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Jul
13
awarded  Yearling
Jul
2
awarded  Curious
Jun
17
comment Diffeomorphism Invariance of General Relativity
"needs to lift to a general covariant transformation of Y, which are not mere coordinate transformations." yes, an example of the actual difference between one and the other would certainly help here
Jun
17
revised Are diffeomorphisms a proper subgroup of conformal transformations?
added 253 characters in body
Jun
17
comment Are diffeomorphisms a proper subgroup of conformal transformations?
Wait.. this could not be right.. General Relativity is invariant under diffeomorphisms, but it certainly is not invariant under conformal transformations, if conformal transformations where a subgroup of diff, you would have a contradiction
Jun
13
awarded  Enthusiast
Jun
7
comment Increasing Earth's albedo
dmckee while this is true in average, enhanced reflectivity in a dense urban area would lower the average absorbed power, and probably the average temperature. But on the other hand, if you are willing to paint all the roofs, why bother? put some solar panels and some solar freakin' roadways: youtube.com/watch?v=qlTA3rnpgzU&feature=kp
Jun
4
comment Are diffeomorphisms a proper subgroup of conformal transformations?
I try to understand this intuitively. A diffeomorphism can arrange points(events) in spacetime arbitrarily. Take a diffeo close to the identity such that it can be written as a small tangent flow on each point. So what you are saying is that if the tangent flow is curly (is not a gradient of a scalar) it can't be written as a conformal close to the identity, which always have their tangent flow to be the gradient of some scalar. Is this correct?
Jun
4
asked Are diffeomorphisms a proper subgroup of conformal transformations?
Jun
2
revised Candidates for holographic QFT of 4D Einstein gravity
bla bla bla
Jun
2
suggested suggested edit on Candidates for holographic QFT of 4D Einstein gravity
Jun
1
awarded  Tumbleweed
Jun
1
comment Dark Energy / Accelerating universe: naive question
Please clarify if this is your question: you are saying that if expansion is accelerating, it means that it was slower in the past, and faster at present. So distant galaxies should be expanding actually slower (at least from each other) that current nearby galaxies are expanding from us at this time. Is that your question?
Jun
1
comment Dark Energy / Accelerating universe: naive question
actually at factor 1 it cannot be 'current time' since other that means spatial distance zero, and there is no 0.5 that is closer than an object that is already at zero distance. I think you meant that at factor 1 we set some 'ruler gauge' distance
May
30
comment Do photons in interferometer violate the law of conservation of mass?
the "weight" (mass) of the photons is zero.
May
30
comment Could a photon travel faster than the speed of light in vacuum?
think about it like this: observations confirm that spatial and time coordinates of events will arrange such that, no matter how you are moving, photons will always seem to travel at the same speed locally
May
29
comment Wormholes with event horizons?
Well, it would, but only in one direction. What I'm asking is if removing the bidirectionality of the horizon removes the exotic stress-energy tensor requirement
May
28
asked Wormholes with event horizons?
May
28
answered Energy Required to Rip Spacetime
May
27
comment Quantum Field Theory with (2,2) metric
relevant: physics.usc.edu/~bars/twoTph.htm phy.duke.edu/~muller/Talks/Columbia_100412.pdf