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[
"How would a toroidal universe affect relativity?"
] |
[
false
] |
[deleted]
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[
"I'm not sure if I understand your question, but it seems to be about whether relativity allows globally periodic spaces or spacetimes. The answer, I believe, is yes. Relativity only makes statements about the ",
" structure of spacetime, not the ",
" structure. For example, general relativity stipulates that your local neighborhood must appear consistent with the laws of special relativity (local Lorentz invariance), and that the local spacetime curvature is related to the local distribution of matter, energy and momentum. But it doesn't say anything about the global structure of spacetime. ",
"There are perfectly valid solutions to Einstein's equations that are periodic. For example, the spacetime commonly used in cosmology to describe our universe allows for space to be periodic, with the geometry of a so-called 3-sphere (like the surface of a 4-dimensional spherical volume). I'm not sure what \"paradox\" you're referring to, there are no logical inconsistencies that arise in compact periodic spaces.",
"Perhaps you're asking whether ",
" can be periodic as well. If so, then again this is perfectly allowed within classical general relativity. Such spacetimes might not be stable against perturbations, since they allow for ",
"closed timelike curves",
", but formally they are perfectly valid solutions to Einstein's equations and are consistent with general relativity."
] |
[
"I also am not sure that I understand your question, but I want to make a few points:",
"what appeared to be a straight path from our perspective actually had the same curvature as the horizontal cross section at the object's latitude",
"The usual model of a 2-torus in 3-space looks curved, but that's just an artefact of the way it's sitting in the space. It does not have what mathematicians call \"intrinsic curvature\", and neither do any of the several analogous 3-manifolds which could correspond to the shape of the universe. Straight lines (mathematically, \"geodesics\") are ",
" straight—they just end up back on themselves.",
"the boundary of the universe",
"There's absolutely no empirical reason to believe the universe has a boundary, and a lot of philosophical reasons to think it doesn't.",
"any path ",
" be periodic in a toroidal universe, right?",
"Actually, no. However, every path in a torus is ",
" periodic ",
" passes arbitrarily close to every point in the torus. Go figure.",
"I remember hearing something about the paradox",
"What paradox?"
] |
[
"Ah, yes, that paradox. Acceleration is the key, because it's what makes our frames of reference different—your velocity from my perspective equals my velocity from your perspective, but we can each measure our accelerations ",
"."
] |
[
"Are we prepared to handle a direct hit from a CME?"
] |
[
false
] |
Are there procedures in place for when the Earth is hit by a coronal mass ejection? What are they? What would be the short term effects? Long term? This is coming from a guy who just learned what a CME is an hour ago.
|
[
"A CME occurs when an explosion on the Sun sends a ball of plasma (and magnetic field) off the surface of the Sun in some direction. If the CME is aimed at Earth it could cause problems when it collides with the Earth's magnetic field, sending showers of high-energy particles through the Earth's atmosphere.",
"The effect depends very strongly on the strength of the CME. An ordinary CME that hits Earth might cause problems for satellites, but would have no really bad effects. But really powerful CMEs like the Carrington Event in 1859 are different. They would be capable of frying all of our satellites (disrupting the Internet), and the giant transformers that are used in our power plants would be ruined. Because these transformers can't be bought off the shelf they would have to be built, and the power outages would last for months. The silver lining would be \"Northern lights\" visible from the equator.",
"These problems can be avoided by shutting down vulnerable electronics or hardening them against CMEs, but doing either would be enormously costly. Shutting down electronics would require reliable forecasts of dangerous CMEs like those we have today for hurricanes to justify the cost. Right now scientists are working on these forecasts. "
] |
[
"Disclaimer: I work with people who do this type of work, but I don't do it myself, so this knowledge is second hand.",
"This topic is something that's undergoing active investigation. Probably the biggest short-term effect would be geomagnetically induced currents in the world's power grids. The large and sudden changes in the Earth's magnetic field that occur during a CME can induce electric fields in the ground, which then disrupt the flow of electricity through power grids and such. This can cause serious problems for transformers, which don't like it when power flows the wrong way through them.",
"There have been a number of instances of CMEs causing problems for power grids - in 1989, a severe CME knocked out power to 6 million people in Canada. More recently, a transformer in New Zealand was destroyed by a CME. The most famous case of a severe CME is the Carrington Event in 1859. This happened before the days of wide-spread electricity, so the effects of the event were relatively small. The storm was the greatest we know of, however, with aurora visible as far south as the Caribbean. ",
"A Carrington level event is the \"worst case scenario\" that we know of. Should one hit today, conceivably we could see transformers being taken out all over the world, which could see hundreds of millions, if not billions, of people without power, potentially for years. Transformers are not easy to replace, and if hundreds of the blew at once, it would take years for them all to be replaced. It's possible that society as we know it would crumble - that's just doom-and-gloom conjecture on my part though.",
"As for what we're doing to prevent such an event occurring - this is a large part of what's currently being studied. The effects of induced currents can be lessened by shutting off and isolating parts of the power grid. The key is being able to predict when a large CME is coming, and taking the appropriate precautions to save important infrastructure. This prediction is not easy, however, and generally we have less than an hour's warning. "
] |
[
"Because that's expensive, and for the most part they don't simply blow up. These events are rare. "
] |
[
"Which newspapers or news organisations do you find report science well?"
] |
[
false
] |
Im only an undergrad but I have found the Guardian and Independent to be quite good ...although the independent did report on some research done by a lecturer of mine and kind of missed the point lol Are there any newspapers more experienced scientists favour?
|
[
"For mainstream American news outlets, there's very few good ones to pick from. The New York Times is an all-around very good newspaper, and they have pretty good science reporting. They're the best I've seen, of the mainstream outlets, at not sensationalizing their science articles. ",
"I also enjoy Scientific American and Physics Today. SciAm has almost no sensationalism, and a significant part of their audience is scientists, so its descriptions and explanations are aimed at science-minded people trying to understand things outside their field. It was at a perfect level for me in high school and undergrad, and it's still good now that I'm in grad school. Physics Today (or whatever is the main publication in your area of interest) is more technical, but still aimed at physicists trying to understand what's going on in other subfields.",
"I stay the hell away from CNN, MSNBC, FoxNews, etc. I also stay the hell away from New Scientist, physorg, and even PopSci. When almost every single article title is completely misleading and sensationalistic, you can bet that the reporter probably doesn't even understand what he's covering."
] |
[
"ars technica has by far the best science coverage\n",
"http://arstechnica.com/science/"
] |
[
"...seriously?"
] |
[
"What makes common bacterium grow slowly in cold temperature?"
] |
[
false
] |
What's the mechanism that starts failing when it gets too cold for a bacteria? And more specifically, why is it failing? A quick google search tells me it's some enzyme required for multiplication, I find this a bit too vague and I couldn't get a much better answer while searching further.
|
[
"Not all bacterial growth is slowed by cold temperatures--some bacteria, including those that live in large bodies of water or in the arctic, do well in cold or even freezing temperatures.",
"Most microbes have an optimum temperature range. Many disease-causing pathogens (although not all) are really well adapted to living in mammal bodies, so their optimum temperature, unsurprisingly, centers around mammalian body temperature.",
"There are many different enzymes and other types of proteins that are involved in cell growth and replication. Proteins are long chains of amino acids that fold up into complicated three-dimensional structures that allow them to perform different functions. Temperature, pH of solution, presence or absence of salts, and other factors all affect the way in which proteins fold, and if they aren't folded the way they are supposed to, their structure is wrong and their functioning will be wrong too.",
"One reason you're getting vague answers is probably that temperature has a widespread, complicated effect of the structures of many different enzymes and proteins inside bacteria, and in cold temperatures, many different cellular systems will all be a bit suboptimal, resulting in slower growth."
] |
[
"All chemical processes slow down as you decrease temperature, this is known as ",
"Arrhenius' relationship",
" . Decreasing the temperature will decrease the rates of multiplication, repair, metabolism, and all processes. Because these functions all depend on each other, they slow even more than any individual chemical step does."
] |
[
"Thank you for your response!"
] |
[
"Given a high-resolution picture of either a sunrise or sunset, can you tell which one it is?"
] |
[
false
] |
Suppose you have an extremely high-resolution (say, on the terapixels scale) picture that was taken at either just after sunrise or just before sunset. Is there a reliable way to determine whether such a picture is of a sunset or a sunrise? For example, are sunrises in some location a characteristically different color relative to sunsets in the same location?
|
[
"Previous thread",
". Sunset looks different to sunrise because there are more particles (dust etc) in the air in the evening."
] |
[
"If you had a sufficiently high resolution, high contrast camera on a very stable tripod, you would be able to see a slight smear, as the sun would move slightly while the picture was exposed.",
"The smear would give the direction of the sun, and from that you could get whether it was rising or setting. "
] |
[
"CCDs typically take longer to desaturate than saturate, resulting in asymmetrical behavior.",
"Film, on the other hand, should give a more symmetrical image where it would be difficult to determine direction of motion. "
] |
[
"Don't know how to title it but it has to do with fire and a glass bottle."
] |
[
false
] |
[deleted]
|
[
"You'll require a lot of alcohol, and I don't think you'll be able to displace enough water in him, but I see no reason not to try it."
] |
[
"A small amount of rubbing alcohol (isopropyl alcohol) has been poured into the bottle and allowed to vaporize, displacing the air in the bottle. The flame can only burn at the gaseous interface between the vapor and the outside air. ",
"Here's another video of the same effect.",
" "
] |
[
"If it were hydrogen the gas would go up.",
"It's some heaver-than-air gas."
] |
[
"How does the body adapt itself to increasing physical activity?"
] |
[
false
] |
For example - in the gym, on day 1, I could barely walk a kilometer without breaking into profuse sweat and being short of breath. After about 15 days now, I can easily walk that much distance in half the time with barely any sweating. How does this happen?
|
[
"Exercise causes damage to your muscle, as part of repair your muscles start developing new blood vessels. Frequent running increases vessel density, which allows the muscles to get more oxygen and nutrients > less stress-induced catabolism."
] |
[
"Classic ",
"r/askscience",
". Most well mannered subreddit I've ever seen."
] |
[
"So why do I sweat lesser now as compared to day 1?"
] |
[
"Confabulation Question: Can one knowingly implant false memories in ones own head?"
] |
[
false
] |
Can one knowingly implant false memories in ones own head? If so, how? It is not important to believe the memory. It is only the vivid recall of this false memory that is important.
|
[
"Try hypnosis, but I don't know how you would hypnosis yourself, and weather or not you could remember when you weren't under hypnosis.",
"Carl Sagan's The Demon Haunted World",
" deals with this and alien abduction as well as religious encounters."
] |
[
"I am not sure what you mean by knowingly, but with enough reinforcement, yes. This is why psychotherapy is/was so controversial with respect to childhood memories. "
] |
[
"Yes. One example I know of: Lieing so much, that you begin to believe the lie yourself."
] |
[
"Why doesn't a compressed spring heat up?"
] |
[
false
] |
I have a limited understanding of physics, and I was wondering why potential energy doesn't always have a thermal byproduct? Is it because it is only potential energy instead of kinetic energy? Or is it still 'actual' energy, just in a stored form? I understand the materials of the spring will eventually break down (due to entropy? Even though large amounts of energy are being spent keeping the spring compressed, it can't hold all of that perpetually) so that it no longer offers any resistance. But why is it that the spring itself doesn't heat up while it's compressed? Where does the lost energy go?
|
[
"Fun experiment at home:",
"Take a (clean...) rubber band and put it against your lips.",
"Extend the band while keeping the contact with your lips. You should clearly feel a small temperature increase.",
"Maintain the band extended a few seconds.",
"Release (slowly, no need to hurt yourself :) ) the tension. The band will feel cold."
] |
[
"A compressed spring does heat up, however, most springs have a fair bit of surface area to dissipate the heat so it is hard to feel it. The ehat is essentially representative of inefficiency associated with the spring's storage, and most springs are pretty efficient. However, if you put were to put a spring in an insulated container full of water and compress/decompress the spring repeatedly, you would see the water warm up."
] |
[
"Why doesn't a compressed spring heat up?",
"As mingy said, it does heat up a very little bit, due to friction, just not enough to be noticable by a human.",
"I was wondering why potential energy doesn't always have a thermal byproduct",
"Because not all potential energy is thermal energy. Thermal energy is the energy a body has which contributes to its temperature, and can include both potential and kinetic energy (depending on the structure of the system; for gasses, it is entirely kinetic, but for most materials it is almost equally divided between kinetic and potential).",
"Is it because it is only potential energy instead of kinetic energy?",
"Thermal energy has its origin in the vibrational movement of atoms -- and in the bonds between atoms -- in the system. So, any potential energy which involves vibration, may contribute to the thermal energy. Any potential energy which doesn't involve vibration (like compression) does not. So the answer to your question is no -- it's not because it is potential and not kinetic.",
"Or is it still 'actual' energy, just in a stored form?",
"It's hard to interpret what potential energy is exactly. Energy, in general, isn't an \"actual\" thing -- energy has no physical form or tangible existance, and it cannot exist independently; that is to say, there is no such thing as \"pure energy.\" Energy, rather, is a ",
" of objects and systems. Even light is not \"pure energy,\" it also possesses properties like momentum and spin.",
"Furthermore, energy is not actually observable or directly measurable -- it is not an \"actual property\" the way speed and position are actual, observable, measurable properties. Energy, on the other hand, has to be ",
" from the observable properties. So even though it is a property, there is a very true sense in which energy is \"not actual.\"",
"The question remains about what potential energy is. Like energy in general, potential energy is a ",
" of a system. But since the amount of potential energy that a system has is relative to the choice of some reference point, only ",
" in potential energy are physically meaningful. So if you have two nearly-identical systems, it doesn't matter whether they have 50J and 60J of potential energy (respectively), or 500J and 510J of potential energy, what matters is that there is a difference of 10J of potential energy between them.",
"I understand the materials of the spring will eventually break down (due to entropy? Even though large amounts of energy are being spent keeping the spring compressed, it can't hold all of that perpetually)",
"Not due to entropy -- due to adding so much (potential) energy to the system that the bonds break and the system flies apart.",
"Where does the lost energy go?",
"It doesn't go anywhere -- it's still part of the system. The energy you apply to compress the spring remains as part of the system as potential energy. Since energy is not a physical thing, that potential energy does not have a physical form, it is just a property of the system, that happens to be conserved when you calculate it."
] |
[
"Is there a physical limit to data transfer rate? Is there a fundamental limit to the interval between the transmission of two bits of information?"
] |
[
false
] |
I have read that information cannot travel at a speed greater than the speed of light. However, is there a fundamental limit on the distance between bits? I guess this should put a limit on the data rate flow. The E-t uncertainty might suggest that transmitting at higher energies wll make higher data rates possible. Is this the right approach to solving the problem?
|
[
"It's important to distinguish between latency and throughput. Throughput (or sometimes called bandwidth but I'd like to avoid that word here for reasons which will soon become clear) measures how many bits can be sent through a channel per second. Measured with megabits per second or gigabits per second or something like that.",
"Latency is the time it takes for someone to receive something you send. Measured usually in milliseconds. Ping times measure latency. Latency is simply bounded by speed of light, there's not much else to it.",
"You can have high throughput and high latency, or low latency and low throughput. A classic example is stuffing a backpack full of DVDs and taking a bicycle ride across Europe. Let's say you have 500 DVDs in your bag and it takes you 30 days to do this ride. You will have achieved a data throughput rate of about 7 megabits per second, which isn't too bad. But your latency is 30 days.",
"For throughput there's the ",
"Shannon-Hartley theorem",
" and some other stuff by Claude Shannon. For any given channel, you can calculate a channel capacity, measured in bits per second. Usually, this will depend on the signal to noise ratio and bandwidth. The signal to noise ratio depends on your transmitting power, how the signal weakens by the time it gets to the receiver end and how much noise interferes with your signal. Bandwidth here has nothing to do with the throughput kind of bandwidth. Bandwidth here is measured in Hz and it's the width of the frequency range you use for communication. This applies both to wireless and wired communication.",
"What's interesting is that the ",
"Noisy-channel coding teorem",
" (also by Shannon) states that for all throughput rates up to the channel capacity, there are encoding schemes that will get you arbitrarily close to error free communication. And conversely that if you try to send data faster than the channel capacity, then no error correction code will let you do error free communication at that rate."
] |
[
"Have a look at the Shannon-Hartley theorem:\n",
"http://en.wikipedia.org/wiki/Shannon-Hartley_theorem",
"It basically says that the information transfer rate is limited by the bandwidth of the channel and the signal to noise ratio. More energy means a higher signal to noise ratio and higher data rates."
] |
[
"This would be considering a packet to be the whole backpack of dvds. For very tiny packets that make up transmissions, latency and throughput would seem to be the same to me. It'd be different if you receive multiple packets at once I guess. What I'm thinking of is something like, let's say a packet is 8kb, and it takes 30ms to receive/send it. You could do 8/30 to find your throughput in kb per ms. Is it really that important to distinguish between the two in this case, if we know the packet size?"
] |
[
"How does Curiosity stay in contact with Earth when Mars is on the opposite side of the sun?"
] |
[
false
] |
I know the Mars rover is equipped with radios. But while Mars is on the opposite side of the sun, wouldn't the signal have to travel through the sun to reach Earth directly? How do they manage to overcome this problem?
|
[
"Mostly, they don't. This event, called a solar ",
"superior conjunction",
" with Mars, is relatively rare, and so we manage to keep mostly-continuous communication with Mars orbiters and probes. Before explaining what happens in a solar conjunction, it's worth taking a deeper look into the MSL/Curiosity telecommunications system. NASA has a nice overview ",
"here",
", and I'll summarise below.",
"There are two ways we can communicate with Curiosity:",
" these are direct uplink/downlink Earth-Curiosity communications, on a frequency band of approximately 7-8.5 GHz (called X-band), using two antennas - the RLGA (Rover Low-Gain Antenna), which cannot be oriented, and the HGA (High-Gain Antenna), mounted on gimbals. Transmission to Earth is done through the HGA, coupled to a solid-state amplifier at 15 W. This nets us data rates of up to 32 kbit/s.",
" these are Curiosity-orbiter (the Mars Reconnaissance Orbiter or the Odyssey) communications, which are the primary system used for surface operations. They use frequencies in approximately 350-450 MHz. The MRO is the primary relay. Both orbiters are in view of Curiosity for about eight continuous minutes per sol (Martian day). The data transfer is done at adaptive data rates, and it can go up to 2 Mbit/s to the MRO. The orbiters can then relay the data to Earth, as well as receive data from us to relay to the rover.",
"During a conjunction, such as ",
"this one",
", there are two problems.",
" while we can still see and communicate with the orbiters (and therefore with the rover), the signal is likely to be degraded by solar flares and the solar corona. In this case, communications are mostly shut down as a matter of precaution (rogue data could, however unlikely, cause undesired after effects in the systems), but NASA uses the time to measure performance of error-correction and redundancy under stress.",
" we can't see neither the rover nor the orbiters. This only happens for a few days (in the superior conjunction of 2023, we expect signals to completely disappear behind the solar disk for one day, and be within two solar radii for three days), and in this case there's really nothing we can do but wait.",
"I don't know if gravitational lensing can have any relevant impact on communications during a conjunction, but I would imagine the effects mentioned above would corrupt the data too much to be of any use even when it got here. Hope this helps!",
"Markovsky, A., Ilott, P., and Taylor, J., \"Mars Science Laboratory Telecommunications System Design- Article 14\". DESCANSO Design and Performance Summary Series. Pasadena, California: Jet Propulsion Laboratory - NASA. November 2009. (",
"PDF",
")",
"Morabito, D., and Hastrup, R., \"Communications with Mars During Periods of Solar Conjunction: Initial Study Results\". IPN Progress Report 42-147, NASA. November 2001. (",
"PDF",
")"
] |
[
"There is a difficulty in these communications due to launch size/weight limitations: the spacecraft always have small, low-power radio dishes. This means that to receive their weak transmissions from across the solar system, we need giant radio dishes to get the signal. NASA uses the huge ",
"Deep Space Network",
" dishes for this. Similarly, because the probes have such small dishes, we need the high-power transmitters of the DSN to transmit signals strong enough for the probes to pick up.",
"So you can see if we had a spacecraft at the half-way point it may not be able to get the job done, because one leg of the signal would be between two spacecraft distant from each other. I'm not sure how big/powerful that spacecraft would need to be (maybe a radio expert could give us an idea), but when we start talking about building a huge probe in a difficult orbit just to buy transmission through a brief phase of the mission, we have to ask whether those resources are better spent building other actual science missions."
