As the late Einstein might quip, “it’s all relative.”

I suppose the standard answer would be 186,282 miles per hour. But it turns out that’s only true in a vacuum under pristine conditions. Would it surprise you to know that light has actually been stopped? How about that it’s been clocked at 38mph?

A research team at Harvard clocked light shining through sodium at -272 C at a mere 38mph. That same team managed to “freeze” light in 2006 by shining it through a Bose-Einstein condensate of the element rubidium. By contrast, shining a light through a diamond slows it to around 80,000mph.

It makes sense, but I have to say, until I read it recently in The Book of General Ignorance, it wouldn’t have occurred to me to even think about the relative speed of light traveling through different substances. If we’re ever going to get that light speed jump, I guess people ought to be doing this kind of research. After all, it would really suck traveling through space at 38mph!