As any science-savvy sixth grader can tell you, looking at the stars means looking at the past. Light from more distant parts of the universe takes longer to get here—billions of years in some cases—and therefore, as telescopes become more powerful, astronomers can see farther and farther back in time.
The roots of this idea date back to the 17th century, but only recently have we understood the full relationship between light and history. A century ago this year, Albert Einstein showed that the speed of light is not just finite but also constant and absolute. In the emptiness of space, light travels at a rock-solid 186,282 miles per second, and nothing can travel faster. In other words, a beam of light from a more distant star can never overtake one from a closer star. The universe's stories never fall out of order.
December nights are particularly good times to explore the speed of light. Earth faces away from our galaxy's dusty center, allowing an unobstructed view of distant objects. Some of the stars you see emitted their light before the time of the Roman Empire. The farthest thing visible to the unaided eye is not a star at all: the Andromeda galaxy. Its light, 2.2 million years old, appears as a smudgy oval floating overhead around 7 p.m. Telescopes can detect other galaxies several thousand times farther still. Indeed, the history lessons posted in the December sky extend nearly all the way to the Big Bang that started it all, courtesy of light's limited speed.
Down here on Earth, things get more complicated. The famous constancy of light applies only when it is zipping through emptiness. Outside a vacuum, all bets are off. Light zips through water at 140,000 miles per second, for instance, and penetrates glass at 125,000 miles per second. The denser the medium, the more light loses speed. Diamonds delay light so much that each wavelength bends off in a different direction, giving the gemstone its distinctive flashes of color. When passing through the peculiar, ultracold collection of atoms known as a Bose-Einstein condensate, light can slow to a mere 38 miles an hour.
In a sense, light never really changes its velocity. Instead, each particle of light, or photon, is briefly absorbed by an atom in the material. A moment later, the atom emits a clone of that photon, which then continues the journey. The fraction of a second it takes for this to happen, repeated over and over, is what slows the light.
This means that it is possible to outrace light wherever it has to jump from atom to atom. When charged particles break the local speed of light, the result is a strangely beautiful blue glow called Cerenkov radiation—the visual equivalent of a sonic boom. On Earth, this radiation shows up in the reactor pools of certain types of nuclear power plants, sparked by high-speed atomic fragments that go shooting through the water.
More surprising, scientists have found that it is possible to exceed even light's top speed of 186,282 miles per second. In 2000 a team at the NEC Research Institute in Princeton, New Jersey, reported that they had pushed a pulse of light energy through a gas-filled chamber at 310 times the speed of light. And in 1997 Swiss researcher Nicolas Gisin studied what could be the fastest process in the universe. When two elementary particles are created together, some of their basic properties—such as the direction of their spin or the orientation in which they travel—become permanently bound together. If the property of one twin changes, the other twin simultaneously changes as well. Gisin's experiments confirmed that this spooky communication happens instantaneously, even across long distances, making it infinitely swifter than light.
Still, it would be misleading to end this Einstein centennial year without affirming that the great physicist was dead-on when he said that no tangible thing can ever outpace light's speed in a vacuum.
Even trying to get close to the speed of light would have some peculiar consequences. An astronaut moving at virtually the speed of light would pass through space but barely pass through time. A photon does not pass through time at all: Traveling at the full speed of light, it experiences being everywhere in the universe all at once.