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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
Feb
8
answered Why does dark energy produce positive space-time curvature?
Feb
6
comment What is the purpose of differential form of Gauss Law?
@EmilioPisanty: as the name says, it tells you about the divergence (and convergence) of field lines and thus also about sources and sinks; mathematically, it's the relative change in volume on infinitesimal transport along the flow induced by the vectorfield
Feb
6
comment What is the purpose of differential form of Gauss Law?
the differential versions are preferred because they get rid of the (arbitrary) domain of integration; I don't think there's anything more to it than that
Feb
1
comment Is thermodynamic reversibility a function of path?
@MarkEichenlaub: you can draw the diagram as soon as the process is quasi-static, but it need not be reversible (all reversible processes are quasi-static, but not the converse)
Jan
31
comment Tangent bundles and $\mathbb{C}P^n$ and $\mathbb{C}^n$
possibly relevant to the question: solder forms