Despite the fact that the speed of light is an absolute upper limit, faster-than-light space travel is deeply embedded in science fiction. Einstein showed that any object with mass cannot reach, let alone exceed, the speed of light. But science fiction tends to overlook this very inconvenient truth simply because the universe is so big. To reach Alpha Centauri, the nearest star after our own Sun, would take more than four years for a spaceship moving at the speed of light, and a jaunt across the full diameter of our galaxy would take 100,000 years. Knowing this, script writers imagine solutions like Star Trek’s “warp drive” that allow the Enterprise to travel around the galaxy at multiples of light speed, or “worm holes” that provide cosmic short cuts.
Although NASA and some scientists have speculated about warp drives and worm holes, no one yet has the slightest idea of how to build an actual device that will beat Einstein’s limit, which may never happen. But although science fiction has to use imagination to move events along, there is increasing interest at the other end of the scale, where light itself is slowed down.
Most people know that light moves at a constant speed of about 300,000 kilometers/second (186,000 miles/second), and are startled to hear that light can also move more slowly; but it can, because the quoted speed applies only to light in vacuum. In a transparent medium like glass or plastic, the speed can drop nearly by half. 150,000 kilometers/second is still enormously fast, but it means that when you observe the world through a pane of glass, you’re seeing events a tiny slice of time later than someone looking through an open window.
That’s not the end of it, because scientists have made light move much more slowly yet. The breakthrough came in 1999, when Danish-born physicist Lene Hau, working at Harvard and the Rowland Institute in Cambridge, brought a ray of laser light to a speed of 61 kilometers/per hour (that’s 38 miles per hour, not per second), almost comparable to a briskly pedaled bicycle. A year later, she dialed the speed down to slower than a walk, 1 mile/hour, and then in 2001 came the ultimate: she brought a light ray to a dead halt.
To do this, Hau introduced the light into an exotic medium called a Bose-Einstein condensate (BEC) – a quantum mechanical superatom formed when a group of regular atoms merges near absolute zero, as predicted by Einstein and the Indian physicist Satyendra Nath Bose in the 1920s. In 1995, Eric Cornell and Carl Wieman, associated with the University of Colorado at Boulder, earned a Nobel Prize in Physics when they achieved the necessary extreme cooling to obtain the first BEC, made of about 2,000 rubidium atoms. Hau understood how passage through a BEC would affect light and created her own BECs for her experiments.
Now slowed-down light has became an active research area as part of a new movement toward light-based technology. Devices that use photons instead of electrons have big advantages in efficiency, miniaturization, and bandwidth. Steady progress has been made in creating and using slowed light for computing and data storage.
In science fiction, the emphasis on the high speed of light has tended to obscure its other fascinating properties. I know of no motion pictures, and only one or two published stories, that explore the possibility of radically decreasing the speed of light; yet, this seems like a perfect opportunity for some stunning movie special effects, especially when joined with the strange and unique properties of the BEC, which is a quantum system observable on a human scale. Maybe it’s time for a science-fiction film that explores the small, slow-moving world of light in a BEC rather than the huge, fast-moving world of space travel.
Readers who want to know more about slow light and BECs can find numerous web articles, including my own “Bose-Einstein condensate,” Encyclopædia Britannica Online, 2009.