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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.