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Cool thread. I had a bit of a negative experience with physics in secondary school, but it's still something that I'd love to have even a basic understanding of. I'm reading the book that accompanies Carl Sagan's Cosmos at the moment, and he explains everything in it in a really simple, straight-forward way that doesn't assume you have any prior knowledge of the principles of physics and so on. It seemed like whenever I was studying physics in school, some things were explained poorly, while others weren't explained at all, which I guess leads me to my question: When a ball bounces on the ground, it gradually loses kinetic energy. Since the principle of conservation of energy says that energy isn't converted or destroyed, what happens to the kinetic energy from the ball after it disperses through the ground? The ball is eventually going to stop bouncing, so presumably its energy has to go somewhere. Does it get stored in the ground somewhere? Is it converted into some other kind of energy? edit: Here are some applets that demonstrate some of the principles of physics if anyone's interested. Dwayne Bensey fucked around with this message at 00:52 on Sep 23, 2010 |
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Sep 23, 2010 00:47
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helopticor posted:Edit: Beaten, and I forgot to post my question: Feynman's Lectures in Physics are probably the best place to start. Your comments about different notation seem to suggest to me that you're doing either quantum mechanics (Dirac notation) or general relativity (covariant/contravariant indices and the Einstein summation convention), and you can look up some stuff about those notations on Wikipedia when you get confused. As for physical intuition, the best way to develop it is probably by watching videos, since they can do so much more than static pictures. Unfortunately there's not a whole lot of good videos for rigorous treatments of quantum mechanics or general relativity, but for classical physics and special relativity I suggest The Mechanical Universe, which you ought to be able to find on some video hosting site. Especially if you like the 80's and Professor David Goodstein of Caltech cracking lame jokes about famous dead physicists. Dwayne Bensey posted:Cool thread. I had a bit of a negative experience with physics in secondary school, but it's still something that I'd love to have even a basic understanding of. I'm reading the book that accompanies Carl Sagan's Cosmos at the moment, and he explains everything in it in a really simple, straight-forward way that doesn't assume you have any prior knowledge of the principles of physics and so on. It seemed like whenever I was studying physics in school, some things were explained poorly, while others weren't explained at all, which I guess leads me to my question: Heat and sound (which also eventually dissipates into heat). Heat, or I should say internal energy, is the energy in all the atoms/molecules of a substance bouncing around randomly. So the orderly bouncing of your ball on a macroscopic level is being converted into a disorderly bouncing around on a microscopic level.
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Sep 23, 2010 01:00
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DontMockMySmock posted:Feynman's Lectures in Physics are probably the best place to start. quote:for classical physics and special relativity I suggest The Mechanical Universe, which you ought to be able to find on some video hosting site. Especially if you like the 80's and Professor David Goodstein of Caltech cracking lame jokes about famous dead physicists.
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Sep 23, 2010 02:52
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helopticor posted:To confirm, I should look at the lectures themselves, as opposed to this "six easy pieces and six not so easy pieces" thing I keep hearing about, right? Ideally, yeah, but theres no reason you can't start with the pieces thing.. Those are 12 chapters straight out of a 125 chapter series. With most of the easy pieces being from the beginning of volume one where math never really comes into play. Unless you have a library card or some cash (~190 new, ~100-30 used) the 6 easy 6 not easy pieces isn't really a bad place to start.
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Sep 23, 2010 03:07
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DontMockMySmock posted:The envelope is mostly held up by radiation pressure, i.e. the charged particles in the envelope are bombarded by enough light such that the light holds them up against gravity. What the hell? Can you explain how this works? That's amazing!
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Sep 23, 2010 03:55
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What are some problems in physics that humans do not yet understand, but are likely to understand within the next 10-50 years? I am curious to see where physics may be headed in my lifetime. Since we are talking about the future nothing of course needs to be concrete, I just want to know what is currently being worked on in the research field.
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Sep 23, 2010 04:38
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Ok, here's something from the Wikipedia article "Observable Universe": The age of the Universe is about 13.75 billion years, but due to the expansion of space we are now observing objects that are now considerably farther away than a static 13.75 billion light-years distance. The diameter of the observable universe is estimated to be about 28 billion parsecs (93 billion light-years),[1] putting the edge of the observable universe at about 46-47 billion light-years away. How the hell does this work? The universe is expanding at a rate faster than the speed of light, but apparently this does not affect our ability to observe radiation from objects that are at a distance greater than the time the radiation has had to travel from them. If I understand right, we can observe objects at, say, 20 billion lights years away, even though they've only been around to emit traces of their existence for a fraction of that time. I visualize this as EM waves "stretching" along with the universe so that the time it takes them to reach us does not actually change, even though the distance does. No idea if this is right.
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Sep 23, 2010 05:19
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Evan Montegarde posted:I visualize this as EM waves "stretching" along with the universe so that the time it takes them to reach us does not actually change, even though the distance does. No idea if this is right. You visualized correctly! Isn't it cool?! Light from older stars/galaxies is redder than younger stars/galaxies.
