# Does gravity affect a magnetic/electric field?

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1. Feb 16, 2015

### PWiz

Since light, a form of electromagnetic radiation, gets bent in a gravitational field even though it does not have any rest mass, it is obvious gravity is a force that does much more than just attract two masses towards each other. Since it affects electromagnetic radiation, it has led me to ask: does gravity(or a particularly strong gravitational field) have an affect on an electric/magnetic field or vice versa? There seems to be some connection here.
P.S. I haven't started with GR yet, so please don't expect me to follow a very advanced conceptual understanding of the theory.

2. Feb 16, 2015

### Orodruin

Staff Emeritus
It is only in Newtonian gravity that the source of gravitation is exclusively masses. Once you go to GR, the source of gravity (really, of space-time curvature) is the energy-momentum tensor - an object that contains both masses and momenta of the contents of the space-time. There is also some feedback, space-time provides the setting for how electromagnetism behaves, so yes, to some extent electromagnetic fields and gravity do interact.

3. Feb 16, 2015

### Staff: Mentor

If you want to work on the effect of gravity on the EM fields then you probably need to start here:
http://en.wikipedia.org/wiki/Maxwell's_equations_in_curved_spacetime
where the difference appears in the $g_{\mu\nu}$ which represents the metric and $g$ which is its determinant.

Sorry that I don't know a gentler answer to the question.

4. Feb 17, 2015

### LUIS FONDEUR

All forms of electromagnetic radiation (EMR) "seems" to interact with gravitational fields, even radio waves are bended by gravitation. As general relativity (GR) does not include forces in its description, what determine what will happens with the EMR in GR? The space curvature. If a beam is collinear with the "force lines" of the curvature, per example, as when a light ray goes against the gravitational field of a massive body, we will have a red shift. (This is not the cosmological red shift caused by the expanding universe). If the light ray is going to a massive body, then it will be a blue shift. (These are observational results). This is interpreted by Einstein as the effect of gravitation on clocks. And the red shift and blue shift are seeing as a result of time accelerating or reducing its ticks. In other words, those phenomena are seen by relativity as an special case of the general effect of Gravitation on time. In the other hand, when the light rays are not collinear with the gravitational field, then we have the bending of the rays. All these phenomena "look" as if "light" interacts with the gravitational field, however, GR forbid us to think that way. Gravitation curves space, and curved space determine the trajectory of objects and the passing of time inside that space. So PWiz your common sense deduction goes against GR. Personally I have never felt comfortable in this schema.

5. Feb 18, 2015

### PWiz

@Orodruin Hmmm, I've heard of the term "energy-momentum tensor" a lot in the past few days. The greedy feeling of wanting to grasp the entire concept mathematically is indescribable (Getting closer by the day though...!)
@DaleSpam Thanks, I've added Maxwell's equations to my must-learn list :)
@LUIS FONDEUR You've given me a new insight on this. Thanks for adding clarity to my concepts.