MattieShoes t1_j27bwfe wrote

You can't see the photons going away from you. The only way to measure the speed of light is by round-trip -- ie. stick a mirror out there and let the photons come back, and measure round-trip time.

... but I'm sure you see where this is going -- if the speed outward was faster, the return trip will be slower, canceling out the difference.

Now, you could stick an observer out there to try and measure the transit time in one direction... but how do you synchronize your watches without relying on the speed of light? Well, you could sync them before moving apart, but the act of traveling apart will make time pass at different rates for you, so your watches instantly become un-synchronized. The equations you might use to cancel out this desyncing all rely on... the speed of light :-D

There's a fun video on it, lemme find it.

EDIT: found it! Veritasium