Faster than a speeding bullet

Perhaps not the most wisely chosen title for this Scientific American newsbite, but very cool research: “Ultra High Speed Camera Records at Speed of Light

They have built a high-speed camera with a high-enough frame rate that they were able to watch a pulse of laser light traveling through a Coke bottle in slow motion. (Pause for a moment to watch this video, for it’s really impressive. I’ve linked to the juicy part.)

You should never read Youtube comments, but on this video, people are actually asking good questions, albeit with the usual Internet rudeness. There are two very confusing things said in the video and article which I think are putting people on the wrong track.

Velocity vs. rate

This can’t possibly be true! Nothing can move faster than the speed of light.

The camera does not record at the speed of light (slightly less than 300 000 000 meters per second, or 1 billion km/h). This confuses two common meanings of the word speed: ‘velocity’ and ‘rate’. It makes no sense to say that a camera records film frames at a particular velocity, much less the velocity of light; velocity means something is moving, and in this case nothing is moving fast at all. (Except for the light pulse itself, which of course travels at the velocity of light.)

Instead, by ‘recording speed’, it really means the camera is recording at the rate of 1 billion images per second (which is not the same thing as meters per second). There is a fundamental rule saying that no object can travel at a faster velocity than 300 million meters per second, but there is no such rule for rates.

(Although, if it were an old-fashioned film camera, the film would have to feed through the camera at a velocity faster than the speed of light, which would be impossible. So it’s lucky we live in the digital age.)

“We can see photons”

The other confusing thing is that the researcher says in the video that they can see photons moving through space — that’s strictly true, but not very helpful, since you are seeing photons moving through space right now too. That leads people to ask:

Hey, I thought you could only see light when it reflected off something into your eye! How can we see the photon moving through the bottle when it hasn’t hit anything yet?

Well, the thing we see moving through the bottle is a laser pulse – not one photon but a clump of trillions of them. Out of those trillions, some hit air molecules and fly off in all directions, and some of those happen to hit the camera. We say “the pulse scatters off the air.” So, it’s true, you can’t see photons directly unless they are flying right at you. What we’re actually watching is the air molecules lighting up as the laser pulse passes by.

None of this takes away from the fact that the front edge of that laser pulse travels with the speed of light — and we are watching that in slow motion! How cool is that?

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