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Sep 23, 2010 06:23
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1. At what point do you become incredulous with regards to the theory? I mean, there are many things that are theorized (and supported with math?) but not (directly) observed. I'm sure most physicists "accept" the existence of black holes, but we don't directly observe them. We look at how things orbit around them and the radiation that is emitted from the accretion disk. The evidence is suggestive (I'm sure there's way more), but is it overwhelmingly in their favor? Should we take their existence for granted in the same way we take the Earth's shape for granted? Is there room for skepticism? What about obscure and exotic things like wormholes and tachyons? Do you "believe" in them? 2. What do you think about dark matter? Do you think there is this unaccounted-for mass or can it be that our current theory is flawed? I've heard of people attempting to "patch up" Newtonian mechanics to avoid this missing mass problem. Hopefully I don't forget about this thread. metainfinity fucked around with this message at 06:52 on Sep 23, 2010 |
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Sep 23, 2010 06:49
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How the hell do they get those pictures you see of the Milky Way from the outside considering that we're inside of it?!? http://www.utahskies.org/image_library/deepsky/messier/m031/M31-RobertGendler.jpg Surely you can't send a probe that far since probably wouldn't even get to Proxima Centauri in the 50ish years since space travel came along. Is it some other galaxy or some sort of artificial construction?
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Sep 23, 2010 09:39
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helopticor posted:To confirm, I should look at the lectures themselves, as opposed to this "six easy pieces and six not so easy pieces" thing I keep hearing about, right? I actually haven't read them, I've just heard they're good from so many people. I guess take Yiggy's advice. As for the science videos thread, that's reminded me of a good one that bears mentioning here: Powers of Ten. Go watch it now if you've never seen it! Its picture of the large-scale universe is a bit out of date, in that it doesn't explore structures as large as we know about today. It's the best way I've ever seen of explaining the incredible differences between the sizes of things we know and the sizes of the astrophysical and quantum worlds. LankyIndjun posted:What the hell? Can you explain how this works? That's amazing! It's pretty crazy, because we don't usually think about light being able to push things, but it's all perfectly logical if you think about it. Light is an electromagnetic wave. Therefore, it can interact with things that have electric charge, such as subatomic particles. An interaction can involve a change in momentum, a little push, if you will. Therefore the light must have momentum to give up to the particles. The amalgam of many such pushes from a lot of light can push it enough to hold it up against the gravity of the star. There has to be a LOT of light, though - that's why it only happens with the particularly bright, dense, hot cores of old stars. Greve posted:What are some problems in physics that humans do not yet understand, but are likely to understand within the next 10-50 years? I am curious to see where physics may be headed in my lifetime. Since we are talking about the future nothing of course needs to be concrete, I just want to know what is currently being worked on in the research field. Here's some questions that come to mind; for each of these there is at least one large group of physicists who thinks its answer is just around the corner. What's the deal with gravitational waves? Do they exist, do they behave as general relativity says? What're the masses of the neutrinos? Do they have mass through the Higgs mechanism or through the Majorana mechanism (don't ask me to explain the Majorana mechanism, I don't know anything about it)? What's dark matter made of? Is the "WIMP miracle" more than a coincidence, and is supersymmetry the answer? How do we resolve the problem with the Higgs boson mass (the Standard Model predicts that it should be infinite), and what IS the Higgs mass? Is it possible to produce a room-temperature superconductor? How did the very first stars in the universe behave? What is the origin of the one-per-billion imbalance between matter and antimatter in the early universe? What is dark energy? Evan Montegarde posted:Ok, here's something from the Wikipedia article "Observable Universe": This is basically right, but it depends on how you look at it. It has to do with our point of view; our reference frame from here on/near Earth. The event of this light's emission is in our past light cone, even though the spatial coordinate corresponding to that event is now forty billion light years away. I visualize it like this (lovely mspaint warning):  Click here for the full 970x1058 image. metainfinity posted:1. At what point do you become incredulous with regards to the theory? Exotic and incredible theories don't gain traction in the scientific community unless they have significant evidence. Let's start with black holes. Black holes are a natural consequence of general relativity. General relativity ALSO predicts things like the gravitational bending of light different from the classical prediction and the orbit of Mercury precessing the correct amount. These are the primary reasons that general relativity was accepted. Black holes were sort of a curiosity until Subrahmanyan Chandrasekhar calculated that a stellar core larger than 1.44 times the mass of our sun would collapse completely unless it was held up by some unknown force. Neutron stars provided a partial solution, but a few years later Oppenheimer among others figured that neutron stars would collapse if they were much bigger than about three times the mass of the sun. What's left? There's no other physical concept we know of to stop that from collapsing into a black hole, and once it does, general relativity says that it is literally impossible to stop it from collapsing entirely into a singularity. So, black holes seemed