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  • 196 votes cast
Apr
15
awarded  Guru
Jan
15
awarded  Yearling
Aug
7
answered Why am I not hearing frequencies of range 14 kHz-15 kHz?
Jul
13
comment Age of the Earth and the star that preceded the Sun
@Rob Jeffries, nothing you have said is in contradiction to what I said. My statement that elements up to iron and Nickle are synthesised over billions of years does not imply that none of them are made in under a billion years. You are reading more into what I said than was there. My point was to distinguish between these elements are produced by different much slower processes than the heavier elements.
Jul
6
comment Is the accelerated expansion of the universe consistent with conservation of energy?
It is the dynamical equations that need to have time shift symmetry for Noether's Theorem to apply and Conservation of energy to hold, and they do. In an expanding universe it is only the solution of the equations that do not have the symmetry, but that does not matter, so long as the energy in the gravitational field is also included in the energy equation.
Jun
27
comment Why is there no absolute maximum temperature?
Here is an example of a question where the further you go down the page the better the answers get. It would be interesting to analyse what went wrong with this site. (see also en.wikipedia.org/wiki/Absolute_hot)
Apr
28
comment Are there any grounds for thinking that the distribution of matter in the universe is unbounded?
If such a model were correct there would be no reason to suppose that we were at the centre so a Copernican principle would not rule it out. In any case there is no reason why a Copernican principle would be relevant.
Jan
15
awarded  Yearling
Jan
11
awarded  Good Answer
Jan
8
revised Is this pseudo science or real: code found in superstring
typo
Dec
26
comment Can a spacetime solution in GR have no Killing vector fields?
@Student4life You could make the Killing field zero outside some patch but it needs to be continuous and smooth at the edge of the patch so it must tend to zero as you approach the edge. You will find that this requires the metric to blow up with a singularity at the edge. Effectively you could then map the local patch to an complete manifold, or conversely you can take a manifold with a Killing field such as Minkowski and map it onto a finite patch to get a local Killing field of the sort you are thinking of.
Dec
26
comment Can a spacetime solution in GR have no Killing vector fields?
@Student4life If you define local symmetry to mean that you have a field that satisfies the Killing equation at a point then that is something different. I think any generator of the Poincare group in a local inertial frame would give you that.
Dec
26
comment Can a spacetime solution in GR have no Killing vector fields?
@Student4life I think your definition of local symmetry may be different from mine so you will need to give your definition to clear up any confusion. My understanding of local symmetry in GR is diffeomorphism invarinace. A background field could only have diffeomorphism invariance if every vector field was a Killing field and that is not possible unless the metric is zero everywhere.
Dec
25
comment Can a spacetime solution in GR have no Killing vector fields?
@Student4life Local lorentz symmetry is a symmetry in the dynamical field equations, but a Killing field is giving you a symmetry in the metric treated as a fixed backgound. The Killing field equation needs to apply everywhere. It does not give any kind of symmetry if it holds only along a worldline.
Dec
25
comment Can a spacetime solution in GR have no Killing vector fields?
@Studemt4life I've not seen the notation used that way before but fair enough if that was what you intended. I have seen $D_\mu$ used in that way
Dec
24
comment How much of theoretical physics research involves contemplation and reflection?
Although it is a soft question I think it is an interesting one that could get some helpful answers, so I voted to reopen.
Dec
23
revised What is “special” and what is “general” in Relativity?
added 1 character in body
Dec
23
comment What is “special” and what is “general” in Relativity?
Fine except that generic co-ordinate transformations are not just $GL(4)$. They are a much larger group of diffeomorphisms.
Dec
23
answered What is “special” and what is “general” in Relativity?
Dec
21
answered Can a spacetime solution in GR have no Killing vector fields?