] |
[
"Not a stupid question at all. They're maximum practical speeds - that is, it's possible, under realistic best conditions, to transmit at those speeds. But actual speed varies according to the link state (signal strength and error rate).",
"In particular, the UHF relay system from the MRO to the MSL uses a special-purpose ",
"software-defined radio",
" called ",
"Electra",
" (and its smaller version, the Electra-Lite), developed by the JPL, which is capable of automatically adjusting the data rate according to variations in signal strength, due to the relative orientation of the antennas and the distance between the MRO and the Curiosity."
] |
[
"Why do I feel so awful when I am sick?"
] |
[
false
] | null |
[
"IL1, for the most part. There are others, though.",
"It's a cytokine made by inflammatory cells fighting off the cold virus and it has global effects. It can make your muscles ache, make you tired, make you nauseous, and give you fever. It has local inflammatory effects, but if you have enough of it circulating there are receptors in the CNS that will respond to it.",
"And yes, it's about keeping you still, not hungry (so you won't go hunt) and away from the rest of the clan/tribe/dorm. The fever is part of the actual defense, though. "
] |
[
"I would think that TNF-alpha would have the stronger inflammatory effect. IL-1 certainly is a part of it though. ",
"And yes, it's about keeping you still, not hungry (so you won't go hunt) and away from the rest of the clan/tribe/dorm. The fever is part of the actual defense, though.",
"Do you have a source for that? "
] |
[
"Hart in the mid-late 80s, and Kluger (Klueger?) around the same time, but a little later. ",
"Here",
" is a review, but just about everybody cites Hart.",
"I listed IL1 because AFAIK it's better documented than the TNFa effects as far as the sickness behavior responses go. IL6, PG's, and a bunch of others are in there, too. It's been getting a bit of attention lately as a target for drugs to offset the side effects of chemo."
] |
[
"Why don't you feel hungry when you're sick?"
] |
[
false
] | null |
[
"When you're sick with something like the flu, your body mounts an attack that includes increasing certain circulating molecules in your bloodstream. This inflammatory response has effects on your nervous system such as making you sleepy, reclusive, and lethargic. It also leads to a decrease in appetite. This phenomenon is called ",
"sickness behavior",
" and may be adaptive from an evolutionary standpoint. "
] |
[
"What is the evolutionary advantage? Shouldn't the body need more energy and nutrients to fight off infections or repair damaged cells when sick?"
] |
[
"My background is in physics not biology, so maybe someone else can handle this more adequately. ",
"Some of the behaviors can be thought of as ostracizing in order to slow infection, and the body is potentially using its resources elsewhere and not wanting to devote resources to digestion. Chronically ill patients have an increased in sympathetic nervous system activity in general which favors the utilization of already stored resources over consuming new food. ",
"A sick body doesn't always do what's normally considered best. For example, in infection, it often sequesters resources at its own expense. We see this with anemia of chronic disease which is a storing of iron in the bone marrow at the expense of healthy red blood cells. This keeps iron away from iron-loving bacteria though. Even the inflammatory response can ultimately be deleterious, but it is done in order to beat down the infection which is the ultimate goal. "
] |
[
"Can you explain me quantum decoherence in simple language?"
] |
[
false
] | null |
[
"To understand this, it probably helps to start with quantum coherence. At a simplistic level, coherence can be thought of as the information that the quantum system holds. More precisely, it is a measure of the relationship between different possible states of the system. Quantum computers rely on this information to function.",
"If the system is isolated, coherence is conserved. Decoherence occurs when the system interacts with its environment. This means that the information that the system contains gets mixed up with its environment.",
"Here's a (limited, but useful) analogy. Imagine I have a small bottle of red ink. I can see that it is red because the pigment is highly concentrated. Let's say I now tip the ink into a large lake or the ocean. At first I can see the colour, but it slowly starts dispersing. After a while, I can't distinguish the ink (system information) from the ocean (environment). The larger background completely overwhelms the smaller system, making it impossible to separate. Quantum decoherence is like that. If you are interested, you may also want to look up quantum entanglement.",
"Source: PhD in High Energy Physics.",
"EDIT: Thank you for the silver and gold kind strangers!"
] |
[
"Wow, just finished my first year of my PhD in physics and I understand coherence/decoherence mathematically but your red ink in a lake analogy definitely made everything click intuitively."
] |
[
"Glad that it helped you! The intuition is often much harder than the mathematics. You can follow mathematical logic and yet have no idea what you are doing.",
"To quote Einstein: \"If you can't explain it simply, then you don't understand it well enough.\" I try to approach my maths and physics with this in mind."
] |
[
"Does gas actually get cooler when it expands in vacuum?"
] |
[
false
] |
For example if I puncture a gas tank, it cools down since escaping the gas is working against the pressure of the environment and the energy is taken away from the internal energy of the gas. The work done by gas is taken away from the internal energy of the gas --> T drops. However if I take the tank to interstellar space where there is nothing for the escaping gas to push against, then the work done by expanding gas (pressure times change in volume) is 0 too and the temperature of the gas stays the same. Is it actually so?
|
[
"What you're describing is called a free expansion of the gas, which is an irreversible process.",
"You're correct in saying the internal energy stays constant, but U is only a function of T only for ideal gases. So for ideal gases free expansion is isothermal, but real gases do change in temperature. ",
"In free expansion you have an entropy increase even though there's no heat exchange, because the process is irreversible. "
] |
[
"What your basically describing is the ",
"Joule-Thomson effect",
", where gas expands through a small hole to a larger volume. It is irreversible, but not quite the same as free expansion (i.e. if you instantly increased the volume of the container holding the gas). You are right that an ideal gas won't decrease in temperature. For a real gas, read below.",
"If you expand an ideal gas reversibly in a piston, it cools because the gas does work on the piston, lowering the internal energy. An ideal gas stores all internal energy as kinetic energy, so if you lower the total internal energy, you lower the average kinetic energy per particle and therefore lower the temperature.",
"In a Joule-Thomson process, the gas expands irreversibly and does no work. As we said, an ideal gas therefore doesn't change temperature. But a real gas might. Why? Because in a real gas some internal energy is stored as potential energy. Potential energy is stored in the Van der Waals potential (approximated by the ",
"Lennard-Jones potential",
"). If you change this average potential energy, you can change the average kinetic energy and change the temperature.",
"Here is where it gets weird: A real gas can either heat up or cool down as it expands in through a hole. After all, if the gas is farther apart on average then you tend to raise the potential energy (the long range part of the Lennard-Jones potential). But if the gas is colliding less frequently, you lose the high potential energy spikes in the Lennard-Jones potential at close range. In practice, this leads to an inversion temperature. Start off with a hot gas, and expanding it makes it hotter. Start off with a cold gas, expanding it makes it colder. Every gas has its own inversion temperature that divides these two regimes."
] |
[
"Real gases in free expansion lose internal energy (Q)",
"No, they don't, the internal energy stays constant. The temperature lowers. Also the symbol for the internal energy is U.",
"and entropy (ΔS = ∫dQ/T, or am I mistaken here?) increases accordingly.",
"That formula is for the variation of entropy due to exchange of entropy with another system, which here is zero. Here entropy increases because of the irreversibility of the process, without any heat transfer.",
"I don't get by which mechanism the gas loses energy unless by work or heat transfer.",
"It doesn't lose energy."
] |
[
"If antibiotics beat your infection, does your body still become more immune to it? Why or why not/to what degree?"
] |
[
false
] |
[deleted]
|
[
"It depends.",
"The immunity you get following an infection results from the presence of antibodies that your body produces to identify and fight the bacteria (In the case of a bacterial infection).",
"If you need antibiotics to fight an infection, there's likely three main reasons:",
"1: You would probably have fought it off eventually anyway, but you wanted to do so faster and with lower risk.",
"2: You may have been able to fight it off, but your immune system was weak, or slow to respond, and the infection became too severe to fight before you died.",
"3: Your body was unable to produce an ",
" immune response at all and even with strong antibiotics, you were only just able to fight off the infection.",
"One thing to note about anti-biotics, though, is that in almost all cases, they do not end the infection themselves. Rather they reduce the number of bacteria to a level that your body is able to manage itself. Totally immunocompromised individuals are very rarely able to survive infections, even with potent antibiotics.",
"With this in mind;",
"In case 1, your body will almost certainly be, at least, more resistant to the infection even after antibiotics; it's learned to identify the bacteria and the antibiotics just helped speed things up.",
"In case 2; your body is usually trying to fight the infection and so will likely be able to deal with isolated bacteria in future. Exceptions occur in bacteria that multiply in locations where antibodies cannot reach, such as the base of nerve cells. (Incidentally, this is why the virus that causes shingles is very difficult to \"stop for good\").",
"In case 3, if your body was able to mount an antibody and antigen response at any point, you should retain that antibody response in subsequent infections of that bacteria. But there is some chance that the bacteria is virulent or effective enough that that antibody response won't be adequate anyway. If, however, your body never mounted an antibody response and instead survived through potent antibiotics and the action of non-selective immune cells (NK cells, macrophages etc), there's a chance that it won't have learned to fight the infection properly and is still at risk of reinfection.",
"Ultimately, anbiotics don't really have an effect on how well your body learns to fight an infection. Which is why the idea that you shouldn't treat babies with infections to \"Boost their immunity\" is so flawed. (There are some points to that argument, but it's complex and worth a thread of its own)."
] |
[
"Wow, thank you so much! That was a fascinating and thorough answer to something I couldn't find by googling.. you rock! The fact about shingles is a fun tidbit too! :)"
] |
[
"Thank you, I added the appropriate flair."
] |
[
"Does a young zero-energy universe which is still sub-plank size violate the uncertainty principle?"
] |
[
false
] |
Layman here, If our universe was once small enough to be susceptible to quantum mechanical effect, if we say that the overall energy in the universe at any time is zero, doesn't that violate the uncertainty principle? It implies that it is impossible to simultaneously measure the present position while also determining the future motion of a particle, or of any system small enough to require quantum mechanical treatment
|
[
"The ",
"zero-energy universe",
" hypothesis states that the total amount of energy in the universe is exactly zero. When the energy of the universe is considered from a pseudo-tensor point of view, zero values are obtained in the resulting calculations. The amount of positive energy in the form of matter is exactly canceled out by the negative energy in the form of gravity"
] |
[
"Well, we don't have a quantum theory of gravity yet, and we'd need one to understand anything going on at sub-Planck length scales. You could even say that the difficulty in answering your question comes from the same problem that's preventing us from creating a consistent quantum theory of gravity: GR needs its certainty in physical properties, while quantum theories need uncertainty at small scales.",
"(The reason the Big Bang theory is still okay is that it just stipulates the rapid expansion and inflation that occurs at scales much much larger than the Planck scale, so quantum gravitational effects don't need to be considered.)"
] |
[
"I don't understand your question :(",
"What is a zero energy universe?"
] |
[
"How does a plant keep track of large amounts of time? Not day/night. For eg. Kurinji plant in Western ghats of South India blooms every 12 years with consistency."
] |
[
false
] |
It is predictable enough that Govt. has put up ads for tourists to watch this in July 2018. Swathes of hills get blanketed by this blue flowers.
|
[
"Each plant possibly has it's own internal timer using some mechanism, but each plant's timer is inaccurate and unreliable, so they take a vote by constantly monitoring each other via the ",
"chemicals they release",
". That way, the few plants that want to flower a year early detect that the other plants aren't preparing to flower and so they themselves hold back, then when the majority of the plants want to flower, they sense that the other plants are doing the same and so proceed with it, and the plants that wanted to flower a year later detect this and go along with the rest by flowering at the same time as them. A similar effect is how an entire orchard can ripen all at once; the plants release ",
"ethylene gas",
" and use it as a signal for fruit ripening, leaf shedding and flower opening."
] |
[
"Basically, there are three questions:",
"They all have similar answers: a process will create a certain subtance, like a protein, that causes an effect when it passes a threshold, but not before. Putting water into a bucket doesn't make you grandma shout, unless you put in enough to make it overflow.",
"Question 1: They measure the length of the day. Basically, they have 2 chemicals (or more, but lets keep it basic). One builds up when it IS light and breaks down when it IS dark, and another that builds up when it GETS dark, and breaks down when it GETS light. So after a really long, bright day, you'll have a ton of chemical 1 built up, and then it gets dark, so you get a peak of chemical 2. Together they \"push a trigger\" and the plant starts to flower. ",
"Question 2: They have a \"ticker\". Instead of flowering during 'question 1', instead, they convert one chemical into another. During the cold of winter, this could again change, making 1 \"tick\". Perhaps they need three \"ticks\" to get sufficient chemical for a \"tock\", uising a mechanism similar as in question 1. Eventually, the plant will again reach a threshold-level, and the \"flower-now\" trigger is pushed. ",
"Question 3: In addition to this whole complex system, there's also an override. If the plants detect a level of trace-chemicals in the air, they immediately start to bloom. And when they bloom, they also release the \"Bloom now\" chemicals into the air. "
] |
[
"Sure. For 1: ",
"https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4414745/",
"And for 2, you're correct. It is mostly speculation. But it's a reasonably widely accepted hypothesis. ",
"https://www.ncbi.nlm.nih.gov/m/pubmed/27477130/",
"Edit: the most useful term to Google is \"mast seeding\" if you want to keep looking"
] |
[
"Has anyone found a practical replacement for lithium ion batteries yet?"
] |
[
false
] |
[deleted]
|
[
"A company in Arizona, called Fluidic Energy, is doing some pretty exciting research on Metal-Air-Ionic-Liquid (MAIL) batteries. They won an ARPA-E grant, and have already been field-testing some of their battery designs. The MAIL batteries would eventually cost about the same as a lead-acid battery, but would store enough energy to drive a car over 400 miles on a single charge.",
"There are other companies doing similar research, but I have heard the most about this one.",
"http://gigaom.com/2013/03/01/fluidic-shows-a-peek-of-its-metal-air-batteries-for-off-and-on-the-grid/"
] |
[
"the only one I know that is being heavily looked at is lithium air batteries for car use. Others can say better then I if they will work out, but someone has to start a comment. "
] |
[
"Were those the ones that electric car companies were planning on using; where you'd just swap batteries once you use one up?"
] |
[
"Why does blue light penetrate deeper than red light in oceans?"
] |
[
false
] |
[deleted]
|
[
"Any given material will respond differently to different wavelengths of EM radiation. This is know as the absorption spectrum of the medium and it's origin arises from the resonant frequencies of the molecules in the medium.",
"For water the absorption spectrum is peaked around the red region of light and is much less absorptive in the blue region. That means that the red light is absorbed faster and doesn't penetrate as far into the ocean. This is also the reason why water is actually \"intrinsically\" blue: if you shine a white light through a long enough sample of water it will become \"bluer\" the longer the distance traveled through water.",
"Reference:\n",
"http://inside.mines.edu/fs_home/dwu/classes/CH353/study/Why%20is%20Water%20Blue.pdf",
"(check out the absorption graph on the first page, the \"red\" absorption peak is the one in 700-800 nm range).",
"Enjoy!"
] |
[
"Right, now I've taken a look at the composition of Earth's atmosphere (Nitrogen is by far the most abundant) but this is without water vapour, and when the do give water they give it not in ppm or ppb but as follows:",
"At any moment, the atmosphere contains an astounding 37.5 million billion gallons of water, in the invisible vapor phase. This is enough water to cover the entire surface of the Earth (land and ocean) with one inch of rain.",
"Or something along those line. I would do a quick calculation but I don't know what they are considering the atmosphere so I'm not sure what volume I would use to do the calc. with. ",
"Is there more N, and O2 than there is water in the atmosphere?! If not, shouldn't the atmosphere essentially act like ocean in that being primarily water it would be the dominant factor? Thanks for the pdf too :) I haven't had time to read it yet but I will."
] |
[
"Hi there, just saw this follow up now.",
"Firstly what you need to know to answer your question is that the strength of the interaction between the EM wave and medium is dependent on the number of molecules per unit volume in the medium. For many cases, including this one, this relationship can be described by a relatively simply law:",
"\"Strength of Interaction\" = (Number of interacting molecules per unit volume) X (\"Strength of interaction\" of a single molecule)",
"The \"strength of interaction\" is actually known as the permittivity of the medium, but it's easier in this context to think of it as a measure of the how strongly the molecules affect the EM wave.",
"Now (as you guessed in your question) we can see numerically why the atmosphere does not in fact behave like the ocean:",
"Number of molecules in water: ~ 3x10",
" molecules of H2O per cubic cm (this is for pure water, we'll assume it's close enough to the number in the ocean as to make no difference). ",
"Number of molecules (all constituent gases) in air at STP: ~ 3x10",
" per cubic cm.",
"Now the number of water molecules in a cubic cm of air is a widely varying figure. It depends on the humidity which itself is relative to the water carrying capacity of the air - a highly variable function of temperature (mostly) and pressure (somewhat). So lets try and maximize the amount of water molecules and take a very hot and humid day, 30C @ 90% humidity. I dug around a bit and got this number: ~ 3x10",
" molecules of water.",
"Which means that we can expect the strength of the attenuation due to H2O interaction to be ~ (3X10",
" / (3x10",
" ) = 10",
" . So even on an extremely humid day, the effect is 100,000 times weaker than when going through ocean water! This means that although the same effect is definitely there (and sensitive instruments on satellites, for example, can actually use it to measure water vapor content in the atmosphere) it is vastly weaker, and long before you would be able to notice it with the naked eye it would have been overtaken by other effects - such as Rayleigh scattering, for example.",
"Hope this helps,\nEnjoy!",
"last edit :) :\nThis page has excellent explanations and a great graph of atmospheric absorption spectra broken down according to the molecule type contributing to the absorption:\n",
"http://wattsupwiththat.com/2008/06/21/a-window-on-water-vapor-and-planetary-temperature-part-2/"
] |
[
"Can single-cell organisms get cancer?"
] |
[
false
] | null |
[
"https://www.reddit.com/r/askscience/comments/3657hz",
"No, not \"cancer\" by definition, but they could get a mutation that makes them \"immortal\" and reproduce without limit. Such a colonly would probably kill itself off by using up all its resources too quickly, though."
] |
[
"Cell biologist here, You dont wanna do that unless you're super sure you can keep the \"starvation\" extremely localised, otherwise the tissue and its surroundings becomes necrotic and the patient will die from toxic shock as there is too much cell debris to be taken care off by macrophages and such. Thats also why you don't want your cancer treatment to be too effective too quickly as the dead cancer cells just dump all their toxins into the bloodstream, poisoning the patient",
"Edit for clarity: Tumour lysis syndrome is what I am describing. Toxic shock syndrome/Septic shock is ususally used for reserved for infections."
] |
[
"Just when you think cancer is not complicated enough, it pulls another card our of his sleeves."
] |
[
"Hey /r/science, question. What is it called when your body gets used to the feeling of your clothing on you, and the feeling subsides?"
] |
[
false
] |
I could be wrong, but I thought that there was some sort of medical term for either what makes this happen, or the state itself. I put on clothing, then I don't notice it after a little bit. Is there a word for this? Thanks!
|
[
"Bingo! You are my new god. Thanks!"
] |
[
"Bingo! You are my new god. Thanks!"
] |
[
"I've always heard it called \"sensory fatigue.\" Same thing though."
] |
[
"Can LEDs emit other types of EM radiation?"
] |
[
false
] |
[deleted]
|
[
"The discovery of IR emitting diodes preceded the invention of LEDs, so yes. ",
"http://theinstitute.ieee.org/tech-history/technology-history/timeline-the-evolution-of-the-led",
"Considering that blue and \"white\" LEDs are relatively new (I was born in the 1980s, so marketable blue LEDs still seem new to me since they hit when I was a teenager), I'd guess that the practical limit is somewhere in the UV range."
] |
[
"As the main difference between near UV, visible light and near IR light is our eyes' ability to see it, many technologies that work over visible range, tend to work in those ranges as well. In case of LEDs, I've found these extremes:",
"232 nm UV-C LED",
"7 μm IR-C LED",
"So roughly speaking, you can have LEDs that emit EM almost ",
"an octave",
" above blue edge of visible light, and over 3 octaves below the red edge of visible light. ",
"Overall, you can choose a LED in a range spanning ~4.9 octaves of EM spectrum (maybe not completely continuously), while the visible light spans around 0.8 octaves. "
] |
[
"The color depends on the band gap size and these are typically at electron volt scales, so with this principle you won't produce much higher energy photons than visible light. Here's an article ",
"https://en.wikipedia.org/wiki/Wide-bandgap_semiconductor",
" talking about wide bandgaps which already correspond to green and blue light so that gives an idea. But maybe an expert knows more about methods to create higher energy photons. "
] |
[
"Why do diatoms have cell walls made of silicon? (biology)"
] |
[
false
] |
I was wondering why diatoms have cell walls made of silicon but no other eukaryote that I came across did as well. What kind of advantages does this give a diatom over other algae and why would the usage of this inorganic element be beneficial over the use of other, assumed more readily available, organic compounds such as cellulose (complex sugars), peptidoglycan, or even something like mycobacterial cell walls with their waxy lipid structure. I know those are all examples of bacterial cell walls, but most other algae have complicated cell walls containing polymers that don't contain silicon and silicon is not really used by any other type of eukaryote or prokaryote that I know of; so I was wondering if anyone knew what made silicon so advantageous to use for this one group of algae as opposed to the rest of organic life.
|
[
"Primarily, defense. Eating diatoms is like eating little bits of glass, not many zooplankton are able to eat that.",
"As for why diatoms arose, it has been hypothesized that the rise of grasses on land contributed to the evolution of diatoms by liberating more silica into the water. I can't remember the reference off the top of my head, but I can look for it later if you're interested.",
"edit: spelling and a factual error"
] |
[
"That's really interesting thanks for the info and sorry I took so long to respond. I was studying for my microbiology class and I thought that it was really interesting that any kind of microorganism would use something like silica for defense and wondered when it actually came about and what environmental influences would have to be present for an organism to utilize a transition metal since I'm fairly certain no other transition metals are utilized by other organisms that aren't multicellular."
