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What do you think of this? http://www.tenthdimension.com/flash2.php quote:where does that information go? When the black hole then emits particles in the form of Hawking radiation, where does the information for those particles come from? The black hole needs to be able to store information somehow. Postgres? epswing fucked around with this message at 05:02 on Sep 25, 2010 |
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Sep 25, 2010 04:52
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DontMockMySmock posted:
Okay, fine, my theories were proved wrong by him, as this isn't such a simple 3D universe (as I thought), but at least I have a common metric unit named after me and all he gets is a synthetic element on the periodic table. (On an honest note, Dont Mock My Smock, I thoroughly enjoyed your argument, and while I still believe Einstein to be one step behind Newton and Faraday, I can understand and respect your argument 100%)
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Sep 25, 2010 04:59
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What is the physical difference between covariant and contravariant quantities? This concept is scattered around physics but usually explained in a somewhat circular manner ("here is how you convert one to the other"). Given that GR uses this concept constantly, please use your excellent explanatory powers to enlighten.
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Sep 25, 2010 06:38
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epswing posted:What do you think of this? Hoo boy. I've seen this before, but I watched it again (that is, this video) to refresh my memory of what about it was so incredibly wrong. This video talks about two different things, neither of which have ANYTHING to do with the ten-dimensional spacetime posited by string theory. First, they talk about the Trousers of Time (the concept that if things had gone differently, that would be another universe branching off from this one), motivated by the many-worlds interpretation of quantum mechanics. The many-worlds interpretation of quantum mechanics is just philosophy. It just says that the whole universe is one big wavefunction which evolves in time like wavefunctions do, with our particular realization of the universe being one of infinitely many possible universes with their different probabilities. Somehow, this video pulls two dimensions out of that. Then, it talks about the concept of a set of different multiverses with different initial conditions. This is something that physicists talk about sometime, even though it's bordering on philosophy, since (by definition) we've only got one universe to look at. Really, this can just be a straightforward extension of the multiverse concept that they just talked about, except with the different initial conditions being different legs of the Trousers of Time also. Instead, they somehow rear end-pull the remaining four dimensions out of it. Toward the end, it doesn't even make sense any more, and they don't even define what the tenth one is, only show us a point in the tenth dimension. Meanwhile, as an aside, they talk about each multiverse with different initial conditions as "Infinity" with a capital I. Not infinity anything, just Infinity. Infinity what? Infinity meters? Infinity grams? Infinity cats? Infinity universes is what they meant, the infinity of possible universes that can arise from the initial condition of our universe. But they say "we often refer to [the infinite possible timelines] as Infinity." No, we don't, hat sentence doesn't make sense. But there's nothing wrong with the many-worlds view of quantum mechanics, nor is there anything wrong with the idea of imagining different initial conditions of our universe. They're decent ideas, and can spark some interesting discussion. They just don't have anything to do with dimensions of spacetime. Okay, so what DO string theorists mean when they're always talking about ten or eleven dimensions? Well, there's one time dimension (like we're used to) and nine or ten spatial dimensions. Here's how to imagine them: Draw a line. It's one dimensional. Draw a line perpendicular to that. Your object is now two dimensional. Draw a line perpendicular to those. Your object is now three dimensional. Draw a line perpendicular to all three of those lines. Your object is now four dimensional. . . . etc etc until you get to ten dimensions. Okay, if you could imagine that, please tell me how, because I have no idea what that means or is like. String theorists offer two explanations for how the universe can be ten or eleven dimensional yet still seem to us like four dimensions. First, and more popular, is the idea of compactified dimensions. Imagine a hose. To a human, if we don't look too closely, the hose looks one-dimensional. But to an ant, crawling on its surface, it's two dimensional - except that one of those dimensions is curved on itself so that if you go around it you'll be back where you started. That's a two-dimensional surface, but one of the dimensions is compactified. Now extend that idea to higher dimensions, and you can in theory imagine a ten-dimensional thing with six of its dimensions rolled up really small. We haven't noticed because