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[
"How do we know, or why do we think that gravitons are massless?"
] |
[
false
] |
This is the same question as "why do we think graviton have a speed of c". I'm not sure which one makes more sense in this context since they're equivalent.
|
[
"This is a deep question, but the simplest explanation is empirical. We see that gravity works over long ranges, obeying a 1/r",
" law (i.e Gauss's law for local low energy situations), this can only happen if the governing field is massless.",
"A massive particle has drastically reduced influence over distance, the most famous being the W and Z bosons which govern the Weak interaction."
] |
[
"According to ",
"this paper",
", the upper limit on graviton mass is 4.5*10",
" g. This works out to about 2.5*10",
" eV/c",
". For comparison, the photon has an upper limit of mass of 3*10",
" eV/c",
", the sum of the masses of the three flavors of neutrinos is about 0.3 eV/c",
", and the mass of an electron is about 500,000 eV/c",
".",
"I don't know how they figured that out though. I admit I didn't bother to try reading the article, but I don't expect to understand it."
] |
[
"Forces are are carried by bosons, which are fundamental particles. The electromagnetic force is carried by photons, the weak force is carried by the w and z bosons, and the strong force is carried by gluons. If a force carrier is massive, then the force associated with this particle will be short range. Because gravity is long range as far as we can tell, them the force carrier associated with gravity, the graviton, must be massless, or incredibly close to massless."
] |
[
"How are \"one way\" functions used in encrypting passwords possible?"
] |
[
false
] |
My computer-savvy roommate was explaining how some software he makes "salts" user inputted passwords and runs them through an irreversible function and stores the output instead of the actual password. I don't understand how any function can be irreversible however. If you add, you can subtract. If you multiply, you can divide. How can a function possibly be made "one way" or irreversible?
|
[
"In one sense, it's trivially easy to construct functions that literally ",
" be \"reversed\". Consider, for example, the function x ↦ x",
". If I tell you that the output of this function is 4, you can't determine whether the input was 2 or -2. Similarly, if I tell you that the output of x ↦ cos(x) is 1, you don't know which multiple of 2*pi I used as input.",
"Now, that's all well and good, but it's not particularly useful for the purposes under discussion because you want to protect data with this password, so you need to be able to compare the stored value with new inputs. If you use something like the above, then you've actually gotten ",
" secure, because someone who knows the output has a very large number of inputs that will give them access.",
"What you're friend is talking about are actually functions that may, ",
", be inverted (that is, they might be such that a given output corresponds to a unique input), but for which the process of ",
" an input that goes with the output is computationally difficult. For example, if I give you two 400 digit prime numbers, it's very easy to find their product. If, on the other hand, I give you a single 800 digit number and tell you it's the product of two 400 digit primes, you're going to have a very difficult time finding them.",
"[edit]",
"To clarify, because there are several other responses that are similar enough for me to be concerned about the ambiguity in my own, the important thing about a one-way function is that it's difficult (in a suitable sense) to find ",
" input that wll give the same output. This is why something like converting the input to a number and pumping it through a function like x",
" or sin(x) isn't a good idea: there are multiple possible inputs that give the same ouput, and an attacker need only find one of them. If your function really does have a unique input for each output, then the goal is to make it difficult to find ",
". However, if your function sends multiple different inputs to the same output, you want to be sure there's no convenient way to construct ",
" input that will generate the same output. For example, the function that takes every input and gives the output \"1\" is completely irreversible: there is no possible way to determine the input from the output, because every input has the same output. But it certainly doesn't qualify as a one-way function, because there are many inputs that will give the same output."
] |
[
"One-way functions have a very particular meaning in crypto, and it has nothing to do with irreversibility. It has to do with the difficulty of reversing the function. The definition we use is that a function f is a one-way function if for all randomized polynomial time (i.e., fast) algorithms A, the probability that f(A(f(x))) = f(x) is negligible (there is a technical definition for this, but just think of it as really small). This means that even though it is ",
" to come up with an input that produces the output that you are given, the odds that you come up with the right answer quickly are really bad. ",
"Its worth adding that we haven't actually proven that one-way functions exist. Pretty much all of crypto depends on them existing, but we aren't actually sure at this point. "
] |
[
"I think people here are generally overcomplicating things. Crypto is a complex subject, but this is a simple question.",
"Take the word \"example\". Translate it to numbers, for instance, a=0, b=1, etc. You get:\n4, 23, 0, 12, 15, 11, 4",
"Now add them up together.\n4 + 23 + 0 + 12 + 15 + 11 + 4 = 69.",
"And that, right there is a one way function. Can you take the result, 69, and get \"example\" back from it? No. ",
"The point of this is that you store the \"69\" in the password database, and somebody who succeeds at looking at it can't tell what the user's password was. The system won't accept \"69\" as the password, you have to give it the unprocessed password, and it then transforms it and compares it to the stored value.",
"Now the problem with this is that an attacker can just precalculate a whole dictionary. Then they have a list where they can look up 69 and find \"example\". So what you do to make that difficult is to add some random junk at the end of the password first:",
"Dictionary attacks are now made difficult as the attacker not only needs a precalculated value for every word in the dictionary, but to do that for every possible salt value too."
] |
[
"Are there astronauts at the ISS at any given time? If not, how is it maintained when no one occupies it?"
] |
[
false
] |
[deleted]
|
[
"The ISS has been continuously occupied since 2000. It's not really designed to be left uncrewed and there's been some concern expressed about the station's condition if that were ever to happen. It should be OK for a short period empty though.",
"If all spacecraft that could take astronauts to the ISS were grounded for long enough, the existing crew would eventually need to depart because of safety limits on how long their return capsule can stay in space. Last year Soyuz, currently the only spacecraft that can do the job, had a launch abort, but subsequent flights went ahead.",
"https://www.google.com/amp/s/www.space.com/amp/42106-empty-space-station-possible-after-soyuz-failure.html"
] |
[
"I think your comment can be quite misleading. It is not gravity that leads to orbital decay, it is drag from the remaining (extremely thin) atmosphere. With just gravity satellites would keep orbiting essentially forever.",
"Graveyard orbits are close to the original orbits, just with sufficient distance to avoid collisions with functional satellites there."
] |
[
"I didn’t know about the Soyuz incident, thanks for the link!"
] |
[
"If dark energy is a repulsive gravitational force, could it be the repulsive portion of the gravitational equation?"
] |
[
false
] |
I could simply be trying to find a relationship that doesn't exist, but: The gravitational equation between two masses is a [(constant) m)]/(r And the Electric field equation is [(constant) q)]/(r The only difference being the electric field equation can repulse and attract, whereas the gravitational equation can only attract.
|
[
"Gravity is NOT described by the simple equation that Newton came up with and that you included in your post; that equation is an approximation that works reasonably well when dealing with situations like the Earth orbiting the Sun.",
"The description from general relativity can be summed up as 16 equations which describe the relationship between the geometry of space-time and the energy/matter content of the universe. Included in the energy/matter content is the pressure. When one works out the equation, it turns out that pressure (which one might think of as something pushing out) actually contribute to the overall attraction. Tension on the other hand (which one thinks of as pulling things) turns out to \"push\" out. (this is all greatly simplified.) It's very counterintuitive but, in all situations where we have been able to perform actual measurements, it has been verified."
] |
[
"It is only the simples Coulomb electrostatic equation and Newton's approximation for gravity that have the similar relations. If one goes beyond those approximations, i.e. using QED for electromagnetic attraction and G.R. for gravitional one, the mathematical relations are completely different."
] |
[
"As ",
"/u/aroberge",
" noted, the equations describing gravity are more complicated than Newton's equation, the simple GmM/r",
" that you learn in high school. That equation is just an approximation to Einstein's equations describing how mass and energy curve the geometry of spacetime.",
", because Einstein's equations do reduce to Newton's equation in certain simple cases, we can still write down a version of Newton's equation taking into account our current best model of dark energy, called a cosmological constant.",
"The reason GmM/r",
" is never repulsive, while the equation for electricity can be, is because masses are always positive, but charges can be negative. But a cosmological constant adds in a new component to the gravitational force law which becomes repulsive at large distances. It looks like this:",
"F = -GmM/r",
" + Λr/3,",
"where Λ is a constant (the cosmological constant).",
"So when r is small, you can ignore the Λr/3 term, and you get the usual Newtonian force law. Because it's negative, the force is attractive.",
"But at large distances, the Λr/3 term grows and the 1/r",
" term shrinks, and eventually F switches from negative to positive, so the gravitational force becomes repulsive. This would mean that very distant objects repel each other rather than attracting, and the further apart they are, the more they repel. This sort of behavior might well be responsible for the accelerating expansion of the universe."
] |
[
"Why do worms cross the sidewalk when it rains?"
] |
[
false
] |
I have always viewed it similar to the Hajj, so I am very careful not to step on them. What are they really doing though?
|
[
"Thank you! As a secondary question, if they \"can survive for several weeks under water,\" is there a reason why I find so many dead in puddles after a rain? They appear to be waterlogged/drowned to the untrained eye. ",
"Thanks again. "
] |
[
"Thank you! As a secondary question, if they \"can survive for several weeks under water,\" is there a reason why I find so many dead in puddles after a rain? They appear to be waterlogged/drowned to the untrained eye. ",
"Thanks again. "
] |
[
"When they respire, they release carbon dioxide. When the carbon dioxide reacts with water, it forms ",
"carbonic acid",
", which they don't like much. At that point they move out of their burrows and try to find another place to eat delicious rotting organic material. ",
"As the other poster mentioned, it has nothing to do with oxygen. In almost any situation in a closed area, the carbon dioxide will cause problems for oxygen respirers long before they run out of oxygen. (Think ISS or Submarine)."
] |
[
"How do a bunch of small noises combine to make a very loud noise?"
] |
[
false
] |
[deleted]
|
[
"Read up on constructive/destructive interference in waves. Basically when sounds are the same frequency and phase the peaks lines up and the troughs line up and then add making the amplitude (volume) bigger."
] |
[
"Exactly. If you got two Identical tones and shift their phase by 180 degrees you will hear nothing at all. "
] |
[
"Exactly. If you got two Identical tones and shift their phase by 180 degrees you will hear nothing at all. "
] |
[
"Do antibiotics (and other medications) maintain efficacy long after its expiration date?"
] |
[
false
] | null |
[
"Most medications do; per an FDA study, 90% of medications were still effective after fifteen years. Manufacturers are required to add expiration dates, but it's not clear that they're generally based on hard data. They basically represent a date at which the medicine is no longer guaranteed, not one where it will certainly start to fail. There's always less risk in a short guarantee than a long one, so companies play it safe.",
"Certain medications do have a known limited shelf life, including insulin, nitroglycerin, and epinephrine; liquids are generally less stable than pills. Tetracycline may become toxic long after expiration, although this is based on a single case report of an older formulation."
] |
[
"They just made a pretty big discovery with this - they found a huge cache of old meds and everything worked amazingly well. Within eighty percent effectiveness. ",
"They put the expiration dates on because they haven't studied it for longer periods of time and so can't guarantee the efficacy"
] |
[
"No. Some antibiotics and medicine can be used up to a year after expiration, but it really depends on the pill. They will generally degrade and decline in availability of the active drug after that point. Solutions/liquids (like the ones for kids medicines) are a different story. They expire within weeks of being prepared at the pharmacy and dispensed. That’s why they are reconstituted (prepared) only when the prescription is given. Bottom line, it’s better to get a new prescription if you have a Med that is expired (or ask your pharmacist specifically about the drug you have in question)."
] |
[
"Let's talk Mars colonization: would the reduced gravity environment be as big a danger as microgravity?"
] |
[
false
] |
[deleted]
|
[
"I'd like to point that the current trend of people just posting links with little content of their own is quite annoying, especially if you're browsing on a mobile phone or something else. It would be nice if people at least quoted selected text from the links they are posting. ",
"Otherwise ",
"/r/askscience",
" turns into a glorified google search engine, well done.",
"EDIT: Still some good links though, just that it would be nice if people posted some text in addition to the links."
] |
[
"They are studying it now",
"Here's a brief synopsis of what is already known",
" IANAE "
] |
[
"Have scientists studied the growth during pregnancy of an animal foetus yet in micro/low gravity environments? ",
"I always have the sinking feeling that Humans simply won't ever be able to colonise Mars because our babies just won't work over there. I guess there's always Venus though, much much more similar gravity. "
] |
[
"Do ticks or other blood sucking bugs (mosquitoes, etc) show preference to certain people?"
] |
[
false
] |
I'm currently on a hike and my friends have found about 1-3 ticks each while I've found 17 and an hour later another 15. Is there a reason that ticks seem to 'love' me much more than my friends?
|
[
"Yes.\nBoth ticks and mosquitoes tend to target people by detecting the CO2 they release from their bodies, so perhaps your metabolic rate is higher than average and produces more carbon dioxide. Blood type O is preferred over A and B for them as well. Additionally, it could be that you were walking in front of them and picking up more because you were leading the trail. "
] |
[
"I hear the Type O thing a lot (in South America they called it \"sweet blood\"), but, while I've read a lot of articles talking about I've never come across any ",
" articles confirming it.",
"Mind you, I haven't looked very hard for those either.",
"The CO2, temperature, pheromones, etc is all pretty well and reliably documented though. "
] |
[
"It took a bit of digging, but yes - according to ",
" research, and not just Newsweek or the like.",
"Here is actual evidence",
"."
] |
[
"How accurate can this video be? (Floating along the milky way)"
] |
[
false
] |
This video is 3 dimensional trip through a portion of the milky way and the author states that it is based on data from the Spitzer Space Telescope. Although the level of detail in varying axes makes me question how much is chalked up to artistic license. Then again what do I know.
|
[
"That does look like Spitzer imagery. The artistic bit is they've put some stars on different layers to give a parallax effect. I can't tell from this animation alone if they've used the real stellar distances for the parallax or just invented something to make it look right. It's a bit funky either way because the original data is not from a telescope moving ",
" the Milky Way, but from one sitting in Earth orbit and looking ",
" the Milky Way, so you shouldn't see any parallax like that at all - it's faked to give a motion effect.",
"The background nebulae look \"flat\", as you'd expect from the original 2D image, so I don't think they've added any extra 3D information to those. And the general picture of clumps and bubbles really is actually what the galaxy does look like in the infrared. The colours are fine, because it's infrared data and you have to invent some visible colour scheme to represent the different bands of infrared light detected by Spitzer.",
"You can see a more direct representation of the data here: ",
"https://www.youtube.com/watch?time_continue=1&v=4b3VlzIjYIY",
" - there's a little icon in the corner showing you which direction along through the galaxy it's looking at any point in time."
] |
[
"Thanks! I figured the layered stars were too good to be true but thought it may have been possible to calculate the distances and then map them in such a way to give the effect. "
] |
[
"That is true, but that would still be a bit fakey - you don't get parallax by standing in one spot and spinning around."
] |
[
"Is there a *theoretical* maximum velocity for a mass with a constant thrust traveling through a vacuum?"
] |
[
false
] |
I understand basic relativistic physics and know that as we approach the speed of light, the the effective mass of an object will increase until it will take an infinite amount of energy to keep accelerating that object. That being said, is there an equation to determine at which velocity does an object's increase in mass render the acceleration generated by a constant thrust negligible? Edit: Maybe I should rephrase my question. Is there an equation to help determine at what point a ship should stop producing thrust because the acceleration generated is no longer worth the energy required?
|
[
"You know how for that situation, assuming Newtonian physics, the velocity is equal to the acceleration times time?",
"In relativistic physics, the velocity as a fraction of light speed is equal to the hyperbolic tangent of velocity times time, and that asymptotically approaches one."
] |
[
"Yeah.",
"I should add that this is relative to the rest frame of the object at t=0."
] |
[
"Yeah.",
"I should add that this is relative to the rest frame of the object at t=0."
] |
[
"Human blood cells are made in bone marrow in the centre of some bones. How do the new red and white blood cells actually get from their stem-cell parents into the blood vessels?"
] |
[
false
] |
Do blood vessels enter the bones to both supply nutrients to the marrow (which I guess is mostly stem cells?) and to pick up the newly-created blood cells? And if so how do the new cells get into the blood vessels? Or is there a diffusion process through the bone and nutrients enter and blood cells leave, at the exterior surface of the bones? My mammalian physiology is limited to feeding bones to dogs, and I don't think I've ever noticed any holes for blood vessels? And if anyone makes it down this far, how/where are blood cells made in invertebrates? Thanks
|
[
"Bone is a highly active type of connective tissue. The macroscopic structure of bone belies the fact that on the cellular level, bone is constantly reshaping and remodeling itself in response to the environment. This tissue has several functions, one of which is housing marrow for hematopoesis. ",
"This",
" is a cartoon depicting the hierarchical structure of bone. As you can see, it is highly vascular in both the cortical (thick outer) bone and cancellous (trabecular) bone. Marrow resides in the trabecular portions of the bone which has room for the ",
"penetrating arteries, #8",
" coming off the large, deep femoral (profunda femoris, #7) artery. "
] |
[
"Yes, they do. Blood vessels walls are made up of cells, and there are gaps between these cells that new cells can squeeze through. (The reverse of that is how white blood cells exit blood to provide immune response in tissue.) Some invertebrates don't generate new blood cells during their lifetime; some have a specialized organ that takes care of it. "
] |
[
"Great links. Thanks. I knew that bone could grow in response to new stresses (engineering sense) but had no idea it was so well penetrated by a vascular system."
] |
[
"Why do we start having a running nose when we are crying?"
] |
[
false
] | null |
[
"Because your tears drain through a sort of downspout embedded in your face that empties into your nose. If you look closely at the nose side of your eyelids, you should see a couple of pinholes called puncta. Those are the drains, and behind them are thin tubes called canaliculi, that in turn empty into the lachrymal sac along the side of your nose and finally into the lachrymal duct and on into your nasal passages. ",
"Tears and mucus are generated at the outside edges of your eyelids and are swept across your eyes by your blinks. Your eyelids don't just blink open and shut like a bear trap; they seal from the outside in like closing a ziploc bag, and this action sweeps gunk towards the drainage ducts where if it is thin liquid it will drain into your nose or if it's goopy it will ball up and fall out or removed manually. I'm sure you've had to pick out lumps of partially dried mucus from the inner corners of your eyes when you've been sick or had allergies.",
"You can confirm this by putting eye drops in your eyes and blinking. Your nose will run and if it runs down the back of your throat you should be able to taste the eyedrops too, since your nasal passages are connected to your mouth as well.",
"Some people can perform a sort of Valsalva Maneuver and force air or liquids back up through the drainage ducts.",
"https://www.youtube.com/watch?v=wCID4BLfQbY",
"https://www.youtube.com/watch?v=G_o9KDb_igY",
"This, however, is not recommended."
] |
[
"Ugh, memories of fluorescein drops at lasik post op exams, burned my nose and tasted awful."
] |
[
"Thanks for the information. ",
"Oh yeah. I do remember that. Wow. Never knew why this happened. Glad I do it now. I can give the eye drop test a go and see what I experience. I don’t remember the last time I had used an eye drop. Thank you once again."
] |
[
"Why is Celsius used more commonly in science if Fahrenheit is more precise?"
] |
[
false
] | null |
[
"Fahrenheit is not more precise -- you can convert from one to the other with a simple formula. You can find out the history of different scales with a simple google search"
] |
[
"If Fahrenheit has more increments of measurement how is it not more precise?"
] |
[
"You can convert any number from Celsius to Fahrenheit or vice versa. It's like saying inches are more precise than centimeters. That makes no sense because you can take any number of centimeters and convert it to inches and vice versa. "
] |
[
"Why do my sinuses get cleared momentarily when something scares me?"
] |
[
false
] |
If I have a stuffy nose and something scares me/makes me jump, my sinuses will temporarily open up.
|
[
"http://www.entnet.org/HealthInformation/stuffyNose.cfm",
"Adrenaline causes constriction of the nasal membranes so that the air passages open up and the person breathes freely."
] |
[
"To be fair, it's probably noradrenaline from the sympathetic nervous system rather than adrenaline from the adrenal glands."
] |
[
"To be fair, it's a quote from a website run by the American Academy of Otolaryngology."
] |
[
"By what scientific principals or phenomena do polygraphs operate?"
] |
[
false
] |
Is there any scientific evidence to support modern polygraph techniques? If there is no evidence why do corporations and governments rely on these methods?
|
[
"Polygraphs are not accepted by mainstream science as accurate. Wikipedia has a lengthy section on its lack of acceptance, and it's full of sources. ",
"Check it out",
"."
] |
[
"I believe polygraphs use frequency of heart beats. The thought being that when someones lies or is hiding something, they will get nervous and thusly their heart will beat faster."
] |
[
"Consider the name \"polygraph.\" \"Poly\" means multiple and \"graph\" means \"to measure.\" So it's multiple measures, not just heart beat. Specially a polygraph uses blood pressure, pulse, skin conductivity, and respiration."
] |
[
"Could any element in the carbon group be the basis of life? Why is carbon so special out of these elements?"
] |
[
false
] | null |
[
"Carbon bonding with things also makes for nice soft wobbly things. Silicon bonding with things makes... well, rock, mostly."
] |
[
"A more viable alternative to carbon-group elements might be Boron; it can form single and double bonds relatively easily, and even ",
"stable triple bonds",
" have been produced. It has the benefit of being much lighter than silicon, and forms bonds with Nitrogen that resemble C-C bonds."
] |
[
"So in a solar system light years away could be boron based life forms?"
] |
[
"As matter is made up of mainly empty space, how big would a sphere containing all matter in the solar system be if all particles were as close as possible without occupying the same position?"
] |
[
false
] |
How about the galaxy? Observable universe? As this is obviously hypothetical, I don't care that as my hypothetical sphere approaches a mass of ~3 of our suns, it will collapse in itself and form a black hole. So basically, how big would a neutron star containing our solar system, galaxy and the observable universe be?
|
[
"All the matter in the solar system is basically the sun. Everything else is insignificant. The sun has a Schwarzschild radius of 3 km. For the galaxy it's two trillion km (not sure if that includes dark matter), and for the galaxy it's about 10 billion lightyears."
] |
[
"\"Atoms touching\" isn't a bad way to explain it to laypeople though. I mean, it's ",
" but is a good way to convey the idea of the exclusion principle without having to get all quantum. "
] |
[
"No, it's not. The Schwarzschild radius associated with a given quantity of stuff is the radius of a spherical volume that would contain that stuff if the information density of such volume were the absolute maximum possible.",
"It's far more fundamental than \"atoms touching,\" which is good since that's not really a sensible thing to consider anyway."
] |
[
"Why don't we all inherit immunity to sicknesses?"
