I know light travels in a straight line and experiment (such as the double slit experiment) had shown photon can take any path so my question is for a single photon, what is it that is actually travelling in a straight line?
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Yes, a photon can take any possible path, however, complex numbers assigned to paths that vary significantly from the classical path make those significantly varying paths cancel each other out:
https://en.wikipedia.org/wiki/Path_integral_formulation#Feynman's_interpretation
So what you're seeing is the photon taking the classical path, or the wave function collapsed into the photon taking a straight line, if you follow the Copenhagen interpretation. If I've missed anything, anyone, feel free to correct me in the comments.
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There is basically two ways to think of this.
the classical EM wave is traveling in a straight line
as per QM, the classical EM wave is built up by a herd of photons, QM entities, the smallest quanta of light, but in this case it does not have a meaning to talk about trajectories of photons, because photons are delocalized as they propagate
Light is described by quantum field theory and can only be fully understood in this context. We sometimes talk about photons and sometimes talk about light rays, but these are only approximations. As a general principle light behaves like a particle when energy is being exchanged with something else, and like a wave when energy is propagating. So light travels like a wave and interacts like a particle.
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