Tagged Questions
0
votes
1answer
95 views
proper variation of action term
I have a term I want to vary by a field, $\phi$.
$$
`S' = \frac{-1}{2}\,\sqrt{-g}\,g^{\mu\,\nu}\,\delta\left[h(\phi)\,\partial_{\mu}\phi\,\partial_{\nu}\phi \right].
$$
Is it correct to get this?
...
6
votes
1answer
100 views
Why is $R^2$ gravity not unitary?
I have often heard that $R^2$ gravity (as studied by Stelle) is renormalisable but not unitary. My question is: what is it that causes the theory to suffer from problems with unitarity?
My naive ...
6
votes
2answers
262 views
Dirac equation in curved space-time
I have seen the Dirac equation in curved space-time written as $$[i\bar{\gamma}^{\mu}\frac{\partial}{\partial x^{\mu}}-i\bar{\gamma}^{\mu}\Gamma_{\mu}-m]\psi=0 $$
This ...
4
votes
1answer
175 views
Is the quantization of gravity necessary for a quantum theory of gravity? Part II
(At the suggestion of the user markovchain, I have decided to take a very large edit/addition to the original question, and ask it as a separate question altogether.)
Here it is:
I have since ...
3
votes
1answer
142 views
Hawking Radiation: how does a particle ever cross the event horizon?
The heuristic argument for Hawking Radiation is, that a virtual pair-production happens just at the event horizon. One particle goes into the black hole, while the other can be observed as radiation.
...
6
votes
0answers
139 views
Does local physics depend on global topology?
Motivating Example
In standard treatments of AdS/CFT (MAGOO for example), one defines $\mathrm{AdS}_{p+2}$ as a particular embedded submanifold of $\mathbb R^{2,p+1}$ which gives it topology ...
6
votes
1answer
128 views
Are group representations possible when the solution space is not a vector space?
As far as I understand, the motivation for using representation theory in high energy physics is as follows. Assume that a theory has some (internal or external) symmetry group which acts on a vector ...
1
vote
0answers
78 views
Massless Dirac equation is Weyl covariant
Does somebody know how to show that the following equation is Weyl invariant?
$$\gamma^ae_a^\mu D_\mu \Psi=0$$
where: $D_\mu \Psi=\partial_\mu\Psi+A_\mu^{ab}\Sigma_{ab}\Psi$ is the spin-covariant ...
1
vote
1answer
199 views
Dirac Equation in General Relativity
Dirac equation for the massless fermions in curved spase time is $γ^ae^μ_aD_μΨ=0$, where $e^μ_a$ are the tetrads. I have to show that Dirac spinors obey the following equation:
...
1
vote
0answers
72 views
Showing that the Ricci scalar equals a product of commutators
I have to compute the square of the Dirac operator, $D=\gamma^a e^\mu_a D_\mu$ , in curved space time ($D_\mu\Psi=\partial_\mu \Psi + A_\mu ^{ab}\Sigma_{ab}$ is the covariant derivative of the spinor ...
3
votes
3answers
220 views
Question on inflation
I have two particular questions regarding the inflationary scenario. They are:
1.) What is the physical origin of the inflaton field?
2.) Why has the potential of the inflation field its particular ...
1
vote
0answers
183 views
Curiosity episode with Stephen Hawking. The Big-Bang
In an episode of Discovery's Curiosity with host Stephen Hawking, he claims the Big Bang event can be explained from physics alone, and does not require the intervention of a creator.
1) His ...
8
votes
2answers
252 views
Wavefunction collapse and gravity
If gravity can be thought of as both a wave (the gravitational wave, as predicted to exist by Albert Einstein and certain calculations) and a particle (the graviton), would it make sense to apply ...
1
vote
0answers
101 views
Extending General Relativity with Kahler Manifolds?
Standard general relativity is based on Riemannian manifolds.
However, the simplest extension of Riemannian manifolds seems to be Kahler manifolds, which have a complex (hermitian) structure, a ...
4
votes
0answers
56 views
gravitational convergence of light
light has a non-zero energy-stress tensor, so a flux of radiation will slightly affect curvature of spacetime
Question: assume a flux of radiation in the $z$ direction, in flat Minkowski space it ...
