Stopping Light

Classical physics teaches us that light travels at 186,200 miles a second in a vacuum and that it slows down only a little bit when it passes through transparent matter, such as air, glass, or water. Generally, light either reflects off an object (so that we see it) or goes through it. It was considered impossible to actually halt light.

In the last year, researchers at Harvard University have done the impossible — they have actually stopped light, in the form of a laser pulse, as it entered a container of atomic vapors. The light was not destroyed or absorbed, but stored, and able to emerge unchanged at the researchers' will.

Scientists have been re-writing the physical "laws" as technology and insight into the workings of the universe have expanded. In previous experiments at Harvard, scientists achieved a considerable slowing down of light. This feat, also, had been believed to be unattainable. The secret of slowing down and stopping light is in using the right combination of lasers and choosing the right transparent medium.

Despite the fact that the laser pulse was kilometers long before it entered the container, it fit into the centimeters-wide chamber. Although this, too, sounds impossible, quantum mechanics, which describes the rules of motion at the atomic and subatomic level, can account for this. Quantum physicists realized last century that light exhibits both wave and particle characteristics at the same time, and that nothing at that level of existence is certain. There is only the probability that a certain condition could exist.

Light is able to travel so fast because the light particle, the photon, has no mass. The Harvard researchers stopped their laser beam by "weighing the photons down" by creating a strong interaction with atoms in the chamber. Physicists call such an atom-photon system a "polariton."

Then they reduced the intensity of the incoming laser until the polariton was 100% atomic, meaning there were no photons left inside the chamber. Yet the imprint of the photons remained — on the atoms themselves, like a code — in the up-and-down patterns of the atoms' spin axes. The light was stored in the chamber until another laser beam was directed through it.

See the above illustration, taken from the NASA site. The caption to this illustration states "As the laser pulse enters the chamber containing the rubidium [or sodium] vapor, the information that defines the light becomes imprinted on the atoms' spin states (indicated by the small arrows). In the moment that the light is 'stopped,' only the spin states exist. This image by Tony Phillips is based on another from the American Institute of Physics."

One possible application for stopping light is the ultra-secure encoding of messages. Since one of the quintessential traits of the quantum world is that observing a system actually alters that system's properties, it would be impossible to intercept a quantum message without the receiver's knowing.

Another application is called "quantum computing," using atoms to process and store information in computers, just as silicon chips are used today. Quantum computers would have enormous power due to the vast numbers of atoms in even a thimble-full of matter.

In a more long-range use, the transporter featured in Star Trek is based on a reversal of this very principle. Rather than converting light to "matter" and then back, the transporter would impress the body's molecular pattern on a laser beam, transmit it to a distant location, and then re-form the light back into matter.

Science may be catching up to science fiction.

For more information see NASA.