √μ) (3)\n",
"where γ is the activity coefficient, Z is the charge of the pH contributing molecule, μ is the ionic strength (which we calculated), and a is the effective diameter of the molecule (which is looked up in a table of values). The activity is related to the activity coefficient by the equation ",
" a = γ[x] (4)\n",
"where a is the activity, γ is the activity coefficient, and [x] is the molar concentration of, say, hydrogen ions.",
"Now that we have activity instead of regular concentration, for a strong acid, i.e. hydrochloric, sulfuric, phosphoric acids etc... you can then substitute activity for concentration into equation (1) and you get a much more accurate calculation.",
"For a weak acid there is another series of steps to follow which I will gladly post if requested.",
"does that better answer your question?"
] |
[
"Someone will want the long answer with equations."
] |
[
"Why can't I remember anything before age 5, but my 3 year old kid can remember things from last year?"
] |
[
false
] |
A local farm has a "fall festival" every year with a corn maze. We went last year, when she was two. This year, we went again. When I suggested going through the corn maze, my daughter systematically recalled some of the sights along the maze, including giant playing cards and a dark room full of disorienting spinning lights. I actually didn't remember any of these details until presented with them again. She also seems to recall details of our old home (we moved last summer). On the other hand, I remember almost nothing about what my house looked like before it was remodeled when I was six, other than what I've seen in photos. What happens to these memories? Why does it seem to be so consistent that most people remember nothing before about age five or six?
|
[
"Childhood amnesia",
". (Not an expert, this is just the answer usually)"
] |
[
"I'll have to find the source, but I do remember an article that somewhat dealt with this. (Might have been a Radiolab episode though.) The major changes the occur in us that allow the retention of memories are advances in defining the world around us. The first big component is language, or at least the ability to clearly label what we are doing and what we are interacting with. Before this, our memories are simply collections of abstract events that we are unable to retrieve in any real manner. The next is an understanding of time. Not the ability to read a clock, or anything as objective as that, but the notion of time between or during events. So, until you have clear labels and a basic ability to temporally stamp events, you have hit or miss memories that are vague and difficult to access. I think that there are significant events that happen when we're younger that sort of buck this trend because so much emotion goes into them, but even those tend to be a bit nebulous.",
"So, as far as your kid goes, I wonder how much she'll remember of those things as the years go by and more concrete memories start to form. "
] |
[
"This sounds like speculation. If you are addressing these questions to an expert you should be clear of your intention, otherwise your response will likely be down voted at lay speculation."
] |
[
"Subscriptions to scientific literature without taking out a loan"
] |
[
false
] |
Does anyone know of scientific journals (for me personally subjects pertaining to nature, geology, astronomy) that will not destroy all my funds. I'm also of course not talking about stuff like National Geographic or Popular Science :). It's not that I'm a cheapass or lack money its just things like this: I'm not about to order a years worth of one journal for $2,483.
|
[
"Well, there's always the ",
"Public Library of Science",
", which is probably the most prestigious open-access journal, or ",
"arXiv",
", but many articles in arXiv are pre-print and thus unreviewed, so be wary.",
"As far as paid journals go, those are intended pretty much solely for institutional subscriptions, since almost anyone with a serious interest in their contents is affiliated with a large academic or industrial institution. Anything cheap enough to be subscribed to by an individual layman is probably not worth your time. If there are particular papers or authors you're interested in, PIs will often have some selected papers of theirs available on their group's page (under their institution's departmental page), and many will be perfectly happy to send papers to an interested party -- you could try just e-mailing them."
] |
[
"You can probably get a membership to a university library. At my alma mater, it's apparently 50 bucks a year."
] |
[
"Thank you, arXiv is a good Astrophysics source it seems (although the rest doesn't strike my fancy). The Public Library of Science doesn't seem to have much of what I want or lack, since I already have full access to PubMed.",
"The thing is of course that I'm old fashioned and love to read actual journals in my hands :), so online sources are never quite as good for me."
] |
[
"What species has the most generations alive at one time?"
] |
[
false
] |
I've heard sea turtles can live something like 150 years, but I don't know how often they reproduce, and I know rabbits reproduce pretty quickly, but what I'm really curious about is what animal/bug/whatever has the most alive generations at a single time. Are there great-great-great-great-great-great-great grandparents out there?
|
[
"Turritopsis Nutricula",
", also.know as the Immortal Jellyfish. Upon reaching 'death', it reverts to polyp stage and goes through life again. "
] |
[
"So theres probably no way to know just how many generations there would be, if that even counts as a generation."
