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#37
Aug1510, 04:03 AM

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#38
Aug1510, 05:37 AM

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Dimensions are strictly mathematical. It may or may not represent the physical reality. The real world is just three dimensional. However, to analyze it, we can use onedimensional or fourdimensional frames.
An expanding system requires a four dimensional frame. As time moves forward, the three space dimensions increase. The spherical surface of the expanding system, or the Gauzian surface described by the three space dimensions, encloses the spacetime. The spacetime can be regarded as the volume at a given time; it is the product of a volume factor and a time factor, ie, it is four dimensional. When the system contracts, the time factor decreases. Mathematically it is time moving back. But in real terms, the direction of time does not change, but the directions of the space dimensions are reversed. 


#39
Aug1510, 06:28 AM

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#40
Aug1510, 06:30 AM

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#41
Aug1510, 04:14 PM

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[QUOTE=Chalnoth;2839984]While true, the empirical evidence for four dimensional spacetime is exceedingly robust.[QUOTE]
Is it really evidence for FOURdimensional spacetime? I think quantum mechanics indicates time must have at least two dimensions. After it's absolute nonsense that the observer determines and outcome. The original evidence for the Big Bang was a uniform background radiation and seeming uniform expansion in all directions. The first was seen as making the formation of galaxies impossible and quickly nonuniformity was found. The second is based on the part of the universe we can observe. Is the world really flat? 


#42
Aug1510, 05:55 PM

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#43
Aug1610, 08:21 AM

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#44
Aug1610, 08:32 AM

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In general we actually have a rather poor understanding of exactly how entropy relates to gravitational systems, so we don't actually know how to write down the entropy of a contracting star. But we can write down the entropy of a diffuse gas, and we can write down the entropy of a black hole. The entropy of the black hole (which can be seen as a far extreme of the contraction of th star) vastly exceeds the entropy of the diffuse gas from which it came. From arguments like this we understand that the universe becoming more clumpy with time is a manifestation of increasing entropy. In fact, it is this fact, the clumpiness increasing with time, and not the expansion, that is the primary increase in entropy since the end of inflation. If it were to be the case that our universe were to recollapse (which today seems manifestly unlikely), then we would still expect our universe to become more and more clumpy as it did so. 


#45
Aug1610, 08:53 AM

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#46
Aug1610, 09:08 AM

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Ultimately, understanding the entropy of most systems where gravity is a significant factor (e.g. stars, galaxies) is likely to require knowledge of quantum gravity. But a black hole is just one of those special cases that is mathematically simple enough that we can be quite sure about its entropy already. 


#47
Aug1710, 04:06 AM

P: 26

One should be sceptical about mathematical models. Mathematics is a tool, in fact, an excellent tool, for analyzing. But of late, it has changed its role, it appears, and has become a shaping tool. Coming back to black holes, does the scientific community accommodate the Doubting Toms even now, or do the Doubting Toms outnumber the others? 


#48
Aug1710, 05:58 AM

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But at the moment my impression of people who do research in the area of black holes has been that the number of people who seriously doubt that black holes are real (or at least are not a very good approximation to reality) is vanishingly small. 


#49
Aug1710, 07:37 AM

P: 153

Obviously, something happens when too much mass gets in one place, ergo some kind of black hole. But math belongs in the experiment phase of the scientific method, not in theory. 


#50
Aug1710, 09:16 AM

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#51
Aug1710, 10:38 AM

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#52
Aug1710, 11:13 AM

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Now, we may speculate wildly as to whether or not the objects we currently call black holes may be something that we haven't thought of yet. And they may be. But there isn't really a good way to actually go out and investigate the vague notion of, "something we haven't thought of yet," so it only makes sense to provisionally accept the theory we have until such time as evidence is presented that contradicts this. Edit: And I'd like to point out that I purposely don't often express this level of skepticism, because it turns out that once a majority of scientists become reasonably convinced something is true within their own field of study, that is almost never overturned. In fact, I can't think of a single situation where this has occurred since the onset of modern science (say, about 150 years ago for physics, give or take). The possibility always remains, of course, but it's just unlikely. 


#53
Aug1710, 11:50 AM

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You haven't given your assertion much thought, have you? 


#54
Aug1710, 12:23 PM

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Yes, there have been many ideas in science that turned out to be false. But it seems to me that the majority of them never passed the level of, "Well, maybe this explains things, let's test it!" Many others stem from prescience concepts, and weren't overturned until we developed the ability to actually investigate them (thus they couldn't be rightly considered conclusions of science, and instead just suppositions). 


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