] |
[
"Here's ",
"the paper",
" I was talking about. I think it's a really interesting hypothesis for the origin of diatoms. The paper's been cited quite a few times since, so maybe looking around through the related literature will be an enlightening experience. Happy hunting!"
] |
[
"Why does Euler's number appear in physics?"
] |
[
false
] |
I understand that Euler's number is an infinite sum of 1/n!, but what I don't understand is why things like the decay and growth of nuclear radiation can be perfectly modeled by this number. I guess you can have this discussion with pi, but to me it is more obvious why pi comes up in physics, whenever the circumference of a circle is divided by its diameter, it's the nature of a circle, which come up often. But why do things in nature work by this infinite sum of reciprocal factorials?
|
[
"It all boils down to",
"d/dx ( e",
" ) = e",
"e is the only base for which the above is true. e",
" being invariant under differentiation means this function will be paramount to the solution of linear differential equations, which are the heart of physics. In fact, you can see that e in physics almost never appears as anything else than the base of an exponential."
] |
[
"There might be a more mundane answer to your question, depending on what you're asking. Is exponential growth in the sense of y = a",
" is reasonable to you, and you're asking why you often see the base a = e in physics? That's actually a choice. It's because whenever you have an exponential you can rewrite it so that the base is e. In particular, a",
" = e",
" (you should check this for yourself!). So we can always freely change the base of the exponential to be e. Since e is a really convenient base to work with for the reasons that the other commenters brought up, we choose to write our formulas with e as the base instead of some other number."
] |
[
"exp(x) is the function which is its own derivative. Any time you have a quantity which changes at a rate which is directly proportional to that quantity, e is going to come up. A non-physics example is population growth.",
"You also see e when you're dealing with complex numbers because it's the polar representation."
] |
[
"When a beam of light passes through a sheet of glass, is it the same photons coming out the other side?"
] |
[
false
] |
So, basically, when light hits a transparent object, the energy from it excites the atom(s) it hits, and is transferred through in a chain reaction of excitations. When the energy reaches the other side, a photon is given off, right? Can it be said that this is the same photon, or would it count as a completely new photon created with the same energy?
|
[
"From what I've read, asking about the identity of such particles is a moot question, they are ",
"indistinguishable",
". We can never tell if any particle photon is the same photon we observed before. This has lead to some pretty funny conjectures like the idea that there is only ",
"one electron in the entire universe",
", but this isn't really taken seriously and is just a to the extreme illustration of this principle.",
"Someone with a fancy flair can feel free to correct me."
] |
[
"To put this to bed, let's have some actual physicists answer the question. ",
"Why is it clear",
" and ",
"what's going on with the refractive index",
".",
"TL;DR, the glass is clear because the band gap is too high for (at least visible) ",
" to be absorbed, and light speed is reduced because the ",
" induce sympathetic EM waves in the atoms they pass which interfere in a manner crudely akin to magnetic eddy currents in copper. Your best bet to answer this will involve both sides of the wave-particle coin. But listen to the guys from Nottingham!"
] |
[
"A few fates of the photon.",
"-Photon has enough energy to promote an electron to an excited state. For molecules of particular structure, the subsequent delayed electron relaxation and photon emission are observed as fluorescence. ",
"-Photon energy is absorbed via atomic dipole and a new photon is re-emitted with very similar characteristics. For a substance like glass, the energy gap is too large for an electron excitation. The re-emitted photon is of the same frequency but slightly out of phase with the initial colliding photon. This accounts for the different speed of light through a medium versus in a vacuum.",
"We use refraction indexes and absorption coefficients to account for light's behavior when traveling through materials of known composition. For a chemist working in the atomic scale, these values/theories suffice and we dismiss them as the same photon. Pragmatic but not technically correct. Scattering, photon identity, occupation in space, and behavior as a boson are typically the concern of physical chemists(quantum mechanics), particle physicists, and the deranged."
] |
[
"Since Hubble is moving at a fast rate in orbit. How does it take pictures with such a long shutter time?"
] |
[
false
] |
Since Hubble is in orbit and moving pretty fast. How does it stay stationary enough to take a picture with a 30 minute open shutter?
|
[
"The objects it looks at are generally so far away that the angle towards them does not change as Hubble moves.",
"You can take a photo of a distant mountain out of a train window, and it won't be nearly as blurry as the trees rushing past right next to you. This is the same principle, except on a much larger scale: the 15000km or so diameter of Hubble's orbit does not matter when you're looking at a galaxy a megaparsec away."
] |
[
"The question's already been answered, but let me just put it in perspective.",
"Let's say Hubble is taking a picture of Neptune. It's really faint so it's going to be a 20 minute exposure, or one side of Hubble's orbit to the other.",
"Neptune right now is 4.374 billion kilometers from Earth. The diameter of Hubble's orbit is 13,879km. That diameter is 0.0000032% of the distance to Neptune.",
"So now let's scale that down. Using the same ratio as above, if I were to take a long term exposure of something that was 10 meters away, the camera would move 32 microns in 20 minutes. Roughly the diameter of a white blood cell.",
"Now imagine how far away a star or a galaxy would be? At that same scale, the Andromeda galaxy would be 57 million kilometers away."
] |
[
"That's still pretty darn far away."
] |
[
"When two objects in space are moving towards each other, do they both have kinetic energy? If so, where does that energy come from?"
] |
[
false
] | null |
[
"Anything that's moving has kinetic energy."
] |
[
"So the energy comes from gravity?"
] |
[
"Not necessarily."
] |
[
"Do animals have any idea of their own mortality?"
] |
[
false
] |
Obviously, most animals don't have any concept of death, but what about animals that mourn their dead, like Elephants? Elephants have graveyards, so they must understand some sort of division between the living and the dead. Are they at all aware of the fact that they, themselves, will die one day? Has much research been done into this question?
|
[
"That's a very layered question!! While it may seem an obvious element, one of the things preventing us from having a definitive answer is that we can't ask the animals! There is simply no way to test it. And I'd be surprised if anyone has actually done any pointed research on this topic because there's so much work to do to get there:",
"What we know:",
"1) Animals do have a sense of self-preservation and reproduction. Animals of all intelligences seek out mates, feed themselves, and avoid things that are threats to their safety. And I mean, these are STRONG drives. Animals travel extreme distances, adapt to new niches to avoid competition for resources, put enormous amounts of energy into attracting mates, and on and on. But in particular, they flee or try to eliminate dangers. So they know if an animal is stalking them, that's bad and you should run away.",
"2) Most animals have a life trajectory. I'm most familiar with primates, so let's stick with that. So an animals is born, it knows mom=safety and food. Then they get curious and start to explore and play. That's their \"job\" as juveniles. This is when they first have to compete for resources, but it's not too serious because mom'll provide whatever they can't get on their own. Then they hit puberty and start having to compete on their own. That's most of their lives. And they seem to recognize this in others because they treat each other differently. Adult male baboons have been known to use babies as sheilds during a fight because adults know not to attack the babies. So they recognize that other individuals are different.",
"What we don't know:",
"1) Do animals have an innate sense of \"self\"\nThis is hard because also, how do you define \"a sense of self\"? Clearly we mean something beyond the self-preservation thing. But do we mean knowing \"I\" is something different from \"You\"? Is it that your \"I\" is different from my \"I\"? People are still working this out. Basically, some species seem to recognize \"what I know is different from what you know\". We can see this when animals are sneaky and practice ",
"tactical deception",
". ",
"But then this raises more questions: Do they all have this cognitive ability? Can this ability be learned or taught? Is it genetic, or based on environment, or even personality? Tactical deception has to be a rare behavior in order to work, so does it then follow that it's a rare attribute, or have we just not caught it as often as it happens? ",
"2) How do you test for a sense of self? \nThere's tactical deception and ",
"intentional teaching",
" that seem to indicate the presence of a sense of self, but can we really test for it? There are the mirror tests, when an animal is anesthetized and marked and then uses a mirror to investigate the mark. But it's not really a very practical test. Most don't pass getting used to mirror in the first place, or just get bored with the mirror and never use it to explore themselves. So, is that self-recognition ability a species trait, or particular to some individuals? ",
"3) If animals do have a sense of self, how do you find out what they know about death? There's evidence that they suffer emotionally when others die. Jane Goodall talked about a chimpanzee mother carrying around the body of a dead infant, there was a group of bonobos that wouldn't let their keepers remove a deceased male who died from their enclosure, and some other anecdotes that may indicate some kind of mourning going on. But again, there's no way to ask them. All you can say is that they're agitated. ",
"So the tl;dr is we don't know, and we're not at a point where we can answer that question. ",
"Further limitations come from not being able to communicate with the animals. Yes, Kanzi can communicate, and some other animals know sign language. But it's not like humans. They seem to have emotions and some seem to have some sense of self, but it'd be hard to even teach them what death is. I mean, how do you explain that in an objective way? It'd be very difficult to do so. And it may not be ethical to make a bonobo ponder their own death, in order to get the answer. \nSo, in sum, not much has been done on the issue, mainly because we don't yet have the knowledge base about cognition to do, and we have no way to test for it. "
] |
[
"A little piece of anecdotical data that seems to indicate that chimps could grasp (some of) the concept of death: ",
"http://ngm.nationalgeographic.com/visions-of-earth/visions-earth-2009?image=2#/1109-chimps-714.jpg"
] |
[
"That is truly haunting."
] |
[
"Does the frequency of a radio station affect the resulting sound quality?"
] |
[
false
] |
I have a radio transmitter that plays my iPod over my car radio and I recently changed the frequency from 107.3 to 90.1 and perceived a change in the sound quality for better. It sounded richer and I was wondering if it was my imagination or if the frequency that a radio station broadcasts would have any impact on the sound quality of what is being broadcast?
|
[
"If there was any difference in the quality of sound, it was likely due to lack of interference from other stations."
] |
[
"It's possible that the reason for this is because of the antenna in your iPod transmitter. This antenna may be better suited for transmitting at the higher frequencies in the FM radio band because of the geometry of the antenna, among other things. "
] |
[
"FM broadcasts modulate the frequency of the carrier frequency, so they need a band to allow the frequency difference from the modulation process. This is the size of the broadcast band (0.2 MHz), and it is fixed. They also add some safe distance between stations to avoid crossover. Because the band width is fixed, all broadcasters can carry only the same amount of information, so the quality difference is not based on the carrier band frequency (107.3 and 90.1 in your example). It was most probably interference, distance to the broadcast station, or bad quality at the source.",
"http://en.wikipedia.org/wiki/FM_broadcast_band"
] |
[
"Are we always seeing into the past?"
] |
[
false
] |
Take the test were in you learn of the speed of sound and the speed of light. A man with a hammer hits down on an anvil and you observe this from 50m away. You see the hammer hit before you hear the sound, but if light has to travel that distance aren't you seeing it in the past as well? At this distance, the time you are seeing into the past is basically zero, but if it is in the past what constitutes the present? Are the atoms of the action moving while we cannot see them? Or am i missing something and just talking out of my ass...
|
[
"Yup, you are seeing the past because light takes travel time. When we started observing the moons of jupiter, they calculated when the moons would go in front of the planet, and found that when Jupiter was furthest away, the moons would cross the planet late. What they pieced together from that was that light was actually taking time to get places.",
"After that, when you want to think about present, that gets into more like biology and out of my domain, but here's at least one article that ",
"discusses it",
". The brain is weird."
] |
[
"Network latency is noticeable at much lower time intervals than your reflex latency. For one, the network latency is added on top of your reflex latency rather than operating in parallel. The more important effect though is that you know when you clicked the button, you aren't reacting to it, so it's far easier to notice a delay.",
"You can even notice delays <10 ms if you know where to look. The refresh rate of your monitor is in this range and you'll notice your cursor feels slightly less responsive on monitors with slower refresh times.",
"When you are playing video games and reacting to something on the screen that you didn't do, like another play moving, that's still a traditional reflex. I believe well trained players operate in the range of 150-200ms for that sort of thing (vs a human average of 250-300ms). 50-75ms seems inhuman and would really surprise me."
] |
[
"I wonder how much in the past? Someone help me out.",
"A good rule of thumb is that light travels about a foot per nanosecond. So you are seeing an object 10 ft away from you as it was about 10 nanoseconds ago."
] |
[
"In units of energy per second, what is the least amount of light it takes for humans to be able to see something?"
] |
[
false
] |
Humans can see outlines of things in dim, even dark conditions. But how dark? Do we know approximately how much light energy striking our eyes it actually takes to make something visible to an average person?
|
[
"As an aside: pupil dilation has very little to do with how you adapt to darkness. It takes less than a second for the pupil to close or open completely (you can try this yourself: look at a mirror in a dark room, then turn on the light while watching your eyes; your pupils will shrink rapidly) but it can take many minutes to completely adapt to darkness. Adaptation is mostly driven by biochemical gain changes in your photoreceptors, which is a slow chemical process."
] |
[
"10 photons per second of yellowish-green light can be detected",
" Roughly 10",
" W/m",
" if I'm doing that conversion correctly. ",
"That, of course, assumes a completely dark room to observe it. "
] |
[
"I once spelunked with a high school group. When we reached the deepest part of the cave, our guides had us extinguish all our lights and wait for our eyes to get to a stable state (still not able to see anything, there was zero visible light, so therefore pupils maximally dilated), and then passed out wintergreen mints to chew with our mouths open. You could see a greenish spark from everyone's mouth, and it seemed like the spark was at the very limit of what was perceivable, and now I'm curious what the quantification of light emissions from chewing a wintergreen mint is. The spark certainly lasted far less than a second, and if it was at the edge of perception (a big assumption), that imply that a single photon was reaching each person's eye!"
] |
[
"How does our body control how much blood flows into specific areas?"
] |
[
false
] |
If I hold my hand up for a minute or two, it becomes pale. If I do a handstand for a minute, my head becomes red. If I stand on my feet however, everything seems fine and I don't get troubles with not having enough blood in the upper parts of my body. How do we control that?
|
[
"Things like this are pretty complex. Blood flow is regulated both locally and systemically through different mechanisms. Locally, metabolic byproducts and other vasoactive compounds cause arterioles to constrict or dilate. As a general rule, byproducts and waste will cause blood vessels to dilate and cause more blood flow to an area. Additionally, pH plays a factor in how tightly oxygen is bound to hemoglobin in the red blood cells; more tightly bound at higher pH like in the lung and less tightly bound in lower pH in the periphery. ",
"Systemically, the autonomic nervous system uses an interplay of neural networks across the body to regulate where blood is shunted on an organ level. Sympathetic drive is best under stood as the 'fight or flight response' and has the purpose of increasing heart rate, contractility, blood pressure, blood sugar, and oxygen uptake in the periphery. The parasympathetic drive is best thought of as a counter balance to the sympathetic nervous system. The two are in a constant balance maintaining homeostasis. ",
"Blood is pumped under pressure, and quite a bit of it too. Average blood pressure is 120/80 mmHg, which means the heart could squirt water about 5 feet straight up. You might run in to a little bit of trouble raising your arms above your head if your arms were 5 feet long. In fact, this is why those with acromegaly or gigantism end up dying of heart failure. It also important to note that veins are very low pressure, and rely on gravity or surrounding muscular contraction to bring the blood back to the heart. ",
"Finally, your blood vessel anatomy is evolved in such a way to function for your body. Valves in the veins prevent back flow of blood in the legs, and with each muscular contraction blood is squeezed upward back to to the heart. Every time you breathe, you create a negative pressure inside your chest, which literally sucks on the inferior vena cava like a straw to bring blood back to the heart. The Aorta is arched with branches coming off the top to go straight up to the head and upper extremities. The more in-line anatomy reduces pressure drop in getting the blood up to the brain and arms. ",
"When you stand on your head, you are forcing your circulatory system to function in a way it wasn't specifically evolved for. It still works, but it is less efficient than the normal physiology. "
] |
[
"The simple answer is that it maintains a more or less constant blood pressure. Since Pressure = Flow x Resistance, each individual organ can control its own blood flow by altering its resistance (at the level of the arterioles)"
] |
[
"You always have to think about which specific body part needs blood in a survival perspective.",
"I do physical evaluation tests, I bring people at their cardiovascular maximum. In order to give these tests, we learn how the body adapts during the effort. For example, the blood flow to the digestive system dimishes while the muscles get filled with blood. You can understand that if you were running away from a lion, you wouldn't want to spend energy digesting diner's steak, you would want the muscles to help you not become one.",
"Same goes for blood flow to the skin, it is related to the external temperature compared to your body's."
] |
[
"What scientific discovery do you think would help progress humanity as a whole the most? And why?"
] |
[
false
] | null |
[
"Grand unified theory, I would assume."
] |
[
"Aliens."
] |
[
"Nuclear fusion. An immediate and outcompeting \"screw you\" to polluting fossil fuel companies whose only claim to legitimacy is that we currently have no other option. I have to commute to work, and I'd love to not screw over the child I want by doing what I have to do to feed him."
] |
[
"Is gravity the strongest at the surface of the earth?"
] |
[
false
] |
My morning shower got me thinkin. If you are on the surface of the earth, you have a huge amount of mass underneath you pulling you down. Now let's say you dig a hole a few miles deep. You have less mass below you and now some mass above you effectively pulling you back up. All of this is speculation and was wondering if this is true. Would gravity be strongest on the surface of the earth?
|
[
"That would be true if the Earth was a sphere of uniform density, but since the core is much denser than the mantle and crust, gravity peaks at the barrier between the mantle and the outer core. ",
"Graphs for comparison.",
"You can think of this as the amount of less dense \"stuff\" that the mantle/crust adds doesn't compensate for the extra distance away from the very dense core."
] |
[
"As an addendum, this is not a unique or even uncommon characteristic for a planet. The most dense materials will tend towards the center of the planet thanks to gravity. The highest gravity on most planets (especially those with a fluid core) is going to be higher beneath the surface. "
] |
[
"The basic model you would see in most intro classes (see xenneract's graph, the constant density line) would peak at the surface of the planet. ",
"Why? Inside a spherical shell of constant density, the net force of gravity on a body is zero. If we treat a solid sphere as a bunch of concentric shells, the only mass that contributes to gravity is the mass from shells between the object and the center of mass. Doing the math (mass goes as density*volume= rho*4/3*pi*r",
", force goes as G*mass*mass/r",
" =bunch of constants*r), inside the planet, the force increases linearly with distance. Outside the planet, the mass is now set, and decreases as 1/r",
". ",
" The mass above you ",
" doesn't pull you back up. For constant density shells (which we're pretending the Earth is), there is no net force from the material above you, because you're ",
" those shells. There is less mass below you, which does decrease the force you'd experience.",
"In reality, there's a huge assumption in that model that's nowhere near true: the material in the Earth is constant in density. If density increases linearly as we go away from the center of the Earth, rho goes like a*r+b, so mass goes as a'*r",
"+b'*r",
", and the force would go like a''*r",
"+b''*r. If density decreases linearly as we go away from the center of the Earth, the leading coefficients (a, a', and a'') are all negative, and you get the linear density line on xenneract's graph. The latter case (decreasing density) is closer to reality. The most realistic model has different densities for different layers of the planet, based on various geophysical measurements (the density impacts propagation of seismic waves, so geologists have developed detailed models of the inner workings of the density of Earth), and lead to the PREM line in the graph."
] |
[
"Can someone give me the \"run through\" on diarrhea?"
] |
[
false
] |
While sitting on the john for the fourth time yesterday evening, the thought entered my head: What causes diarrhea? I know some general information (Large intestine usually should absorb water, but for some reason fails to), but was hoping someone would give me dirty details. Thanks.
|
[
"Some bacteria ( like C.diff) produce enterotoxins, which cause havoc with the cellular machinery of the epithelial layer in the gut. The end result being much more water being pumped out into the lumen of the intestine, causing looser stools. ",
"The cramps and explosive events are also caused by spasms in your intestine, as a response to infection, the desired result being flushing all the bacteria out of your gut. For the bacteria, it can be beneficial because they get to leave the body and find another host. "
] |
[
"It can also happen with certain infections when your body tries to force as much water back through your intestinal lining and speed up your peristaltic contractions (which hurts like shit, I might add) to flush it out as fast as possible.",
"That's why something like norovirus gives you high-pressure, crampy, liquid shits for so long."