] |
[
false
] |
[deleted]
|
[
"That's an interesting question. Ok so basically the answer to your question comes in two different parts based on what I was taught. First it is important to know the method in which our bodies learn immunity to certain sicknesses. Now, there are a lot of different types of cells that play a role in our immune system, but the two we are going to be talking about are B memory cells and T memory cells. These cells basically are formed when our bodies begin to recognize the antigens of whatever may be causing the sickness. This causes these T and B cells to become specific to that one antigen. Most of the T and B cells will die off after you have healed, but some of them stick around and form your immunity. This is because they have already been specified for that particular antigen, which is actually where the whole idea of letting your kids get dirty to prevent allergies and sickness later in life comes from. This is also why getting even mild illnesses when you're older can be so detrimental as your body doesn't produce as many of the \"universal\" T and B cells and pretty much relies on its immunity and past exposures to keep healthy.",
"Now that you understand the basics of how your immunity is formed we can tackle the question of why it isn't passed on to your children. So like I said before, it is these T and B memory cells that basically form your bodies ability to say, \"Hey I recognize these antigens\" and fight off the infection. Well because it is only found in those cells, that means it isn't going to be found in your DNA, which means you won't be able to pass it along to your kids. Even if it was incorporated into the DNA of the cell, only the DNA in your reproductive organs has the chance to be passed on to your children, so that's another barrier. Not to mention immunity would only be able to be passed down through the father because a woman's ovum are developed incredibly early (I forget exactly when but) way before they would be able to incorporate their own immunity into their eggs.",
"Now that probably answers the question for most people, but there are maybe a couple of other people out there that would ask why some of these memory cells wouldn't be able to pass into the baby from the mother through the umbilical cord. That's actually not too hard to understand or explain. The body has a natural defense that prevents a lot of things from being able to go into the unborn child from the mother, one of the biggest being the cells in the mother's immune system. This is actually really important and here's why. We all know about blood types right, and we all know that if you are given the wrong blood type you'll basically have a bad allergic reaction to the blood. Why doesn't that happen if a baby has a different blood type than the mother? It's because of this barrier, and this barrier is then the reason that the mother's immune cells can't give the baby immunity."
] |
[
"The short answer is that the formation immunological memory occurs in somatic cells (more specifically B and T lymphocytes) and ",
" germline cells, so it isn't passed on to the next generation.",
"The long answer involves an understanding of how immunological memory is formed. B and T lymphocytes make up what is known as your adaptive immune system; when these cells are first generated from stem cells in your bone marrow and thymus, they undergo a very unique process in which a part of their DNA is literally reshuffled. The details of this reshuffling are complex (and fascinating!), but suffice it to say that the end result is that each T and B cell ends up with a unique sequence of DNA that encodes a receptor specific to that cell (called, unsurprisingly, the T Cell Receptor (TCR) in T cells, and the B Cell Receptor (BCR) in B cells. Antibodies, for all intents and purposes, are BCRs that aren't physically attached to B cells and that are instead secreted into the bloodstream). Each cell has only one type of TCR or BCR, and each cell's TCR/BCR is different that any other cell.",
"When a virus or bacteria or other nasty foreign thing (aka. pathogen) enters your body, bits of it are taken to your lymph nodes and presented to the T and B cells living there. If the bits of the pathogen stick to a cell's BCR/TCR, that cell becomes activated, and starts making copies of itself. Because each cell has one random BCR/TCR, the first time you encounter any given pathogen, you only have maybe a handful of cells at most in your body that can become activated by it. Once activated, though, those handful of cells can multiply into the tens of millions, and thus are able to fight off the pathogen. After the pathogen is gone, most of the cells die off, but a fraction (maybe 1%-10%) remain... A smaller population, but still orders of magnitude more than the original handful. The next time you see that same pathogen, you now have a bigger pool of cells that recognize it, so you'll mount a faster, bigger response. This is the essence of immunological memory. (I'm skipping over some key details, but this is the general gist of it).",
"This happens to every pathogen you encounter in your lifetime. If your cells tried to encode all that information into your germline DNA, your genome would be impossibly huge and essentially comprised of nothing but TCR and BCR sequence. This is why the system relies on each cell generating a unique, random receptor... You get the benefit of having a (practically) infinite capacity to recognize pathogens, while making sure each cell only has to worry about a single TCR or BCR gene. And this is also why immunological memory isn't passed down to offspring... The memory bank, so to speak, doesn't reside in the DNA but rather it resides in the pool of cells present in your body. Those cells aren't transferred to the baby (indeed, this would be a bad thing, since those cells might treat the baby as \"foreign\" and start responding to it as they would a pathogen), and so immunological memory is not transferred.",
"As a side note, as was pointed out, a mother does transfer antibodies (the soluble forms of her BCRs) to her baby via the umbilical cord and via breast milk. This happens because it takes some time for an infant's immune system to fully mature, and the mother's antibodies provide some degree of passive protection (incidentally, this is why it's not a good idea to be around newborns if you are sick and not the newborn's mother). Those antibodies, however, are just protein, and will eventually degrade once the baby stops breast feeding. They don't provide lasting immunity because the cells that make the antibodies reside in the mother. Their purpose is simply to protect the newborn until it is old enough that it's own immune system can start producing its own memory."
] |
[
"Acquired immunities are not inherited. In some cases (like with HIV and the CCR5 protein), a person has a variant of a gene (ie. a mutation) that interferes with some part of how a pathogen infects its host. In these cases, the person is \"immune\" because the specific disease is blocked from infecting the person. These kind of immunities can be inherited, but they are rare, and essentially random. It is completely different than the kind of immunity you get when you get sick and acquire an immunity against future exposures to that same disease."
] |
[
"Have diseases such as the bubonic plague, smallpox, and measles gotten less deadly over time?"
] |
[
false
] |
I've heard with the Omicron variant, people were mentioning that it has become less deadly, which evolutionary speaking, helps it spread more. I've also heard a similar account for Influenza - that the Spanish Flu became much more 'mild' (relatively speaking) simply because it was more 'fit' if it had a lower mortality rate. I'm wondering how this relates with regards to other diseases. For example, is this a factor in what made the successive black plague outbreaks less deadly (with the first one in the mid-14th century being infamously horrible)? And not to downplay how bad smallpox or measles is today, but was there a time when it was even deadlier, and has since become less deadly? Or do some diseases always stay at some mortality rate? Edit: I should add I'm interested because my "common understanding" of why the black plague became less bad is acquired immunity. But given this recent insight from COVID-19, I'm wondering if this "evolving to less deadly" was relevant for the black plague as well. And as well for other diseases...
|
[
"There’s a widespread misunderstanding that pathogens evolve to become less virulent. It’s not true; there are many counterexamples. This is a very well studied field for over 50 years; by far the major driver of pathogen evolution is transmission efficiency, but that is an immensely complex driver with tons of feedback from the host and environment. ",
"The point about host and environment also confuses the question of whether the pathogen actually is less virulent than its predecessors. With omicron, for example, you’re comparing the virulence of the original virus ",
", with omicron in a population that is very widely immune due to vaccination and prior infection. ",
"Similarly, with measles, you’re comparing the virulence of a virus in a 19th century population — crowded, poorly nourished, lacking vitamins, unsanitary — with a virus in the 20th century, with vitamin supplements, high-protein diets, and so on - even before the vaccine was widespread. ",
"So the general answer is that yes, sometimes pathogens become less virulent; sometimes they don’t; sometimes they look as if they do, even though they don’t. But there’s no general rule for this, it’s a spillover from actual drivers of selection, and external factors."
] |
[
"Thank you for adding some nuance to the discussion. It's essentially become a meme at this point.",
"Also, there isn't really any direct evolutionary pressure for SARS-CoV-2 to become less virulent. In large part due to the fact that most transmission occurs in early stages of infection, while hospitalisation tends to occur from the second week onward, when viral titres are declining, and its entirely possible that by the time someone is suffering from ARDS, that they are PCR negative by that stage. So if a mutation arose that increased fitness through transmissibility or some other method, but as a side effect, also increased the severity of the disease, it would still be a completely plausible candidate to outcompete other potentially milder variants."
] |
[
"this widespread misunderstanding may stem from the even more widespread misunderstanding of how evolution works. people tend to cling to this idea that organisms / pathogens etc are evolving towards a goal"
] |
[
"What is fabric softening?"
] |
[
false
] |
What does fabric softening actually do? I'm interested in both the addition of a softener during washing, and a dryer sheet. Is there something that actually affects clothes by doing this, or is it a marketing gimmick?
|
[
"Fabric softeners are basically a \"greasy\" substance that sticks to the surface of your clothes after the wash, making them feel smoother. When I say \"greasy\" I do not mean like greasy fat, even though the early softeners were actually based on oil (olive, corn or tallow oil).",
"There is a new type of silicone-based softener that literally lubricates the fibres, but most modern fabric softeners are based on quaternary ammonium salts and work better and last longer than the original oil-based softeners. Softeners normally contain antistatic agents too, making synthetic clothes less prone to collect static electricity.",
"So it does actually help and is ",
" a marketing gimmick."
] |
[
"Dryer sheets will never soften anything. No softening atributes. Check P+G or Unilever if you doubt my claim. Dryer sheets are about static cling... nothing more or less.",
"Fabric softeners do work. There are a couple of quats (actives) that are in the game but for the most part it's about fragrance. People are buying a scent... so it's not scientific.",
"I'm an expert on marketing homecare products (15 years) but I've been deleted when commenting about this topic on this sub.",
"Mods - Whatcha think? Worthless contribution or knowledgeable and insightful?"
] |
[
"Question - does this \"lubricant\" build up over time? Does it have any downsides, i.e. cause degradation of fibers, mess up colors, etc.?"
] |
[
"Would the blue visible light seen on top of a mountain top be more stimulating than the light seen everyday at sea-level?"
] |
[
false
] |
My question was conceived after reading about blue light increasing activity in the brain. I'm curious if the change in of visible light at different elevations is significant enough to cause a noticeable change in neural stimulation? Specifically, the difference in elevation from sea-level to a tall mountain peak (let's say 8,000 meters)? Thanks in advance, y'all!
|
[
"Retinal neuroscientist here, although a little late! I like your question and find it interesting. So here's the deal. our retinas detect light and turn it into signals that can be understood by our brain. This, of course, allows us to see and perceive the visual world. But our retina also detects light and sends signals to \"non-image forming\" areas of the brain. These areas receive the information about light in our environment and use that information to drive our daily rhythms (",
"Circadian Rhythm",
")... best example sleep and wakefulness. As it turns out, there are a particular set of cells in our retina that are specifically designated to respond to light and send the information to the brain to inform our circadian rhythms. These cells are the most sensitive to blue light (ie, light at a wavelength of about 470 nm can activate them at a lower intensity than any other \"color\" of light). ",
"But, and here's where the scattering of light comes into play, these cells require a very high intensity of light (think daylight at noon). and as retinal neuro-people, we measure the intensity of light by the number of photons that hit a defined region or area. therefore, scattering would essentially reduce the number of photons that would land in the area, ultimately reducing the intensity. So, while the wavelength may be similar at both locations, the difference in intensity (due to scattering) would cause different activation of these neurons and then result in different brain activity. So really, its simply that the intensity, or number of photons available to activate the retinal cells, is a bit diminished. "
] |
[
"Thank you so much for the detailed answer! I had asked this question before and it went by without much (any) traction. I had read some of the research papers about how different wavelengths of light affected different areas of the brain, but I'm not sure if the heart of my questions was made clear enough.",
"As you touched on scattering in the last paragraph, perhaps I have a misunderstanding of the phenomenon. My current assumption is that there is more of an abundance of low wavelength light at higher elevations. This causes me to wonder if there would be a greater cognitive impact that happens at higher elevations due to this lack of scattering.",
"If I am obfuscating what I'm trying to ask, let me rephrase: would the assumed greater intensity of blue light seen on a mountain top (Everest, K2, etc.) have a noticeable effect on cognitive processes, compared to the daylight (and presumably lower-intensity blue light) seen at sea-level? More abstractly, would simply seeing the sky at 25,000 feet make you feel more alert/active compared to at home?",
"Thank you again for the response, and please feel free to correct any errors in my line of thinking or understanding!"
] |
[
"I'll be honest, I don't know enough about the composition of light at high altitude vs lower altitudes to know whether there is an abundance of short wavelength at either one. Based on the wikipedia page you cited, it seems like the reason we perceive the sky as blue is because the blue (or short) wavelengths are much more likely to be scattered and so its as if there are many more \"sources\" of blue light than any other color, making the sky seem blue. ie if you had a wall of many colored bulbs, but a majority of them were emitting blue light, the wall would look blue ish and that would be the dominant color. ",
"But, if we set that aside and assume your assumptions are true, then yes. When the intensity of blue light reaches a threshold value of some level then it would promote those effects that are mentioned in the Psych Today article. So if the intensity of the blue light was drastically larger at the higher altitude than at sea level, then it may drive the wakefulness circuits in your brain. But remember, your brain is only interpreting electrical signals that are sent from your retina. So the color selectivity or preference that your brain may \"exhibit\" is a direct response to retinal input. So really, it still comes down to whether or not the retina is responding to the light and to what extent. What I'm getting to here is that, although the intensity of blue light may be more on top of everest than in your backyard at home, the increase may not result in any different response from your retina. you can reach a saturation threshold where increasing the intensity no longer results in a change in cellular function. The psych today article doesn't really account for differences in intensity of blue light, but instead reports the difference of absence or presence. so the cognitive effects they are reporting probably wouldnt change if you upped the intensity of light."
] |
[
"Does the thickness of a sheet of steel affect its maximum temperature when exposed to sunlight?"
] |
[
false
] |
Say you had a sheet of 2m x 2m x 2mm black steel bolted to a 2m x 2m x 1m concrete platform. On a 30C (86F) sunny day with no wind, this sheet would reach a certain max temperature, probably enough to burn you if you touched it. But what if you bolted nine more equivalent sheets on top of the first so that they're effectively one 20mm sheet of metal – would it now have a higher maximum temperature? And does the increased mass mean that that maximum temperature takes a lot longer to reach or is this difference negligible? Thanks in advance!
|
[
"This is not a trivial problem. Increasing the thickness of the steel sheet is going to have at least three effects, two of which will tend to ",
" its maximum temperature and one of which will to tend to increase it: ",
"(1) It will increase the lateral conduction of heat (which would affect the equilibrium temperature). However, this factor seems small because even the built-up steel is still not very thick relative to its length and width. In other words, most of the energy is leaving by convection on one side of the flat part and conduction to the concrete on the other side of the flat part. One a small amount is leaving from convection on the sides because of their relatively small surface area.",
"(2) It will increase the heat capacity of the steel in total (which will increase the amount of time required to reach its maximum temperature).",
"(3) It will increase the thermal barrier between the top surface of the steel (which is absorbing the radiation) and the concrete (which is sinking heat). However, this factor also seems to be negligible. The thermal conductivity of steel is two orders of magnitude larger than that of concrete, so heat transfer through the steel isn't a problem.",
"Note that I'm assuming that there's initially enough steel to burn you. Consider aluminum foil, which doesn't have enough thermal mass to burn you at any temperature. If you increase the thickness of aluminum foil, the hazard would increase even at a constant temperature. However, 2 mm steel is certainly already thick enough to transfer a harmful amount of heat to your finger.",
"Thus, once there's enough high-thermal-conductivity metal to burn you if you touch the surface, adding more metal thickness in this case wouldn't seem to increase the hazard; if anything, it lowers it because the additional metal requires more time to heat up."
] |
[
"So you also asked specifically about the time required for the steel to heat up when it's exposed to the sun. This calculation is also not trivial, since the steel is losing heat through convection to the atmosphere and also to conduction to the concrete. We can say that the 10× thicker steel will take 1-10 times longer to heat up. ",
"For example, a few rough calculations indicate that the ",
"Biot number",
" Bi of the steel, even the built-up steel, isn't much larger than 0.1, so I'm going to treat the steel as being at a uniform temperature. In this case, the ",
"lumped-capacitance",
" time constant is ρct/h, where ρ is the density, c is the specific heat capacity, t is the thickness, and h is the effective convection efficient. Using ρ = 8000 kg/m",
" c = 490 J/kg-K, t = 2 mm, and h = 10 W/m",
"K, I get a time constant of about 13 minutes. In other words, after a few time constants, or 40 minutes, the steel surface will be as hot as it's ever going to get. You would want to verify this experimentally. Under this model, the built-up steel with a thickness of 20 mm would take 10 times as long to heat up. This represents the upper bound. "
] |
[
"Hey thanks for the comprehensive answer, that really helps!"
] |
[
"If a single atom is vibrating (the random motions due to heat) and that atom is isolated in a vacuum, does the atom slowly cool down (vibrate less), or does it always have the same temperature? And if it DOES vibrate less over time, then how is it doing that?"
] |
[
false
] |
Atoms cool down by bumping neighbor atoms that are vibrating less. How would a single atom cool off then?
|
[
"With a single atom, it's hard to talk about vibration, as that relies on bumping off other atoms to change direction - if you're the only atom, you can't hit another one, and will continue moving in your initial direction forever. Equally, Temperature is only really valid when talking about groups of atoms - if it's a single atom, all you're discussing is its kinetic energy. Again, that will stay constant unless it hits something else or is acted upon by an external force."
] |
[
"That atom is not \"vibrating\" as mentioned earlier. All its real \"temperature\" is in its translational motion and electronic energy. The latter can emit a photon to cool but in the absence of a field the transnational energy will remain.",
"If you make this a molecule though (where vibrations can happen) vibrational energy will naturally decay down into V=0 (the lowest vibrational state). The time is takes to randomly emit a photon is proportional to the cube of the frequency being emitted and the square of the \"transition dipole\"(how coupled the two energy states are). For typical vibrations the average lifetime is between picoseconds and low nanoseconds."
] |
[
"What about internal energy due to motion of electrons and nucleons? I know they're not technically \"moving\" (quantum mechanics and all that), but they do have kinetic energy of their own that doesn't contribute to the motion of the atom as a whole."
] |
[
"Why are some diseases such as chickenpox more dangerous to adults than children?"
] |
[
false
] |
I've read that the symptoms can be different in diseases like chickenpox, but why are they different, what is actually causing it to be potentially worse to an adult?
|
[
"There's a significant element of \"we don't know\" involved in this response. For a short answer, I can say that adults seem to develop more severe sequelae on average than do children. Adults with chicken pox seem to develop severe pneumonia, for example, more frequently than children in the same position. This is a proximate cause, though, not a mechanism, and no real, single mechanism has been proposed.",
"We're also taking about relatively small absolute numbers here if you're wanting to extend the \"more dangerous\" part of your question to death as an endpoint. Between 1990 and 2001 (a period that covers the introduction and expansion of chicken pox vaccination programs in the U.S.), total annual deaths listing chicken pox as an underlying cause dropped from 105 to 39. Most of the gains seen in that drop in mortality (89-92% of it) was in the 9 years old and younger age group, not in adults contracting it for the first time.",
"So, the picture is complex: adults may experience more severe symptoms from a later-in-life challenge, but they also may be less likely to die than children experiencing their first challenge (or at least that was the case prior to the advent of a vaccine).",
"(Reference-wise, ",
"start",
" here and work your way outward into their references)."
] |
[
"It has to do with how your immune system works, specifically the part that deals with the cell mediated immunity that fights intracellular pathogens, like viruses such as varicella (chickenpox). As you get older your body gets worse at cell mediated responses, just a natural part of aging. As a consequence certain diseases, like chickenpox, go from being a relatively minor problem as a kid to a potentially fatal.\nTL;DR As you get older cell mediated immunity decreases"
] |
[
"Cell-mediated immunity is the part of your immune system that involves cells going out and fighting the infection. This includes macrophages, T-cells, NK-cells, etc. These are your first line defenders. ",
"Humoral immunity is the part of your immune system centered around the ability of your B-cells to make antibodies against things in the long term. This is your heavy artillery. It can completely obliterate an infection but takes a while to rev up. That's why we use immunizations (like the varicella zoster vaccination for chickenpox/shingles) to prime the humoral immune system. ",
"The cell-mediated immune system is very important for preventing things like pneumonia. The reason that chicken pox kills people when they're older is not that they are getting really itchy, it's because varicella zoster is starting to affect other parts of their bodies. The two biggest concerns are encephalitis (inflammation of the brain) and pneumonia."
] |
[
"If you have asthma are your lungs always somewhat constricted?"
] |
[
false
] |
Also, does taking asthma medicine such as rescue inhalers make the airways comparable to a normal person's or are they still somewhat constricted? I'm wondering if people with asthma feel its effects even when it's not a full-on asthma attack.
|
[
"Thanks that was really informative! I kind of want to check what my regular FEV-1 is now...",
"Any reason why 1 second of exhalation is the standard used? Does the percentage increase of air exhaled increase after 1 second and is thus hard to standardize?"
] |
[
"Thanks that was really informative! I kind of want to check what my regular FEV-1 is now...",
"Any reason why 1 second of exhalation is the standard used? Does the percentage increase of air exhaled increase after 1 second and is thus hard to standardize?"
] |
[
"So FEV-1 is your forced expiration volume in one second. That's usually paired with a simultaneous measurement called FEV, forced expiratory volume. ",
"A normal FEV-1/FEV ratio is about 80%, so most of the air is usually expelled from your lungs in that first second. Changes to these values, and the ratio between them, can indicate changes in thing lung tissue. Different lung diseases have characteristic patterns. ",
"By far the biggest impact on your FEV-1, won't be your asthma btw, in case you get your hands on a spirometer. It's mostly going to be your size. "
] |
[
"Why do parallel mirrors that face each other produce infinite reflection that curve *upwards*?"
] |
[
false
] |
I've never been able to parse out this phenomenon, and I hope askscience will be able to. Here's an example photo: Googling other examples suggests that the phenomenon is ubiquitous. Any explanations?
|
[
"It doesn't. This one curves to the right for example. \n",
"http://www.flickr.com/photos/jfpower/198315425/",
"\nIt's a matter of how close the mirrors are to parallel.\nAdditionally, you cannot observe infinite depth due to the fact that you must obstruct the mirrors with an observing device."
] |
[
"Might work, but since you're looking through a 1 way mirror, you have reduced its reflectivity, your viewing distance is greatly reduced due to the escaping of photons through your end of the mirror (hence your ability to see through it)"
] |
[
"Might work, but since you're looking through a 1 way mirror, you have reduced its reflectivity, your viewing distance is greatly reduced due to the escaping of photons through your end of the mirror (hence your ability to see through it)"
] |
[
"What material property causes substances to be transparent?"
