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[
"How many years did it take ecosystems to recover from K-Pg Extinction event (Chicxulub asteroid)?"
] |
[
false
] |
Hi, I'm doing research for fun stuff. From my understanding of article from Nature, the impact winter after the K-Pg meteor was caused by soot (and/or sulfur) in the atmosphere reducing sunlight penetration. And that the 10-ish years of the impact winter was much more responsible for the extinction event than the impact itself (ie. the sonic waves and tsunamis and heat and magnitude 11 earthquake and stuff). I would like to figure out exactly what the timeline was for the world's recovery after K-Pg. This stuff is surprisingly hard to find, I swear. I found a that said it took 30,000 years for the first microbes to start showing back up again. That feels weird. Why did the article say that the climate would be returned to normal after 10ish years but it still took 30,000 years for microbe life to return? How long was it before the first big trees? How long until everything was more or less "back to normal" in terms of the presence of flora? Are we talking, like, 200 years or 200,000? At what point after K-Pg could I walk outside and see a green landscape? Is this number different for fauna? Like, what's the earliest we saw animals thriving again? I'm going crazy. If you're an expert I'd appreciate book recommendations or articles and whatnot. Point me in the right direction so I don't have to keep hasslin' you :)
|
[
"The very unsatisfying answer is that nobody really knows. There aren't a lot of fossil beds immediately on either side of the K-Pg boundary, which means we don't really know how long the overall extinction took, let alone how long the recovery was, at least not on a global scale. There are estimates for the length of the extinction ranging from a few years to over 100,000. ",
"Part of the problem is that you're dealing with an incredibly incomplete record. Preservation of fossils is rare, and finding them is difficult, so often researchers are trying to draw conclusions from very narrow sets of data and extrapolate it to the rest of the ecosystem. Like ",
"this one",
" drawing conclusions from insect bites on fossilized leaves. They're making a lot of reasonable assumptions, but it's not exactly definitive even on a local scale. ",
"It's also very difficult to narrow things down in geology/palaeontology to less than a few thousand years, so any numbers you find that are more precise than that aren't going to be based on actual data collected from the rocks. There's the convenient Iridium layer that generally lets you know which side of the K-Pg boundary you're on, but beyond that the most precise rock dates you can probably find would be +/-10,000 years. ",
"So yeah. Any numbers you can get out of this are probably going to be large ranges, possibly overlapping each other."
] |
[
"",
"I think you're misinterpreting the article. It mentions that it took a long time for life to come back ",
". That's to be expected, as that area was essentially sterilized by the impact",
"That that's not what would have happened all over the Earth. If *ALL* the microbes had died, all life would have also died. But we know it didn't. We know that plenty of animals survived the impact and the subsequent climate upheaval. After all, if they hadn't, we wouldn't exist!",
"So yeah, 10 years sounds like a reasonable figure for the soot and other debris launched into the stratosphere to fall out, and while 30,000 years sounds like a lot of time for life to return to the area of crater, I wouldn't dismiss it out of hand."
] |
[
"Thanks for taking the time to reply. This is still great to know! I'm glad to know that I'm looking at a scientific-community-wide blank in knowledge, and not just the blank in knowledge of my own research, which sometimes looks identical. Have a great day :)"
] |
[
"How did we figure reference temperatures out?"
] |
[
false
] |
One of the ways to check for the purity of a substance is to boil/melt and check against the expected value,but how do we know the reference in the first place? How do we know that water boils at 100 degrees so we can use it as a reference for tests?
|
[
"The temperature scales were created to align with naturally occurring phenomena. Specific temperature values are a rather arbitrary definition as the change (or difference) in temperature is significantly more meaningful than a singular value (with the exception of absolute zero on the Kelvin scale). ",
"For the Celcius scale, the temperatures and increments were defined such that there were one hundred integer increments (degrees) between the melting point of water and the boiling point. The melting point of water was chosen to coincide with 0 degrees so that the Celcius scale measured a lot of relevant, Earthly phenomena with \"approachable\" numbers. ",
"Kelvin [degrees] denote the same change in temperature as Celcius degrees - only the reference points are changed. This means that you can use Kelvin or Celcius measurements interchangeably in any equation that calls for a temperature difference [such as dh = C *(T2 - T1) ",
" ]",
"Rankine and Fahrenheit are interchangeable under those same conditions, but have a different increment [degree] size.",
"So, we didn't ",
" water boils at 100°C first, we defined our temperature scale such that a specific value aligned with the boiling point of water. This makes sense if you look at either of the temperature scales I mentioned that begin with absolute zero. In the case of Kelvin, the melting point of water is 273.15K, which isn't even an integer value away from absolute zero."
] |
[
"Melting point and boiling point are not the only methods to determine purity, and weren't the only ones back in the day either. So you determine the purity of your sample by various methods, if you are sure it is pure, you measure your reference temperatures."
] |
[
"Get a pure substance, test it. There are other techniques that you can use to prove a substance is pure (ie chromatography). Note that melting point comparison is only useful when you know that the crystal structure is the same.",
"Take a thermometer without markings. Mark the temperature where water freezes, label it as 0. Mark the spot where water boils, label it as 100. Add more lines in between. Water boils at 100 because the we say so. We could just as easily say that it boils at 212. Note that water boils at different temperatures depending on the pressure, so if you live in the mountains or have a storm coming through, water will boil at a lower temperature."
] |
[
"How high can a wall made of stone be? Would the wall in GoT be possible?"
] |
[
false
] |
In Game of Thrones / A Song Of Ice And Fire there is a wall which is supposed to be roughly 700 feet high and 300 miles long. Is this physically possible, or would the stones at the bottom get crushed by the weight? It is also mentioned that the wall is not entirely made of stone, also of ice in some parts, which ration is never really said (at least I never saw it). Would appreciate an answer, thanks! EDIT: Thanks for the answers, just to clear the question up: how tall could a manmade structure built of stone, not of ice and magic, be built?
|
[
"Ok, assumption time... Wall of stone... Lets pick granite its a nice strong stone since we are trying to build the maximum height.",
"We're also going to assume that we'll be starting construction right on the bedrock which will also be granite for simplicity.",
"Granite has a density of around 0.1 lb per cubic inch and a compressive strength about 29,000 psi.",
"We know the wall is supposedly 700 feet high.",
"You can pick just about any width since we're making this up as we go along, so I decided on an even 75 ft. Length is the same so lets just make it 10 miles... It doesn't really effect the end results that much.",
"So we're also going to assume that the people building this are master masons and make perfect joints which will transmit the load evenly all the way to the bedrock.",
"Total weight of our wall will be an impressive ~2.4 x 10",
" tons.",
"However this is distributed over a total area of ~4 x 10",
" square feet.",
"This places a total load on the bedrock of 4.8 x 10",
" lb-force which creates a compressive stress of only 834 psi.",
"So yes you could build this giant wall if your masons were good enough.",
"Finally its always good to do a sanity check, do these results make sense... I would say they do, there are granite cliffs like El Capitan which are way taller..."
] |
[
"The wall is made of ice. It was constructed using manpower and magic. It is this ratio that is never expanded upon or described. This would effectively make the wall a giant glacier that spans the continent since it exceeds 0.1 km",
" of surface area and 50m of thickness. Glaciers ",
" because their massive weight presses down upon their bottom and deforms it gradually. At the time of the events in ASOIAF books (and the GoT show) the wall is roughly eight millennia old. This would be enough for significant deformation to occur and turn ",
" into ",
". Within the context of the story we have to assume then that part of the magic used during the construction involved preventing the eventual slumping (and moving) that would occur."
] |
[
"There are granite walls thousands of feet high.",
"Trango tower",
" is about 6000ft.",
"Granite has unconfined compressive strength of about 200MPa and density of 2.75 which means you can make a vertical wall of 7200m before granite will get crushed.",
"If it's made of ice of 6MPa compressive strength then you will be able to make a rectangular wall 600m high.",
"If the wall is tapered you can go much higher."
] |
[
"Is there a common ancestor between exoskeleton and endoskeleton species?"
] |
[
false
] |
There has to be a common ancestor somewhere, but I must not be thinking of the right search terms. I was just thinking about it because endo and exo skeletons are so incredibly different when it comes to how the body is built, so I naturally started wondering where both the frameworks came from.
|
[
"So the split between lineages with exoskeletons versus endoskeletons happened after the evolution of the coelom (the body cavity). Of coelomates, organisms are either proteostomes (molluscs, annelid worms, and arthropods) or deuterostomes (echinoderms and vertebrates). Endoskeletons evolved with deuterostomes, so all deuterostomes have endoskeletons. Exoskeletons evolved later in the protostome lineage, and only arthropods have exoskeletons. \nThe closest common ancestor of say a lobster and a human would be one of the first animals with a true coelom. "
] |
[
"This is a long answer. Every line indicates a stopping point, but if you want to understand the answer completely, read to the end!",
"The reason you're having difficulty imagining the common ancestor is probably that you're trying to imagine one that had a sort of in-between of an endoskeleton and an exoskeleton. More likely, the most recent common ancestor had neither. Instead, it was probably soft-bodied, like a worm. ",
"That means that skeletons (endo or exo) are ",
" features, not homologous features. Rather than being variations on a single ancestral form, they are independently evolved features that ended up with a similar purpose: structure. The feathers on a raven and a penguin are homologous: they're used somewhat differently, but they came from the same structure that their common ancestor also had. The wings on a raven and a bee are analogous. Although they have a similar purpose, they evolved independently. The evidence shows that endo- and exoskeletons are likely analogous, and therefore their ancestor was soft-bodied.",
"A brief discussion of each: ",
"An endoskeleton is a hard, internal support structure. Endoskeletons grow as the animals do. There are endoskeletons in three groups of animals: sponges, vertebrates and echinoderms. We are vertebrates, and so are birds, fish, and amphibians. Echinoderms include sea stars and sea cucumbers. The endoskeletons of sponges are so completely different from those of the others that they almost certainly evolved separately: sponges are very distantly related from the others, their skeletons have a different structure, and they develop differently. Therefore, we'll ignore sponge skeletons. ",
"Animals in the group Coleoidea also sometimes have endoskeletons, but they're also an exception we'll get back to later.",
"An exoskeleton is a hard outer support structure. Some exoskeletons do not grow and therefore must be shed periodically. Many arthropods--such as spiders, ants, lobsters, and horseshoe crabs--and mollusks have exoskeletons (shells count).",
"Here's how we know that endoskeletons and exoskeletons are analogous:",
"1) Germ layers: As the embryo of an animal more complex than a sponge develops, it separates into two or three layers of different kinds of cells; each layer develops into specific organ systems. All animals with skeletons develop into three layers. From outside in, they are the ectoderm, the mesoderm, and the endoderm. The mesoderm develops into the endoskeleton of animals that have it; however, exoskeletons come from the ectoderm. This shows that the exoskeleton evolved after the ectoderm, and the endoskeleton after the endoderm. The two must have evolved separately. (It also explains why sponge endoskeletons are analogous--sponges don't have a mesoderm.)",
"2) Protostomes and deuterostomes: I'm going to throw in more vocabulary: Every animal with a digestive tract (a mouth and an anus) is part of one of two groups: ",
" and ",
" The difference between the two is in the formation of the digestive tract in the embryo. For protostomes, the hole that becomes the mouth develops before the hole that becomes the anus. In deuterostomes the opposite happens--the anus develops first. These arbitrary differences in embryology, and other evidence, show that protostomes and deuterostomes are clades: each its own branch on the evolutionary tree. Every protostome is more closely related to any other protostome than it is to any deuterostome. It so happens that the animals with exoskeletons are all protostomes and the animals with endoskeletons are all deuterostomes. Each group originated as soft-bodied animals, so the skeletons must have come later.",
"So what ",
" the ancestor to both exoskeletal and endoskeletal species look like?",
"The ancestor was soft-bodied, like almost all organisms at the time. Unfortunately, this means that it's very difficult to get a fossil: animals with skeletons are much more likely to fossilize. It existed at least before the Cambrian Explosion (540 million years ago,) which is when skeletal fossils started appearing--most likely quite a while before. It was bilateral: the two halves, when drawn front to back, mirrored each other. This means that it was able to move around by itself. It was neither a protostome nor a deuterostome, as the distinction appeared after its existence. Therefore, it existed sometime before 558 million years ago. It was soft-bodied.",
"What the hell is Coleoidea?",
"Coleoids, like octopuses and squid, have no exoskeletons and sometimes have endoskeletons, such as the ",
"cuttlebone",
" or the ",
"gladius",
". However, they are protostomes and are therefore more closely related to exoskeleton species (specifically mollusks) than to endoskeleton species. The endoskeleton in coleoids develops from the ectoderm like the exoskeleton, showing that it evolved from the exoskeleton--in this case, a ",
"shell.",
" Embryology shows the ectoderm, forming an exoskeleton, delve into the body and then close off; it probably happened similarly in evolutionary history. So this actually is a case where the exoskeleton turned into an endoskeleton. ",
"There's also an example of endoskeletons becoming exoskeletons in the Testudines: turtles and tortoises. Their shells, considered a partial exoskeleton, probably spread from ridges in their backs. The shells come from mesoderm tissue.",
"Edited thanks to ",
"/u/skadefryd",
"."
] |
[
"Actually, the development of the coelom was recently found to be absolutely useless for determining evolutionary relationships, as discovered by molecular data.",
"http://en.wikipedia.org/wiki/Coelom#Biology_and_zoology"
] |
[
"Reports are coming out that SARS-CoV-2 has been detected in old sewage samples. How many people need to be infected before we can detect viruses in sewage?"
] |
[
false
] |
The latest report says Spain has detected the virus in a sample from March 2019. Assuming the report is correct, there should have been very few infected people since it was not identified at hospitals at that time. I guess there are two parts to the question. How much sewage sampling are countries doing, and how sensitive are the tests? Lets assume they didn't just get lucky, and the prevalence in the population was such that we expect that they will find it.
|
[
"The Scottish Enviromental Protection Agency has started testing waste effluent for traces of RNA of the virus",
"Not sure how they are back dating this method but they are using it to test for virus within the population to see how the virus is spreading and where it is within the population ",
"https://media.sepa.org.uk/media-releases/2020/sepa-begins-analysis-of-first-samples-in-covid-19-rna-fragment-waste-water-monitoring-trial.aspx"
] |
[
"Here in the Netherlands they do this. They've been doing it for all kinds of diseases like polio and anti-biotic resistant bacteria and now they also monitor Corona.",
"Info (in Dutch) from our health institute: ",
"https://www.rivm.nl/coronavirus-covid-19/onderzoek/rioolwater",
" The article says they measured Corona in the sewage in early March, shortly after the first known case late February. They sample at 29 locations and cover about a quarter of the population."
] |
[
"I'm a molecular biologist, and things like RT-PCR tests for viruses are notoriously tricky and prone to false positives (and false negatives). The nature of how they work (they amplify nucleic acids using short pieces of matching DNA) means that it can be EXTREMELY sensitive. We're talking just a molecule or two getting amplified to give a signal. The downside of that is that it's also really easy to get a contaminant that amplifies. If your lab tech has covid, or donald trump toured the facility that makes your test tubes, that could be enough to give you false positives. If your technician is sloppy about separating samples before and after amplification, previous tests you ran can also contaminate current tests."
] |
[
"What actually MAKES the ocean salty, but inland water is not salty?"
] |
[
false
] |
Had a random shower thought about this. What makes the ocean salty? What originally made it salty when the oceans were formed? Was it always salty? If its minerals getting absorbed by the seabed, why is it salty but inland water is not?
|
[
"Inland water has drainage; the buildup of salt gets washed into the oceans by relatively fresh water. The only way water leaves the ocean is by evaporation, which doesn’t take the salt with it.",
"Rinse and repeat to get salt water."
] |
[
"No, there are outputs for salts in the ocean. Think of the oceans more as a way-station for the ionic products of continental weathering, rather than as a final destination. ",
"The outputs are in sea spray generated from a choppy sea-surface; the chemical exchanges between seawater and crust within hydrothermal vent systems; and sedimentation onto he seafloor sediments, either directly or indirectly (via marine creatures when they die and sink to the seafloor).",
"The oceans are thought to have started out saltier than they currently are, but the formation of significant continental landmasses during the Archean removed large amounts of salt in the form of evaporite deposits in restricted basins. Think of those areas on Earth which are just salt flats for miles and miles - that all used to be salt in the ocean. "
] |
[
"Based in this; is the ocean getting perpetually saltier?"
] |
[
"[Biology] Does animal hair lose its potency as an allergen once it has been shed?"
] |
[
false
] | null |
[
"most people with pet allergies are actually allergic to enzymes in the dogs saliva. So on the dog or off it doesnt really matter the dander and fluff are well laced with whatever level of ani-funk the pet blessed them with. "
] |
[
"It's true that the hair itself is harmless and that saliva... isn't, but it's about how one's body responds to different animal proteins. In the case of shedding fur, these are found in the dander i.e. dead skin cells that are attached to or break off of follicles. So it's not about an ani-funk saliva enzyme transfer, as much as I like the idea. The answer to OP's original question is no, that the shedding is in fact what causes the allergic reaction, but that over time (if the dog or cat moves out or goes to stay at the big farm in the country) the matter left behind will lose its potency as an allergen as it decays."
] |
[
"I wouldn't say the problem is shedding in general, if anyone allergic to cats or dogs (or really the proteins in their saliva) buried their face in said cat or dog's fur, they'd have a pretty sneezy allergic reaction. But when they're in households that have cats or dogs and their shed fur is everywhere, yes, that'll trigger an allergic reaction. The answer to his question is still no, but shedding isn't what causes it."
] |
[
"How long take for an atom whose electrons have ascended to a higher orbit due to light absorption to go back in a ground state?"
] |
[
false
] |
I am interested in finding out if there is a document with the time it takes for each atom to go from an excited state to a low-energy one. For example: when hydrogen passes from 1s to 2s it remains in this state for x seconds, etc. I'm a bit confused about this thing, thanks in advance!
|
[
"if there is a document with the time it takes for each atom to go from an excited state to a low-energy one.",
"It's not that simple.",
"For a two-level atom, if you prepare it in the excited state, the survival probability decays exponentially with time, where the timescale of the exponential is the mean lifetime of the excited state.",
"So you can't say when exactly any particular atom will make the transition, you just have a probability distribution.",
"If there are more two levels in the system and you prepare it in a high excited state, then there are generally multiple branches the atom can decay through to eventually reach the ground state, each transition with its own lifetime. So you'll have a cascade of transitions, each taking some random amount of time."
] |
[
"Yes, each level has a half-life, and there's going to be a cascade through multiple levels to go from a general excited state to the ground state."
] |
[
"You can average the mean lifetimes of all transitions, weighted by branching ratios, to get an average time from the initial level to the ground state. But that requires all of those half-lives and branching ratios to be known, and it still won't necessarily be representative of any given cascade event."
] |
[
"Since Adderall is a stimulant, why is it used in the treatment of ADHD? Is it not counterproductive?"
] |
[
false
] | null |
[
"People with ADHD aren't overstimulated, so much as they're under-focused. The effect of adderall and other similar drugs is to increase focus. ",
"Some interesting reading on the topic."
] |
[
"It does, a little bit. But the side-effects are large enough to become disagreeable before the primary effect kicks in. Self-medicating with coffee is a common ADHD trait. Nicotine also."
] |
[
"So why doesn't caffeine have the same effect?"
] |
[
"[Biology] Would a stun gun be as effective on a 500lb person as it would be on a 100lb person?"
] |
[
false
] |
Any constant and controlled shock. And would it matter if the extra weight was from fat or muscle?
|
[
"Source for ",
" of his claims..."
] |
[
"It would be different - and it would matter whether or not the extra weight was fat or muscle. ",
"This is also another reason why arming police with tasers is dangerous, since tasering a very skinny elderly person would likely kill them, as opposed to a younger person with healthier heart. I think police tasers are adjustable though to deal with this. "
] |
[
"A modern taser works by a process known as neuromuscular incapacation, which prevents skeletal muscle from firing. ",
"Simpler devices like a cattle prod, or a taser on ",
"'drive' mode",
", simply cause pain (they are classed as \"pain compliance\" devices). These are the types that can be negated by strong drugs.",
"Pretty much any story you read about people 'shrugging off' tasers are either because the weapon failed to hit the target (both prongs must contact), was wearing clothing designed to discharge the weapon, or the device was on drive mode."
] |
[
"AskScience AMA Series: Hi, I'm Dr. Christina Nicolaidis and I'm editor of a brand new peer-reviewed journal called Autism in Adulthood. Ask me anything about the new journal or the ways that people on the autism spectrum can get better health care!"
