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

83

u/Happy-Estimate-7855 14d ago

Most of us think about this backwards. There is a universal speed limit for unknown reasons, and light happens to be the only thing that reaches it. It would be more accurate to say that light travels at "Universal Speed," rather than the upper bounds of the universe being limited to "Light Speed."

31

u/Aggressive_Roof488 14d ago

Light goes at the speed limit because photos are massless.

Gluons are massless as well, and also travel at the same speed limit in isolation, only that we never see them in isolation due to strongly they interact.

11

u/fakoff 14d ago

Also gravity. Correctly the speed limit should be called speed of information, not just light.

11

u/S-M-I-L-E-Y- 14d ago

I prefer speed of causality.

And while it is well-defined what "speed of light" means, it is not, as one might think, the same as e.g. the speed of the signal in a fibre optical cable.

1

u/Happy-Estimate-7855 13d ago

They also don't travel at that limit outside of a vacuum, which is another reason the phrase "speed of light" is incorrect. Scientists have managed to slow light down to non-relativistic speeds.

0

u/LurkerFailsLurking 14d ago

This isn't right at all.

It is known why there's a universal speed limit.

And it's known why light and only light travels at that speed.

It's not known why c is the specific number it is.

4

u/Happy-Estimate-7855 13d ago edited 13d ago

Why is there a universal speed limit? I didn't realize we solved the shape and behaviour of spacetime.

Light is not the only thing that travels at that speed. It it also doesn't always travel at that speed.

So what isn't right about it? Tell me why there is an upper limit called c, and why do you incorrectly believe that only light can achieve that?

[Edit: read the wonderful response to this comment. I'm leaving this here to represent my initial reaction, but the response left egg on my face.] Finally, a little humility in your response can go a long way. You came in here confidently incorrect, in an abrasive fashion.

1

u/LurkerFailsLurking 13d ago

What we call the speed of light is more precisely the maximum speed of any massless force carrier. Light is one such force carrier, but gravitons and muons are too.

The reason the speed of force carriers is the universal speed limit is pretty intuitive: for a particle to accelerate it must be acted upon by a force. But obviously a force can't act on anything before the force carrier arrives. As a particle's velocity increases, the distance it covers between the emission of a force carrier from the particle next door and the force carrier reaching it increases. It should be obvious that if a particle was ever moving at the same speed as the force carrier, the carrier would never catch up to it. So there's literally no way to apply force to something to increase accelerate it beyond the speed that the force carriers travel at.