inevitable from our understanding of thermodynamics, statistical quantum mechanics, and general relativity. By twenty years later, black holes and neutron stars became the subject of theoretical research, although neither had ever been observed. People were willing to hypothesize their existence because they followed from theories that had been well-confirmed in other ways. Since then, we've seen pulsars, and found convincing evidence for them being neutron stars. We've seen quasars, and found convincing evidence for them being accreting supermassive black holes. And most importantly of all for this discussion, we've been able to look at the very center of our own galaxy, where we can see a cluster of stars all seeming to orbit a single supermassive object. But when we look where that object ought to be, we see nothing. Blackness. An object small enough to fit within our own solar system, yet over four billion times more massive than our Sun, emitting nothing at all. Sure sounds like a black hole to me. Dark matter has a similar story. Its existence follows from general relativity and the observations we have made. We like general relativity a lot, so we are loathe to abandon it (although some physicists do try - someone should). But even if GR is wrong, there is a good reason to think dark matter will still be around. The reason is this: there are parts of the universe with small amounts of regular mass and large amounts of gravitational lensing, and parts of the universe with large amounts of regular mass and less gravitational lensing. The former must have something that's making it lens more than the latter. Dark matter is a remarkably simple solution. antwizzle posted:How the hell do they get those pictures you see of the Milky Way from the outside considering that we're inside of it?!? The filename tells me that this is M31 - Messier object number 31, commonly called "the Andromeda galaxy." It is the nearest large spiral galaxy to the Milky Way, the other major player that dominates our local group of galaxies. It's generally thought for various reasons that the Milky Way and the Andromeda galaxy are very similar, and if you could get outside the Milky Way and look at it, it would look like this. Consequently, sometimes it is used as a "picture of the Milky Way," which it is not. Getting far enough away to get a view of the Milky Way like this, even if you are traveling at the speed of light, would take tens of thousands of years - longer than human civilization has been around.
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Sep 23, 2010 09:59
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DontMockMySmock posted:Dark matter has a similar story. Its existence follows from general relativity and the observations we have made. We like general relativity a lot, so we are loathe to abandon it (although some physicists do try - someone should). But even if GR is wrong, there is a good reason to think dark matter will still be around. The reason is this: there are parts of the universe with small amounts of regular mass and large amounts of gravitational lensing, and parts of the universe with large amounts of regular mass and less gravitational lensing. The former must have something that's making it lens more than the latter. Dark matter is a remarkably simple solution. 
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Sep 23, 2010 10:45
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pigdog posted:So... could you explain what exactly is dark matter? Can we somehow see or interact with it, or if not then why not? Perhaps it is something physicists just sorta made up to make the calculations match. I assure you if DontMockMySmock could answer this question he would be sipping expensive wine and laughing all the way to the Nobel committee. DontMockMySmock posted:The filename tells me that this is M31 - Messier object number 31, commonly called "the Andromeda galaxy." It is the nearest large spiral galaxy to the Milky Way, the other major player that dominates our local group of galaxies. It's generally thought for various reasons that the Milky Way and the Andromeda galaxy are very similar, and if you could get outside the Milky Way and look at it, it would look like this. Consequently, sometimes it is used as a "picture of the Milky Way," which it is not. Getting far enough away to get a view of the Milky Way like this, even if you are traveling at the speed of light, would take tens of thousands of years - longer than human civilization has been around. I have been lied to all my life and feel a little violated.
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Sep 23, 2010 10:51
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pigdog posted:So... could you explain what exactly is dark matter? Can we somehow see or interact with it, or if not then why not? Perhaps it is something physicists just sorta made up to make the calculations match. That's pretty much what it is. We observe, through gravitational effects, that there is matter there - but we can't see it. Hence, "dark matter." Since the Milky Way has a lot of dark matter in it, if dark matter is a previously-unknown subatomic particle (as is commonly thought), there's dark matter passing through your body all the time. But it interacts with normal matter so scarcely that it's near-impossible to detect. People are trying to detect it, but have been so far unsuccessful. The dark matter problem could also be resolved by the dark matter particle being created directly in an accelerator, which is among the stronger motivations for the LHC.
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Sep 23, 2010 10:54
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Can you explain super fluids and rollin films? And why helium doesn't freeze at 0K?
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Sep 23, 2010 15:19
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Cymbal Monkey posted:Can you explain super fluids and rollin films? And why helium doesn't freeze at 0K? I'd take a wild guess and say that for it to freeze it would require the electrons in the outer shell to not move, violating the uncertainty principle, so their continued "jiggling" prevents helium from freezing even at 0K. I would imagine. I'm probably wrong though.