we haven't looked closely enough - we need much larger particle accelerators first. Second, there is the Flatlander idea - that we are constrained somehow to a four-dimensional surface inside a ten-dimensional universe, just as Flatlanders are constrained to a two-dimensional surface in a three-dimensional universe. I'm not sure how much this idea is accepted in the string thoery community. And neither of those concepts of ten dimensions have anything to do with anything discussed in that video, despite the video directly invoking string theory as justification for its ideas. String theorists are all crazy anyway. Sir Isaac Newton posted:Okay, fine, my theories were proved wrong by him, as this isn't such a simple 3D universe (as I thought), but at least I have a common metric unit named after me and all he gets is a synthetic element on the periodic table. http://en.wikipedia.org/wiki/List_of_things_named_after_Albert_Einstein  Sandu posted:What is the physical difference between covariant and contravariant quantities? In general, things that are physical are contravariant vectors/tensors. Energy/momentum, space/time, four-velocity, electromagnetic field tensor, stress-energy tensor, etc. When you multiply two vectors (or tensor components) together, you need to have some definition of the inner product. In ordinary vector calculus, it's a.b = sum of (ai*bi) over all coordinates i. In general relativity, it's a.b = sum of (gij*ai*bj) over all combinations of coordinates i and j. Now we can introduce a new shorthand: all vectors are written as va, the metric tensor is written with lower indices gij, and any indices repeated as both upper and lower indices are summed over ("contracted"). So the inner product between a and b is gijaibj (with an implicit sum over i and j since they are repeated as both upper and lower). Now we can introduce the concept of lowering indices, which is this: gijai = aj. It's a covariant vector. It's not a physical thing, even though ai was. It's just this thing which, if you contract it with bj (i.e ajbj, with the implicit sum over j), will give you the inner product between vectors a and b. So that's how I think about it. It's just a shorthand for properly defining the inner product between vectors. On the other hand, the more math-inclined people keep going on about dual vector spaces and some such thing as a way to define covariant vectors, and my eyes just glaze over. They say things like, Wikipedia posted:The contravariant components of a vector are obtained by projecting onto the coordinate axes. The covariant components are obtained by projecting onto the normal lines to the coordinate hyperplanes,
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Sep 25, 2010 10:20
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DontMockMySmock posted:Nothing is really known about quark-gluon plasma. The strong nuclear force manages to be pretty ineffable, for all we've studied it, so there's not a whole lot of theory to go on. There isn't any experimental data either, although there are people at the Relativistic Heavy Ion Collider who are doing their best to get some. If I may chirp in, I've worked with my school's RHIC group for almost two years now and from what I've seen and heard (only an undergrad so maybe I'm misunderstanding!) RHIC actually has a great deal of experimental results relating to flow, jet quenching, and thermodynamics, among other properties of the medium created, which is presumably a Quark-Gluon Plasma at this point. I say presumably because while the crossover temperature of ~175MeV predicted by Lattice QCD (and pQCD I think) has been exceeded in Au-Au collisions, some of the results are unexpected and difficult to explore in QCD calculations. One interesting result is that the medium created, if it is a QGP, seems to behave more like a perfect liquid with ultra low viscosity than anyone had originally predicted. The LHC is going to start one-upping RHIC soon though, as Pb-Pb collisions start at the end of the year.
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Sep 25, 2010 17:32
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EscalatorThief posted:My information was probably pretty out of date; I haven't heard anything about it in a couple years I think. Interesting stuff!
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Sep 25, 2010 18:18
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Hello! I'm an undergrad doing research in your field. My advisor is terribad at answering emails, so I'm curious if you could answer my question. Why can the (rho + 3*P) term in the second Friedmann equation be volume integrated to give a gravitic potential? This popped up in a paper that showed a certain form of dark energy (with w < -1) could yield a big rip in the long term. I feel like the answer is something obvious like "That's exactly what that term represents" but I'm not really seeing it. Calef fucked around with this message at 19:03 on Sep 25, 2010 |
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Sep 25, 2010 18:55
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Calef posted:Hello! I'm an undergrad doing research in your field. Can you link the paper in question? I'm not sure I've ever seen anything like that. It's been a while since I did cosmology, though.