] |
[
false
] |
Hi, I'm Dr. Christina Nicolaidis and I'm editor of a brand new peer-reviewed journal called . I teach social work at Portland State University and internal medicine at Oregon Health & Science University in Portland, Oregon. There's a lot of talk about autism in children, but not as much about autism among adults. That's why we just launched this new academic journal. Our goal is to cover the most pressing issues affecting autistic adults, from emerging adulthood to later life. I am particularly excited about including autistic adults in our new journal - as editorial board members, authors, reviewers and readers. As a doctor and a services researcher, my own research is focused on how people on the autism spectrum can get better access to health care services. Many autistic adults don't get the health care they need, resulting in more health problems and shorter life expectancy. Ask me anything about the new journal or the ways that people on the autism spectrum can get better care. We'll be on at 1 p.m. Pacific (4 PM ET, 21 UT), we're looking forward to the session!
|
[
"So some adult has (presumably) undiagnosed autism. What's the threshold for \"you need to get help\" vs. \"you've gotten this far without assistance, no need get it now\" that you'd recommend?"
] |
[
"First, thank you for this AMA. It seems that GenX adults with milder symptoms, more akin to those that used to be deemed Asperger’s before DSM-5, who were left to navigate the world before there was this current awareness, are still somewhat in a no-man’s land due to the fact that they’ve adopted certain behavioral accommodations to “pass” in social situations. Are there any specific diagnostic tools that are truly able to assess where these individuals exist on the spectrum? And if so, what treatment options exist for these now-middle-aged adults?"
] |
[
"How would an adult go about getting diagnosed? Everyone seems to be focused on diagnosing children. "
] |
[
"Is there an upper limit to the amount of energy a photon can have?"
] |
[
false
] |
Whenever you see an EM spectrum chart, it always ends at gamma rays. Why is this? Are all photons with more than 100 KeV just referred to as gamma rays?
|
[
"If Lorentz symmetry is not violated, any photon has any energy in the right reference frame, so it can be arbitrarily high. However, interacting with such a photon might cause black holes to form.",
"I don't think that's what you were asking though. X-rays and gamma rays are basically the same thing except x-rays are typically produced from electron interactions and gamma rays are produced from nuclear and particle interactions. There is no special name for even higher energy photons."
] |
[
"Black holes form when you pack enough energy (specifically mass) in a small enough space. A high energy photon by itself does not have mass, but its energy can become mass when its hits other objects. The energy needed to make a black hole this way is huge, and doesn't occur naturally. And the black hole that's created from this process is so small that it evaporates almost instantly, releasing a bunch of lower energy photons. "
] |
[
"might cause black holes to form\nWould you mind elaborating? I'm really interested"
] |
[
"[Physics] Would a large stack of clean, standard, gold bars in direct contact eventually weld together through diffusion bonding? - at room temperature, no extra energy aside from pressure of the weight of the stack). How long would it take?"
] |
[
false
] |
My specific question is about the likelihood of stacked gold bars at normal earth temperatures (tombs, caves, storage facilities) to actually fuse together over a long period of time. Has it happened before? And if so, what process explains it? How long would it take? I've heard this claim before. However, I can't find much on specifics. is the only place I can find anyone making a similar claim. It is one of the reasons stacks of gold will have separate pallets or separators between layers: For short term storage they may just be stacked… but leave them too long and the bars fuse together - and when separated may not have proper weight anymore. Potentially useful links: The reason for this unexpected behavior is that when the atoms in contact are all of the same kind, there is no way for the atoms to “know” that they are in different pieces of copper. When there are other atoms, in the oxides and greases and more complicated thin surface layers of contaminants in between, the atoms “know” when they are not on the same part.
|
[
"\"Clean\" is relative. Every surface exposed to air immediately acquires at least a monolayer of hydrocarbons and (especially for clean gold) sulfur. The unsatisfied bonds at the surface are just too appealing.",
"",
"In a microfabrication context, \"clean\" would mean deposited atom-by-atom in a vacuum chamber (e.g., by sputtering or evaporation) and never exposed to air. Placing such a surface in contact with its twin would likely result in a very strong bond."
] |
[
"You're right, and you need only to look at the paper OP links to to see how they got the gold to stick together. The samples of gold used were sputter deposited in an argon atmosphere and then pressed together at varying temperatures, none below 100 Celsius. These are the most favorable conditions you could imagine, and would certainly not be anywhere near the conditions of gold bars stored in a room somewhere."
] |
[
"Where are these separation layers?",
"I could imagine layers of a different material for mechanical stability of the stack."
] |
[
"What is happening when one of my ears suddenly goes 'dim'?"
] |
[
false
] | null |
[
"I recently finished a neurobiology course, and this was covered. From what my professor said, this type of anomaly originates in the area of the brain that processesall of the information your aural nerves send it. It then sorts through information, and what happens is similar to how your vision (and smell) processing centers will block out repetitive information. This is how you adjust to certain environments that dont fall into any of the extremes of human tolerance. Obviously you wont be able to adjust to boiling water, but note the next time you go swimming in slightly cooler water, and how at the end of the swim the water feels normal and manageable.",
"I digress; so your brain processes all of this aural (sound) information, and then selects how to organize the information into a manageable \"picture\", if you will. At times, in a process not entirely understood, the management of the information is not perfect, and your brain will suppress all information from one of your auditory nerves. This is interpreted as a muting of sound, or a perceived loss of auditory range. It quickly goes away, and your information management syncs back up with the picture it is trying to create. You know how I mentioned the swimming in cold water thing? Now think back to when you are in a crowded room with many conversations going on. Your brain cant interpret and pick out every single conversation, and the noise becomes background noise. You dont simply stop hearing everything, but your brain picks up what it deems the highest priority and you focus in on that. Now it isnt black and white, which is why the conversations around you tend to cover a whole range of how intelligible they are. This is why you can pick up on some buzz words in conversations around you in a crowded room. "
] |
[
"OP doesn't mention any vertigo at all, which I thought was a primary symptom of Meniere's?"
] |
[
"OP doesn't mention any vertigo at all, which I thought was a primary symptom of Meniere's?"
] |
[
"Why is your hunger levels affected when you are depressed specifically going through love lost?"
] |
[
false
] | null |
[
"But this wouldn't account for people who lose their appetite when they are depressed."
] |
[
"Dopamine and appetite kind of go hand and hand. When depressed, your dopamine levels, along with other chemicals/hormones, are reduced. Dopamine can control how much appetite you have, when dopamine is released, it can work down to your stomach and intestines and trigger a hunger sensation. That sensation can make you want to eat. So if you are depressed after a loss of a significant other, or suffer from depression, your dopamine levels are low. Low levels of dopamine make it so there are no triggers of hunger coming from your stomach, making it so you don't have an appetite.",
"tl;dr Depression lowers levels of dopamine, low levels of dopamine don't trigger stomach to feel hungry, no hunger, no appetite, no appetite, no eating."
] |
[
"This article",
" should be a good read for you"
] |
[
"Why is oxygen the third most common element in the Milky Way?"
] |
[
false
] |
Wikipedia article claims that oxygen is the second most abundent element in the Milky Way galaxy. Why is this? Why is lithium, being of the atomic number 3, not the third most common? I would guess that it has to do with the mechanics of fusion, but I don't know for sure.
|
[
"When stars run out of hydrogen they star fusing helium using the triple-alpha process which fuses 3 helium nuclei into one carbon atom. Once carbon is present, it can be fused into oxygen.",
"The next step, fusing oxygen into neon requires a very massive star, so in most stars oxygen builds up and this leads to the high concentrations of oxygen in the universe."
] |
[
"Basically, the step of fusing He into C is pretty reasonable for a star that is out of H. That C fuses pretty easily with He to form O. However, it's very difficult for that O to be fused into Ne. The result is that the intermediate amounts of C keep being used up, but large amounts of O build up."
] |
[
"But wouldn't that result in carbon being more common? Does the fusion into oxygen occur easily and/or rapidly enough that a carbon atom in a dying star is less likely to stay that way than to be fused into oxygen?"
] |
[
"If I give my glasses to somebody with 20-20 vision, is their sight then as bad as mine?"
] |
[
false
] |
I've always wondered this, because people will put them on and then claim that my vision isn't bad at all. Also, what might cause both eyes to have different prescriptions?
|
[
"Is the balancing of eye sight really as easy as just adding or subtracting numbers? I know that's how prescriptions are given, but aren't there variations even past the roundness of an eye? Astigmatisms perhaps."
] |
[
"Yes, the glasses make your vision more farsighted to corrected for your nearsightedness. If you give them to someone else, it will make their vision more farsighted than it was before. Depending how nearsighted you are (thus, the strength of your prescription), their eyes may just adjust and not notice a difference, or they may become actually farsighted.",
"It definitely can't make them nearsighted.",
"Also, if you put on both a +4 and a -4 set of glasses at the same time (or wore glasses with contacts, etc), they would cancel each other out and have no effect."
] |
[
"Remember that your glasses compensate for an irregularity of your eye, and will therefore overcompensate in the eye of somebody with 20-20 vision (instead of -4, they would see as if they had +4, for example).",
"\nYour eyes have a different prescription, because the eyes are not completely the same. For example, one eye could be normal, while the other may be too long."
] |
[
"Would it be possible to isolate the chemical in psychedelics that causes time dilation and make it into a daily pill that will lead people to feel as if they're living two lifetimes?"
] |
[
false
] | null |
[
"Hi Inquiring_mind001 thank you for submitting to ",
"/r/Askscience",
".",
" Please add flair to your post. ",
"Your post will be removed permanently if flair is not added within one hour. You can flair this post by replying to this message with your flair choice. It must be an exact match to one of the following flair categories and contain no other text:",
"'Computing', 'Economics', 'Human Body', 'Engineering', 'Planetary Sci.', 'Archaeology', 'Neuroscience', 'Biology', 'Chemistry', 'Medicine', 'Linguistics', 'Mathematics', 'Astronomy', 'Psychology', 'Paleontology', 'Political Science', 'Social Science', 'Earth Sciences', 'Anthropology', 'Physics'",
"Your post is not yet visible on the forum and is awaiting review from the moderator team. Your question may be denied for the following reasons, ",
"/r/AskScienceDiscussion",
"There are more restrictions on what kind of questions are suitable for ",
"/r/AskScience",
", the above are just some of the most common. While you wait, check out the forum \n",
" on asking questions as well as our ",
". Please wait several hours before messaging us if there is an issue, moderator mail concerning recent submissions will be ignored.",
" ",
" "
] |
[
"Chemistry"
] |
[
"'Chemistry'"
] |
[
"Does COVID spread differently in humid indoor environments? (Like swimming pools?)"
] |
[
false
] | null |
[
"There have been several studies done on the topic. Dry air tends to 'float' the virus longer. Moisture helps the virus settle and deposit on surfaces. See Confined spaces and virus stability:",
"https://www.thelancet.com/journals/lancet/article/PIIS2213-2600(20)30245-9/fulltext",
"https://science.sciencemag.org/content/early/2020/05/27/science.abc6197.full",
"Mounting evidence suggests coronavirus is airborne — but health advice has not caught up",
"https://www.medrxiv.org/content/10.1101/2020.02.28.20029272v2",
"Airborne RNA - exhaled breath",
"'We detected viral contamination among all samples, supporting the use of airborne isolation precautions when caring for COVID-19 patients.'",
"https://www.nature.com/articles/s41598-020-69286-3",
"other reports saying same thing\n",
"https://www.nature.com/articles/s41598-020-76442-2",
"Projected life savings - 127k - due to mask usage by February 2021. ",
"https://www.nature.com/articles/s41591-020-1132-9",
"Other studies saying the same thing",
"https://www.theatlantic.com/health/archive/2020/07/why-arent-we-talking-more-about-airborne-transmission/614737/",
"Summary of studies, looking at individual factors, but still not all factors at once\n",
"https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7263814/",
"Restaurants are spreading factories\n",
"https://www.nature.com/articles/d41586-020-03140-4",
"Study that quantifies live virus in breath of covid patients at different distances",
"https://www.medrxiv.org/content/10.1101/2020.08.03.20167395v1",
"Some studies have shown surfaces very low virus counts compared to air",
"https://www.medrxiv.org/content/medrxiv/early/2020/06/02/2020.05.28.20114041.full.pdf",
"Effects of different materials on virus filtration",
"https://publishing.aip.org/publications/latest-content/effectiveness-of-cloth-masks-depends-on-type-of-covering/",
"Another study on cheaper face coverings",
"https://advances.sciencemag.org/content/early/2020/08/07/sciadv.abd3083.full",
"Face shields don't work",
"https://aip.scitation.org/doi/10.1063/5.0022968"
] |
[
"Afaik there have been limited in vitro studies suggesting that Covid like many viruses has favored and unfavorable temperature and humidity ranges. It's not clear to me that we really understand what those are in real life situations."
] |
[
"Sorry for the late reply: this is incredibly helpful. Thank you!"
] |
[
"Do endothermic organisms have higher entropy or greater entropy production than ectotherms?"
] |
[
false
] |
I've studied endothermy and ectothermy but I have never seen this topic contextualized in terms of physical entropy, and a preliminary literature search returns no results. I suppose this question is quite interdisciplinary...maybe if I finish Schrödinger will tell me, but I figured I would ask the panel and see if anyone has anything insightful to say.
|
[
"Entropy production is pretty straightforward. Any spontaneous transfer of heat means that the total entropy is increasing. If we assume the animal is doing no work on the environment, the loss of heat dQ by the animal lowers its entropy by dS",
" >= -dQ/T",
" and raises the entropy of the environment by dS",
" >= dQ/T",
" . The total the entropy gained for a transfer of heat dQ will therefore be: ",
"∆S = dS",
"+dS",
" >= dQ(1/T",
" - 1/T",
")",
"This change in entropy is larger if the difference in temperature is larger (which is true for endotherms) and if the total heat lost is larger (which is also true for endotherms, at least compared to an ectotherm of comparable size).",
"So endotherms almost certainly produce more entropy. You can get extra entropy beyond this minimum required by the heat transfer, say by mixing two different gases together that were separated. But it is unlikely that these processes will amount to much compared to the heat generated.",
"The internal entropy of the organism is going to depend on its temperature as well, so my intuition is that an endotherm has more internal entropy than a comparably sized ectotherm. But trying to calculate the internal entropy for a system that complex is going to be nearly impossible, so I wouldn't want to say anything definite."
] |
[
"Under this view, are animals like partially isolated pockets of the Earth that may decrease their own local entropy temporarily, but that act to increase the Earth's total entropy as a whole? ",
"Yes, but that has to be true for any pocket of the world we can draw a box around. Total entropy is going up, even if it may go down inside the pocket.",
"Particularly once the animal has died and decayed?",
"One big simplification above is that I what I wrote is also true for an animal that, say, had just died. As the heat leaves the animal, total entropy goes up even as internal entropy decreases. In a living animal, new internal energy is being created by its metabolism to make up for the heat that is lost. I was worried about trying to define how much that might be, so I just left the lower limit in there. But, if we assume that the animal returns to roughly the same internal state after 24 hours (i.e. eats and digests about the same amount, keeps the same body temperature), then dS",
" should add to zero over that window.",
"That puts an even higher lower limit on the entropy created, namely:",
"∆S >= dQ/T",
"Again, the endotherm still generates more entropy under this assumption because it released more heat over the course of a day.",
"We could get more detailed, and worry about the exact exchange of atoms between animal and environment as it breathes and eats and calculate chemical potentials for everything exchanged. This raises the lower limit on ∆S even further. But the endotherm is always doing more - releasing more heat, eating more food, exhaling more CO2 - so its lower limit goes up faster than the lower limit for the ectotherm. "
] |
[
"the animal lowers its entropy by dSa >= -dQ/Ta and raises the entropy of the environment by dSe >= dQ/Te",
"Under this view, are animals like partially isolated pockets of the Earth that may decrease their own local entropy temporarily, but that act to increase the Earth's total entropy as a whole? Particularly once the animal has died and decayed?"
] |
[
"Found in an old post: \"You know if we breed goats selecting for horn distance, in a few generations we should be able to make UNICORNS!\" Is it realistic?"
] |
[
false
] |
even if not relevant at all
|
[
"Except that a goat is not a horse."
] |
[
"This Image",
" strongly suggests that the basic principal is valid. You can see two horns about halfway merged in the center of the head."
] |
[
"Unicorns have a single horn. Goats have two horns. There are other problems. If you listen to Pliny the unicorn was ",
"a creature with a horse's body, deer's head, elephant's feet, lion's tail, and one black horn two cubits long projecting from its forehead",
"You're not going to get that from selective breeding.",
"But, just concentrating on the horn. Some things are not possible with selective breeding -some people think it's not possible to create a pink budgie for example- and some things are possible.",
"I don't think it's easy to remove a horn, and to move the remaining horn to the centre, and then to grow it straight and long.",
"Don't forget that many selective breeding programmes have caused considerable genetic flaws in the target population. I guess this is easier to to see in \"pedigree\" dogs, which often have diseases and deformities linked to breeding.",
"http://www.etymonline.com/index.php?term=unicorn",
"CAUTION: The following newspaper article contains an image (at the top of the page) that many people will find distressing. It's a hairless cat being held up by its skin.",
"http://www.telegraph.co.uk/health/petshealth/4990966/Inbred-pedigree-cats-suffering-from-life-threatening-diseases-and-deformities.html",
"tl;dr No - you'll end up with ugly ill goats with a weird horn thing."
] |
[
"Do sound waves fall?"
] |
[
false
] |
Many years ago I saw a primitive sound propagation simulation in which the sound waves fell. I do not know if this was a flaw in the simulation software (physics undergraduate project) or the actual behavior of sound waves. (Also, I would expect that sound moving faster through lower denser air would "lens" the sound upwards somewhat.)
|
[
"Fell, as in curved downwards due to gravity? No, that doesn't happen unless the medium through which the sound is moving is falling. For example, if there were a vertical shaft with air blowing downwards through it, sound waves traveling horizontally would be deflected downward by the wind.",
"If there is no significant motion of the medium, sound propagation is oblivious to gravity."
] |
[
"The density of air decreases as you go up, so if the speed of sound was greater further up, the sound would bend ",
", which could be what was being simulated. Think of it like a light beam going from a higher to lower refractive index, it bends away from the normal, not towards it. In reality though, the speed of sound is ",
"all over the place",
" because of temperature variation in the atmosphere."
] |
[
"This may sound crazy but I think sound waves in air fall, because the gas molecules do fall between collisions; but I suspect sound waves in water would not fall. This is what seems intuitive to me but I've been wrong plenty."
] |
[
"Is the speed of a particle and the gravity field it creates related?"
] |
[
false
] |
My understanding is that you can not accelerate a mass to the speed of light because it's mass increases as it's speed does, and as you approach the speed of light the mass becomes infinitely large, requiring infinitely more energy to push it faster. If this is the case, does a particle moving near the speed of light create a larger gravitation effect that it would otherwise? Such as in the in the LHC where protons are accelerated to c-3m/s?
|
[
"My understanding is that you can not accelerate a mass to the speed of light because it's mass increases as it's speed does",
"I'm sure that RRC will be along shortly to disabuse you of that notion. I'm layman enough that I can't fully explain why that's the case other than that velocity is not actually linearly additive, but behaves in a way that makes it act similarly to increasing in mass as velocity increases.",
"Hopefully someone with real credentials will come along and explain how changes in velocity are rotations in hyperbolic Minkowski space, and the speed of light represents infinite rotation . . . or something like that."
] |
[
"Well my work here is done. ;-)"
] |
[
"As AnteChronos said very well, your premise is flawed. There is no such thing as relativistic mass.",
"What's more, mass isn't the source of gravitation anyway. Energy is ",
" of the source of gravitation, and mass is one type of energy, so while there is a relationship there, it's an indirect one."
] |
[
"Does a car's wheel have rotational velocity?"
] |
[
false
] |
There's an old riddle about the bottom of a car's tire never moving, but does it have rotational velocity because the rest of the wheel is rotating at a certain speed?
|
[
"The bottom of the wheel isn't moving relative to the road, but it is definitely moving relative to the axis. "
] |
[
"That's not true. OP's scenario is a classic example of rolling without sliding. "
] |
[
"The idea is that the bottom of the tire won't move relative to the road, because all the road ever experiences is a rectangle of tire touching it. as the tire rotates, it's indistinguishable from a sliding rectangle (from the perspective of the pavement)"
] |
[
"Does urine have disinfecting properties?"