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Sep 23, 2010 19:23
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Argh, I totally made a long post about phase diagrams and absolute zero but it didn't get posted somehow; here goes take two. First of all, regarding superfluid helium, I can't really explain much more than I already have. I've never studied it in detail. But anyways. Absolute zero. Absolute zero is not a total cessation of motion. From a naive point of view, you can think of it as a cessation of translational motion. Vibrational and rotational motions can still go on. But you can't ever get to absolute zero, because the closer you get something to absolute zero, the harder it gets to cool it. So you can only get closer and closer, never reaching it. Besides, as Gavrilo Princip points out, once you take a quantum mechanical view, there's always quantum jiggling. All of this applies to solids cooled towards 0K as much as it does helium; the atoms in solids are all moving about, they're just doing it while loosely constrained to some sort of lattice. Anyways. Helium does have a solid phase, it just only occurs at pressures you would probably consider high:  "Helium I" is ordinary liquid and "Helium II" is superfluid. Pressure is on the vertical axis and temperature on the horizontal axis. You can see that at low temperatures and high pressures, there is a solid phase, but at temperatures as low as atmospheric pressure it just goes superfluid. Compare this to water:  When water is at low pressure, it goes straight from gas to solid, and at even lower pressures, it doesn't leave the vapor phase at all (helium does that too, everything does, it's just not shown on the phase diagram I used). Basically, the moral of the story is, different materials behave differently at different pressures, and your intuition is entirely based on water at one atmosphere, which isn't the whole story.
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Sep 23, 2010 19:41
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Speaking about phase diagrams, I skimmed the wikipedia page, and it didn't really help much: What's up with the critical point where the line between liquid and gas just stops? The article almost seemed to imply that there should probably be a vertical line and a horizontal line coming out of it to represent stuff that's not really a liquid or a gas, but even if that's the case, it seems weird that it would be a perfect rectangle on a phase diagram.
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Sep 23, 2010 20:37
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helopticor posted:Speaking about phase diagrams, I skimmed the wikipedia page, and it didn't really help much: What's up with the critical point where the line between liquid and gas just stops? The critical point is where there is no distinction between liquid and gas and it becomes a supercritical fluid. I'm not sure how it's measured though but I do know those fluids are pretty cool!
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Sep 23, 2010 21:19
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helopticor posted:Speaking about phase diagrams, I skimmed the wikipedia page, and it didn't really help much: What's up with the critical point where the line between liquid and gas just stops? It's kind of like this: imagine that the density of a substance as a function of pressure and temperature is represented by the height of a surface. When there is a phase change, there is a discontinuity - a jump from low density to high density. A stair or cliff, if you will. This is represented by a line on the phase diagram. But, as you move along the line, the difference in height (density) between the two regions can change. So take the liquid/gas phase change of a substance. As you increase the pressure/temperature and follow the phase change, the liquid gets denser and the gas gets denser. But the gas gets denser faster; the density of the liquid changes hardly at all (it's not very compressible, this is why we call it a liquid instead of a gas). So the height difference of the cliff is getting smaller even as both levels are increasing in height. At some point, the two heights are going to meet, and the cliff disappears. That's the critical point. Here's a plot I hacked up in Wolfram|Alpha to illustrate how a discontinuity can vanish smoothly like that, if you're having trouble visualizing it. If you are changing the temperature/pressure of a gas, you can change it in a path that takes it around the critical point, so there's no phase change, but you end up with a thing that looks like an ordinary liquid to you. If you took a different path, you'd cross the phase change, and it would be obvious that you've just changed gas to liquid. When you're dealing with critical phenomena, it almost doesn't make sense to think of phases as separate entities - only think about phase transitions, which is just an interface in the pressure-temperature curve where there is a discontinuity in the properties of the substance. Here's a video where they try to get water as close as possible to the critical point. It starts as a supercritical fluid, which behaves qualitatively like a gas. As it cools, and small bits of liquid condense, you see the liquid and the gas mix together in crazy ways because they have nearly the same density. Then it eventually becomes clear that there is some gas and some liquid, because it is cool enough for there to be a liquid.
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Sep 23, 2010 21:29
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Has there been any news on the general topology of the universe? I read a book a while ago that mentioned evidence that it's shaped like a dodecahedron with opposite faces rotated and identifed (I forgot by how much it was rotated by. 1/10, 3/10 and 1/2 a turn all yield different structures). Any news on this?
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Sep 23, 2010 22:15
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Willheim Wordsworth posted:Has there been any news on the general topology of the universe? I read a book a while ago that mentioned evidence that it's shaped like a dodecahedron with opposite faces rotated and identifed (I forgot by how much it was rotated by. 1/10, 3/10 and 1/2 a turn all yield different structures). Any news on this? I had never heard of this, but I did some reading and it apparently isn't nearly as crackpot as it sounds. Here's my take on it. Most cosmologists believe that the universe is flat and essentially infinite. From this model, you can make predictions of the variations in the cosmic microwave background (CMB) at large scales (small scale variations are caused by matter and galaxies and other messy things, but the large scale is dominated by the overall shape of the universe). Our observation of the CMB by the Wilkinson Microwave Anisotropy Probe (WMAP) showed a small but distinct deviation from those predictions at the largest scales; an unexpected lack of variation - the CMB looked smoother than it ought to have at those large scales. Apparently, this data fits in with a model of the shape of the universe that is a Poincare sphere that is basically a closed sphere with pentagonal regions (arranged like a dodecahedron) that are linked to the opposite pentagon, or something like that. I'm not sure I really understand it. In any case, it's not the only explanation, and until the anomaly in the data is confirmed by future experiments, I wouldn't get too worked up about it.