] |
[
"If you want some general information why don't you just read the ",
"wiki article",
" on it?"
] |
[
"Do astronauts have internet in space? If they do, how fast is it?"
] |
[
false
] |
Wow front page. I thought this was a stupid question, but I guess that Redditors want to know that if they become a astronaut they can still reddit.
|
[
"They do, the ISS has it's own network with ",
"68 specially modified ThinkPad A31s and 32 ThinkPad T61ps",
" connected to the station's ",
"wireless down link",
" to the Earth via ",
"Ku band satellite relay",
". The speed is roughly equivalent to that of consumer DSL (10Mbps down, 3Mbps up) but with considerably higher ping.",
"\nAlso, ",
"Relevant XKCD"
] |
[
"Mostly thermal modifications, without gravity you don't get heat convection and so they need additional cooling to radiate heat away from the inside bits. They also had to be adapted to use the ISS's power grid, they don't use standard 110 or 220v AC up there, their main power is 28v DC. "
] |
[
"What kind of modifications have to be made to your standard ThinkPad to make it space-worthy?"
] |
[
"Can stress or intense fear actually cause your hair to turn grey?"
] |
[
false
] |
I feel this is a common pop culture concept (Futurama, Twin Peaks, etc) but I don't know if it is science based at all. Admittedly the examples I provided are both from non-serious shows, but I believe I've seen this concept in more reality-based media before. To clarify for those who asked, I meant "Can stress cause your hair to grow in grey in the fututre" as opposed to spontaneously switching colors.
|
[
"scientific american article on this ",
"http://www.scientificamerican.com/article.cfm?id=fact-or-fiction-stress-causes-gray-hair",
"Excerpt: ",
"Does stress accelerate this demise of the melanocyte population? \"It is not so simple,\" Fisher says, noting that the process of graying is a multivariable equation. Stress hormones may impact the survival and / or activity of melanocytes, but no clear link has been found between stress and gray hair.",
"NB Melanocytes are where melanin (brown pigment) is produced "
] |
[
"Trick question!\nWhen your hair goes grey, it's really actually white and just appears grey contextually. There's no such thing as ",
"grey hair",
"."
] |
[
"I can think of tonnes of anecdotal examples, but wonder if this isn't mostly coincidence due to the fact that people tend to reach the apex of their careers (and hence get in to high pressured, stressful situations) in their late middle age, which just happens to be the age when the grey starts for most. "
] |
[
"Why can't certain animals catch certain viruses?"
] |
[
false
] |
So the idea for this question came when I saw somewhere in the world they were using dogs to deliver beer to people quarantined in their homes. Some research on google showed me dogs, chickens and certain other aninals can't catch the coronavirus. Why is that?
|
[
"Viruses are essentially hyper complex organic compounds, they require a specific chemistry and conditions to do what they do. ",
"It's like asking why not every key can open every lock, only viruses are keys that destroy the lock once it opens creating more keys...",
"The simple explanation is that the viruses just didn't evolve to effect that species, it just doesn't have what it needs to affect the very specific chemistry of cells from other species... Unless a mutation allows it to later.",
"When a virus spontaneously mutates allowing it jump to another species, it's because this mutation has now been made compatible with this new species cells. Like if a chip in a non matching key made it match a lock it wasn't designed for."
] |
[
"Viruses have to become very very specialized in order to efficiently exploit the extremely complex cell machinery of a species. In addition viruses have to become very very efficient in avoiding a large number of barriers and countermeasures.\nIt is like a band of movie bankrobbers. They plan everything to the detail how to rob a specific bank. The plan would not really work in a completely different bank.",
"However, some viruses are actually capable of infecting multiple species but usually the infection is quite incomplete in other hosts than the primary.",
"In simpler words: viruses are almost perfect solutions to enourmosly complicated problems. It is rather unexpected when viruses actually can infect multiple organisms. Similar as when your key randomly fits the lock of a friend of yours..."
] |
[
"Ohh. Now I get it, thanks so much man. Btw I had always had trouble thinking why viruses were non living things since every layman is told germs cause you to get sick and you can kill germs, so I was puzzled if you can kill something, isn't it alive. Now it kinda makes sense. So basically can I think of virus the same way as a poisonous compound? I know this is a weird metaphor, but bear with me. Poison isn't something that multiplies in your body, it's just compounds that cause damage to your body, can we think of viruses as the same thing, the only difference being it's creating copies when it destroys a cell?"
] |
[
"How can a microSD card be so compact?"
] |
[
false
] |
[deleted]
|
[
"A standard microSD is 165mm",
" Subtracting a bit of area under pins and such, you're left with closer to 130mm",
" If you convert that straight to nm",
" you have 1.3*10",
" nm",
" A 32nm transistor takes up 1024nm",
" approximately, so just dividing that in, you can fit ~125 billion transistors on a chip. The reason you can't though is because you need wiring to connect the cells and a bit of other logic.",
"The reason intel and other companies can't put as many transistors in is they're running at speeds tens to hundreds of times faster than an SD card and the wiring involved is far more complex, both of which eat up die space and power.",
"Edit:: I guess I should answer the question, yes there are 86 billion transistors in them + a few thousands for clock/logic/addressing functionality."
] |
[
"This is an excellent response. I'll just point out that on a CPU the power consumption is only partially due to transistor switching. Just as important is that those switching transistors have to drive a signal through interconnect, and the resistance and capacitance of the interconnect is responsible for much of the power dissipation. (There's also a portion due to static leakage, but it is less than the other two factors.)"
] |
[
"This is an excellent response. I'll just point out that on a CPU the power consumption is only partially due to transistor switching. Just as important is that those switching transistors have to drive a signal through interconnect, and the resistance and capacitance of the interconnect is responsible for much of the power dissipation. (There's also a portion due to static leakage, but it is less than the other two factors.)"
] |
[
"My baby daughter puts anything and everything in her mouth...how is this behaviour so common when it seems like such a massive evolutionary disadvantage?"
] |
[
false
] |
Given that the vast majority of things are inedible, and potentially lethal through choking or toxicity how come babies are always trying to put things in to their mouths?
|
[
"Evidence is showing that exposing a baby to the environment (putting things in their mouths) allows the baby's immune system to develop appropriate to the environment. Putting the baby in a germ free environment ",
"seems to cause allergies in later life",
"This is going to hurt most parents, but studies are showing the healthiest adults were fairly sick babies and ",
". In other words, keeping babies healthy and disease free will cause all sorts of problems in their adult life as their immune systems do not develop appropriately. Most parents don't like hearing the best thing they can do for their children is let them get sick."
] |
[
"Yup, I've read that it could be seen as a \"sampling\" behaviour alongside the Hygiene Hypothesis",
"https://en.wikipedia.org/wiki/Hygiene_hypothesis"
] |
[
"Well most things are edible in that they can be consumed and won't poison you. Many, many things are not digestible but it's somewhat benign to swallow something that isn't digestible that you later pass.",
"Choking hazard aside, there's typically not a great number of things that are toxic in your environment. Many poisonous plants are toxic only in fairly substantial doses and many toxic plants are very bitter, so an infant would usually stop consuming them before they were at risk of fatally poisoning themselves. Alongside this if you live in an environment with acutely toxic things which an infant may come across you're likely to be fairly vigilant about what they are doing, so they have that extra layer of protection. With those things in place it's clearly somewhat more advantageous to explore objects in the world with your mouth than it isn't.",
" Or to put an evolutionary spin on it, clearly putting things in mouths isn't sufficiently fatal for it to have an impact on the evolution of humans, thus there is likely little evolutionary pressure selecting the behaviour out of the population. The corollary prediction here would be that it's not true that the vast majority of things infants put in their mouths could be lethal."
] |
[
"Magnets stick together one way and repel the other. If you had something that could clamp some big magnets together while they were trying to repel. Overtime is there any reaction?"
] |
[
false
] | null |
[
"Yes. The magnets would start to demagnetise each other. That is one of the ways we use to demagnetise permanent magnets, by placing them in an opposite magnetic field."
] |
[
"Wouldn't it be easier practically to heat up the magnet?"
] |
[
"Yes. Unless you want your magnets to have a certain strength, then using an external magnetic field would make it easier to control."
] |
[
"I've heard of antiparticles (positrons, antineutrino, etc) and antimatter but i'm still not sure if we have confirmed it's existence. Do they exist? If so, what is the process we went through to discover them?"
] |
[
false
] |
[deleted]
|
[
"Yes, antiparticles have been confirmed to exist. They are observed in a wide variety of particle physics measurements, usually as a by-product or intermediate step in a physical process being studied. One extremely common antiparticle is the ",
"positron",
", if you want more detailed information reading the wiki page should give you a good overview."
] |
[
"As a continuation of that, there's even a good use for the properties of positrons. OP may want to check out ",
"PET scans",
"."
] |
[
"You're welcome, though I am very surprised that a professor teaching quantum mechanics would not be aware of the regular observations of antimatter in experiment. Was there some more technical question being discussed that wasn't covered here?"
] |
[
"If you received a blood transfusion from someone with blood cancer, would you get blood cancer?"
] |
[
false
] | null |
[
"It's very, very unlikely. While there are a ",
"few famous case reports of cancer being transmitted",
" from person to person by something like an accidental needle sticks, cancer risk for blood transfusion is probably not an issue.",
"For example, this large study of almost 350,000 transfusions showed only 3% of blood recipients even received blood from donors that may have had cancer at the time of donation. Among those 12,000 recipients, ",
"there was no increased risk of cancer",
"."
] |
[
"It is unlikely for a few reasons:",
"1) your immune system is much more likely to prevail against the few immune cells in a transfusion of whole blood. This is why Tasmanian devils can give STD cancers (they lack alloimmunity)",
"2) many blood transfusion products have been pre-treated to prevent complications (namely graft vs host). This includes irradiation to halt division of cells and removal of WBCs.",
"Technically, any cancer could be present in the blood. But this doesn't mean it is likely to be a problem for transfusions. The exception to this is in immunocompromised people"
] |
[
"Donated blood is separated into its constituent parts by centrifugation. The red blood cells that pellet to the bottom and the plasma that rises to the top are used for patients, but the lymphocytes, which migrate to an oily looking layer in between called the \"buffy coat\" are discarded or used for research. A few cells may fail to migrate out of the clinically usable layers, but if these cells are malignant, they are extremely unlikely to establish cancer unless the recipient is immunocompromised, because the cells will be recognized as \"non-self\" if they're still expressing their major histocompatibility complexes, and if they're not, they're likely to be killed by natural killer cells in a healthy recipient. ",
"Protocols for experimental graft cancers typically require special strains of immunocompromised mice because normal mice will just reject the graft."
] |
[
"When something painful happens in a dream, what is happening when you wake up? Example inside"
] |
[
false
] |
So I just had an absolutely CRAZY dream. is a link to it because I don't want to explain it again in that great of detail. In said dream, I lose my middle finger and the tip of my index finger on my left hand. They while I was dreaming, and after waking up, they continued to hurt for a little bit. Now, approximately 30 minutes later, my middle finger feels stiff and the tip of my index finger just doesn't feel right somehow. My middle feels like it hasn't been used for a while somehow, even though I was dreaming for only an hour tops (actually much less than that because REM cycles are so short). Why and how does this happen? It is not the first time that I have had residual pain from dreams. Does this happen to other people as well?
|
[
"Just last week I was dreaming my leg was in pain and when I tried to stand up, my feet couldn't touch the ground because I was levitating. I woke up immediately and realized I had a cramp in that leg and walked it out.",
"My point being that our mind recognizes pain that occurs while we are asleep and blends it into the dream. It wasn't the dream that caused the pain, it was the pain that impacted the dream."
] |
[
"From my experience, pain in dreams is the direct result of a pain stimulus in actuality (like rolling on to painful surface while sleeping). I've had dreams where I've been stabbed and shot, but they didn't hurt because no actual stimulus was attached to them."
] |
[
"Pain is partly a construct of the brain and partly actual pain. So, the pain you felt in your dream could be either of the two cases - ",
"you were actually hurting your fingers and the dream incorporated it.",
"your brain physically manifested the pain from your dream.",
"For ex. ",
"http://www.ncbi.nlm.nih.gov/pubmed/12405613?dopt=Abstract"
] |
[
"How big are nebulae, what causes them, and what do they form later on?"
] |
[
false
] |
I'm just really curious to know about nebulae, and as the topic says, I'm trying to grasp a few basic concepts about them. Any additional information you can provide about them would be helpful! Also, do all stars become nebulae, and are all nebulae formed of stars?
|
[
"Not a full answer to your question, but it should be pointed out that there are sources of nebula other than stars.",
"\nIn the early universe there was only nebula, and remnants of that form the largest known nebula, the size of galaxy clusters: ",
"http://en.wikipedia.org/wiki/Lyman-alpha_blob_1",
"."
] |
[
"Nebulae do come in all different sizes, and even their creations vary. There are those created by stelar explosions (i.e. the Crab Nebula). These are created when medium to large stars (eventually even our star) end their lives. When the stars explode, their debris is scattered around. You may have heard of the as \"clouds\" but know that these are essentially a denser concentrations of gases, mainly hydrogen, helium, and small amounts of heavier elements than most of space. However, they are still very sparse. To get a sense of how much matter is really in these nebulae, the amount of gas in a volume the size of Earth would weigh about a kilogram. In the cases of bigger nebulae, they are formed through the gravitational attraction of the interstellar medium, the matter in between stars. Remember, space is not a total vacuum so gas and plasma can become attracted together to form these structures. Some of these include the Eagle Nebula or even the monstrous Tarantula Nebula. And now, you may have heard that these are often referred to as stelar nurseries. The gravity between the gases could eventually become so great that they begin to orbit each other more and more. As they do, their speeds become greater as more gas is being pulled in. Soon, so much mass has acquired and the temperature due to its motion inside the primitive star so great that it has enough energy to induce nuclear fusion, combining hydrogen nuclei to form helium. Thus, a star is born. Multiple stars can be born in the same nebula. However, not all nebulae do form stars. Planetary nebula, such as the Carb Nebula I mentioned above will slowly disperse as there isn't sufficient mass to keep it together for long. Finally, nebulae can range from less than a light year to ~650 light years. They are very diverse. Hope this helps."
] |
[
"formed through the gravitational attraction of the interstellar medium, the matter in between stars",
"Is this matter which is present in what we call empty space? Like empty space between Earth and any other planet?\nAlso, what causes them to attract if the matter is just spread out? ",
"Also, great answer."
] |
[
"Why don't we just keep probes in orbit around objects, instead of doing fly-bys?"
] |
[
false
] |
We have Cassini-Huygens orbiting Saturn right now, couldn't we have done this with other probes? New Horizons is going to Pluto, why not just keep it there to do science for a while?
|
[
"Two reasons, really: ",
"One is that in order to get to Pluto in a tolerable amount of time (~10 years) NH has to move very fast, using a gravitational slingshot. This means that if we wanted to slow it down to orbit Pluto, we'd have to expend a huge amount of propellent (which we don't have) in order to get rid of all this spare kinetic energy in time to go into orbit around Pluto. ",
"The second reasons is a little more technical. Pluto is tiny, with a very small gravity well into which we'd want to drop our spacecraft. In order to avoid overshooting, we'd have to start decelerating essentially immediately after slingshot to the planet, thus burning a lot more propellent and extending the duration of the mission substantially. ",
"In the end, it comes down to tradeoffs. Do you bump up the cost of the mission hugely for these more complicated propellent systems, or do you fire a leaner, cheaper, faster system and try to maximize what you can get as you pass by? In the end, the decision came down on the side of the latter with NH. "
] |
[
"Money. ",
"It takes way more delta-v (fuel) to deliver a probe into orbit than it does to do a flyby. Especially when you are sending crap all the way out to ",
" Pluto. To establish orbit around a body, you need to begin decelerating once you are out there. This takes fuel. To get the fuel out there, you need to get that fuel into orbit, which requires a BIGGER launch vehicle. And I think NH already used one of the largest launch vehicles at the time just for the flyby (Atlas V)... so if you wanted to get put it in orbit, you would need to spend all that money to make a new, one of a kind launch vehicle - just for NH. "
] |
[
"The USSR launched about 40 reactors in orbit and the US launched one"
] |
[
"Why can i pick up AM stations from long distances at night?"
] |
[
false
] |
I'm in Alabama, last night in my car I picked up a talk radio station from Cleveland Ohio and listened for a little while. What factors cause those radio waves to travel so far?
|
[
"\"During the day, MF (medium frequency) signals travel by groundwave, diffracting around the curve of the earth over a distance up to a few hundred miles (or kilometers) from the signal transmitter. However, after sunset, changes in the ionosphere cause MF signals to travel by skywave, enabling radio stations to be heard much farther from their point of origin than is normal during the day.\"",
"Skywave just refers to the propagation of radio waves reflected back from the ionosphere. Since the ionosphere is not limited by the curvature of the earth "
] |
[
"AM stands for Amplitude Modulation. FM stands for Frequency Modulation. AM radio waves tavel through the ground and the air. FM radio waves only travel through the air. That's why a mountain (or a long tunnel) will block an FM signal but not an AM one. FM is \"line of sight\" transmission."
] |
[
"Read the day/night info about ",
"the ionosphere",
"."
] |
[
"How did multicellular organisms originate?"
] |
[
false
] |
[deleted]
|
[
"It makes sense, doesn't it? Basic symbiosis -- two cells together have a better chance of filling both of their metabolic needs than both on their own.",
"There are a few factors that limit cell size. Most notably, the ability of a cell to intake nutrients through its surface area increases as the square of its radius, whereas its volume (that is, its size and thus how much nutrition it requires to survive) increases as the cube. If you increase the radius too much you become incapable of acquiring enough nutrition to support the cell, and it dies (is not evolutionarily selected for.)",
"So, if we want to optimize the process, we would find the maximum volume a single cell could reach, and then in order to become more \"fit\" to survive, cells would increase the total surface area through which they can receive nutrients by working together instead of by growing. This is simply because when you add another cell you double your surface area and double your metabolic need, as opposed to doubling your surface area and increasing your metabolic need by more than double.",
"This process goes on and on, and suddenly you have multicellular organisms."
] |
[
"Something to consider - bacteria are known to communicate with one another, ranging from ",
"quorum sensing",
" to forming ",
"biofilms",
". Also, for example, cellular differentiation is observed with cyanobacterial ",
"heterocysts",
". The delineation isn't quite as sharp as one might initially suspect, I'd say. "
] |
[
"Another example of where drawing the line between unicelluar and multicellular is blurred: the group Myxogastria. ",
"http://en.wikipedia.org/wiki/Myxogastria",
"tl;dr lives freely as single cells, then congregate to form large reproductive structure which produces spores."
] |
[
"Why aren't fish as large as whales?"
] |
[
false
] |
[deleted]
|
[
"I suspect that it has to do with a couple of factors, including:",
"A cursory internet search suggests that there isn't quite enough research to ascertain which of these factors is most important, but it's likely that they all play a role.",
"Consider the reverse question; why is the ",
"smallest marine mammal",
" still somewhat large, when there are so many smaller fish, as small as ",
"a few millimeters",
"?"
] |
[
"Some of them are as large as whales, but whales come in different sizes well ( many of which are actually fairly small ). None of them as big as a blue whale, the biggest of whales. But the Whale Shark and also the Basking Shark can be in found in the 25-42 feet range typically. That's on par medium whales. Though supposedly some of them can get up to 65 feet, no one's recorded measurement of one of that length.",
"Other fish can measure up similar lengths but typically have the same height and width dimensions."
] |
[
"So guess my question is more why is the biggest organism in the ocean a mammal when fish are more diverse and populous? "
] |
[
"Does NASA or any other space organisation have satellites orbiting other planets? if not, why not?"
] |
[
false
] | null |
[
"Don't forget Venus, Ceres, the moon, and the comet 67P. Also, the MESSENGER probe around Mercury just recently concluded operations and is no longer in orbit."
] |
[
"There are scientific probes orbiting Mars, Saturn, and Mercury."
] |
[
"Absolutely - ",
"here's",
" a list on Wikipedia of all spacecraft that ever orbited planets other than Earth. Perhaps the most notable one today is Cassini, a probe that's been orbiting Saturn since 2004. It is responsible for many of the beautiful photographs of Saturn and its moons."
] |
[
"What is the effect of impurities on boiling point?"
] |
[
false
] |
My understanding is that the boiling point of a mixture is lower than the boiling point of its constituent compounds (although I could be wrong). Why is this? Do the boiling points of the compounds have to be different for this to work?
|
[
" This is exactly what I was looking for! Thank you!"