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Sep 26, 2010 00:56
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Sure, http://www.slac.stanford.edu/spires/find/hep/www?irn=5482470 Paper title sounds like something out of a bad sci fi movie
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Sep 26, 2010 01:22
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Calef posted:Sure, Hmm, I'm as confused as you are. Ask one of your advisor's grad students, would be my advice. Whoever is in their office at the time. Of course, you might have a hard time getting an answer until Monday. "Phantom Energy and Cosmic Doomsday" is now my favorite serious research paper title.
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Sep 26, 2010 01:35
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How can superconductors have no resistance? Wouldn't that essentially create infinite energy?
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Sep 26, 2010 03:12
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seo posted:How can superconductors have no resistance? Wouldn't that essentially create infinite energy? How do you figure? Resistance is just the dissipation of energy. If you flow current in, say, a superconducting ring of wire, it will just continue to circulate without stopping, but that doesn't mean you're "creating infinite energy". The minute you try and do any work with that current, it will be used up, just like normal current.
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Sep 26, 2010 04:03
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right well that solves that. what do you think of fermi's paradox?
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Sep 26, 2010 07:26
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If you cool a CPU so that all the wires become super-conducting, could you then increase the clock rate indefinitely without it getting unstable or hotter? Like can you clock it at 1 terahertz if it is superconducting?
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Sep 26, 2010 08:38
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seo posted:right well that solves that. what do you think of fermi's paradox? Well, even back in Fermi's day, he had a valid point. And now, we've been able to search so much more since then, and still no result. Something must be wrong with our assumptions that lead to the paradox. For those who are not familiar with Fermi's paradox, it goes like this: Fact 1: There are a fuckton of stars in our galaxy. Something like 400 billion. Assumption 1: Most main sequence stars probably have planets, except possibly the really old, small ones which formed before there was a lot of heavier stuff for planets to form from. Assumption 2: A small but appreciable portion of those planets will have conditions suitable for life. Liquid water is probably a good start. Robust weather, seasons, and tides are probably pretty necessary, meaning the planet has to have a tilt and a moon. But there's probably plenty of those, too. It's got to have an abundance of carbon, oxygen, nitrogen, water, sulfur, phosphorus, and other trace elements, but that shouldn't be so hard since these are among the most common elements in the universe apart from hydrogen and helium. Even if one hundredth of one hundredth of planets fit this description, that still leaves 40 million life-ready planets out there. Assumption 3: If you get the chemicals necessary for life, in an environment suitable for life, in large quantities, primitive life (self-replicating chemistry) is going to spontaneously happen amongst the random bouncing of an ocean's worth of organic molecules. Assumption 4: Once you get life, it's only a matter of time before it evolves to the point of intelligence. There has been enough time for life on Earth to evolve three times over, so we can probably safely assume that there are other planets where life arose where it evolved to intelligence. Assumption 5: Long-distance communication practically necessitates the use of radio. Our radio broadcasts are loud enough to be heard throughout the galaxy. So if there are any intelligent beings in our galaxy, we should hear them. Assumption 6: Intelligent beings don't stop broadcasting radio of one variety or another. Result 1: Even by the most conservative odds, there should at least be several civilizations we are able to hear. Fact 2: We haven't heard them. Paradox! So something is wrong with our assumptions. One of these six scenarios (or some combination of them) must be the case, corresponding to our six assumptions: Scenario 1: Rocky planets are actually really rare, for astrophysical reasons that are not understood. I find this scenario unlikely and unsatisfying. Scenario 2: Life is actually a lot pickier than we realize. It can't arise unless the conditions are juuuuust right. Maybe the abundance of some rare element needs to be fine-tuned; maybe the