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Sep 24, 2010 01:41
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DontMockMySmock posted:It's kind of like this: imagine that the density of a substance as a function of pressure and temperature is represented by the height of a surface. When there is a phase change, there is a discontinuity - a jump from low density to high density. …When you're dealing with critical phenomena, it almost doesn't make sense to think of phases as separate entities - only think about phase transitions, which is just an interface in the pressure-temperature curve where there is a discontinuity in the properties of the substance. This made a lot of sense; thanks. Also, the Poincare sphere thing is kind of interesting; it's a weird shape, to say the least.
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Sep 24, 2010 02:55
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helopticor posted:Also, the Poincare sphere thing is kind of interesting; it's a weird shape, to say the least. Actually, maybe you can explain that to me. I really don't get it; I've never done any topology or anything formally.
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Sep 24, 2010 03:01
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I am glad I have finally found someone who has found the same "spiritual" resolve from the utter unfathomability of the cosmos. I was also raised Christian and have finally completely detached. I was 16 when I stopped following any of the rules, but it took a lot more for me to comfortable believe in a world without an all-powerful cloud-riding giant. What were we thinking? I tried to write one question, but it led to more: - Is there a general consensus among scientists about the relativity of time, or does its definition rely on that of the universe's form, like string theory VS loop quantum gravity? - If our entire lives exist at the same time / all the time, couldn't it be assumed this is true for the entire universe? If so, rather than there being a beginning point (Big Bang), wouldn't all of time exist harmoniously together?
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Sep 24, 2010 04:00
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Don't Mock my Smock, I'd like to tip my hat to your abilities to answer and field such a breadth of physics questions, you really deserve the Lucasian chair when Hawking passes it along. My quintessential question is: Am I the greatest physicist to have ever lived, or could you argue that there is someone else who has ultimately had a more critical role in laying down the foundations of modern science than I? Sir Isaac Newton
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Sep 24, 2010 04:11
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I was watching what I thought was a bullshit doc on the Science Channel and Leonard Susskind said that the Holographic Principle was now mainstream physics. How did this come to happen? Mind=blown.
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Sep 24, 2010 05:21
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czarmonger posted:- Is there a general consensus among scientists about the relativity of time, Spacetime is relative. Any given situation has different ways of formulating space and time, all equally valid, depending on your point of view as an observer. This is more than consensus, it is fact. czarmonger posted:or does its definition rely on that of the universe's form, like string theory VS loop quantum gravity? I'm not sure I understand what you're getting at here. How you properly formulate your spacetime depends on what's in it, according to Einstein's Equations. An successful string theory of gravity or loop quantum gravity theory both ought to reproduce the same equations. czarmonger posted:- If our entire lives exist at the same time / all the time, couldn't it be assumed this is true for the entire universe? If so, rather than there being a beginning point (Big Bang), wouldn't all of time exist harmoniously together? At the risk of sounding like a broken record, I'm not sure I understand what you're getting at here, either. I think what you're trying to say is, "what if there really isn't any time, and the entire universe, start to finish, is one harmonious instant?" I'm not sure that question can ever be in the domain of physics - it sounds more like philosophy, and particularly pointless even for philosophy (sorry, I call 'em like I see 'em). I experience a progression of events in a regular manner, and I choose to call this experience "time," and here's a set of physical laws using that concept. I am also confused why, when talking about time, you make a distinction between our lives and the universe. A life is not a measurable thing; matter flows from place to place, and momentarily comes together to be you; there's no reason why time should be different for your life than for the entire universe. Josh Lyman posted:I was watching what I thought was a bullshit doc on the Science Channel and Leonard Susskind said that the Holographic Principle was now mainstream physics. It's mainstream, and often accepted, but it's not proven and probably won't get proven for a long time. It really only has an impact one way or another in the field of quantum gravity, so only quantum gravity people bother wondering whether it's true. It's certainly well-known and there's no big arguments against it, so "mainstream" might as well be the word for it.