] |
[
"Do we count bacteria?"
] |
[
"Can the equations of fluid dynamics be used to describe/model the flow of electrons?"
] |
[
false
] | null |
[
"In low-Reynolds number flow (slow or constricted flow), fluid circuits can be treated the same way as basic electrical circuits, with pressure instead of voltage, flow rate instead of electrical current, and a fluidic resistance that is analogous to electrical resistance except depends differently on geometry. The analogies generally break down if you move away from this regime."
] |
[
"Add to that the fact that the pressure-voltage analogy only works in horizontal flow with no change in section area. That's because the energy of a fluid can be stored as gravitational and kinetic as well as \"pressure energy\" (which is actually work done at the boundaries). To solve a hydraulic circuit, we have to work out the dissipative or irreversible effects, which have a fair analogy with ohmic resistances in electrical circuits if we restrict it to the laminar regime as you mentioned, ",
" also the reversible transformations, which are the redistribution of the total mechanical energy into pressure, potential energy and kinetic energy according to the changes in height and cross-section area. For those, there is no fair analogy as far as I'm aware."
] |
[
"Almost. The incompressible, Newtonian Navier-Stokes equations for fluid momentum are very similar to the drift-diffusion equations that govern the electron/hole density in materials. One major difference is that the drift-diffusion equations also require solving the Poisson equation for the electric field inside the material."
] |
[
"Just watched \"Sugar: The Bitter Truth\", how much of what is said about the hazards of fructose in this lecture is strongly supported by scientific evidence?"
] |
[
false
] |
So I just got done watching . (Its about 90 minutes long.) While he seems passionate and knowledgeable about the topic, he uses "shock and awe" words, makes causation out of weak correlation, and frequently falls back on conspiracy and anecdotal evidence. So my questions is just how much is what he said actually supported in the scientific community. What information should I take with me when choosing my food and what is just a matter of opinion?
|
[
"http://www.alanaragonblog.com/2010/01/29/the-bitter-truth-about-fructose-alarmism/"
] |
[
"Fructose may not elicit an insulin response but it's byproducts certainly do. ",
"Source: I'm a type I diabetic and also a biochemist. If you need sources I'll pubmed it in the morning."
] |
[
"The rebuttal shifts the argument mostly to the total caloric content, but to be fair, there ",
" a marked difference between fructose and glucose. Whether it's as significant a difference as Lustig wants to have is another question, but I think Alan went a little overboard on dismissing it.",
"Here some key differences to glucose:",
"fructose malabsorption",
"To sum it up: the thought that the chemistry of fructose ",
" be responsible for obesity is both intriguing and plausible. Of course, Lustig can't back it up sufficiently, but neither does Alan point to direct evidence to the contrary."
] |
[
"Heard an 'alarming' statistic for the first time this weekend... can someone please untangle this mess and explain? Water Vapor as Greenhouse Gas?"
] |
[
false
] |
I can't really recall what I was watching that mentioned this. Basically they were referring to water vapor in the atmosphere causing a positive feedback loop and having global temps rise as a result. To be honest, I had never heard of water vapor being described as a greenhouse gas, but after thinking about it a bit, I could see where that thought was coming from. So I decided to research a bit and of course, good old Google sent me to . It appears to be a few years old, so I'm sure the theories have advanced a bit since. I started to read the comments and there is a great back and forth that starts up and I can't really tell who's right or wrong. With all the knowledge here, I hope to be able to walk away from this with a better understanding of the theory.
|
[
"Water is an extremely strong absorber of infrared light. It is the bane of my existence. It is present in almost every infrared spectrum ever measured.",
"So basically warmer planet --> more water evaporation --> more water in the atmosphere--> more infrared light being trapped --> warmer planet.",
"But the planet isn't this simple, for starters more water in the atmosphere should lead to more clouds, clouds are white so they will reflect more light back into space before it reaches the ground.",
"So yes water is a greenhouse gas and a very potent one but I can't comment on how climate models predict water will affect the warming it is probably very complex."