] |
[
"Actually it's ",
"boiling-point ",
"."
] |
[
"Epistaxis is correct about boiling-point elevation, but another effect which has to do with liquids only. The boiling point for mixtures can either increase or decrease depending on the intermolecular forces between the components.",
"Here's some phase",
" diagrams of mixtures where the boiling point of the mixture is actually lower than the boiling point of either pure liquid.",
"One way to think about this is that as the mixture becomes less pure in either liquid there are repulsive forces between the molecules. The system is of a higher energy and as such require less thermal energy to vaporize or a smaller pressure drop to vaporize.",
"This is the case with water and ethanol mixtures where the boiling point of the azeotrope (lowest point in the diagram) is actually lower than both the boiling points of pure water or ethanol. Conversely you need a higher pressure to keep them liquid at a given temperature.",
"However, the exact opposite effect also exists. Here's a ",
"phase diagram",
" where the boiling point is actually higher than either the pure liquids. This is because of attractive forces between the molecules in the liquid phase.",
"Now these two diagrams require completely mixed and single phase liquids. For things like oil and water which only mix for ",
" concentrations, this graph would be a lot more complicated because it'd include multiple liquid phases, but the idea is the same.",
"So mixtures can either raise or lower the boiling points or do all sorts of things in between. It depends on what liquids you're dealing with and how they interact which each other and in which compositions."
] |
[
"Are there any materials with variable weight or mass? Can anything be done to achieve this?"
] |
[
false
] |
I saw a funny picture on the internet here: , and it got me thinking. Is there any naturally occurring substance that has variable weight? (I now realize mass is probably hard to change.) Are there ways to change the weight of a material such that, if it was placed on this picture, the change in weight would truly generated perpetual motion?
|
[
"The simple answer is no. The accurate answer is, a little. All matter is subject to ",
"conservation of mass",
". However, ina non-closed system, you could insert energy, which increases mass by m=E/c",
" The reverse reaction happens in nuclear fission: mass is reduced and energy is dissipated."
] |
[
"Anodes and cathodes (eg: in batteries) can change mass, but that mass has to be added or subtracted from the surroundings. I believe it also requires net electrical energy input in order to get a cycle happening as per your diagram.",
"Perpetual motion machines are impossible. Those (proposed) that involve changing mass to energy and back again usually neglect slight redshifting of shining the light back up again against gravity."
] |
[
"Are there ways to change the weight of a material ",
"I'm not sure if this counts:",
"Wikipedia: ",
"Woodward effect",
"As I understand it (my understanding may be wrong) the Mach Effect or Woodward effect theorises that when electrical energy is temporarily stored in a capacitor or inductor of a tuned circuit the mass of that capacitor or inductor is changed according to the equation E = mc",
" where m is the change in mass, E is the energy stored and c is the speed of light.",
"If this effect is real it could potentially be harnessed to generate a thrust from electrical energy without using ",
"reaction mass",
".",
"Experiments designed to test if this effect is real",
" have so far been inconclusive. Wikipedia: "
] |
[
"Friend who has a concussion just told me that she can't sleep for longer than 2 hour intervals, or she might die. Is this true? Can someone explain?"
] |
[
false
] | null |
[
"We can't really comment on anecdotes / isolated incidents without resorting to speculation which we try to avoid."
] |
[
"That's fair. In general, is this a common problem associated with concussions though? Or does it have a name?"
] |
[
"I recommend posting such questions to ",
"/r/askdocs",
" instead"
] |
[
"Is it possible that there are beneficial contagions, like in that episode of Red Dwarf?"
] |
[
false
] |
It stands to reason that happier, healthier people are more likely to, , exchange fluids with other people. A well adapted, hard to detect organism would at least have a good evolutionary reason to take care of their course. Of course everyone knows there's beneficial bacteria in our bodies, but is it possible these symbiotes could be affecting things beyond our digestive tract, such as mood, general health or even sexual attractiveness. Also: Can our white blood cells differentiate bacteria, or are all foreign bodies treated equally? How much is the discomfort of sickness caused by our immune response and how much is caused by the disease?
|
[
"Absolutely a possibility. They appear to have an effect on ",
"fly mating preference",
".",
"So far it's been hard to study (because of the diversity of the bacteria, the fact that many bacteria don't grow in lab conditions, etc), but bacteria have been known to produce vitamins and hormones that our bodies respond to, so there's no reason that they couldn't cause major changes to our fitness."
] |
[
"People with Sickle Cell Anemia are immune to malaria. I know this isn't a contagion, but it's an example of an advantageous disadvantage."
] |
[
"Brundlefly",
" upvotes!"
] |
[
"What would happen if a water-tight but not air-tight container filled with air was submerged in water?"
] |
[
false
] |
The first question is whether the container is rigid. If it's not then the container would contract until the pressure is equal inside and outside of the container, and I think this would be enough for most or all of the gases to dissolve in the water, depending on the pressure and depth. I'm not sure what would happen in a rigid container, with atmospheric pressure inside the container, perhaps only the carbon dioxide would dissolve. Please correct me and / or explain why I'm right or wrong. I don't think there would be any real differences if the media were switched, could you tell me though?
|
[
"If it is a rigid container nothing would happen as water can't get it. Air can get out but the only way that would happen is if the pressure inside the container is greater which won't happen if the container is submerged. ",
"If the container isn't rigid then as the pressure increases it would force the container to collapse and then push the air out till the inside volume reaches zero. Although I don't understand how you would have a material that is water-tight but not air-tight while not being rigid as well. "
] |
[
"Although I don't understand how you would have a material that is water-tight but not air-tight while not being rigid as well.",
"It would be like a rubber balloon made out of a porous material with gaps big enough for N2/O2/CO2 molecules but not for H2O. "
] |
[
"I'm sorry, but your understanding on this issue is incorrect. what matters is the partial pressures. also, Gore-Tex is a material that is water tight but not air tight while not being rigid. There are a variety of other materials that can do this as well, many of them are used in the making of coats."
] |
[
"If it is now possible to reconstruct visual information in the brain using fMRI, can we record dreams?"
] |
[
false
] |
Recently, there was a link posted on that showed the reconstruction of images from a person's brain using fMRI. I was wondering if this technology means we could also reconstruct the visual activity during REM sleep. From this YouTube video: The left clip is a segment of the movie that the subject viewed while in the magnet. The right clip shows the reconstruction of this movie from brain activity measured using fMRI. The reconstruction was obtained using only each subject's brain activity and a library of 18 million seconds of random YouTube video. (In brief, the algorithm processes each of the 18 million clips through a model of each individual brain, and identifies the clips that would likely have produced brain activity as similar to the measured brain activity as possible. The clips used to fit the model, those used to test the model and those used to reconstruct the stimulus were entirely separate.) Brain activity was sampled every one second, and each one-second section of the viewed movie was reconstructed separately. Here is the relevant paper:
|
[
"Hey - I actually work in the lab that did this work, and though I was not an author on this paper I know the study extremely well, so I think I can clear some things up.",
"1) Reconstruction of the images are from the visual system. Not whole brain, though I believe, the whole brain data was used (I'll have to double check).",
"This was not done using whole brain data. Whole brain coverage was sacrificed in the interest of image resolution and sampling rate (i.e. we need a sampling rate of 0.5Hz to get whole brain images, but this study was done with 1Hz images.. in practice this doesn't make a huge difference because of the temporal low-pass properties of the BOLD signal, but anyway..). The images used in this study covered what is usually considered visual cortex: V1-V4, LO, MT, and their surrounds.",
"fMRI is blood flow",
"Sure it is, but I don't really see how that's relevant. If anything it makes the result more impressive.",
"Now, this isn't exactly a \"reconstruction\". This is an extremely complex statistical process to build back approximations by knowing a few things, due to the experimental design:\n1) The researchers know exactly what pixels are being watched at exactly what time during the experiment.\n2) The sampling happened over hours by a small set of people. This may not generalize at all.\n3) Reconstruction occurs through knowing pixels at give times and voxels (bloody brain pixel-cubes) at given times. Then match the two, basically.",
"I'm not really sure what you're saying here. Of course it's reconstruction! We have an algorithm that, given a series of fMRI images of the brain, generates an estimate of the video the subject was watching. This algorithm is tailored to the specific subject, but it does not require prior knowledge of the video that is being reconstructed.",
"The algorithm is built using a large set of data for which we ",
" know both the stimulus and the response. But the actual reconstruction is done with zero knowledge of the stimulus.",
"You're right that it's very difficult to generalize the models from person to person. Simple anatomical mapping from one brain to another is all but worthless for this purpose. More complex functional mappings may make this problem much more tractable, however. (If you're a Haxby-ite, you may be very aware of such methods!)",
"I was wondering if this technology means we could also reconstruct the visual activity during REM sleep.",
"So, to answer that question no one knows. Except I do know that this same group is working on that exact idea.",
"I'll add that it's a hard problem. It's unclear to what degree \"seeing\" in dreams actually manifests as activity in early visual cortex. Visual dream reconstruction may prove easy, or may be entirely impossible, it's just too early to tell."
] |
[
"Hey - I actually work in the lab that did this work, and though I was not an author on this paper I know the study extremely well, so I think I can clear some things up.",
"1) Reconstruction of the images are from the visual system. Not whole brain, though I believe, the whole brain data was used (I'll have to double check).",
"This was not done using whole brain data. Whole brain coverage was sacrificed in the interest of image resolution and sampling rate (i.e. we need a sampling rate of 0.5Hz to get whole brain images, but this study was done with 1Hz images.. in practice this doesn't make a huge difference because of the temporal low-pass properties of the BOLD signal, but anyway..). The images used in this study covered what is usually considered visual cortex: V1-V4, LO, MT, and their surrounds.",
"fMRI is blood flow",
"Sure it is, but I don't really see how that's relevant. If anything it makes the result more impressive.",
"Now, this isn't exactly a \"reconstruction\". This is an extremely complex statistical process to build back approximations by knowing a few things, due to the experimental design:\n1) The researchers know exactly what pixels are being watched at exactly what time during the experiment.\n2) The sampling happened over hours by a small set of people. This may not generalize at all.\n3) Reconstruction occurs through knowing pixels at give times and voxels (bloody brain pixel-cubes) at given times. Then match the two, basically.",
"I'm not really sure what you're saying here. Of course it's reconstruction! We have an algorithm that, given a series of fMRI images of the brain, generates an estimate of the video the subject was watching. This algorithm is tailored to the specific subject, but it does not require prior knowledge of the video that is being reconstructed.",
"The algorithm is built using a large set of data for which we ",
" know both the stimulus and the response. But the actual reconstruction is done with zero knowledge of the stimulus.",
"You're right that it's very difficult to generalize the models from person to person. Simple anatomical mapping from one brain to another is all but worthless for this purpose. More complex functional mappings may make this problem much more tractable, however. (If you're a Haxby-ite, you may be very aware of such methods!)",
"I was wondering if this technology means we could also reconstruct the visual activity during REM sleep.",
"So, to answer that question no one knows. Except I do know that this same group is working on that exact idea.",
"I'll add that it's a hard problem. It's unclear to what degree \"seeing\" in dreams actually manifests as activity in early visual cortex. Visual dream reconstruction may prove easy, or may be entirely impossible, it's just too early to tell."
] |
[
"I'll answer this by addressing what I think is a misconception. This study did not do \"pattern matching\" in the way that I think you imply. Let me start by saying what I think you're saying, and then I'll say what actually happened. So what I think you're saying is this: to reconstruct a video from a brain image one would go to a database of known brain image-video pairs, find the closest brain images to the known brain image, and then combine those to get the video. What actually happened in this study is a bit more nuanced.",
"We started out with a big set (2 hours) of training data: videos and their corresponding brain images. Just using pixel values from the videos to predict brain data would be silly (we know the visual cortex doesn't work that way) so the videos were first ",
" into a representation that we think is more like what the brain is doing. The particular transformation used for this study (a motion energy Gabor filter) seems to capture a lot of what the brain is doing, so it lets us build very accurate models. So then we can build a model that predicts the brain activity given the Gabor-transformed video.",
"Now here's the problem: the motion energy Gabor transformation is not even remotely invertible. So given the Gabor transformation of a video clip, it's impossible to exactly recover the original video clip. This is great for modeling the brain because (at a gross level) the brain is invariant to the same things as the transform. But it makes reconstruction a bitch.",
"So what do we do? Using the model, we can quite accurately predict how the brain will respond to a given video, but how do we use that to reconstruct an actual video? Here's where the 18 million seconds of youtube videos come in! To reconstruct a second of video from a particular brain image, we run each of the 18 million seconds of youtube video through the model and get the predicted response. Then we take the few seconds of video whose predicted responses best matched the actual response and average them together.",
"So it's not a matter of pattern matching, really. It's a matter of inverting the transformation that we put the videos through to predict brain activity. You wouldn't need a database of every possible image to reconstruct something that you hadn't seen before as long as you can mix images together."
] |
[
"Why do I always see the same repeating pattern when I close my eyes?"
] |
[
false
] |
[deleted]
|
[
"It is most likely a form of closed eye halucination (can be seen even with eyes open). I don't know the mechanism, but I get them as well.",
"http://en.wikipedia.org/wiki/Closed-eye_hallucination"
] |
[
"Those might be ",
"phosphenes."
] |
[
"Floaters",
" perhaps?"
] |
[
"Are nootropic drugs a scam?"
] |
[
false
] |
I've been hearing a lot of about these; the companies pushing them seem to be very good at marketing, hiring celebrities to give testimonials and the whole kaboodle. But is there any scientific evidence proving they can make a real difference, or is it all smoke and mirrors? I'm also wary of anything that's not a controlled substance... if it worked so well, why aren't pharmas picking them up? Can anyone give the lowdown?
|
[
"Nootropics aren't a real drug class. They are a sort of made up term for things that ",
" help cognitive function. ",
"The things on onnit.com are supplements/nutraceuticals that are branded in a way that are more brain health-focused. I'm not saying they have no science to back them up, but the science they show are not efficacy studies.",
"For example, the studies they cite may say...\"l-tryptophan levels are linked to depression\". Whereas the company says, \"we put lots of l-tryptophan in our pills as it's proven to have a cognitive benefit\". It's very misleading. And they do this often.",
"Unless you see a study that clearly states that a significant amount of people had a benefit in the parameter they were testing from the \"drug\" vs. placebo, then the study doesn't verify anything. Every real drug has to go through the FDA's clinical trials where safety and efficacy vs placebo has to be shown. Health supplements don't have to do this."
] |
[
"If you're looking for things that make you visibly smarter, then you probably won't find them. There are substances that are harmless enough and, like HoboZoo said, ",
" help, but it won't be something out of a science fiction movie. Research and healthy skepticism is probably most useful at this point, as there isn't a resource that will tell you reliably \"this will work, this won't\".",
"From what I've read over time:",
"racetams (piracetam etc) may help some with intelligence",
"eating and sleeping well has about the same effect, and it's more reliable. ",
"there are more pills that affect concentration and motivation, and they are a lot more reliable. Caffeine, L-Theanine (tea), kava-kava, modafinil (provigil), adderall are pretty well-tested. There are also amphetamines etc, but side-effects are usually not worth it.",
"again, good sleeping and eating habits do about as well long term. In my personal experience, meditation works about as well as modafinil and with a lot less side-effects",
"some substances affect mood. Alcohol and cannabis of course, but there's also St John's Wort which works reasonably well on depression. Look and you may find others. ",
"But again, trick is to look not at sites selling them, but at peer-reviewed studies, and always read three times the side-effects section."
] |
[
"Yeah that site is mostly bullshit and regardless a huge waste of money. If you do the research yourself though (or check out ",
"/r/nootropics",
") there are substances which have no known negative affect and have been shown to promote brain function. Again caffeine itself is the basic example."
] |
[
"Can someone explain the relationship between frequency, wavelength, voltage and energy across a band gap?"
] |
[
false
] |
I'm working on a lesson (student teaching[not physics]) and physics is absolutely not my forte. My goal is to have students take voltage across a resistor connected to a photodiode that has an LED shining light into it. Obviously there will be a different voltage depending on the wavelength of the light, but I'm not sure if graphing it this way makes sense. Can we assume that, for example, a red LED is hitting the diode with the same number of photons as an ultraviolet LED, but the energy within that photon is different, or are we seeing more photons hit thereby increasing said voltage? Ultimately what I would like to suggest is that shorter wavelengths of light have higher energy, but I don't know if I'm comparing apples to oranges. Any input would be greatly appreciated.
|
[
"I don't think you're going to get the results you want. While there is a relationship between light frequency and energy, the photodiode will not respond linearly to this. ",
"http://en.wikipedia.org/wiki/Photodiode#/media/File:Response_silicon_photodiode.svg",
"The 2nd problem you'll have is that a bunch of different coloured LEDs won't necessarily put out the same amount of light, and you won't be able to say that the same number of photons are being produced."
] |
[
"There's a reason that it took people a long time to work these things out.",
"Generally speaking, the voltage that the LED puts out is related to the color, and the current is related to the intensity of the light, but LEDs aren't exactly simple devices. (If you're confused about how LEDs work, how are you planning to handle questions from the students if they ask about that?)",
"The thing that compelled people to think that photons of a particular wavelength have a particular energy is probably Einstein's explanation of the photoelectric effect, but I'm not sure that's the sort of equipment that you\ncan hand to students to use in a lab."
] |
[
"The energy per photon depends on the frequency according to ",
"E = h nu ",
"where E is the energy per photon, h is Planck's constant, nu is the emission frequency.",
"The wavelength is related to frequency by",
"nu = c / lambda",
"where c is the speed of light and lambda is the wavelength. So you could also write",
"E = h c / lambda.",
"So that is all about the energy per photon. However (to simplify things a little bit) if you drive the LED with more current it will produce more photons. The more photons that strike the photodiode the more current it produces. ",
"And the more current produced by your photodiode, the more voltage will be developed across the series resistor.",
"shorter wavelengths of light have higher energy",
"Shorter wavelengths have higher energy per photon. But that means if the total energy is kept constant, the number of photons is lower for the shorter wavelength source. So the photodiode will respond with ",
" output current.",
"(Up to a point. If the wavelength gets too long, and the photon energy gets too low, then there won't be enough energy in a single photon to excite an electron into the conduction band of the photodiode, so the response will drop dramatically when the photon energy goes lower than the band gap energy)",
"Here's a graph on Wikipedia that shows the typical behavior:\n",
"http://en.wikipedia.org/wiki/Photodiode#/media/File:Response_silicon_photodiode.svg"
] |
[
"Could someone please attempt to explain entropy to me in a fluid dynamics context?"
] |
[
false
] |
I have recently started a job as an aerothermal engineer and have had trouble understanding exactly what entropy is in a physical sense. I've encountered it in a few cases, for example, viewing entropy plots to locate pressure shocks over aerofoils, but I can't grasp exactly what it is.
|
[
"Entropy (both in computer science and physics) is a measure of the number of possible states of a system.",
"Creating entropy somewhere means you have taken a certain path in reality, out of a great number of others. The greater the entropy increase, the greater the number of possible states your system can have. The sheer number of these new potential other states basically makes it impossible to go back to a point where the number of states were comparatively hugely smaller. Creating entropy means there is no going back in time.",
"Entropy can also be seen as a measure of disorder: the more the possible states of a system, the more you'll see random behavior appear.",
"When your fluid is laminar, little entropy is being created: your fluid has a very low number of ways to evolve. When your fluid is not laminar, when there are pressure shocks, a great number of possible states are being created. And you're likely to see turbulences.",
"When plotting entropy, you are basically viewing where the action takes place (where a single reality is chosen over a great number of potential states)."
] |
[
"A thought experiment:",
"You have two thermally isolated volumes of regular air separated by a a magical membrane.",
"Initially, the two volumes are at thermodynamic equilibrium; that is, they have the same concentration of each type of gas on both sides of the membrane, at the same pressure and temperature. This is a \"disordered,\" \"high-entropy\" configuration.",
"Now, I spend energy, and using my magic membrane, I make the two volumes different. Maybe I remove all the air from one volume and put it all in the other one. Now the system is \"more ordered\" and is a \"low entropy\" system.",
"Note that the \"low entropy\" scenario is less energetically favorable. I have to expend energy to separate out the air, and if I \"turn off\" my magical selective membrane, we would expect the system to return to a low energy, high-entropy mixed state."
] |
[
"High entropy means that a lot of microstates can give rise to the same macrostate, while low entropy means that the number of microstates giving rise to the same macrostate is small. To take the (probably faulty) example of sand, a beach is high in entropy, because the exact location and orientation of the sand grains doesn't matter as much for the observed macrostate. A sand castle is lower in entropy because the locations of the sand grains making up the castle is much more restricted."
] |
[
"Does your brain go to a physical place when you think?"