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Sep 24, 2010 05:29
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This next answer deserves its very own post. Sir Isaac Newton posted:Don't Mock my Smock, First of all, let me say I am honored to receive such praise from the second ever Lucasian Chair, the founder of mathematical physics, and the co-inventor of the calculus. Second of all, let me say how happy I am that you donated Lowtax  .Thirdly, I was under the impression that you were more humble of your work in the natural sciences, focusing instead on mathematics and theology. To even suggest of yourself that you are the greatest ever physicist takes a lot of balls. Now, allow me to address your actual question. I'm afraid you lose the title of Greatest Physicist Ever to a man you may have heard of in recent decades. His achievements include revolutionizing three different fields of physics in a single year when he was only 26 years old, revolutionizing a fourth ten years later, and having a sweet mustache and hairstyle. I know what you're thinking. "I had my own annus miribalis when I was only 23 years old, not 26, I invented the reflecting telescope, and there's no way his mustache has anything on my rock-hard chin. Who is this poser who would dethrone me?" That man is the father of modern physics. The one, the only, Albert Einstein.  I know what you're thinking again. "That loser? He only did so much by expanding on MY theories! His general relativity would be nothing without my Universal Gravitation, his special relativity would be nothing without my Mechanics, his theory of the photoelectric effect would be nothing without my Optiks! Besides, he pissed away his success by wasting time arguing about whether the God he didn't believe in 'rolled dice.'" Well, I'll admit that your Universal Gravitation was an amazing discovery, and you deserve much praise for a leap of logic that no other could have made. But mechanics and optics were ripe for the picking, their secrets laid bare to anyone who had access to a measuring-stick and a glass grinder. I don't mean to diminish your achievements, far from it, I just want to illustrate a contrast: Einstein discovered things that no one else could discover, just by sitting around in a Swiss patent office thinking really hard. Everything he did was as awesome as your Universal Gravitation. He had such an incredible sense of how physical reality arose from mathematical beauty. He took the things that other people couldn't explain, explained them, and turned reality one hundred and eighty degrees while doing so. As you point out, his physical intuition failed once - with regards to quantum mechanics, the field he helped found with his theory of the photoelectric effect. But for this, he must be praised also. In attacking quantum mechanics so efficiently and thoroughly, he managed to find all of its little weaknesses, so that it could be corrected and strengthened. As a final point, you may also note that part of why his theories were so revolutionary is because they proved all of your theories wrong. Ladies and gentlemen, Albert motherfucking Einstein. DontMockMySmock fucked around with this message at 17:22 on Sep 24, 2010 |
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Sep 24, 2010 05:29
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Two quick questions 1) In the entirity of history, has any matter gone into the Event Horizon of a black hole? 2) I set up a collosal double-slit experiment involving electrons. Since electrons have mass and thus gravity, running two lasers in parallel to the length of the set up for an appropriate time/length will cause one of them to shift more than the other - allowing me to assertain which-path information from the gravitational interaction. Will or will I not get an interference patten? I'll leave out the 'speed of gravity' questions because debates about it tend to get a bit heated!
spikenigma fucked around with this message at 11:32 on Sep 24, 2010 |
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Sep 24, 2010 10:25
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DontMockMySmock posted:
The view taken by almost all human beings including yourself is that there's a 'start' to time at t=0 and then an 'end' to time at t=f or t goes to infinity. And there's some sort of arrow called the present that moves from t=0 to t=f and whatever instant it is pointing at is the 'present' and all other instants just don't exist. So you could draw the whole of time out like a line with a star indicating the present. |===*==========| The present time 3 o'clock and some time later |=========*===| The present time 10 o'clock. Some of the wording you use above is quite interesting. You say that this is not a matter for physics and that you call to choose this experience time. Now the thing is with those two diagrams above is that what clock is there that defines the present? It is quite a Newtonian view of having x,y,z coordinates defined in space and a big, holy clock outside the universe somewhere that keeps ticking away that defines the present. But physics should only be the science of things that physically exist, that you could in principle reach out and touch. You could never reach out and touch this clock, or point to it so in what way can we say that it exists? What is it that makes the 'present' somehow more valid than the past or the future? Could all instants from t=0 up to t=f just exist without the need for an external clock? Now you say that you experience time and that is obviously quite correct. But, if your brain is just made up of matter in a particular configuration the same as anything else in the universe, then at this instant you could think "Now is the present time." But many instants ago, say a year ago, there is also matter made into the configuration that we call your brain and that brain will be thinking "Now is the present time." In this way time flowing is just a matter of your perception. So we end up with a picture like this |*************| All instants of time are equally valid and 'past' and 'future' just describe the way they are arranged. Gosh I hope that made some ounce of sense and that you don't now think I am a drivelling moron. There are a lot of people who don't accept this view and I'm not demanding that you do but it would be nice if you would approach the idea with an open mind and give an honest thought to it. Incidentally I believe that Einstein subscribed to this view of the universe but my source for this is an episode of In Our Time (a BBC radio show about science, philosophy and history that isn't usually about time despite the title).
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Sep 24, 2010 10:26
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FightingMongoose posted:… What do you mean by "valid" here? Depending on what you mean, the answer might be "nearly every physicist subscribes to that latter view", "that's a question that physicists generally don't think about because it's more of a philosophy question", "that's a question that physicists generally don't think about because it's more of a question for neurologists", etc. I think you need to pin down what you mean by this more precisely.
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Sep 24, 2010 13:54
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By valid I mean that the present exists but the past no longer does and the future doesn't yet.