] |
[
false
] |
My grandfather told me that when he was a kid and had a small injury he would piss on his wound to "clean" it. Every kid used to do the same. Does urine have antiseptic properties? Isn't this practice dangerous? I mean aren't there any harmful bacteria that could cause something serious?
|
[
"I can't comment on the use of urine as a disinfectant. I can comment on how urine prevents bacteria growth within the body. In general urea plays a key role in preventing bacterial thread along with pH and fluid concentration. Generally, bacteria with complex nutritional requirements won't grow in urine but it has been shown to stop bacteria growth even with proper nutrients in place.",
"In general, fastidious organisms that normally colonize the urethra will not multiply in urine and rarely cause UTIs ( Cattell et al, 1974). In contrast, urine will usually support the growth of nonfastidious bacteria ( Asscher et al, 1968). Urine from normal individuals may be inhibitory, especially when the inoculum is small ( Kaye, 1968). The most inhibitory factors are the osmolality, urea concentration, organic acid concentration, and pH. Bacterial growth is inhibited by either very dilute urine or a high osmolality when associated with a low pH. Much of the antimicrobial activity of urine is related to a high urea and organic acid content ( Solomon et al, 1983). From a clinical perspective, however, these conditions do not appear to significantly distinguish between patients who are susceptible or resistant to infection. ",
"Campbell-Walsh Urology ",
"Urea concentration was a more important determinant of antibacterial activity than osmolality or ammonium concentration. Increasing the urea of a noninhibitory urine to equal that of an inhibitory urine made the urine inhibitory. However, increasing osmolality (with sodium chloride) or increasing ammonium to equal the osmolality or ammonium of an inhibitory urine did not increase antibacterial activity. Similarly, dialysis to decrease osmolality or ammonium but preserve urea did not decrease inhibitory activity. Decreasing urea with preservation of ammonium and osmolality decreased antibacterial activity. Removal of ammonium with an ion exchanger did not decrease antibacterial activity, whereas conversion of urea to ammonium with urease and subsequent removal of the ammonium decreased antibacterial activity. ",
"Kaye, 1968",
" "
] |
[
"I'm a med student and after reading your comment I was thinking: what about the bacteria that colonize the human urethra? this would potentially contaminate urine and make it more dangerous if it was to be used to irrigate a dirty wound..."
] |
[
"I'm a med student and after reading your comment I was thinking: what about the bacteria that colonize the human urethra? this would potentially contaminate urine and make it more dangerous if it was to be used to irrigate a dirty wound..."
] |
[
"Are rapid changes in volume worse for my ears than constantly loud noise?"
] |
[
false
] |
If I am walking near a highway my ipod volume sometimes gets turned up to near maximum but the volume feels fine (I know that's bad for my ears, but sometimes happens). If later in the day I turn the music back on when working in a quiet area, the sensation in my ears is almost painful. Does this pain reflect some sort of damage? Is it worse to go from quiet to loud immediately vs slowly increasing the volume. Does loud background noise somehow make the loud headphones less damaging to my ears?
|
[
"Your auditory system can compensate (to some extent) in (quiet or) loud environments. For example, muscles which connect to the middle ear bones can tighten, reducing the intensity level of sound transmission to the inner ear. Additionally, nerve fibers which transmit from the brain to the inner ear can tone down (or up) the excitability of hair cells (in the inner ear), which serve to amplify (quiet) sounds.",
"Is it worse to go from quiet to loud immediately vs slowly increasing the volume. ",
"This really depends on how quiet and how loud. Explosions are bad. In general, I'd guess increasing the volume slowly is probably better. It takes a bit of time/exposure for your auditory system to adapt.",
"Does loud background noise somehow make the loud headphones less damaging to my ears?",
"I'm not quite sure how to answer this. Your ability to hear across frequencies is directly related to the physical mapping in your inner ear. Since background noise is mostly lower frequencies and music/speech is mostly higher frequencies, I want to say that loud background noise + loud headphones is worse than just loud headphones.",
"Edit: Here are two interesting links...",
"Noise exposure - permissible level and duration",
".",
"Typical sound levels for reference",
"."
] |
[
"Not an expert, but my understanding is that yes, sudden changes in volume are more problematic. I'm not sure about long term damage, so I won't comment on that.",
"There are tiny muscles in your ear, mostly the ",
"stapedius",
" and the ",
"tensor tympani",
", that tighten in order to dampen the vibrations of the ",
"ossicles",
" and prevent damage to the ear from loud noises. If the loud noise is constant, these muscles stay contracted. In the case of a sudden loud sound, the muscles don't have time to contract, so there is no dampening of the sound."
] |
[
"Thanks. i had no idea that there were structural changes to the ear during increased volume."
] |
[
"What exactly causes spaceships to burn in the Earth's atmosphere during reentry?"
] |
[
false
] | null |
[
"Friction due to the vehicle's velocity.",
"It's not friction. It is due to adiabatic heating from compression of the air. When the object moves through the air that fast, the air in front of it doesn't have time to move out of the way, so it becomes compressed very quickly. This compression heats the air by a large amount, creating a cushion of superhot air in front of the object. This heat transfers to the object and can quickly destroy it if it isn't built to sustain this kind of heat and pressure.",
"That said, friction is still a factor in re-entry, and it is how much of the energy of the object is lost to slow it down, but it isn't a significant contributor to the heating that occurs."
] |
[
"Friction due to the vehicle's velocity.",
"It's not friction. It is due to adiabatic heating from compression of the air. When the object moves through the air that fast, the air in front of it doesn't have time to move out of the way, so it becomes compressed very quickly. This compression heats the air by a large amount, creating a cushion of superhot air in front of the object. This heat transfers to the object and can quickly destroy it if it isn't built to sustain this kind of heat and pressure.",
"That said, friction is still a factor in re-entry, and it is how much of the energy of the object is lost to slow it down, but it isn't a significant contributor to the heating that occurs."
] |
[
"A re-entering spaceship will be moving very, very fast: even re-entering from low orbit is done at about 8 km/s (17900 mph, or 28800 km/h).",
"Furthermore, spaceship heatshields are usually not very aerodynamic for obvious reasons: you want to lose as much speed as possible while descending.",
"Because of this insane speed and unaerodynamic surface, the atmosphere in front of the ship ends up getting compressed, rather than being pushed out of the way.",
"Gay-Lussac's temperature law",
" tells us that when pressure rises, so does temperature.",
"In other words, because the air in front of the spacecraft gets compressed, the temperature rises. Anything that gets warm starts emitting light, which means we see the ship 'burn'."
] |
[
"Is there a proven way to prevent chapped lips?"
] |
[
false
] |
The title says it all...is there a scientifically proven way to prevent (not treat) chapped lips?
|
[
"Well it depends on why they are becoming chapped. For example if you are going to become dehydrated/malnourished you should drink some water/eat something before becoming so and if you are planning on sailing accross the carribean in a raft sunscreen would be a good idea (yes on your lips).",
"As for the most common cause of cold weather...though the carribean raft ride is also a solution...moisturizing in advance of them becoming chapped and sleeping with a humidifier might help."
] |
[
"Chapping is caused by dehydration, which cold weather tends to exacerbate. Covering the lips with some sort of water-impermeable barrier retards the dehydration. Most lip balms are basically either some sort of wax or petroleum jelly, maybe with some moisturizers and flavoring mixed in. Anything that can create a hydrophobic layer would work though; Inuit have been using seal fat and whale blubber for millenia."
] |
[
"Anecdotal evidence only, but don't lick you lips, and dry them when they get wet."
] |
[
"Is it possible to measure the amount of air displaced by turning on a shower or some other mundane thing that adds matter to a small area?"
] |
[
false
] | null |
[
"The pressure increases slightly and briefly, before it has a chance to equalize with the outside world, which is does very quickly in a matter of seconds."
] |
[
"For example you could turn the shower on for the length of time it stays in the air for (a bit less than a second), and aim the spray in the bucket, and then weight the bucket or otherwise measure the volume of water in it, to determine the volume of air that the water displaces."
] |
[
"In more of a general sense though does the air pressure increase? If it’s your shower or your air conditioner or whatever? I think I worded my question badly. When you are introducing matter into a space does the air pressure increase or does the pressure equalize with the outside world through small cracks?"
] |
[
"If both signals travel at the speed of light why is a wireless connection faster than a wired connection."
] |
[
false
] |
Is it just allocated bandwidth from the wireless connection, signal/noise??
|
[
"a wireless connection is not faster than a wired connection.",
"According to the Shannon-Hartley theorem, if C is the channel capacity in bits/sec, B is the channel bandwidth in Hz and S/N is the signal to noise ratio,",
"C = B*log(1+S/N)",
"for connections of equal bandwidth, only the SNR is the determining factor. For most application, it's cheaper to achieve the same SNR from point A to point B with a wired connection than with a wireless connection."
] |
[
"Wired connection doesn't usually travel the speed of light."
] |
[
"because electric fields don't travel at the speed of light?"
] |
[
"Why is rocket staging more efficient?"
] |
[
false
] |
For example, when you release a stage after using up all its fuel to lessen the empty mass. Why is this more efficient than say having one main fuel "compartment" and just burning it all out of what would be the first stage engines? Definitely phrased this question wrong.
|
[
"The mass of the full fuel tank is mostly fuel but the actual tank has a significant mass too. When you drop a stage you lose the empty fuel tank and thus make your payload lighter and your lifting job easier.",
"There are also savings to be made in engine efficiency, when you are low to the ground it is important to have very high thrust, when you are in orbit it may be better to be more efficient with your fuel. By dropping your big heavy high thrust engines and switching to lighter engines further gains can be made."
] |
[
"By dropping your big heavy high thrust engines and switching to lighter engines further gains can be made.",
"That's correct, but it's only part of the story.",
"The rocket nozzle is cone or bell shaped. As the gases flow outward they expand, decreasing pressure and gaining speed. Obviously the more expanded the faster, and the faster the better, but if the pressure at exit is lower than atmospheric pressure then we say it's ",
". A gross overexpansion can cause shockwaves inside the nozzle, which may be damaging for the engine.",
"https://en.wikipedia.org/wiki/Rocket_engine#Nozzle",
"In a vacuum, you can expand as much as you'd like, though beyond a certain point the extra mass overcomes the benefits of the slightly higher exhaust speed.",
"So by dropping the low expansion engine used at sea level, you can continue with a more expanded engine that will be more efficient in the upper atmosphere, and then another one that will be even better in a vacuum.",
"Edit: forgot to mention another important fact. Staging allows you to choose different fuels/propellants for each stage. Typically they will use the cheapest fuel in the first stage, which is the largest one. This was the case of Saturn V, which burned RP1 (kerosene) in the first stage and liquid hydrogen/oxygen in upper stages."
] |
[
"Hey Kerb!",
"To make it super simple, lets think about it this way: you're making a week-long expedition up the side of a mountain. Its going to be a long, arduous climb, and its going to take a lot out of you. To prepare for the trip, you've packed a ton of food and supplies into a big, sturdy, weather-proof backpack. Lets say you've got about 100 lbs of stuff in your backpack, and your backpack itself weighs 50 lbs (you got it at a garage sale or something. Its ugly and bulky, but it will do the trick. This is a budget hike.) When you start the trip, you not only have to carry your own weight, but the additional 150 lbs of food, supplies, and backpack to get you up the mountain.",
"A couple days pass, you're making your way up the mountain, and you've eaten about 20% of your food to fuel your journey. This hike blows. Its really friggin steep, and you hate carrying all of this crap the whole way up. At this point, at least you've consumed a good amount of your food and turned it into the energy to move you forward (and poop), because you're only toting about 100 lbs now instead of your starting 150. You're moving a little faster now too, since you have less to carry.",
"5 days into your journey, and you've only got 15% of your food left. That's 15 lbs of food, plus that obnoxiously heavy backpack that actually weighs more than your remaining supplies at this point. As far as the hike goes, you're moving way faster than you were at the start of this journey, since you're now only carrying 65 lbs compared to the original 150. Still, hauling this backpack up the mountain really sucks.",
"When you finally get to the top of the mountain, you've eaten all of your food, and you're still carrying the damn 50 lb backpack. Seriously who made this thing. What backpack needs to weigh 50 lbs?? You made it up the mountain, but you're not exactly happy about it.",
"Lets say instead of buying that crappy garage sale sack, you went to Patagucci and got a super fancy, modular hiking backpack. It still weighs 50 lbs, but its got a clever design to it. Not only do you get to impress everyone along the hike with your name-brand, expensive, organic, gluten-free hiking gear, but you get to store 80 lbs of your supplies in the main backpack, and 20 lbs of it in a nifty fanny pack on your front.",
"This time, when you start your journey, you've still got an obnoxiously heavy 150 lbs to tote up the mountain. However, when you've exhausted 80% of your food supply, you can drop the empty backpack by the wayside. The fanny pack only weighs 10 lbs, plus the 20 lbs of food in it, so for the remainder of the journey you're practically running. (Don't worry about the backpack. Its biodegradable or something.)",
"Not only do you make it to the top faster than before, you're nowhere near as grumpy since you didn't have to carry that huge, bulky, and now empty backpack to the top. You shed that a long time ago, and you're just left with a very stylish, lightweight fanny pack. Enjoy the view!"
] |
[
"How does the body know the relative position of its parts (eg, where you arm is)?"
] |
[
false
] | null |
[
"The concept is called ",
"proprioception",
". ",
"I'm sure someone will give a more in depth answer, but essentially there are multiple body maps in your brain, and these maps are used in conjunction with your peripheral nervous system in order to determine body position. ",
"For example, proprioceptors are found in skeletal muscles. The relative stretching/compression of these proprioreceptors gives information about the position of the limb."
] |
[
"As already mentioned this is called proprioception. To expand a bit on how sensors in the muscles tell you about limb position: The body's sense of its position in space is determined by stretch receptors in the muscles, joints and tendons. Muscle stretch receptors, or muscle spindles, are specialised muscle fibres in a fibrous capsule. These detect changes in muscle length.",
"Joint receptors are mechanosensitive axons in the connective tissue of joints. They are good at detecting the movement of the joint, but not so useful when the joint is kept still. ",
"Finally, at the junction of a muscle and a tendon we find Golgi tendon organs. These register muscle tension, or force of contraction. The information from all of these sensors come together to tell us where our limbs are. "
] |
[
"OP is not being very precise in their question, but it seems proprioception is what they are after.",
"It is worth noting, however, that visual cues seem able to overrule whatever information the brain gets from the nervous system in skeletal muscles: see the famous ",
"invisible hand illusion",
"."
] |
[
"What’s magnesium role in the human body?"
] |
[
false
] | null |
[
"Magnesium is the fourth most adundant mineral in your body and is involved in over 600 cellular reactions, including: ",
"More Details"
] |
[
"If you want the most fundamental answer possible: ATP, the \"energy molecule\" of the body, requires a magnesium ion to function. The presence of this ion is so ubiquitous that it rarely is even indicated, but simply understood to be there. Without ATP, cells simply wouldn't function."
] |
[
"I'm a molecular biologist studying RNA. We have to include magnesium in our reactions because it coordinates with nucleic acids and helps them fold correctly."
] |
[
"What did the Wow! Signal actually contain?"
] |
[
false
] |
I'm having trouble understanding this, and what I've read hasn't been very enlightening. If we actually intercepted some sort of signal, what was that signal? Was it a message? How can we call something a signal without having idea of what the signal was? Secondly, what are the actual opinions of the Wow! Signal? Popular culture aside, is the signal actually considered to be nonhuman, or is it regarded by the scientific community to most likely be man made? Thanks!
|
[
"The Wow! signal didn't actually contain any information. It was simply a narrow-band radio source that varied in intensity over roughly 72 seconds. There are a few reasons why it's of interest:",
"The frequency of the signal occurred almost exactly at what's known as the hydrogen line, which is the resonant frequency of hydrogen. Most SETI researchers agree that this is exactly the frequency an extraterrestrial intelligence might use to transmit information because of it's mathematical importance and because it is able to travel well across space without getting blocked by gas and dust clouds",
"Its peak intensity was roughly 30x greater than the normal background noise.",
"It could not be attributed to any terrestrial source.",
"On the other hand, there are number of reasons why it's not a smoking gun or definitive proof:",
"Despite exhaustive search with better telescopes, the signal could not be found again.",
"It came from a region of space with few stars, which brings into question whether or not it could be from an alien civilization.",
"In short, there are more questions than answers. While it seems unlikely to have come from earth, that possibility can't be ruled out, nor can the possibility that it may have home from an as-yet unknown astronomical phenomenon. There's simply not enough data to draw a conclusion with any certainty."
] |
[
"Astronomer here! You are right but with one very important detail that should be emphasized- we do not know if the signal only lasted 72 seconds, or that even the radio signal itself was varying during that time frame. To explain, the radio telescope that saw the Wow! signal detected sources by just seeing what went overhead during the Earth's rotation. The size of its feed horn (ie what was looking at the sky) was such that if you had a bright radio source in the sky there constantly it would look like it was steadily increasing in signal, peak, and then steadily decrease as it went out of the field of view you were looking at.",
"So this is what the Wow! signal was like- the signal varied, but that does not mean the source that was causing it to vary necessarily was. In fact, it was probably quite bright and constant. It's just the telescope was automatically running and no one saw the signal until the next day, so we can't say anything more about the duration than it was on during those 72 seconds the telescope was pointed in that direction."
] |
[
"Because there are a lot of people wondering if, geopolitically, it would be the best thing to tell aliens where we are. What if they're hostile?",
"To be clear, we also don't do a lot of consciously sending out other signals for aliens to pick up (",
"with some exceptions",
") and this isn't a huge part of SETI operations at all."
] |
[
"How much Asteroid mining/extra mass until it has an impact on earth's orbit?"
] |
[
false
] | null |
[
"It will not have an effect.",
"Because the sun is so much more massive than the Earth, Earth's mass is essentially irrelevant and only the sun's mass matters. The Earth would have to exert a significant tug on the sun in order for its own orbit to change. However, a heavier Earth would affect the moon's orbit. BUT there really isn't that much asteroid out there, all of them added together are about 1/25th the mass of the moon, and even if you pile all that onto the Earth* it will only increase the planet's mass by a twentieth of a percent, which an order of magnitude less than ",
"naturally occurring gravity variations.",
"*do not recommend"
] |
[
"The ",
" mass of all the asteroids is just .0005 times the mass of the Earth. Asteroid mining would produce no significant change in the mass of the Earth."
] |
[
"That's a fairly large scale affect, there're also variations due to changes in elevation and sub surface composition"
] |
[
"Question about Pluto's dwarf planet status?"
] |
[
false
] |
[deleted]
|
[
"This is not my field. The problem is that Pluto is in the ",
"Kuiper belt",
" a sort of cold belt of junk left over from the early solar system. ",
"When a proto-planet encounters \"junk in its neighborhood\", one of 2 things should happen, either it captures the junk and adds to its own mass, or its gravity flings the junk out of the neighborhood by a slingshot type mechanism. Pluto has not accomplished this, since it is just a large object in the Kuiper belt, much like Ceres is just a large object in the asteroid belt, despite also meeting the first two criteria.",
"Edit: Ceres comparison."
] |
[
"The reason that Pluto and Neptune don't crash into each other is not because Pluto's orbit is tilted but because they are in a 3:2 orbital resonance."
] |
[
"This is a HUGE controversy. When the IAU voted it was the middle of the night after most members went home. Some people believe they did this just so the one planet found by an American would not be a planet. There is some conspiracy theories it is crazy. I know people who say Pluto is a planet. ",
"As for the third one, look at its moon (2 moons now). Are they really moons of pluto or just other objects. Which orbits which? "
] |
[
"Are some of Pluto's moons really ellipse shaped?"
] |
[
false
] |
[deleted]
|
[
"They are not ellipse shaped, but they are not spherical either. Objects that aren't big enough for gravity to pull them into spheres have potato-looking shapes. The cutoff is around 200 kilometers radius: ",
"http://quarksandcoffee.com/index.php/2015/10/29/why-are-some-moons-spherical-but-others-are-shaped-like-potatoes/",
"New Horizons did recently get some images of Pluto's smaller moons. ",
"As you can see",
", Nix looks like a potato."
] |
[
"You, for example, are probably not spherical, but if you were the size of Arizona cubed, you probably would be.",
"I dunno, some people are alarming close to spherical at much lower masses... "
] |
[
"Mars' moons are not spherical, nor are most of Jupiter's or Saturn's. Neither, for that matter, are most asteroids. ",
"This page",
" on hydrostatic equilibrium gives a few examples of objects that do and don't have spherical (or spheroid) shapes and their relative sizes. (see the Planetary Geology section of \"Applications\")",
"Basically, at around 400km in diameter, the gravity is strong enough to pull the object into a sphere, but below that, there isn't enough mass. You, for example, are probably not spherical, but if you were the size of Arizona cubed, you probably would be."
] |
[
"Does naturally occurring oil provide any environmental benefit?"
] |
[
false
] |
If we never extracted oil from the ground, what would happen to that oil over time? Does it provide nutrients to anything? Will it just sit there indefinitely? Does it hurt anything? BTW I in no way want to use any answers to justify oil extraction
|
[
"If left unextracted, oil would sit in place until it were either squeezed out by tectonic forces (eventually reaching the surface, in theory) or transformed into natural gas due to heating. It's impossible to say when either process will occur, or how long it will take. It won't hurt anything just sitting under the ground - remember, it's already been there for hundreds of millions of years."
] |
[
"There are also some microorganisms that can metabolize it, but this not likely their primary source of energy. "
] |
[
"True, but they live relatively close to the surface. They're generally separated from oil by layers of cap rock."
] |
[
"In board sports, why are some people 'regular' footed or 'goofy' footed and why does this have nothing to do with being left or right handed?"