] |
[
false
] |
Why can I see through certain substances compared to other opaque substances even though they are both solids?
|
[
"It is mostly to do with the electronic structure of the material. Every material, due to the different amount of electrons and their shell structure that the individual atoms bring as well as the type of bonding it forms when it makes a solid, has a unique spectrum of states at different energies and momenta where electrons can occupy. ",
"More specifically, in a given solid, the electrons fill up to a certain energy level, or band, of states. The top filled band of states is called the valence band. Above there, there might be a gap (no allowed states due to quantum mechanical rules) and then more states that are mostly empty. The lowest most empty band is called the conduction band because any electron in that band contributes to conduction (the reasons for this are beyond the scope of this answer). Now, the distance in energy between these bands is the crucial part here. In most materials the amount of energy needed to jump from valence to conduction band is small or even zero (metals). This energy can be provided by visible light and so visible light photons get absorbed and bump the electrons up. Those electrons eventually decay back down through different mechanisms that usually end up distributing the absorbed energy as heat. So that is when the atoms start vibrating as doctorbong alluded to. ",
"Conversely, in clear materials, the gap between conduction and valence band is larger than the energy of all visible photons (which is about above 2.8 eV) so no visible photons get absorbed because there is no quantum mechanical transition available. They just pass through, for the most part unchanged."
] |
[
"For something to be transparent, there must be an uninterrupted path from one side of the material to the other side for photons to follow. Two things can interrupt such a path: Absorption or scattering.",
"For absorption to take place, the photon must be able to excite a mode (I think that's the right English word?) in the material. There are a whole lot of things that can be excited, such as the rotational motion of a molecule, the vibration of bonds or the spin of the nuclei in the material, but only electronic orbital transitions have energies in the visible range. Basically, photons have a certain amount of energy which they can donate to an electron. However, the electron must be able to accept that energy and the energies available to electrons in materials are restricted. This is because an electron has a wavelength dependent on it's energy and it must be able to form a standing wave in the material. Too much or too little energy and the electron doesn't have a standing wave, so those energy values are forbidden. This means that there are a limited number of transitions possible, dependent on the electronic structure. For example, electrons in metals almost do not feel the lattice, so to speak, so they have a lot of freedom in their energy. This means that the electrons in the metal can absorb photons of all colors and most metals have a grey color as a result of this. On the flip side, in glass, the electrons are restricted to the silicon-oxygen bond they belong to, which puts severe restrictions on the energies these electrons can attain. Glass is therefor transparent in visible light and you need to go to UV wavelengths before it starts to absorb.",
"Scattering occurs when light is transferred over an interface. It's essentially just reflection, although you don't get a neat reflection of the interface isn't flat. Internal scattering happens when there are a lot of internal interfaces, which happens when a material is made up of a lot of tiny crystals, or when tiny foreign bodies are dispersed in the material. This explains why Titanium Oxide looks white even though it should be transparent. It consists of a loooooooot of tiny crystals and light scatters all over the place instead of being able to pass in a straight line. Glass is amorphous, it doesn't contain any crystals whatsoever, while quartz can be obtained in large, pure single crystals. That's why both glass and quartz are transparent. Sandstone, even though it's made from the same material, is nanocrystaline and contains all kinds of foreign impurities, which results in internal scattering (and the impurities result in absorption, giving the stone it's color), making it opaque."
] |
[
"It has far less to do with the bonds vibrating and much more to do with the electronic spectrum of the material. Electrons are by far responsible for the interaction with incoming visible photons in any material."
] |
[
"Why are neutrons a little heavier than protons?"
] |
[
false
] |
Intuitively I would think that a proton would have to be heavier, since it needs some additional energy to localize charge.
|
[
"Actually, they both contain charges. Both Neutrons and Protons contain ",
" and ",
" quarks.",
"up quark: +2/3 charge",
"down quark: -1/3 charge",
"Proton = up/up/down = +1 charge",
"Neutron = up/down/down = 0 charge",
"You can think of neutrons as being a bit heavier because they may have a bit more energy, E=MC",
"Free neutrons",
" (neutrons not part of atoms) will ",
"spontaneously decay",
" (half-life of approximately 10.4 minutes) to a Proton and release an electron. The released electron is carrying some mass energy, some kinetic energy, and the -1 charge to balance out one of the down quarks changing to an up quark. That energy (plus some recoil kinetic energy in the resulting proton) is your mass difference between protons and neutrons."
] |
[
"That's right, most of the difference in mass comes from the fact that the up quark has (as far as we can determine) less mass than the down quark (the proton has two valence up quarks and one down quark, the neutron has one up quark and two down quarks)."
] |
[
"The extra mass of the neutron more than makes up for it. Keep in mind that the mass of a neutron ",
" the rest energy. The mass of the neutron is not the sum of the masses of the quarks. It's ",
" larger, because it ",
" takes the binding energy of the strong and electromagnetic forces into account."
] |
[
"Can you get parasites from eating rare or medium rare steak?"
] |
[
false
] |
[deleted]
|
[
"Yes. The protozoan ",
" and beef tapeworm (",
"). The former resolves on its own after a few days; the latter sets up home in the colon and sheds a gazillion eggs, but is generally without symptoms unless one is heavily colonized with worms."
] |
[
"Hygiene and sanitation. Beef, pork, and other meat in the United States are relatively safe.",
"Also note that many parasites don't do well when frozen. Beef tapeworm dies at -5C, for example."
] |
[
"Hygiene and sanitation. Beef, pork, and other meat in the United States are relatively safe.",
"Also note that many parasites don't do well when frozen. Beef tapeworm dies at -5C, for example."
] |
[
"Why is chlorophyll green? Extensive double bond-conjugation or complex ion formation?"
] |
[
false
] |
I had a school test recently and there was a question asking to explain at a molecular level why chlorophyll is green. In my course there were to explanations of coloured molecules: either a d-block element forms a complex ion, and the ligands split the d orbitals, or there is an extensive conjugation of double bonds over a molecule. I looked at the molecule of chlorophyll and there a lot of alternating single-double bonds, so my mind immediately though of extensive p orbital delocalisation. However, after the test I remembered that there is a central Mg ion and I though that maybe the rest of the molecules act as ligands for it. However, Mg is not a d block element so there are no s orbitals to be split so Mg should have nothing to do with wavelength absorption? Is my reasoning correct? TL;DR: Are conjugated bonds in chlorophyll the only reason the molecule is coloured or does the Mg ion has anything to do with it?
|
[
"\" all land plants contain chlorophyll a and b. These 2 types of chlorophyll are identical in composition apart from one side chain, composed of a -CH3 in chlorophyll a, while in chlorophyll b it is -CHO. Both consist of a very stable network of alternating single and double bonds, a structure that allows the orbitals to delocalize, making them excellent photoreceptors.\" from ",
"http://www.webexhibits.org/causesofcolor/7A.html",
"Sounds like it's the orbital delocalisation, but this is more a chem/biochem question sorry I can't help more."
] |
[
"Thank you! When you say orbital delocalisation, you mean electrons are delocalised over several bonding orbitals? Not that orbitals themselves are split (such as the case with transition metals). \nI changed the flair to chemistry."
] |
[
"Orbital delocalisation means that the orbital is spread over many atoms, so the 0-2 electrons in the orbital are also spread over many atoms. So for something like benzene or hexatriene the orbitals are spread over all 6 atoms so the electrons in those orbitals are also spread over the 6 atoms. Hope this makes sense, I'm not sure how much molecular orbital theory you will have seen so this is a fairly simple explanation, can explain further if you like.",
"\nAlso just checking that you mean the set of orbitals is split, and not a single orbital."
] |
[
"Does size matter? Could human level intelligence exist in something the size of a squirrel or an ant?"
] |
[
false
] | null |
[
"This question is a bit vague. Humans don't have the largest brains. What do you mean by \"human level intellgience\"? Non-human animals can engage in all sorts of complex behaviors from tool-use to problem solving to communication."
] |
[
"Like, could a very small brain produce intelligence as refined as a humans?"
] |
[
"What does \"an intelligence as refined as humans\" mean?"
] |
[
"Why doesn’t chemotherapy have a 100% success rate?"
] |
[
false
] |
[deleted]
|
[
"Chemo targets actively replicating cells in your body. That's why you lose your hair and have GI issues (those cells replicate a lot). The primary tumor is usually active, but some early disseminated cells might be hiding dormant at metastatic sites. They get passed over and only start proliferating many years later. We don't know what causes early dissemination, dormancy, or the reactivation of metastatic cancer cells. "
] |
[
"Traditional chemo targets the replicating DNA, so you need the cell to be proliferating. That being said, metastasis causes 90% of cancer related death. With a local tumor, you cut it out and you're cancer free. They usually just give you chemo after surgery to try to make sure, because we can't see if there are micrometastases or not. "
] |
[
"Might be a silly question, so if the cancer is actively replicating then the chemo will have an affect? And if this is true is that why catching it early makes such a big difference?"
] |
[
"Why does visible light not cause eye damage?"
] |
[
false
] |
Since visible light is higher on the EM scale and has more energy than Infrared, then why does visible light not damage our eyes while infrared does (at long exposures)
|
[
"Because you blink if it's too bright. Infrared doesn't make you blink. In fact, if you're in the dark, your iris could be wide open letting in more infrared. ",
"You start to notice problems after the damage is done. It needs to be very intense to cause damage. Anything which is incandescent enough to burn your eyes will also be bright in the visual range. ",
"If the source is a laser diode, there's increased danger because the coherent light can be focused onto a very small area. "
] |
[
"Light damages the eye in two ways: thermally and chemically. IR and visible light heat the retina until it essentially cooks, while UV light damages the photoreceptors by chemically denaturing them.",
"The amount of heat damage depends in part on the energy, but also on the power, the area it's confined to, and the exposure time. As whitcwa says, the exposure time is generally small for visible light. And as cantgetno197 says, it's possible to increase the visible power high enough (like in a flashbang) to cause discomfort or damage. Blackbody IR radiation doesn't hurt us because it doesn't focus to a small area, and because it's typically not very intense.",
"Staring at the sun is dangerous primarily because of possible UV damage. "
] |
[
"Visible light can damage the eye.\n ",
"Here",
" is a list of damage mechanisms based on the wavelength of light: ",
"180–315 nm (UV-B, UV-C): photokeratitis (inflammation of the cornea, equivalent to sunburn)",
"315–400 nm (UV-A): photochemical cataract (clouding of the eye lens)",
"780–1400 nm (near-IR): cataract, retinal burn",
"1.4–3.0μm (IR): aqueous flare (protein in the aqueous humour), cataract, corneal burn",
"3.0 μm–1 mm: corneal burn"
] |
[
"I can close my eyes and recall a vision, or focus hard enough and hear a song in my head, but why can't I do the same with a taste or smell?"
] |
[
false
] | null |
[
"Sometimes when I look at a picture of roses i can smell roses. I used to work for the medical examiner or coroner picking up bodies and sometimes when i see a picture of a corpse or even imagining it in my head I can smell the decomposition. I haven't worked that job in several years.",
"It is possible. You just haven't experienced it yet. "
] |
[
"It's still somewhat contentious of a subject as to whether or not people \"truly\" can recall smell memories without any other sort of sensory cues, since there's evidence for both. I personally think the evidence is stronger for a lack of mental visualization for our sense of smell, but it all depends on who you ask. While the average person would be hard-pressed to honestly claim they could mentally imagine smells, expert smellers such as perfumers claim they are able to mentally imagine smells, but of course it's impossible to objectively test a purely subjective claim.",
"As for your question, a somewhat recent Frontiers article argues that a lack of representation for our sense of smell in our neocortex relative to our other senses is what's responsible for our inability to recall smells. Other senses have, throughout evolution, taken up more and more space in our neocortex since our species has evolved to prioritize vision, hearing etc over our sense of smell. This in turn leads to less direct connections between sensory and semantic memory brain locations/processes.",
"Stevenson and Attuquayefio. \"Human olfactory consciousness and cognition: its unusual features may not result from unusual functions but from limited neocortical processing resources\" Frontiers in Psychology 2013",
"http://www.frontiersin.org/Journal/10.3389/fpsyg.2013.00819/full"
] |
[
"I have no problems recalling tastes or smells as long as I'm in a neutral environment. If I imagine the taste and smell of very sour lemons I start to salivate and can feel my mouth pucker. I can imagine lime too, and the various differences in flavour and texture compared to the lemon. But it would be very hard to test for if not impossible to prove scientifically, other than perhaps measuring saliva in the case of the lemon."
] |
[
"How are colors determined when coloring pictures taken with an Electron Microscope?"
] |
[
false
] |
I understand that colors are added in to Electron Microscope images but I am curious as to how the colors are chosen. Are they colored to imitate what the actual color is? Are they chosen randomly? Any information would be awesome!
|
[
"It's entirely up to the user. ",
"Quite often, in biologic samples anyway, we already know the true colour of the sample we're looking at. So most of the time it's based off that.",
"If the colour is unknown, people tend to stick to high contrasting colours which look friendly to the eye.",
"Have a google of electronmicrographs of red blood cells for a good set of examples. "
] |
[
"As far as i know it depends on what they're trying to show/demonstrate - just as with other types of false colouring. So it could be used to identify one type of atom/molecule from another, one type of structure, size or a variety of other characteristics. "
] |
[
"I've done some electron microscopy. Generally you either pick a color scale that highlights the feature that you're looking for; or for artistry, to get people interested in your work. For instance, if you have several types of surfaces and you want to show how they're organized, you might want to make them contrasting colors: ",
"http://www.nature.com/nature/journal/v469/n7330/images/nature09718-f2.2.jpg",
" Or if you have two of them, just two colors: ",
"http://www.elmitec.de/css/Bilder/LEEM-DF-Si100_gr.jpg",
" On the other hand, if you have mountains or something and you want to show how high they are, you probably shouldn't make them all the same color: ",
"http://vig.extremetech.com/media/images/245432.jpg?thumb=y",
"If you have subtle features that you want to enhance, then you need color scales with subtle gradients. There's a whole theory and/or controversy regarding appropriate color scales for different tasks, here's an article I found interesting on the topic:\n",
"https://jakevdp.github.io/blog/2014/10/16/how-bad-is-your-colormap/"
] |
[
"Would there be any \"bad\" consequences if tomorrow, mosquitoes suddenly became extinct?"
] |
[
false
] |
I'm sure they serve a purpose , right? Like on the food chain or something? But still, won't animals that rely on mosquitoes as a food source find alternative sources of food? Other insects that mosquitoes competed against could feed those animals, right? As far as I'm aware, there aren't any plants exclusively pollinated by mosquitoes, correct? All I can think of are good things as a result of their extinction, although something tells me that's wrong.
|
[
"Interestingly enough, while the answer at first seems like it would be undoubtedly \"yes,\" there's an interesting debate in the scientific community about this. ",
"Here's a Nature",
" article I came across last year. Definitely worth a read!"
] |
[
"I read an interesting opinion piece on this matter a while ago. The author suggested that Mosquitoes serve a very important purpose that most people overlook: they protect ecosystems from us. ",
"Native peoples usually don't have a problem with mosquitoes in the South American rain forests, for instance, because they don't cut down the trees. But when people try to cut down those trees, the mosquitoes living in the upper canopy are released causing a high risk of disease to those doing the cutting. This is one of the primary dangers of cutting down trees in the rainforest, apparently, so you could think of the mosquitoes as being an important part of the forest's defenses. "
] |
[
"Sounds like you're confusing \"serving a purpose\" with \"occupying a niche.\""
] |
[
"Deploy Chute!!!"
] |
[
false
] |
I realized the other day while on an airplane just how much flying actually terrified me sometimes. I've always thought about how it would be nice to have a parachute with me in case the plane were crashing so I could just jump out of the plane, but how possible would something like this be? I'd like to know what the possibility of jumping out of a commercial Boeing 737-700 or similar aircraft and deploying a parachute would be? I imagine it would be quite difficult considering the g-forces you would experience, how the aircraft was plummeting (straight nose dive, barrel roll, etc) and what altitude you are at since above a certain altitude there would not be enough oxygen to survive.
|
[
"First, remember that most crashes happen at low altitude ",
"Actually, they all happen at zero altitude. :)"
] |
[
"Not going to comment on the physics of this (not my specialty), but just as a note, there are a variety of devices that will ",
"open",
" a ",
"chute",
" ",
"automatically",
"."
] |
[
"First, remember that most crashes happen at low altitude with little warning. A parachute won't do you any good if the pilot flies the plane into a mountain. Or encounters a microburst on approach. Or suffers an engine failure on takeoff. Or if the pilot stalls the plane in a storm. Or if the plane suffers a catastrophic structural failure.",
"The only case where this would potentially help is if the plane suffers a full engine or control failure at altitude. This is super rare. Provided the pilots can keep the plane under control, you can in principle bail out. The doors won't open in flight, but that's an engineering problem. ",
"Lack of oxygen won't kill you by itself; you'll splat if you pass out and don't open the chute, but if you're carrying a parachute, you might as well take a portable emergency oxygen system too.",
"In practice, a bunch of untrained passengers can't evacuate the plane safely. No airline is going to make bailing out a policy for this reason, and they're also not going to give you the option. Engine failure is quite survivable anyway -- the plane will probably be a loss at the end of the day, but that doesn't mean it won't glide well. The only case where I would want to use a parachute would be the total loss of flight controls, and that has happened only five times, ever (seven if you count hostile action). [",
"1",
"]"
] |
[
"Will the genes I pass to my son be influenced by my life experiences ?"
] |
[
false
] |
That is to say, will his genes be natures mix of the genes I was born wit and the genes his mother was born with ? Or have our (mine and those of his mother) genes evolved during our lives such that the next ''iteration' of our genes will be influenced by our nurtured characteristics ?
|
[
"Evolution is not primarily caused by the mechanism that ",
"/u/Forestman88",
" described. In evolution, it is the DNA sequence that changes over time, whereas in the process described above, the actual DNA sequence of the offspring remains unchanged.",
"In contrast, evolution is primarily caused by random mutations in the DNA itself. Whenever a cell divides, there is a chance that a part of the DNA gets copied incorrectly. This is a mutation, and can cause the DNA of the offspring to be slightly different that you would expect from a mix of the DNA of its parents.",
"Mutations can be beneficial, detrimental, or neutral. An individual with a beneficial mutation will have a slightly higher chance of reproducing, and thus a higher chance of passing on the mutation to the next generation. In this way, over the course of many generations, beneficial mutations will become more prevalent across the gene pool, while detrimental mutations become less prevalent. This is the mechanism of natural selection. "
] |
[
"Epigenetics are actually relatively well understood. We know the mechanism, and there are plenty of well validated studies that show this type of effect, if not this actual effect. I don't think I've seen one on height, but the concept in the example used are pretty well established."
] |
[
"Epigenetics (via DNA methylation) sort of counters this. Experience can be passed down if it results in gametes getting DNA methylation. ",
"In a weird way, it is very Lamarck-like, but not how he really described it. Obviously, 3000 years of Judaism shows that you can't just lop off a body part and expect it to be absent or smaller in the children, but some more subtle things like extreme trauma can get passed down via DNA methylation."
] |
[
"Is there a way to achieve \"true\" random?"
] |
[
false
] | null |
[
"This depends on context, but 'true' randomness does exist as far as we know. What he may be talking about is that there are no true random number generators in computers as any way of generating numbers tends to give off a deterministic sequence of numbers, which repeats eventually. As for generating a single random numbers this can be done by using processes which are inherently probabilistic. ",
"An example is radioactive decay, the decay of an individual atom is a random process (as far as we know), with a small catch. Usually you won't have a single atom of the radioactive material, you are going to have a large sample because a single decay will be rarer the fewer atoms you have, the number of decays in a given time period will be your random number, or rather the difference from the average number of decays in that time period. The catch? It isn't 'random' in the sense that each number is equally likely to occur, it will typically follow a distribution. Usually you can account for the distribution and transform it to something uniform, you can also use the number generated by this process as a \"seed\" for a pseudorandom number generator, which is a deterministic sequence I mentioned before. These sequences can be quite good so if you have a reliable way to generate a random seed it's actually quite useful to use a randomly generated seed and then a deterministic sequence if doing computer simulations that depend on randomly generated numbers.",
"Interesting video on random numbers from numberphile."
] |
[
"Of course you'll be able to spot a pattern, with sufficiently many rolls it should start approaching some distribution, hopefully a uniform one for a roulette wheel. The existence of a pattern isn't the question with 'randomness', it's more if you can have an a priori prediction about the most likely future event based on previous events. Something completely deterministic, like the output of a pseudorandom number generator, is completely predictable. If I know the nature of how the numbers are generated I can tell you the exact sequence of numbers that will get generated and how often it will repeat. With a roulette wheel I can't do that, I can tell you approximately how many times 15 black shows up in 10 million rolls, but I can't say what will be rolled after 15 black. The radioactive decay is even better at generating a random number than a roulette wheel, in principle if you knew the exact nature of the surface of the roulette wheel, the angular momentum of the wheel, and the initial position and momentum of the ball, you could predict how it lands. To the best of our knowledge radioactive decay is not a deterministic process, so even in principle the number of decays in a time period couldn't be predicted exactly, we could just say it would be approximately m of them, and we would be close to that most of the time."
] |
[
"Whether the output is uniform or not doesn't matter, as you can always feed your random sequence into an ",
"extractor",
" that converts it into a uniform, i.i.d. sequence.",
"All you need is a sequence with some positive lower bound on the amount of entropy per element."
] |
[
"Hey all, I just got a motorcycle and was wondering.. Why is it easier to balance the faster I am moving?"
] |
[
false
] | null |
[
"To all of those people who responded angular momentum; that's not the sole cause of balancing. I believe it is currently an open question as to how exactly the bicycle / motorcycle stay balanced. ",
"This video",
" shows a cleverly designed riderless bicycle that stays upright (at sufficient speeds) with absolutely no angular momentum. ",
"See: \n",
"http://en.wikipedia.org/wiki/Bicycle_and_motorcycle_dynamics",
"\n",
"http://www.sciencemag.org/content/332/6027/339.abstract"
] |
[
"Its a stable dynamical system. ",
"A lean of one way causes a reaction ",
"camber thrust",
" in the front wheel causing it to turn in the direction of the lean. This causes the bottom of the bike to move back under the center of gravity. ",
"The amount of camber thrust varies with speed and lean angle, so the restoring moment is prevalent at speed. However, past a certain point, it is not enough, and the bike will be laid down or low-side. At slow speeds, the camber thrust can be so severe as to cause the bike to lean the other way and capsize, otherwise known as a high-side. "
] |
[
"The answer, apparently, is that the real parts of the eigenvalues of the linear system are negative. The authors of the paper said they have no idea what that actually means, physically, though..."
] |
[
"Is there a material that blocks or prevents magnetic attraction?"
] |
[
false
] |
Do all materials block it slightly better than just empty space filled only by air molecules and are there any known materials that block it better than others. ie. if I put a strong magnet on one side of my hand and another on the other side they can potentially hold each other in place, is there any material that would prevent this from happening?
|
[
"are there any known materials that block it better than others",
"Magnetic shielding"
] |
[
"Yes, ",
"mu metal",
" for example. You want a metal with high magnetic permeability to direct magnetic field lines away from a certain area."
] |
[
"So from reading that it sounds like it distorts the magnetic field emanating from said magnetic object, would this eliminate attraction created by said object in all directions or could it be used to only block it in 1 direction.\nIf I attached one of these \"magnetic shields\" to the right side of a magnet would the magnet still be able to create an attraction force with an object on its left side?",
"sorry if this is basic stuff, my physics understanding kind of fell off the deep end when I hit electricity and magnetism."
