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17h
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Can general relativity be completely described as a field in a flat space? @Anixx: the universe is decidedly NOT Minkowskian in the standard FRLW picture. It is a FRLW model with spatially-flat 3-d submanifolds when you chose constant time in comoving coordinates. |
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May
14 |
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Would this be a metric? You can work consistently with null submanifolds in spacetime, and there is nothing wrong with the intrinsic metric, but you are stuck with a non-linked metric and a cometric, and if you want the full differential geometry machinery, you'll be stuck with some extrinsic geometry. |
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May
13 |
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Excluding big bang itself, does spacetime have a boundary? @BenCrowell: sure. But I would argue that epistomologically valid statements about physics should really only apply to knowable things. "We have seen no evidence for edge effects" is a better statement than "there is no edge." I gave an example of a model that trivially shows that it's possible to have an edge that we haven't seen and that is in prinicple unseeable by us. If it were an interesting problem, I'm sure less stupid models could be constructed. |
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May
13 |
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Excluding big bang itself, does spacetime have a boundary? @MoziburUllah: that last solution is exactly what Big Bang Cosmology does. |
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May
13 |
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Excluding big bang itself, does spacetime have a boundary? It is certainly possible that the universe has a spatial boundary. It's just that we've found no evidence of one. A boundary sphere that, relative to us, expands at a rate equal to or greater than our cosmological horizon, and which lies beyond our cosmological horizon would be, in principle, undetectable. |
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May
13 |
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A theoretical problem on Mechanics Are the only forces acting on them due to each other? |
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May
12 |
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Anti-Matter Black Holes Yeah, me either. But I did want that thrown out there. |
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May
12 |
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Collision of charged black holes the only hedge against radiating away a significant fraction of the mass of black holes is the area increase theorem. In some cases, this is not all that stringent of a restriction. |
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May
12 |
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Anti-Matter Black Holes And I balk at the claim that charge neutralization will be unknown to the outside world--black hole horizons carry a charge, and that charge IS externally visible. |
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May
12 |
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Anti-Matter Black Holes @annav: And anyway, if the black hole is shrinking , no true event horizon forms. I wouldn't be surprised to see a theorem showing that in the case of a charged BH that eventually evaporates, that every geodesic escapes the interior. |
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May
12 |
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Theoretical physics and education: Does it really matter a great deal about what happens inside a black hole, or about Hawking radiation? In 1920, it would have seemed stupid and pointless to study quantum mechanics, but much of our modern electronics are dependent on an understanding of QM. You odn't do basic science to get technology, but technology is impossible if the basic science isn't already done. |
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May
12 |
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iPhone compass not being affected by current A part of me doubts that the iphone compass uses a magnet--it is probably finding true north using the GPS device in the phone. |
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May
12 |
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Similarity between the Coulomb force and Newton's gravitational force Of course, it should be said that <b>any</b> Lorentz-invariant theory of gravity will have gravetomagnetic effects, by the standard argument used to show why the magnetic field is necessary. |
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May
9 |
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awarded | Civic Duty |
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May
9 |
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What methods can astronomers use to find a black hole? @Everyone: a few orbits. This plot of the neighborhood of the center of the Miky Way : hera.ph1.uni-koeln.de/~heintzma/U/b1/GalZ-ghez.jpg Is pretty famous as being a set of observations showing the existence of a black hole. |
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May
8 |
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Neutron decay and bet on the honor of physicists This question is ill-posed, as it is posed using classical concepts, but asks for conclusions about an inherently quantum system. One could make debatable opposing definitions of the terms in all of these statements. |
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May
4 |
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Bound State of Only Massless Particles? Follows a Time-Like Trajectory? @MichaelBrown: you can't use non-relativistic QM to discusss massless particles. |
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May
3 |
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is the nature of particle beam weapons in science fiction true to the reality of particle physics? Of course, the statement is nearly tautalogous--different particle beams will affect targets differently--that's how we know that they're different particles, after all. |
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May
2 |
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Production vs. Collection, and Contaminants vs. Depositions, what might be missing in cold fusion research I should also point out that NASA, of late, has been prone to premature sensationalistic claims. |
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Apr
30 |
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answered | Does the unit of a quantity change if you take square root of it? |
