Alright, when 2 photons from the same source of light are traveling in the same direction (at the speed of light of course), the photons relative to each other are not moving. Or, if there is a small angle between the waves, the photons would be moving toward or away from each other at a very small speed.

 

Does SR only concern itself with objects with mass?

 

If so, the photons still carry energy and information. Mass is a form of energy.

If lightbeams are bent by heavy objects then the photons should be "pulling" on the heavy objects too.

 

Mass and Energy both bends spacetime, equivalent by E=mc².

 

The question is will the two photons attract each other or outrun the gravity ?

 

Gravity is travelling with c too and since the photons have a head start, I think they succed.

(Unless they are moving against each other.)

Alright, when 2 photons from the same source of light are traveling in the same direction (at the speed of light of course), the photons relative to each other are not moving. Or, if there is a small angle between the waves, the photons would be moving toward or away from each other at a very small speed.

 

You need to be slightly careful here since photons don't have a rest frame, so it is meaningless to talk about the velocity of one as seen by the other (which I think is what you mean). If you consider a frame which is moving at 0.999999999999...c in the same direction, then relative to that frame, both photons are still moving at c. (Weird huh?)

It seems to me a bit disingenuous to say that a rest frame cannot travel at c just because our equations break down. Perhaps it just means that the equations need fixing? Or does it not matter simply because we cannot accelerate an observer to that speed?

It seems to me a bit disingenuous to say that a rest frame cannot travel at c just because our equations break down. Perhaps it just means that the equations need fixing? Or does it not matter simply because we cannot accelerate an observer to that speed?

 

The photon simply doesn't have a rest frame... so we can't talk about the photon's travel from the point of view of the photon.

 

OK... The photon doesn't have a point of view, but for the sake of this discussion, I'll word the answer as if the photon did have a point of view: From the photon's point of view, it's entire journey is instantaneous... it doesn't travel from one point to another because, from the point of view of the photon, the two points (origin and destination) occupy the same space... there is zero distance to travel. So you cannot say 'the photons, relative to each other, are not moving' Because it implies that the photons have a rest frame.

It is not that the equations break down. It is a postulate of the theory that electrons travel at c in all frames, so they cannot have a rest frame.

But it is still possible to measure how fast two photons are heading towards or away from each other.

 

In the observers frame they have a speed "relative" each other.

But it is still possible to measure how fast two photons are heading towards or away from each other.

 

In the observers frame they have a speed "relative" each other.

 

It's not like two trains travelling toward or away from one another. Photons always travel at c. It's not intuitive, but that's the way it is.

It's not like two trains travelling toward or away from one another. Photons always travel at c. It's not intuitive, but that's the way it is.

If I have two flashlights and point them in opposite directions, the photons will travel away from me with c.

 

You are not allowed to put an observer on a photon and measure their relative speed, as Severian said.

 

But in my frame, standing between the photons, they are travelling at c in opposite directions, like trains.

 

The photons will "distance" themselves with 2c, from my point of view.

 

(Maybe I should have said "In a valid observers frame"...)