A Brief History of the Speed of Light

[MerryBlooms]

One night over drinks at a conference in San Jose, Miles Padgett, a physicist at Glasgow University in Scotland, was chatting with a colleague about whether or not they could make light go slower than its “lawful” speed in a vacuum. “It’s just one of those big, fundamental questions you may want to ask yourself at some point in the pub one night,” he told BBC News. Though light slows down when it passes through a medium, like water or air, the speed of light in a vacuum is usually regarded as an absolute.
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Image: Flickr user Steve Oldham, adapted under a Creative Commons license.

This time, the pub talk proved to be a particularly fruitful exchange. Last month, Padgett and his collaborators made headlines when they revealed their surprising success: They raced two photons down a one-meter “track” and managed to slow one down just enough that it finished a few millionths of a meter behind its partner. The experiment showed that it is possible for light to travel at a slower speed even in free space—and Padgett and his colleagues did it at the scale of individual photons.

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