Achieving faster than light speed is one of the dearest dreams of science (and science fiction). But what about slowing light down? In the last ten years, scientists have done some amazing things by slowing down the speed of light.
The speed of light is a constant. The exact speed is 299,792,458 meters per second. Nothing can move faster than that without violating fundamental physical laws (and possibly traveling backward in time). But light can be slowed down – light speed is a surprisingly inconstant constant.
Light in a vacuum
The commonly cited speed of light only applies to light moving through a vacuum, where it won't run into any atoms. This value (the speed of light in a vacuum) is called "c," one part of Einstein's famous E=mc² equation. In a non-vacuum, photons randomly run into atoms, which absorb the photon, then re-emit it. This absorption/re-emission cycle induces a delay, slowing down the light. In this respect, the slowing of light is an illusion. The photons still travel from atom to atom at c, but they make brief stops along the way.
Transparent substances have a refractive index, a measure of how much that substance affects the speed of light passing through it. The formula for calculating this is v=c/n, where v represents the actual velocity of the light and n is the medium's refractive index. A vacuum has a refractive index of 1, and Earth's atmosphere has a refractive index of 1.00029. That's pretty close to full speed.
When light slows down
Water, on the other hand, has a refractive index of 1.330, and different types of glass have indices between 1.4 and 1.9. Light moves significantly slower through these substances. Diamond makes light even slower, with a refractive index of about 2.4.
Bob Shaw's 1968 short story "Light of Other Days" proposed an interesting use for materials with a high refractive index – use slow glass with a refractive index so high that light takes a year to pass through it to "record" pleasant nature scenes. Then people living in drab cities could place these "scenedows" in their houses and enjoy a year's worth of lakeside bird watching instead of the laundromat across the street. L. Sprague de Camp had weaponized slow glass technology earlier, in "The Exalted," printed in a 1940 issue of Astounding. He used a rod that trapped light, then released it all at once to explosive effect.
Lasers and quantum computers
Modern researchers have found an even more powerful use for slow light. In the last ten years, scientists have managed to slow light to a crawl and even stop it using special gas diffusions excited in particular ways with laser beams. When scientists stop light, they aren't actually halting a photon – they're embedding its quantum state into the nearby atoms. Later, they use another laser pulse to activate those atoms and make them emit an identical photon. And by later, I mean almost instantly, because the quantum state starts to decay in less than one second. The ability to slow, stop and generate photons in this way is a major step on the road to developing quantum computers.
"Single photons step into the slow light." Physorg.
"Researchers Demonstrate Slow Light Effect in Semiconductor-Based Nano-Structures." Azonano.
"Scientists put a light wave on hold." MSNBC.