Source of photon propagation

[Loren Booda]

Imagine a massless, classically zero curvature, infinite spacetime in which propagates a single actual photon. What eventually motivated this photon to propagate, and does its trajectory or energy vary due to the spacetime vacuum energy?

[Fredrik]

It's impossible to answer the first question. I believe the answer to the second is that the vacuum energy would be completely irrelevant in a universe without gravity. If the photon actually has a trajectory depends on the state it's in. If it's a momentum eigenstate, its position is smeared out over the entire universe.

[nitin]

Given an infinite time, we can use the uncertainty principle to argue that energy is borrowed from the vacuum, a particle-antiparticle pair for example is created and the particles then annihilate into 2 gamma photons which travel in opposite directions, instead of going back into the vacuum. Then, if you want, you can ignore one of them (I wonder if this makes sense!?!). I can't answer the second question.