This paper on the possible discovery of faster-than-light neutrinos has created a lot of discussion in the physics community and the world at large about the speed of light, the speed limit of the universe. Although I agree with most physicists that this experiment is probably the result of some kind of mistake, this seems like a good opportunity to point out that actually there are “things”—not neutrinos (probably) or electrons or any Standard Model particles, but “things”—that do in fact travel faster than light. Naturally, these “things” are a sort of trick, since causal influence never propagates faster than light. The first example is a laser on a wall. If you shine a laser on a wall, and the wall is far enough away, by rotating the laser you can drag the laser dot across the wall faster than the speed of light. Another example is an array of a million light bulbs in a long line timed just right so that they turn on one after another in quick enough succession such that the wave of them turning on travels faster than light.

So “things” can travel faster than light. But isn’t there still some kind of limit? Sometimes people say that “information can’t travel faster than light,” thinking that perhaps this is the real limit. But this too is false. If you project a message on a wall—say the message “information can travel faster than light”—and if the wall is far enough away, you can rotate your projecter to move the message across the wall faster than the speed of light. That’s information traveling faster than light under any reasonable definition of information.

But of course there is still a limit: in all cases of “things” traveling faster than light, causal influence always propagates at or less than the speed of light. This is true by definition. If it ever appears that causal influence is traveling faster than light, it is always possible to reinterpret the situation to understand that causal influence actually never traveled faster than light. Suppose Person A sent Person B a message faster than light. There would be a frame of reference where Person B received the message before it was sent by Person A. In that frame of reference, Person B could then send a message back to Person A, who would receive it before having sent their original message. Paradoxes could result. But the only way to arrange this scenario would be to plan it all out in advance. Thus we see that if we trace the scenario to its origin, the planning stage, no causal influence ever propagated faster than the speed of light.

“Things” that travel faster than the speed of light make perfect sense in relativity. For instance, gamma, the proportionality constant that shows up all over relativity, can be calculated for such a thing and is imaginary. These “things” are properly called tachyons.