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At the third beep, the Atomic Clock will be 60 ... imprecisely

The science that makes GPS (and a whole lot more) possible

Feature The atomic clock celebrates its 60th birthday this year. John Watkinson looks at the story of this enabling technology that lies behind GPS and DAB and discusses its roots in quantum mechanics and relativity. Readers are warned that they might need to lie down for a while after reading this article.

The story probably begins with the state of physics towards the end of the nineteenth century. The mathematics of the propagation of light and other electromagnetic waves had been set down, but it was still thought by many that electromagnetic waves needed a medium in which to travel, rather like sound needs air.

The medium would have to be pretty thin, because the planets were tearing through it in their orbits round the Sun without ill effect - and for all that was known the Sun could also be in motion. This flimsy hypothetical medium was dubbed the "ether".

In the 1880s, an experiment was designed by Michelson and Morley to measure the velocity of the Earth through the ether. They sent beams of light from the same source in two directions at right angles and added them together when they reflected back.

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The whole apparatus floated in mercury so it could be turned. The theory was that if one of the light paths aligned with the direction though the ether, it would show a difference in the velocity of light in comparison with the other path.

Whilst their apparatus was adequately sensitive to measure what they were looking for, they couldn't find any effect. However they turned the apparatus, whatever the season and whatever the time of day, they couldn't pick up any velocity at all. As more people repeated the experiment with more accuracy and more determination, the only result was that scientific fingers went in more scientific mouths.

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Some describe the experiment as a failure, but that is not the case. In fact the experiment was a milestone because by showing that the existing theories were incorrect it led to new ones. What Michelson and Morley had found required that it could not be possible to detect linear motion from within a closed experiment. It required the laws of physics to display symmetry, meaning the same result would be obtained irrespective of the axes employed or of whether the origin of those axes was moving in a linear manner.

The Dutch physicist Hendrik Lorentz then proposed the theory that space contracted its dimensions as a function of motion so that the light would appear to travel at an unchanged velocity. Albert Einstein picked up on this and set out the ideas that led to relativity.

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According to Einstein’s theory, obtaining symmetry would not only require a contraction in dimensions with motion, but mass would also be required to increase. Time would also be modified by motion and gravity. Einstein argued that Newton’s Laws needed correction terms at high velocities, high in the sense that they were a sizeable fraction of the velocity of light, which became known as relativistic velocities.

Nevertheless, the sensationalist media reported that Newton’s Laws had been overthrown, thereby setting an equally enduring standard of accuracy that today explains why something that is obviously bright orange is called a black box. If anything was overthrown, it was the concept of the ether.

Next page: Beat the clock

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