] |
[
false
] | null |
[
"I'm not sure what you mean. Can you rephrase the question? Are you asking if you imagine being in some other place?"
] |
[
"Like when you think of anything, does your brain sometimes take you to a physical setting that you’ve visited before to think about whatever you were going to think about?"
] |
[
"I'm still not quite sure what you mean. Like if you are thinking about something that happened on your front steps do you go there to think about the event?",
"There's nothing explicit like that, but it is the case that being in the same environment where you learned something makes it easier to remember (",
"Smith and Vela 2001",
" <- pdf)."
] |
[
"Was there gravity before, and during, the Big Bang?"
] |
[
false
] |
I've been going down the rabbit-hole of the FAQ and the answers on the Big Bang seem to all mention that the Big Bang happened everywhere and with infinite matter. I'm having a hard time understanding if all matter was just a hot plasma before the Big Bang, why it didn't collapse due to gravity? Why didn't the universe turn into a giant black hole?
|
[
"There probably wasn't gravity as we know it during the first moments after the big bang. At high energies, fundamental forces cannot be described as separate interactions -- for example, shortly after the big bang, the electroweak interaction split into the electromagnetic and weak interactions. During the ",
"Planck epoch",
", the first 10^-43 seconds following the big bang, we think that gravity was probably unified with the unified force (EM+weak+strong interactions), but we don't yet have a sufficiently descriptive theory of quantum gravity to deal with the associated energies."
] |
[
"Also, the inflationary period happened immediately after the separation of gravity from the other forces. This distributed the mass of the universe so that it became smooth and homogeneous. This also means that the mass of the universe was spread out enough so that gravity became significantly weaker than the other forces.",
"Correct me if I'm wrong, just finishing undergrad."
] |
[
"My current understanding is that it ",
" infinite but we don't have any proof or proving it is impossible. We can't detect any curvature to the universe but if it's big enough maybe we just aren't using sensitive enough means."
] |
[
"What dictates how big our atmosphere is?"
] |
[
false
] |
I tried searching and found some interesting topics, but nothing about this actual question. So what dictates how big our atmosphere is? Is it random? Is there some set proportion for all planets? Thanks!
|
[
"there are some questions about this. ",
"Venus lacks a significant magnetic field",
", is closer to the sun, but has significant atmosphere."
] |
[
"Planetary mass (gravity) and magnetic field. The magnetic field deflects incoming solar wind, which will strip away the atmosphere, as happens with ",
"Mars",
"."
] |
[
"One thing to keep in mind, the gasses that get 'sucked' in by gravity aren't always able to stick around. Earth is too small to support the atmosphere it came with (Hydrogen and Helium) so they bled off into space.",
"Thankfully, volcanoes decided to spew out massive amounts of CO2, which worked out pretty well for our atmosphere.",
"But then some jerks called Cyanobacteria figured out a neat trick where they could break the carbon out of CO2 and combine it with photons to make sugar, so they started polluting the world with unusable O2.",
"Joke's on them though, we figured out how to ",
" the stuff."
] |
[
"How quickly do humans lose weight when fasting?"
] |
[
false
] |
I realize there is a lot of variability in humans and what "fasting" means. Make whatever assumptions you want to facilitate an interesting answer. Thanks.
|
[
"You ",
" lose a good deal of water weight and some lean mass as well.",
"15lbs of fat can easily be 25-35lbs scale weight."
] |
[
"A rule of thumb is that to gain or lose 1 pound of fat is the equivalent of ingesting or burning about 3600 calories of food. A second rule of thumb is that an average, sedentary human being needs about 2000 cal/day to maintain body weight.",
"If you're fasting,that means you're consuming no calories for the duration of fasting. This is equivalent, roughly, to burning 2000 calories per day (assuming you aren't overly active). This works out to 0.55lbs per day. ",
"Obviously your mileage may vary depending on your metabolism and level of activity. But that should be a rough estimate for you."
] |
[
"Again, this is assuming that you live an otherwise sedentary lifestyle"
] |
[
"What's the difference between the \"heat death\" and \"Big Rip\" models of the ultimate fate of the universe?"
] |
[
false
] |
(Prefacing this with: I'm not a professional science person, I just love me a bitta space. I'm working on a prose-poem-y thing about anonymous sex and end-of-universe scenarios and I like to think I know a reasonable amount about both, but I only really have first hand experience of one of them and want to be as accurate as I can in the other.) I thought I understood the idea pretty well: flat curvature, accellerated expansion, increased ratio of dark energy to regular matter and dark matter, things getting further away from each other, galaxies and solar systems and individual objects split apart, shit burns out, shit Hawking radiates away, atoms split apart, individual particles are on their own and to far away from each other to possible have any further causal relation to each other, everything is cold and boring and sad, no more pizza, yadda yadda yadda. But I keep seeing them listed as two distinct models. Is it that they're kind of similar or overlap? Or are they mutually incompatible in some way that's gone over my head?
|
[
"The heat death of the universe is when the universe reaches maximum entropy, and no motion is ever possible again.",
"The big rip is the idea that as space keeps expanding, everything will keep expanding until the point that sub-atomic particles are torn apart. ",
"The difference is in that in the big rip matted is torn apart while in the heat death matter stays together but decays(I don’t know if that’s the right word) into radiation.\n(This is all from my understanding and I could definitely be incorrect)",
"I’d highly recommend ",
"this video.",
" "
] |
[
"Seems like you're committing a bit of a fallacy. You're saying since change stops, time stops. But that doesn't follow. Change needs time to happen, but time doesn't need change to happen. Time just allows FOR change to happen, but it doesn't require it. "
] |
[
"A big rip could happen so distant in the future that the universe could be considered dead at this point.",
"Unlike the big rip, a heat death is a very slow process."
] |
[
"R. Feynman said once we don't understand why 1/2 spin particles obey Fermi-Dirac statistics enough to reduce it to a freshman level. Has it changed since? Can we do it several decades later?"
] |
[
false
] |
Some things are, if not understandable, at least 'graspable' to both freshman and a layman - like Pauli's exclusion principle. But can 'we' (a collective, education-oriented science world) explain it like I'm 18 and clueless about higher mathematics?
|
[
"Well, I think you have to take as your starting point the idea that fermions have an antisymmetric wavefunction. This tends to be thrown out as just \"a brute fact\" that this is the way the universe works. The pauli exclusion principle and fermi-dirac statistics both follow from this fact. It's easier to show the origin of the exclusion principle by taking the simple case of a two fermion wavefunction:",
"psi(1,2) = phi(1)theta(2) - phi(2)theta(1)",
"That is that the wavefunction is composed of two parts- it doesn't really matter what those parts are, your wavefunction just needs to be antisymmetric. The consequence of this is that if particle 1 = particle 2 then the whole wavefunction goes to 0- the particles can't exist in the same state. ",
"In contrast Bosons have a symmetric wavefunction, so it won't go to zero when two particles are in the same state. ",
"Just armed with this information we can imagine what the difference in statistics will look like between fermi-dirac and bose-einstein. At zero temperature fermi-dirac will look like a step function, where every state is filled up to some point that's dependent on how many particles you have (and how many equivalent energy states there are), then drops to zero. In contrast bose-einstein will just have every particle in the ground state ",
". ",
"Now if you add temperature the effect is to \"smear\" the distributions out a little bit. That extra energy from temperature might remove a particle from the energy level it ",
" at, into a higher level state. In fermi-dirac this can only happen around the \"step\" at the fermi energy- states at lower energies don't have anywhere to move into if the other states are occupied. ",
"I've sort of gone off topic here, but the point is that fermi-dirac statistics arise because of the pauli exclusion principle, which happens because fermions are antisymmetric, and spin 1/2 particles are all fermions. ",
"I think having a physical explanation of \"why\" fermions are antisymmetric would be much trickier, although there's a nice attempt to explain it ",
"here",
" if you can follow the argument. "
] |
[
"TL;DR- ",
". :)"
] |
[
"well, when you want to put it like ",
"."
] |
[
"When you shave down a magnet on one of its poles, does it naturally adjust while conserving net magnetic attraction or what?"
] |
[
false
] |
[deleted]
|
[
"It helps to understand what actually makes a magnet magnetic.",
"Every atom has something called a magnetic moment. Basically, every atom behaves like a tiny (very weak) magnet with a north and south pole. In most materials the atoms are all oriented randomly, but if the arrangement isn't completely random, then the magnetic fields of the individual atoms will add up to give a much stronger effect. This is what's going on inside a ferromagnet like the one you describe.",
"If you were to shave a little off the end then the rest of the atoms would be pretty much unaffected. You would still have a magnet, but it would be (very) slightly weaker due to having fewer atoms contributing to the overall field. If you were to break a magnet in half you would end up with two smaller (and weaker) magnets for the same reason. Each one would have two poles like the original one did."
] |
[
"While I have no doubt this answer is correct, I feel like I know less about magnets for having read it."
] |
[
"Thanks, but knowledge just ruins the magic. ",
"..Like the day I found out where eggs came from."
] |
[
"Can architecture influence (human) decisions/behavior?"
] |
[
false
] |
For example: Can we design bridge which will make people change their mind if they decide to commit suicide?
|
[
"Yes, and I can think back to a NYTimes article where they mention how the architecture of two bridges affected suicide rates. It wasn't visual/psychological like you'd expect, it was a difference in the height of the fences. You can climb over both and expect to die but one was slightly higher and the difference in height caused a suicidal person to take a few seconds longer to climb and jump over but those few seconds caused them to think a little bit longer about what they were doing and that would cause many of them to change their minds. I know this isn't the answer you were looking for, but maybe it's a start.",
"http://www.nytimes.com/2008/07/06/magazine/06suicide-t.html?_r=0"
] |
[
"Vision scientist in training here! My intuition is that, as others have said, the only easy way to do this is to change certain aspects of the bridge that, through how they appear visually to people, alter peoples' ",
" for their environment--that is, what can the bridge/environment be used for? A really tall fence might mean that the bridge doesn't afford suicide, and so it might not occur to people at all.",
"In terms of the illusions you're talking about, my guess is that yes, there probably are ways we could alter peoples' decisions, but they are not the sorts of things we could easily (or perhaps even at all feasibly) discover. They might also be subject to high levels of individual differences, unlike classic, lower-level visual illusions. The best I can think of is using something like what you've said (a light path vs dark) to make people less likely to approach the edges in the first place."
] |
[
"Thank you very much! Very interesting reading."
] |
[
"Why do bees/wasps/flies and other flying insects get trapped by windows?"
] |
[
false
] |
I know it might seem silly, but I was just wondering if there was a scientific reason that flying insects seem to easily find their way into an open window, but spend the next couple of hours slamming head first into the part of the window that is closed. Is it easier for them to fly in than it is to fly out?
|
[
"Entomologist here. Couple things to note...",
"While certain groups of insects are capable of latent learning (e.g. using landmarks for orientation and the like), they are completely incapable of insight or transfer learning. That is to say, in many situations, insects are unable to apply previous experience. ",
"For instance, let's say we place a bee into a simple maze. At the end of this maze, there is a sweet reward of nectar. Eventually, the bee will navigate the maze and find the reward. If you were to then place another sample of nectar at the beginning, the bee would not simply turn around and go the same way; it will again wander about until it stumbles across the reward.",
"The case of your fly is very similar. The fly came across the open window and entered your home. Upon attempting to exit, though, it did not \"realize\" that the same path could be used to escape."
] |
[
"And now we know why ",
" went into entomology!"
] |
[
"That is really interesting! I clearly misunderstood bee navigation, I thought that they could 'remember' where specific locations are for collecting nectar?"
] |
[
"If we see \"worst case scenario\" climate change, which species will die and which will thrive?"
] |
[
false
] |
Also someone should define what is the worst case scenario.
|
[
"We cannot say for sure what will happen. But the categories of species thought to be most at risk include:",
"alpine species, which may get \"pushed up off the mountains\"",
"polar species (\"pushed off the planet\", basically)",
"especially, arctic species dependent on pack ice (polar bears, ringed seals and a few others)",
"coastal species in areas where they may not be able to shift inland rapidly due to heavy human construction above the current high-tide line.",
"small terrestrial species that may be unable to migrate poleward easily due to certain human barriers (particularly, species that can't cross interstate highways easily (amphibians etc) or rivers (small mammals)",
"slow-growing sessile marine species sensitive to sunlight and water depth - esp coral reefs",
"marine species highly affected by pH, e.g. those with calcium carbonate skeletons. Corals again, molluscs, others.",
"any species that for whatever reason is using specific cues to timing nesting/migration such that the cues are not changing rapidly enough to keep pace with local climate changes. (Partly hypothetical example: sea turtle nests earlier, using water temperature as a cue about when to nest; but beach sand is warming faster than water. This appears to be happening with Kemp's ridleys right now)",
"species that are at highly reduced population size compared to their historic numbers; this reduces genetic diversity and makes species less able to adapt (evolutionarily) to sudden environmental change.",
"species that have a highly restricted range compared to their historic range; this typically makes species more vulnerable to fluke weather events within that small range and also makes it more difficult for them to move to a very distant new range.",
"species in fragmented habitats that do not have \"habitat corridors\" available for them to migrate from one place to another.",
"some forest tree communities are thought to not be able to shift range fast enough to keep up with current warning trends.",
"I'm sure I'm forgetting some categories. Can post cites later; there's been a bunch of papers on this issue."
] |
[
"Generalists always do well in times of rapid change. Rats, seagulls, roaches, pigs, humans, dandelions. Being able to live in many different habitats and eat many different foods pays off in the long run.",
"Overspecialization can be an evolutionary dead end. E.g.: cheetahs are the fastest land animal, but sprinting across the savannah is all they are good at. As humans rapidly change the savannah, and shoot them with guns they can't outrun, they are vanishing faster than any other top African predator."
] |
[
"Generalists always do well in times of rapid change. Rats, seagulls, roaches, pigs, humans, dandelions. Being able to live in many different habitats and eat many different foods pays off in the long run.",
"Overspecialization can be an evolutionary dead end. E.g.: cheetahs are the fastest land animal, but sprinting across the savannah is all they are good at. As humans rapidly change the savannah, and shoot them with guns they can't outrun, they are vanishing faster than any other top African predator."
] |
[
"Why do soft drink bubbles attach themselves to the outside of the cup?"
] |
[
false
] |
As above, I've also noticed they stick to a straw as well.
|
[
"The bubbles nucleate around small imperfections in surfaces. Its how the bubbles start. But mostly look up nucleation of gasses on glass.",
"Edit: even seemingly smooth surfaces are rough when viewed under microscope. You can check out the internets plethora of information on both subjects. Science rabbit holes are far cooler than youtube rabbit holes."
] |
[
"Awesome thank you"
] |
[
"Do you know why the same phenomenon doesnt happen with beer?"
] |
[
"Do you have to cook potatoes?"
] |
[
false
] |
When I was a kid my grandma would feed me the raw potato bits she did not use. I know you have to cook the bacteria and stuff out of other foods like meat, but you do not have to cook carrots, so is it okay to eat raw potato?
|
[
"http://www.utsandiego.com/news/2009/aug/05/1f5focusm195324-pros-and-cons-munching-raw-potatoe/",
"The potato plant produces a number of defensive (toxic) substances in the upper plant (leaves, stems and above-ground fruit), but they are not in the below-ground tubers, which are the “potatoes” we're familiar with. However, a potato (tuber) that's been injured, exposed to sunlight or stored for an extended period of time might begin to sprout or develop patches of green. The green is chlorophyll, produced by photosynthesis.",
"Chlorophyll is not toxic, but if photosynthesis has begun, it means that toxic alkaloids, such as solanine, will be also present. Green potatoes and especially potato sprouts, should never be eaten. Cooking does not deactivate these toxins.",
"The raw potato also contains antinutrients that act as enzyme (protease) inhibitors. This might be a consideration if you don't eat well and crunching on raw spuds is a regular part of your diet.",
"To reiterate, the poison is in the green patches, or sometimes purple depending on the potato. This is why you're supposed to cut all the eyes out before you cook them, and why most potato peelers have that little protuberant ovoid blade on the end."
] |
[
"As noted by many people here, raw potato tuber is not toxic to humans. Follow the linked advice from ",
"/u/mutatron",
". ",
"However, speaking as a home cook (that has learned the hard way on underdone potatoes): potatoes contain a pretty solid carb load of ~12.44 g per 100g of potato (according to the USDA National Agricultural Library, ",
"http://tinyurl.com/clblydw",
"). Most of that carb load is in starch. Uncooked starches are comparatively hard to digest, and can (depending on the person) result in stomach pain, indigestion, and gas. ",
"Carrots, in comparison, contain almost no starch. Their carb load is in the form of cellulose and various sugars. (simple reference: ",
"http://en.wikipedia.org/wiki/Carrot",
") Cellulose is essentially passed through the body with minimal digestion. ",
"So, tubers raw aren't toxic, but are not a great idea."
] |
[
"Actually potatoes do not need to have turned green or sprouted to have measurable and sometimes high levels of solanine present. Although it is rare for potatoes to accumulate enough solanine to be actually toxic.",
"Humans are also able to tolerate a lot more solanine than you would normally expect in a mammal of similar size. So even if potatoes do accumulate small amounts of solanine we usually don't find out about it because people don't react very often and people are almost always fine if they happen to eat some green potatoes.",
"source: I was a tomato/Solanaceous plant biologist for 4 years."
] |
[
"Some Cameras in space appear to degrade quickly over time, why is this?"
] |
[
false
] |
In videos such as: You can clearly see artifacts on the camera appearing over time, from dots to a large line of missing pixels. What causes this? Is it just stray radiation? Why does this not effect some other cameras?
|
[
"This could definitely be damage from cosmic radiation. It looks similar to the damage that happens to cameras in radiation therapy vaults. I believe the damage is usually from radiation in the form of nuclei hitting atoms in the semiconductor hard enough to knock them out of place and screwing up the crystal lattice, which makes them stop working correctly.",
"Different cameras may be affected differently because of the construction of their semiconductors. Larger crystals/pixels may lead to better longevity because it takes more lattice defects to make them fail."
] |
[
"Yes sir. Cameras in space are often damaged by higher energy particles that normally get filtered on its way through the atmosphere. ",
"What you notice on nice dslr photos from space is they have a tooooon of hot pixels, sensor spots that always receive power. I think NASA releases the raw images and the monkeys or enthusiast clean them up for them. ",
"Source: Am digital monkey",
"Edit: Some cameras are more effected simply because they have been up there longer. There are also several new cameras that have internal firmware that can clean up hot pixels before the photo leaves the camera. Since the hot pixels are persistent regardless of photographic content, you take a couple photo with the lens cap on and the camera ignores or clones those hot pixels with different algorithms. "
] |
[
"One more fun fact, the computers on the ISS are prone to blue screening due to their lack of em and particle shielding. Some scientists also believe that charged particles are to blamed for many random terrestrial blue screens. That's just conjecture but makes me feel better when my tower flips out :D"
] |
[
"When I walk while wearing a sweatshirt, how come the drawstrings swing side to side instead of bouncing in the direction I walk?"
] |
[
false
] | null |
[
"One factor is that drawstrings can sway side to side with very little loss in energy: any side to side motion you may have while walking is allowed to build up in a resonance. But for front to back, the drawstring loses nearly all its energy every time it hits you: cloth doesn't bounce very well. So even if you would expect to have more front to back from how you are moving, that motion gets stopped every single cycle."
] |
[
"When you walk your speed forward is pretty much constant, you have no acceleration. But when stepping you do a fair amount of side to side motion, especially as high as your shoulders. Because this keeps changing, they swing. "
] |
[
"As you walk forward they move with you. Initially they’ll appear to move backward, they are in fact wanting to stay where they are. After you drag them with you they’ll appear to just be moving at the same speed as you. They are swaying backwards and forwards as you walk, just to a small degree.",
"Side to side though is a different matter. As you walk you sway to make room for your foot to move. This movement, on most people, is rhythmic. It’s in time, at regular intervals. This back and fourth motion will give the strings regular pushes and pulls, the movement will build up in the same way a swing does each time you push it. Your movement side to side imparts the same movement in the strings.",
"So, they are probably moving in ellipses it’s just you don’t perceive the forward backward motion as the side to side is so exaggerated."
] |
[
"Hi, I enjoy Word Finds and I was curious if they exercise or help \"sharpen\" anything in your brain?"
] |
[
false
] |
[deleted]
|
[
"I read something ages ago that psychologists tested it and found that word puzzles aren't beneficial, but playing chess can decrease your chances of alzheimers "
] |
[
"Cool, thank you. I guess I know what I'm switching too."
] |
[
"They'll increase your vocabulary at the very least. Any activity that requires extensive thought \"sharpens\" your brain. If you really want to increase your brain power, play strategy games and puzzles. There's sites devoted to things like this and actually let you evaluate yourself, and track your progress over time. Google \"brain exercise games\"."