] |
[
"Is there a plant that can survive high doses of radiation and survive?"
] |
[
false
] |
The bacteria Deinococcus radiodurans can survive doses up to 5,000 Gy. Is there a plant or other life form that can also survive that much?
|
[
"How about this chart?",
"Looks like thermococcus gammatolerans beats it out."
] |
[
"This illustrates why a \"general\" rule is not the same as an absolute rule."
] |
[
"The general rule to develop from the chart seems to be that the 'higher' (more evolved) the organism, the more vulnerable it is to radiation. "
] |
[
"Why does a camera convert infrared to visible light?"
] |
[
false
] | null |
[
"It doesn't actually change the wavelength of any photons. The camera sensor is sensitive to wavelengths in the IR that your eye is not, so when it sees light coming from the IR diode on the TV remote it sees it as signal in the red channel. The camera electronics then outputs this signal on the screen on the back the same as red visible light. The camera can't tell the difference between the two - it just sees signal in the 'red' channel. Because the wavelength range that the sensor can detect is different than what your eye can detect, you end up seeing some things with a camera as the detector that are not visible by eye. "
] |
[
"It would be unnecessarily costly to develop a CMOS sensor that had the exact same cut-on and cut-off of the human eye. In fact, those values vary from person to person, therefore, it's beneficial for a sensor to have a bit wider range than the average eye so as not to leave anything out.",
"\nFurthermore, there's quite a bit of roll-off beyond these wavelengths. If you created a sensor that had dead stops at 390nm and 750nm (typical human cuton and cutoff) their sensitivity at the ends of this spectrum would be horrible and give a crappy color picture due to the inherent roll-off of a sensor. Therefore, it's best to have a sensor that has nearly identical gain at all of the wavelengths from 390nm - 750nm which causes the sensor to actually absorb photons of shorter and longer wavelength.",
"\nTypical IR remotes frequencies are at 940nm which is probably just inside the roll-off of the sensor. Gain here will be small for this wavelenght, but the amount of intensity that the average remote control has is quite powerful - more powerful than ambient lighting. So although the sensor is weak at this wavelength, the intensity of the IR light from the remote is strong.",
"I would bet that you might see a difference in sensed IR intensity with varying price ranges of cameras. The more sensitive a camera is to UV and IR, the more \"noise\" it is sensing. I would venture to guess that more expensive cameras will not sense the IR remote as well. They will be tuned for better \"noise\", and therefore better picture."
] |
[
"Good clarifications. ",
"The more expensive cameras often have a 'heat mirror', or filter that rejects IR light to prevent this from happening. Cheap cameras lack this feature.",
"And the human eye has plenty of what you're calling roll-off, too. I know I can see, with greatly decreased sensitivity, 900 nm photons. "
] |
[
"Could someone explain what causes the angle and positions from the center of a diffraction pattern of the reflections"
] |
[
false
] |
Im an undergrad hoping to go into X-ray crystallography, and each time I try to understand the diffraction pattern it bends my mind. I understand the position relates to the frequency of the X-rays, but not sure where these different frequencies come from Could someone explain it using a vivid analogy to help me picture it Thanks in advance
|
[
"I'm not a crystallographer (maybe one can come in and really lay down some truth), but I think that the question you're asking is related to the uncertainty principle. Even if you have only one frequency in your crystal (say, a point scatterer every ",
" meters), you will still see more than one spot on the film/readout/whatever, because the crystal is of finite size.",
"Is that your question?"
] |
[
"EDIT: Wow, didn't answer your question at all in first draft of response. Ok. This delves pretty deep into Fourier series/transforms, which are some pretty crazy stuff (back when it was discovered, they actually thought that Fourier made a deal with the Devil to get is formula, because it is so powerful).",
"Imagine a sound wave. You sample the sound over a finite interval (from time 0 to time ",
"), with some finite number of measurements ",
". Because you only sample a finite number of times, you don't ",
" know what the sound you measured was. ",
"There are many options",
".",
"Here is where my understanding gets a little fuzzy. Basically, the same effect happens with a finite crystal. But you will just have to take my word for it because I don't understand it well enough to explain it.",
"As the crystal gets more complex, there are other periodicities in the material. Imagine, for example, that your crystal has a repeating pattern of two point scatterers like so:",
"P P P P P P\n",
"Then, there are ",
" frequencies, the \"small frequency\" (distance between P's in a pair) and the \"big frequency\" (between pairs). Both will be in the final readout."
] |
[
"so an atom bound by a covalent bond will make up the high frequencies as they are close in real space?",
"Yes! Exactly. And those high frequencies will show up ",
" from the center of the diffraction pattern, where the low frequencies will appear ",
".",
"Are these frequencies in parenthesis as they are not frequencies in the sense of wave frequency?",
"I'm not sure what you're talking about... you mean that I \"small frequency\" and \"big frequency\" in quotation marks?"
] |
[
"If you cut a flat spiral coil into sectors, do the pieces together still act like a flat coil? Image included."
] |
[
false
] |
is an illustration of my question. If a flat coil is cut up into sectors, does it still act as an uncut coil (with reduced effectiveness)? Or, by cutting it, does it become completely useless? Can it still interact with an electromagnetic field in a meaningful way?
|
[
"For a first approximation, consider the two ends of the wires as the plates of a ",
"parallel plate capacitor",
". There, we have the formula for the capacitance C = epsilon * epsilon_0 * A / d, where epsilon is the dielectric constant of the medium (1 for air), epsilon_0 is the vacuum permittivity (8.85 pF/m ~ 10 pF/m because I just feel like doing orders of magnitude), A is the area and d the distance.",
"So in order to get a 10 pF capacitor from a wire with 0.1 mm",
" (= 10",
" m",
" (about AWG 26 or 28), the distance has to be d = 10",
" m = 100nm. So your wire should better be thick, so the distance becomes manageable.",
"The sizes of the capacitor you want also depends on the frequency of the current, for microwave signals instead of radio frequency, you might be happy with something around 100 fF, which gives you another two orders of magnitude to work with."
] |
[
"What happens if you cut a light cord? ",
"If you cut a wire (coil or not) anywhere, you've created an open loop. Since no current can flow through the gap in the wire, whatever the wire was a part of has lost it's function.",
"Very high voltages would arc across the gaps, but it would rapidly wear the surfaces where the arcs begin and end. "
] |
[
"If you put the ends of two cables very close to one another, then the gap between the wires acts as a capacitor, which blocks DC but passes high frequency current.",
"So if you cut your spiral but place the ends of the pieces you cut very close to one another, you could get some sort of bad LC circuit."
] |
[
"How can gravity escape a black hole?"
] |
[
false
] |
If gravity isn't instant, how can it escape an event horizon if the space-time is bent in a way that there's no path from the inside the event horizon to the outside?
|
[
"It is \"changes to gravity\" that _propagate_, not gravity in itself. It's not like the blackhole is \"emitting\" gravity at the speed of light.",
"Gravity in itself is a property of space-time created by existence of matter. If the matter changes suddenly (a sudden disappearance), the \"before\" and \"after\" of the gravity in that local region goes through a change, which then propagates at the speed of light.",
"Edit: a lot of folks have been reading my response as a statement about GR supremacy over graviton/QG. I chose to explain using GR strictly because what it does explain fits well with experiments, while QG is still in a hypothetical territory. But in the spirit of not spreading partial information, more details follow below.",
"My original explanation above is based on GR (General Relativity), a theory of physics that helps explain the fabric of spacetime in the universe. What GR does not explain though, is the underlying mechanism of gravity itself, in the sense that how/why does existence of matter/energy warp spacetime.",
"On the quantum side, a hypothesised particle called the graviton is used to explain the underlying mechanism of gravity. However, this is in deep hypothetical territory right now, and unlike GR, has not made predictions in a way that help us get closer to validating/invalidating its existence (",
"research continues",
"). It may or may not turn out to be the underlying mechanism. That is the reason why I shied away from using QG to explain.",
"So the real answer is:"
] |
[
"The speed of gravitational waves in the general theory of relativity is equal to the speed of light in a vacuum, c.[3] Within the theory of special relativity, the constant c is not only about light; instead it is the highest possible speed for any interaction in nature. Formally, c is a conversion factor for changing the unit of time to the unit of space.[4] This makes it the only speed which does not depend either on the motion of an observer or a source of light and / or gravity. Thus, the speed of \"light\" is also the speed of gravitational waves, and further the speed of any massless particle.",
"https://en.wikipedia.org/wiki/Speed_of_gravity",
"Yes, as near as we can tell, gravity and light travel at the same speed, which is the speed of causality.",
"Edit: slight clarification, gravity and light both travel at the same speed, but that speed limit is not intrinsically related to light. It's more so just that they both obey the same speed limit."
] |
[
"edit: as far as we know:",
"Gravity isn't escaping a black hole because the black hole doesn't emit gravity.",
"Gravity, the curvature of spacetime, is a property of the spacetime itself effected by the mass of objects in space, not natively emitted by the objects themselves."
] |
[
"To give some perspective about the size of the universe, how would you most accurately complete this analogy: \"An atom of hydrogen is to the size of the earth, as the earth is to the size of...\""
] |
[
false
] |
I imagine the answer is some ginormous celestial body.
|
[
"Well, the \"size\" of a hydrogen atom isn't particularly well defined, but we can go with the ",
"Bohr radius",
" for this example.",
"So the radius of a hydrogen atom is approximately 5.2917721092*10",
" m",
"The radius of the Earth is (on average) 6.371*",
" 10",
" m [",
"/u/RobbyObby",
"]",
"So the Earth's radius is roughly 1.204 * ",
" 10",
" times larger than that of a hydrogen atom. Multiply that by the radius of the Earth, and you get 7.67 * ",
" 10",
" m.",
"This would equate to an object with a radius of ",
" 81 ",
" light years. Or, if it helps you visualize it, ",
" 810 times the radius of the Milky Way galaxy."
] |
[
"Is there anything that better approximates the size of 81 light years?",
"Not really. Distances that huge are very difficult to grasp. I'd normally use a comparison such as \"",
" 81 million light years compared to the earth is like the earth compared to a hydrogen atom\", but then we're back to where we started.",
"Perhaps it would be more useful to compare the size of a hydrogen atom to something a bit more managable. Say, a US quarter.",
"The ratio of the radius of a quarter (1.213 * 10",
" m) to the radius of a hydrogen atom is 2.292*10",
" ",
"If we scale up a quarter by that amount, it would have a radius of 2780 km. Imagine a gigantic quarter stretching out past the horizon in every direction, 100 km tall and covering an area the size of all of North America, packed top-to-bottom and end-to-end with a sea of quarter-sized atoms (we'll pretend that quarters are made of hydrogen). That should give you a better feel for how incredibly small atoms are."
] |
[
"You're off by a factor of 6. The radius of the Earth is 6.371 * 10",
" m (as ",
"u/gaj1985",
" correctly stated). Meaning the actual ratio is a million times larger than what you stated. "
] |
[
"Why was entropy so low at the beginning of the universe?"
] |
[
false
] |
I was recently reading and began thinking about entropy and why our universe had such a low entropy at the beginning. It occurred to me that, before the big bang, the universe was supposedly condensed into a singularity with space just barely surrounding it. Theoretically, this is the lowest entropy state possible because if all matter is compressed into one spot and space is just barely big enough to contain it there are zero other possible microstates (but I would think that entropy itself would not exist due to the space boundary). Even if it was not a singularity and just a very small space, there would still be very few microstates and a small of possible entropy. It would then make sense to assume that when the big bang occurred and space expanded, the number of possible microstates increased on an exponential level due to expanding space (like "room to move around" and occupy more states) and, ultimately, lower entropy. When I begin to think about this, it seems to me like the only way you could confidently decrease entropy would be to expand space. If you look at the universe, there is a lot more emptiness than there is matter so naturally we would be a very low entropy universe if the big bang theory is accurate. Please let me know what you think or correct me where I am wrong.
|
[
"We have no idea! This is one of the great mysteries of cosmology. You'll hear it argued that this is even the reason we have an arrow of time: time goes in a particular direction (that is to say, forward) because of the second law of thermodynamics, but we need to start off in a special low-entropy state in order to even have that evolution in the first place.",
"For now, that low entropy has to take the form of an initial condition. One would hope that whatever Final Theory of Absolutely Everything we find one day will explain that in a nice and natural way. But that is a long way off.",
"Sean Carroll's book \"From Eternity to Here\" is supposed to be an excellent and well-explained discussion of this for laymen. In fact, he loves talking about this stuff, so you might consider reading his ",
"FAQ",
" on it (not to mention his many blog posts on the subject) or a ",
"video",
". Because frankly, Sean's been thinking about this stuff for a lot longer than I have and will explain it much better!"
] |
[
"I can't speak to your other points, but your interpretation of entropy is incorrect. Entropy is proportional to the log of the number of microstates. So increasing the number of microstates ",
" the entropy of the system."
] |
[
"Without a deeper analysis, that's circular reasoning. Or maybe it's just rephrasing the question. Honestly I don't know, it's late here. It may even be a correct implementation of \"begging the question\" which is so often used incorrectly. Poor logical falacy.",
"Either way it simply translates the question into \"Why didnt the composition of the early universe have a higher entropy?\" or even (ooh, this is better) \"What allows the steady increase in entropy between t=0 and now?\"",
"If you blow up a perfect balloon and then tie it up, there is no inherent increase in entropy for that balloon or the air inside it. It has N microstates that it switches between. Entropy increases when N increases, such as when we introduce a needle to the system.",
"A lot can be explained by noticing one of the OPs flawed arguments: The expansion of the universe increases entropy. This is slightly better, it relates to one of the reasons why the composition of the universe is different now than it was then. ",
"One may be tempted, then, to say: \"Woohoo! An answer!\" but I would advice caution. Let's look at what we'v said so far:",
"If we look at at a 4D volume around a corner of spacetime, entropy will be lower than it is further away from the corner. We will have high pressures and temperatures.",
"As we move further away from this corner, temperatures decrease and matter starts condensing into cooler states. Entropy just keeps increasing.",
"We seem to have found something that couples the arrow of time with a direction near a spacetime corner. The existence of a \"big bang\" region causes us to have an arrow of time.",
"However, by that argument we should see similar behaviour if we started collapsing space. By that I mean: If the arrow of time is set by the proximity to the big bang, then moving closer to another corner (a big crunch) should reverse the arrow of time. If the universe started collapsing, however, suns would not start absorbing radiation instead of emiting.",
"So, what do we have? Entropy can increase because the universe is expanding, but entropy increase is not neccesarilly ",
" by the expansion of the universe.",
"Geeze, I'm really tired. I don't even know why you're still reading this, so I'll try to get to some kind of point.",
"I've gone of track a little bit, as you may have noticed. I'm mostly talking about things connected to the arrow of time here, rather than pointing out why translating the question isn't an answer.",
"My goal is to illuminate a strange thing:",
"If we start in a big bang, and end in a big crunch (we won't but the universe could have that 4D shape and still act just like it does) we would have near-identical start and end compositions.",
"Entropy would also be very low at the start and the end.",
"Local entropy (essentially the arrow of time) would, however, tend to increase until matter became hot enough to undergo another state change. In other words, if you looked at the whole universe in all its 4+D glory, it would be obvious to you which direction in time humans would consider \"forward\" and which \"backward\". Entropy increase on the local plane (say, a galaxy) would clearly unsymetric.",
"Even though the composition of the universe is nearly identical at both ends, and ",
" (that we know of) ",
".",
"\nNo."
] |
[
"Help me to write Q&A for a kids science quiz game."
] |
[
false
] |
My daughter turns 8 this weekend, and she's having a science-themed party at home. I'm going to be the mad-scientist master of ceremonies for her, and I've got a bunch of cool science tricks and activities planned. One of them is going to be a science "quiz show" that the kids will compete in. I need enough good questions for two teams of five 7/8-year-olds each. I'd love some help if anyone wants to volunteer some good questions (and correct answers) that would be challenging, entertaining and fun for kids in this age group. Whaddaya got?
|
[
"I don't know how much brain/neuro stuff you may want to include, but if you want to check out the ",
"Brain Awareness Week",
" website, they have a ton of great and simple trivia type questions/activities.",
"Hope this helps!"
] |
[
"Q. Why does Australia have summer when we have winter?",
"A. The tilt means that the southern half of the globe has longer days.",
"assuming you're in the northern hemisphere\n"
] |
[
"Or in a simpler version: \"Why do we have summer and winter\", and then yours as a follow-up. :-) Like it."
] |
[
"Do radioactive elements decay at the same rate if said elements are traveling near the speed of light?"
] |
[
false
] | null |
[
"Good question. They decay at the same rate ",
" This means that to an outside observer who sees the element going really fast, the decay happens much slower due to time dilation.",
"This is a well observed phenomena; cosmic muons have observed lifetimes that are quite a bit longer than the relatively slower ones we can get in a lab."
] |
[
"IIRC, muon decay was the first ",
"experimental test of time dilation",
"."
] |
[
"We also confirmed it by flying atomic clocks around the world (one east and one west) and then comparing the time once they'd done one revolution, sure enough, time was slightly behind on those clocks.",
"EDIT: ",
"Source"
] |
[
"To what extent does the planet Jupiter actually \"shield\" the earth from meteor impacts?"
] |
[
false
] |
I've been reading reddit today, and found out that a large object collided with Jupiter, and was by an amateur astronomer. Not long after, I came across claiming that Jupiter may very well have "saved" the Earth from a catastrophic impact. If my physics and astronomy lessons still mean anything, shouldn't Jupiter still be incredibly small compared to the vast size of the Solar System, regardless of it's gravitational pull? Also if my memory serves correctly, there are innumerable asteroids floating around the solar system, both inside and outside the asteroid belt, both near and far from Jupiter. I wanna know whether or not Jupiter has any measurable effect on capturing rogue objects hurtling through the solar system which would otherwise impact Earth, or whether or not this is indeed measurable.
|
[
"For gravitational interaction, a good measure of how close things get is the ",
"Hill Sphere",
". Jupiter's Hill Radius is about 50 million kilometers (5x10",
" km). Generally, if something gets within about 3 Hill Radii that's considered a close encounter and the object will get a good kick.",
"Now, about hitting Earth: Since these things are typically going pretty fast when they're in the inner solar system, gravitational focusing isn't very effective. Thus, to hit Earth an object will have to come within about one Earth radius (6400km). If you wanted to be generous you could consider everything that comes within one lunar distance (the distance from Earth to the Moon, 3.8x10",
" km).",
"So, let's compare the numbers:",
"If you take 3 Hill Radii in either direction from Jupiter and divide that by the circumference of its orbit (its semi-major axis is 5.2 AU) that's 0.065 = 6.5%.",
"If you take Earth's diameter and divide that by the circumference if its orbit (semi-major axis of 1 AU) that's 1.3x10",
" = 0.000013 = 0.0013%. Doing the same thing with one lunar distance gives you 0.00080 = 0.08%.",
"So if something crosses both Jupiter's and Earth's orbits and (for simplicity) assuming that all the orbits are on the same plane (ok for asteroids, ok-ish assumption for short period comets, bad assumption for long period comets) then, very roughly, the object is 6.5/0.0013 = 4800 (or generously 6.5/0.08 = 80) times more likely to encounter Jupiter than hit Earth. (If you wanted to allow non-coplanar orbits it would be more appropriate to do comparison of Jupiter's Hill Radius and Earth's radius to the surface area of the spheres defined by their orbits, in which case encountering Jupiter is even more favored vs hitting Earth.)",
"That begs the question, how many encounters with Jupiter does it take to fling something out?",
"The solar system ",
"escape velocity",
" at Jupiter's distance is about 18.5 km/s. Jupiter's orbital velocity is about 13 km/s. If the object's orbit's semi-major axis is 12 AU (intermediate between Saturn and Uranus) it's going about 16 km/s at Jupiter's distance from the Sun (from the ",
"vis-viva equation",
").",
"Now a ",
"gravity assist",
": Basically, you take the relative velocity between the object and Jupiter (in this example 3 to 19 km/s depending on geometry), spin it in some direction, and vector add that to Jupiter's velocity (relative to the Sun). So, if the geometry of the encounter is such that at the end the object's velocity relative to Jupiter is in the same direction as Jupiter's velocity relative to the Sun then it's pretty easy to get to escape velocity and that object will leave the solar system.",
"For giant planets in general the story is more muddled. Saturn, Uranus, and Neptune tend to throw things inwards, towards the inner solar system, rather than outwards, and these planets could thus be considered detrimental. There are ",
"several papers by Horner and Jones",
" that begin to address this, but further study is needed (I'm uncomfortable with some of their assumptions for initial conditions and the way in which they quantify impact flux).",
"Edit: Refined link for Horner and Jones papers."
] |
[
"Truth be told, had Jupiter not existed, the object would have probably missed Earth anyway. Gravity is cool, but it's still quite weak on a very large scale, such as our solar system. It doesn't really make the \"bulleye\" that's necessary much bigger.",
"Now that that's out of the way, imagine trying to shoot a basketball flying through the air with a bullet, except you're blindfolded and only get one bullet every few years.",
"EDIT: More stuff",
"Also, keep in mind that Jupiter's gravity well is big, but not big enough to pull any objects that are about half a planet length into hitting it. Jupiter's \"Shield\" size is not very big.",
"Also, keep in mind that we have entire periods of time when Jupiter is on the other side of the sun. The amount of time that Jupiter would actually shield Earth is minuscule."
] |
[
"There had been a longstanding notion that Jupiter played a role over the history of the Solar System to clear away things that would have hit the earth. But apparently that picture has recently been studied via simulations, and the conclusion of that study is that there was no net plus or minus. You can read about this set of ideas and the recent study at ",
"http://www.dailygalaxy.com/my_weblog/2012/03/is-jupiter-earths-guardian-or-ultimate-destroyer.html"
] |
[
"If someone cover their eyes for a year straight without seeing any light, would it just be really bright when they take it off then slowly adjust back to normal or would it have a permanent affect on the persons vision?"
] |
[
false
] | null |
[
"Damien Eccles, who was imprisoned on death row for 18 years for a murder he did not commit; had his vision affected by being kept in low light for many years. It's been ten years since he was released and he says his vision has improved but initially he couldn't focus on anything more than 5cm from his face. I expect the subject would experience a similar inability to focus after not using their eyes for a year.",
"Edit: Echols not Eccles"
] |
[
"My understanding was that his issues were more to do with a lack of depth of field from the small cell he was kept in twenty-three hours a day, not the lighting."
] |
[
"When I went caving, they said that if we got stuck in the cave, we should turn our torches off (to save their battery) and close our eyes. Iirc they said it was better for our eye health to close them, than to have them open in pitch blackness trying and failing to focus."
] |
[
"What are the differences between a standard wind tunnel and a recreational indoor sky diving tunnel?"