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Sep 24, 2010 14:40
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spikenigma posted:Two quick questions Matter falls into event horizons all the time. Within our own galaxy we've observed many "X-ray binaries," systems where a black hole is accreting matter from a companion star. The in-falling matter releases X-rays as it falls in. In distant galaxies, we've observed active galactic nuclei, where the same thing is happening on a colossal scale at the center of a distant galaxy. As for your double slit experiment, there will be no interference pattern on your detector. However, this isn't due to gravity in this case - the electromagnetic effects are so much stronger. That is, if they're strong enough to detect at all - if your interferometer can't show the difference, then the electrons will interfere. If you used a hypothetical chargeless electron instead, your interferometer would have to be so much more sensitive, but if it could be, there would be no interference pattern. You can't know which slit it went through and have an interference pattern. FightingMongoose posted:Likewise at sounding like a broken record, if you feel I'm making GBS threads up your thread I can stop. No, it's fine, you needed to explain yourself better. I think I understand what you're getting at, which is this: the present clearly exists, but does the future and past also exist? Well, I've got good news and bad news. The bad news is, "present" isn't clearly defined in relativity. Any given observer has a moment they call the "present," but it's different for any observer. For example, imagine you are standing next to a train track and another person zooms by on the train. At the point where your paths cross, where you are (essentially) at the same point in space and time, you still have wildly different ideas of what's the "present." His "present" includes events that you would consider in your "future" and events in your "past." Which brings us to the good news. When you look at it like this, the pragmatic point of view is that your "past" and "future" DO exist, since they're in someone else's present. Any other view borders on solipsism.
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Sep 24, 2010 17:39
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DontMockMySmock posted:Matter falls into event horizons all the time. Within our own galaxy we've observed many "X-ray binaries," systems where a black hole is accreting matter from a companion star. The in-falling matter releases X-rays as it falls in. In distant galaxies, we've observed active galactic nuclei, where the same thing is happening on a colossal scale at the center of a distant galaxy. Could you expand on this. Take us as observers, we will never see anything cross the event horizon. If you just use our time co-ordinates does the object actually cross the event horizon?
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Sep 24, 2010 18:53
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Unparagoned posted:Could you expand on this. Take us as observers, we will never see anything cross the event horizon. If you just use our time co-ordinates does the object actually cross the event horizon? Okay, so, technically speaking, if you use asymptotically flat coordinates, and you observe from infinity, which is a good approximation of our point of view, for a non-spinning, non-charged black hole, nothing ever falls past the event horizon from a distant observer's point of view, because in that point of view, the horizon is a coordinate singularity. But that's just an approximation; real black holes spin and have charge, we're not infinitely far away, and the space we sit in isn't quite flat. From any practical point of view, stuff falls into black holes all the time.
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Sep 24, 2010 19:13
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I am quickly getting out of my depth here but I think it is possible to describe spacetime as a set of spacelike surfaces*. It is each of these surfaces that would make up an instant of time. So while you are right that an event that is in the present for one observer might be in the future of another observer you could look at spacetime from a different angle if you like and define surfaces that contain only events that are in the present for all observers on the surface. *ADM formalism, wikipedia and scholarpedia seem to have good write ups on this, although wikipedia seems to be missing a crucial reference see "Quantum Gravitation: The Feynman Path Integral Approach" section 4.3
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Sep 24, 2010 19:57
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FightingMongoose posted:So while you are right that an event that is in the present for one observer might be in the future of another observer you could look at spacetime from a different angle if you like and define surfaces that contain only events that are in the present for all observers on the surface. It's the present for all observers on the surface who aren't moving relative to it. If they're moving, they would have a different idea of what the "present" is. Besides, the concept of "present" isn't very well-defined anyway; I'm using it to mean "a set of events that are simultaneous in a particular coordinate system." Since coordinate systems are arbitrary, so is the concept of a "present." But if you're an observer the natural choice is to choose a coordinate system where you are at rest, and if two different observers do this they will have two different ideas of what's a simultaneous set of events. You might even say that spacetime, and hence simultaneity, is relative, and you might describe this through some theory of relativity. (okay maybe that was a little too facetious.) Seriously though, your intuition is just completely wrong here, which is perfectly normal. All your real-life experience, cultural influence, and brain wiring goes against this concept, but that doesn't stop it from being true. The ADM formalism, for any given spacetime, is equivalent to some other formalism that doesn't conceptually split spacetime into 3 + 1 (space and time) coordinates. It's just useful because it provides a framework for numerically simulating general relativity, since the physical information at any slice depends only on the information on the slice that came before. There's not really any conceptual benefit to it. DontMockMySmock fucked around with this message at 21:40 on Sep 24, 2010 |
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Sep 24, 2010 21:38
