system
Given the following post, respond as if you're commenting on Reddit:
human
Eli5: Why does light travel so fast?
gpt
As far as we can tell, the universe seems to have a max speed cap on how quickly anything can affect anything else. We call that the speed of causality, or *c*.

There's a fundamental property that some particles have and others don't, called *mass*. You might know it as the thing that causes the sensation of weight, or makes things hard to lift or move. Whether or not a particle has mass determines how it interacts with this universal speed limit:

* If something has mass, it can travel at any speed it likes, as long as it's less than *c*.
* If something does not have mass, it must *always* travel at *c*, all the time. No exceptions\*\*.

^(** Assuming it's in a vacuum)

Light happens to be massless, so it always moves at the speed *c*. This is why *c* is more typically called "the speed of light", even though it really doesn't have anything to do with light.

Answers to some possible followup questions:

> "Why does the universe have a max speed cap at all?"

We don't know.

> "Why is the speed of *c* what it is?"

We don't know.

> "Why does the universe have some particles that have mass, and some that don't?"

We don't know.