] |
[
"If you decrease the volume of a star while keeping it's mass the same, is the star going to have a higher gravitational pull?"
] |
[
false
] |
[deleted]
|
[
"It's because when you shrink the radius of an object, you can get closer to the ",
" mass. Let's say you were 1 m from the center of the Earth. That means that all the Earth mass outside of 1 m doesn't affect you (it all balances out), and therefore, only the tiny bit of mass within 1 meter of the Earth's exact center would be pulling on you gravitationally. ",
"However, now let's shrink Earth to the size of 1 meter. Now, you have ALL of Earth's mass within 1 meter, so you feel ",
" of gravitational force. ",
"However, to the Moon, in either scenario, there's exactly the same amount of mass below it in either scenario, so it feels the same gravitational pull in both."
] |
[
"It's because when you shrink the radius of an object, you can get closer to the ",
" mass. Let's say you were 1 m from the center of the Earth. That means that all the Earth mass outside of 1 m doesn't affect you (it all balances out), and therefore, only the tiny bit of mass within 1 meter of the Earth's exact center would be pulling on you gravitationally. ",
"However, now let's shrink Earth to the size of 1 meter. Now, you have ALL of Earth's mass within 1 meter, so you feel ",
" of gravitational force. ",
"However, to the Moon, in either scenario, there's exactly the same amount of mass below it in either scenario, so it feels the same gravitational pull in both."
] |
[
"Its gravitational \"pull\" depends only on the mass, with the exception that if the radius is decreased then it will be possible to get closer to the center of mass without part of the mass of the star being above you and cancelling out the field."
] |
[
"Why didn't whales evolve gills?"
] |
[
false
] |
When the first mammals returned to the sea, why did they evolve blowholes instead of (re)evolving gills?
|
[
"while a whale with gills may have more fitness than a lung-breathing whale, this does not mean that the trait has to evolve. when the mutations which are the basis for evolutionary change occur, they occur randomly. the genome does not \"know\" to mutate in a certain way to make the animal more adapted to the environment. in whales, the necessary mutations to start them off on a path towards evolving functional gills never occurred, simply by random chance."
] |
[
"Mammals burn and consume energy much faster than other animals.",
"Although I haven't done any calculations, I suspect the surface area of the gills required to support the mammal would be far too large. The oxygen content of water is much lower than the oxygen content of air."
] |
[
"The short answer is really just that they didn't need to. They already had the entire lung architecture built, and all that had to change was for the nostril's to migrate up to the top of the head, which is probably a pretty small change, in terms of the underlying genetics.",
"Gills, however, would have had to be completely reinvented. Evolution can generally be said to take the path of least resistance in the short term (most of the time). There just was no need to re-evolve gills."
] |
[
"How does the body know there is no antibody for an antigen?"
] |
[
false
] |
So I’ve been reading an immunology textbook and I have gathered that APCs bring antigens into the lymphatic system to present them to lymphocytes, and if there is no existing antibody then the process of generating random lymphocytes from stem cells is triggered. But how does the body know? I assume it can take time before an APC can bring an antigen to the correct lymphocyte (if there is one), so is there a timer or something when the body gives up trying to find a match? Or is it the buildup of one kind of antigen in the lymph nodes that triggers the process to create a new antibody?
|
[
"It’s my understanding that the antibody discovery process is always ongoing, so the presence of an antigen on APCs will always stimulate cognate naive T/B cells. Once all the circulating naive cells that have a matching receptor have been stimulated, there are no longer matching naive cells",
"Likewise naive cells with random T/B cell receptors are always being produced. ",
"When they recognize the antigen, then they start mutating their receptor for affinity maturation"
] |
[
"The production of random (naive) lymphocytes is continuously ongoing, there’s no trigger required. You always have a reserve of naive lymphocytes in your body, mostly concentrated in the lymph nodes. APCs present antigens to these naive lymphocytes, and any lymphocyte with relatively good affinity (produces antibody that binds well with the antigen) will be stimulated into proliferating."
] |
[
"Oh wow, OK so it’s an ongoing process. I asked a related question below, but to rephrase here: it sounds like the intense reaction we have to a new virus (or a COVID vaccine for example) is an already existing process happening on a larger scale, not a whole new process being triggered, is that remotely correct?"
] |
[
"If a waterfall is always flowing quickly how does one freeze?"
] |
[
false
] | null |
[
"The same way that even though water is constantly dripping from the ceiling of a cave, stalactites can still build up - little by little. A small amount of water will freeze at the very top of the waterfall, and this will provide a point at which more ice can build up on top. Over time, the ice will reach the bottom of the waterfall, giving the impression that the waterfall froze instantaneously."
] |
[
"I find this a bit hard to imagine. By chance, would you have a time lapse of this?"
] |
[
"But with the extreme force of the waterfall, one would think that any newly-formed ice would simply go with the stream."
] |
[
"Why aren't neutrinos \"supposed\" to travel away from us?"
] |
[
false
] |
I read somewhere that neutrinos were observed leaving Earth instead of towards it, which could mean a parallel universe. I mean, if neutrinos come to us from all directions and barely interacts with anything, shouldn't we see equal amounts of neutrinos looking as if it is coming towards and away from us?
|
[
"I'm assuming you're asking about the article that's been floating around Facebook the last couple of weeks which says scientists discovered proof of a parallel universe from particles traveling \"backwards in time\".",
"These articles are basically pseudoscience. The experiment is real but what the articles are claiming is not at all what the researchers found. ",
"The experiment is the Antarctic Impulsive Transient Antenna.",
"Essentially, the experiment looks for radio waves that are generated by cosmic rays interacting with the Antarctic ice. Every now and again they see \"bursts\" of radio waves due to a cosmic ray shower in the upper atmosphere. Since their instrument, which is floating on a giant weather baloon, is only pointed at the ground, the bursts are seen after they bounce off the ground and go back up toward the detector.",
"Like any kind of light wave, radio waves have a specific polarization. When the waves bounce off of a surface, the polarization is flipped. ",
"The radio waves being generated by the cosmic ray showers are always polarized in the direction of Earth's magnetic field. Since these \"bursts\" are bouncing off of the ground before detection, they should always have flipped polarization relative to the local geomagnetic field.",
"However, the scientists detected three \"anomalous\" bursts that were not flipped. The implication is that these bursts never bounced of the ground, and instead shot straight up from underneath.",
"The scientists tried to determine if these anomalous bursts could have been caused by cosmic rays that penetrated the earth from the other side and passed all the way through, but they found that the liklihood of that happening is too small to be plausible for any particle that we currently know of (in particular, neutrinos).",
"There are two likely explanations, neither of which are nearly as sexy as parallel universes: the first is that the anomalous bursts actually did bounce off the ice, but through some process, did not have their polarization flipped; this is completely plausible since there are known mechanisms by which this can happen. The second, and more interesting, is that the cosmic rays were composed of as-yet-unknown particles that are outside the current standard model of physics; this would be exciting because it would mean rethinking our entire understanding of particle physics."
] |
[
"Wow thank you for this very in depth reply, it certainly clears things up."
] |
[
"Enjoying reading the answer. It got me thinking, and I had some doubts - ",
"",
"It's entirely possible that I'm missing something obvious, but isn't it possible that the waves underwent another bounce off some large vertical thing like a cliff (which caused their polarization to flip back)?",
"",
"Do they bounce off the physical earth (as in solid) or off a layer of the atmosphere, since if it is the former, it would be pretty hard to predict how many bounces the rays went through before reaching the detector (and it would be unlikely that all the particles in the rays go off in the same direction since even a small difference in landing spot could result in wide differences in final direction of travel). ",
"",
"I guess my question is - what exactly is it that the rays bounce off so that we can consistently expect only 1 bounce?"
] |
[
"Can we observe two galaxies that cannot observe each other due to distance in space/time?"
] |
[
false
] | null |
[
"Yes. Two galaxies diametrically opposed at the edge of our observable universe with us at the center would see our galaxy at the edge of their respective observable universes."
] |
[
"You want an example? It's going to be two unnamed galaxies in catalogs that are identified by where they are in RA and dec. How does that help?"
] |
[
"Oh, I know. I was just trying to demonstrate the possibility with the most extreme example. "
] |
[
"Is it true that people who live in dirty environments have better immune systems?"
] |
[
false
] |
So I was over at some guys' place today and it was (in my opinion?) very dirty. A lot of pets, a foul smell throughout his house and it was very messy and unhygienic. But then I wondered, do people living in that kind of environment have more developed immune systems than people who live in cleaner environments?
|
[
"To some extent.",
"First, we need to clarify what a strong immune system is. Very simply put, the immune system is a set of cells and proteins that react to foreign bodies and pathogens (e.g. bacteria, viruses). ",
"For instance, there are five types of ",
" (white blood cells) with different functions (",
" kill bacteria and fungi, ",
" poison parasitic worms, ",
" release histamine and anti-coagulation substances, ",
" become macrophages that consume debris and microorganisms, and ",
" differentiate into B, T, or NK cells that do different things such as producing antibodies and killing damaged cells.) My favorite part: neutrophils can release ",
", which means they release a small cloud of dangerous molecules that kill the bad guys. Picture of that ",
"here.",
"Then you have ",
" (also known as immunoglobulins, or Ig) that are produced by B cells. These antibodies recognize foreign bodies, or rather, a unique portion of foreign bodies (known as antigens), and either tag them for destruction or neutralize them directly. And on the topic of proteins, there is the ",
", which help antibodies and leukocytes eliminate pathogens. For instance the membrane attack complex is when a bunch of different complement proteins are signaled to rupture a target cell.",
"Then there are so many more parts (e.g. platelets, cilia, cytokines) to the immune system that it would take a textbook to describe them all.",
"My point is (apart from pointing out that neutrophils are like superheros) that what does it mean to boost the immune system? Which part? How? More antibodies? White cells? More effective rupturing? As long as you do all those things are you're supposed to (eat, sleep, and exercise well), your immune system will be fine.",
"Now, onto the topic of \"clean living\": advocates for eating dirt (literally) want to promote a regulated immune system - not stronger or weaker, just \"calibrated\". Whether or not your immune system responds a little bit or a lot (in the case of allergies) to antigens (anything that isn't part of your body) depends on previous exposure. Your immune system can learn that some compounds (like pollen) don't require a set-fire-to-the-drawbridge response, but only if it's been consistently exposed to pollen before. And then there's the immune memory; once you're exposed to an antigen (like chickenpox), your immune system keeps some specific leukocytes handy so once your body encounters the antigen again, the leukocytes can just multiply and take out the invader, without calling for a full-fledged response (all the symptoms of chickenpox again).",
"So essentially, if you don't encounter a lot of antigens (in an overly clean environment), your immune system might not calibrate itself correctly and can overreact when exposed to unfamiliar antigens. In addition, there's the possibility that your immune memory library might be lacking, resulting in more severe symptoms when you come into contact with pathogens.",
"However, that's it for the \"dirty living\" argument. As long as you socialize regularly, you should be fine. But it definitely isn't healthy either to live in an environment where salmonella or fleas can breed."
] |
[
"He's say yes clean environments = bored immune system leads to getting drunk and attacking self"
] |
[
"That link to the photos of the ",
" is probably the coolest thing I have seen in months. ",
"Hard to fathom that individual cells have basically evolved the ability to create a destructive \"force field\" to keep us healthy. "
] |
[
"Why are 1, 3, 7, and 9 the only numbers whose multiples can end in any digit?"
] |
[
false
] |
Obviously 1 can multiply into anything. Multiples of 7 can end in any digit 0 through 9 07 14 21 28 35 42 49 56 63 70 Same for 3 03 06 09 12 15 18 21 24 27 30 And 9 09 18 27 36 45 54 63 72 81 90 But 2, 4, 5, 6, and 8 won't do it.
|
[
"Quick and easy: ",
"Because 1,3,7,9 are the only numbers less than 10 that share no prime factors with 10. On the other hand 2,4,6,8 share a factor of 2 and 5 shares a factor of 5. The reason that we can check if a number is divisible by 2 (aka even) or divisible by 5 really quickly, by just looking at the last digit, is because they divide 10. And this extends to them not being able to end in any digit.",
"Long and fun:",
"A funny thing happens with two numbers that share no common factors: If A and B are integers with no common factors, then there are two other integers M and N so that ",
"AM+BN=1\n",
"Moreover, if A and B happen to be two numbers that we can find such an M and N, then A and B must share no common factors. That is, being able to solve AM+BN=1, where M and N are integers, is the same as A and B sharing no factors. A quick way to see this is that if both A and B are divisible by 2, then so should 5A+3B and any other combination made like this. Since 1 is not divisible by 2, we cannot solve AM+BN=1. For example:",
"For A=1 and B=10, we can take M=11 and N=-1. This gives 1*11+10*(-1) = 1.",
"For A=3 and B=10, we can take M=7 and N=-2. This gives 3*7+10*(-2) = 1.",
"For A=7 and B=10, we can take M=3 and N=-2. This gives 7*3+10*(-2) = 1.",
"For A=9 and B=10, we can take M=9 and N=-8. This gives 9*9+10*(-8)=1.",
"But for A=6 and B=10, the best we can do is with M=2 and N=-1 to give 6*2+10*(-1)=2. In general, the smallest number we can get will be the greatest common divisor.",
"But look what this allows us to do. Rearranging all these gives",
"For A=1 and B=10, we have 1*11=10 + 1 = 11.",
"For A=3 and B=10, we have 3*7 = 1+20 = 21.",
"For A=7 and B=10, we have 7*3 = 1 +20 = 21.",
"For A=9 and B=10, we have 9*(9) = 1+80 = 81.",
"This finds for us a multiple of A that has last digit 1. We can then get ",
" final digit from these. If I want to find a multiple of 3 that ends in 7, all I have to do it multiply 3*7=21 by 7 to get 3*49 = 147. This means that being able to find a multiple of A that ends in any digit is the same as being able to find a multiple of A that has final digit 1. But ",
" is the same as being able to solve AM+BN=1 and ",
" is the same as A and B not sharing any common factors. The equation AM+BN=d is called ",
"Bezout's Identity",
".",
"With this generality, we can extend this to other bases. If we work with base B, then the only numbers that can have a multiple whose final digit be anything are those that share no common factors with B. So, in base 7, ",
" number that is not a multiple of 7 can do this. In base 12, the numbers that can do this are 1,5,7,11. ",
"Digression:",
"This is a small part of something bigger. If we fix some base number B, we can stretch the meaning of equality to be a lot more generous. We can say that two numbers X and Y are \"equal mod B\" if they have the same remainder after dividing by B or (equivalently) if X-Y is divisible by B. We can then do arithmetic with this generous kind of equality. This is called ",
"Modular Arithmetic",
" and it's how clocks work.",
"For instance, if B=12, then 3 hours from now and 27 hours from now will show the same time. This is because 27-3 =24 is divisible by 12. If B=7, then we can say that \"16=58 mod 7\" because 58-16=42 is divisible by 7. The fun thing is that if B=10, then \"X=Y mod 10\" exactly when X and Y have the same last digit in base 10. So \"2663=393485723 mod 10\".",
"Now, in just the integers we can't to arbitrary division. I can divide 25 by 5, but I can't divide 16 by 5, otherwise I'd have to leave the integers. In other words, there is no ",
" that equals 1/5. Said another way, there is no integer X so that X*5=1.",
"But if I work mod 7 then, in a way, I can say that \"1/5 = 3 mod 7\" and this is because 3*5=15 and \"15=1 mod 7\". This means that I have \"3*5=1 mod 7\". That is, I can find some integer X so that \"X*5=1 mod 7\". We say that \"5 is invertible mod 7\". ",
"If we have a base B that we mod by, what are the numbers, A, that are invertible mod B? It is exactly the numbers that share no factor with B. This is because we can then find integers AX+BY=1 which means that AX-1=B*(-Y) so that AX-1 is a multiple of B which means that \"AX=1 mod B\". If we work with B=10, then our invertible numbers are 1,3,7,9 and their inverses are 1,7,3,9 respectively: \"1*1=1 mod 10\", \"3*7=1 mod 10\", \"7*3 = 1 mod 10\", \"9*9=1 mod 10\".",
"Note that if I can find an X so that \"AX=1 mod B\", then for ",
" Y, I can find a Z so that \"AZ=Y mod B\". This works the same as above. Just take Z=XY. If B=10 and A=3, then X=7. So if I want a multiple of 3 that ends in 7, I can just take Z=7X=49 so that \"3*49 = 7 mod 10\".",
"What happens if we look at 6 mod 10? What goes wrong? Why can't we invert it mod 10? The reason is that 6 is potentially a zero mod 10 and dividing by it is practically the same as dividing by zero. What do I mean? By saying that 6 is potentially zero, I mean that I can find a number Z that is ",
" = 0 mod 10, yet we get \"6*Z=0 mod 10\". Just take Z=5 and 5 is not = 0 mod 10, by 5*6=30, which means that \"5*6=0 mod 10\". This is something that never happens in the integers. If XY=0 in the integers, then we know that either X=0 or Y=0. But, mod 10, I can find XY so that \"XY=0 mod 10\" where neither X or Y were 0 mod 10. So 6 is potentially zero in that a nonzero multiple of it is zero. We say that 6 is a ",
"Zero Divisor",
" mod 10. We can't invert zero divisor for the same reason we can't invert zero itself. That is, Z is a zero divisor mod B, then there ",
" be an X so that \"XZ=1 mod B\". When B=10, this means that there are no multiples of Z that have final digit 1.",
"This ",
"multiplicative structure",
" is very important in number theory and one thing we can learn is why some numbers have multiples that end in any digit and others do not. Another thing that this can do is cryptography. The way multiplication mod B works allows many cryptographic systems, that encode your information online, work."
] |
[
"Even + even = even, so no way to get an odd number, ruling out 2,4,6,8. And 5 doesn't either because it's halfway between 10, so it only gives 5 or 0.",
"Aditionally, odd + odd = even, then even + odd = odd. By multiplying odd numbers, you can get both even and old sums."
] |
[
"Note that these aren't the \"only numbers\" with that property -- they are just the only numbers smaller than 10 with that property. We also have, say, the multiples of 13"
] |
[
"How do we know what is below the crust of the Earth?"
] |
[
false
] |
[deleted]
|
[
"Siesmology uses the waves created by earthquakes to measure the rate of propegation of those waves through the structure of the Earth. The deflection of waves and the time it takes them to get to any given siesmograph around the Earth is then used to calculate the structure of the earths interior. ",
"http://geophysics.ou.edu/solid_earth/notes/seismology/seismo_interior/seismo_interior.html"
] |
[
"You may know this, but just for clarification...",
"We are taught in school that there is liquid rock underneath the crust",
"There is indeed ",
" liquid rock beneath the crust, but the mantle is almost entirely solid rock - there is just a very tiny percentage of molten rock.",
"The outer core of the planet is indeed liquid, and the inner core is solid.",
"Mostly though, the Earth is solid.",
"These are common misconceptions, so even if you were aware of this others might appreciate the clarification.",
"Sycosys is exactly right that seismology is the primary method we have of studying the interior of the planet. There are other clues and inferences we can make based on other evidence, but it all is quite agreeable with the conclusions we draw from seismology. "
] |
[
"Fascinating, thank you."
] |
[
"How do racing motorbikes go so low when turning without falling?"
] |
[
false
] |
Seriously what's that about? It looks like they should just fall.
|
[
"When turning, you are accelerating sideways toward the inside of the turn. This acceleration is applied at the ground, so if you are on a motorcycle and you don't lean, you will fall over toward the outside of the turn due to the torque. Because they are traveling at very high speeds, the turning acceleration is much higher, and since the angle is proportional to the ratio between the turning acceleration and gravity, the higher acceleration means that you need to lean farther in order to balance the forces.",
"This is a simplified explanation. For more specific/numerical analysis, I can offer an example calculation upon request."
] |
[
"Thanks you very much. That's exactly what I was looking for. "
] |
[
"For a really simple analogy, think about how you get pushed to the right of your car when you make a left turn. It's exactly the same force."
] |
[
"What resolution of digital audio would surpass the ability of our ears to discern between digital and analogue?"
] |
[
false
] |
I am a bit of an audiophile, but I've never looked into this subject. Basically, if there are cameras out there that are able to take pictures with a greater resolution than our eyes can perceive, then it seems logical that there would be a digital audio stream that would be greater than our ears can perceive too. Is it possible that digital is actually better than analogue? Is it practical?
|
[
"'Resolution' in the audio sense would be the sampling frequency. As it turns out, in order to perfectly recreate a sound, you need to record at a sampling rate of at least ",
"twice",
" the highest frequency. Since the typical human hearing goes up to about 20 kHz, anything over 40 kHz should be about enough. A typical CD recording has a sampling rate of 44.1 kHz."
