Ask a Physicist

It appears that the speed of light isn't constant. At least it can be demonstrated that light can be manipulated by humans at room temperature (up to c times 1.4).
Submitted by Tatheg

You hear a lot of claims like this in the news, and they often come with a claim of breaking the rule that nothing can move faster than the speed of light, "c". But they're not really breaking the speed limit.

With any signal you can talk about two speeds called the phase velocity and the group velocity. It's the "phase velocity" that can be faster than c. The phase velocity basically tells you how fast a pure sine wave (with a single frequency) moves. But there's no such thing as a pure sine wave, because it extends infinitely far ahead and behind and lasts forever too.

Realistic signals are made by summing sine waves of lots of frequencies, so that after some finite number of wavelengths they interfere destructively and you get a signal of finite width and duration. When you figure out how fast a real signal (group of sine waves with different frequencies) goes, that's the "group velocity." And that's always less than c.

Then there's the fact that light slows down in matter as compared to vacuum. So if the speed of light in some medium is 0.5c, you can make matter particles go 0.6c, and you can call them faster than light if you want to impress your friends. You do get neat effects like Cerenkov radiation (that lovely nuclear reactor glow), but you're not violating causality or any of the other features of the light-speed limit because they're tied to the vacuum speed c.

And there's the searchlight effect, where you shine a light on a screen far away and move your arm in a circle. You can technically make the spot "move" faster than c across the screen, but no real physical information is actually carried that fast from one point on the screen to the other.

Since Einstein proposed the limit a hundred years ago, we've been looking hard for ways to break it. But every test has made it seem more universal.