] |
[
false
] | null |
[
"Skydiving tunnels have a taper (except in a short section at the entrance) to reduce windspeed with height, that way you can enforce a maximum height by reducing windspeed to some un-flyable level. Never seen such a taper in the few research tunnels i've seen.",
"Other than that? Not much."
] |
[
"Skyventure lists the average power draw of a 10' diameter recirculating tunnel (their smallest) at 205kw (275HP). They're powered by 2x 450hp motors. ",
"The tunnel I have used is 1200HP, and has a typical power draw of ~400HP. IIRC it uses 4x 300HP electric motors.",
"Baldor lists a TEFC general purpose 300HP motor at just shy of $40,000. 300HP VFD's list for $32,000 each. Then you need to get 1000kw service into the building. No idea how much that would cost, but I suspect it's something like \"a lot\". ",
"Then you need the hardware which can handle conditioning that volume of air (it's not totally recirculating - more like 80% i think).",
"Then you have to figure the thing might run, I dunno, maybe 11 hours a week on average? Not much time to make up some pretty big capital costs, on top of the more usual overhead like employees, insurance, advertisement and so-on. At $15/min that works out to ~$500k/y in revenue, so in two years your revenue has covered the hard costs (guessing it's a flat million bucks to build the thing) and start generating profit over your recurring costs. Maybe turns $2-300k/y in profit? Dunno. Sounds about the right sort of scale though.",
"Plus, it's a novelty so you can charge a lot for it. :-)",
"EDIT: also keep in mind that it's still really cheap compared with an actual skydive, and being able to do several minutes in a row offers a huge advantage for training."
] |
[
"Recreational air tunnels have significantly less airflow conditioning. The 'standard' air tunnel layout is a big rectangle, with the test section on one of the long legs and the fan directly opposite. This gives the designers the ability to include a large number of various flow conditioning devices to ensure that the airflow within the test section is as smooth and uniform as possible. Also, the tunnel is likely going to be a closed loop, so that the air in the tunnel stays in the tunnel and doesn't blow the researcher's papers around.",
"The indoor sky-diving facility I've been to (the one in Pigeon Forge, TN) had the fan operating directly below the cage/mesh floors and appeared to be open-loop, but upon some research I see that there can also be recirculating indoor skydiving facilities.",
"The key difference, then, is like the difference between a consumer-focused device and a scientific-focused device: your scientific device is going to be much, much more accurate and cost much, much, much more money."
] |
[
"Found an unusual rock while hiking yesterday. Possible meteorite?"
] |
[
false
] |
[deleted]
|
[
"Standard disclaimer: one cannot be positive about rock identification from a photograph. That being said, it really doesn't look like a meteorite to me: meteorites don't have vesicles (gas bubbles). Instead it appears to be basalt with some volcanic glass.",
"/meteorite expert"
] |
[
"Who thinks that!?"
] |
[
"Couldn't say... glacial perhaps? It never ceases to amaze me how rocks travel around to exotic locations... one encounters so many out-of-place samples. "
] |
[
"Eyes - How do our eyes compensate for movement in our head? How do they track moving targets automatically and smoothly? Why can't I follow the same path as smoothly without a target?"
] |
[
false
] |
Examples: First question - I can look myself in the eyes in a mirror and rotate my head in any direction without having to think about moving my eyes the opposite direction. Second question - I can watch a car drive down a road without thinking a about moving my eyes, and the movement isn't "jumping." Third question - I can't follow the path an imaginary car go as smoothly as if a car actually was there, it's very jumpy.
|
[
"This is called the ",
"vestibulo-ocular reflex",
" (VOR). Basically your inner ear senses the movement of your head and causes your eyes to reflexively move to compensate. This actually occurs independently of vision, and is present in newborns.",
"See number 3.",
"You have two main types of eye movements: saccades, and pursuits. Saccades are short, fast, jerky, ",
" movements. These occur when you don't have a specific target in mind, and you're just seeing what's out there. Pursuits are smooth, slow, ",
" movements. Our minds are very attracted to moving targets, and we evolved very precise eye movements to follow them (gotta keep an eye on those lions on the savannah). It's such a strong impulse, actually, that we literally cannot avoid following moving targets with our eyes to some degree (\"",
"Optokinetic nystagmus",
"\"). It's impossible to perform a smooth pursuit eye movement without a target. If you try, what you end up with is actually just a serious of short saccades (this is what happens when you read). Now, you mentioned how you can watch a moving car without the image jerking. Your pursuit eye movements are accurate only to a certain speed. Once the target is moving faster than a certain angular velocity (the specific number escapes me right now), the pursuits are replaced with the faster (but less accurate) saccade, and the image will be jerky.",
"Source: me (optometrist)"
] |
[
"Yes. And then some. When you're intoxicated your motor system control is screwier, and your pursuits tend to turn into saccades. Also, when you try to fixate on a target way off to the side, your eyes \"wiggle\" a little. This is called ",
"end-gaze nystagmus",
" (nystagmus = eye wiggling), and it is WAY more pronounced when you're drunk. ",
"More here",
".",
"Happy to help!"
] |
[
"Hey. I just want to say thank you. \nI have always wondered this as a child. \nI had asked my dad, my mom, my grandpa, my grandma all throughout my childhood, and they couldn't answer my question. ",
"When someone is intoxicated, and doing the DUI \"follow my finger\", are the officers then looking for Saccade eye movement to show intoxication? "
] |
[
"How does a substance like soot absorb light so well?"
] |
[
false
] |
It's easy to understand where photons go when they're reflected, or just allowed to pass right through. But what happens exactly when light is absorbed? Where does the energy of the photon go? Does it largely get transformed into disorganized heat? How does that happen exactly? What's absorbing it exactly? Just electrons? Or are atomic nuclei ever involved? Why it reflect or re-transmit the light? In addition, if the absorptive properties of soot for instance is due in large part to its content of carbon, how come a pile of amorphous carbon absorbs light so well, but organized structures of carbon such as diamond does not?
|
[
"Absorbed light does one of a few things to molecules (depending on energy): it will flip nuclear or electronic spins (in the presence of a magnetic field), it will cause molecule(s) to rotate, it will cause bonds to vibrate, it will add energy to an electron (promoting it to a higher shell) or it will add energy to an electron causing it to eject.",
"Where and how well it absorbs light is a bit funky. ",
"Where: these are the energylevels as specified by quantum mechanics. They, in gas phase,are delta functions (in frequency) but in bulk systems they begin to broaden and blur over wide ranges as all the atoms are in ever so slightly different environments and are no longer degenerate.",
"How well: This is something called the transition dipole. When light interacts with matter it is (generally) an interaction of electric fields (yeah yeah spins are magnetic). Mathematically its the overlap integral of the upper and lower state energy levels of the transition (the probability to transition between them. When a photon passes its probability of being absorbed is a function of this transition dipole, length of the interaction time, and frequency of the incoming photon. ",
"These dipoles and energy levels are different for every molecule making some different colors and other the same color but more transparent. If these are not met for absorption it will scatter or ignore each other.",
"Soot vs diamond: Well both are carbon but diamond is very homogeneous. This means that soot has many many many different molecular species and environments causing it to greatly increase the number of possible molecular transitions. This causes a very broad absorption.",
"Diamond however is transparent in the visible because it only has a very select few transitions and these transitions just so happen to be outside of this range. It WILL absorb light, just not visible."
] |
[
"To clarify a bit for the less mathematically equipped, absorption of visible light is largely the domain of electrons. When light is absorbed, an electron goes form a low energy state to a higher one, and this conserves the energy of the photon. What happens after is a different question. It might remit the light, it might give off some heat then first, it might lose all the absorbed energy as heat, or we might collect that energy in a solar cell. It is very materials, device, and conditions dependent. As for why is soot black and diamond clear, diamond doesn't have any empty and filled electronic states that are separated by the energy visible photons, so they can't be absorbed. This isn't so much because of the fact they are ordered but rather how they are ordered -- they could be ordered a different way to produce something black: graphite. "
] |
[
"But how does it go from \"electron in higher energy state\" to heat? Does it move back to some lower intermediate state between the first two states, and emits a photon with a partial amount of energy to the original, which may \"distribute\" the photon into energy that is more spread out?",
"I guess maybe this might be more generally asked as: What does ",
" ",
" at a quantum mechanical level in solid material? How much are the nuclei moving/vibrating in respect to the electrons? How much do the electrons move around from atom to atom, how much do they vibrate in place? How much do those aspects vary between different elements/materials? And so on.."
] |
[
"How does the Y chromosome attain genetic diversity?"
] |
[
false
] |
If it's the same Y chromosome passed down the male side, how does a person achieve genetic diversity like they do with X chromosomes?
|
[
"The Y chromosome doesn't achieve large-scale diversity from one generation to the next like the X chromosome in females does or autosomes (in males and females) do, it is passed down more or less intact from father to son.",
"On an evolutionary time scale it achieves diversity due to the accumulation of mutations. In fact the Y chromosome may be more susceptible to mutation than \"paired\" chromosomes: on a paired chromosome, if a mutation occurs on one homolog (one of the paternal or maternal copies of a chromosome) the mutation will often be repaired using the other homolog as a template.",
"The Y chromosome is therefore similar to mitochondrial DNA which is passed down more or less unchanged through the maternal lineage, and just like there's a suspected ",
"Mitochondrial Eve",
" there's a ",
"Y-chromosomal Adam",
". The Y chromosome contains a ",
" of repetitive sequences, suggesting its evolution features a lot of duplication events, so it's not evolving simply by small-scale point mutations (though those do happen, and even happen from one generation to the next to generate a little diversity in each generation).",
"There is one small exception to the lack of large scale diversity discussed above, about 5% of the Y chromosome consists of what's called pseudoautosomal regions. These regions have matching sequences on the X chromosome and they recombine like regular autosomes. About 20 of the 80 or so genes on the human Y chromosome are in this region.",
"Sources:",
". Cortez et al, Nature 2014",
". Bachtrog, Nature 2013"
] |
[
"Thank you very much, curiosity sated."
] |
[
"X And Y sex chromosomes are the last pair of chromosomes in the human genome (23 pairs in total). They help determine gender by encoding different hormone productions (dont do much for diversity). In other words, a mother and father pass down 23 chromosomes each to make a total of 46 chromosomes. A mother (females) only have XX sex chromosome pairs, as a father (male) has XY. Genetic diversity does not come from the sex chromosomes, it comes from the combinations of the 46 chromosomes from your parents, and mutations/crossing over during gametogenesis. A mother can only pass down X chromosomes so every child gets their X from her. Fathers determine the gender of a child because he can pass down his X (making XX) or Y (making XY). The X and Y encode for many different genes! But females have something called barr bodies that actually neglect one of her Xs from getting transcribed (people with trisomy ex XXY or XXX can be normal people because of barr bodies). The Y chromosome (does not get neglected) encodes for certain hormones that make a person develop male characteristics (evident at 60days conception). Example Sry gene on Y chromosome encodes testes determining factor which develop testicles (which produce testosterone). I hope this clears things up. "
] |
[
"If there was a tank that could hold 10000 tons of water and had a finger - width hole at the bottom and you put your finger on/in the hole, would the water not drain or push your finger out?"
] |
[
false
] | null |
[
"That depends on the shape of the tank. What matters is the pressure at the bottom of the tank, which only depends on the height of the column of water above the bottom. It turns out that 34 feet of water produces roughly atmospheric pressure, about 15psi. I'm pretty sure you can hold 15 psi with your finger, but I'm also pretty sure you'll have trouble with 10 times that. So if the tank is 100's of feet tall or more, you should worry about it leaking. If it's 30 feet tall or shorter, probably not a problem, though you'd be better off finding a cork than using your finger."
] |
[
"The amount of water makes no difference. Only the height of the column of water. The taller the column the higher the hydrostatic pressure at the bottom. ",
"Not sure how much you could contain with your finger. Maybe 50-100 psi. ",
"For a column of water weighing 8.3lb/gal pressure=0.052 x 8.3 lb/gal x depth",
"Assuming you can hold 100 psi then using the above formula, anything over 231 ft deep you couldnt hold back the water pressure any more.",
"These are calcs I use in the oilfield all the time."
] |
[
"You're right, by \"shape\" I meant \"skinny and tall\" vs \"wide and not tall\", but my usage can be taken the wrong way. Thanks for the clarification."
] |
[
"Is a sound mirror possible?"
] |
[
false
] |
[deleted]
|
[
"https://science360.gov/obj/video/1338647f-08cf-4036-8038-19220c9d4263/thin-engineered-material-perfectly-redirects-reflects-sound"
] |
[
"Perfect reflection is impossible - there will always be ",
" loss. But sound mirrors are a thing. Some were built on the south coast of Britain to warn of approaching enemy aircraft, before radar was invented. \"Whispering dishes\" are also a common demonstration in science museums.",
"https://en.wikipedia.org/wiki/Acoustic_mirror"
] |
[
"Yes, an echo is just a reflection of sound off a hard surface. The more smooth and rigid a surface is, the better it reflects sound. Mirrors can even be shaped to ",
"focus distant sounds",
". There is no way to perfectly reflect sounds just like you can’t perfectly reflect light, but you can do it pretty well. The biggest complication if you want a precise reflection is reducing reflection from other objects, which is why you mostly hear echoes off of big empty spaces like canyons."
] |
[
"Help Identifying a Scientific Instrument"
] |
[
false
] |
Hello ! A couple days ago while cleaning my grandfather's attic out I came across and I'd like some help identifying it. My grandfather bought it in the late 50's and he says that it is used similarly to a in that it would count a constant time for making waves on a smoke barrel in order to track fluctuations in stimulated muscle tissue. Does anybody know what this instrument is? I'd also like to see if I could get it working; apparently it's an electric-pendulum so that it swings back and forth on its own with assistance from a battery. If anyone has any tips or an idea about where to hook a battery up I would be very interested in such info. Thank you all for your help!
|
[
"Try asking in ",
"/r/whatisthisthing",
" "
] |
[
"I already asked them, waiting for a reply. I'll post here. :)"
] |
[
"They replied: ",
"Hello Andrea,",
"I have checked in with a colleague that has been with the company MUCH longer than I have. He is not positive exactly what it would be used for today, but it is some time of event time marker. This was not a standard product that would have been included in any of the catalogs; it would have been a custom built machine. Based on the location of the company on the sticker our best guess is that this equipment is from the Late 1950’s, early 1960’s. I must same I am quite impressed with the condition it is in! It looks like it has been taken really goo care of; the company sticker is not even ripped up at all! J",
"Best Regards,",
"Sarah Walsh",
"Senior Technical Support Specialist"
] |
[
"What can I do for self inflicted photophobia?"
] |
[
false
] | null |
[
"We don't offer medical advice. See the guidelines."
] |
[
"I'm not asking for medical advice, kind of life hacky type things I can do to help adjust. "
] |
[
"Not the right sub for that. "
] |
[
"Is transhumanism scientifically possible, or a hoax?"
] |
[
false
] |
The which was featured in Popular Science, spearheaded by some Russian millionaire purports that technology can be used to extend human being's lives longer than what they are now, even towards infinity ("Immortality") by cybernetics. Is this an attainable goal (as technology advances, some say towards a "technological singularity), would this hypothetical mechanism run against the laws of Thermodynamics, or is it all a scam or some sort?
|
[
"There's nothing in physics that says humans can't live indefinitely... With the exception of heat death.",
"However, the means by which you would need to go to make a human last an indefinite period of time might very well violate our definition of what we think a human to be today.",
"The technology of today is no where near the levels they would need to be though. ",
"The jury is out. Nothing says it's impossible, but that doesn't mean it is possible."
] |
[
"Look at the timescales they are suggesting. The thing isn't even up and running yet, but they are predicting brain-controlled avatars in the next 8 years. Have a read of ",
"this summary",
" of the last 40-odd years of research and see how confident you are about their first milestone. Then look a tthe ones they've set next.",
"It's absurd."
] |
[
"True immortality is probably impossible - eventually the universe (or the solar system) will run out of useful energy, but there's certainly nothing fundamentally impossible with the idea of extending our lifespan well beyond what it already is. That said, whether this can be achieved by 2045 is anyone's guess. "
] |
[
"What does it mean to say \"Dogs have better hearing than humans\"?"
] |
[
false
] | null |
[
"If you are asking in general it can mean either of those things."
] |
[
"Actually I was asking in the sense of why it's cruel to burst crackers near animals (with respect to hearing). \nSomeone spewed facts at me claiming that \"Dogs have 7 times the hearing of humans. If this noise is loud to you, imagine what it is for them.\""
] |
[
"Then I would ask a more specific question. Perhaps something like \"how sensitive is dog hearing and how does it compare to human hearing\""
] |
[
"Do glass lenses bend infrared and ultraviolet light in a similar way to their effects on visible light?"
] |
[
false
] |
Do lenses work on radio waves and X-rays or gamma rays?
|
[
"There's lots of catalogs of glass index of refraction and absorption online (try the Schott website for example). Long story short, yes they are bent or refracted just like visible light, but at a slightly different angle due the the wavelength dependent change in the index of refraction (see said web material). In general, common glasses do not change immensely in their index over the UV to IR range, so most visible lenses focus well at those wavelengths too. Absorption is a different story though, as UV is readily absorbed by most common glasses. Get yourself some fused silica or sapphire instead of something standard though, and it transmits well. The cost is appreciable, but in most high volume applications (windows, spectacles, etc) it's enough cost difference that a more commingling glass (like BK-7) is used. ",
"Source: optical engineer, this is my everyday life. ",
"Good question though, got any follow ups?"
] |
[
"Yes. Indexes of refraction are actually dependent on wavelength, it decreases as wavelength decreases. So x rays are refracted, just not as much. There are many uses for RF lenses and mirrors, in telescopes for example."
] |
[
"No, x-rays would be refracted more (shorter wavelength=more refraction). And this is ignoring whether glass is actually transparent to X-rays which I do not know."
] |
[
"I'm sitting in a wagon on a frictionless surface and have three bricks. If I throw each one, one at a time at 10 ft/s my final velocity will be higher than if I threw them at 10 ft/s all at once. How?"
] |
[
false
] |
To clarify, I'm sitting in a wagon, or am floating out in space somewhere, and I have 3 bricks. If I launch each one away from me at 10 ft/s relative to myself one at time I will be moving faster in the end than if I launched all three at once at 10 ft/s away from me. In my engineering dynamics class some time ago, the professor did this as a example question in the momentum chapter. He claimed that energy is not conserved in this system, so my question is, why?
|
[
"Think of it in terms of thrust. Assuming it takes 1 second to throw the bricks. In the first instance we have: Thrust = Mass ejection rate X Speed of ejection. So F = 30 kg/s X 10 m/s = 300 N.",
"Since F=ma, where we're trying to find the resultant velocity of ourself, we get a = 300 N / 100 kg = 3 m/s",
" Since it took 1 second to accelerate, that means our final velocity is 3 m/s.",
"OK, so if we throw one brick at a time: Thrust = 10 kg/s * 10 m/s = 100 N.",
"a = F / m = 100 N / 120 kg = 0.833 m/s",
" for 1 second = 0.833 m/s.",
"Then another brick F = 100 N",
"a = F / m = 100 N / 110 kg = 0.909 m/s",
" for 1 second = 0.909 m/s + 0.833 m/s = 1.742 m/s",
"Then the final brick, F = 100 N",
"a = F / m = 100 N / 100 kg = 1 m/s",
" for 1 second = 1 m/s + 1.742 m/s = 2.742 m/s",
"Why is this lower? Because it requires more force to launch 30 kg at 10 m/s every second (even though you only do it once) than it does to launch 10 kg at 10 m/s every second (despite doing it for 3 seconds). So the energy of the 30 kg/s exhaust system is higher than the 10 kg/s exhaust system.",
"That's why launching rockets into orbit is so expensive. If you could have a controlled release of all the fuel at one instant, it would require a lot less fuel to get into orbit. But since you can't, you wind up expending energy to accelerate some of your fuel (in our case bricks) just to throw them in the opposite direction.",
"In an inertial reference frame (take the origin of the wagon+bricks system), only the first brick will actually be travelling at 10 m/s, in that frame. The second brick will actually only be travelling 9.167 m/s and the third brick even slower at 8.257 m/s. You can actually calculate the momentum of the final system around this arbitrary stationary point in the system by doing (10 kg)*(10 m/s)+(10 kg)*(9.167 m/s)+(10 kg)*(8.257 m/s) = (100 kg)*V to solve for V which works out to 2.742 m/s just as calculated above.",
"Edit: And if you're wondering why energy isn't conserved, it's because these are inelastic events and kinetic energy is not traceable through series of inelastic collisions. ",
"http://hyperphysics.phy-astr.gsu.edu/hbase/inecol.html#c1"
] |
[
"AH, I see what's going on. He's taking the velocity difference between you and the previously thrown brick to always be 10 ft/s (or m/s, in this case the units are interchangeable). That changes the frame of reference and adds a slightl complexity to the momentum equation. We've all been assuming that this is the reverse of an inelastic collision. That is to say, you could look at this in reverse as a 10 m/s brick of 10 kg hitting a 120 kg boy with 2 bricks heading at the brick at a speed such that they'll be stationary when they hit and stick together. This was an incorrect assumption on our part.",
"What this does is add the second equation seen to the system of equations. That is the Vb - Vw = 10 equation. ",
"This will let us solve for both V brick and V wagon/Boy, while conserving momentum.",
"I'm doing my own calculations at the moment and will post them in an edit for you when I'm done.",
"Edit: And here's the Imgur album with my calculations to confirm your professors, along with some final thoughts. ",
"http://imgur.com/a/DFGzk",
"The moral of the story is, perspective is everything, and your prof was correct that energy is not conserved because this is an inelastic scenario."
] |
[
"But you throw each brick at 10 m/s relative to yourself. So for the second brick it would not be 10 m/s but 10 + 0.8333 m/s.",
" (Throw bricks 1 at a time)",
"First Brick: 10kg x 10 m/s = 120kg (2 bricks plus you) x V. V=.8333 m/s",
"Second Brick: .833 m/s x 120 kg + 10kg x 10.8333m/s = 110 kg x V. V=1.894 m/s",
"Third Brick: 1.894 m/s x 110 kg + 10kg x (10m+1.894)/s = 100 kg x V. V=3.27 m/s",
" (Bricks all at once)",
"30kg x 10 m/s = 100kg x V. V=3 m/s.",
"3.27>3.0 \nHow does this work in terms of energy conservation?"
] |
[
"Do other planets have techtonic plates? Why or why not?"
