Why do massless photons always travel at light speed instead of accelerating from rest?

[hover]

This is an interesting question that I can't seem to sort out. Since photons are massless, they travel at the speed of light(3*10^8 m/s). Why must photons travel at the highest possible speed (the speed of light)? Why is it when a photon is created, it doesn't start off at a speed zero and then once a force(whatever the magnitude) is acted upon it cause it to move at the speed of light? How could this happen?

Thanks!

 

[vanesch]

Well, if that bothers you (I don't see WHY it should bother you - you could just as well ask why massive objects DO accelerate, right ?), think of photons as having a very very tiny mass. They will then accelerate as you propose, but they will do so extremely swiftly, so that the time for them to "get up speed" is very small. Their speed will then by a tiny tiny fraction below light velocity, but that will not be noticeable. Now think of them to be "lighter and lighter". In the end, there is no observable phenomenon that will be noticeably different from the photons without any mass.