system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: If Time 'Stops' at the Speed of Light, How Does Anything Happen To Light Itself?

As you get faster and faster time "slows down".

Photons have no mass and travel at this 'top speed', so how does anything happen (bounce off surfaces, being detected by your eye, travelling through space etc.) when it is not affected by time?
gpt
Nothing does happen to light. The only "things" that can "happen" to light (and that's using the terms loosely) is that it can come into existence, and it can stop existing … and in the frame of reference of light, those two events happen at both the same instant and the same point in space.

Say you have an atom. That atom has electrons. One of those electrons, we'll say, gets excited. We don't care how; it just does. Sooner or later, that electron is going to drop back to its lowest possible energy state. But the energy it has to shed to do this doesn't just disappear; it can't, because energy has to be conserved in this situation. So the energy radiates off as light. Like … kpew. Specifically, a *quantum* of light, which we call a photon.

Sometime later (in the frame of reference of an observer) that quantum of light interacts with some other charged particle, almost certainly another electron in some other atom. A variety of things can happen at this point, but they all involve that quantum of light ceasing to exist. For example, it can be absorbed by the electron, knocking it up into a higher energy state. At this point, the quantum of light stops existing; it just goes away. The momentum it carried becomes part of the electron.

If you imagine what that experience was like for the light quantum — which you really can't do, strictly speaking, because the math doesn't let you, but we can *imagine* it qualitatively just for fun — you find it went like this: At some time which we'll arbitrarily call t1 and point in space we'll arbitrarily label x1, the light quantum came into existence. At another time t2 and point x2, the light quantum stopped existing. In the imaginary, not-real frame of reference we've concocted for the light quantum, t1=t2 and x1=x2. That is, the photon is emitted and absorbed at the same instant and in the same place.

To a distant observer, however, t1 does not equal t2; some time elapses between the emission and absorption events. Also x1 does not equal x2; the events happen at different places. However — and this is really basically key to understanding all of modern physics — t2-t1 = x2-x1. Always. For every observer, no matter what. In other words the amount of time it takes for a quantum of light to cross between two points is always exactly equal to the distance between those two points. All observers everywhere will always agree on this. Even the photon itself would "agree" about this, in our imaginations, because if t1=t2 and x1=x2, then t2-t1 = 0 and x2-x1 = 0, which means t2-t1 = x2-x1.

That's what makes light special. It always propagates between two points in the time equal to the distance between the two points. This is universally true for all observers, no matter what, no exceptions ever.