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Let's say that we can somehow bent spacetime, can this produce electromagnetic fields? or EMF can only be emitted from charged particles of matter?

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    $\begingroup$ @JohnDuffield That's nonsense unless you elaborate. $\endgroup$ – Ryan Unger May 5 '16 at 0:44
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    $\begingroup$ @JohnDuffield You obviously do not understand the mathematical structure of the theories. The Yang-Mills curvature of the $\mathrm{U}(1)$ bundle over spacetime has nothing to do with the Riemannian curvature of the tangent bundle of spacetime. Unless, of course, you postulate that there is some equation linking the two (like the Einstein equations in GR). Please do your own research Mr. Duffield. $\endgroup$ – Ryan Unger May 5 '16 at 13:21
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    $\begingroup$ @JohnDuffield Sigh...how do you explain that space is perfectly flat in Maxwellian electromagnetism? $\endgroup$ – Ryan Unger May 5 '16 at 22:16
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    $\begingroup$ @JohnDuffield Maxwell didn't know anything about differential geometry, try again. $\endgroup$ – Ryan Unger May 6 '16 at 13:26
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    $\begingroup$ @JohnDuffield Unfortunately I do not see where any topological properties are investigated in that paper, so I don't understand what he means by that sentence. (But the curvature he refers to is not spatial curvature.) $\endgroup$ – Ryan Unger May 6 '16 at 13:51
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Only from charges. It's the quantum property that couples to photons.

Gravity couples to anything, but there has to be some charge acccelerated to create photons. So gravity could accelerate it, but it also would depend on what is your frame of reference as to whether you'd see it.

Another way to create photons from charges that normally don't exist is to accelerate a virtual charge in vacuum (there are virtual particle-antiparticles around, and when there is a (say) black hole horizon nearby it'll capture one of the particle's and the other one is free to accelerate and radiate). This is the so called Hawking black body radiation from a black hole. Still, in some point of view, the charged particle was there.

At energies above the electroweak symmetry breaking possibly an electroweak charge can do it, create an electroweak field, which when at lower energy would split into electromagnetic and weak. In principle is should, not sure exactly how it would work. Similalrly if grand unification with gravity happens at the Planck scale again some mix including a gravitosomething may do similarly.

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  • $\begingroup$ so accelerated,charged virtual particle from vacuum (casimir stuff) becomes a real particle with mass? this leads to production of matter out of energy used to make time varying spacetime distortion? and if the distortion stops what will happen to this real particles? they disappear? $\endgroup$ – szufla May 4 '16 at 15:26
  • $\begingroup$ No. Once the virtual particle becomes real (and that's one way to describe the quantum process In Hawking radiation but it's a little figurative) it just moves in the spacetime, which if it is curved will make the particle fiollow its geodesics. The next question is whether radiation is produced from that particle. In its reference frame, no, because it does not feel it is accelerated, by the equivalence principle, so it feels it is in an inertial frame. In the frame of another observer who is say an accelerated motion wrt the particle, it will see the particle radiate wrt him $\endgroup$ – Bob Bee May 4 '16 at 17:09
  • $\begingroup$ And the energy comes from the black hole which thus slowly evaporates, as Hawking predicted. It takes billions of years, the larger the slower. Only a miniature black hole would evaporate fast. $\endgroup$ – Bob Bee May 4 '16 at 19:01
  • $\begingroup$ yes,that make sense in terms of frame of reference but does the newly "created from vacuum" particle will disappear (becomes virtual again) when bending space stops or not? I know it can be detected and seen only by observer that is outside of black hole that bends. $\endgroup$ – szufla May 4 '16 at 19:21
  • $\begingroup$ No it's there for good until any interactions affect it, just like any other particle. $\endgroup$ – Bob Bee May 4 '16 at 22:37
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I assume that it is a time varying "bent" , which is a gravitational wave.

Only charges radiate electromagnetic waves.

At the elementary particle level photons will be produced only during the interaction time of changing space, i.e. graviton-charged_particle interaction.

Classically, accelerated and decelerated charges radiate, so a classical gravitational wave will produce electromagnetic waves from charged matter on its path.

Now if by "bent" is meant the distortion of space as happened with the LIGO merger of two black holes, then it is gravitational waves that are radiated from masses in general, charged or neutral.

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  • $\begingroup$ Anna, if you drop an electron, it doesn't radiate. $\endgroup$ – John Duffield May 4 '16 at 12:11
  • $\begingroup$ @JohnDuffield changing electric fields do tapir.caltech.edu/~teviet/Waves/empulse.html . google.gr/… it will be a very very low frequency physics.stackexchange.com/questions/70915/… though $\endgroup$ – anna v May 4 '16 at 13:22
  • $\begingroup$ @JohnDuffield found this arxiv.org/pdf/1509.08757.pdf " It seems a controversial issue, which he solves by a different interpretation. Anyway, since it seems to be an open question , I am with the ones who say it radiates. $\endgroup$ – anna v May 4 '16 at 14:01
  • $\begingroup$ @JohnDuffield this gives a window for observing falling charges into strong gravitational fields arxiv.org/pdf/1301.1559.pdf . "A general method has been developed to find radiated electromagnetic multipole fields for the free falling multipoles into a black hole (including higher order mul- tipoles quadrupoles, etc.). The electromagnetic spectrum can be compared with observational data from stellar mass and smaller black holes." $\endgroup$ – anna v May 4 '16 at 14:08
  • $\begingroup$ the falling charge doesn't radiate. If you're with those who say it does, then with respect, I'm sorry, but you're wrong. Ask a question about this and I'll explain why. $\endgroup$ – John Duffield May 4 '16 at 22:28

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