i understand that the photon when emitted is already travelling in the speed of light but how can it go from 0 to speed of light instantaneously. is it because photon is a mass less particle and the maximum speed of casualty is the speed of light?
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Similar questions have already been asked and answered here: physics.stackexchange.com/what-starts-the-movement-of-a-photon and physics.stackexchange.com/do-photons-have-acceleration. Please read these answers and update your question if you are still unsatisfied. – Mark Mitchison Nov 17 '15 at 10:49
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Take as an example the electron positron annihilation Feynman diagram:
Before time t' there exists no photon. After that the photon exists conserving energy, momentum and angular momentum for the reaction, and has to have velocity c in special relativity, because of its zero mass.
The concept of acceleration needs the photon to exist before it does, and has no meaning for zero mass particles in special relativity. Note that dv/dt in special relativity is different than in classical physics.
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