] |
[
"Yes, it's widely accepted that water vapour is a greenhouse gas, and probably the most potent in the atmosphere. ",
"Wikipedia",
" actually gives not too bad a concise summary.",
"You got it right when you say that water vapour is a feedback response to temperature increase caused by carbon dioxide. In fact, if you look at ",
"this chart",
", you can see that water vapour absorbs a high proportion of the radiation that Earth emits outwards (7 micrometers and beyond), much more than carbon dioxide. That being said, we must keep in mind the lifetime of these two greenhouse gases in the atmosphere. Carbon dioxide can stay up there for decades or centuries (we don't know for sure), water vapour has lifetimes of days or weeks.",
"Two modelling studies last year showed that water vapour accounts for about 50% of the greenhouse effect, with clouds taking the second place of 25% and carbon dioxide at 20% (",
"link",
" and ",
"link",
"). However, the second study also interestingly showed that an atmosphere with carbon dioxide suddenly removed (with water vapour as the only primary greenhouse gas) will see temperatures crashing because of water vapour's short-lived timespan in the atmosphere.",
"I apologise for not being able to give you good layman sources/citations. All I have is a huge bunch of research articles!",
"Edit: I think ",
"this Wikipedia article",
" and references therein are worth a look as well."
] |
[
"Yeah, the complexity is what got me. I started to read up on all of this and got a headache :)"
] |
[
"Why is pig tissue used for human transplants instead of monkey tissue?"
] |
[
false
] |
My sister made a snide comment at dinner, "isn't that interesting" implying it was evidence that we did not share a common ancestry with apes/chimps. I'm curious for information on tissue evolution and why pig tissue might be more compatible than our closer relatives.
|
[
"From ",
"the Wikipedia on xenotransplantation",
":",
"Pigs are currently thought to be the best candidates for organ donation. The risk of cross-species disease transmission is decreased because of their increased phylogenetic distance from humans. They are readily available, their organs are anatomically comparable in size, and new infectious agents are less likely since they have been in close contact with humans through domestication for many generations. Current experiments in xenotransplantation most often use pigs as the donor, and baboons as human models."
] |
[
"Tissue engineer here - the main reason for the use of porcine tissue as a tissue source is historically based on the intial experiments and due to tissue availability. ",
"If you are refering to acellular matrices, then the cells and most immunogenic components are removed so the species should in theory have little impact on the acceptace of the material. The materials are all over 80% collagen and these proteins are highly conserved in all mammals. It should be noted that whilst pig is used (for example OaSIS, by Healthpoint) an increasingly wide range of source species are also utilised including human cadaver (alloderm), sheep forestomach (endoform), cow skin and horse pericardium. There are concerns about the disease risk of using cow for example due to prions (think mad cow) which cannot be eliminated during processing.",
"Historical Basis\nThe initial experiments using decellularised extracellular matrix where performed by Stephen Badylak in the late 80s using pig small intestinal submuscosa. The porcine SIS was the first product to be comercialised and hence has the most data.",
"Availability\nTo make these products economically viable, the tissue must be very cheap and therefore to date this has limited tissue sources to human and farmed animals. In terms of the science, chimps would be a perfectly fine source. Development costs, ethical concerns and availability would likely prohibit the commercial sucess of such a product."
] |
[
"The other problem is not compatibility, but rights. There are several well supported movements (from scientific and political standpoints) that are ",
"pushing to grant rights of \"personhood\" to great apes",
".",
"Killing them simply to cut out a heart valve is considerably closer to murder than with pigs.",
"Additionally, a pig is an adult after about a year (and are therefore easy and quick to raise), while chimpanzees ",
"don't hit puberty until 8-10 years old",
"."
] |
[
"At the cellular level, what is happening when we get callouses?"
] |
[
false
] |
I've been rock climbing a lot and I'm getting callouses on my hands. What is different in those cells?
|
[
"Just for more specification. Your stratus corneum is the outer most layer of your epidermis. It is comprised of dead epithelial cells. Your epidermis is composed of 5 layers, the outermost of which is the stratus corneum. The stratus corneum is essentially made up of dead epithelial cells that are keratinized. Friction can cause this layer to become more pronounced. So, essentially your callous is just the build up of the top layer of your skin adapting to the constant friction and becoming more pronounced in a protective fashion."
