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seen Feb 11 '12 at 3:07

Apr
6
comment Can you see yourself in a mirror when you are riding on top of a light stream?
@dbrane I think it is assumed the mirror is not moving relative to you (ie. you are holding the mirror, not travelling at a large relative velocity towards the mirror). So aberration and doppler shift will not cause any effect for the person holding the mirror, no matter how fast they are moving according to an arbitrary choice of coordinate system.
Apr
5
revised Are these two quantum systems distinguishable?
found mistake
Apr
5
revised Are these two quantum systems distinguishable?
added 6 characters in body
Apr
5
revised Are these two quantum systems distinguishable?
typos
Apr
5
comment Are these two quantum systems distinguishable?
I see you marked an accepted answer. I find this question fascinating and would like to see it discussed further. I really was hoping people would add more analysis. Since no one did, I added what at initial glance (may be wrong) appears to be a counter example to hopefully at least get more discussion going.
Apr
5
answered Are these two quantum systems distinguishable?
Apr
4
comment Does the foam on top of boiling maple sap affect the rate of evaporation?
For a potentially complicated system like this, it seems the best solution would be experiment. Can't you just try it both ways for the sake of curiosity?
Apr
3
comment Angular deficit
Basically, in the line defect example at least, the angular deficit appears to be a topological quantity and therefore we should be able to define it in a coordinate system independent manner. So if we add other matter to the background of that line defect situation, we can still calculate the angular deficit even though we don't have a nice flat spacetime 'background' anymore. Ultimately it would be nice to have a definition in terms of an integral over an area or some paths, and involving only the metric or curvature.
Apr
3
comment Angular deficit
@Roy No this isn't specifically about cosmic strings, but they give the only introduction to this that intuitively describes angular defects. But even there, instead of viewing it as a wedge removal, we can scale the coordinates to get back to the usual $0 \leq \theta \leq 2\pi$ and get the metric: $g^{\mu\nu} = dt^2-dz^2-dr^2 - k^2r^2 d\theta^2$ where $k$ is the scaling parameter to map theta back to the usual range. It seems that angular deficit isn't clearly defined unless comparing directly to flat spacetime. If there was any other matter, could we even still define an angular deficit?
Apr
3
comment Can GR be derived by postulating a maximum force?
@Deepak Thanks for clarifying. Your initial comment seemed to come off as saying you agree with Schiller's claim here, which took me aback. I hope we can both agree the claim is incorrect. I considered writing an answer, but am glad someone else at least posted something. It would be nice to see an answer attempting more analysis of Schiller's actual derivation and conterarguments though.
Apr
3
asked Angular deficit
Apr
2
revised Why can different batteries with the same voltage send different currents through the same object?
removed obvious statement
Apr
2
comment Is it possible for wind to break the sound barrier?
Speed is relative, and the 'sound barrier' speed is usually relative to rest frame of the fluid (air in this case). So it sounds like you want to measure the speed of the air with respect to itself which is confusing. Can you clarify what you mean? Give an example setup maybe.
Apr
2
revised Why can different batteries with the same voltage send different currents through the same object?
added 17 characters in body; added 6 characters in body
Apr
2
answered Why can different batteries with the same voltage send different currents through the same object?
Apr
2
comment Can GR be derived by postulating a maximum force?
@Deepak I think most people know of Jacobson's work, or at least I would consider his math uncontroversial. Although the tentative conclusions and hints some extrapolate from it, like Verlinde and his entropic gravity, are, umm, controversial. Jacobson's work itself is fine. Looking at the linked paper, all of the, umm, 'hand-waving' as John puts it, is in trying to get to the start of Jacobson's work. @John It would be very very difficult to answer this politely, so it almost comes off as a trolling question. I don't think you are going to get many(any?) responses. Maybe Lubos will do it.
Mar
30
revised Is there an energy density limit in GR?
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Mar
30
comment How can you tell if a critical energy density is actually a black hole?
Hopefully this isn't the case, but I believe currently there is no known simple answer. What you essentially want is a full proof of the hoop conjecture which there is none that I know of. While it makes intuitive sense that if we shove enough mass into a region a black hole forms, it is difficult to discuss precisely. It is even difficult to define what we mean by mass contained in a local region in a completely general case en.wikipedia.org/wiki/Mass_in_general_relativity
Mar
30
comment Is there an energy density limit in GR?
As a clarification for John, what Lawrence is essentially referring to here is the hoop conjecture ( en.wikipedia.org/wiki/Hoop_Conjecture ). However the mass referred to here can't simply be related to the energy density, otherwise the hoop conjecture is trivially wrong. We can always change coordinates to add as much kinetic energy as we want and it clearly doesn't change whether a black hole forms or not. So if speaking of only the energy density -- no there is not a limit. Mass in general relativity can be a tricky concept en.wikipedia.org/wiki/Mass_in_general_relativity
Mar
30
revised Is there an energy density limit in GR?
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