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Mar
24
answered Many times speed of light
Mar
15
answered Why can we assume torsion is zero in GR?
Feb
27
awarded  Nice Question
Feb
24
comment How far could the LHC “fire” a particle
@Zephyr: the value I was getting at was at solar system galactic radius; you're right that the relevant quantity is actually the root of the sum of the appropriate escape velocities (earth from surface, sun from earth orbit, milky way from solar system) squared, but the first two are negligible
Feb
24
comment How far could the LHC “fire” a particle
LHC protons are ultrarelativistic and should basically be able to escape anything that's not a black hole; as to the escape velocity of the Milky Way, according to Wikipedia it's less than that of the sun and thus well below the speed of light (though I cannot confirm right now as it's nightime where I'm sitting ;)).
Feb
24
answered Is there any mathematical structure modelling parallel universes (multiverse)?
Feb
24
comment wave functions of Klein Gordon particles
the problem of negative probabilities of the Klein-Gordon field gets naturally resolved in the framework of pseudo-Hermitian QM; see arxiv.org/abs/quant-ph/0307059 and arxiv.org/abs/0810.5643 for more background information
Feb
21
comment If Black Hole never forms, how important will be to study Black Hole paradoxes?
@kleingordon: black holes are observationally indistinguishable from dark gray ones
Feb
17
comment Can space be torn?
see arxiv.org/abs/hep-th/9109030 for a related paper from the arxiv; extending general relativity to degenerate metrics, some apparently violent processes sometimes can be realized as smooth deformations; also, I believe string theory has some things to say about the subject as well...
Feb
15
awarded  Nice Answer
Feb
14
comment What might be a satisfying criterion to qualify something as having physical existence?
my criterion would be energy-momentum, but I wouldn't want to formalize that argument while avoiding circularity; also, compound system (like, say, the universe as a whole) can have zero total energy-momentum...
Feb
14
revised Why does dark energy produce positive space-time curvature?
deleted 5 characters in body
Feb
14
comment Why does dark energy produce positive space-time curvature?
@DanielMahler: I just added some missing pieces
Feb
14
revised Why does dark energy produce positive space-time curvature?
add some missing pieces
Feb
13
comment Total energy of the Universe
@BenCrowell: I'll see your Misner, Thorne, Wheeler and raise you a Giachetta, Mangiarotti, Sardanashvily: "Advanced Classical Field Theory"
Feb
13
comment Total energy of the Universe
note that contrary to the number of votes, I believe Philip Gibbs's answer is the correct one; the appropriate approach to energy-momentum in general relativity is the Komar superpotential (see eg this math-y paper and Komar's original one), which yields a zero total energy for Friedmann-type universes (see this paper), but not necessarily Gödel-type ones (see this paper (PDF))
Feb
8
comment Why does dark energy produce positive space-time curvature?
@DanielMahler: I added a note about that; see also en.wikipedia.org/wiki/…
Feb
8
revised Why does dark energy produce positive space-time curvature?
elaborate on negative pressure
Feb
8
comment Why does dark energy produce positive space-time curvature?
@DanielMahler: your guess for $u$ is correct, and I added a link to the corresponding Wikipedia page; you're also right that the horizontal circles aren't geodesisc, but we can use them to model a closed universe that's contracting in the lower half and expanding in the upper one
Feb
8
revised Why does dark energy produce positive space-time curvature?
add link to explain stress-energy notation