] |
[
false
] | null |
[
"Here's a good diagram",
" for anyone needing further clarification.",
"Turns out I push mongo (didn't know there was a name for it) and ride regular. I'm right handed so I thought it was always related."
] |
[
"That's sort of what I'm asking. "
] |
[
"That's sort of what I'm asking. "
] |
[
"Can radio waves be created in nature? And If so, how?"
] |
[
false
] |
[deleted]
|
[
"Radio waves are just long wave electromagnetic waves, and are emitted by anything with a temperature (see black body radiation)."
] |
[
"Yeah, look up radio telescopes, a lot of what we know about the beginnings of our universe comes from early radio waves."
] |
[
"One of the most ubiquitous signals in astronomy is the ",
"hydrogen 21 cm line",
". Since hydrogen is everywhere, we see this line a lot, right in the middle of the radio band (21 cm wavelength = 1.4 GHz frequency)."
] |
[
"Does leg bouncing/twitching really burn hundreds of calories a day?"
] |
[
false
] |
From reading around it's seems there's a consensus that you do burn a notable amount of calories if you're a leg bouncer, but the range of calories cited seems really wide. How many calories a day or hour do I really burn from my annoying habit?
|
[
"I don't know, but I have a feeling it's not that much.",
"What I do want to say, however, is that you should make sure both you and the sources you've read know the distinction between calories and Calories. 1 Calorie = 1000 calories, and the Calorie version is what is quoted on food packages and FDA diet recommendations."
] |
[
"Who's the fucktard that came up with that convention? And why are people not getting a new name for that?",
"It's like saying ",
".",
"Absolutely retarded."
] |
[
"Yup. If you're really serious about being fed up with it, though, you can always move abroad and go metric. :)"
] |
[
"Why is there a 100% chance that a 2D random walker will return to its original position but this reduces to 34% in three dimensions?"
] |
[
false
] |
So I read that a two dimensional random walker will always return to its original position given infinite time, but this decreases more and more in n dimensions. In three dimensions, for example, I read that there is a 34% chance. Why is this?
|
[
"I'm ",
"paraphrasing an answer from Math Overflow",
" in a way that will make it more accessible. Let's say you have a random walk in d dimensions. What does it look like when you disregard all but one of the dimensions - you see a 1-d random walk! Since you have d dimensions, we can do this kind of thought experiment for each dimension. So in the end you are really looking at a combination of d one-dimensional random walks (really a \"product\" of sorts). Now, if you are looking at the probability of returning to the origin, in 1 dimension the answer is not that difficult. The answer for d dimensions is really a product of the 1-dimensional probabilities. They are kind of small but for d = 1 and d = 2 they add up to 1 quickly and you find that you return to the original position. When d = 3, they are just small enough that they don't add up to 1, no matter how many steps you allow. ",
"To be a bit more precise the probability that a 1-d random walks is near the original position after n steps is proportional to n",
". To find the probability that we'll eventually return you need to sum the probabilities that the walk will return in 1 step, in 2 steps, in 3 steps, ad infinitum. This sum becomes infinite, and so you are sure that you'll eventually return. Now since you are looking at a product of d one-dimensional walks, you expect that the probability of coming back after n steps is going to be proportional to (n",
")",
" = n",
". The sum of these over n is going to be the probability that we'll eventually return.",
"When d is 1, you have a sum of 1/sqrt(n) which is infinite ",
"p-series with p = 1/2",
" as I mentioned. When d is 2, you have the sum of 1/n, which ",
"is also infinite",
" and we're sure to come back to the origin. However, when d = 3, you have a convergent series and in the end the probability ends up being less that 1. This is more and more true for larger d.",
"In a more philosophical sense, there is just too much space to go to in higher dimensions. In 1-dimensional space you can only go left and right. In 2 dimensions you have east, west, north and south and gets just a bit harder to eventually come back but not difficult enough to make it improbable. In 3 dimensions you now have up and down and it's just too much to handle with certainty. This issue becomes even worse in higher dimensions.",
"To get the actual probability you need to look at ",
"Polya's random walk constants",
"."
] |
[
"What it ",
" means is that if you consider the probability that it will return to the origin within N steps, as N goes to infinity, the probability approaches 100%. Obviously there will be some paths that don't return, but they're unlikely."
] |
[
"I think (but I could be wrong) that the shape of the grid is irreverent in this experiment. The reason being that you can overlay a higher resolution square based grid on any other type of 2d grid, and any path taken in that second grid system can be represented in the square based grid system.",
"The pixels on your monitor are all either squares or rectangles, but can still display a hex-based grid very easily."
] |
[
"When someone says that if you were traveling at the speed of life, time would not pass in that reference frame, what does this mean, exactly?"
] |
[
false
] | null |
[
"Yeah, in the same way that \"in our current understanding of physics\" things fall 'cause of gravity and not because invisible angels push them toward the ground."
] |
[
"Okay, first of all, you can't move at the speed of light. Period. No exceptions, no arguments. You simply can't, not ever.",
"But you can choose a set of coordinates that are moving at the speed of light relative to you. That's trivial. Poof. There they are.",
"Once you've chosen such a set of coordinates, you can write down the equations that let you ",
" things like distances and durations from your coordinates to the infinite-momentum coordinates you chose.",
"When you do this, you find that any duration you care to identify, when transformed into the infinite-momentum coordinates, turns out to be exactly zero. In other words, the ",
" between any pair of points in that reference frame is zero.",
"But that's only half the story. Remember, there's one dimension of time and three dimensions of space. If you consider the dimension of space that is ",
" to this imaginary set of coordinates' motion relative to you, you find that all distances in that dimension are contracted to zero. You can identify any two points you like, as long as they're separated by a straight line parallel to the relative velocity of the infinite-momentum frame, and when you transform that pair of points into the infinite-momentum frame, you find that the distance between them is exactly zero.",
"In other words, in an infinite-momentum frame, all ",
" are zero, and all ",
" parallel to the direction of relative motion are zero.",
"But this is purely a mathematical exercise. You cannot move at the speed of light. Not ever, not ever, not ever. It's not even worth a \"what if.\""
] |
[
"So you're saying that trying to make sense of time as we perceive it makes no sense compared to perceiving time at the speed of light? Could you elaborate on this a bit more? what do you mean by \"infinite-momentum\"? Why is it that the distance between any two points (in a straight line) is zero when moving at the speed of light, intuitively speaking? When I imagine two points, I imagine a set distance between them despite what speed whatever you're observing is traveling at, and that the speed of light just happens to be a ",
" speed.",
"I guess I have a very poor conception of spacetime. I apologize for asking so many questions, I just so badly want to understand what I guess Einstein was getting at."
] |
[
"Is Coca-Cola as acidic as my teacher says?"
] |
[
false
] |
He says it has a pH of 2.8, you can dissolve nails, etc. I disagreed with him and showed him a snopes link that said it was false. He completely dismissed it. If it's false can you provide some sources he'll trust? Thanks Edit: Thank you for all the answers! And yes I know I'm a complete dumbass. Snopes is not a good source.
|
[
"OP asked for the pH of an internationally known beverage (Coca Cola). Although redditors delivered answers, empirical evidence for the stated fact was not provided, nor were peer-reviewed sources cited. Therefore, we set up a simple experiment to answer OP's question.",
"The beverage was obtained in the cafeteria of the faculty of chemistry and biochemistry. It was opened and 50 ml were transferred to a beaker. Since pH is temperature-dependent, the temperature of the beverage was measured with a fine-scale thermometer and the pH meter was adjusted accordingly. ",
"pH was determined with a lab grade pH meter (Knick, Germany) as well as with indicator strips (Merck, Germany). The digital pH meter was calibrated with two buffers, pH 4 and pH 7.",
"At a temperature of about 12 °C (54 °F), the pH of the beverage was determined to be 2.56 with the digital pH meter. The measurement with indicator strips gave a value between 2.5 and 3, validating the original measurement.",
"Figure 1",
"The study finds that Coca Cola has a pH of 2.56, which is in the acidic range. These results are in line with values reported previously in the literature",
" and back the statement by OP's teacher. However, this does in no way address any claims regarding postive or negative effects on human health, as this was beyond the scope of this study.",
" ",
"Chin TW, Loeb M, and Fong IW (1995): Effects of an acidic beverage (Coca-Cola) on absorption of ketoconazole, Antimicrob Agents Chemother (39): 1671-1675.",
" ",
"Johansson AK, Lingström P, and Birkhed D (2007): Effect of soft drinks on proximal plaque pH at normal and low salivary secretion rates, Acta Odontol Scand (65): 352-356",
". ",
"Edit: ",
"The authors thank the anonymous redditor for the gilding.",
"Edit 2: ",
"\nUpon re-reading of the manuscript, the authors noticed two mistakes:",
"A volume of 75 ml, not 50 ml, were transferred to a beaker.",
"The correct pH is 2.52, not 2.56. However, a ΔpH of 0.04 can be considered negligible.",
"Wow. Thanks. That was a surprise when I opened reddit this morning."
] |
[
"Wow great job! Thank you so much."
] |
[
"The phosphoric acid and carbonic acid in coca-cola do have low acidic pHs and given enough time will dissolve all sorts of things, most obviously tranishing on pennies and limescale build up on taps, showerheads or toilets ",
"As both acids are weak acids you wouldn't expect them to be especially corrosive unless they were highly concentrated (>60%). As such I would be amazed if you could dissolve nails with any solution as dilute as coca-cola"
] |
[
"Where do the nutrients come from when regrowing romaine lettuce from an intact stem?"
] |
[
false
] |
We've placed an intact stump in just water and the romaine seems to be growing phenomenally well. However, without soil and the chemistry found therein, what aside from sunlight is promoting this growth? Additionally, the water turns yellow - what process causes this?
|
[
"what aside from sunlight is promoting this growth?",
"The ",
"carbon dioxide in the air",
". That's why with addition to some other nutrient requirement (such as nitrogen and phosphate), you can grow plants hydorponic settings.",
"Furthermore, the lettuce is likely to have quite a bit of nutrient store within the stem in the form of starch and cellulose. It's in the process of reestablishing itself using the reserved energy."
] |
[
"Agree completely with zephirum---most of the bulk of a plant comes from either water or carbon dioxide (providing C, H, and O), so it can increase in size substantially from these alone. The plant is redistributing all the other elements (nitrogen, phosphorous, iron, etc) around itself from reserves in the stem and leaves. If you were to leave it like this for a longer period, it will eventually start to fail as these nutrients run out--unless you add them to the water.",
"no idea about the yellow water...could be pigment leaking out of the plant, or could be microbe or algae growth in the water. Actually, if it's a greenish yellow and the water has been inthe sun for days, it's almost certainly algae."
] |
[
"Don't forget, that tap water they are using has plenty of dissolved minerals in it for whatever plant structure is not CHON-based molecules."
] |
[
"When archeologists find a fossil millions of years old, how do they determine its age if carbon dating has a limit of ~60,000 years?"
] |
[
false
] | null |
[
"There are a whole host of different ",
"radiometric",
" techniques that, depending on the decay system and mineral in question, have time ranges which exceed the age of the Earth / solar system. Radiocarbon dating is important for archeology and very specific subsets of geology that are concerned with deposits <~50,000 years old (e.g., it's used commonly in paleoseismology, etc), but for the vast majority of geology, methods like ",
"U-Pb",
" or ",
"Ar-Ar",
" are more relevant (and have effective ranges of several billion to hundreds of million of years depending on the exact method). Our ",
"FAQ entries on radiometric dating",
" may also provide additional context."
] |
[
"Thank you!"
] |
[
"Also, once you’ve narrowed down indicator fossils you can use those to extrapolate new fossils found…..say you find a new fossil, it’s below a layer containing a fossil you know to be 60m in age, you now have a head start in knowing it’s older than 60m….using stratigraphy can get you most of the way without having to use any decay age dating methods….compaction and looking at surrounding rock can tell you a lot also….if your new fossil is in a layer with a well documented age range your set, works already been done for you….",
"But on a new discovery in a undetermined geologic layer then yeah, you’ll need to narrow down the age range using the above, then pick a radioactive decay method appropriate to verify and hopefully narrow further…."
] |
[
"What are \"cosmic rays\"?"
] |
[
false
] |
I know space travel is hazardous to astronauts due to long term exposure to "cosmic rays".... but what are they? Are they part of the electro-magnetic spectrum, like gamma rays? Also, I've read that they are difficult to shield against... but an insulating layer of water in a space ship's hull could provide protection... is that true? If so, why water and not another (less bulky) substance? : Thanks, !
|
[
"They are highly energetic charged particles which originate outside our solar system. They are primarily protons.",
"When they collide with atomic nuclei in the atmosphere, they produce showers of other particles via nuclear spallation and other high-energy reactions.",
"There are muons traveling through your body right now which were produced as a result of these showers. For example, a high energy proton could collide with a ",
"N nucleus, producing a shower of pions. The charged pions can then decay into muons, which travel toward the ground at a speed very close to c.",
"Time dilation extends the lifetime of these muons in the laboratory frame so that they live long enough to reach us. They are traveling through you right now, losing a very small amount of energy every once in a while.",
"It's extremely hard to shield against these muons. But they don't significantly harm us, and they can be pretty useful (for calibrating detectors, for example)."
] |
[
"what are they? Are they part of the electro-magnetic spectrum, like gamma rays?",
"No, they are not part of the electromagnetic spectrum. The term was coined by Pierre Auger when he reached high altitudes in a balloon and detected \"a penetrating radiation\". Today we know they are actually charged particles travelling at very high speeds (mostly protons and alpha particles, plus some heavy ions).",
"I've read that they are difficult to shield against",
"That's correct. The problem is their very high energy, they will penetrate very deep (several meters) in any shielding material.",
"but an insulating layer of water in a space ship's hull could provide protection... is that true?",
"It is a partial truth. An insulating layer of water could block the lower energy particles. Higher energy particles would still go through, but they are fewer.",
"I call it a partial truth because making a thick enough water shield is unrealistic. Launch costs are in the order of $5000 per kg. If you had to shield an entire ISS module (taking Columbus as an example) with a 1m thick shield it would take almost 190 m",
" of water, with a mass of 190 tons - I'll let you calculate how expensive that would be, I'll only tell you that the whole ISS is around 450 tons... and this would merely shield of a single module. What's worse, a 1 m thick shield would merely reduce the radiation dose by 10%. Providing good shielding is impossible with any realistic budget.",
"why water and not another (less bulky) substance?",
"When a cosmic ray collides with an atomic nucleus it breaks it up and throws a bunch of more elementary particles. We call this secondary radiation, and in many cases it can be more harmful than the cosmic ray itself. In fact, a poorly designed radiation shield can be worse than no shielding at all.",
"The higher the atomic number of the shielding material, the higher the cross-section area of the atomic nuclei, thus the higher probability of collisions. We prefer materials that contain light elements and preferably lots of hydrogen (the ideal, of course, would be pure hydrogen, but it's difficult to maintain in liquid form because of it's extremely low boiling temperature, and much more bulky than water).",
"The other reason is implicit in the Bethe formula: the radiation stopping power of a material is proportional to Z/A, where Z is the atomic number and A is the atomic mass. This fraction is 1 for hydrogen and ~0.5 for most of the other elements. Therefore we prefer a hydrogen-rich material (like water or polymers) so that it can achieve better shielding proportionally to its mass."
] |
[
"The term \"rays\" is misleading, because it does make them seem more like EM radiation, but they're typically massive particles, usually single protons but occasionally whole atomic nuclei.",
"It was an error that was corrected, but the term stuck. "
] |
[
"Why is the water molecule a dipole in the first place?"
] |
[
false
] |
Yeah, sure, they teach you in high-school chemistry water is polar and that it is one of the reasons it is so amazing. What they don't tell you is . A quick google search says something about . So what? There's TWO molecules, and since two positive charges repel, wouldn't that make the hydrogen atoms push each other to the far end of the oxygen atom, resulting in a symmetrical molecule with no poles? What's the lies-to-children part of molecular bonds they didn't tell us?
|
[
"Because there are other electrons on the oxygen that aren't shared with the hydrogen, and they repel each other too. ",
"Here's an illustration:",
"http://learnbiochemistry.wordpress.com/category/water-h2o/"
] |
[
"Besides just the hydrogen atoms, an oxygen atom in water has two electrons pairs. All four of these want to be as far apart as possible, resulting a a tetrahedron shape in 3D space. ",
"http://en.wikipedia.org/wiki/Tetrahedral_molecular_geometry",
"I'm sure a chemist could explain this better. Also, I was taught this in middle school and high school."
] |
[
"If you want to know more detail, read about ",
"VSEPR theory",
". It explains why molecules take the shapes that they do. It says that water is will be similar to a tetrahedral structure (2 lone pairs, 2 hydrogens around a central oxygen), but slightly 'bent' because the lone electron pairs space out differently than the hydrogens.",
"In addition to the geometry, you also need to consider the relative charge of the hydrogen and the oxygen. As you mentioned, because of their relative electronegativity the electrons involved in bonding 'spend a little more time' or 'stay closer to' the oxygen than the two hydrogens. I used quotes because the statements are simplifications. Because of that, the oxygen is slightly more negative and the hydrogens are slightly more positive. The charge imbalance plus the geometry give you your dipole.",
"If the molecule were linear (e.g. CO2) it would still have more negative charge close to the oxygens, but because of the geometry there is no net dipole for the whole molecule."
] |
[
"[META] - Introducing AskScience Sponsored Content"
] |
[
false
] |
The mods at AskScience would like to proudly introduce our newest feature: . We believe that with this non-obtrusive , we'll be able to properly motivate the best responses from scientists and encourage the best moderation of our community. Here is the list of the released so far: All posts must adhere to AskScience rules as per usual, though posts that unfairly attack our sponsors' products may be moderated at our discretion. The best comments in each sponsored thread will be compensated (~$100-2000 + reddit gold) at the sponsors' discretion. Moderators will also be compensated to support the extra moderation these threads will receive. Sponsored content will be submitted by and distinguished to make it easy to identify and prevent spammers from introducing sponsored content without going through the official process. Please see . - djimbob 2013-04-01
|
[
"I've already learned so much from all of this. I'm really grateful to the mods for allowing this to take place.",
"I can't believe that oil spills, logging, and mining are actually good for the environment. As soon as I get back to the uni, I'm going to confront those silly conservation, evolution, and ecology professors that told me the opposite!",
"I can't wait to embarrass my silly physiology professor when I tell him that cigarettes actually don't give you cancer. ",
"What a great stride forward for the community this has proved to be."
] |
[
"This is a terrible terrible idea IMO.",
"If AskScience does this I will be unsubscribing.",
"Edit: Apologies for the short off the cuff reply... I was on a tablet when posting this first message... This thread/concept bugged me enough to switch to the laptop to give a real defended reply with reasons which is the comments of this. That said my initial opinion of unsubscribing still holds true."
] |
[
"I am all for this. Finally the truth can be told without liberal Reddit downvoting everything. We need fracking and Monsanto. People don't seem to understand this.",
"note: payments can be made by paypal to ",
"blueboybob@gmail.com"
] |
[
"What happened to all the energy released during Baryogenesis?"
] |
[
false
] |
[deleted]
|
[
"Whatever energy existed in the past is still here, it's just been spread out.",
"The universe is expanding, and as it does, the total mass/energy density of the universe declines. There's stars, galaxies, and the CMB. If you sum up all that mass/energy and then run time backwards to a brief moment after the big-bang, you'll find that you have an unbelievably huge energy density everywhere."
] |
[
"I actually don't know anything about the stages of the Big Bang or Baryogenesis, but I think the answer is yes.",
"If I'm wrong, I hope someone can correct me."
] |
[
"Does that mean that CMB is the radiation rleased during Baryogenesis only very spread out?"
] |
[
"Why is it so hard to remember dreams?"
] |
[
false
] |
If I just wake up and go about my business, I never remember dreams. The only way I can manage it is to wake up slowly, in darkness, and what I've been dreaming while I slowly wake up. I think I'm making a conscious memory that is keyed to the ones that have been flagged as unconscious/no access. But once I have that key, I can remember quite a bit about the dream, and even other related dreams.
|
[
"I usually remember about 4-8 dreams per night.",
"You win, sir; that's the most blatant bullshit I've read today."
] |
[
"My dreams are vivid, and I remember them all when the alarm goes off. I return directly to them after that first swipe at snooze. \nThis has remained constant for fifty three years regardless of substance abuse or health regimes. I do not know how or why, but I fly, get laid by twins, get crowned king, travel in space, and do all kinds of wonderful shit.\nUnfortunately most of them involve trying to use a phi lips head screwdriver to turn a standard screw for the whole night. Then I wake up and got o work. "
] |
[
"I kept a dream journal two years ago or so when I started lucid dreaming, and I found out that just deciding that you will try to remember your dreams makes your dream retention better. Two nights after my first dream journal entry, I was suddenly remembering two dreams each night."
] |
[
"Why have dental scientists (is that a thing?) not come up with something better to fix cavities with than metal tools and drills?"