amount of cosmic radiation from the planet's sun has to be perfectly fine tuned somehow to stimulate chemical reaction without breaking down any semi-complex molecule that arises. This is one of the scenarios that I consider pretty likely. Scenario 3: Even when conditions are good, the odds of life arising are very very slim. It took a while on Earth, so maybe it could have taken a lot longer. Amino acids, phospholipid bubbles, and other things that are the primitive building blocks of life have been produced in facsimiles of the early Earth, but we've not seen anything like a complex nucleic acid show up. Maybe a planet-wide version of those experiments isn't always enough, either. This, too, seems like it could be the case. Scenario 4: Intelligence is not the pinnacle of evolution. It may be that intelligent life is super-rare even among planets with life. While intelligence on the human level certainly is a great evolutionary advantage (as our dominance over Earth validates), maybe the things you have to evolve to get to the position where intelligence CAN evolve aren't so useful and so aren't always explored by the evolutionary process. This also could be pretty likely. We don't know a whole lot about how intelligence evolves, although we are learning more all the time. Scenario 5: Aliens are out there, they just aren't communicating by any means we can detect. Considering the pace of scientific and technological development compared to cosmic timescales, it wouldn't surprise me at all if there are plenty of alien civilizations, but they know something we don't and don't need this dumb radio poo poo. Scenario 6: Intelligent life doesn't last very long. Basically this. This is the most depressing option. Bonus scenario 7: Some combination of these scenarios points toward very slim odds that we would ever detect an intelligent civilization. Bonus scenario 8: We're right, and we're likely to detect another intelligent civilization, but likely is not certain, and we rolled a great cosmic snake eyes. People have been trying to figure out the answer, but it's not really a paradox anymore because there are enough answers and we just don't know which one is right. It's certainly interesting to think about. rawstorm posted:If you cool a CPU so that all the wires become super-conducting, could you then increase the clock rate indefinitely without it getting unstable or hotter? Like can you clock it at 1 terahertz if it is superconducting? CPUs, or logic circuit, rely on the properties of semiconductors, and the concepts of semiconductors and superconductors are mutually exclusive (since a superconductor has zero resistance but a semiconductor has resistance that varies according to voltages applied and stuff like that). While I do know a lot about the properties of semiconductor devices, I don't have any idea what the limiting factor in CPU speed is. I'd imagine it mostly has to do with the switching time of logic gates.
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Sep 26, 2010 08:45
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DontMockMySmock posted:While I do know a lot about the properties of semiconductor devices, I don't have any idea what the limiting factor in CPU speed is. I'd imagine it mostly has to do with the switching time of logic gates. Right now the heat produced by the resistance in the "wires" is the thing that limits clock rate, because if it gets too hot the CPU melts. I'm thinking that if you could cool the CPU so that everything that conducts electricity inside has no resistance, then you could raise the frequency a lot more since you don't have to worry about melting the CPU.
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Sep 26, 2010 09:20
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Most of this thread seems to contain exotic questions, so I'll try a more mundane one: How does stirring your cup of coffee create a vortex? (could never wrap my head around centri-forces) Is it also related to cooling down the hot coffee?
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Sep 26, 2010 09:42
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Can you tell me a bit about the Many-World Interpretation of QM? I've had QM classes but everything was always with the collapsing wave function and MW still confuses me.
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Sep 26, 2010 13:06
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Regarding time distortion as you get near the speed of light, as a way of wrapping my head around it: is it correct to say that the speed of light is a constant, always covering 300 million metres a second (or whatever it is), and that as the observer approaches that speed light itself still covers 300 million m/s but does so by time changing? In other words, if speed is distance over time and light always moves at light speed to the observer, is light keeping the same speed by altering time?
