Most people learn in physics class that light goes one speed: faster than anything else. Because of its long, rich history, this 300 million meters per second is generally treated as an established fact. In the last few decades, though, scientists have been playing around with light's speed. But, as that history noted, researchers have started playing around with exceptions, based on the premise that "nothing in normal space can go faster than light, but if you can do funny things to space, you can go faster than light."

Conveniently, a recent review in Science delves into how light can be slowed down or sped up. It discusses the equations that allow us to tune the velocity with which light passes through material media, known as its group velocity.

Changing the group velocity of light depends on a factor called the group index, which is the sum of the refractive index of the material the light is passing through and the frequency of the light multiplied by a term called the "dispersive contribution," which relates the refractive index to the light's frequency. When you change these things about the light or its environment, you change the group index and effectively make the light go faster, slower, or even backwards.

The very words "fast light" sound suspiciously like the unraveling of relativity itself. However, portions of a signal can be superluminal (faster than the speed of light) without actually breaking light's speed limit. Light moves in a pulse that, for most purposes, looks like a hump that trails off at either end, like a bell curve or a sleeping brontosaurus. The front part of the pulse, where the transmission is officially no longer zero, moves no faster than light in a vacuum. It is the body of the pulse, or more specifically, the highest point in the pulse, that can move at a superluminal velocity.

The the easiest way to arrange a group index that can create a superluminal velocity is to have a dispersive contribution that's negative, meaning either the refractive index or the frequency of the light is getting smaller. If it's negative, the group index will be less than one, so when it divides the speed of light, the group velocity is larger than light's speed in a vacuum. The effect can't last for long periods of time, however: if the signal travels long enough for the body of the pulse to overtake its own front, it will distort.

Slow light, on the other hand, was first studied by passing light through complicated materials like Bose-Einstein condensates. Scientists found they could drop light down to speeds as languid as a few dozen meters per second. It turns out that slow light can happen a couple of different ways, even in materials kept at room temperature.

One method is stimulated Brillouin scattering, which is produced by passing a strong and a weak laser beam through through a transparent material, along with an acoustic wave generated by the laser beams themselves. The interactions between these beams cause the light inside the medium to move slower than its regular speed.

Another method, called coherent population oscillation, shines two light beams of slightly different frequencies on a crystal like alexandrite. At the right frequencies of light, the atoms inside the alexandrite are driven consistently between the ground and excited states. Because the two different frequencies of light become preoccupied with changing the states of the particles, with each frequency constantly undoing the work of the other, the total group velocity is reduced.

At other frequencies, alexandrite can even become what's termed an inverse saturable absorber. Even though it's a relatively transparent crystal, the same coherent population oscillation effect will result in some frequencies of light being spit back by the atoms in the opposite direction the light came, usually at velocities as high as 800 meters per second. The light is most easily slowed when using mediums with a high dispersive contribution.

Now that slow light and fast light are better understood and can be reproduced in a wider variety of experimental configurations, researchers are trying to figure out what to do with them. Most of the potential practical applications have focused on communications technology.

To begin with, significantly slowed light has the benefit of reducing noise in transmissions. It might also come in handy for optical switching—a slow-light medium could act as a buffer for packets, allowing a switch, which can only handle one packet at a time, to deal with each packet separately. Missed packets might become a thing of the past.

The ability to control light could also come in handy in microwave photonics systems that translate radio- or microwave-frequency signals into optical signals. These devices are now integrated into systems like broadband WiFi networks. Microwave photonics hardware needs continuously tunable time delay to separate out parts of an optical waveform, and a combination of both slow and fast light buffers might be able to help the devices sort and process the signals more easily.

So, it's possible that violating one of the fundamental rules of physics may actually produce some marketable devices.

Science, 2009. DOI: 10.1126/science.1170885