system
Given the following post, respond as if you're commenting on Reddit:
human
Does light actually travel at "the speed of light" in a vacuum, or do interactions with virtual particles slow the light down?

I have only a rudimentary understanding of physics, but I gather that photons traveling through matter will be delayed as a result of interactions with the particles that make up the matter, effectively slowing down the light wave.  I also understand that empty space isn't really empty, but is full of virtual particles that constantly snap in and out of existence.  Do photons travelling through empty space interact with these virtual particles?  If so, does a light wave slow down, even when traveling in a vacuum?
gpt
It isn't obvious at all that photons shouldn't acquire an effective mass from quantum corrections.

However, it is found in quantum field theory that the mass of the photon is protected from any quantum correction by gauge invariance. This is a nontrivial result.