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Sep 26, 2010 14:16
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rawstorm posted:Right now the heat produced by the resistance in the "wires" is the thing that limits clock rate, because if it gets too hot the CPU melts. I'm thinking that if you could cool the CPU so that everything that conducts electricity inside has no resistance, then you could raise the frequency a lot more since you don't have to worry about melting the CPU. Well, then, I suppose that if you made the wires out of a high temperature superconductor, then put the thing in liquid nitrogen, you could do some cool things. Eflas posted:Most of this thread seems to contain exotic questions, so I'll try a more mundane one: How does stirring your cup of coffee create a vortex? (could never wrap my head around centri-forces) Is it also related to cooling down the hot coffee? A vortex is just a spinning fluid. When you stir your coffee, you usually do so with a circular motion, and your coffee spins. I think what you might be asking is, "why does the coffee bunch up around the edge and form a depression in the center?" This has to do with the centrifugal effect. The simplest way to explain it is that when you push the coffee around, the coffee wants to go in a straight line (all things tend to go in a straight line if nothing interferes). It can't go in a straight line, because the coffee cup is keeping it in a circle. So the walls of the coffee cup have to push it to bend its path into a circle. The result of this is that the coffee tends to go to the edge. MS Paint Time!  Namarrgon posted:Can you tell me a bit about the Many-World Interpretation of QM? I've had QM classes but everything was always with the collapsing wave function and MW still confuses me. So, a system of two quantum mechanical things can be represented by some composite wave function. Take this to its logical extreme and the entire universe can be represented by some incredibly complicated wave function. This wave function evolves naturally according to Schrodinger's equation, and there's no need for collapsing of the wave functions of particles or anything like that. We, as conscious beings, seem to observe one of the infinitude of possibilities described by this universe wave function. So the other possibilities would basically be other universes. Don't think too hard about it, it's not "Sliders," and it doesn't give any predictions different than the Copenhagen wave-function-collapse interpretation. Hydrolith posted:Regarding time distortion as you get near the speed of light, as a way of wrapping my head around it: is it correct to say that the speed of light is a constant, always covering 300 million metres a second (or whatever it is), and that as the observer approaches that speed light itself still covers 300 million m/s but does so by time changing? In other words, if speed is distance over time and light always moves at light speed to the observer, is light keeping the same speed by altering time? Time and distance both distort when you travel at speeds comparable to the speed of light, such that the velocity of two observes relative to each other is the same from either one's point of view. Say you see a spaceship go past at 60 percent of the speed of light, on a journey 60 light-years long (I chose a number that lets me do all the math in my head). That person's clock will tick 8 times for every 10 of yours, from your point of view, because of time dilation. From your point of view, it will take him one hundred years to get to his destination (60 c-yr / .60 c). From that person's point of view, the universe is zooming past him at 60 percent of the speed of light, on a journey 48 light-years long. It's shorter because of the "Lorentz contraction" effect of special relativity that makes distances look shorter in the direction an observer is traveling. His journey, then, takes him eighty years (48 c-yr / .60 c). So his eighty years corresponds to your hundred years, just as time dilation predicted. So speed is distance/time, but in special relativity, from the mover's point of view, it's (shortened distance)/(shortened time), and since distance and time are shortened by the same factor, both the mover and the non-mover agree on how fast they are moving relative to one another. Okay, now let's talk about light. When an observer gets closer and closer to the speed of light, the "boost factor" that relates time to dilated time and distance to contracted distance gets closer and closer to infinity. So if a light beam could be an observer, it would observe (zero distance)/(zero time). This is indeterminate, but we can do the limit with calculus and it's plain to see that the limit converges to the speed of light. So even light thinks it's going at the speed of light. Convenient! Of course, light can't observe things, so that's really all academic. The crux of the matter is this: the speed of light is ALWAYS the same in flat space, regardless of where it's coming from or what it's doing or who's observing it. This is the principle that you use to derive all of the equations of special relativity. The speed of light is determined from electromagnetism, and since the laws of physics are the same for everyone, everyone will derive the same speed from E&M (this was the assumption that led Einstein to his discovery). The time dilation and length contraction effects describe how you distort spacetime while keeping the speed of light constant.
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Sep 26, 2010 18:21
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DontMockMySmock posted:So, a system of two quantum mechanical things can be represented by some composite wave function. Take this to its logical extreme and the entire universe can be represented by some incredibly complicated wave function. This wave function evolves naturally according to Schrodinger's equation, and there's no need for collapsing of the wave functions of particles or anything like that. We, as conscious beings, seem to observe one of the infinitude of possibilities described by this universe wave function. So the other possibilities would basically be other universes. Don't think too hard about it, it's not "Sliders," and it doesn't give any predictions different than the Copenhagen wave-function-collapse interpretation. You lose me here. If we take the universe as a single super complex wave function, how does it follow that we observe one possibility out of almost infinite? Do we even speak of 'other possibilities' when looking at a single wave function? I'm assuming here that the statistical approach to QM is only used in Copenhagen interpretation, is that false?
