In classical mech since light has no mass it had no p cause p=mv with m at 0 you have no momenum nor a force correct, by take the der. But in GR, how is light considered to have momentum, I read somehting about Einstein saying that in the GR field EM spectrum could now be called matter. Wow interesting but how... just becuase EM now has momentum. Also found p = E/c which holds for a light-speed massless particle. is that E energy and if so pot or K or both just take - of the other.?? can someone derive this? also in GR since the definition of momentum changed did also the defintion of a force. That would include all Em spectrum to apply a force also light shined from a flashlight. I know of radiation pressure felt by such things is this kind of like the force that is felt by a burst of photons falling on an object. By no means am I qualified in any of this stuff just trying to learn, so feel free to hammer anything I have messed up in assuming or if my definitons are lacking horribly. But if anyone could answer a little of this I would be greatly appreciative.