The speed of light in a vacuum, commonly referred to as C, is widely accepted as a universal constant. This means that it is unchanging, no matter what conditions are applied to it; it remains 299,792,458 metres per second. Similarly the speed of light through other materials such as air or water are also known to be constants, although they’re less than C: for example the speed of light as it passes through a diamond is 124,000,000 metres per second.
However, in recent years some physicists have suggest that the speed of light may not be a constant after all. Last week a team of researchers from the University of Glasgow and Heriot-Watt University announced that they had managed to slow the speed of light after passing photons (particles of light) through a liquid crystal device called a mask. This changed the shape of the photons and their speed remained slower than the speed of light even after they had exited the mask. This is possible because, as the researchers explained in this article, the photons in a beam of light can move around like cyclists in a race. Although they keep travelling forwards as a group, individuals may sometimes drop back or speed up as they travel.
As if this wasn’t interesting enough, I discovered while researching this article that not only have other physicists managed to slow the speed of light even more, they’ve managed to stop it altogether. Yes, you read that right.
In 1999 a team at Harvard university, led by the Danish physicist Lene Hau, managed to slow the speed of light to just 17 metres per second (38 mph). They did this by passing a beam of light (called a coupling laser) through a cloud of atoms that had been supercooled until it was a billion times colder than the space between stars, and then suspended in a chamber where the air pressure was 100 trillion times lower than at ground level. A second beam of light was fired into the atoms (now in a state of being known as a Bose-Einstein condensate) and it slowed to just 17 metres per second. At the time Ms Hau was quoted as saying “In this odd state, light takes on a more human dimension; you can almost touch it.”
Just two years later, in 2001, Hau and her team managed to bring a beam of light to a complete standstill before restarting it again. They did this by waiting until the beam of light was contained within the Bose-Einstein condensate and then switching off the coupling laser. Once it was switched on again the beam of light continued on its way. The light was stopped for just one thousandth of a second, which doesn’t sound much until you realise that at its usual speed the light would have travelled 300 km in that time. This work has tremendous implications for the future of quantum computing, and I can’t wait to see what Lene Hau and her team do next.