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Sep 26, 2010 18:31
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Namarrgon posted:You lose me here. If we take the universe as a single super complex wave function, how does it follow that we observe one possibility out of almost infinite? Do we even speak of 'other possibilities' when looking at a single wave function? I'm assuming here that the statistical approach to QM is only used in Copenhagen interpretation, is that false? It's still statistical. We are observing one possibility for the universe, just as you might observe one possible location of an electron out of infinite possibilities. The "many-worlds" part is just the fact that there's nothing special about the possibility we observe, and so the other possibilities are also like universes. It's kind of like the "Trousers of Time" concept jacked up to eleven: the trousers have infinite legs, and they branch every time a quantum mechanical interaction happens. It's important to keep in mind that it makes no predictions different than the Copenhagen interpretation.
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Sep 26, 2010 18:48
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DontMockMySmock posted:It's still statistical. We are observing one possibility for the universe, just as you might observe one possible location of an electron out of infinite possibilities. The "many-worlds" part is just the fact that there's nothing special about the possibility we observe, and so the other possibilities are also like universes. It's kind of like the "Trousers of Time" concept jacked up to eleven: the trousers have infinite legs, and they branch every time a quantum mechanical interaction happens. With a poetic licence, am I then correct in saying that in the MWI the entirety of the universe as we have it now is in essence then 'one observation' of the possibilities?
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Sep 26, 2010 18:51
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Namarrgon posted:With a poetic licence, am I then correct in saying that in the MWI the entirety of the universe as we have it now is in essence then 'one observation' of the possibilities? Pretty much. It's all philosophical, anyway; don't get too excited.
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Sep 26, 2010 19:03
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Tell me about perspective. When I witness the destruction of something big (compared to me), like a building, it looks like it happens in slow motion. When I flick over a small tower made of lego, it looks like it happens quickly. It just falls straight over, immediately. Does an ant witness the toppling lego tower in the same "slow motion" as I do a large building? When I see small insects running along the ground, sometimes they dart left and right faster than I can follow. Is their decision making process and motor control function that fast? Or, to them, I'm just a big slow-moving building? I'm having a hard time describing this, hopefully you understand what I mean.
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Sep 26, 2010 19:26
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epswing posted:Tell me about perspective. This is actually more of a philosophy of mind question than a physics question. A better question to ask would be "Does the ant have the same phenomenal experience of 'witnessing an object' that I do?" Or even "Do ants actively cogitate about their movement decisions?" To those two questions, I would answer 'almost definitely no' to both. So, in a sense, their "decision making process" is "faster", but only insofar as they don't even really think about their decisions, it's almost purely reflexive. On the other hand, the reason the building looks like it's falling slower is because, in a relative sense, it is. The time it takes for a lego block to fall from the top of your lego tower to the floor is much shorter than it takes for a brick on the top of that building to fall and hit the ground. Both objects are being accelerated the same amount (actually, you run in to air resistance problems for objects that are larger and that are falling longer distances, which only makes the process seem even more relatively "slower").
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Sep 26, 2010 20:42
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Calef posted:A better question to ask would be "Does the ant have the same phenomenal experience of 'witnessing an object' that I do?" Or even "Do ants actively cogitate about their movement decisions?" To those two questions, I would answer 'almost definitely no' to both. I meant "from the perspective of someone as small as an ant," rather than to examine the cognitive quality of ants. vvv dude what the gently caress? epswing fucked around with this message at 23:25 on Sep 26, 2010 |
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Sep 26, 2010 22:40
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If you have an airplane and it goes on a giant treadmill (belt that you run on) how do they know to set the speed on the treadmill? For example, if I walk I will put it on 2.4, but airplanes don't have speedometers, so do you have to experiment, or is there a manual? When you shoot a gun (regular one, not a handgun or pistol) how do they keep it safe for the shooter? Does the bullet have a sensor to tell it how to go, or is it magnetic?