] |
[
false
] | null |
[
"Nothing that's quite like Earth. ",
"Mercury's surface basically hasn't moved in four billion years, except for a small amount of planetary shrinkage. ",
"Venus's crust moves, but it stretches and compresses rather than moving as solid plates, perhaps because of its higher surface temperature. ",
"Mars has some interesting deformation from its volcanoes, but no global plates. One possible explanation is that it's too small to retain enough heat.",
"Europa is probably the closest to having plate tectonics, with rigid mobile blocks that move around its surface, but when they come together one flows ",
" the other rather than under, and they both melt off the bottom in a process called \"subsumption\" rather than subduction.",
"Most of the other outer worlds are either boring, or have lots of crazy stuff going we haven't figured out yet. Pluto is definitely in the latter category.",
"https://www.nature.com/articles/ngeo2245",
"https://www.jpl.nasa.gov/news/news.php?release=2014-300",
"http://education.seattlepi.com/mercury-plate-tectonics-5919.html",
"https://arstechnica.com/science/2014/04/venus-crust-heals-too-fast-for-plate-tectonics/",
"https://en.wikipedia.org/wiki/Tectonics_of_Mars"
] |
[
"Just about everything regarding Titan's geology is an open question at this point."
] |
[
"Do we know about Titan? Seems a possible candidate given the lubricating hydrosphere and tidal stresses, no?"
] |
[
"How much do we know about hybrid birds, fish, and bugs? Also, how much is the topic of hybrid animals studied?"
] |
[
false
] |
I know many animals can hybridize, and most hybrids are sterile. Examples include ligers, tigons, wholphins, etc. And i know there is a lot that is unknown in this territory, such as which canids can produce hybrid offspring. My question is this: Many bugs, birds and fish seem very similar. How much do we know about their hybridizationability (is that a word?)? For example, can a bald eagle and a golden eagle mate successfully? What about an eagle and a hawk? Carpenter ant and a fire ant? Or two species of shark?
|
[
"A lot is known. You can pretty much go into Google Scholar and search for \"<type of animal> hybrid\" or \"<type of animal> reproductive barriers\". It's much better studied in some types of organisms than others. I couldn't find any examples of Bald or Golden Eagles making hybrids with anything, but other birds will."
] |
[
"Thanks!"
] |
[
"Thanks!"
] |
[
"How do I explain to my homeopathically inclined wife that vaccines are not made of poison, and should be required for our future children?"
] |
[
false
] |
[deleted]
|
[
"You're on the right track, but your explanation needs some adjustment.",
"You see, a vaccine is really a preemptive homeopathic remedy. What they do is start with a pure strain of the disease causing agent, and repeatedly dilute it to the point where it is no longer harmful. Then, they inject it into your body, and, by the standard homeopathic principal which is well known to all, causes the body to become familiar with the pathogen and able to fight it off in the future. "
] |
[
"Vaccines are easy to explain. They're not some wonder drug (or really a drug at all). What they do is they weaken the virus that they wish to inoculate against, put it in a solution and inject you with it. The virus doesn't cause autism, so why would it when weakened and put in saline?"
] |
[
"It's one thing to fear vaccines. I think it's natural to be worried about treating someone for a problem they don't even have yet (and hopefully never will). Reasonable people often dislike the notion of taking life-saving statin medications because they don't ",
" sick. It's OK to have such concerns, so long as one has a critical mind and evaluates the evidence that is available for themselves (or trusts the experts' evaluations of the evidence).",
"It's another to fear vaccines, reject all of medicine, and embrace homeopathy (the practice of diluting poison until there is nothing left but water and then calling that \"medicine\"). That is a much more alarming, harmful stance. It's also self-fulfilling. If you tell me that you will not accept evidence from the medical establishment, then all that's really left is bunk \"evidence\" produced by quacks who are trying to sell you something.",
"If she rejects the medical establishment irrationally, then I suspect rational arguments and things like evidence won't convince her. In that case, you should consider looking not at ",
"/r/askscience",
", but instead at ",
"/r/rhetoric",
" or something like that."
] |
[
"I've been working on a form of string theory. Could i get some help on understanding what I've found?"
] |
[
false
] | null |
[
"Hello,",
"We don't evaluate personal theories here.",
"Cheers."
] |
[
"you know where i would post this then?"
] |
[
"You could try ",
"/r/HypotheticalPhysics",
"."
] |
[
"Does pure methamphetamine produced in a laboratory setting still cause the loss of teeth and other physical maladies seen in photos all over the net?"
] |
[
false
] |
I've wondered if perhaps these meth guys looked like that because the product was bad due to it being produced in sketchy trailers by meth heads rather than chemists in a lab causing it to be tainted with harmful adulterants or if meth itself just caused it. Not that I'd want to do meth, I just wondered since it seems amphetamines, albeit different ones, seem to be prescribed to people daily.
|
[
"Correct me if I'm wrong, but I believe the physical maladies seen on meth addicts (scrapes, spots, missing teeth, etc.) is to do with the lifestyle that goes along with being a typical meth addict - getting roughed up for not paying for a score, stealing, not maintaining personal hygiene due to being high, etc.",
"I absolutely don't doubt there's many many meth addicts out there who you wouldn't even know were addicts from looking at them as they avoid the certain lifestyles that I mentioned above. (Prescription patients included)"
] |
[
"It's not any one thing really. Meth users are very often severely dehydrated and malnourished as they spend ever-increasing amounts of time/energy/resources in pursuit of the drug. They ",
" also live a more dangerous lifestyle. This, coupled with the dehydration/malnutrition, makes it much more difficult for them to cope with/recover from injury/illness.",
"Finally, yeah, a lot of it does have to do with the production method. Home meth labs use all kinds of crappy reagents, from battery acid, to drain cleaner, antifreeze, lantern oil, red phosphorous, ect. A lot of the intermediates and waste products are ",
" flammable &/or caustic/corrosive, and the makers are not the best lab technicians when stone cold sober. The production of meth also comes with a lot of highly toxic waste products. Outside of a legit lab, these often aren't properly extracted from the final product. Then there's cutting, where the final product is diluted with other compounds, to hit a desired concentration &/or boost sale profits (ie, 5g of meth becomes 10g of cut meth, doubling the amount of 'meth' you can sell)."
] |
[
"Meth teeth are caused by the drug drying up the saliva glands. Meth users constantly have dry mouth which interferes with the mouth's natural state, causing the teeth to rot from the inside out. So yes, perfectly pure lab made meth would still rot the teeth.",
"As for the face lesions / scratches....those are caused by meth driving the user to pick and scratch at the skin neurotically. Which would also still exist in \"pure\" meth users.",
"If anything, the negative effects would be worse, as the drug would be much stronger in the absence of \"cut\" (impurities)"
] |
[
"Can someone help me understand the relationship between light and gravity? I know that nothing can travel faster than the speed of light, so why is it that the gravity of a black hole can exert enough force to prevent light from escaping?"
] |
[
false
] |
[deleted]
|
[
"It's a perfectly legitimate view that a black hole is something like a waterfall - once you've passed through the horizon, space-time inevitably flows into the singularity. The event horizon is special in the sense that it's the point where space-time flows at the speed of light. Farther out, space-time is flowing slower than the speed of light and coser to the singularity, space-time flows faster than the speed of light. In extreme situations like around a black hole, space-time can flow faster than the speed of light.",
"See: ",
"http://jila.colorado.edu/~ajsh/insidebh/waterfall.html"
] |
[
"What units would spacetime flow have?"
] |
[
"Gravity doesn't exert a \"force\" on the light. The mass of an object (such as a black hole) is literally warping space and light is following that curvature. ",
"Check out ",
"Gravational lensing",
" for another amazing phenomenon that is a consequence of gravity bending the path of light."
] |
[
"How does the discovery of the universe's accelerating expansion affect calculations of its age?"
] |
[
false
] |
I know that other data affect this calculation, but if backwards extrapolation in time based on measured expansion rates is one of the main methods by which we estimate the age of the universe, how is our estimation of ~13.75 billion years affected by the acceptance of the fact that such expansion is accelerating, rather than decelerating as Newtonian physics would predict?
|
[
"The acceleration has only set in relatively late, so it doesn't influence the age too much. Well, it's around 3 billion years, so the order of magnitude stays the same. You can see a plot ",
"here",
". Before 1998 we thought the curve labeled \"Omega_M = 0.3\" describes the expansion history of our Universe. In that case, the scale factor becomes zero around 11 billion years ago - an event usually called the \"Big Bang\". But since we found out that the expansion history is actually more like the curve labeled \"Omega_M = 0.3, Omega_Lambda = 0.7\", the Big Bang was really 13.7 billion years ago.",
"Also, note that Newtonian physics isn't really applicable here. But you are right in the sense that general relativity, the theory that replaced Newtonian gravity, also predicts a decelerating Universe - but only if there is no dark energy. "
] |
[
"I am by no means a physics expert, but it's my understanding that the findings are fairly well accepted. The three astronomers behind it won Nobel Prizes last year. ",
"http://www.reuters.com/article/2011/10/04/us-nobel-physics-idUSTRE7931ET20111004"
] |
[
"This is correct.",
"http://www.nobelprize.org/nobel_prizes/physics/laureates/2011/"
] |
[
"How did scientist come up with and prove carbon dating?"
] |
[
false
] | null |
[
"So, as you may know, the number of protons in an atom determines what kind of element it is. In it's stable form, carbon has 6 protons and 6 neutrons, and is otherwise known as carbon 12. Nitrogen, the next element on the periodic table, is most stable with 7 protons and 7 neutrons, aka nitrogen 14. ",
"Sunlight in our atmosphere causes atomic particles, like neutrons, to be blasted around (I can explain this more if you'd like). When normal Nitrogen 14 in the atmosphere comes into contact with a free flying neutron, it causes that nitrogen atom to gain the neutron, but also to immediately lose a proton. Since the atom now has 6 protons, it is officially carbon, but since it also has 8 neutrons, it is an unstable (and radioactive) form of carbon, Carbon 14. Carbon 14 behaves just like regular carbon, but since it is radioactive, it slowly decays into stable Carbon 13. This decay can be detected using a Geiger counter and its relative abundance can be quite easily measured.",
"Carbon 14 is generated in the atmosphere at a very constant rate, making it's concentration both in the air and inside every LIVING thing quite predictable (about 1 per trillion carbon atoms). However, when organisms die, they stop recycling carbon, so they no longer collect new Carbon 14. The Carbon 14 that they do have slowly decays, so the organism's concentration of the radioactive isotope is also slowly depleted.",
"Depending on when an organism lived (whether it's a tree 50,000 years ago or a squirrel 30 years ago) it will have some amount of Carbon 14 remaining. As such, the ratio of carbon 14 to stable carbon atoms can give us a very accurate measure of how long ago this organism stopped taking in new carbon (died). This is the basis of carbon dating.",
"TL;DR - carbon 14, a radioactive isotope of carbon, is generated at a constant rate in our atmosphere. Its concentration in the atmosphere is mirrored in all living organisms. When an organism dies, it's concentration of c14 slowly depletes. Depending on the ratio of remaining radioactive carbon to stable carbon, we can quite accurately estimate how long ago the organism lived.",
"Edit: Arg, sorry. As a number of people have pointed out, I am wrong about how carbon 14 decays. One of the extra neutrons actually decays into a proton, returning the element to a stable nitrogen 14 atom (not carbon 13). Apologies. Carbon 13 is stable, but forms in a different manner."
] |
[
"Two chemists, Martin Kamen and Samuel Ruben, were looking into ways to essentially radio-tag carbon so they could track it performing various metabolic tasks in living animals. This is a fairly common technique to this day - I've used radio-tagged steroids, for instance, injected into living things to see where they ended up, since radioactive things are relatively easy to detect in very small quantities. Kamen and Ruben bombarded nitrogen with radiation and some of the atoms turned to radioactive forms of carbon. C-11 turns out to be not so useful as it has a half-life (the period it takes half of the atoms to decay into other stuff) of about 20 minutes. That's not long enough to study much of anything as it takes time to run experiments. C-14 now, that's a solid 5500 years or so, which is also not great studying processes in living things as it decays too slowly (in lab-time).",
"Another chemist named Willard Libby realized that naturally produced C-14 in the atmosphere would only enter organisms while they were alive (all else equal, but that's another story). That sounds promising because it would essentially put a 'clock' on any suitable living thing, as, after they die, they stop picking up new C-14 and just cook off steadily. And a 5000 year half-life is pretty useful too, as lots of really interesting stuff happened with humans over the last 40-50,000 years (several half-lifes out). Or so we thought, as before C-14, we didn't have a very good idea how old most things really were. Sometimes we had written records and people had laboriously worked out things like tree-ring sequences, but every living thing has carbon in it so this could potentially work on virtually ",
".",
"Libby was right, and won a Nobel Prize in Chemistry in 1960. C-14 remains the gold standard for dating although debate continues about how far back it works, and how dates can end up looking 'too young' or 'too old' because of various things like contamination. ",
"EDIT: hey, thanks for the gold!"
] |
[
"Carbon dating is often the best dating method when it comes to human history. That is, its time frame and uses fit very well with what we are trying to discover about our past. Most geology uses different kinds of radiometric dating, as C-14's limit of 100,000 years is way too small to be useful for the entire span of earth history. ",
"Samarium-neodymium and rubidium-strontium were some of the first methods to really take off, since they can provide ages for rocks that are billions of years old. Nowadays, U-Pb is preferred by most geologists when it is applicable, as there are two different isotopes of uranium that both exist with sufficient abundance in nature and decay to lead. This allows more sophisticated analysis of ages, and leads to a very impressive accuracy for some very old materials. There have been zircon crystals over 3 billion years old dated with a margin of error of less than a million years.",
"Other isotopic systems are often used, such as argon-argon dating, rhenium-osmium, uranium-thorium, lutetium-hafnium, etc. Other systems have been used for very specific investigations, such as the use of an extinct isotope of tungsten--tracked by looking at concentrations of its daughter product--in determining how quickly the earth's core formed. Wikipedia actually has a very good run-down of ",
"radiometric dating",
".",
"While carbon 13 is a stable isotope and thus does not undergo radioactive decay, your instinct is correct in that scientists must be wary of other elements that can decay into either the parent or daughter product in question. In such cases, care must be taken to either use these finicky methods where the third element will not be present to come into play, or to conduct further analysis in order to separate contributions from radioactive decay from populations initially present.",
"Hope that makes sense. Good questions!"
] |
[
"Does someone with a compromised immune system become vulnerable to diseases that they had been vaccinated against?"
] |
[
false
] |
For example, will a person who is vaccinated for polio, diptheria, whooping cough become susceptible to the diseases if they contract HIV/AIDS?
|
[
"Yes.",
"Vaccines don't actually do anything by themselves. They sensitize the immune system so that it can respond early enough and aggressively enough to eradicate an infection before the person gets sick.",
"In a patient who is immunosuppressed, the immune system may not be able to respond aggressively enough even if it is already sensitized. So people who are immunosuppressed may still be vulnerable to infections even after prior exposure or vaccination. They depend on \"herd immunity\" to protect them.",
"This is one of the biggest and least-discussed problems with the anti-vaccination movement. People who are unvaccinated not only risk their own health and that of other unvaccinated people, but also the health (and possibly the lives) of people whose immune systems are weakened by AIDS, leukemia, anti-rejection drugs for organ transplants, and other conditions, even if those people have been vaccinated.",
"(Note, however, that an HIV-infected patient may still have a healthy immune system. Many people with HIV have normal T-cell counts and undetectable or nearly-undetectable viral loads thanks to modern antiretroviral drugs. Only patients whose HIV infection progresses to AIDS are at risk.)"
] |
[
"Adding on the the question, as many compromised immune systems stem from a deficiency in T/B white cells, would recent infection and the resultant high antibody levels present in serum result in a better response to disease in the immunocompromised? (Activation of complement). ",
"[Opsinization of pathogens notwithstanding.]"
] |
[
"From what I understand, if your adaptive immune system has been exposed to a particular antigen and developed antibodies for it - that does not guarantee that the cells of the adaptive immune system will be able to neutralize the threat. They may be unable to effectively localize to the targets, phagocytose the target or continue to produce the proper signaling cascades necessary to recruit more cells to assist.",
"Lastly, many threats such as fungi or bacteria, actually fight back and so the cells of an immunocompromised organism may not effectively be able to do this, despite having the proper tools at their disposal thanks to vaccines."
] |
[
"If a photon leaves the sun travelling in one direction, is another photon leaving on the opposite side of the sun travelling at twice the speed of light relative to the first photon?"
] |
[
false
] | null |
[
"The term \"relative velocity of B with respect to A\" typically means \"the velocity of B in the rest frame of A\". Since photons do not have rest frames, it is meaningless to talk about the relative velocity of anything with respect to a photon.",
"You are likely thinking of \"mutual velocity of A and B\" which is simply the difference in the velocities of A and B, as seen from some third observer C (sometimes C is required to be inertial, sometimes not). So if you see a photon moving directly away from you and a second photon moving directly away in the opposite direction, then the mutual speed of the two photons (according to you) is 2",
".",
"Mutual velocity is really a special case of so-called \"coordinate velocity\" because it arises simply as the time derivative of some spacetime coordinates. In the example above, the first photon has coordinates (t, ct, 0, 0) and the second photon has coordinates (t, -ct, 0, 0). The difference in their coordinates is (0, 2ct, 0, 0), and the time derivative of the spatial part is (2c, 0, 0), which is just a velocity of magnitude 2",
". The magnitude of coordinate velocity (as compared to ",
") is rather meaningless because coordinate velocity can really be anything. You can always choose some wacky coordinates. A famous example is the coordinate velocity of galaxies in a FLRW cosmology. The so-called recessional velocity (the speed given by Hubble's law) can be arbitrarily large.",
" And just to be clear, relative velocities do not add linearly, in case the opposite was the assumption that prompted your question in the first place. If the relative velocity of B with respect to A is ",
" and the relative velocity of C with respect to B is ",
", then the relative velocity of C with respect to A is not ",
" = ",
"+",
", but rather",
" = (",
"+",
")/(1+",
"/",
")"
] |
[
"Great explanation"
] |
[
"The FLRW model is a simplified cosmological model. It approximates the large-scale geometry of the universe based on the assumption that the universe is homogeneous (every point looks the same as any other point) and isotropic (at any point, each direction looks the same) on very large scales.",
"The FLRW metric basically has three geometries: a flat universe, a spherical universe, and a hyperbolic universe, based on how dense the distribution of matter in the universe is."
] |
[
"Has any research been done on human asexuality?"
] |
[
false
] |
That is, why are some people asexual? Why do some people never have sexual desires?
|
[
"There's a quite comprehensive bibliography of studies ",
"over here",
", if you're interested. But research into asexuality is pretty new, and given that we haven't found a definitive explanation for why people have any other sexual orientation, we're probably going to be waiting a while for this one."
] |
[
"so is it only the lack of desire for physical intimacy or do you not want to have kids either(by using a carrier or artificially inseminating yourself or your partner). If the latter case is true(no kids), how do you feel about the abrupt end of your genetic line?"
] |
[
"No, I'm not interested in having kids, either. I don't really see the end of my genetic line as being a huge issue, but it bothers me slightly."
] |
[
"Are there different constellations on Mars?"
] |
[
false
] | null |
[
"No, they are the same as on Earth. You have to go very far away (comaprable to the distance to the stars in the constellations) before they look different. If you were to try to measure the parallax of stars (the slight change in apparent position between closer and farther stars due to the rotation of the planet around the sun), it would be greater on Mars than on Earth."
] |
[
"Am I correct to assume that the difference would be negligible even on Pluto?"
] |
[
"Yes"
] |
[
"I'm helping a friend of mine with a test for her master thesis on psychology. I need your opinion on the validity of the test. [XPOST from askreddit]"
] |
[
false
] |
[deleted]
|
[
"You are asking this person to trust data someone else collected to make a subjective judgement or measurement that these images were 'scary'",
"As I said, you would ONLY claim \"this is what people believe\" and would not ever claim that it is objectively true. Certain things, like \"fear\", have no objective meaning as they are based completely on the arbitrary subjective experience of a person. You can still make completely objective statements about the population of human beings. There is no reason to claim you are making an objective statement about the nature of reality if you are actually not doing so. Of course, then your claims become much less falsifiable and everyone realizes you're not even trying to do real science any more... so psychologists, and \"researchers\" who work in other fields try desperately to trick people into writing off the difference between 'people drawn from this population have been shown to believe...' and 'it is this way.'"
] |
[
"her idea is to create a test in which the correct answers are created by the users itself similar to a sort of Data Gathering game.",
"I'm not sure why this would be necessary. From the example you provide - the cat picture - surely it would be enough to say that the ",
" answer is \"cat\", regardless of what the majority answered? Yes, that means that potentially only a minority could answer with the genuinely correct option, but that's not exactly novel in psychology. ",
"especially since the test will deal with emotions, and they are extremely subjective.",
"How is the test going to \"deal with emotions\"? If the pictures themselves are going to be labeled as somehow emotional, then you or your friend may need to run a pre-experiment so that a decent sized sample of people can rate how emotional they find a particular image.",
"I, as a potential experimenter, might find myself in agreement with the majority of potential participants when rating a picture of a crying puppy as very sad; but I might find myself in violent disagreement with them when I rate a picture of a crying Justin Beiber as an image which causes me joy.",
"(trolls and \"hacekrs\" will be accounted statistically).",
"How? If the test is multiple-choice (and your cat example doesn't make it clear whether this is the case), then outliers might be difficult to determine. Even if the test ",
" multiple-choice, how are you or your friend going to decide whether somebody responding that the cat is a dragon are a) trolling or b) just mistaken? "
] |
[
"Every question you ask had better begin with \"In your opinion\" or something similar. And any conclusions ever drawn had better make it absolutely clear that no truth was involved in the test. Truth only exists when things correlate to reality. When you just correlate them to one another, you just end up with borderline meaningless correlations. Yes, stupid people love to use correlations like divining rods to try to lead them to wisdom, but they get similar success rates.",
"In order to make it more rigorous, actually test something. Ask a question which has a factually true answer. And if the answers given are not factually true, judge them false, regardless of opinion. When 99% of the world believed that rats spontaneously generated from dirty rags in a corner, it did not make that belief true. It just meant that most of society believed in wrong things. ",
"Dealing with emotions does not escape you from a burden of objectivity, in fact it heightens it. If you are talking about \"fear\", for example, then you need to get together a bunch of people (probably hundreds), subject them to things which they independently classify as 'frightened', figure out what physiological states correlate with fear, etc then you can come up with an objective test for what most people would call fear. If you do not do this, and instead take the easy way out and just rely on peoples intuition or on the opinions of the majority, you're just helping prove that psychology is still a century or more away from actually being a real science. Whatever you discover will be worthless. Luckily, your friends commission will likely be made up of people who have too much personal investment to admit that, really, what was done was not science and is, at best, profoundly misleading."
] |
[
"If light moves only in space (and not in time), is there something that moves only in time (and not in space)?"
] |
[
false
] | null |
[
"Is there a better way to think about it? And what is an example of a stationary object? In a universe with gravity, is a stable stationary object even possible?"
] |
[
"Is there a better way to think about it? And what is an example of a stationary object? In a universe with gravity, is a stable stationary object even possible?"
