# Why does light travel at finite speed? [duplicate]

We have known for quite some time now that light travels at a finite speed of 3x10^8m/s (approx) through vacuum. But why is that? Why can't the speed be infinite? Or at least higher than the current value? What is limiting its speed? I mean, the speed of sound is limited by the density of the medium. Then, what limits the speed of light in vacuum? Does finiteness of speed of light mean the possibility of existence of a omnipresent medium like aether, which, may be is intangible (or doesn't interact with the particles we know of currently)?

I do know that light is propagated by photons. That only makes me more curious as to why the speed is fixed to the current value? Especially as to WHAT is limiting the speed of a particle with no inertia?

• The speed of light is a "free parameter". We don't know why it is the speed it is. I'm pretty sure various anthropic-based arguments suggest the speed of light has to be at least a lot faster than everyday speeds or the universe, galaxy, and solar system would be too different from what they are today. – Brandon Enright Dec 27 '13 at 9:17
• Have a look at this question and answers on why questions in physics physics.stackexchange.com/questions/90164/… – anna v Dec 27 '13 at 9:44

## 1 Answer

Light travels at this speed because it is propagated by a massless particle, the photon. It follows that it travels at the universal speed $c$ which is always observed to be the same from all frames of reference.

Why is this? We can partly answer this question by making basic symmetry and homogeneity arguments about our universe and then one can derive the form of all possible co-ordinate transformations for the relativity of inertial frames that are consistent with these basic symmetries and homogeneities.

To understand how this is done, see the section "From Group Postulates" on the Wikipedia Page "Lorentz Transformation". (Also see my summary here).

Now it so happens that Galilean relativity is consistent with these basic assumptions, but not uniquely so: the other possibility is that there is some speed $c$ characterizing relativity such that $c$ is the same when measured from all frames of reference. Time dilation, Lorentz-Fitzgerald contraction and the impossibility of accelerating a massive particle to $c$ are all simple consequences of these other possible, non-Galilean relativities.

So we have two possibilites: infinite $c$, which is equivalent to Galilean relativity, or a finite $c$. We know that we live in a universe with the latter relativity, because we have observed a speed, namely that of light, that is the same in all reference frames. Then the relativity that follows assuming a $c$ with the value of the speed of light has correctly foretold all experimental tests. It is a highly falsifiable proposition, but it has withstood all experimental attempts to falsify it.

So why is $c$ not infinite? This is simply an experimental fact as far as I know, (which is roughly to a basic working knowledge of General Relativity). There is no basic underlying reason why we have a finite $c$ and not Galilean relativity aside from that the latter is patently falsified by experiment and the former is not.

One consequence of a finite $c$ is that it stops a causal chain of events spread throughout space from happenning all at once. An infinite $c$ would mean that events here on Earth could provoke other events instantly in the middle of M87, and this is not what is observed to happen.