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Sep 26, 2010 23:23
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I'm pretty sure airplanes have speedometers.
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Sep 26, 2010 23:25
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rawstorm posted:Right now the heat produced by the resistance in the "wires" is the thing that limits clock rate, because if it gets too hot the CPU melts. I'm thinking that if you could cool the CPU so that everything that conducts electricity inside has no resistance, then you could raise the frequency a lot more since you don't have to worry about melting the CPU. Maybe it is true that currently heat is the limiting factor, but if you do an awesome job with cooling then something else will become the limiting factor. People have done overclocking with liquid nitrogen cooling.
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Sep 26, 2010 23:46
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M R GALLETA posted:If you have an airplane and it goes on a giant treadmill (belt that you run on) how do they know to set the speed on the treadmill? For example, if I walk I will put it on 2.4, but airplanes don't have speedometers, so do you have to experiment, or is there a manual?  TROLL ALERT nothing to see here, move along epswing posted:I meant "from the perspective of someone as small as an ant," rather than to examine the cognitive quality of ants. A tower a meter tall will take longer to topple than a tower a centimeter tall. So if, to this hypothetical short person, a tower a centemeter tall toppling looks "normal," then a tower a meter tall will look like it takes "a long time" in comparsion.
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Sep 26, 2010 23:48
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I'll shed some engineering insight on a few of these, if you don't mind.epswing posted:Tell me about perspective. This is related to the dimensional scaling of objects. As objects get bigger or smaller, certain properties change because not every scales at the same rate. For example, if you scaled up a mouse to the size of an elephant, the mouse would have much thinner bones. This is because the the strength of bones increases with their cross-section area (i.e. changes with the square of their size) whereas weight changes with the volume (i.e. with the cube of their size). The acceleration due to gravity, g, is a constant. You can define a "falling over time", and because g is constant, bigger things need more time. As a result, a 1m ball falling from 100m is not the same as a 1cm ball falling from 1m: their size makes them fall differently. Another less appreciated constant is the speed that nerves conduct impulses. Nerves are electrical, but they are limited to a speed of maybe 100 m/s. For a human with a 1m arm, that means it takes 10 ms just to tell your arm to do anything. For an insect with a 1mm leg, the signal reaches then leg in 10 microseconds instead. When you add in the eye->brain->thinking part, it's no wonder than humans have slower reflexes than a tiny bug: it's a natural feature of being big. This leads to some weird stuff happening. For example, a reflex action, like pulling away from something hot, happens before your brain knows about it. I think that gets handled in your spine, but it's not something I know a lot about. Note I'm probably butchering physiology here, if anyone who knows more about insects wants to correct me. Eflas posted:Most of this thread seems to contain exotic questions, so I'll try a more mundane one: How does stirring your cup of coffee create a vortex? (could never wrap my head around centri-forces) Is it also related to cooling down the hot coffee? Stirring coffee does helps cool it. To a decent approximation, when something cools in air the cooling is proportional to the speed that air moves across it. When you stir coffee, you create a sort of fake wind where the coffee meets the air: this helps the air take in heat, as well as helping the hot air move away so that cold air can come in and absorb more heat. Evaporation also increases and you can see this when you stir a steaming cup of coffee: you can make a bigger 'steam' cloud (the visible stuff is actually tiny liquid water droplets, but whatever) by stirring rapidly. Stirring also moves hot coffee to the very edges of the cup. Ordinarily when things cool their edges cool off first, which slows down cooling. By stirring, the coffee is mixed up and the hot coffee can't hide in the centre. rawstorm posted:Right now the heat produced by the resistance in the "wires" is the thing that limits clock rate, because if it gets too hot the CPU melts. I'm thinking that if you could cool the CPU so that everything that conducts electricity inside has no resistance, then you could raise the frequency a lot more since you don't have to worry about melting the CPU. For the CPU you actually have in your computer, most of the heat is generated in the transistors anyway, so superconducting wires wouldn't help that much. Even with extra cooling though, you can't boost clock speed that much. Liquid nitrogen, for example, will definitely keep heat from being an issue no matter what you set the voltage and clock speed to. It's not the heat that limits the clock speed, although resistance is a part of it. Transistors are limited by the rate at which they can force a current through themselves: this is related to their size, and to certain semiconductor properties. It was announced a few years ago that someone built a 500 GHz transistor that operated at 4 Kelvin, but it still ran at 350 GHz at room temperature. There's also the issue of the speed of light. If a CPU works by signals getting across it and back, then the electrical signals, which travel at a large fraction of the speed of light, need to be able to cross that distance. A 500 GHz CPU, for example, can't be larger than 0.6mm if the signals travel across the chip at the speed of light. The reason you don't have a CPU at 300 GHz is that they can't build you a few billion of those transistors on a 1 mm square.
