Physics: Principles and Problems


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For use with Chapter 16
Light

Slower then the Speed of Light
Posted September 1st, 2001

Would you like to run faster than the speed of light? Well, it can be done... if you slow down. The speed of light is a constant in the universe, but its actual speed depends on what type of material it is traveling through.

Light travels through the vacuum of space at approximately 3 x 108 m/s. However, it travels through water at less than one quarter of 3 x 108 m/s. This reduction in speed is a result of water’s refractive index, which is what causes light to bend as it moves through a material.

Lene Vestergaard Hau and her team at Rowland Institute have used these principles to slow light down to less than 60 km/h. They began their research efforts in May 1998, and this year have been successful in making light slow down to the point where it stopped completely.

Hau and her team achieved this slowing – and eventual stopping – of light by aiming a concentrated beam through a gaseous mixture that is kept only a few millionths of a degree above absolute zero. This gas’s refractive index can be manipulated with laser beams, magnetic fields, and radio waves to alter the speed of light traveling through the gas mixture.

Hau and her colleagues first slowed light down to the speed of a commercial airliner, then down to less than the speed of freeway traffic, then down to the speed of a cyclist, and finally to a complete halt. They are able to "store" a particle of light for as long as a millisecond.

One interesting point that Hau and her colleagues noticed while researching the methods of slowing down light, was the fact that light does not amplify as it piles up on itself. The intensity of the light remains constant as it transmits much of its energy to the atoms in the surrounding gas.

The ability to slow light down and stop it completely has overwhelming applications in modern science. One of the biggest potential applications of this is in building "quantum computers." A traditional computer stores information as a binary series of ones and zeros on a magnetic disc. A quantum computer replaces the magnetic disc with atoms that are in one of two quantum states.

In a quantum computer, the hardware pieces have been made very small, allowing technicians to build a machine that can do incredibly powerful calculations – much more powerful than even the most advanced supercomputers today – and small enough to fit in the palm of your hand. Slowing light down to the stopping point may be an effective way of "storing" data in the atoms of a quantum computer.

Another application of this frozen light is the ability to study phenomena associated with black holes. Black holes have such a strong gravitational field that light cannot escape. By being able to dramatically reduce the speed of light in a lab, and even being able to freeze light in its tracks, scientists will be able to study this similar black hole phenomenon.

Activity
Use the Internet to research quantum computers. In your Science Journal, compare and contrast quantum computers and regular computers.

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