Things that cheatFive things that go faster than light, or look like it, and break no rule at all. In each one the trick is the same: ask what is actually movin', and whether you could send a message with it.

The speed limit is not on motion. It is on cause getting from here to there — on one thing making another thing happen. Plenty of patterns beat it. A pattern cannot carry a message, so nobody minds.

1. The shadow on the barn

Hold your hand in front of a flashlight and throw a shadow on a barn wall a mile off. Twitch your wrist. The shadow's edge sweeps the wall at whatever speed the geometry says, and if the wall is far enough it sweeps faster than light, easy.

What is movin': nothing. There is no object called the shadow. The wall is dark at one spot, then dark at another spot, and the news that made each spot dark came from your hand at exactly light speed, no quicker. You cannot signal the far end of the barn any faster than the near end. Same trick works with a laser pointer swept across the face of the Moon — the dot outruns light, the photons do not.

2. Scissors

A very long pair of scissors, closing. The crossing point of the blades runs out toward the tips faster and faster, and with long enough blades it goes superluminal.

What is movin': a point of intersection, which is a place, not a thing. And the scissors cheat a second time: real steel does not close rigidly. When you squeeze the handle, the news travels up the blade at the speed of sound in steel — about five thousand metres a second, which is sixty thousand times slower than the limit you were worried about.

3. The wave that outruns its own light

In some materials the crests of a light wave move faster than c. This is the phase velocity, it is a measured, ordinary thing, and it is not a loophole: the crests carry no information, because a pure endless wave says the same thing forever. Chop the wave to make it say something and the chop — the envelope, the front — travels at or below c. Any actual message rides the front.

4. Space itself

Distant galaxies recede from us faster than light and they are not moving. The space between is getting bigger, everywhere at once, and far enough away there is enough between-ness accumulating per second to beat c^[29]. Nothing is being pushed. Nothing local exceeds anything. The rule is about motion through space, and expansion is not that — which is also the only known way anybody could get somewhere faster than light, and the arithmetic for it wants a kind of matter nobody has found^[30].

5. The entangled pair

Two particles prepared together, carried apart, measured. The outcomes match, and they match whether the measurements happen a mile apart or a light-year apart, with no time for a signal between them.

What is movin': still nothing. Each side alone sees plain noise. The correlation only shows up when the two sides compare notes, and comparing notes takes a phone call, and the phone call goes at light speed like everything else^[31]. You cannot make your side come out heads on purpose. The universe is coordinated, and it is not a telegraph.