] |
[
"That's right, and makes sense! I guess to be more specific, what I'm wondering is this: just like four-velocity of a beam of light is from an observer's point of view constant regardless of the position of the observer, and the proper time is zero between 2 events from the point of view of that beam of light, is there something that has the inverse property? Ie. An object with a four-velocity of 0 from the point of view of any observer, and a correspondingly large distance between any two events along its world line?"
] |
[
"What is \"different\" about a photon at the peak vs the valley of the wave?"
] |
[
false
] |
Hi all. What is the difference in a photon's electric and magnetic properties (if any) at the peak vs the trough of the two aspects the wave? Does something different happen if a photon hits an object at the peak vs the trough? I'll thank you in advance though I have a feeling I'll have follow-ups.
|
[
"To get to the heart of your question, I don't think this is a very productive way to think about wave-particle duality. However, the direct answer is that at a trough the electric field is -A, where \"A\" is the amplitude of the wave, and at a crest it's +A. But getting back to, I believe, the spirit of your question, for 99% percent of cases there is no reason to think of light as anything other than an electromagnetic wave with the quirky feature that its amplitude, or intensity, only depletes in certain clumps (given by E=hf). It's just a wave through and through but its brightness only changes in small discrete amounts.",
"The only time any \"particle\" nature of light comes into play is if you dial down the intensity/brightness on your light source so far down that the power it's outputting per second falls below that of a discrete jump. That's the only regime where things get weirder beyond the \"EM wave that depletes in whole clumps\". In that case one will find the ",
" intensity still behaves exactly as expected from our EM wave picture, it just shows up ",
"little points at a time",
". That's the fringe case where we must entertain the idea of assigning particle-like properties to light, even though the OVERALL, AVERAGE shape is that of our wave as before.",
"However, such behaviour is unfortunately not as simple as saying \"our wave was particles all along!\". As I said, each of those points is really statistically sampled from our EM wave and if you trace back its trajectory it would have no connection to that of what we might think of as a particle-like trajectory. Furthermore, repeating the exact same experiment would produce a new set of points but the same set of AVERAGE wave shape. And that's why your attempts to, kinda, staple - kicking and screaming - wave properties onto a particle leads to a very confusing mental picture. Wave-particle duality is what it is and one thing we do know about it is that we're never going to wake up one day and find out that some rogue genius has found a flaw in our thinking and secretly it was just particles behaving with funny rules all along (stating this more concretely, we do know that quantum mechanics is not secretly a so-called local \"hidden variable\" theory (i.e. there are funny local rules going on that are deterministic, we just don't know about them))."
] |
[
"\"A photon being at its peak\" isn't a thing. An electromagnetic wave always has peaks and troughs behind each other, and you'll always be hit by all of them in sequence. For a single photon these concepts become ill-defined."
] |
[
"informed more by intuiting from days at the beach that the crest has more energy than the trough because that was what I was hit with",
"Well teeeccchhhhnnnnniiiccccaaaaallllyyyy it's the nodes (the points where E switches from positive to negative and is zero) that have zero energy. The crest and trough both have maximal electric field magnitude, just with opposite signs, but essentially yes.",
"So is the \"fabric of the universe\" or some quantum field the medium in which the wave is propagating (I'm aware the ether theory has been long disproved)?",
"I think people maybe push back too hard about the idea of the \"ether\". Both in classical and quantum physics light and electric and magnetic fields are excitations of an \"EM\" field that permeates all space. However, crucially such fields don't behave in a \"Galilean way\" where one can have motion relative to them, but rather a \"Lorentzian way\". So much intuition one has about such a medium, like \"air in a room\" or \"water in the ocean\" will be wrong."
] |
[
"How fast is the Universe expanding?"
] |
[
false
] |
And how could we calculate it? I think I heard the observable universe is not the edge of everything.
|
[
"Lots of related stuff here, you should take a look.",
"Don't think of the universe as a single object that's expanding (i.e. like a ball that gets bigger). This gets confusing because the universe is boundless, and most likely infinite in size: there's no real meaning to something infinite \"getting bigger\".",
"Instead, think of the expansion of the universe as \"everything getting further from everything else\". If the average distance between galaxies is 1 million light years at one point in time, then at some time in the future the average distance between galaxies will become 2 million light years, and so on.",
"As for the actual speed of expansion, there is not exactly one single number for it. The speed at which something is receding from us depends on how far away it is. In a not-very-correct-but-sorta-metaphorical-sense, space is expanding, and the more space there is between us and the distant galaxy, the more space there is to expand, and the faster it's moving away from us.",
"So we can measure this, and the answer is what's called the Hubble Constant. It's about 70 km/s per megaparsec. A megaparsec is a bit over 3 million light-years. For every 1 megaparsec of distance, the expansion of space is causing the galaxy to move away at 70 km/s. So if something is 5 megaparsecs away, space is expanding to make it move away at 350 km/s. If it's 10 megaparsecs away, it's moving away at 700 km/s. And so on."
] |
[
"The Hubble constant tells you that. Its equal to roughly 70 kilometers per second per megaparsec. A megaparsec is about 3 million light years, so what that means is take a distance of 3 million light years, wait one second, its now 3 million light years + 70 kilometers long."
] |
[
"That's basically right. This is where it get a little more tricky: you can't actually go faster than light, but this result is correct because this is not an actual ",
". The distance between these objects is increasing faster than the speed of light, but their ",
" relative to any object is always slower than the speed of light: you don't get weird time-travel contradictions because you can't actually use this method to travel between two events faster than light.",
"But note that this is just the radius of the observable universe (i.e. how fast something 93 billion light years from us receding). There's more universe beyond the observable universe: our measurements tend to point towards the universe being infinite. So there are objects that are receding at 1000 times the speed of light. We will just never see them."
] |
[
"Is rotation a relative or absolute movement?"
] |
[
false
] |
We may only measure velocity in relation to some other object, but this doesn't seem to hold for rotational velocity. Why is that?
|
[
"This is a really deep question. I can't really give a good answer beyond no, rotational velocity is not a relative measurement.",
"The fancy-pants reason is that while velocity transformations ('boosts') are a symmetry of nature, rotational velocity transformations ('twirls'?) are not.",
"In Newtonian mechanics, if you 'twirl' into a rotating frame of reference, 'fictitious' forces appear that were not present in the original frame (e.g. centrifugal force); this is the sign of a non-inertial reference frame in which Newton's laws don't hold. (Modified versions that include the fictitious forces do, though.) This implies that there is some universal 'non-rotating' reference frame where all fictitious forces vanish. On the contrary, boosting into a moving frame of reference leaves Newton's laws invariant and all the forces are the same. (This goes over in relativity too, with the addition that the speed of light is also the same in all boosted frames.)",
"The deep part, at least to me, is that this universally non-rotating frame is also the one in which distant stars appear stationary. In any other frame, distant stars would appear to be moving (rotating around you) faster the further away they are. This is the sort of thing that's obvious until you think about it hard. Many bright people think that there is some deep connection here such as ",
"Mach's principle",
". By some accounts Einstein was trying to embody Mach's principle while formulating general relativity, but he didn't really succeed (he found that ",
" matter determines local spacetime geometry, rather than distant matter like stars).",
"I'd better stop before having to call a philosopher. In short, that's the way the universe rolls (or rather, doesn't)."
] |
[
"That's a question that has been bugging philosophers and physicists for a long time; ",
"debates about it can be traced back to the times of Newton",
".",
"As we currently conceive it, velocity is relative ",
". If you're accelerating, you're doing it in every inertial reference frame. Rotation implies an acceleration. Even if the ",
" of the velocity vector doesn't change in a uniform circular movement, the direction of the vector changes constantly. Acceleration can thus be defined as the derivative of the velocity vector with respect to time.",
"From the above you may deduct that ",
". Without such force objects would move in a straight line according to the law of inertia, not rotate. In the case of a rigid body spinning on its own axis, centripetal force is provided by the material's resistance and it does cause a strain. ",
"In fact they shatter if they spin fast enough",
". This is certainly not relative, it will happen in every reference frame."
] |
[
"Velocity has both magnitude and direction. An acceleration is a change in velocity, and therefore an object experiences acceleration if we change the magnitude of its velocity or the direction of its velocity. A race car gaining speed on a straight track is an example of a change in the velocity's magnitude (a linear acceleration). A race car traveling at constant speed but turning a corner is an example of a change in the velocity's direction (a centripetal acceleration). An object sitting in a rotating reference frame experiences centripetal acceleration. For both kinds of accelerations, the inertia of the object is being forcibly changed, and therefore in the rest frame of the object the object experiences an inertial force. For the race car picking up speed, you are pressed back against your chair. For the race car turning a corner, you are pressed against the outer side of the car (this is the centrifugal force).",
"The presence of these inertial forces indicate that the rest frame of the object being accelerated is now a special frame that is not equivalent in a simple way to other reference frames. I would not call such a reference frame \"absolute\" since that word infers an underlying absolute universal single preferred reference frame that you have managed to tap into, which is not the case. I would simply call such a reference frame \"non-inertial\" meaning that there is now a preferred direction as a result of the acceleration. A person in a race car gaining speed can run an experiment (e.g. let go of a ball) and find out the preferred direction (towards the back of the car) and therefore the direction of acceleration. Similarly, a person in a race car that is turning a corner can run an experiment and find the preferred direction (towards the side of the car) and therefore find out the direction of acceleration. ",
"A non-inertial reference frame is internally absolute in the sense that there is an absolute preferred direction determined by the acceleration, but it is not absolute in the sense of being tied to some fixed, underlying, absolute, preferred reference frame.",
"People often misunderstand this and say that the earth rotating about its axis relative to the fixed starts is equivalent to the stars rotating about earth's axis while the earth stays fixed. This is incorrect. The earth experiences inertial forces from its spinning. The stars do not experience inertial forces from being uniformly rotated about earth's axis. Put another way, hurricanes are shaped by the Coriolis force, which is an inertial force caused by Earth's rotation. If the earth were really still and the stars were rotating, the Coriolis force would not be present and hurricanes would not have the shape they do. In this way, acceleration breaks the symmetry between reference frames and makes them not equivalent. The stars may look to be the ones rotating in the sky when viewed from earth's surface, but we can do local experiments in earth's spinning reference frame to detect that the earth is spinning and in which direction it is spinning. In this sense (the non-equivalence of accelerating reference frames), a rotating reference frame can be thought to be absolute.",
"General Relativity complicates this picture by stating that an accelerating frame in flat spacetime (no gravity present) is equivalent to a non-accelerating frame in a particular curved spacetime (gravity present). But this does not change my argument because whether it be due to acceleration or due to gravity, such a reference frame has a preferred direction and is in no way equivalent to a non-accelerating frame in flat spacetime. "
] |
[
"Are there clear examples of subgroups in a species diverging on a clear path to become different species?"
] |
[
false
] |
We see tons of examples of the “Before and After” with documented cases of evolution from a single parent group like the Finches of The Galápagos Islands. I am looking for examples of us observing the “during”. What I am looking for are subgroups within a species where individuals can still mate across groups to produce fertile offspring but are distinct enough to indicate that they will one day be distinct species. I definitely understand that evolution and taxonomy are both fluid and more of a spectrum than a binary condition but wanted to see if there were any examples of a species with separate sub groups or populations that fit that description!
|
[
"Hindsight is 20/20. While we can look at things that are clearly distinct species right now and figure out (with varying degrees of accuracy) how long ago they probably separated, there really is no guaranteed way to identify as definite speciation event while it is happening. I think ",
"this figure",
" does a really good job of highlighting the possible outcomes that can occur in a situation like you describe. In your scenario, we have two populations that have diverged in some way. This might be because they were physically separated for some time (i.e., allopatry; studies of speciation tend to focus on this because it's relatively \"clean\"), or maybe they have stayed in the same area but still managed to split into overlapping groups with some distinctions (this concept, sometimes called sympatric speciation or more generally just \"speciation with gene flow\" probably can and does happen in the real world, but is a lot messier to study). For now though, let's just go with the more standard allopatric model in which the two populations were physically separated for a while but later reunited. As outlined in the above figure, there are generally four options for what happens next, which I'll briefly go over:",
"reinforcement",
"display mating preferences",
"Dodd, 1989",
"region of hybridization between",
"Aguillon et al. 2018",
"Selz et al. 2014",
"Larsen et al. 2010",
"So to wrap up, there are a variety of fates that can await two populations of individuals in the situation you describe, and it is very difficult to know exactly what the future will hold for them. As you correctly point out, speciation really is a somewhat fluid spectrum and a process rather than an event that can be directly pinpointed (well, except for certain polyploid plants I guess). That said though, and depending on how far along they are, we may be able to pick up on trends going on in the lineages in question to get some idea of which direction they are headed in. If you're actually just looking for a list of examples, ",
"this website",
" has a decent number (though it hasn't been updated in some time), though again a lot of these are still too early to really say for sure if they will become distinct species or not."
] |
[
"This is an awesome reply! I looked up allopatry and it seems like examples of Allopatric Speciation, as you pointed out, are probably the easiest to point to a candidates for the divergence that I was looking for. I had not thought much about the hybrid zones and the article on flickers was very interesting. It feels like there should be more Galapagos-ian examples of clear allopatry with a species diverging before us. I will dive into the list you shared!",
"Apart from geographic separation due to distance or natural barriers, I wonder if there are emerging examples of species being separated due to emerging urban centers, where urban and non-urban counterparts develop different traits (specifically thinking of species like rats)"
] |
[
"Yeah, I'm sure there are plenty of other examples out there. Another very famous example (which is included in the list I linked) is apple maggot flies in genus ",
", which appear to have undergone speciation based on two different host plants (with one group feeding on native North American hawthorn, and another having switch to apples after they were brought over from Europe). This system is a bit complex though, since it is sometimes used as an example of sympatric speciation, but other instead argue that it is more accurately described as \"heteropatric\", since the two groups of flies overlap at large spatial scales, but probably do not interact at small spatial scales if that makes sense.",
"The idea that human urbanization may be driving speciation has been explored to some extent too. Coincidentally I actually ",
"just answered a question",
" about evolution in moths in response to light pollution. If you have access to it, ",
"Thompson et al. 2018",
"30200-3?rss=yes) is a nice review of some of the theory behind how speciation might progress in these environments, with a few examples mentioned too. And also with respect to rats in particular, check out ",
"Combs et al. 2018",
", which found that there are measurable genetic differences between rats in uptown and downtown Manhattan (though not nearly enough to be considered distinct species yet at least)."
] |
[
"If you had a bunch of charged particles in a sealed room, and you didn't know which were pos. and which were neg., how would you figure it out?"
] |
[
false
] |
The "rules" in this situation are that you can't use any matter of known charge. So if you want to make an electromagnet, you have to use the unknown charges to do it. The root of the question is: is there anything fundamentally identifiable about positive and negative charges, or are they only defined by their relationship to each other?
|
[
"If they were all fermions, you place them all in left-handed chiral states. The ones that undergo weak interaction are negatively charged."
] |
[
"You wouldn't be able to distinguish between quarks and antiquarks this way."
] |
[
"The only thing that is coming to mind is building a particle accelerator to produce ",
"CP-volating",
" particles."
] |
[
"How do ELISA assays detect protein concentration?"
] |
[
false
] |
I understand the idea behind the ELISA, that you bind antigen to plate, antibody to antigen, and then another antibody to that antibody with a color indicator. That second antibody is bound to an enzyme and you add the substrate, which then causes a color change as the bound enzyme consumes the substrate. What I don't understand, is how do you test for the presence of another protein using the test? I did one with the sandwich method where the top antibody was bound to Horseradish Peroxidase with an indicator, added HRP substrate, and then we somehow are supposed to detect the presence of eukaryotic initiation factor 4? How does that work?
|
[
"What do you mean test for the presence of another protein? ",
"In your example you were looking at EIF-4 using a sandwich ELISA. So, to make this very basic:",
"First primary antibody is on plate, let's say that it corresponds to amino acids 1-15 and we'll call it mouse anti-EIF4. ",
"You then add your antigen (likely cell or tissue lysate which should contain EIF-4).",
"Next you add your second primary antibody, let's say this recognizes amino acids 60-80 of EIF-4, we'll call it rabbit anti-EIF4.",
"Now, this second primary may be HRP-conjugated or you may need to add a secondary antibody. Let's say it's unconjugated. You then add HRP anti-rabbit (which binds the second primary).",
"Add luminescent reagents so that the peroxidase can do it's thing.",
"Based on absorbance values, you can either \"quantify\" relative amounts (Does sample X have more EIF4 than sample Y?) or you can quantify using known concentrations of EIF4. Either way you want to do a standard curve - either, for example, 1x 2x 4x 6x or 1M 2M 4M 6M (these are random numbers) so that you can get a linear range and an idea of the sensitivity of your assay.",
"Does this answer your question?"
] |
[
"Yes, it does. ",
"For some reason it never dawned on me that the \"antigen\" is actually the protein. Thank you!"
] |
[
"No problem! Terminology can definitely take time to get used to."
] |
[
"Are halogen ions (Cl-, I-) toxic to microbes or is it just the elemental forms (Cl2, I2) ?"
] |
[
false
] |
For example, would iodized salt in a isotonic solution be very toxic to bacteria?
|
[
"It's also worth mentioning that most of the halogens do exist in organisms in some ionic form, and that anything in too high concentration would eventually kill bacteria. 10g/L helps the bacteria grow, but if you increase too much you'd see cell lysis since the salt would be too highly concentrated."
] |
[
"Chloride isn't. In fact, the most common growth medium, LB, contains 10g NaCl per litre. Iodide isn't tolerated as well and exhibits some toxicity. The antibacterial iodine preparations used have a way higher concentration of iodide and elementary iodine than iodized salt, though."
] |
[
"Of course. Besides being a biochemist, I also cure bacons, sausages and hams as a hobby, which is where that comes in. Ultimately, that's not straightforward toxicity, though, but rather an effect of osmosis."
] |
[
"Is there a difference between how the human body processes added sugars vs naturally occuring sugars?"
] |
[
false
] |
[deleted]
|
[
"Glucose is the body's fuel. Fructose is processed differently. Most of it does get turned into glucose, but it takes time and has ",
"similar metabolites to alcohol",
", which is why a diet high in fructose causes non-alcoholic fatty liver disease and cancers.",
"So the high-fructose corn syrup added to processed foods is probably bad for the body in ways that naturally-occurring sugars are not, due to the higher levels of fructose without the accompanying fibre to slow absorption. ",
"This article",
" has some good references to look through."
] |
[
"Note: HFCS is typically not any higher than 55% Fructose. It's only nominally \"worse\" than Sucrose.",
"The issue is quantity.",
"The USDA estimated sugar consumption in the US to be over 150 lbs per year on average in 2014.",
"This is over three times as much as 100 years earlier."
] |
[
"So just to clarify, \"High Fructose Corn Syrup\" is high in fructose compared to regular corn syrup, which is mostly glucose. HFCS is about the same as table sugar."
] |
[
"Does light actually slow down when traveling through an optical thicker medium? Does this hold true in general relativity?"
] |
[
false
] |
Also: how much truth is there in saying that photons get constantly absorbed and re-emitted when traveling through a medium?
|
[
"But when they move through a solid they constantly hit things and get absorbed and re-emitted (just as you said",
"That is actually a common misconception. ",
"This",
" is a good article about that topic."
] |
[
"Thanks, great article."
] |
[
"As natty_dread said, the absorbed and re-emitted thing was actually not true. I've heard that before and wanted more information about it."
] |
[
"How do scientists benefit from Foldit?"
] |
[
false
] |
I get it that they find out the structure of the proteins or stuff, but how does that actually help? As you can see I am not versed on this, so I can't even proper question. Please and thanks.
|
[
"Basically, ",
" a particular protein sequence doesn't necessarily mean we know the shape it will take. Simulation programs like Folding@Home uses your computer to attempt to find the lowest energy configuration - so imagine your protein as a long chain that's ",
" sticky towards each other (hydrophobic interactions can lead those residues to clump up). While that's good, sometimes you get into a \"local minimum\" in energy - that is, you get stuck in a shallow valley. This gives a plausible, but not necessarily the correct protein conformation.",
"Foldit allows for human input to bypass this. As the program teaches the player a simple set of rules (i.e. \"these beta sheets can clump together, etc.), players can recognize the \"likely\" conformation a protein may take. Note that features like \"jiggling\" in the game is still similar to using thermal energy to find the lowest energy conformation.",
"The reason that this is necessary is that often proteins isn't synthesized as a long linear chain that folds according to these \"rules of hydrophobicity.\" Often there are helper proteins - such as ",
"chaperonin",
" - that alters the environment the protein is in and allows for folding that doesn't normally occur in a free protein in solution.",
"Finally, knowing the correct protein conformation will give us ideas on how it works. In enzymes, for example, the shape of the active site is exactly what allows for catalytic activity.",
"Addendum: As per discussion below:",
"The reason Foldit is explored is because humans can excel at what machines cannot - novel strategies at exploring the conformational landscape of proteins, often requiring dramatic changes in the backbone positions of proteins in question."
] |
[
"Spent a year running MD simulations of proteins and small peptides, and I just want to point out that \"Molecular dynamics simulations can avoid this problem\" is also a little misleading.",
"Basically, MD works by simultaneously simulating many (say 50+) proteins in more or less identical environments, but different temperatures. Then at every time step (say femtoseconds), there is a chance to swap your simulated protein with the one in the higher or lower temp environment, effectively raising or lowering the temperature. At the end this generates an ensemble of proteins that are thought to represent the dominant native structures. ",
"Key point. This process is SLOOOOOW. Not starting up crysis slow. WAAAAY slower. For reference, I was running a TINY peptide (15 Amino Acids), and it took about 36 hours to get 10 nanoseconds of simulation on 40 nodes of a super-computing cluster. ",
"The big issue for larger proteins is that not only do the simulations take longer (the algorithms actually scale fairly well), but that you have to simulate WAAAAY more time. There are proteins known to fold in the SECOND timescale. Extrapolating, this means that a single simulation can take MONTHS of supercomputer time. MONTHS!!!!",
"TL;DR Yes MD simulation can achieve the same results as Fold-It users. Is it cheap? No. Is it convenient? No. Is it fast? No. "
] |
[
"Spent a year running MD simulations of proteins and small peptides, and I just want to point out that \"Molecular dynamics simulations can avoid this problem\" is also a little misleading.",
"Basically, MD works by simultaneously simulating many (say 50+) proteins in more or less identical environments, but different temperatures. Then at every time step (say femtoseconds), there is a chance to swap your simulated protein with the one in the higher or lower temp environment, effectively raising or lowering the temperature. At the end this generates an ensemble of proteins that are thought to represent the dominant native structures. ",
"Key point. This process is SLOOOOOW. Not starting up crysis slow. WAAAAY slower. For reference, I was running a TINY peptide (15 Amino Acids), and it took about 36 hours to get 10 nanoseconds of simulation on 40 nodes of a super-computing cluster. ",
"The big issue for larger proteins is that not only do the simulations take longer (the algorithms actually scale fairly well), but that you have to simulate WAAAAY more time. There are proteins known to fold in the SECOND timescale. Extrapolating, this means that a single simulation can take MONTHS of supercomputer time. MONTHS!!!!",
"TL;DR Yes MD simulation can achieve the same results as Fold-It users. Is it cheap? No. Is it convenient? No. Is it fast? No. "
] |
[
"If I cut a piece of wood in half, why can't I put it back together?"