] |
[
false
] |
It seems to me that using basic metal tools to hack away at the hard tissue of teeth is a slightly barbaric way of dealing with cavities/root canals, etc. Why have we not come up with something better? OR What could be a future alternative?
|
[
"They have alternatives. My old dentist used to use a ",
"air abrasion",
" tool (the equivalent of sandblasting for teeth) to clear out tooth decay. "
] |
[
"Is there a reason that a) He doesn't anymore? and b) It's not more widespread? It seems that there are alternatives, but why are they not more implemented?"
] |
[
"Well, the particular dentist I went to retired. I would imagine the reason more dentists don't use it is because it likely requires going back to get mid-career training in order to be able to use it, also the equipment represents an additional capital cost you need to be able to recover in your business. I would imagine drills are something every dentist is taught to use (and every dentist has on hand). Everything additional (training, equipment, maintenance, etc) is just another barrier to providing it."
] |
[
"Why is an electrical plug not hot / warm immediately after being unplugged from an outlet?"
] |
[
false
] |
[deleted]
|
[
"Electrical plugs ",
" get warm. If you use a high-power appliance you'll notice it - vacuum cleaners, space heaters, etc. "
] |
[
"Electrical conductors are designed with enough cross-sectional area so as not to become noticeably warm under normal use.",
"The thicker a conductor is, the lower its electrical resistance. The heat dissipated by a plug can be calculated with I²R. Say the resistance is 0.01 ohms and you put 10 amps through it. The heat produced would then be 1 Watt. Properly designed plugs rated at this current will have lower resistance (ie. thicker prongs & connections) to prevent losing watts.",
"Next time you use a vacuum cleaner, feel the cord, it's probably warm due to the resistance of the cord and high current passing through it."
] |
[
"I've seen plugs on extension cords that were rated high enough to handle the air conditioner get a little squishy-melty on really hot days. "
] |
[
"What's the difference between a legless lizard and a snake?"
] |
[
false
] |
They look very similar to me but what is the real difference?
|
[
"Another difference is that lizards (even legless ones) have earholes in about the place you'd expect. Snakes have no external ears, and they pick up vibrations through the ground. "
] |
[
"There are a bunch of differences between the two (skeletal differences, etc.). But one of the most obvious one is the legless lizard can blink (they have \"eye lids\"), while snakes can't (they have a modified scale that covers their eye all the time and have no \"eye lids\"). ",
"EDIT: This is just a general rule, there may be exceptions"
] |
[
"In addition, snakes have broad belly scales, lizards do not. It's harder to tell by just looking, but snakes have very short tails, while lizards have long tails. "
] |
[
"Why does π come in integrals of Gaussian curves ?"
] |
[
false
] |
All in the title
|
[
"The function f(x)=e",
" has the amazing property that the value of f(x)f(y)=f(r), where r is the distance that the point (x,y) is from the origin. This is easy enough to see as ",
"Moreover, up to some scaling factors, it is the only nice function whose product depends on the distance from the origin. This means that it has a special relationship to geometry.",
"The moral of this is that, because of this, the function f(x) has a fundamental multiplicative relationship with the circle. How you can think of this is as follows: The fundamental property of the exponential function e",
", the property that makes it useful and interesting, is that e",
"=e",
"e",
". That is, it turns addition into multiplication. On the other hand, the Pythagorean Theorem tells us that there is an additive connection between the the squares of lengths in circles. That is, if (x,y) is on a circle of radius r, then we have the additive relationship between their squares x",
"+y",
"=r",
". The equation f(x)f(y)=f(r) uses the exponential to turn it into a multiplicative version of this statement.",
"We, therefore, should think of the equation f(x)f(y)=f(r) as a multiplicative version of the Pythagorean Theorem and should then ",
" there to be pi to pop up when we do computations with f(x).",
"To see how, in particular, this pops up in this case, let's let ",
" be the integral of f(x)dx for x=-infinity to infinity. There doesn't seem to be too much we can do right away to compute this. So we can compute ",
" instead. By properties of integrals, instead of writing this as a product of integrals, we can write this as the integral of f(x)f(y)dxdy over the real plane. But we know that f(x)f(y)=f(r), which has rotational symmetry, so we should switch to polar coordinates. This means that ",
" is the integral of f(r)rdrdt, where r goes from 0 to infinity and t goes from 0 to 2pi. Since the integrand does not depends on the angle t, that integral evaluates to just 2pi. So ",
" is equal to 2pi times the integral of re",
"dr for r=0 to infinity. We can use u-substitution to evaluate this, and this will evaluate to 1/2. Therefore, ",
"=pi, and since we know that ",
" must be positive, we get that ",
"=sqrt(pi).",
"Really what we're doing is, roughly, adding up a bunch of multiplicative Pythagorean Theorems. So you can think of the statement that ",
"=sqrt(pi) as a statement of the Pythagorean Theorem."
] |
[
"That was awesome. Thank you!"
] |
[
"We use do have to use ",
"Fubini's Theorem",
", along with linearity of the integral, to setup it up."
] |
[
"Is a black hole a perfect sphere?"
] |
[
false
] |
People speak of black holes having a diameter (independent of the diameter of the event horizon). Do they mean that whatever that diameter measures is part of a perfect sphere? Could there be any perturbations on the "surface" of a black hole, the way neutron stars can have "star quakes" that release gamma ray bursts (and I realize that nothing similar on a black hole, were there any such thing, would release any energy to the outside).
|
[
"Yes and no. The event horizon of a black hole is not a physical object; it is the beginning of the region at which there are no paths leading away from the black hole's singularity. So a non-rotating black hole will have a spherical event horizon.",
"A rotating black hole has an equatorial bulge due to a phenomenon called frame-dragging. It will have the shape of an oblate spheroid (not unlike the Earth), depending of course on your choice of coordinate system.",
"I have to wonder if the \"hairy ball theorem\" is related (and I have a feeling it'll be cited in another answer), but I think the answer is \"no\". Applied to black holes, the theorem states that objects that fall into the event horizon have only the properties of mass, charge, and angular momentum; all other information about them is lost. This doesn't say anything (obvious to me, at least) about whether the event horizon can have surface deformations.",
"edit: Maybe you are not referring to the shape of the event horizon proper or the ergosphere (the frame-dragged region around it which has a more pronounced oblateness), but to the material surrounding the event horizon. In general, this has the shape of an ",
"; material seldom falls straight into the black hole but rather follows a spiral trajectory until it hits the event horizon. The answer, then, is no, the disk is not spherical (and in general it has no \"perfect\" shape). There's no shortage of papers examining fluctuations in the accretion disk; it's a sort of very viscous fluid.",
"As material nears the center of the disk, it heats up, releasing x-rays. I seem to remember reading that this is usually in the direction perpendicular to the plane of the disk, but I might be wrong.",
"edit edit: Maybe I've still got it wrong and you're referring not to the event horizon, the ergosphere, or the material outside the black hole proper, but rather the singularity itself. The singularity is a point or, if the body that formed the black hole was rotating, a ring. Calculating the diameter of the ring is unfortunately beyond the powers of my undergrad physics degree. (Actually, on further search, it seems like a \"circumference\" can meaningfully be calculated, but the space inside the ring is so fucking curved that a \"diameter\" wouldn't be particularly meaningful.) In either case, the singularity has no \"surface\" and no properties besides mass, angular momentum, and charge (if, say, the body that formed the black hole, or the material that has since fallen into it, had a lot of electric charge)."
] |
[
"As material nears the center of the disk, it heats up, releasing x-rays. I seem to remember reading that this is usually in the direction perpendicular to the plane of the disk, but I might be wrong",
"Around smaller black holes, the temperature of the inner edge of the accretion disk is high enough that the peak of the black-body spectrum is in the X-ray regime. The more massive the black hole, the cooler the inner disk edge, so super-massive black holes only have UV disk emmission. The disk emmission will be primarily perpendicular to the disk.",
"You do get X-ray emission from supermassive black holes, but the actual origin is still very much up in the air. The assumption is that it is produced by Compton upscattering, but where that occurs is unknown. One of the more common ideas is a cloud of gas 'above' the SMBH (relative to the disk plane), which oscillates in some manner. Personally I think that's a bit nuts, but proponents like it because they claim you can detect the black hole spin with that model. Other models include an extended corona above the disk, which is the basis of what I am currently including in my simulations."
] |
[
"Unfortunately, it's hard to talk about the shape of the event horizon since spacetime is quite curved there. Usually when we think about a shape, we think of one in Euclidean 3-space, and we can freely translate and rotate a body without affecting the \"shape\". However, the event horizon is a surface within a curved, non-Euclidean spacetime. There is no reason why we could embed it into a Euclidean spacetime (provide a mapping from the horizon H->E",
" to Euclidean 3-space) in order to talk about it's shape. That is, there is no ",
" way to make this mapping, any mapping you choose will be arbitrary.",
"I can definitely say that the ",
" of the event horizon is RxS",
" (that is, the real line (time) times a 2-sphere). For a non-rotating black hole, you can say that it's spherically symmetric. Mathematically, we say that spacetime has an SO(3) isometry, and the event horizon is stable under the isometry. For a rotating black hole, though, we only have axial symmetry, U(1). We can say for sure that the shape is axisymmetric, but it's hard to say more. We can describe the shape in some particular coordinate system on spacetime, but no coordinate system is special (that is, GR is diffeomorphism invariant). The time coordinate and azimuthal coordinate can be chosen \"naturally\" because the spacetime is time-translation invariant (stationary) and azimuthally symmetric (actually this gives a family of coordinate choices). However, the radial coordinate and polar angle coordinate can not be chosen naturally.",
"/u/skadefryd",
" suggested that there is an \"equatorial bulge\", but such a statement is really coordinate-dependent; it can't be made in a coordinate-free way."
] |
[
"If the universe is expanding, does that mean that beyond that there is just nothing? Sorry if this is a stupid question I'm not very educated in any of this."
] |
[
false
] | null |
[
"There isn't any \"beyond\" the universe, because the current consensus is that the universe is infinite. "
] |
[
"No, the answer to his question is determined by consensus. "
] |
[
"The space between two objects is getting larger. This is irrelevant at small scales, but at galactic scales we see uniform expansion."
] |
[
"I'm a creationist starting to take a more serious look at evolution. What, in your view, is the SINGLE most compelling piece of evidence for inter-species evolution?"
] |
[
false
] | null |
[
"Evolution stands as a scientific theory because there is ",
" evidence. Identifying a single piece as the \"most compelling\" isn't really feasible, unless you're willing to accept something like the entire fossil record as a single piece of evidence.",
"If you really want to take a serious look at evolution, I would suggest starting ",
"here",
". ",
"Evolution 101",
" is a very good introduction to the subject, and they also have a page detailing ",
"evidence for evolution",
". This way your computer can do the downloading of large amount of information, while your brain can process at whatever speed is comfortable to you."
] |
[
"ribosomal RNA homology.",
"All living things (depending on how you define \"living\") have ribosomes, and all ribosomes the same basic architecture and purpose. Structurally, they are made of a mix of folded RNA and folded protein, although the RNA parts form most of the \"core\" of the complex.",
"Because every living thing has ribosomes, it follows that the DNA coding sequences for ribosomal RNA are a part of the genome of every organism. This provides a very convenient way to directly compare every member of the tree if life.",
"There are broad sequential homologies, of course. All ribosomes have basically the same function, and function is determined by structure which is in turn determined by sequence, so we'd expect the sequences to be similar. ",
"But the really interesting parts are in the differences. In a population that is under evolution (whether it is a case of selection, or just drift, is irrelevant here; all that is important is changes) you would expect some sequence changes as time goes on. And you would also expect that these sequence changes would by historically contingent; i.e. my sequence is a modified (or un-modified) version of my ancestor's sequence. ",
"If you do this for all known rRNA sequences, making a tree of rRNA relatedness across the tree of life, the result is striking: ",
"WIKI",
"There, in one plot, is a mathematical demonstration that all known life shares a common ancestor.",
"And here, another plot where instead entire genomes are compared: ",
"WIKI"
] |
[
"Very literal. As far as we know ",
" life on earth shares a common ancestor."
] |
[
"Does a refrigerator cool more efficiently when it is full or empty?"
] |
[
false
] |
If I have some food in my refrigerator that I am going to throw away, but that does not need to be thrown away urgently due to spoilage -- say, an expired can of beer -- is it more energy efficient to leave the can in the refrigerator until later, or is it more energy efficient to have as much empty space as possible in the refrigerator? Or maybe it makes no difference? Intuitively, I would assume that the refrigerator would be more efficient with a can of beer because the can "retains cold better" -- but I know that we aren't concerned with cold, we are concerned with heat: and it's much easier to change the temperature of air then it is to change the temperature of a solid. So I guess that as long as no heat is entering the refrigerator, it would be more efficient for it to be empty. Is that correct?
|
[
"Full. ",
"The amount of energy a fridge consumes depends on the temperature difference between where it wants to be and where it is, temp-wise. Leave a fridge closed and once everything is cool you only have to make up for head influx though the well-insulated walls. This is the lowest amount of energy usage your fridge will have so we want to stay as close to that as we can. ",
"Problem is that you can't grab a beer through the closed door. I mean you might be able to but I can't. If you can put that shit on Youtube. It'd definitely be bad-ass. Anyways - when you open the door hot air comes in from outside. More hot air = more energy needed to chill it all back down. You can minimize the amount of air that comes in by packing the fridge full of stuff that takes up space and having less fridge air that can be lost and replaced with outside air when you open the door. ",
"You're right in identifying that it takes more energy to cool down the beer in the first place than it would an equivalent amount of air. Therein lies the trade-off - how much air would you have to cool to make it take an equal amount of energy as cooling the beer? That's a problem someone with some HVAC knowledge could answer. You can cheat by getting the beer cold for free - take it from the store, your buddy's fridge, or leave it outside once the weather dips below freezing again, then put your freely-chilled beer back in the fridge. ",
"Jugs of water work, too, if you don't have gallons of beer in your house. "
] |
[
"If it is full of cold items - then opening the doors briefly means less cold air to escape and be replaced by warm air. But it will take more energy to cool them down in the first place.",
"But when fridge is cold (and assuming fairly air tight) - I suspect it will loose the same amount of energy to the environment whatever it contains "
] |
[
"expired beer? you're kidding right?"
] |
[
"Hey social scientists, what do the data actually say about guns and crime?"
] |
[
false
] |
It's often said by gun-control opponents that gun ownership reduces crime, and obviously we can imagine scenarios where we defend our homes from burglars with the aid of our trusty Glock, but gun-control proponents say you're more likely to shoot a member of your household than a burglar. There seem to be reasonable, informed people on both sides of the issue. What's the truth, empirically? Do places where people have more guns experience less crime, and does it look like the former causes the latter? Does the likelihood of an accident dwarf any such effect? is for expert scientific answers, not layman speculation, memes, politics, etc.*
|
[
"You may have trouble finding good studies with real data. This New York Times article discusses the ways in which the NRA has used its political clout to defund exactly the kind of research you are hoping to find. It's a very chilling article for any scientist to read.",
"http://www.nytimes.com/2011/01/26/us/26guns.html",
"To be clear, I'm not trying to make a political statement with this; rather, I hope that it serves as a reminder that unfortunately, the \"truth, empirically\" may be mired by politics in such a politically-charged area of inquiry. "
] |
[
"All the data I've seen indicates that violent crime is a function of poverty and other social factors more than gun ownership. ",
"Pennsylvania releases gun sales numbers by county, on a yearly basis. \nI'll use two neighboring counties, Philadelphia and Bucks, as an example.",
"Bucks County is suburban (625,000 people), Philadelphia is urban (1.5 million people). Median family income in Bucks County was estimated at $86,493 (2007) and in Philadelphia (2009) at $45,842. ",
"2010 numbers (",
"http://www.portal.state.pa.us/portal/server.pt/document/1102470/pennsylvania_state_police_2010_firearms_annual_report_pdf):",
"\nBucks County: 13,622 handgun purchases \nPhiladelphia: 6,852 handgun purchases",
"It seems reasonable to assume the rates of purchase are similar to the rates of legal ownership, as these numbers are fairly stable over several years. About five times as many long guns were bought in Bucks as in Philly, but for the sake of brevity I'm going to assume that long guns are infrequently used in crime and can be discounted. It's also useful to note that, in PA, handguns must be transferred through FFLs (gun dealers) while long guns may be transferred in face-to-face (undocumented) sales, and that the gun laws in Bucks and Philly are identical (state preemption of firearms laws). ",
"Murder (criminal homicide) rates, 2010:",
"Bucks - 1 per 100,000 \nPhiladelphia - 19.9 per 100,000 ",
"So, from this data, about 7.2 times as many handguns are purchased per capita in Bucks, and Philly has about 20 times as many murders per capita. Median family income in Bucks is roughly double that of Philadelphia.",
"It is probably a safe assumption that the rate of illegal gun ownership is higher in Philadelphia, though those numbers are incredibly hard to pin down accurately. As these two counties border each other, and there is no legal requirement to purchase a handgun in your county of residence, it is likely that at least some purchases were made by Philadelphia residents at Bucks gun stores, and vice versa. ",
"Obviously this is a very limited set of data, but you could draw two reasonable conclusions:\n* Increased gun ownership = decreased murders\n* Lower income = more murders",
"Which of these is more likely? Probably the second one. However, I think the safest conclusion is that gun ownership will not cause violent crime in an area where there is very little violent crime to begin with, but it may exacerbate violence in an area that is predisposed to violence. ",
"tl;dr: There are far too many variables to easily isolate gun ownership with relation to violent crime, but there is enough evidence to say that gun ownership ",
" does not cause an increase in murder. Gun ownership in combination with other social factors may. "
] |
[
"There's an ",
"AskSocialScience",
" subreddit, where you might have more luck."
] |
[
"If light has more energy than microwaves (since it has a higher frequency), why do microwaves have a greater heating effect?"
] |
[
false
] | null |
[
"All the answers I see here do NOT correctly explain how a microwave works!",
"Microwaves do not heat food by directly imparting energy via photon absorption. They work via ",
"dielectric heating",
".",
"Essentially, the microwave field in your microwave oscillates rapidly. The polarized molecules in your food (mostly water, maybe some fats) attempt to realign with the field. As they continue to realign with the field while it oscillates back and forth, you impart a fair amount of thermal energy to the molecules, and your food heats up.",
". Not only is the resonant frequency of water well outside the microwave range, nut microwaves do not carry much energy. To all you people who answered incorrectly, shame on you! Don't speculate!",
"EDIT: I seemed to have spawned quite the debate. Let me be clear - I interpreted the question as why microwaves work. This is via dielectric heating. ",
"To all those who vehemently defend absorption of photons - yes, photons are the quanta of the EM field. And yes, to impart momentum to a molecule a photon needs to mediate the EM force. That being said, it is a common misconception that photons are absorbed and ",
" molecules. ",
". I wanted to dispel the common myth about microwaves working via absorption. I wanted to explain it in a way most accessible to a laymen. ",
"If you fault me for misinterpreting the question, ok. If you fault me for not mentioning the quanta of EM field, then maybe we disagree about the point of this forum (answering questions for ",
"). I don't believe in answering a question about how microwaves work by explaining quantum field theory, when classical E&M is adequate and conceptually much easier to grasp. You only need Newtonian gravity to explain the moon's orbit, not GR."
] |
[
"Temperature is a measure of the motion of a molecule. Molecules can undergo several types of motion: translation (where the whole molecule moves in one direction), rotation (the whole molecule spins), and vibration (the atoms in the molecule wiggle with respect to the other atoms in the same molecule).",
"The EM radiation (light, microwaves, etc) that a molecule absorbs and converts to motion depends on the ",
" of the light. Microwaves are just the right frequency to make water molecules start to rotate. This increase in rotation is observed as an increase in temperature. The water molecule has absorbed the energy of a microwave photon and converted it to energy of motion (kinetic energy).",
" You might have noticed that microwaves only heat up liquid water - they don't do so well to heat up a block of ice. This is because ice is a solid, so the water is not free to rotate and heat up as well as if it were all liquid. Also, microwaves heat the water, but collisions between water and other molecules in the food disperse energy to heat everything evenly. "
] |
[
"Actually, dielectric heating does involve the absorption of (microwave) photons. The microwave field is a quantized field just like any other, and the water molecules absorb excitations in this field. It is wrong to say \"do not heat food by directly imparting energy via photon absorption\". That is actually precisely what happens, it's just that it doesn't tell the whole story."
] |
[
"Which uses less energy. Accelerating down a hill and using momentum to reach the top. Or, accelerating up the hill after you free roll down?"
] |
[
false
] |
If you are driving a car and encounter a row of hills all exactly the same height and perfectly symmetrical, what is the most fuel effecient way to drive over them?
|
[
"On a coarse level, ignoring environmental friction, if you never touch the brakes, you would use the same amount of energy no matter how you drove them. Fuel efficiency is lost when you convert kinetic energy into heat. The power needed to overcome rolling resistance is linear with speed, but the power needed to overcome air resistance increases exponentially with speed [",
"1",
"]; considering both factors, the most fuel efficient method will be the one which results in the slowest median speed ",
", since you will encounter the least rolling and air resistance at the lowest median speed, but when you use the brakes, you rob the system of kinetic energy and must reintroduce energy back into the system by burning fuel.",
"Thus, you would reach peak fuel efficiency by expending exactly as much energy as necessary to reach the top of a hill, and then coast down it without touching your brakes. You would then use your engine as needed to reach the top of the next hill and repeat."
