Question

Why can't anything travel faster than the speed of light?

Answer 1

The name "speed of light" is a little misleading. It's not that the speed of light is magic. It's that there exists an "ultimate speed limit" faster than which nothing can travel, and it so happens that light travels at that speed, in a vacuum.

As for why there is any upper speed limit at all -- that is unknown. It may be logically necessary from an anthropic principle point of view, since without an upper speed limit, it would not be possible for any portion of the universe to be practically isolated from the rest.

For example, consider your DNA replicating as your cells divide. In order for that process to complete, the behaviour of the atoms and molecules under the influence of the local electric fields must be consistent, and in order for such a system to evolve, it must be predictable and consistent. But if there is no upper speed limit in the universe, then it will always be possible for a sufficiently high-energy event very far away -- in the Andromeda galaxy, say -- to instantly affect local events, and make them unpredictable and inconsistent. Our local universe would be chaotically affected by events from all over, out to the most distant stars, and it is unlikely any form of complex repeating chemical behaviour, e.g. life, would evolve. Which is to say, it may be that in any universe in which there is NOT an upper speed limit, conditions would be too chaotic to permit the evolution of intelligent life to observe it.

I've heard the argument that the entire evolution of the universe can be considered to be an evolution of the speed of light, from infinity to zero. For example, when the speed of light is infinite, everything is instantaneousy connected to everything else, so, in effect, the "size" of the universe is zero. There is no "distance" between events. This is the Big Bang. As the speed of light drops from infinity, things acquire "distance" because the time it takes events here to affect events there increases, and the more the speed of light drops, the greater the distance gets. This is the expansion of the universe. When the speed of light drops to zero, there are no longer events, because nothing can affect anything else. Roughly speaking, the universe evolves from having time by no space dimensions to have four space dimensions, but no time. From everything happening at once to nothing happening at all.

It's a pretty silly idea, but it isn't wholly stupid, and for all we know there could be some shred of truth in there somewhere.

Answer 2

Incidentally, the reason light travels at the ultimate speed limit is because it's the only "substance" that has a rest (inertial) mass of exactly zero. So it has a resistance to acceleration of zero, and will therefore travel at the ultimate speed limit. Anything with a nonzero rest mass, and therefore a nonzero resistance to acceleration, will have to travel slower.