I just read about this but… can quite wrap my head around it. Is it even possible?
In: Physics
Imagine we shine a powerful laser pointer at the surface of the Moon, so that it produces a visible red dot. Now suppose we point the laser pointer at one edge of the Moon, and then quickly move it to the opposite edge, crossing through the center. If we do this in one second, the laser dot will move across the entire visible surface of the Moon in a second. This is a distance of about 5000 km. So the laser dot has moved at a speed of 5000 km/s.
It wouldn’t be hard to flick the laser pointer faster. For instance, say we used a machine to spin the laser very fast, so that the laser dot sweeps across the surface of the Moon in 10 ms. This wouldn’t be hard – the machine wouldn’t have to spin that fast (on a scale of what is possible). But it would mean that the dot “traveled” across the surface of the moon at 500,000 km/s. For comparison, the speed of light (in a vacuum) is “only” 300,000 km/s.
The important thing to note is that no light actually traveled at 500,000 km/s. Each photon in the laser beam left Earth and traveled to the moon at about 300,000 km/s (or a bit slower as it passed through Earth’s atmosphere). It’s just that one photon arrived at point A on the moon, and another photon arrived at point B, and their arrivals were spaced very close together in time. However, *no photons actually traveled from A to B*. So nothing actually moved at 500,000 km/s.
Importantly, this also means that no *information* actually traveled faster than light. Suppose we used our laser to send information to the Moon. For instance, let’s say we can change the color to mean different things: red means “Earth is under attack” and green means “Earth is fine”. As we spin the laser, this means that the information about Earth’s safety reaches point A 10 ms earlier than point B. If point A had sent this message *to point B* in 10 ms, that would have broken the speed of light, since that would have required the information to travel at 500,000 km/s. But that’s not what happened, and indeed we cannot use our spinning laser pointer to send information from A to B – only to send some information to A and some information to B.
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