] |
[
"Further specification/clarification: Skin cells are actively dividing down at the basal layer, where the cells directly touching the basement membrane (a layer of non-cell connective material, we have different kinds of this stuff around all sorts of different organs). So these newly created cells are then being pushed out to the surface as more cells are created by division underneath. As they rise they undergo certain chemical changes at certain levels, which gives rise to the layers of skin, until the outside stratus corneum which are dead keratinized cells that are sort of overlapping each other in layers 5-10 cells thick or more in the case of a callouse (keratin is a structural fibrous protein, also in our hair and nails). So in response to the friction your body starts pushing more cells out in that location and in such a way that they form hardened callouses. "
] |
[
"The friction causes the strateus corneum to develop."
] |
[
"How does the \"I am not a robot\" checkbox know I'm not a robot?"
] |
[
false
] | null |
[
"Mouse position, clicks, scroll, keystrokes(not all), screen swipes in touch screen displays, ip address, os, browser, extensions, resolution, fonts installed, timezone, language and some other stuffs I can't remember from the top of my head are detectable by sites without any permission. Caches and cookies are normally site specific i.e. a site can only fetch and save its own."
] |
[
"The verification is based on many different factors such as-",
"Mouse movements, clicks and keystrokes: Many of the bots work by directly generating a click on a certain position of the screen or requesting a browser to send a click to a certain element of the website (button, link etc); they don't bring the mouse cursor to that location. We on the other hand bring the cursor to the buttons, links etc before clicking them and our response time to do so varies as well. The pattern in which we hover the cursor is random and hard to imitate for bots. An ai system is trained to look for these cues. ",
"Our browser caches, cookies and other information sent through browser request: Google has its trackers on countless websites in the form of google analytics, google ads etc so, it can create a basic idea about the sites you visit. Bots on the otherhand are made to do certain tasks on certain websites. This can be used to detect them as well. ",
"The model checks for many other factors and create a score of how likely are you to be a bot. If the chances are low, it predicts you to be human. If the chances are high, it proceed to show an image based captcha."
] |
[
"Wait, so if i make erratic mouse movements, or take a while to click the box, then i won't have to identify busses and fire hydrants for the machine gods' amusement?"
] |
[
"I read that the fastest way to thaw something is cold running water. Why is this so?"
] |
[
false
] | null |
[
"Water has a very high heat of fusion. That is, the energy to freeze the liquid or melt the solid is quite large in magnitude. So even if the water is cold, it still has quite a bit of ice-melting energy on account of it being a liquid.",
"Why running water? Because by constantly replacing the water there is a steady stream of more energy coming in. Standing water would end up coming to equilibrium with the frozen food and might not quite melt everything.",
"Why not hot water? Because you want to keep your food cold while it is sitting out so it doesn't grow bacteria. Hot water does have more energy to melt the ice, but compared to the heat of fusion the extra energy in the hot water is actually quite small."
] |
[
"Ah, I remember now! This was the reason Alton Brown gives in \"Good Eats\". Thank you!"
] |
[
"Care to share the source? I would suspect hot running water would achieve the same results, but faster."
] |
[
"Does the Doppler effect have any noticeable consequences on wireless Internet connection?"
] |
[
false
] |
Let's say I'm downloading a file and run towards my router at significant speed, will that make the file download faster, or cause errors? Does it matter whether the signal is AM or FM?
