system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why does light have speed? Why isn't it instantaneous?
gpt
The speed of light is the speed of causality. It is the fastest speed by which any event can affect anything else in the universe. It is also the defining property of the universe that gives rise to time.

Let me explain.

Imagine a clock. This clock is made by holding two perfect mirrors exactly parallel to each other and bouncing a photon between the two. Every time a photon hits mirror A, one tick has happened. This is our photon clock, and it is a perfect measure of time.

Now, let's say the photon bounces up and down and you can move the clock left and right at any speed you want. Even light speed.

Now, let's take for granted that light moves **ONLY** at light speed. I'll explain why we can do this in a second, but for now, just accept that.

If we move the clock, the photon now has to move diagonally to bounce between the mirrors. The photon's absolute speed must be equal to C, the speed of light. This, we could imagine as the hypotenuse of a right triangle. But, in order for the hypotenuse to be C, the X and Y velocities must satisfy X^2 + Y^2 = C^2 (the Pythagorean theorem). Thus, in order for C to be the speed of light while X is not 0, Y must be less than C.

This means it takes longer for the clock to tick. The faster you go, the bigger X gets, meaning the smaller Y must be and the slower time goes. This is Einstein's Relativity.

I bet you can see where this is going.

If X = C, then Y *must* equal 0 or X^2 + Y^2 =/= C^2. In layman's terms, anything going the speed of light *cannot* experience time.

So, if you were the photon, your trip would be instantaneous. You own clock would never tick.

But, for us, we are the product of light-speed particles moving back and forth and interacting with each other. We are a ticking photon clock. Time is an emergent property of this.

Now, consider what would happen if light was faster or slower. Our own perception of time, which is entirely dependent on the speed of light, would accelerate or decelerate by the same amount. If light was 2x as fast, we'd think 2x as fast and our clocks would tick 2x as fast... which means we'd see it cover 2x the distance it would per *old* clock tick, but our *new* clock ticks are 2x as fast. 2/2 = 1. No matter how fast light goes, we, by definition, must see it go *exactly that fast*, or else we don't experience *any* time at all.

It is also for this reason that light speed is constant to every observer. If you go faster, your clock slows down by an amount that keeps your perception of the speed of light constant.