] |
[
"mp3 uses a few tricks to pack most of the useful audio information into a significantly smaller size. for most people constant bitrate mp3 @ 320 kbps with a good encoder should be perceptually identical to the original CD. this is also said to be true for the variable bitrate V0 mp3 which runs at about 260 kbps (if I remember correctly)",
"lossless codecs such as FLAC are able to compress sound by about 50% while being able to decompress to a bit-for-bit copy of the original"
] |
[
"This is true as far as time sampling goes, but we should also consider the fact that analog audio has effectively continuous amplitude, while digital audio is ",
"quantized",
". Very low amplitude digital audio signals can be pretty distorted by this quantization, so digital audio is often ",
"dithered",
" by adding small-amplitude noise to the signal. This reduces the quantization artifact but raises the noise level, obv. For well-mastered audio with a reasonable dynamic range this isn't a problem, but it's another way that digital and analog audio differ."
] |
[
"The Yankees and Red Sox are both blaming the air in London Stadium for their breaking balls staying up instead of falling. How does different air change the drop?"
] |
[
false
] |
Both teams told Buster Olney yesterday that the lack of air circulation in London Stadium is causing breaking balls not to drop. Wouldn't less resistance give it a BETTER chance at dropping?
|
[
"This sounds dubious at best. Air density, temperature and humidity are all similar in Boston, NY, and London. The respective stadiums are only double digit meters above sea level. Temperatures may be slightly cooler in London vs across the pond. This could affect play by increasing air density slightly, but I doubt it would be significant.",
"Humidity affects play largely because it affects the friction on the leather covering of the ball. Balls are coated with a thin coating of dried mud/clay to minimize this effect. But the humidity is similar in Boston, NY, and London.",
"The only major league stadium where altitude is high enough to make a significant difference in play is Coors Field in Denver CO, with an attitude of 5211' or 1.59 km. It's sometimes known colloquially as \"Mile High Ballpark\" not to be confused with Mile High Stadium with respect to the Denver Broncos. This mainly effects the distance batters can hit by reducing air drag modestly. Home runs are statistically more likely at Coors Field. Air density also effects very long throws from outfielders."
] |
[
"Air circulation is key to making a ball 'break'. I'd advise you to google 'The Magnus Effect' to learn more, but basically a 'breaking pitch' like a curve ball rotates forward(towards the batter if you will) and it's the air flow coming from under the ball and pusing against the seams that make a pitch 'break' so no air circulation would theoretically mean less 'break' in a pitch. In the case of a Fastball, which rotates backwards(towards the pitcher if you will) it's the air flow coming from over the ball against the seams that make it go upwards. Air flow is also the reason why a Knuckleball is so hard to hit as they usually rotate about between a quarter rotation to half a rotation between the time they are released and the time the reach the plate so it offers no resistance to the air flow on any side. Hope that sums it up."
] |
[
"Has there ever even been a definitive answer as to whether breaking balls actually \"break\", or curve much at all?"
] |
[
"I witnessed this a while ago. How does this happen and why is it such a rare occurrence?"
] |
[
false
] |
Well, my news feed just exploded with "OMG THE SKY IS SO RED". I am just curious as to how it happens and especially, why is it a rare occurrence? Here is the picture I got from my news feed:
|
[
"I can't answer for why it appears \"more red\" than other sunsets, but I can explain why it appears red. In short, blue light scatters more than red light. The light from the sun has much more atmosphere to travel through when it's on the horizon, and all the blue light scatters before it reaches our eyes, so all we see is red. ",
"EDIT: If I had to guess as to why it is more red than other sunsets, it'd probably be due to the atmospheric conditions. Maybe humidity, atmospheric pressure? Maybe someone else can elaborate on that "
] |
[
"There more particles in the air the more light is scattered. The redness could be due to this being a city which has medium or high amounts of air pollution"
] |
[
"Manila, 4 - 6:30 pm"
] |
[
"Is there a fundamental limit to bit rate? Or, is there a limit to serial data transfer?"
] |
[
false
] |
[deleted]
|
[
"There is something known as the ",
"Shannon Channel capacity",
" which is the maximum rate at which data can be transmitted through a channel.",
"The formula is: ",
"Capacity = Bandwidth * log(1 + SNR)\n",
"Bandwidth refers to the bandwidth of the channel, for example, ",
"ADSL downloads",
" use 138kHz to 1104kHz. Increasing the bandwidth is not always possible for things such as wireless communications where the spectrum is regulated. Higher frequencies also attenuate more, lowering the SNR, which limits their usefulness in long distance electrical signalling (ADSL).",
"SNR is the signal to noise ratio. Noise in an electrical circuit refers to interference from other communication channels or power cables. You can increase the SNR by increasing the power used to transmit your signal (ie, increase the voltage of the signal). At some point this becomes impractical. It is also possible to increase the SNR by decreasing noise through shielding and using better cabling."
] |
[
"There are hard physical limits (involving trying to encode pulses smaller than the planck length, which happens as you try to get a bit rate of 2x10",
" bits/second (1 bit per planck time)), and since flipping or encoding a bit takes kTln(2) J (3.7x10",
" J/bit) you could only get a certain bit rate with a given power source (about 2.7x10",
" bits/second/watt). Nonetheless, we're nowhere near the theoretical physical limits as of right now."
] |
[
"USB 3.0 is nowhere near the speed of 100Gbps Ethernet over G.652 / G.655 / G.657 type singlemode fiber, which uses four simultaneous, parallel yet separate 25 Gbps wavelengths. ",
"Even if you ignore 50GHz grid DWDM and use one fiber pair per 100GbE circuit, the capacity of simultaneous 100GbE circuits that can be run over a 72-fiber singlemode cable is incredible."
] |
[
"What makes Diet soft drinks 'bad' for you?"
] |
[
false
] | null |
[
"are not natural, they're man made. Drinking them in high quantities is potentially harmful to you.",
"Non sequitur. There are plenty man made drinks that are safe and much much more natural drinks that are extremely poisonous."
] |
[
"In one perspective, Splenda or sucralose as its generic name is pretty much glucose with two of the hydroxyl groups replaced with chlorines. Basically in your body the substance is inert and will pass right through the same form it came in, but under ideal/unideal conditions these alkyl halides become reactive to cell membranes and DNA and potentially leading to cancer. ",
"Maltitol, which is in a lot sugar-free gums in excessive amounts can exacerbate or induce glaucoma.",
"There's still a lot of debate but artificial sweeteners in diet soda in general can raise insulin levels and affect sensitivity as well as induce hypoglycemia. That chased with caffeine can induce jitters to seizures depending on your threshold. "
] |
[
"I know some diet sodas contained an artificial sweetener called Saccharin, which was thought to be able to cause cancer. I do not know if this is still the case."
] |
[
"Could making yourself excessively dizzy have negative consequences?"
] |
[
false
] |
[deleted]
|
[
"Um... vomiting maybe?"
] |
[
"Immediate consequences = falling over and breaking your wrist, arm"
] |
[
"Seizures."
] |
[
"Is Richard Dawkins respected among the scientific community as an evolutionary biologist?"
] |
[
false
] | null |
[
"I'm a microbiologist that does a fair amount of evolutionary biology, and well, the argument can be made that he's more of a personality or public policy advocate than an actual \"scientist.\" Most of what he publishes isn't original work, but rather reviews or collections of others' work. I kind of lump him into the same category as Bill Nye, Neil Tyson, and that douchey physicist at NYU. They don't really do science (anymore); they just teach science. And in that regard, sure, Dawkins is respected."
] |
[
"Origin of Species"
] |
[
"He hasn't published in a while, but that's mainly because for a long time, his job was to literally inform the public about science. He was a public education chair at Oxford, I think. So all those books were actually just him doing his job. As to whether or not he's respected, I'd certainly say so, but I'm not a biologist."
] |
[
"Which spinoff technology from NASA has contributed the most to everyday life?"
] |
[
false
] |
[deleted]
|
[
"here are a couple of \"top 10\" lists which might give an idea of some of the options",
"One",
"Two",
"Some of the highlights in my opinion - insulin pump, water filtration, anti-corrosion coating. I think the biggest one that has affected our everyday life is \"communication satellites\" but it would be unfair to give all the credit to NASA for that."
] |
[
"Aerodynamic tractor trailer cabs"
] |
[
"No question in my mind its the memory foam mattress. Changed my life. "
] |
[
"Do pet pythons recognize their owners and show any animal traits that dogs/cats do?"
] |
[
false
] | null |
[
"Generally, pythons only coil around things aggressively if that thing is small enough to eat. There aren't many (any?) pythons large enough to eat a typical adult human. That being said, very large pythons, boas and other constrictors can pose a danger to their handlers, but it's pretty rare for people to actually be ",
" by them.",
"To answer your question: Snakes really aren't particularly intellegent. They may become ",
" to an owner, but unlike a dog, they don't have any sort of real \"loyalty.\" Snakes are not pack animals and they do not make friends. If a snake thinks you are either food or a threat, it may attack you no matter how long you've known it.",
"From personal experience, my pet ball python seems a lot more comfortable climbing around on me than other people, but my gut says that's mostly due to me being more comfortable with him (not the other way around).",
"Also, just FYI, you may get other responses from ",
"/r/snakes",
". They're pretty active and tend to know quite a lot of specialized snake information. I know more than most people, but some of the guys in ",
"/r/snakes",
" are amazing."
] |
[
"Well, you'd certainly get a lot more anecdotes, but those guys are nothing if not brutally honest when it comes to snake-related information. Some of them actually own constrictors large enough to be dangerous and can offer quite a bit of information from that front. There was actually really good information coming out of that picture of the Reticulated Python on a leash from last week."
] |
[
"Snakes really aren't particularly intellegent. They may become acclimated to an owner, but unlike a dog, they don't have any sort of real \"loyalty.\" Snakes are not pack animals and they do not make friends. If a snake thinks you are either food or a threat, it may attack you no matter how long you've known it.",
"True for all other members of the Reptile class, I suppose?"
] |
[
"Hi r/askscience, two questions about nutrition"
] |
[
false
] |
[deleted]
|
[
"Water contains no calories because there is not much you can break down water into while getting energy out of it. Everything breaks down into water and CO2, so for practical considerations and biological, we can consider them as having 0 calories. ",
"Who said the water doesn't \"stay\" in your body. By weight 70% of your body is water. Its everywhere, it just doesn't look like it from the outside!",
"The no. of calories ARE different depending on the source. Here's a quick list from ",
"http://www.nutristrategy.com/nutrition/calories.htm",
" :",
"Fat: 1 gram = 9 calories ",
"Protein: 1 gram = 4 calories ",
"Carbohydrates: 1 gram = 4 calories",
"Alcohol: 1 gram = 7 calories"
] |
[
"After you eat food, you combine it with oxygen you inhale to extract energy from the chemical bonds. This energy (typically measured in calories) is what you use to move, grow, etc. Now, even though we cannot metabolize water for energy, the vast majority of reactions in the body are carried out in an aqueous (water) environment, and its also very important to help clear out toxins and various other things you need to excrete. ",
"The amount of calories in a given food is dependent on how much energy you can extract from it. It should be noted that the tabulated values aren't literally correct, but just the average. For example, different amino acids (protein building blocks) yield different amounts of energy, so its possible you could go over or under 4 calories/gram. ",
"On the other hand, just for fun:",
"1 Calorie=4.184J, 1 kCal (food calorie)=4184J",
"Since E=mc",
" ",
"4184J=E=(mass)(3x10",
" m/s)",
" -->",
"Mass of 1 Food Calorie=4.65x10",
" kg ",
"To put that in perspective, its about a single bacteria. "
] |
[
"A calorie is just a measure of energy. One food calorie (1 kCal, 1000 calories), is about 4.2 kJ. It's about the amount of energy you need to raise a kilogram of water by 1 degree C.",
"So it doesn't make a lot of sense to ask what they weigh. But, you can talk about the energy density of different foods - how much energy you get per gram of that food. Fat, for instance, gives you 9 Calories/gram, carbohydrates and proteins give you 4, and alcohol gives you 7."
] |
[
"Why can't ocean water be purified?"
] |
[
false
] | null |
[
"Because the countries that are at high risk for fresh water 'running out' are often third world countries that can hardly afford the expensive desalination processes. Plus that much, much water is needed for agriculture and transporting water from the sea to inland farms is very expensive. For supplying coastal cities with drinking water it may work fine, but for agriculture fresh water shortages will still be a huge problem. "
] |
[
"Desalination",
" of ocean water for industrial purposes, as well as consumer drinking is widespread and available in areas of the world with limited fresh water availability. Desalination, though, is more energy intensive, and so costs more, than just using fresh water where available."
] |
[
"like he said, it is energy intensive and for that reason expensive.",
"In my home of California urban users consume only 10% of our water. We need a ton of water, and especially here in Los Angeles where to pull it from is a looming issue."
] |
[
"What does the \"moment\" in moment of inertia mean?"
] |
[
false
] |
Title
|
[
"It might help to think about the problem this way: if you knew the all the moments of the mass distribution over 3D space...first, second, third...up to infinity--then you would know the entire mass distribution. ",
"The laws of motion are generally only applicable for point masses. (This is why physicists sometimes joke \"let's approximate the cow by a sphere.\") For real-world objects, they have mass distributions that are generally non-spherical. The usefulness of moments in physics is because some calculations don't require knowing the whole mass distribution, just one moment or another. Moments are just a mathematical way to reduce the distribution of mass within an object (which is a scalar function in 3 dimensions) to a single scalar value. It's just like statistics. Moments are generally useful for rigid bodies and fixed mass distributions.",
"For example, ",
"for kinematics you can assume the whole object is just a point mass concentrated in the center-of-mass of the object. For the equation F=Ma, you don't care how the matter is positioned within the object, once you add up the vector acceleration experienced by every single particle in the object the motion is the same as if the thing was just one point with mass M at the \"center\" of the object. ",
"For rotational motion, you need to know if the heavy parts of the object are close to the center or farther out. The 2nd moment of mass, moment of inertia tells you this. ",
"staying with rotational motion, a wheel that has an imbalance might rotate smoothly while horizontal on a table-top, but when vertical it will shudder. The 3rd moment, skewness will reflect this...a symmetric object will have 0 skewness. ",
"One wheel might have a different distribution of matter within it, one might have a beefier rim than the other while saving mass in the middle. Even though they may have the same moment of inertia, their 4th moment will be different. ",
"the fifth moment might tell you if the wheel was balanced at the factory or had to be balanced by putting weights along the rim...2 wheels that are both balanced, but were balanced differently, would have different 5th moments of mass.",
"the sixth...well you get the picture. ",
"And if you knew the exact mass distribution of the wheel, and a powerful piece of software to do the finite element modelling for you, you would not need to care about these quantities...they are just like statistics in that --given certain assumptions--they make certain calculations easier. For example, 6 numbers above will tell you everything you need to know about how a wheel would behave under rotation, whether it's a $2000 set of rims or a piece of junk. The alternative to this would be a 2D image showing the mass distribution of the wheel...which would tell you a lot more about how the wheel behaves when rotating, but is a lot more data. "
] |
[
"It might help to think about the problem this way: if you knew the all the moments of the mass distribution over 3D space...first, second, third...up to infinity--then you would know the entire mass distribution. ",
"The laws of motion are generally only applicable for point masses. (This is why physicists sometimes joke \"let's approximate the cow by a sphere.\") For real-world objects, they have mass distributions that are generally non-spherical. The usefulness of moments in physics is because some calculations don't require knowing the whole mass distribution, just one moment or another. Moments are just a mathematical way to reduce the distribution of mass within an object (which is a scalar function in 3 dimensions) to a single scalar value. It's just like statistics. Moments are generally useful for rigid bodies and fixed mass distributions.",
"For example, ",
"for kinematics you can assume the whole object is just a point mass concentrated in the center-of-mass of the object. For the equation F=Ma, you don't care how the matter is positioned within the object, once you add up the vector acceleration experienced by every single particle in the object the motion is the same as if the thing was just one point with mass M at the \"center\" of the object. ",
"For rotational motion, you need to know if the heavy parts of the object are close to the center or farther out. The 2nd moment of mass, moment of inertia tells you this. ",
"staying with rotational motion, a wheel that has an imbalance might rotate smoothly while horizontal on a table-top, but when vertical it will shudder. The 3rd moment, skewness will reflect this...a symmetric object will have 0 skewness. ",
"One wheel might have a different distribution of matter within it, one might have a beefier rim than the other while saving mass in the middle. Even though they may have the same moment of inertia, their 4th moment will be different. ",
"the fifth moment might tell you if the wheel was balanced at the factory or had to be balanced by putting weights along the rim...2 wheels that are both balanced, but were balanced differently, would have different 5th moments of mass.",
"the sixth...well you get the picture. ",
"And if you knew the exact mass distribution of the wheel, and a powerful piece of software to do the finite element modelling for you, you would not need to care about these quantities...they are just like statistics in that --given certain assumptions--they make certain calculations easier. For example, 6 numbers above will tell you everything you need to know about how a wheel would behave under rotation, whether it's a $2000 set of rims or a piece of junk. The alternative to this would be a 2D image showing the mass distribution of the wheel...which would tell you a lot more about how the wheel behaves when rotating, but is a lot more data. "
] |
[
"It might help to think about the problem this way: if you knew the all the moments of the mass distribution over 3D space...first, second, third...up to infinity--then you would know the entire mass distribution. ",
"The laws of motion are generally only applicable for point masses. (This is why physicists sometimes joke \"let's approximate the cow by a sphere.\") For real-world objects, they have mass distributions that are generally non-spherical. The usefulness of moments in physics is because some calculations don't require knowing the whole mass distribution, just one moment or another. Moments are just a mathematical way to reduce the distribution of mass within an object (which is a scalar function in 3 dimensions) to a single scalar value. It's just like statistics. Moments are generally useful for rigid bodies and fixed mass distributions.",
"For example, ",
"for kinematics you can assume the whole object is just a point mass concentrated in the center-of-mass of the object. For the equation F=Ma, you don't care how the matter is positioned within the object, once you add up the vector acceleration experienced by every single particle in the object the motion is the same as if the thing was just one point with mass M at the \"center\" of the object. ",
"For rotational motion, you need to know if the heavy parts of the object are close to the center or farther out. The 2nd moment of mass, moment of inertia tells you this. ",
"staying with rotational motion, a wheel that has an imbalance might rotate smoothly while horizontal on a table-top, but when vertical it will shudder. The 3rd moment, skewness will reflect this...a symmetric object will have 0 skewness. ",
"One wheel might have a different distribution of matter within it, one might have a beefier rim than the other while saving mass in the middle. Even though they may have the same moment of inertia, their 4th moment will be different. ",
"the fifth moment might tell you if the wheel was balanced at the factory or had to be balanced by putting weights along the rim...2 wheels that are both balanced, but were balanced differently, would have different 5th moments of mass.",
"the sixth...well you get the picture. ",
"And if you knew the exact mass distribution of the wheel, and a powerful piece of software to do the finite element modelling for you, you would not need to care about these quantities...they are just like statistics in that --given certain assumptions--they make certain calculations easier. For example, 6 numbers above will tell you everything you need to know about how a wheel would behave under rotation, whether it's a $2000 set of rims or a piece of junk. The alternative to this would be a 2D image showing the mass distribution of the wheel...which would tell you a lot more about how the wheel behaves when rotating, but is a lot more data. "
] |
[
"If space itself is expanding, is anything happening to time?"
] |
[
false
] |
[deleted]
|
[
"Time being \"personal\" simply means there is no universal time, as everyone from Newton to Einstein believed. It's exactly what special relativity proved to us. ",
"If you are standing next to me and we then synchonise clock, and you then move away from me at some significant speed and come back, you will have had a different experience of time to me - shorter. That is what is meant by time is personal. Many well-known physicists describe it exactly this way."
] |
[
"Space isn't relative in the same way time is. For example, the ratio between the average distance between galaxies and the average size of a galaxy is changing, and no similar comparison can be made for time."
] |
[
"Space isn't relative in the same way time is. For example, the ratio between the average distance between galaxies and the average size of a galaxy is changing, and no similar comparison can be made for time."
] |
[
"How consanguinity is handled for sperm donors?"
] |
[
false
] |
If a man donate his sperm, how can he be sure that his kids won't end up marrying each other?
|
[
"Because there would be an increased chance of genetic abnormalities when they in turn have kids."
] |
[
"The coefficient of relationship for half-brother/half-sister is 12.5%. The better sperm donation organizations will do genetic tests on their donors, so add in the low chance of two offspring from the same donor becoming partners and the risk isn't really high enough to try to violate privacy concerns.",
"Keep in mind that the sperm is standing in for what was likely an unfortunate genetic pairing in the first place."
] |
[
"The coefficient of relationship for first cousins is 6.25% and a lot of people consider it in-breeding.",
"The risk at 12.5% is actually quite high for the progeny to get the same alleles."
] |
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