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Sep 27, 2010 00:01
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Dilb posted:I'll shed some engineering insight on a few of these, if you don't mind. Go right ahead! This doesn't have to be a one-man show. Dilb posted:There's also the issue of the speed of light. If a CPU works by signals getting across it and back, then the electrical signals, which travel at a large fraction of the speed of light, need to be able to cross that distance. A 500 GHz CPU, for example, can't be larger than 0.6mm if the signals travel across the chip at the speed of light. Huh, I never thought of computing devices as being speed-of-light limited, but as soon as you run the numbers, it makes total sense. That's pretty cool.
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Sep 27, 2010 00:20
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Can you explain why Hawking radiation doesn't violate the law of conservation of mass-energy? The way I understand, two virtual particles pop into existence all the time and annihilate each other right away, so there's been no gain or loss. But if, in the case of Hawking radiation, the particles don't annihilate and one is free to float about, isn't that a net mass gain?
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Sep 27, 2010 00:46
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Raere posted:Can you explain why Hawking radiation doesn't violate the law of conservation of mass-energy? The idea is that the antiparticle falls into the black hole, and annihilates some of it's mass. That's why Hawking radiation causes black holes to "evaporate".
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Sep 27, 2010 00:50
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Dilb posted:The idea is that the antiparticle falls into the black hole, and annihilates some of it's mass. That's why Hawking radiation causes black holes to "evaporate". This isn't right (and not just because of a misplaced apostrophe); antiparticles have normal positive mass-energy. The particle and antiparticle, when they are created, rob energy from somewhere. In this case, it's the gravitational field of the black hole. Since the gravitational field of the black hole is tied to the mass of the black hole, the mass-energy of the particle/antiparticle pair actually comes from the mass-energy of the black hole. Then, if one of the particles falls into the black hole and one doesn't, only half of the mass robbed by the particle/antiparticle pair is returned to the black hole, and so the black hole has suffered a net loss of mass. A theory of quantum gravity would, in general, provide a mechanism for a particle/antiparticle pair to be created from the gravitational field's energy. But we don't know the particulars of that mechanism, and so Hawking radiation is still just a hypothesis.
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Sep 27, 2010 01:28
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can you explain negative mass and negative energy density
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Sep 27, 2010 01:53
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Huh, I hadn't realized black holes can emit antiparticles and photons. Neat.
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Sep 27, 2010 02:15
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seo posted:can you explain negative mass and negative energy density No, I can't. As far as I (or anyone) knows, it doesn't exist. Dilb posted:Huh, I hadn't realized black holes can emit antiparticles and photons. Neat. Can they? If you can prove they can, you've just earned yourself and Stephen Hawking a Nobel prize.
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Sep 27, 2010 02:22
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DontMockMySmock posted:Can they? If you can prove they can, you've just earned yourself and Stephen Hawking a Nobel prize. According to the theory, whatever. Unless you're making a more subtle point that I'm missing? Hey, actual question: I understand dark matter is thought to exist because of the problem in the galaxy rotation curve. How do you measure that, so that you know things are rotating too quickly? I can imagine that you can measure position and velocity, but how do you know what actual orbit a star is in when you're looking at a whole other galaxy, or anywhere really? The sun's orbit in the galaxy is supposed to take ~200 million years, so I would guess you can't just watch it happen the same way you can watch a planet orbit around the sun.
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Sep 27, 2010 02:59
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