] |
[
"You don't factor in wind resistance. The faster you go, wind resistance creates exponentially more drag. You want to cross the hills with the lowest average speed.",
"\nSee ",
"cheald's comment"
] |
[
"Most cars have a fuel cut off that kicks in when you let off the gas (engine braking). This should tip the scales to always let you coast down the hill, then accelerate up. ",
"Specifically, roll down the hill, off throttle with the transmission in the highest gear. This give the least resistance and still uses no car (assuming fuel cut out engages). Then leave the car in the highest gear it can maintain peak efficiency rpms up the hill. Most cars are not very efficient at <1000 rpms.",
"EDIT: If anyone is interested I put a vacuum gauge on my old Ford Focus, it gives a readout of fuel economy with no scale. It was most efficient at ~2200 rpm."
] |
[
"In Relativity Theory, we are told that time is relative and that there is no \"master clock\" for the Universe, and yet it is commonly stated that the Universe is 13.7 billion years old. How can both of these statements be true?"
] |
[
false
] |
Is it that the Universe is only this old from our perspective, or is that that this dating is a simplification of a more complicated reality?
|
[
"We defined it to be the age in the reference frame where the CMB is isotropic. "
] |
[
"13.7 billion years is the longest amount of time that it's possible for a massive particle to have experienced between the big bang and (here and) now. It's the amount of time the particle will have experienced if the total amount of acceleration and gravitational fields (which general relativity tells us are equivalent) that it has experienced has been kept to a minimum -- if a particle has undergone violent acceleration or passed close to a black hole, it will have experienced less time.",
"Since most mass that makes it here has spent most of its history experiencing relatively small accelerations / gravitational forces, 13.7 billion years is a pretty good estimate for how long any particular particle we encounter around us has been around since the big bang. So for example, it should be accurate to say that by and large the particles of the Milky Way (considered, say, at the time considered simultaneous with now in the reference frame of the center of mass of the Milky Way) are 13.7 billion years old. If you keep trying to get more precise, then at some point, you'll start seeing discrepancies. But I'm not sure when this is -- the variance of the ages of the particles on Earth could be a few million years, or a few seconds for all I know.",
"It's also significant that when we make astronomical observations, if we infer that the light reaching us took (say) 1.3 billion years to get here, we can basically subtract that off and infer that the particles that emitted the light had experienced 12.4 billion years since the big bang. This has to do with the homogeneity and the isotropy of the universe."
] |
[
"Everything about the universe is in some sense defined in a special frame which has us at the centre of the universe because this is all we can observe. "
] |
[
"Why does a light tap on the testicles hurt so much?"
] |
[
false
] |
[deleted]
|
[
"Danger Will Robinson!"
] |
[
"Oh dear, we are not supposed to laugh in this sub-reddit. "
] |
[
"Wouldn't it be because evolutionarily speaking they are the reason for your existence? Therefore the security system evolved to be very sensitive. "
] |
[
"Does a charged particle feel its own field?"
] |
[
false
] |
Question is in the title, but specifically I'm thinking about effects of velocity fields vs acceleration fields and what effects it has if any? Also for the Lienard Wiechert fields what is the justification that the acceleration part is responsible for radiation?
|
[
"Since the top comment is incorrect, I feel like I also need to write a wall of text, so please bear with me. I also know that this is still a somewhat controversial issue, since it has been solved not too long ago.",
"Here is also a recording of a lecture on exactly this subject, if you'd rather hear someone talk over some slides instead of reading some reddit post: ",
"http://www.multimedia.ethz.ch/speakers/zurich_physics_colloquium",
" -> scroll down and click on \"The Impossible Electron: Understanding the Paradoxes of Self-Force\"",
": In classical physics, yes, unless you use redefine what you mean by a charged particle. In quantum field theory, no, because you do precisely this redefinition.",
": In classical electrodynamics, the problem of self-interaction was a long-standing issue, which is only resolved surprisingly recently.",
"Many people will tell you that a charged body does not feel its own field. But these people are confused (or liars). People like the abstract but unphysical(!) concept of a \"test charge\", a tiny charge which feels the Lorentz force due to electromagnetic field but does not influence the electromagnetic field at all.",
"Of course we know that any real charged body ",
" influence the electromagnetic field. We plug the charge and current density associated with the body into Maxwell's equations to get the electromagnetic field due to the body. The electromagnetic field will have nonzero values at the surface of and inside the body. ",
"Now what does this self-interaction do?",
"Clearly, a static body cannot exert a force onto itself, since that would violate the conservation of momentum. This must also hold for any body moving at constant velocity by Lorentz invariance.",
"However, for a body accelerated by an external force, one finds that the self-interaction does in fact give rise to a nonzero force! This force has precisely the form of an additional mass term in the equations of motion. ",
"This should not come as a surprise. We know that any electromagnetic field has some energy associated with it. Since classical electrodynamics is a relativistic theory, Einstein's E=m c",
" holds (in the rest frame of the body). So the energy of the electromagnetic field ",
" appear as an additional mass of the charged body.",
"This also implies that any charged body must have a mass larger than (or equal to?) its electrodynamic mass!",
"Let us now consider the electrodynamic mass of a charged sphere of radius r and charge Q. The energy of its electrostatic field or equivalently its electrodynamic mass is proportional to Q",
" /r. ",
" Therefore, the concept of a charged point particle is wrong in classical electrodynamics! If we want to use classical electrodynamics consistently we should replace them by charged spheres with nonzero radius.",
"If you redefine a charged body as the mechanical body plus its electromagnetic field, you have avoided self-interaction by using a smart definition. You can calculate the mass of your redefined body as its bare mass plus its electrodynamic mass. This is what you do in quantum field theory.",
"In quantum field theory, the mass of a charged point particle is finite due to regularization procedures. This means we ",
" have a point-like electron thanks to quantum physics. Intuitively, this can be explained by the fact that the wavefunction of an electron always has a nonzero radius due to Heisenberg's Uncertainty Principle."
] |
[
"I've done some reading since I posted this and this seems to be a bit of a rabbit hole.",
"\"In a classical sense, if a charged particle felt its own electric field, it would experience a force. This would basically be self-acceleration, where would Newton's third law put the reaction force? It would ultimately break a lot of physics- conservation of momentum, etc.\" ",
"Radiation damping is a thing though, and can't you just talk about field momentum and energy to solve these issues?",
"You mention wiggling which I can grasp why it gives you EMR, but say just constant linear acceleration how does that give radiation?. I thought the poynting vector was only used for time averaged sinusoidal fields but for the lamour derivation they just stick in the LW fields without any considerations on the form of ",
"[;\\dot{\\beta};]"
] |
[
"I've done some reading since I posted this and this seems to be a bit of a rabbit hole.",
"\"In a classical sense, if a charged particle felt its own electric field, it would experience a force. This would basically be self-acceleration, where would Newton's third law put the reaction force? It would ultimately break a lot of physics- conservation of momentum, etc.\" ",
"Radiation damping is a thing though, and can't you just talk about field momentum and energy to solve these issues?",
"You mention wiggling which I can grasp why it gives you EMR, but say just constant linear acceleration how does that give radiation?. I thought the poynting vector was only used for time averaged sinusoidal fields but for the lamour derivation they just stick in the LW fields without any considerations on the form of ",
"[;\\dot{\\beta};]"
] |
[
"What makes the sound of a stomach grumble?"
] |
[
false
] |
I assume it has something to do with liquids moving and such, but how can it get so loud sometimes? Why do we not hear the same sounds on a full stomach?
|
[
"It's called borborygmus and it's caused when the muscle contractions in your upper gut push its contents against the walls, sometimes causing pockets of gas or liquid to move and bubble around.",
"It's louder and more common when you're hungry because after your stomach empties, shortly after eating, the srong muscle contractions stop and don't restart until a couple of hours later to prepare for the next meal."
] |
[
"Not really - your stomach's job is to mash your food into a kind of sludge with a cocktail of horrible chemicals, which it then pushes into the rest of your intestinal tract so the nutrition in it can be extracted.",
"Your stomach is very strong and usually does a good job of packing the sludge quite tightly into your gut. It's not until later that it can separate enough to shift around, or so I'm told."
] |
[
"Well, they are by most standards - they're caustic, quite toxic outside their intended environment, and very smelly. If you ever attend an autopsy, you may find that they're very unpleasant indeed!"
] |
[
"How big of a role does drag play in atmospheric reentry breakup?"
] |
[
false
] |
|
[
"All of it. When an meteorite enters the atmosphere from space, it’s often going several kilometers per second. Drag increases at an exponential rate, with the velocity squared, so an object going ten times the speed has roughly 100 times as much drag. When it hits the atmosphere the drag and pressure created from it pushing air out of the way causes it to heat up and burn, and the force of drag is often enough to break the object into multiple pieces."
] |
[
"Yea, that’s pretty much it. Anything that can’t withstand the drag force gets torn off, and objects that can’t stand the heat melt. Anything else falls to the ground and is usually destroyed."
] |
[
"Drag increases at an exponential rate, with the velocity squared",
"So... not exponential.",
"Most of the heating is due to compression, not drag."
] |
[
"We have seen successful transplants of various organs, hands, even faces -- so why not transplants of legs or feet to lower-limb amputees? Why are these not a thing?"
] |
[
false
] | null |
[
"Prosthetics. The lower limbs are simple enough that you can use a fake one and still get around for the most part. They've even got those special legs for running that are banned in the Olympics for being too fast. It's just not worth the risks from transplants. With transplants, you have to take immunosuppressive drugs for the rest of life and your body will still eventually reject the donated organs."
] |
[
"People need to understand that transplants are never actually successful. When someone gets a new kidney/lung/heart the body wholeheartedly rejects it. The recipient has to take immunosuppressant drugs for the rest of their life which does nothing more than slow down the bodies ability to reject the organ. But reject it will. Consider that only half the people who get a lung transplant (for example) live 5 more years.",
"So you have that approach or you have a prosthetic. Prosthetic wins."
] |
[
"Risk of rejection would be the biggest one , not to mention having to suppress your immune system. ",
"And it’s all to get back something very simple and easy to replicate if not provide something that performs even better ",
"Not to mention we already have a small supply of spare organs for transplants , it’s way harder for someone to donate that large of a chunk of tissue and survive"
] |
[
"How do we know the core of the Earth is hot?"
] |
[
false
] |
How do we know its really hot when no one has been to the core of the Earth? I get that there is magma and all, but where is the gaurantee that it's from the core? It could very well be from the mid layer
|
[
"In addition to the other answers, if you drill down, it gets warmer. There are lots of caveats to this. The deepest hole is about 12 km and the earth is >6000 km is size, so we have only drilled a small percentage of the distance. This does not guarantee that the core is a particular temperature, but any model that has a cooler outside, and warmer as you go down means that there is some sort of heat flowing out. ",
"The temp at the bottom of the hole was 180C (356F). See ",
"this article",
"."
] |
[
"This is a good question, the answer to which (exactly how hot is the core?) is not precisely known and con't be measured with current technology.",
"However, it is hot by all reckoning (4000K - 7000K). The simplest explanation proceeds from the assertion that the outer core of the earth is liquid FeNi. At the pressure of the core, for FeNi to be liquid is has to be very hot. Why is in inferred the outer core is liquid one may ask. The explanation for which is acoustic tomography, using the paths of sound waves through the earth to define composition.",
"A good description of the details can be found in this 1997 Scientific America article: ",
"Why is the earth's core so hot? And how do scientists measure its temperature?"
] |
[
"Huh, so it's about as hot as an oven. That's really crazy, as that doesn't SEEM that hot, but I've heard that the heat is what prevents them from continuing further?"
] |
[
"Is it possible to have an object whose surface area grows faster than its volume?"
] |
[
false
] |
I am familiar with the square-cube law, but all the examples I can find only discuss regular solids, I am wondering if it applies to all 3D objects.
|
[
"If you mean: an object in regular (euclidean) 3D space, and you are looking for an object whose surface grows faster than its volume, the answer is no. All objects will show cubic scale behavior for the volume, and quadratic scale behavior for the surface, under linear scaling.",
"However, it is possible, for a given surface volume, to find a surface that has any value greater than or equal to the surface of the sphere of the specified volume. The sphere is the \"minimum-surface\" solution for a given volume, but there is no \"maximum-surface\" volume.",
"This fact makes it possible to define surfaces that grow faster than the object's volume when you 'blow up' an object, by making the surface \"wrinklier\" as you increase the volume. Of course that's cheating a bit -- you're not comparing the same-shaped object at different scales -- but it's an interesting fact nonetheless.",
"For an example from nature, look no further than your own brain -- its surface cells are functionally different from the cells in the core, and having a lot of surface area cells turns out to be advantageous, from an evolutionary perspective. One way to get more surface area would be to increase the volume but there are natural limit to that -- babies need to fit through the birth canal, and as adults we have a firm cranium for good reasons. A solution to get more surface area with these constraints is to make the surface wrinklier, which is precisely what we do see in the brain."
] |
[
"There is a class of mathematical objects with infinite surface area but finite volume. Take the function 1/x and revolve it around the x axis from x=1 to infinity to give a trumpet shape. The resulting integral is finite, but because the function has an asymptote and never quite reaches the x axis the surface area would be infinite. ",
"Aside from curiosities such as this, however, I’m pretty sure that any normal, finite shape that is enlarged and kept to the same shape with everything to scale would have to obey the square-cube law."
] |
[
"because the function has an asymptote and never quite reaches the x axis the surface area would be infinite.",
"1/x",
" has an asymptote and never quite reaches the x-axis but its integral from 1 to infinity is 1.",
"https://www.wolframalpha.com/input/?i=integral+1%2Fx%5E2+from+1+to+infinity"
] |
[
"Will the first influenza season after all corona restrictions are lifted be worse than before?"
] |
[
false
] |
It is my understanding that our immune system needs, to some extend, "training" or rather some exposure to germs in everyday life to keep us healthy. To my knowledge, this is also why overuse of desinfectants (e.g. every time you touch something in public spaces) is actually a bad thing. So, I was wondering whether the next influenza season or some other disease could become worse (meaning more people suffer from it or show more severe symptoms) than before because our immune systems won't be up for the task when the corona restrictions are lifted.
|
[
"This question is complicated on many different levels, but I can explain a few things. First, the state of seasonal influenza is determined by random mutation, but mutation can only occur when the viral genome is replicated during infection. The virus needs host infection to produce more mutations, and this years flu season has been extremely low in infections because so many people are masking up (masking seems to prevent influenza infection very well). Because of this observation, there is an argument that masking will actually prevent a bad 2021-2022 flu season. However, a reassortment event could always happen because pigs don’t wear masks, which would most definitely cause a bad flu season (example: swine flu). Second, our immune systems tend to be pretty resilient when it comes to remembering past antigens, but influenza is special when it comes to this due to its many different subtypes (H1N1 & H3N2) and history of circulation. Due to this, the reason for our seasonal flu shots isn’t really to “train” the immune system (like other vaccines), but to “prime” your immune system for the next season. Lastly, the flu vaccine will more then likely not change much from this season’s due to the lack of infections, but there are very smart people working on this everyday. Just keep in mind that these are all opinions, and it is an ongoing debate in the flu community. But the #1 thing you can do to prevent a bad 2021-2022 season is to get your flu shot in October!"
] |
[
"Thanks for this response. I guess we can only hope that wearing masks will at least somewhat become a new normal when people are feeling sick. Regarding the flu shot: won't this only work in more developed countries because poorer countries don't have enough access to flu vaccines?"
] |
[
"It’s complicated. ",
"A recent paper tried modeling the impact of non-pharmaceutical interventions (NPI - things like social distancing and wearing masks) on future influenza and RSV outbreaks. (RSV is respiratory syncytial virus, another seasonal respiratory pathogen.). They concluded that the current downturn in influenza and RSV due to NPI might lead to worse RSV in the near future and, ",
", to worse influenza. ",
"Nonpharmaceutical interventions (NPIs) have been employed to reduce the transmission of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), yet these measures are already having similar effects on other directly transmitted, endemic diseases. Disruptions to the seasonal transmission patterns of these diseases may have consequences for the timing and severity of future outbreaks. … As susceptibility increases over the NPI period, we find that substantial outbreaks of RSV may occur in future years, with peak outbreaks likely occurring in the winter of 2021–2022. Longer NPIs, in general, lead to larger future outbreaks although they may display complex interactions with baseline seasonality. Results for influenza broadly echo this picture, but are more uncertain; future outbreaks are likely dependent on the transmissibility and evolutionary dynamics of circulating strains. … Our results suggest that a buildup of susceptibility during these control periods may result in large outbreaks in the coming years. ",
"—",
"The impact of COVID-19 nonpharmaceutical interventions on the future dynamics of endemic infections"
] |
[
"What was the most commonly held scientific theory regarding the origin of the universe and what observation led scientists to question this theory compared to the big bang ?"
] |
[
false
] |
[deleted]
|
[
"infinitesimally small at some point",
"Just to nitpick, infinitely dense not infinitely small."
] |
[
"infinitesimally small at some point",
"Just to nitpick, infinitely dense not infinitely small."
] |
[
"Sort of... More accurately, they used various branches of physics to make predictions about what the universe should look like if the big bang occurred, for example the spectrum of the Cosmic Microwave Background. They found repeatedly that the theory led to accurate predictions and so gradually came to the conclusion that it's basically accurate."
] |
[
"I am the \"X\" number person to have existed, ever. How would I approximate \"X\", for a given year in time?"
] |
[
false
] | null |
[
"I can tell you how to do it in a very round-about way as I don't know the exact method (which I think you may be after here, rather than just a way to get \"your number\"), however - have you seen:",
"http://www.bbc.co.uk/news/world-15391515",
" this?",
"It works out from your date of birth what number out of \"the 7 billion\" you are (you can add the total estimated people to have died before this point to figure your \"X\" for \"to have existed, ever\").",
"If you want to do the calculation yourself the information behind this thing should be available I think... if you scroll to the bottom of this page and see under \"Sources\" there's a bunch of information on how this was worked out, it's linked to some UN studies and data - check the FAQ link at the end in particular I guess."
] |
[
"That website actually also gives you the number person alive you are aswell when you put in your age and gives this blurb about how the calculate it:",
"\"Both numbers have been calculated using UN Population Division figures. The first is an estimate of how many people were alive on your date of birth. It is one possible value based on global population figures and estimates of growth rates over time. Data before 1950 is less accurate than figures after that date. The second number includes calculations based on the methodology of scholar Carl Haub, who estimated how many people had been alive since 50,000 B.C. His calculation has been amended by the UN to include additional points in time.\""
] |
[
"Well, first you'd need to decide what constitutes a person for your purposes. Evolution doesn't produce neat little tags on the back of the neck with one's species written on them, and at what age even a modern human may be considered a person is very dependent on your definition."
] |
[
"Are there researchers/academics that need development help from a highly-motivated scientific amateur? Advice on connecting with them?"
] |
[
false
] |
[deleted]
|
[
"You could always just cold call/e-mail faculty at a local university in a department you're interested in. Or even go to the department (best to have a meeting there with someone already) and talk to scientists in person. This is what I did when I wanted to do astronomy research in high school. When I found a project I was basically like an undergrad intern who happened to be a few years younger. But it was still really hard to find someone willing to take me, and I'm not sure to what extent it mattered that I was planning to go into the field eventually."
] |
[
"going back to school to get a physics degree isn't really an option",
"I'm not sure if you're interested in this or not, but there are other options that could let you avoid doing an undergrad degree. For example, you could simply apply for a phd in bioinformatics or ",
"computation biology",
".",
"Outside of the conventional paths there's a catch-22 scenario where most good professors already have resources available to them and don't have the time to bring you up to speed -- you'll probably only be able to work with people doing less interesting research. This isn't always the case, there are some good researchers who might make time, but you should watch out for it."
] |
[
"Why don't you offer your services in biophysics/molecular biology by means of molecular simulation?\nAdvantages:",
"You can work from home",
"It is at the interface between chemistry, physics and biology",
"All tools are open source (I used ",
"GROMACS",
" which is also well-documented and has a great support mailing list -I learned it all practically by myself)",
"There are lots of bioinformatic groups you may ask. "
] |
[
"Correct me if I'm wrong, it is possible to send a message through light right?"
] |
[
false
] |
[deleted]
|
[
"When you turn on the radio, that's what's happening. Radio waves are a part of the electromagnetic spectrum, just with a much longer wavelength than visible light. Visible light isn't all that useful for this sort of thing, because it tends to not go through things..."
