# B How can gravity act on a photon?

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1. Mar 12, 2017

### Simon Peach

Black hole or more correctly the event horizon will not let anything past through it, from inside to outside, ok.
But then we get to a particle with zero mass eg. photon. How can gravity act on that particle, surely for gravity to act there must be mass? Or does it act on the electromagnetic force? If so then the EMF must have some form of gravity, that is to say the wave form it's self. Help?

2. Mar 12, 2017

### Buzz Bloom

Gravity acts on mass-energy. A photon with energy hμ has mass equivalent hμ/c2.

3. Mar 12, 2017

### Drakkith

Staff Emeritus
My understanding based mostly on what I've read here at PF or from links given to me:

The key here is to understand that General Relativity (the theory which predicts black holes and our current models of gravity and such) is a geometric theory. The presence of mass or energy literally changes the geometry of spacetime in such a way as to alter the paths that objects or disturbances take through spacetime. When there is mass or energy present, we say that it causes a curvature of spacetime. This curvature changes spacetime from being flat, like regular Euclidean geometry you learned in middle or high school, into a non-Euclidean geometry.

In non-Euclidean geometry, two objects (or disturbances like EM waves) which initially start out along parallel paths can end up having their paths converge or diverge despite the fact that no forces are acting on them. Gravity is the former, where objects get closer together despite having no attractive forces between them because their paths through spacetime are curved in such a way as to converge. The steeper the curvature, the faster the paths converge (i.e. the stronger the gravity, the large the acceleration on each object). To continue moving in a straight path, you would need to apply a force, perhaps by using a rocket engine, to counteract this curvature. Much like how you need to turn your steering wheel slightly in order to keep moving in a straight line on roads that are angled slightly. The stronger the curvature, the more force needed to counteract it.

For a black hole, spacetime is curved so strongly beyond the event horizon that there is no force that can be applied that can completely counteract it. You may be able to delay your arrival, but all paths eventually converge to the singularity. So even light itself cannot escape a black hole because there are no paths through spacetime that lead back to the "outside world"!

You can find a great amount of information of on SR and GR at the following link, but it is rather advanced: https://www.preposterousuniverse.com/grnotes/

4. Mar 13, 2017

### Simon Peach

Thanks I think I understand now. The light cannot escape, not because it is held back by gravity, but because the gravity is so strong that it curves spacetime to the extent that the light is curved back on it's self.

5. Mar 13, 2017

### Drakkith

Staff Emeritus
Pretty much. A minor clarification I'd like to make is that gravity doesn't cause spacetime curvature, curvature causes gravity. In this context, gravity is the effect that curvature has on objects moving through spacetime, namely that it causes them to appear to attract each other via a conventional force.