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DontMockMySmock posted:Actually, maybe you can explain that to me. I really don't get it; I've never done any topology or anything formally. Alright, I'll try to explain this with as few mathematical asides as I'm capable of, since this is a thread about physics. First, I'll explain the idea of "gluing" with some examples. As soon as you feel comfortable with the idea of gluing things, you can skip to the  . Consider a square (filled in, so that it's like {(x,y)|0≤x,y≤1} in the plane). Alone, this is a bounded region. If you start somewhere, and you keep going up, you'll hit the top of the square. We can fix this in a number of ways. One way to do this is to "glue" the top edge with the bottom edge (in the same orientation). Now, if you keep going up, you'll come out the bottom edge with the same x-coordinate. This is usually represented schematically like this:  . Now, if you think about it, since if you go right or left, you hit a wall, and if you go up or down, you go around. This means that this is roughly like the shape of the surface of a horizontal cylinder of finite length with open ends. If you're familiar with Moebius strips, then you can get the rough shape of one by gluing the top edge with the bottom edge in reverse orientation:  . If you go up to the point (x,1) and keep going up, now you end up at (1-x,0) instead of (x,0).However, in both of these examples, there are still the x=0 and x=1 boundaries. If we do more gluing, we can get rid of them. One thing to do is to take the "cylinder", and glue those two sides of the square to each other (in the same orientation):  . Now if you go right you pop out the left side at the same height. If you think about it hard enough, this is like gluing the two circular ends of the cylinder, which yields the surface of a torus (donut-shape). [Nerd note: This is why the maps in old RPGs that wrap around this way imply the game world is a torus, not a sphere.]A torus is a weird space, so as a final example, here's how you can get (roughly) the surface of a sphere:  (the green line means nothing). To help you see why this is similar to the surface of a sphere, you can fold the top left triangle into a cone so that the red sides match up, and similarly for the bottom right triangle, to get the surface of this double-cone thing:  -----------------------------Now that you understand the idea of gluing in 2D, I can describe the Poincare sphere. Instead of gluing the edges of a filled in square, we'll glue the faces of a filled in dodecahedron:  Click here for the full 256x256 image.. Each face will be glued to the opposite face. However, there's a problem with this: the opposite face does not line up right (unlike for a cube). To fix this, glue each face to its opposing face after a 36ο twist. This is hard to visualize, but if you ignore the extraneous triangles drawn on the faces, the second Java applet on this page does a pretty good job. Each numbered point (all points are on the boundary surface) is glued to each other point with the same number. Since the inside of a filled-in dodecahedron is a 3D space, the "Poincare sphere" is 3D as well, just a little weird (a bit like the surface of the earth is a 2D space, just a little weird). dirby fucked around with this message at 23:29 on Sep 24, 2010 |
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Sep 24, 2010 23:24
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| # ? Apr 25, 2024 16:08 |
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DontMockMySmock posted:Okay, so, technically speaking, if you use asymptotically flat coordinates, and you observe from infinity, which is a good approximation of our point of view, for a non-spinning, non-charged black hole, nothing ever falls past the event horizon from a distant observer's point of view, because in that point of view, the horizon is a coordinate singularity. But that's just an approximation; real black holes spin and have charge, we're not infinitely far away, and the space we sit in isn't quite flat. I think the question was alluding to the fact that it takes infinite schwarzschild time for an object to fall into a black hole. However an observer in freefall is not a schwarzschild observer and so can see objects fall into a black hole in finite proper time. FightingMongoose posted:I am quickly getting out of my depth here but I think it is possible to describe spacetime as a set of spacelike surfaces*. It is each of these surfaces that would make up an instant of time. Not all spacetimes can be described like this. For example this certainly won't work if "time travel is possible". The technical term for those spacetimes that admit a well behaved description of this sort is "globally hyperbolic". DontMockMySmock posted:The ADM formalism, for any given spacetime, is equivalent to some other formalism that doesn't conceptually split spacetime into 3 + 1 (space and time) coordinates. It's just useful because it provides a framework for numerically simulating general relativity, since the physical information at any slice depends only on the information on the slice that came before. There's not really any conceptual benefit to it. It may just be a matter of the different points of view of mathematicians and physicists, but I completely disagree with this statement, especially the bolded part. Physical theories only gain predictive power (i.e. the ability to predict the future) when they admit formulations as initial value problems. A priori, most physical theories only describe reality as the solution to some differential equation. That we want the former and are given the latter is often readily ignored because the resolution in classical mechanics is so simple: for Newton's equations of motion, just give the initial position and velocity of everything and you can extend that to a solution, at least until something catastrophic happens. But the situation in general relativity is much more delicate. The ADM formalism is important because it says that for a certain class of spacetimes, an initial value formulation does exist. I think physicists often don't appreciate how profound this result is because they usually don't study the quite difficult mathematics behind it. I think it is also interesting to note that the difference between physics as an initial value problem and physics as a global solution to a differential equation resolves many standard time travel paradoxes. For example, the grandfather paradox arises mathematically by considering a spacetime where the causality structure has a closed timelike loop. Then there are initial data on spacelike hypersurfaces that don't extend to global solutions. I.e. you could consider initial data such that when the solution is extended "forward" you go back in time and kill your grandfather, while when you extend it backwards, you and your father are both born. It is clear that these solutions won't agree where they meet, and thus there won't be a global solution. Thus we see that the grandfather paradox only arises from our mistaken view that physics has an initial value formulation where we are free to specify initial conditions as we please. Any global solution to Einstein's equations won't have "grandfather paradoxes".
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Sep 25, 2010 00:48
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