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Wait, I'm still confused why information can't travel faster than the speed of light.

I read that objects could move faster than the speed of light (as long as no information is delivered during the process). But two particles can travel away from each other (for instance during the expansion of the universe) at faster than the speed of light. Isn't that "information?"

Are there any Earth-Scale examples?

Inflation is one example of an instance where an omnipotent observer could pick out two particles in the universe and say, 'these are traveling apart from each other at faster than the speed of light.'

But appreciating why 'no information can travel faster than the speed of light' is a bit amazing and a bit mundane. It's amazing because this same principle, when incorporated into our daily interactions with objects moving relative to one another, leads to the mind-bending Theory of Special Relativity. A sort of *if* particles or spaceships move in a consistent way, then the ticking of ones clock or the measuring of distances must appear to change with velocity.

On the other hand, the 'no information faster than the speed of light' is a bit straightforward. (Though it seems never to be explained that way.) The amazing bits above simply put a speed limit on the universe: nothing moves faster than the speed of light. End of story. Consequence: If you're moving at 0.6 the speed of light in one direction, and your friend is moving at 0.6 the speed of light in the other direction, how could you transmit information to one another? There's no mode that will make up the growing distance between the two of you. So, while many observers could record the information you beam out from your spaceship, your friend can't. It's not that information is or is not, but that information reception is relative.

Are there any Earth-scale examples? Check out Quantum Entanglement. It's basically the "two particles travel in opposite directions quickly" problem but scaled down to shorter times. For QE, it's actually a bit like breaking the information limit, which is why its studied so often. Imaging two balls, one is red and one is blue. (The catch here is that the color of the ball represents a particular quantum state, which is a universal fact, and so everyone in the world knows that these balls must be red or blue. And if one is blue, the other is red.) The two travel at opposite distances near the speed of light. If an observer measures the color of one of the ball -- red -- he or she knows for certainty that the other ball, well beyond the information horizon, is blue. One could argue that that's a statement that about the universality of those quantum states though, so its a bit of a cheat.

You might get a kick out of comparing the 'information horizon' to 'breaking the sound barrier.' Sound is the propagation of pressure waves. But more related, its the speed at which physical information can be sent in a solid. If you whack a balloon, sound waves bounce back and forth inside the balloon. But until a sound wave strike the far side of the balloon, that other side has no idea the former side has been whacked. Breaking the sound barrier is like breaking the information barrier: that large sonic boom is a result of traveling faster than the speed of sound can keep up.

Thanks for writing in to Ask an Astronomer!

June 2011, David Bernat (more by David Bernat) (Like this Answer)

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