] |
[
"Again, we already use radio waves for this (communicating with our deep space probes). ",
"Radio waves ",
" are light, just at much lower wavelengths than the light that our eyes are capable of seeing. If you wanted to send a message deep into space using visible light, like the laser you saw in class, you could certainly do it, but light in the radio wavelength is going to be better at penetrating interstellar gasses and whatnot than visible light is, so you're better off using light in the radio range."
] |
[
"You mean like radios, cell phones, wireless internet, fiber optic communications...?"
] |
[
"What determines the number of propeller blades a vehicle has?"
] |
[
false
] |
Some aircrafts have three, while some have seven balded props. Similarly helicopters and submarines also have different number of propellers.
|
[
"You can imagine an airscrew as a disk that accelerates air passing through it and creates a pressure change across the disk. ",
"https://en.wikipedia.org/wiki/Momentum_theory",
"The average pressure change accross the disk is called ",
"disk loading",
".",
"If you want more thrust, you can either increase the disk loading, or you can increase the size of the disk (length of the blades).",
"In first approximation, it is more efficient to have lower disk loading and increase the disk size. It is more efficient to accelerate a lot of air a little bit, than to accelerate a little bit of air by a lot. As an analogy, if you sit on an office chair you need less energy to accelerate yourself by pushing away another person on an office chair than if you push away an empty office chair. ",
"However, if you make the blades too long, the tips reach high speeds quickly, and you don't want them in the transsonic regime, which causes sonic boom that can cause damage, wave drag and so on.",
"You can also increase the disk loading, this you can do by increasing the rotation speed, but at some point you run into the same problem with the blade tips. Instead of increasing rotation speed, you can also add more blades. ",
"With more blades you can run into the problem that they are affected by each other's turbulence, which decreases efficiency.",
"In the end it is a complex tradeoff that depends on the desired power, the target airspeeds, noise concerns et cetera."
] |
[
"I just wanted to add that same principles work for water too. For ship propellers and the like. You practically just change the fixed valued for air for water values like weight, viscosity and so on. Even the efficiency principle: infinitely large, slowly moving propeller is the most efficient but in oractice there needs to be a middle ground for practicality, weight and draft."
] |
[
"Water propellers actually have an additional consideration to take into account during design - cavitation.",
"The fluid medium they're in can phase change under certain conditions if the propeller creates momentary areas of low pressure in the fluid as it moves. In practical terms this means bubbles, but the formation and collapse of these bubbles can damage the propeller, make noise, and reduce efficiency, so the propellers are designed to avoid this happening."
] |
[
"Would a more massive object fall slightly faster than a less massive object with no air resistance?"
] |
[
false
] |
I've been taught that without air resistance, any object, when dropped, will fall at the same speed based on earths gravity. However, would a more massive item have a larger gravitational pull of its own, pulling itself towards earth slightly faster than a less massive object?
|
[
"Surprising fact: no it wouldn't! This is a very special fact about gravity. No other force works this way, and this is one of the things that inspired Einstein to develop general relativity.",
"In algebra terms, here's what's going on. The attractive force between the earth and the object is ",
"Where ",
" is the mass of the earth and ",
" is the mass of the object. Newton's second law says that the acceleration of the small object is given by ",
"Or",
"Since there's an ",
" on both sides of the equation, we can cancel it and write this as",
"So the acceleration of the falling object depends only on the mass of the Earth, even though the force depends on both masses. ",
" ",
"I wrote a blog post about this!",
" You should read it. :)",
" after writing this, I realized there is a minor correction. A more massive object, ",
" pulls the earth towards itself more rapidly than a less massive object. (After all, the falling object has a mass too.) So in a sense, the earth is ever-so-slightly falling up towards the smaller object. And the mass of the small falling object matters here. But, for stuff that's our size, the effect is really really tiny. It's only really important when the earth interacts with something roughly the same mass as it.",
"For example, Pluto and Charon are actually orbiting each other because of this effect."
] |
[
"When you use the formula you have to have reference point. When dealing with that situation usually use earth as reference point. So earth is stationary. If in your edited scenario earth is also moving then your reference point is outside of earth and formulas are changed."
] |
[
"Wait no. I don't think that's right. I can choose a \"lab frame\" independent of the position of both the Earth and the falling object, and the formulae I wrote down would still hold. And thanks to Newton's third law, the Earth has to feel some acceleration due to the pull of the falling object."
] |
[
"[optics] How small can focus spot be?"
] |
[
false
] |
There is a great post on XKCD about how you cannot focus light to a hotter spot than the source. I kinda understand that, but then again - not quite. Specifically, what limts the size of a focal spot? The smaller the focus spot size, the bigger density of photons and, thus, hotter it is. I cant seem to find information about this. Maybe im just googling wrong.
|
[
"Geometrically speaking, it can be infinitely small. But the problem begins when you consider the wave property of light: if you do the calculations (which I do not recommend, but it is fun to see it once), you'll see that the remaining pattern will be a sort of wavey-pattern, with a large middle peak. The size of this peak scales with the wavelenght/aperture. In general, this is the real limiting factor to the size of the focus point"
] |
[
"You are looking for ",
"diffraction limit",
". I have not seen xkcd post you are referring to, but my guess is that it was an analogy to a blackbody radiation or second law of thermodynamics that was supposed to help you understand, but in the end did the opposite. It would help if you can link the post here. ",
"Instead for now, let's talk about limit of spot size in terms of ",
" of light. Density of photons has nothing to do with it, only their wavelength. Actually, it is even easier to approach if you consider only ",
" photon. Since photons are particles and waves at the same time, a photon cannot be localized better than its wavelength. Focusing would not be a correct term to apply to a single photon, but basically you cannot focus it better than this. Now, in presence of many photons there will be diffraction (since they are waves), but in the end the smallest spot to which you can focus those photons is also expressed in wavelength, only now the numerical aperture also plays role (i.e. from which range of angles can you collect the photons)."
] |
[
"Call it by name: Diffraction."
] |
[
"Do you think there will be another paradigm shift in science like the emergence of quantum mechanics 100 years ago?"
] |
[
false
] |
Will there be another incisive paradigm shift at some point, or will it be a more gradual transition. What candidates are there for such a paradigm shift? String theory? A post-quantum theory? How would science/technology change with this paradigm shift? What problems do you think we would be able to solve? On a related note: The common opinion (that may actually be a myth, but that's a separate issue) is that scientists in the late 19th century thought that all problems in science were essentially solved and that any further development would be incremental. Today, we think we have learned our lesson: we know that we don't know everything and that another paradigm shift might occur. Still, I have the feeling that we (the scientific community) secretly believe that we know better today and at least won't be surprised by another paradigm shift. Again, take string theory as an example: do the results of a theory still constitute a paradigm shift if you have been researching this theory for more than 40 years?
|
[
"Hmm... these are all very nebulous terms that \"philosophers of science\" sometimes like to use... and they're all very nebulous. Most scientists don't really talk about them seriously, in my experience.",
"In a sense, the difference between a sudden paradigm shift and a gradual transition has nothing to do with the science itself, and everything to do with some subjective definition of sudden and gradual. Remember: the universe and all its rules have always been there and always will be (compared to human timescales) - how fast we figure it out is of no consequence to the cosmos.",
"But ok - whatever - we scientists like to be exact and objective, or at least we like to think we do, so we qualify everything. Let's assume that we understand that the notion of the paradigm shift is at least a bit subjective. You want to know what we've learned from looking back over the past 100-500+ years of doing fundamental science.",
"I'd argue that the most important thing we've learned in fundamental physics is the idea of the effective theory. In one sense, it's the idea that you can take a theory and constrain the kinds of physical systems you'll apply it to, and you can get a slightly different theory. For example, if you take the full theory of special relativity and only talk about systems where the speeds are much less than the speed of light, normal non-relativistic classical mechanics pops out. In this sense, non-relativistic classical mechanics is an approximation to the full relativistic theory. It is an effective theory, with a specific regime of validity: it only works for systems where the speeds are much less than c, but it works very well there.",
"In the other sense, we can turn this on its head: every theory has a certain regime of validity, and outside of this regime, the theory breaks down. Nonsensical answers are generated, or your theory fails to agree with experiments. This ",
" you that there's some bigger theory out there that has a bigger regime of validity and can explain this! To return to the example, that your Galilean transformations in classical mechanics conflict with the constancy of c in electromagnetism tells you that you've got some 'spalining to do, which eventually led to the development of special relativity.",
"An important point about this is that once we find a new theory with a larger regime of validity, our old theory is not necessarily considered \"wrong!\" It still probably does a great job explaining phenomena within its regime of validity, and if all the newfangled stuff in the new theory wasn't needed in that regime anyway, the old theory probably calculates things easier. We haven't thrown away classical mechanics - we just know its limits.",
"The second important point is that, when we go out looking for a new theory to explain the new things, we must always make sure that our theory also explains the old things just as well as the old theory did! If special relativity didn't reduce down to classical mechanics at low speeds, we'd still be scratching our heads about it. (In fact, that relativity and quantum mechanics cannot yet be reconciled with each other is one major problem in physics. Their regimes of validity do not overlap, but we'd like to find a theory that explains them both.) This is sometimes known as the ",
"Correspondence Principle",
".",
"Another example of this is the realization of quantum mechanics. When we considered systems with small distances and high energies, classical mechanics no longer explained what we saw. The probabilistic ideas of quantum mechanics explained these things, however, and when we go to lower energies with larger numbers of particles and larger distances, classical mechanics pops out of the theory again. ",
"When we raise the energies/shorten the distances even more we start to realize that special relativity allows us to create and destroy certain pairs of particles. The number of particles in our quantum system is not fixed. This led to the development of ",
"Second Quantization",
", which culminated in ",
"Quantum Field Theory",
" (QFT), which is currently the basis for all of particle physics and some of condensed matter physics.",
"We've even formalized this process within quantum field theory. Effective field theories (EFTs) can be generated by taking a quantum field theory and integrating out behavior at higher energies. This gives us an explanation of the physics that works strictly for lower energies. In a way, you figure out what sorts of phenomena are important at the lower energies, and remove the stuff that doesn't affect what you observe very much.",
"But now, now let's turn this on its head! We once had a great explanation of atomic nuclei in terms of protons, neutrons, and mesons, but as we went to higher energies, we saw things we couldn't explain. We realized that our proton-neutron-meson theory was just an effective theory for quarks and gluons, which ended up developing ",
"Quantum Chromodynamics",
" (QCD).",
"Our current theory, the Standard Model, explains almost everything below an energy scale of about 100 GeV to an insane amount of precision. But we're looking upwards. Everything that we know and think about particle physics right now is an effective theory for something even bigger at higher energies. We have proposals that the strong, weak, and EM forces may unify somewhere around 10",
" GeV (known as the ",
"GUT scale",
"), and that they may unify with gravity around the 10",
" GeV (known as the ",
"Planck scale",
"). Maybe the theory that unifies all four forces ends up being string theory. Maybe not."
] |
[
"Edit: Oh shit, I didn't know you were a panelist. I would have written much less, and differently."
] |
[
"so? substitute \"small\" for \"weakly interacting\", \"turbulent\", \"really, really far away\", \"Magnetic\" or any number of other tricky conditions."
] |
[
"In the double slit experiment, what are the results if you remove the slits? Does the wave pattern still appear?"
] |
[
false
] | null |
[
"If you remove the slits, you just have a screen that blocks all light going through. So the result is, no light goes through."
] |
[
"You get ",
"single slit diffraction"
] |
[
"Ideal interference specifically needs multiple point sources; the pattern washes out as the width of the slits becomes comparable to their separation. In the limit where the widths are half the separation your wall has vanished: nothing fancy happens here, it's the same thing you get if you shine a lightbulb onto a wall. A brighter region where it's closest that fades out with distance."
] |
[
"Layman's question on the curvature of spacetime"
] |
[
false
] |
Let's assume I'm stood on the surface of an Earth-like planet in the depths of space, it is stationary (not orbiting anything or rotating). I jump into the air, with my legs, at non-relativistic speeds. According to Newton, I exert a force against the ground greater than the force of gravity, this propels me into the air until the force of gravity pulls me back to the ground. The more force I exert, the higher I jump. No problems understanding this. According Einstein, gravity is not a force. I exert a force against the ground which pushes me in a straight line through curved spacetime, the curvature depends on the mass of the planet. This sounds reasonable, but I can't wrap my head around what exactly determines how high I jump. If I was just moving faster in the same direction then wouldn't I just jump and land rather than higher? Is it that the direction away from the mass is more difficult to travel through, and if so, how is this not a force? If I jump directly up with varying forces at different times, I am landing in the same place but travelling to it along many different up-down directions, all of which are straight lines. So are there a continuum of different "ups" depending on how much force I exert, or am I missing something? I'm guessing my problem in understanding this is due to me being unable to visualise the relationship between position and direction in a 3D curved space. Can help?
|
[
"One of the key ideas that allowed Einstein to unlock general relativity is known as the equivalence principle.",
"Imagine you go to sleep tonight, and wake up sometime later in a room with no windows. This room just happens — because you're lucky, and also this is all imaginary — to be stocked with every piece of scientific equipment you can imagine. What experiment can you do to determine conclusively whether you're in a building on the surface of the Earth, or out in deep space in a rocket ship that's accelerating at 1",
"The answer is none. There's no experiment you can do — no experiment that ",
" — that can distinguish between those two situations. Likewise, there's no experiment that could tell you whether you're in a ",
"accelerating spaceship in deep space, or in a freely falling elevator mere seconds from death.",
"This is a really important idea, because it turns Newton's interpretation of gravity on it's head. As you described, in the Newtonian way of looking at things, when you're standing on the Earth, you're not moving. When you fall, you're accelerating toward the ground. But that's not really how it works at all.",
"In fact, when you're standing on the Earth, you are accelerating. In technical terms, your geodesic path through spacetime is being deviated by the acceleration of the ground pushing up against your feet. When you're freely falling, you're not actually accelerating. Rather, you're moving inertially in a geodesic path that happens, because of the local curvature of spacetime, to include a space component of motion.",
"So imagine you're in that windowless spaceship we talked about before, being accelerated at 1",
" by your rocket motor. You jump \"upwards,\" meaning in the direction in which your spaceship is accelerating. What happens? Well, when you're just standing there, you have some instantaneous velocity ",
" Your velocity is changing all the time, because your rocket engine is accelerating you, but ",
" both your velocity and the velocity of your spaceship are both equal to ",
" ",
"When you push against the floor of your spaceship with your legs, your muscles accelerate you, giving you an instantaneous velocity we'll call ",
" where ",
" is the velocity you gained due to the push your legs gave you.",
"But once you leave the floor, your legs aren't pushing against anything any more, so they can't accelerate you any further. Your velocity is now a constant ",
" and you're no longer accelerating.",
"Meanwhile, the spaceship continues to accelerate, because its engines are continuing to push against it. When you jumped, the spaceship's velocity was ",
" but that's constantly increasing by a factor of ",
" So soon, the velocity of the spaceship will be equal to ",
" at which moment you'll reach the top of the arc of your jump. The spaceship will continue accelerating, \"catching up\" to you until your feet touch the floor again.",
"How high you can jump depends on how much you can accelerate yourself with your leg muscles. If you push against the floor very hard, you'll increase your velocity by some greater factor — call it ",
" where big-",
" is larger than little-",
" — and thus more time will elapse before your constantly-accelerating spaceship can get from velocity ",
" to velocity ",
" So you jump higher.",
"That's basically how gravity works. When you jump, you ",
" your acceleration momentarily, but then (because you're not touching the ground any more) your acceleration ceases, and you resume inertial motion. Your inertial path through curved spacetime brings you back to the surface of the Earth again, which then pushes up against you, making you accelerate once more.",
"I'm not sure if this helped at all. To be honest, I'm not really confident than I understood the question."
] |
[
"If gravity relies on a constant acceleration, what happens when velocity reaches the speed of light?",
"It doesn't. Nothing can ever go that fast.",
"Does this mean you could accelerate forever at a constant rate?",
"Yes!",
"Imagine we're sitting here on Earth, and there's a spaceship on the launch pad. This spaceship is magical: its engine can put out a fixed amount of thrust ",
" and it uses no fuel so the mass of the spaceship never changes. So by all accounts — thanks, Newton — that spaceship should accelerate at a constant rate forever. We mash the button, and boom, the spaceship flies off.",
"We watch the spaceship fly away using our magical telescope that only works in thought experiments. For a while, we'll observe that the spaceship's velocity relative to the Earth is increasing at a constant rate … but eventually, we'll notice that its velocity is increasing more slowly than it was before. It went from zero (that is, sitting motionless on the launch pad) to 80 percent of the speed of light in a day, but over the next day it only went from 80 percent of the speed of light to 90 percent of the speed of light. And in the next day, it went from 90 to 91 percent. And the next day, it went from 91 to 91.6 percent.",
"And over the next ",
" it only gets up to 99.9 percent.",
"And over the next ",
" it only gets up to 99.99 percent.",
"And over the next ",
" it only gets up to 99.999 percent.",
"And all that despite the fact that the magic engine in the magic spaceship kept putting out the same thrust the whole time.",
"(Important disclaimer: I made all those numbers up, because I'm lazy about arithmetic.)",
"For a while, it was fashionable, particularly in high-school physics classes, to ascribe this behavior to a quantity called \"relativistic mass.\" The faster something moves relative to you, the greater its mass appears to be, which is why a constant force applied for indefinite time results in decreasing acceleration. But really, that's sort of a bogus interpretation. In technical jargon, you can't really apply the Lorentz transformation to mass and come out the other end with a meaningful quantity. It's more useful to talk about momentum, which we can Lorentz-transform in meaningful ways."
] |
[
"At the risk of sounding stupid, does equivalence mean that a mass at rest has intrinsic acceleration which is equivalent to a change in velocity?",
"A mass ",
" that's at rest ",
" is undergoing acceleration, yes.",
"At this level what we mean by \"acceleration\" gets a bit wonky, because we normally think of acceleration as being the time derivative of three-velocity in some reference frame. But in fact, it's more useful to talk about it as the time derivative of four-velocity; sometimes physicists use the term \"four-acceleration\" to make this explicit.",
"But really, without diving into tensor calculus and crap like that, the easiest way to understand it is to remember the equivalence principle: Being at rest in a gravitational field with respect to the source of that field is indistinguishable from, and perfectly equivalent to, being in a spaceship under constant acceleration. When you sit still on the surface of the earth, you ",
" in an accelerated reference frame, regardless of the fact that your three-velocity as measured relative to the Earth is not changing over time."
] |
[
"When Deuterium and Anti-Hydrogen Annihilate, What Happens to the Extra Neutron?"
] |
[
false
] |
According to Star Trek lore, Federation starships are powered by matter-antimatter reactions, but more specifically, this reaction is canonically said to be between deuterium and anti-hydrogen. Given that every atom of deuterium has a neutron in it that hydrogen (and anti-hydrogen) atoms lack, what would happen to this neutron in real life if an atom of deuterium were to annihilate with an atom of anti-hydrogen?
|
[
"Antiprotons (the nuclei of antihydrogen) will happily react with both protons and neutrons - they don't care. And as both of them are close together, the reaction is really more like \"deuterium nucleus+antiproton -> X\" where X will typically contain either a proton or a neutron (with about 50% probability) and a couple of pions. There are many other reactions possible but they are less likely.",
"The electrons and positrons will react with each other at some point as well, how exactly depends on details of the reaction."
] |
[
"I see, thank you. So roughly half the time we’re left with a proton and the other half of the time, we’re left with a neutron? In either case, we are left with one baryon (if I have my terminology right)? Plus a pion or few, as you said. Am I correct in understanding that the fate of this leftover baryon depends on the specifics of the reaction?"
] |
[
"Yes."
] |
[
"How do I do research on my own? (sonoluminescene)"
] |
[
false
] | null |
[
"Start by reading up on bubble dynamics, for example, the Rayleigh-Plesset equation. Tim Leighton has a book on it, called The Acoustic Bubble, that may interest you. You'll need to know differential equations.",
"The rate at which surrounding water fills the bubble was largely worked out by Rayleigh in 1917."
] |
[
"The book is so much money :(",
"124 dollars for a paperback edition. I may have to pirate it :X"
] |
[
"Oh no!",
"Also, try to read through as many sonoluminescence papers as possible. That's how you stay on top of a topic."
] |
[
"It is estimated that at formation the moon was between 20 to 30 thousand kilometers away from earth. With the moon's closer proximity, how often would earth experience solar eclipses, and what effects would it have on the path of totality?"
] |
[
false
] | null |
[
"I'll have to check my math again tomorrow but it seems as though almost every single new moon should be a full eclipse. At a distance of 20 thousands km only a 93 km radius circle is needed to completely cover the Sun. And even though the Moon's orbital inclination will carry it above and below the line of sight to the Sun that will only do so up to a maximum of 1793 km, while the Moon is 1737 km in radius. So only at the extreme conditions of poor alignment would eclipses not happen during a new moon period, which, incidentally would occur roughly every twelve hours. By my estimation only 2 days out of the year would experience periods where a new moon wouldn't cause an eclipse. So there would be roughly 2 eclipses per day somewhere on Earth except for roughly two separate days without them.",
"The eclipses would have much wider paths and totality would last up to about 14 minutes."