3
votes
2answers
217 views
Hawking radiation and black hole entropy
Is black hole entropy, computed by means of quantum field theory on curved spacetime, the entropy of matter degrees of freedom i.e. non-gravitational dofs? What is one actually counting?
2
votes
0answers
67 views
Is eternal inflation Lorentz invariant?
Start without general relativity. Consider a metastable vacuum over good ol'-fashioned Minkowski space. It decays. A bubble forms and the domain wall expands. The domain wall is timelike, and ...
0
votes
3answers
246 views
Laws of gravity for a universe that only consists of two objects?
So, we know that when two objects of normal matter get away from each other, the gravitational pull they feel from each other, decreases.
I wanted to see how that would work. And in my ...
1
vote
3answers
181 views
Theory that gets rid of dark matter/energy
Is there any physics theory that either groups together gravity and dark energy/dark matter or eliminates dark energy/dark matter by modifying standard understanding of gravity or any force? If so, ...
12
votes
3answers
310 views
Are gravitomagnetic monopoles hypothesized?
My understanding is that gravitomagnetism is essentially the same relativistic effect as magnetism. If so, why is it that I've heard so much about magnetic monopoles, but never gravitomagnetic ...
2
votes
1answer
286 views
General Relativity - Einstein field equation and quantum field theory
Einstein field equation has many solutions. Out of them, is there any solution that is incompatible with quantum field theory?
Also, what solutions of Einstein field equation would be incompatible ...
4
votes
1answer
280 views
Interpretation of the Einstein-Hilbert action
Everyone knows the famous Einstein-Hilbert action $S_{EH} = \int d^4x \sqrt{-g} R$. I'd like to know if, after we first explicit the Ricci scalar in terms of the metric, it could be possible to ...
4
votes
1answer
108 views
Hawking radiation: direct matter -> energy conversion?
When a black hole evaporates, does it turn all the matter that has fallen in directly to energy, or will it somehow throw back out the same kind of matter (normal or anti) that went in?
4
votes
6answers
155 views
General Relativity research and QFT in curved spacetime
A naive question:
Are these subjects, i.e. classical GR and QFT in curved spacetime, being worked upon much anymore?
Who is researching this and what are the problems within these fields? Any ...
10
votes
3answers
3k views
Why you need a graviton when you have the higgs boson?
Since I studied General Relativity I had this question running on my mind. As I see it (just taking lectures of Quantum Field Theory right now)
"Why you need a gauge boson for gravity when the higgs ...
3
votes
1answer
235 views
Why is Dirac Lagrangian in Curved Spacetime Weyl Invariant?
Are there any references on the Weyl invariance of the Dirac Lagragian in general spacetime?
Thanks a lot!
0
votes
0answers
254 views
Gravitation and the QFT vacuum
I'm asking this to get yet another lessson in the inability of QFT and GR to cohabit. Many people believe GR must yield to quantization. The question here is as to why the activity of the vacuum ...
10
votes
3answers
359 views
Why is there a flux of radiation in the Hawking effect but not in the Unruh effect? (and other questions)
This question is slightly related to this one Do all massive bodies emit Hawking radiation?, which I think was poorly posed and so didn't get very useful answers. There are several questions in this ...
11
votes
3answers
605 views
Does the Unruh effect violate Mach's principle?
Mach's principle says that it is impossible to tell if something is accelerating unless there is something else in the universe to compare that motion to, which seems reasonable. However, if you had ...
2
votes
1answer
334 views
Why is GR ghost-free?
I wonder how one can show that general relativity is ghost-free? By ghost I mean the negative norm state that breaks the unitarity. I think it is a well-known "fact" but I just couldn't find any ...
12
votes
3answers
701 views
No hair theorem for black holes and the baryon number
The no hair theorem says that a black hole can be characterized by a small number of parameters that are visible from distance - mass, angular momentum and electric charge.
For me it is puzzling why ...
1
vote
0answers
256 views
Quantization of Gravitational Field: Quantization conditions
I'm begining to study Quantization of field with the second quantization formalism. I've studied phononic field, electromagnetic field in the vacuum and a generic relativistical scalar field.
I ...
7
votes
2answers
628 views
Do traversable wormholes exist as solutions to string theory?
There has been some heated debate as to whether the laws of physics allow for traversable wormholes. Some physicists claim we require exotic matter to construct wormholes, but then others counter the ...
