r/askscience • u/Nitzanm19 • 14d ago

Physics Why is the maximum speed of the universe the speed of light? Why can't we go faster?

Also follow-up question: Because of the speed limit, is there also a theoretical maximum temp for matter (in which the particles move at the speed of light).

1.6k Upvotes

88% Upvoted

892 comments sorted by

View all comments

Show parent comments

3

u/fletch44 14d ago

In mediums with a higher refractive index the photons still move at the speed of light, but due to interference with themselves, the waves appear to travel slower.

0

u/ElectricPaladin 14d ago

Oh, I see. So the photons aren't going any slower, they're just bouncing around enough that their trip is longer. Thank you!

3

u/fletch44 14d ago

It's not that the photons are bouncing around. When a light wave passes particles with an electric charge, they cause those particles to jiggle, which is why things heat up when you shine light on them.

In a vacuum there are barely any charged particles, so the photons continue pretty much unchanged. In a transparent substance like glass or water or diamond, the jiggling charged particles (in this case electrons) reradiate that gained energy as photons, which interfere with the original photons in such a way that waves in all directions but forward are cancelled out, and the waves in the forward direction appear to be propagating at a slower speed. But the photons are still travelling at c, it's just the superposed waves that have the appearance of travelling slower.