# Slowing the speed of light with a Bose-Einstein condensate vs. relativity

I recently watched an interesting documentary from NOVA called Absolute Zero. They dedicated a large portion of the show to talking about the B-E condensates and their discovery, then went on to showcase a female scientist (whose name I forget) who was doing experiments that involved shooting laser beams through a condensate.

Remarkably, they claimed that when the laser hit the condensate, its speed slowed from the speed of light down to “about the speed of a bicycle”. They had a cool CGI graphic to illustrate this phenomenon.

This seems like it would contradict the theory of relativity to me, which I understand to state that the speed of light is constant and independent of the observer. What am I missing?

As far as I know, but I’m not absolutely certain, relativity only depends on the speed of light in a vacuum. In any case, lots of materials slow down the speed of light traveling through them.

Ah, right. That’s what I missed.

Yep. Almost any medium will slow down light- slowing down light is what creates that bent-straw effect in a water glass.

I’ve heard it explained, and I’m not sure if it’s correct, that light actually always travels at the same speed, regardless of medium. But the medium could cause the light to bounce around on a very small level which results in light taking effectively a longer path than a straight line through the medium, and hence, its speed from point to point is lower than it is with a vacuum, even though it’s travelling at the same speed. Is that correct?

Following on from Lene Hau’s initial experiments, she and others have managed to completely halt light entirely (Liu et al. 2001) (.pdf). Special Relativity only places an upper bound on the speed of light, not a lower one.

First off, special relativity deals with the constant c, which might be referred to as Einstein’s constant. One of the predictions of relativity is that all massless particles must travel at exactly c, and photons appear to be massless (or at least, very close to it). But if it turns out that photons do have some (extremely small) mass, then light travels at slower than c, but it’s still c that matters for relativity. It doesn’t have to be the speed of anything: The equations all work just as well if there aren’t any massless particles at all in the Universe.

Quoth SenorBeef:

Close. It’s not so much that the photons take a crooked path, as that they get momentarily absorbed and re-emitted by atoms. The re-emission is very quick, but not instantaneous. So it’s not like a winding road, but more like a straight road with traffic lights.

And appleciders is correct that all media will slow down light some. The Bose-Einstein condensates that slow it down to a few MPH are impressive, to be sure, but they’re qualitatively no different than any other material. Light only travels about 3/4 as fast as normal in water, and about 2/3 as fast as normal in glass (with some variation depending on the kind of glass).