Science: Search Engines: Serendipity A trick of the light

THOMAS Young was one of the most remarkable scientists of the 19th century. His most famous experiment involved shining light at a partition containing two very thin parallel slits. On the other side of the slits, the light fanned out and illuminated a screen with several light and dark stripes. Young was perplexed, because he expected to see just two light stripes, projections of the slits in the partition.

Young conducted much of his research at Emmanuel College, Cambridge, and it is said that he eventually worked out what was happening in his light experiment while wandering around the college duck pond.

The inspiration for his discovery was the sight of two ducks swimming next to each other. Behind them, two sets of ripples fanned out and interacted, creating a whole series of rough and calm patches. If the crest of a wave from one duck met the crest of a wave from the other, the result was an even bigger wave. On the other hand, if the crest from one duck met the trough from the other, they cancelled each other, and the result was a patch of calm.

Young believed that exactly the same thing was happening in his light experiment. He argued that light was a form of wave, and light waves emerging from the two slits were akin to the two sets of ripples from the ducks. If two light peaks coincided, then the result was a bright stripe, and if a light peak met a light trough, then the result was a dark stripe.

Thanks to his encounter with the ducks, Young had made a brilliant discovery. However, in the 20th century Young's experiment has led to a strange paradox. Scientists have repeated it using a light source that is so dim that it emits only one wave of light at a time. The result is another striped pattern, as before, but how can that be? Young's original explanation required two waves to interact simultaneously, and so how can waves travelling one at a time cause the same thing?

This result can only be explained by quantum mechanics, one of the weirdest areas of science. According to one interpretation of quantum mechanics, when the single light wave is confronted by two slits, the universe splits into two. In one universe the light passes through one slit, and in the other universe it passes through the other slit. The two universes then interact to create the final pattern.

This week the Institute of Physics is holding a seminar on the prospect of a quantum computer, a device that attempts to exploit the same effect. It would perform long calculations by entering a multitude of universes and then doing a separate part of the calculation in each one. Quantum computers are currently one of the hottest research topics, even though the idea sounds totally absurd. As the Nobel prize winner Niels Bohr once put it: "Anyone who can contemplate quantum mechanics without getting dizzy hasn't understood it."