- Travelling at the Speed of Thought: Einstein and the Quest for Gravitational Waves by Daniel Kennefick
Princeton, 319 pp, £19.95, May 2007, ISBN 978 0 691 11727 0
When yachts set sail with the tide, or people gather to witness a total eclipse of the Sun, they are trusting in Isaac Newton’s theory of gravity. For more than three hundred years his theory has proved so accurate in describing the universe that it has enabled us not only to predict tides and eclipses, but even to send spaceships to Jupiter, Saturn and beyond. One of Newton’s assumptions is that the effects of gravity are transmitted instantaneously. However, it is worth asking what ‘instantaneous’ means in this context. Since Einstein, the speed of light has been recognised as a natural limit; what, then, is the speed of gravity?
In Einstein’s universe, space and time are intimately entwined. ‘Space-time’ acts like a medium in which objects are distorted and the passage of time slowed when massive bodies such as planets and stars are present. In the jargon, space-time is ‘warped’. When galaxies collide or stars explode, the warping changes and the disturbance spreads out in ‘gravitational waves’ travelling at the speed of – what? The story of gravity, and especially of gravitational waves, has been far from clear-cut; as Daniel Kennefick points out in Travelling at the Speed of Thought, Einstein’s ‘first reaction on the completion of his theory was to conclude that gravitational waves do not exist’. The majority opinion now is that they do, even though no definitive observation of them has yet been made. Kennefick’s historical account shows that even today, as satellites carrying gravitational wave detectors are preparing for launch into space, the nature of what they are looking for is still not fully understood.
Unlike the electrical attractions and repulsions resulting from positive and negative charges within atoms, which cancel one another out, the gravitational attraction exerted by each and every particle in a large body adds up. Objects larger than about 500 km in diameter exert a powerful pull. The Sun, no bigger than a thumbnail when viewed from Earth, traps the planets in a cosmic waltz across hundreds of millions of kilometres of space. Newton posited that gravity’s pull between two bodies diminishes as the square of the distance between them increases, and that a massive body such as the Sun sends out its gravitational tentacles in all directions uniformly. His was a clockwork universe, where planets orbited permanently in regular repetitive orbits; the design seemed to accord with the perfection expected from a divine creator. But this ideal would not last.
Near the end of the 17th century, Edmond Halley examined records of medieval and ancient solar eclipses back to the time of Ptolemy. He discovered that when he used the position and trajectory of the Moon to determine retrospectively when solar eclipses should have occurred, the times calculated differed from the actual ones by up to an hour. Halley deduced that in the past the Moon must have moved across the sky from east to west more slowly than in his own time. This was a far-reaching, even heretical assertion. For the Moon to have changed its motion in such a way would imply that its course through the heavens did not repeat in periodic orbits. Such ‘secular’ changes in its orbit could eventually cause the system itself to disappear, and the Moon to fall into the Earth or escape into space. For many philosophers, to theorise that the cosmos could decay in this way was a slur on the Almighty, as it implied that God was such an unskilled craftsman as to have constructed a system of stars and planets that could fall into ruin and disorder. Nonetheless, Halley was right, as even the fundamentalists were eventually forced to concede. The question now became: what causes the secular acceleration of the Moon?