system
Given the following post, respond as if you're commenting on Reddit:
human
What would be different if the speed of light were some lower number?

Or, to put it another way, if the speed of light suddenly slowed down, would we be able to measure this?  And if so, how?
gpt
The "speed of light" isn't a physical constant. It's a consequence of the fact that we use different units for distance and duration. We have to have a conversion factor to go between them, and the number *c,* the "speed of light," is that conversion factor.

You can change the definition of either your unit of distance or your unit of duration, of course; that would give the conversion factor a different numerical value. But you wouldn't be changing anything about the *universe.* You'd be changing your numbers. If you then set up an experiment to measure the "speed of light," you'd get a different numerical answer … but not because the "speed of light" changed. Because your units of measurement did.

I know it's hard to think of the "speed of light" as anything other than, well, a *speed.* But it isn't one. It just appeared to be hundreds of years ago, when the first experiments into the nature of light were being done. Once it became clear that light didn't propagate instantaneously, it seemed obvious that it must move at some speed. But that's only because we were still thinking, in those days, of our universe as being fundamentally the same as what we observe around us in day-to-day life. In fact it isn't, and the fact that we still call the conversion factor *c* the "speed of light" is a remnant leftover from those days.