Does a Photon accelerates?

Aaron F · FollowPostdoctoral Fellow at Center for the Theory of Quantum Matter (2020–present)Updated Mar 25Does a photon instantaneously gain the speed of light or do they accelerate from 0?

A good question! The direct answer is simple: photons do not accelerate! The in-depth answer is kinda cool. Photons do exactly three things:

1. They can be emitted by particles with charge or magnetic moments
2. They can travel at exactly the speed of light
3. They can be absorbed by particles with charge or magnetic moments

That’s it! There’s no acceleration.

This may seem weird, but this is how massless particles like photons behave. What it means for a photon to be “massless” is literally that it has no “rest energy.” Does this mean it’s energy is zero if it’s at rest? No! It means it can’t be at rest. In fact, it can only travel at the speed of light.

I should note that other waves are also massless, and propagate only at a constant speed determined by their medium. This applies to sound, e.g., whose quantum excitation is called a phonon, which travels at exactly the speed of sound. Massless particles are a bit weird, because our intuition generally applies to matter, which is made up of massive particles. For sound modes, the masslessness is perhaps more confusing, because they propagate in media comprising matter, and a phonon even relates to the collective motion of massive constituents. Despite this, the sound modes themselves are massless, in that they obey a massless dispersion E=pcE=pc, where c is the speed of the wave / mode. Likewise for water waves.

Photons, however are particularly strange because: (1) they propagate in the vacuum (i.e., without a medium made of matter); (2) they do not involve any collective motion of matter (instead, they are disturbances of electromagnetic fields); and (3) they are fundamental particles that are not themselves part of “matter.” Rather, they are the worker bees that carry electromagnetic information between massive particles .