] |
[
false
] |
If the atoms are naturally attached to each other, and I separate them, why can't I put it back together? Also, why does this work with liquids, but not with solids? Why is it that if I have two separate pieces of iron, I can't stick them back together, but I can melt it and freeze it and it will be attached to each other? EDIT: Spelling
|
[
"So there's actually a lot going on in your questions...",
"Atoms can attach to each other only if they are in compounds (covalent bonds). Atoms can also be associated with each other through electrostatic interactions (ionic bonds). Molecules can be associated with each other through a host of interactions, such as hydrogen bonding, dipole-dipole, london dispersion, etc.",
"When you cut a piece of wood in half, you are not cutting atoms away from each other...what you are doing is destroying the cellular architecture of the wood, and once you do that, there is no way to piece it together since it was originally a very dense matrix of cells that adhered to each other. This adherence was originally mediated by electrostatic interactions, hydrophobic bonding, protein-protein interactions, etc when the tree was still alive. Once dead and dried, these interactions are essentially non-existant. So when you cut it, its cut for good.",
"You can easily separate liquids into two containers, and then pour them together again because its an inherent property of liquids - liquid molecules are able to move about freely, interact, and intermix.",
"Solids however, are rigid and their molecules are immobile. In the case of a piece of metal, you can imagine it as protons of the metal floating in a sea of electrons, everything ",
" moving. So when you separate a piece of iron, there is no ",
" way to put the two together immediately, since the molecules don't move and intermix. When you heat it, you turn the solid metal into a liquid, granting the molecules the ability to move freely, to intermix, and when solidified, you are back to one piece.",
"Edit: fixed wording thanks to all the redditers that brought cold-welding to my attention!"
] |
[
"In the case of a piece of metal, you can imagine it as protons of the metal floating in a sea of electrons, everything not moving. So when you separate a piece of iron, there is no way to put the two together immediately, since the molecules don't move and intermix. ",
"Actually, this isn't true. 2 pieces of metal will fuse together in vacuum in what is called ",
"Cold Welding",
"."
] |
[
"This will answer your question about why you can't just stick two pieces of iron together.",
"The short answer is: in a vacuum, you can.",
"http://en.wikipedia.org/wiki/Cold_welding"
] |
[
"Is it possible for a kinetic impact or explosion to induce a solar flare powerful enough to disable satellites? Not that I'm planning on making a doomsday device or anything..."
] |
[
false
] |
Actually, I'm trying to harden up some science-fiction I'm working on by determining plausible realistic scenarios. I thought the question was interesting and difficult enough that it would be worth asking here where it might get some attention from fully-fledged astronomers and physicists. Given how energetic solar flares are, I don't even know if a small amount of added energy is enough to 'prime' a solar flare that is perhaps orders of magnitude greater in energy - is this even possible to begin with, and what sort of energy levels would be required? If this energy were to come from a falling object, how big would the object need to be in order to not melt before striking? Would targeting an area where the sun's magnetic field is already unstable reduce the required energy? If this energy were to come from a massive nuclear or anti-matter explosion (for instance, from a large solar-powered anti-matter factory in close solar orbit) rather than a kinetic impactor, would the different types of energy released be more likely to induce the magnetic effects required for a solar flare? I won't be overly disheartened if the required energies prove implausible to fit into my fictional setting because I'm nonetheless curious about the astrophysical implications of my questions.
|
[
"I sincerely doubt you could do anything at all that would effect the sun in the least, let along force it to give on a flare. The sun is fucking ",
" compared to us.",
"Plus, the flares come from weird twisting of the sun's magnetic field, so I doubt chucking explosives into it would make a big difference. The best you might be able to hope for would be to possibly throw Jupiter into the sun in order to try and make a splash, but even that's a but iffy."
] |
[
"Looks like the orbital energy of Jupiter is about 1.62*10",
" J, and from wikipiedia, a solar flare is \"a large explosion in the Sun's atmosphere that can release as much as 6 × 10",
" joules\" So, if Jupiter were to be suddenly directed into the sun, it would release close to 10 orders of magnitude more energy than I'm talking about. ",
"Put another way, a solar flare is something on the order of 10 billion fusion bombs. So, I'm not just talking about chucking explosives at the sun - I'm wondering if I have explosive equivalent to a mere 10 million fusion bombs, for instance, either from a massive antimatter explosion or dropping a large asteroid or comet into the sun, could I set off a solar flare by disturbing the surface of the sun?"
] |
[
"Even if you were to do that, what resulted would most likely ",
" be a solar flare. You get a huge splash of solar material, which would probably fuck everything up, but it still wouldn't be a solar flare."
] |
[
"You go back in time to visit Mozart. You play him some Daft Punk. Does he recognize it as music?"
] |
[
false
] | null |
[
"I would play him Rock Me Amedeus"
] |
[
"But then he would know when he was supposed to die and Marty McFly would have to save him!"
] |
[
"This question may be better suited for ",
"/r/asksciencediscussion",
" or ",
"/r/askhistorians",
" if rephrased"
] |
[
"Do lithium ion batteries (such as those found in laptoips) lose storage capacity from charged while the device is being used, and if so, why?"
] |
[
false
] |
So I've heard that lithium ion batteries lose storage capacity from being overcharged and from using the device while charging. Are either of these true, and if so why do they occur? EDIT: Sorry if this should be flared under something else, I wasn't really sure where to put it.
|
[
"Hi, first we need to understand what the batteries themselves are made of. In the case of lithium ion batteries (modern ones) they work on intercolation chemistry.",
"Intercolation chemistry is where you get a material that has spaces in it's crystal structure, sort of like a sandwich with no filling, two layers of material and a gap to put in your bacon or sausage for example. Manganese dioxide is an example of such a material and is used in many batteries.",
"http://en.wikipedia.org/wiki/Manganese_dioxide",
"Now, the battery works just like a normal battery, you have an anode, a cathode and an ion medium in-between to allow the passage of ions from anode to cathode and back again. The first generation of Lithium ion batteries used an electrode of lithium and a graphite anode as the intercolation compound. The problem with these batteries was that the lithium electrode would grow dendritic structures, little arms of Lithium that would stretch across the ion medium. Here is an example of this happening:",
"http://www.youtube.com/watch?feature=player_embedded&v=MZ0mM7ktmvE",
"Note how the lithium (silvery stuff on the right hand side of the video) grows as the video goes on and eventually short circuits the battery. Also note that the graphite (on the right) cracks at the bottom.",
"http://lithiumbatteryresearch.com/Dendrites-and-Fracture.php",
"To combat this, the sony rocking chair battery was developed, where both anode and cathode are intercolation compounds to prevent this sort of thing happening. Here's a paper discussing it:",
"http://rsta.royalsocietypublishing.org/content/354/1712/1577.abstract",
"And a comment I made on a similar thread:",
"http://www.reddit.com/r/askscience/comments/16reh2/since_electricity_is_so_fast_how_come_it_takes/c7yrdu1",
"Now, we've tackled the problem of side reactions (dendritic growth and electrode cracking) killing or battery, or have we?",
"The short answer is no, there is still side reactions that occur which will lower the storage capacity of the battery, such as reactions of the ion medium and reactions in the intercolation compounds themselves meaning they just don't work as efficiently. Here are some papers that look at these issues:",
"http://jes.ecsdl.org/content/145/10/3647.short",
"\n",
"http://www.sciencedirect.com/science/article/pii/S0378775301008217?np=y",
"So, to answer your questions:",
"However, modern batteries come with safety measure like \"fast\" and \"trickle\" charge, so it'll charge really fast to say 80% and trickle from there on. If you keep charging it after the 80% that's when most of the damage starts to happen (though it'll still get damaged anyway, using the battery damages it)",
"Lithium ion batteries work best under partial charge conditions, never let them run dry and don't worry about using them whilst charging. The built in systems will mean they don't go through charge and discharge cycles rapidly. i.e it'll stop charging then wait to drop to a certain percentage of power before it charges again.",
"Here's an interesting paper that gives a good overview of lithium ion batteries:",
"http://www.nature.com/nature/journal/v414/n6861/abs/414359a0.html",
"Hope this helps :) Feel free to ask questions etc.",
"Yes overcharging with damage them. No using whilst charging will not damage them."
] |
[
"Lots of factors go into why batteries lose capacity over time, but specifically for lithium ion batteries:",
"Repeated charging/discharging (general use) and heat in general can cause whatever electrolyte used in the battery to decompose and increase the internal resistance of the battery. Overcharging increases the rate at which this occurs (depending on the method of charging) by causing additional heat buildup. Lithium ion battery capacity has been found to be particular affected by temperature compared to other battery types.",
"Many things about batteries: ",
"http://batteryuniversity.com/learn/article/discharging_at_high_and_low_temperatures"
] |
[
"Wow, thanks for the detailed response - I certainly learned something :)"
] |
[
"What is the process of \"turning on\" a nuclear reactor?"
] |
[
false
] | null |
[
"The only non-military graphite moderated reactors in our country produce tritium and weapons grade plutonium for our military. In our country, in commercial reactors we moderate with boric acid. With the boron concentration at 2400 ppm, The control rods are withdrawn and then the process of de-borating (dilution) begins until the reactor becomes critical (self sustaining reaction). Whoever wrote the previous pulled it from an online text and has no personal experience, though it does read well. Chernobyl type reactors are graphite moderated. When they get too hot, they burn. Three Mile Island partially melted, but was of very little danger to the public though it was a long and difficult clean up. I refueled commercial nuclear reactors for 25 years. They are safe. When you think about how many people die in the oil and gas industry daily, it's more than have ever died in the history of nuclear power in our country. I really enjoyed my job. All my coworkers were smart, responsible and honest. It's probably the only industry that has such a high caliber workforce. S.O.N.G.S. San Onofre Nuclear Generating Station 1981 to 2006. ",
"As a footnote... Any coal fired power plant puts more radioactive material in the air in one day than a commercial nuclear plant is allowed to release in a year. Due to there being no requirement to report this in the 10 CFR ( Code of Federal Regulations - Energy), the public is not informed. Those power producers need to be closed due to the many pollutants they produce. Use less energy. That's what we can do individually to change our impact on the environment."
] |
[
"The only non-military graphite moderated reactors in our country produce tritium and weapons grade plutonium for our military. In our country, in commercial reactors we moderate with boric acid. With the boron concentration at 2400 ppm, The control rods are withdrawn and then the process of de-borating (dilution) begins until the reactor becomes critical (self sustaining reaction). Whoever wrote the previous pulled it from an online text and has no personal experience, though it does read well. Chernobyl type reactors are graphite moderated. When they get too hot, they burn. Three Mile Island partially melted, but was of very little danger to the public though it was a long and difficult clean up. I refueled commercial nuclear reactors for 25 years. They are safe. When you think about how many people die in the oil and gas industry daily, it's more than have ever died in the history of nuclear power in our country. I really enjoyed my job. All my coworkers were smart, responsible and honest. It's probably the only industry that has such a high caliber workforce. S.O.N.G.S. San Onofre Nuclear Generating Station 1981 to 2006. ",
"As a footnote... Any coal fired power plant puts more radioactive material in the air in one day than a commercial nuclear plant is allowed to release in a year. Due to there being no requirement to report this in the 10 CFR ( Code of Federal Regulations - Energy), the public is not informed. Those power producers need to be closed due to the many pollutants they produce. Use less energy. That's what we can do individually to change our impact on the environment."
] |
[
"reactor pumps are also turned on to increase decay heat in the reactor vessel, bringing it to operational temperature.",
"No, it is literally the fuel melting. The fissile material (i.e., the uranium or plutonium) is encased in a cladding so that it cannot mix with the coolant (water in most reactors). When a plant has a \"meltdown\" it is due to the fuel getting too hot - at a high level this is either because the nuclear reaction has gone supercritical (uncontrolled) or there is some issue with the power plant such that the coolant is not flowing over the core at a rate sufficient to remove the heat being generated by the fuel. In either case, the fuel will get hotter and hotter as it creates energy that is not being removed, and eventually the fuel will reach the melting point of the cladding and the core will \"meltdown\""
] |
[
"X-rays from CRTs"
] |
[
false
] | null |
[
"Depends on the power of the CRT. If your electric potential is less than 10kV you will not get x-ray discharge no matter how long you keep the beam in one place. Most x-ray vacuum tubes require electric potential >30kV."
] |
[
"The easiest way to test for x-rays is with a geiger counter, but if you don't have one of those, you can use your cellphone. x-ray emission makes a distinctive white noise effect when your camera is exposed to it. ",
"http://youtu.be/HbQU8-LOg00?t=10s"
] |
[
"As I recall color TV's (what I use) are in the 25kv range. It seems I may be on dangerous ground."
] |
[
"During beta decay, when a neutrino and a beta ray are created, where does it's mass come from?"
] |
[
false
] | null |
[
"The binding energy of the daughter nucleus is larger than the binding energy of the parent. The extra energy is released as the masses and kinetic energies of the particles in the final state."
] |
[
"Any resource which covers beta decay should cover the energetics of it. I like the book ",
" by K. Krane. It's written at the level of an undergraduate studying nuclear physics for the first time."
] |
[
"What can I read to learn more about this? "
] |
[
"How does radiometrically dating rocks work if all radioactive isotopes came from super novae millions of years ago? Wouldn't all rocks have the same date?"
] |
[
false
] | null |
[
"The key aspects are ",
" when does a rock or mineral incorporate either a radioactive isotope or the child isotope created during decay into its structure and ",
" when does a rock or mineral become a closed system such that the child isotope accumulates and no more parent is incorporated. In short (and expounded on at length below), the formation of a new mineral or rock creates new mixtures of elements and isotopes (and critically either includes or excludes radioactive isotopes and their decay products at different ratios, based on the mineral in question), which will of course be limited by the composition of what it forms from, but will be enriched/depleted in different elements and isotopes depending on the structure / chemistry of the mineral in question. This, along with how radioactive decay products either accumulate or don't in a given crystal, is why they do not all have the same age.",
"For ",
", this almost always is when crystallization of a mineral from a melt occurrs. Imagine a magma, i.e. a liquid, with lots of elements/isotopes floating around. As it cools, minerals starts to form (and there is a relationship between temperature along with the composition of the magma as to which minerals form at a given time, e.g. ",
"Bowen's reaction series",
"). Different minerals will incorporate (or exclude) different elements based on the minearls ",
"crystal lattice structure",
" (we'll skip over the chemistry that controls which elements make up the lattice, etc), meaning that some minerals incorporate radioactive isotopes into their structure, either as a trace component (e.g. ",
"U, ",
"U, and ",
"Th in the mineral ",
"zircon",
" where some of the sites in the lattice normally occupied by zirconium are replaced by a uranium or thorium) or as a main component (e.g. ",
"potassium feldspars",
" which have potassium is a primary component within the lattice, some of which will be ",
"K). Ideally, when the crystal forms, no child isotope is incorporated (which is effectively the case for zircon, as the products of decay of uranium and thorium, lead, does not fit easily into the zircon crystal lattice) or the child isotope leaves the crystal easily until some point (more on that later, but this is the case for ",
"K, which decays to ",
"Ar and diffuses out of the crystal efficiently at first). While this assumption of no child isotope seems naive, we have a variety of ways to check that this is the case (e.g. for U-Th-Pb systems, checking to see that both uranium-lead isotope pairs yield the same age) and methods to correct for incorporation of some unknown amount of child isotope (e.g. through the use of ",
"isochron",
" diagrams).",
"For ",
", crystallization of the mineral from a melt gives you a particular starting mixture which will start to change as radioactive decay progresses, but the \"clock\" doesn't actually start until the child isotope is retained within the crystal. At high temperatures, while solid, the rates at which elements can \"diffuse\" out of minerals will be high (e.g. ",
"this set of notes on generally how diffusion in minerals works",
"). The rates at which a child isotope diffuses out of a mineral depends on the mineral, the isotope in question, and the temperature, with rates of diffusion decreasing as a function of temperature. At a certain point, the temperature drops below a threshold where diffusion of the child isotope out of the crystal effectively stops (i.e. the mineral becomes a closed system with respect to that particular set of parent and child isotopes). This is referred to as the ",
"\"closure temperature\"",
" or ",
"Dodson, 1973",
" if you want the original paper. For some minerals/systems, the closure temperature is effectively the same as the crystallization temperature (e.g. U-Pb in zircon), but for many minerals/systems this closure temperature is ",
"well below",
" the crystallization temperature (e.g. (U-Th)/He in the mineral apatite does not become closed until ~75 C). This means that the \"date\" we calculate may represent crystallization or the time at which a mineral cooled below a certain temperature (which could be recording a variety of geologic processes). The ability to date the timing of when minerals cool below certain temps forms the basis for ",
"thermochronology",
"."
] |
[
"There's a great in-depth explanation here already, but I'm going to try for a short and simple one, okay?",
"If you think about the history of a particular ro k, it's been sitting there in basically the same state for probably millions of years. But at some point, the rock formed. However it formed, through various chemical and physical processes, it incorporated a few radioactive materials in it. But once it's formed, it can't take any more. Importantly, if it's incorporating a radioactive isotope, it probably doesn't incorporate that isotope's decay product, because those products don't have the same chemical or physical properties. So, the only decay product that should be there is the stuff that decayed. Find the ratio of radioactive isotope to decay product, and you can math out how old the rock is."
] |
[
"Ultra-short answer: the atoms may have come from supernovae or other celestial events, but the *rocks* didn't form until much later, and it's the formation of the *rock* that we're attempting to date.",
"(The long answer already here goes into nice detail on what moment in the formation of the rock we're dating, and how.)"
] |
[
"Would it be possible to accelerate a space craft on Mars into a low Mars orbit without chemical propulsion?"
] |
[
false
] |
As a thought experiment, I was wondering if a Mars base could be a better choice than Earth as a hub for space exploration. Mars has all the resources of a planet, i.e. minerals like Thorium, Iron, Aluminium, Magnesium etc., and it has three advantages over Earth: Lower gravity, a much less dense atmospere, and no-one on the ground who could get hurt by accidents. I understand that the big problem with current spacecraft is the chemical propulsion required to escape Earth. Rocket fuel is dangerous and bulky, and rocket engines are inefficient. I heard about development of high-energy Ion engines which are significantly more efficient, put have very little thrust. In order to remove chemical engines from spacecraft, would it be possible to design a spaceport on Mars that could electromagnetically catapult a spacecraft into a low Mars orbit, or maybe even just to a height where the low-thrust Ion engine could begin spiralling the craft out of the gravity well? Both the spaceport and the spacecraft would be nuclear powered, of course. What would be the required speed to enter a low Mars orbit? Is the idea even remotely feasible, or would the spacecraft (aerodynamically shaped) burst into flames from friction, even in the thin Martian atmosphere? What would be the approximate mass of a suitable spacecraft that has a nuclear reactor, a cluster of Ion engines, fuel and a payload? How long would an electromagnetic rail need to be to get the craft to the required speed? 1 km? 100km? I know that there are plenty of SciFi scenarios that make use of space elevators, but I think building a 10km railgun for spaceships is way easier than a space elevator...
|
[
"Well, my reasoning is this:",
"moving any kind of serious payload from Earth into space using current rocketry tech is expensive, and the launching devices and propellants are not reusable.",
"I doubt we will be able to do continuos interplanetary exploration (as in: weekly launches of reusable vehicles) or even asteroid mining if we have to move every gram of spacecraft matter from Earth into space.",
"We will not be able to do this kind of space travel with chemical propellants. For interplanetary travel within reasonable timeframes we simply need an Ion drive, and they are already in development, so that's great. But if we need and will get an Ion drive anyway, why add chemical propulsion if we can find a way to do it without?",
"Chemical propellant is a large amount of mass, it is dangerous, difficult to manufacture, and it is lost after use. Ion engines are a magnitude more efficient, and some will take a variety of materials as propellant.",
"The method I mentioned would have an initially high startup cost, but then every start would be comparativly dirt cheap, and would not use anything but electrical energy."
] |
[
"So, if your criteria for a \"space base\" is metal resources, then an asteroid is far far better than any planet. Gigatons of metals, and no significant gravity well to take them out of.",
"What you're talking about is a launch loop and you can read all about it on wikipedia. While the energy needed to reach LMO from the Martian surface is less than the energy needed to reach LEO from Earth's (making the power demands of a Martian launch loop less than those of a terrestrial one), the industrial capacity to construct such a system just doesn't exist on Mars. One would need to either industrialized Mars, which means colonize Mars, before that could happen, or bring all those construction materials from Earth, an extraordinarily expensive proposition and one not likely to occur while the asteroids are so close with their cheap resources."
] |
[
"Yes, the same thing could be done on an asteroid. I suggested Mars because Mars has some gravity and confirmed resources that would allow for a rather large human colony (especially water), and because it is the closest planet/location where a colony would be thinkable.",
"Obviously, some kind of human colonization would be required. The idea wasn't meant as an immediate project, but as the endgoal of a step-by-step colonization attempt. Basically a reasoning for the colonization of Mars: \"If we built this, could it be the key to opening up the solar system to humankind?\"",
"Thanks for the launch loop point, I had not actually heard that term before.",
"Despite the effort of building an industrial base on Mars, I am still wondering if the idea is feasable at all..."
] |
[
"Can plasmids form in the shape of a mobius strip?"
] |
[
false
] |
And if so, would this have any consequences for their transcription or replication?
|
[
"The classic way to form a Mobius strip is to start with a ribbon and join the ends. But given a ribbon, there's two ways to do that: ",
"A ------------ B",
"C ------------ D",
"If you attach A to B and C to D, you just end up with a circular ribbon, while if you flip one end and attach A to D and C to B, you get a Mobius strip.",
"When we're talking DNA, though, you have to remember that a strand of DNA is not just like a piece of string. It has ",
"directionality",
". So if you think about forming a circular DNA starting with a linear piece of DNA, it will look like this:",
"5' ------------ 3'",
"3' ------------ 5'",
"Now if you want to join the two ends together, there's only one way to go about it: 3' to 5', giving you a boring old non-Mobius circular DNA."
] |
[
"You nailed it. The structure and chemistry of nucleic acids does not allow the formation of a proper Mobius strip.",
"Double helixes of nucleic acids are always anti-parallel. There is no known replicative polymerase that can synthesise DNA in the 3' to 5' direction."
] |
[
"If I'm reading your comment correctly, then it's no longer a mobius strip. It'll have two distinct sides."
] |
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