Boys and Girls! You too can make something travel faster than light!

Can't say I had realised this before, but physicist Bee has a fascinating post that starts:

If you sweep a laser pointer across the moon, will the spot move faster than the speed of light? Every physics major encounters this question at some point, and the answer is yes, it will. If you sweep the laser pointer it in an arc, the velocity of the spot increases with the distance to the surface you point at. On Earth, you only have to rotate the laser in a full arc within a few seconds, then it will move faster than the speed of light on the moon!

Now a bit more explanation:

This faster-than-light motion is not in conflict with special relativity because the continuous movement of the spot is an illusion. What actually moves are the photons in the laser beam, and they move at the always same speed of light. But different photons illuminate different parts of the surface in a pattern synchronized by the photon’s collective origin, which appears like a continuous movement that can happen at arbitrary speed. It isn’t possible in this way to exchange information faster than the speed of light because information can only be sent from the source to the surface, not between the illuminated parts on the surface. 

Oh, and your average laser pointer won't still be visible on the moon, and I have my doubts a laser strong enough to be visible is available from scientific supplies shops.

But, it's still fascinating.

(And it's posts like this that I sometimes re-read years later and think "Geez, I do run a great blog!")