|
[
"Yes it can actually have a pretty significant effect on communications. ",
"In particular, if S(f) is the spectrum of your original signal, S(",
"f) will be the spectrum afterwards (PDF).",
" Here ",
" is relatively close",
" to 1, basically 1 - v/c where c is the speed of light and v the velocity of the object. At most the shift will be a few Hz.",
"The shift in Hz is not a problem, in fact ",
"phase lock loop",
" are already usually applied so that the receiver can reap the gains of using coherent demodulators (PDF) ",
"see here (PDF)",
". Phase lock loops help to account for artifacts such as signal drift, and so on. The point being this is not where the problem comes in.",
"The big problem is what occurs with multi-path communications. If your signal has multiple reflections (like, if you are in a room of some sort instead of in an open field). When moving now, there will be a different velocity shift depending on the angle of arrival of the wave. And because of this the signal, being the sum of the different waves, can experience a wide arrange of channels. ",
" This is problematic though as the capacity (maximum amount of bits/second that can be sent over it) basically defaults to the minimum rate achievable over all possible channels simultaneously (see ",
"Theorem 1 (PDF)",
"). ",
" While the variation due only to doppler shift will be small (about 80 Hz for 5 mph over 5GHz), it is just on the edge of what is considered to not be completely trivial.",
"Anytime you transmit a code with a higher rate than the capacity of the channel, an error will occur. As a result a moving source does increase the probability of packet error, with the probability increase depending on the speed and strength of the reflections. ",
"The same principal applies to AM and FM radio. Although both of which are horribly inefficient in terms of transferring information, so it is unlikely that it would have a noticeable impact. To put it in perspective, a WiFi router is trying to transmit at the very edge of comprehension, while AM and FM require the signal to be easily understandable. Adding a little bit more noise to the WiFi signal will move it past the point of comprehension, while for AM and FM the added noise will still result in a large enough SNR for it to not make a difference. ",
"Edit-- ",
"1 - correction due to ",
"/u/devman0",
" ",
"2 - Added later, as was pointed out by ",
"/u/dirtyuncleron69",
" and ",
"/u/dayzstu",
". Crossed out what is not directly attributable to the doppler shift. ",
"Also, while on the edit, for a much more in depth discussion about how we model movement and the impact this has on communication, please see ",
"Wireless Communications by Andrea Goldsmith",
". She is the most prominent authority on the subject. Chapter 3 in particular talks about this exact scenario in realistic environments, and chapter 5 the impact on capacity. "
] |
[
"so, this is shift from reflections being out of phase with the original signal, due to a longer path, and those reflections changing with time due to the source moving.",
"If there were zero reflections, say I'm just in an open field with a router and my phone, would this effect still occur?"
] |
[
"This is not the doppler effect.",
"You know, you are right. That is not the doppler effect. Yeah my bad. But I am not saying other factors, the doppler effect in and of itself still reduces the capacity of a channel in a multi-path environment in a way that does not occur with LOS.",
"The doppler effect in multi-path environments can be modeled as a random time-varying channel. This is primarily because it would be very hard to determine the velocities relative to every individual reflection arriving at the receiver. In essence then, the signal will be as if there was a random perturbation in the frequency. Albeit small, this random perturbation is then a source of noise which generally decrease the capacity of the channel. Thus the combination of doppler and multi-path do in fact work together to degrade the channel. With LOS this is not the case as any constant movement would only invoke a constant doppler shift. Furthermore, the capacity of any arbitrarily varying channel is generally less than the minimum of the capacities of the possible respective channels."
] |
[
"Is it possible there is a super nova we don't see yet within our galaxy, that is close enough to our solar system, that once it appeared would be bright enough to illuminate the night sky for the rest of human existence?"
] |
[
false
] |
This of course assumes that the event happened several hundred thousand years ago, and light from the event has not yet reached Earth. Based on a program I saw on TV the other night, it seems quite plausible, though the program did not attempt to paint this scenario at all.
|
[
"Supernovae don't last very long. The initial burst of energy can be as brief as only a few hours, depending on the type of supernova, (edit: but will often take weeks/months to burn out entirely).",
"So, taking IK Peg as an example, which is only 150LY away, we can do a little maths with known values to figure out how bright it'll be here on earth:",
"IK Peg will be a type 1a supernova, if it does explode, which will have an absolute magnitude of around -19. (Magnitude is a measure of brightness, essentially. Lower values are brighter.) So, we use the formula:",
"Apparent Magnitude = -5 + Absolute Magnitude + 5log(d)",
"...where d is the distance in parsecs. 150LY is about 46 Parsecs, we get:",
"Apparent Magnitude = -5 -19 + 8.31 = -15.69",
"Now, remembering that with magnitudes, lower numbers are brighter, we can compare this to a few things we're familiar with. The maximum brightness of the moon is magnitude -12.94. The sun is around -26.74. It might not be particularly visible during the day, given the presence of the sun, but at night, it'd easily be the brightest thing in the sky.",
"If we examine the case of the second closest potential supernova, Spica; this star will end in a type 2 supernova, due to its composition. At a distance of 260LY, this event will have an apparent magnitude of -12.58, which still places it on-par with the moon in terms of brightness."
] |
[
"For Spica I used a magnitude of -17, since that's the typical absolute magnitude of a type 2. I guess maybe I should've mentioned that detail."
] |
[
"Related:",
"The nearest star that could go SuperNova is 150 light years way, though one at 260 is more likely",
"That would be entirely possible then, though there would be some lead up to the event that we would notice."
] |
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