Slowing down light

By use of a Bose--Einstein condensate, Danish physicist Lene Vestergaard Hau (Harvard University) succeeded in slowing a beam of light to about 17 metres per second, and, in 2001, was able to momentarily stop a beam completely.

About a decade ago, Hau started playing with BECs — for a physicist, that means shooting lasers at them. She found that lasers of the right wavelengths could tune the optical properties of a BEC, giving Hau an almost supernatural command over any other light shined into it.

Her first trick was slowing a pulse of light to a crawl — 25 kmph as it traveled through the BEC. Since then, Hau has completely frozen a pulse and then released it. And recently she shot a pulse into one BEC and stopped it — turning the BEC into a hologram, a sort of matter version of the pulse. Then she transferred that matter waveform into an entirely different BEC nearby — which emitted the original light pulse. That's just freaky. Hey, Einstein may have set that initial speed limit of light, but he only theorized about BECs. "It's not breaking relativity," Hau says. "But I'm sure he would have been rather surprised."

This clip was taken from BBC documentary Absolute Zero.

How long is a piece of string?

How long is a piece of string? It sounds like a ridiculous question... but it isn't. This hour long journey to find out the length of a certain piece of string will blow your mind. This trip takes you from the classical to the quantum and leaves you with more questions than answers. Easily one of the best episodes from BBC's Horizon. I know that an hour of your time is a lot to ask for, but believe me, it's worth it.