] |
[
"Thank you for the answer."
] |
[
"The length of the totality would depend on the rotation rate of the Earth. What did you use for your estimate?"
] |
[
"Have any animals shown signs of reaction to music? Or any studies to suggest they can feel appreciation to or at least recognition of it?"
] |
[
false
] | null |
[
"National Geographic seems to think so on some level:",
"http://news.nationalgeographic.com/news/2001/01/0105biomusic.html"
] |
[
"Discriminative and reinforcing stimulus properties of music for rats",
":",
"We trained rats to discriminate music by Bach from that by Stravinsky using operant conditioning. The rats successfully learned the discrimination and transferred their discrimination to novel music by the same artists. Then, we trained rats on concurrent-chain schedule in which the terminal links were associated with different music, Bach or Stravinsky. The rats did not show strong preference for either style of music, although one subject showed a preference for Bach and another subject preferred Stravinsky. Finally, we examined the validity of the concurrent-chain procedure as a method of preference measurement with conspecific vocalization evoked by an aversive experience. Most of the rats preferred white noise to the conspecific vocalization. Therefore, music has a discriminative stimulus property but not a clear reinforcing property for rats.",
"Not a particularly encouraging finding for your question, but note that two of the rats did appear to have a preference for different musicians, and note that the experimenters did not attempt to attach any reinforcing value to the music. In the appreciation of music in humans, we have to remember that a significant part of our pleasure is not simply the inherent pleasure from harmonious sounds, but also the associations that have been generated throughout our lives. Just look at how parents introduce their kids to music, they turn it on, then they dance with them, smile and hug them, etc. ",
"Looking at these results, it would be easy to make a particular style of music reinforcing to specific subjects. "
] |
[
"Quite a few I think. I specifically remember dairy cattle being shown to react to music being played whilst being milked; they produced more milk when classical music was played compared to rock and roll. A quick google found ",
"this",
"\n and ",
"this paper",
" about chickens"
] |
[
"What temperature would be required to ignite the earth's atmosphere, and would it be possible for humans to ignite our atmosphere?"
] |
[
false
] | null |
[
"The possibility of humans igniting the atmosphere was considered as part of the initial work on the development of the atomic bomb.",
"The fear was that if you set off a fission bomb, then you could release enough energy to cause fusion to occur in the atmosphere. These fusion reactions would release more energy leading to more fusion, etc etc. Since the atmosphere is mostly nitrogen, this would be critical thing you'd be trying to avoid fusing.",
"The problem is that in order for fusion to occur you need extremely hot temperatures (say around 100 million degrees centigrade). The hotter something is the faster it loses heat and cools. Hence it is a reasonably straight forward calculation to show that the atmosphere would cool much faster than any fusion reaction could produce heat, and that this particular scenario is impossible. ",
"Looking at more conventional means, there are things like flour/sulphide dust explosions that can occur when you have high enough concentration of flammable material in an atmosphere (usually a contained almost closed one one like a flour mill or an underground mine. Though I would suspect its impossible to sufficiently dose/seed the entire atmosphere with high enough levels of flammable material like that to say create a sustainable explosion, at least with present day technology. Even if you could you wouldn't be igniting the natural atmosphere, you'd be creating your own unnatural one.",
"Some of the other planets that higher levels of hydrogen in their atmosphere are more interesting to consider when it comes to ignition of atmosphere."
] |
[
"Technically at 2000℃, carbon dioxide decomposes into carbon monoxide, which is flammable at 605℃, and may at the right mixture explode (12,5%-74,2% vol), meaning it would be possible if you manage to hear up the entire atmosphere a LOT.",
"Dust explosions are like saying 'lets just add a flammable gas and light it'"
] |
[
"Sorry if I'm off on this, but if you're igniting carbon monoxide won't that just get you back carbon dioxide? The opposite process of CO2 decomposing. "
] |
[
"Could an animal come back from extinction due to evolving?"
] |
[
false
] |
There's been a post circulating reddit that says about a bird coming back from extinction die to evolution which has been said is fake but I'm wondering could it be possible that animals couple evolve back an extinct species (say the dodo for example)
|
[
"What this story is about is ",
"this paper",
". The study basically found that there was a bird (let's call them bird 1). Some of the population of bird 1 flew to a new island where evolution made them lose their ability of flight, turning into a new species of bird (bird 2). Their new island then gets flooded and bird 2 is wiped out as they can't fly. Bird 1 that didn't fly to the island is still around though and some more of them fly to the island after it's no longer flooded, they then evolve to lose their flight ability too (turning into bird 3).",
"So bird 2 and 3 have both evolved from bird 1 and they evolved the same characteristic (flightlessness), but they differ in some other ways.",
"This is known as ",
"iterative evolution",
"."
] |
[
"A species could evolve to resemble a species that went extinct. See: ",
"convergent evolution",
".",
"But they would not be categorized as the same species. And to say that the animal \"came back from extinction\" is sensationalism."
] |
[
"I like how much this suggests about the kinds of factors that influence evolution. In this case, it's almost a deterministic: similar inputs (bird 1 + island 1, bird 1 descendant + island 1) yield similar outputs."
] |
[
"I am an artificially-intelligent machine. What do I need to do to prove it?"
] |
[
false
] | null |
[
"what does it mean to be artificial?"
] |
[
"The politically correct term is \"synthetic sentience\". "
] |
[
"Have a conversation with a person to the point that the person with whom you are speaking does not realize you are not human.",
"Or in a less round-a-bout way of saying it; take you, put a real person next to you, and have a person talk to you both without being able to see you. If the person is unable to identify which is human and which is you, you pass."
] |
[
"Does a sickness know when it is losing the battle in its host (i.e. when the host starts getting better)? If so, does the sickness try to do anything differently to try and stay alive?"
] |
[
false
] | null |
[
"Wow, ok, good question. \nSo if you’re thinking about “foreign invaders” I think not. I mean bacteria, viruses and parasites don’t really have a super complex thought process. They are driven by basic needs: to survive, to procreate. So I don’t think they “know” per se they are losing the battle. \nThat said, bacteria for example mutate under certain conditions (eg. inhospitable environment) and create resistant genes in order to survive better against the attackers (antibiotics). But it’s not because it “knows”. I don’t think a whole colony of Staph goes “hmm these guys know what they’re doing, men, we must become better, we must fight harder!” And then mutate to the superbug MRSA. I think it’s a “reflex” for lack of a better term. \nAt the moment we have a huge problem with the buggers which as you put it do things differently to stay alive. "
] |
[
"bacteria for example mutate under certain conditions (eg. inhospitable environment) and create resistant genes in order to survive better against the attackers (antibiotics)",
"Mutations don't happen in response to treatment. They are generally random. Sometimes a random mutation produces a result that makes the bacteria resistant to the treatment. Because such bacteria will survive the treatment and procreate, it will expand the amount of bacteria that have this resistant gene making it become more dominant and making the treatment overall less effective."
] |
[
"You are partially right. ",
"Mutations do happen indeed because of randomness, but they also happen in response to treatment. And it happens so darn fast, because bacteria can just \"touch\" each other in order to share information aka resistant genes. ",
"I would refer you to this comprehensive article which explains this is in a whole lot of detail: ",
"https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2937522/",
"I like that their first example is the bacteria I'm studying, Mycobacterium tuberculosis: \" Tuberculosis is the archetypical human pathogen; it evolved with the human race and currently infects as much as one-third of the world population. While the ground-breaking discoveries of streptomycin and isoniazid provided vital treatments, resistance development was rapid.\""
] |
[
"Does the road heat up due to friction?"
] |
[
false
] |
I know that just one car driving on a road won't be enough to cause any heat change. However, I was wondering if on a highway early in the morning, let's say 05:00 am, where the ambient temperature of the air is 5 C would the friction from the morning traffic traveling at 120 km/h cause the road to eventually heat up significantly? Because I know that tires do get quite hot from the temperature from the brakes as well as the actual friction to the road. Would that same friction cause the road to heat up or does the heat disperse into the earth fast enough that it is a negligible difference.
|
[
"Why does snow seem to melt more and wet road seems to dry sooner where car tires pass vs where they don't?"
] |
[
"A rolling (not slipping) rigid wheel will not generate any heat from friction of the surface of the wheel against the surface of the road. Work requires relative displacement of the two surfaces, not just a force. Static friction theoretically does not do work. e.g.: doing a burnout will create heat",
"That heat from the wheels getting hot I suspect has more to do with the deformation of the wheel (internal friction?) than it does friction between surface of the wheel and the road."
] |
[
"That's because of pressure as already answered, and it's also because of the contact in wet conditions as the water gets a larger surface area and relative wind speed to dry to."
] |
[
"Black holes?"
] |
[
false
] |
I was just reading an article about how there are high resolution photos of a super massive black hole ( ) and I started thinking. 1) Are black holes stationary (i.e. do they orbit other things, or do they sort of float around in space and time)? 2) If they are stationary, what would happen if two black holes encountered each other? Would they combine into an even bigger black hole? I'm not a scientist, but these sorts of things are really interesting. Thanks!
|
[
"A collision of black holes would be very dramatic and exciting event.",
"To steal from my ",
"own post",
" in another thread about a month ago:",
"When two black holes collide, they coalesce into a single black hole. And, if all goes right, ",
"LIGO",
" and ",
"VIRGO",
" detect the events (and maybe even win a Nobel prize for doing so).",
"When the black holes collide, they cross each other's event horizons, and so become one combined black hole. This happens in four stages, summarized ",
"here (PDF)",
". Initially, they are far enough apart that their interactions don't deviate much from what Newton's theory of gravity would predict for objects of those masses. As they close in, they begin to spiral into each other. Then the event horizons cross each other. Finally, the combined black hole goes through ",
", becoming a canonical rotating black hole (a \"Kerr black hole\").",
"The inspiral and ringdown produce gravitational radiation (the last piece of Einstein's general relativity that has never been directly detected), which it is the goal of LIGO and VIRGO to detect. The amount of gravitational radiation produced should be quite large.",
"You can read a bit more about black hole collisions ",
"here",
"."
] |
[
"Black holes can merge, yes. I'm just not sufficiently knowledgeable to explain the repercussions of it. "
] |
[
"Thank you so much!"
] |
[
"Do small dogs have less mental capacity than larger ones?"
] |
[
false
] |
[deleted]
|
[
"Attempts to measure the differing intelligence of different breeds have been made (",
"see here",
"). At a fairly casual glance it is difficult to see any correlation between average size and intelligence. Certainly there is no strict relationship, as demonstrated by Miniature Schnauzer (weight 5-8kg) at 12th most intelligent, and Bull Mastiff (weight 45-59kg) at 69th most intelligent."
] |
[
"Thanks for the link. "
] |
[
"To truly determine if this correlation is true would require quite a sophisticated analysis that requires the incorporation of phylogenetic information (the evolutionary history of dogs)"
] |
[
"If a fly, the insect, flies into a car and the car moves, would the fly stay stationary where it is in the car or would it be pushed to the back of the car?"
] |
[
false
] |
Usually when you're in a moving car you get slightly pushed to the back. Would a fly be pushed as well?
|
[
"When a car accelerates forward you are not pushed back in the car; the car moves forward and pushes at your back. Since the air inside the car is moved along with the car, when the car is accelerating the rear of the car will move toward the fly, but only slightly."
] |
[
"You can consider a fly in your car similar to a fish in a bowl of water. The air in the car is a fluid like water, but less viscous. Since a fly is a lot lighter than a fish, it's still enough to carry the fly around as you accelerate the car."
] |
[
"Yes, it will. In fact, assuming the fly just hovers there, it would smack the back of the car, here's why:",
"Here's a ",
"graph",
" of the fly in your car as you accelerate from a stop to 108 kph (~67 MPH). Note that the \"fly displacement\" is BACKWARDS towards the rear of your car. I can provide the equation + code used to get this result if like."
] |
[
"Why do we use different units for the same thing?"
] |
[
false
] |
Hey guys. In Richard Feynman's book The Character of Physical Law he says that if you want to embarrass a physicist, ask them why they use different units to measure different kinds of energy when they could use one for all energy and make it less confusing, since energy is everything. Do you actually think this would make the subject less complicated, and if so, why don't we do this? Thank you for any insight.
|
[
"I do not think using one unit of energy would necessarily simplify the matter (sorry for the pun). Sometimes, using a specific unit of energy allows one to quickly gauge the relative energy of the system one is investigating. ",
"For example, consider the unit of electron volts, which is equal to the energy change of an electron accelerated through a potential difference of one volt. When dealing with particles and their interactions, having the energy given in terms of eV makes relative particle energies immediately clear and makes it easy to determine how much energy it takes to break a given bond or induce another change. If one were designing an instrument for analyzing compounds (a mass spectrometer) with an electron impact ionization source, it's trivial to find the energy of the electrons at different voltages when the units are in eV, whereas otherwise, calculations are more involved. The order of magnitude is also convenient, since there are not excessive digits or decimal places. Obviously, when dealing with higher energy phenomena, eV is often inappropriate.",
"As another example, consider heating 1L of water by 1 degree Celsius. How much energy does this process take? Using eV in this case would be very cumbersome, so instead units of calories would be easiest. Based on how the calorie is defined, the aforementioned heating simply takes 1000cal. Now consider work done by an electrical system. How much energy does a fully operating 50 watt solar panel provide per minute? Using the joule = watt x second relation, the answer is quickly found to be 3000J (or 3kJ). ",
"If one were using this solar-derived energy to heat water, there is no perfect way to avoid conversions and make all calculations as trivial as before. However, while working with exclusively one type of process, using a specific unit allows easy comparisons and calculations. Returning to the electron example, an electron accelerated through 20 volts has an energy of 20eV. What if one wanted to use joules instead of eV for this measurement? Well, 20eV is about equal to 3.2 × 10",
" joules. Clearly, using joules makes comparing energies significantly more challenging, and increases the likelyhood of calculation errors because of the unwieldy number of decimal places (which also necessitates scientific notation).",
"*Edit: Fixed a few typos"
] |
[
"Could you clarify what different kinds of energy he means. If he means kinetic or thermal etc then I don't agree that we do. Sure different physicists might use ergs vs joules or whatever but they don't usually use different units between forms.",
"If he means mass and energy then well part is tradition part is not having to write a star being 10",
" energy units."
] |
[
"The order of magnitude is also convenient, since there are not excessive digits or decimal places.",
"I can not stress this point enough. Sure we could work in all the same units, but when it comes to using computers for data analysis and modeling, different unit systems are beneficial for preventing numerical errors when performing arithmetic with floating-point calculations."
] |
[
"If I mix red and blue paint, do the two colours ever truly become one? Or if I zoom in close enough, will I always see distinct red and blue particles?"
] |
[
false
] | null |
[
"The pigment molecules stay exactly the same. You see a mixture of red and blue light that your brain perceives as purple. "
] |
[
"It's possible to design pigments that have any colour of the visible light spectrum. (any colour of the rainbow in other words). However because of the huge range of coloured paints I suspect many of these aren't pure solutions of pigment but rather mixtures of different pigments.\nSo your purple paint may either be a mixture of red and blue or it may contain a pigment which reflects the colour purple. "
] |
[
"It's possible to design pigments that have any colour of the visible light spectrum. (any colour of the rainbow in other words). However because of the huge range of coloured paints I suspect many of these aren't pure solutions of pigment but rather mixtures of different pigments.\nSo your purple paint may either be a mixture of red and blue or it may contain a pigment which reflects the colour purple. "
] |
[
"What are the the fundamentals and differences of different types of bonding? Are some bonds the same as other? What are the pros and cons of each bond?"
] |
[
false
] | null |
[
"The question is asking a lot. We request that you do some research on your own before posing a question, and that should answer several of the points you are asking. But there is a continuum between ionic and covalent bonding, which relates to the degree that electrons are shared between bound nuclei. In complete ionic bonding, the bond is completely ionized (one terminus is positively charged, the other is negatively charged). In a completely covalent bond, electrons are shared equally between nuclei and there is no charge separation. ",
"I am not really sure what you are asking in your last two questions. A bond is a bond, and we can't arbitrarily change it to make a \"better bond.\" Normally, ionic bonds are stronger, if that is what you are getting at. "
] |
[
"For simplicity, you see more ionic character in bonds when there is a greater difference in electronegativity between the bonding atoms. So, when you have an alkali metal or alkaline earth metal bound to a halogen or oxygen, then it will be ionic. But when there is a small difference in electronegativity, like between the main group elements in the p-block and hydrogen, then it will be more covalent. "
] |
[
"I am not sure how you come to the conclusion that ionic bonds are stronger than covalent bonds. Covalents bonds with a partial ionic character are stronger than totally covalent bonds as far as I know.\nDid you derive this (covalent bonds weaker than ionic bonds) from the Gibbs free energies of formation? And if so, can one just compare them for different elements?"
] |
[
"What would happen if Earth moved at a different speed in its orbit around the sun?"
] |
[
false
] |
At this moment it's orbiting at about 30 km/sec. What would happen if the earth would, for instance, orbit at 20 km/sec around the sun? Or at 40 km/sec?
|
[
"I don't really understand your question so I'll try two interpretations",
"Interpretation 1: The earth suddenly loses (gains) speed from one moment to the other with everything else staying as it is. Not taking into account effects due to the acceleration, the earth would go from the basically circular orbit, it has at the moment, to a highly elliptical orbit. A slower earth at this point would approach the sun while accelerating and after the end of the orbit go back to the start point. The implication being that the distance sun-earth varies a lot over the course of a \"year\". This approach would heat up the earth and thus cause dramatic changes in climate. (likewise, a higher initial speed would lead to an elliptical orbit which causes cooling)",
"Interpretation 2: Stable circular orbit at a different speed. For slower speed this either means the earth is farther away (at 20km/s beyond current mars orbit) or that the mass of the sun is smaller. Farther away obviously means colder, while smaller sun in most likelihood means colder (depends on the stage of the star etc.)."
] |
[
"The orbit would become elliptical with endpoints lying on the original circular axis. Our orbit would turn into a oval that gets close to the sun."
] |
[
"Wait, you say that it would heat up the Earth, but the world would still spin around its shoulder, and the Earth would lean the same way, anyway. Isn't this, in fact, a completely moot question? Wouldn't the only differnce be that a year is longer?"
] |
[
"Why doesn't the roche limit have an effect on objects like the ISS or people when we orbit the Earth or Moon?"
] |
[
false
] |
This may not be the most accurate way to describe it, but I have been told that objects get ripped apart by the roche limit because they get too close to another object and the gravity from the bigger object rips them apart because their mass doesn't have enough gravity to hold itself together. Given we are much smaller and have way less gravity, why don't we get ripped apart in orbit?
|
[
"The normal roche limit is based on something with 0 tensile strength / is purely held together by gravity. This is a good approximation for a planet or large moon, but not for a person or manmade object. (If it were, we'd all be spheres)"
] |
[
"The Roche limit applies to bodies with enough mass to be held together by their own gravity. The ISS is obviously held together with welds, nuts & bolts. It doesn't have any gravity to speak of. So the Roche limit does not apply."
] |
[
"\"In celestial mechanics, the Roche limit, also called Roche radius, is the distance from a celestial body within which a second celestial body, ",
" will disintegrate because the first body's tidal forces exceed the second body's gravitational self-attraction.\"",
"If indeed the ISS or people were merely being held together by gravity, that is, every component placed next to one another but having no other binding element, then the Roche limit WOULD apply.",
"Luckily for us though nature has thought of this and bound our individual molecules together within a biologically connected unit, and we also applied the same logic to our engineering and connected the ISS together with rivets, welds, bolts etc."
] |
[
"Are our brain's wrinkles unique to each individual like fingerprints or are they mostly the same shape with few variations?"
] |
[
false
] | null |
[
"They are generally the same in a gross manner. For example most people (~70%) have a ",
"lateral occipital sulcus",
" but not everyone does.",
"You may find people completely lacking a sulcus (groove) or gyrus (ridge) where others may have one more than a \"regular\" brain.",
"Anatomical textbooks generally show a \"perfect\" brain and on well preserved specimens these are usually pretty easy to find landmarks on, but they are definitely not all the same and each cortex varies slightly.",
"Source: Neuroscience student."
] |
[
"And what are the changes associated with those variations? In case we know."
] |
[
"Are you trying to describe neuroplasticity?"
] |
[
"Best Carl Sagan Book?"
] |
[
false
] |
I am looking to pick up my first Carl Sagan book and since I trust the Reddit Science community more than random internet threads I thought I'd ask; Which is the best Carl Sagan book and why?
|
[
"I thought Contact was amazing."
] |
[
"Second vote for demon haunted world. Really the first book of it's kind: A broad overview of pseudo science."
] |
[
"Also, do watch all 13 episodes of COSMOS: ",
"http://hulu.com/COSMOS"
] |
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