# Speed of light

## Main Question or Discussion Point

Why does light or anything have a finite speed? In E=mc2, it gives the constant of the speed of light but I don't understand why light has a maximum speed. Is there something that slows down light?

Related Other Physics Topics News on Phys.org
cepheid
Staff Emeritus
Gold Member
It's not that c is light's maximum speed. It is that c is its only speed. The simplest theory of what light is -- electromagnetic theory -- describes light as a wave consisting of oscillating electric and magnetic fields. This theory shows directly that c is the speed at which those waves propagate (in a vacuum).

The result that nothing with mass can reach c comes from Special Relativity (SR). To get SR, we postulate that the laws of physics must be the same for all inertial observers. From this we conclude that there must be a universal maximum speed for the propagation of information, otherwise it would be possible to violate causality (effects could precede causes) and there goes the logical foundation for everything. It just so happens that this max speed is the speed of light, but we would come to the same conclusion from the same starting assumptions in a universe without light. It's built into the structure of spacetime.

I thought that light travels different speeds depending on what medium that it is in. Is it possible light goes a set speed because a vacuum is just another medium with things that slow light down? or is it just a random speed that light or whatever energy moves at?

cepheid
Staff Emeritus
Gold Member
I thought that light travels different speeds depending on what medium that it is in.
Yes, electromagnetic waves travel at different speeds in different media. This arises naturally from electromagnetic theory. Another (quantum) way of looking at light, however, is as photons -- particles that have wavelike properties*. Now, photons *always* travel at c, and the explanation for why light is slowed down in materials is that although each individual photon travels at c, photons are constantly being destroyed and recreated during their interactions with matter. So, the overall progress of the light through the medium is slowed down.

*note: all particles have wavelike properties, not just photons.

Is it possible light goes a set speed because a vacuum is just another medium with things that slow light down?
No, they don't travel at c because "something" in a vacuum is preventing them from moving any faster. They travel at c because that is a fundamental property of electromagnetic waves (as prescribed by the theory). Electromagnetic waves travel at c in a vacuum. Or, if you want to look at it from the photon picture, photons are massless particles, and special relativity restricts massless particles from moving at any speed other than c (they can neither move slower nor faster).

or is it just a random speed that light or whatever energy moves at?
Yes and no. Yes, that is just the speed at which light travels. No, it is not "random" in the sense of being arbitrary. It is predicted by theory and holds up in experiment.

I think you might be missing a fundamental point that I tried to make before, which is that the idea of a maximum possible speed for the propagation of information (and, as it turns out, particles) arises naturally in the formulation of physical theories.

Thank you very much, this explanation helps me greatly.