The Science of Discworld Revised Edition

predicting it and then see what they agree about. According to the mathematics, you can be pretty sure
those
things are right.
    One of the most striking results is that the solar system is due to lose a planet. About a billion years from now, Mercury will move outwards from the Sun until it crosses the orbit of Venus. At that point, a close encounter between Venus and Mercury will fling one or the other, possibly both, out of the solar system altogether – unless they hit something on the way, which is highly unlikely, but possible. It might even be the Earth, or the passing Venus might join with us in a cosmic dance whose end result is the
Earth
being flung out of the solar system. The details are unpredictable, but the general scenario is very likely.
    This means that we’ve got the wrong picture of the solar system. On a human timescale it’s a very simple place, in which nothing much changes. On its
own
timescale, hundreds of millions of years, it’s full of drama and excitement, with planets roaring all over the place, whirling around each other, and dragging each other out of orbit in a mad gravitational dance.
    This is vaguely reminiscent of
Worlds in Collision
, a book published in 1950 by Immanuel Velikovsky, who believed that a giant comet was once spat out by Jupiter, passed close to the Earth
twice
, had a love affair with Mars (giving rise to a brood of baby comets), and finally retired to live in peace as Venus. Along the way it gave rise to many strange effects that became stories in the Bible. Velikovsky was right about one thing: the orbits of the planets are not fixed forever. He wasn’t right about much else.
    Do other solar systems encircle distant stars, or are we unique? Until a few years ago there was a lot of argument about this question, but no hard evidence. Most scientists, if they had to bet, would have backed the existence of other solar systems, because the collapsing dustcloud mechanism could easily get going almost anywhere there’s cosmic dust – and there are a hundred billion stars in our own galaxy, let alone the billions upon billions of others in the universe, all of which once
were
cosmic dust. But that’s only indirect evidence. Now the position is much clearer. Characteristically, however, the story involves at least one false start, and a critical re-examination of evidence that at first looked rather convincing.
    In 1967 Jocelyn Bell, a graduate student at the University of Cambridge, was working for a doctorate under the direction of Anthony Hewish. Their field was radio astronomy. Like light, radio is an electromagnetic wave, and like light, radio waves can be emitted by stars. Those radio waves can be detected using parabolic dish receivers – today’s satellite TV dishes are a close relative – rather misleadingly called ‘radio telescopes’, even though they work on very different principles from normal optical telescopes. If we look at the sky in the radio part of the electromagnetic spectrum, we can often ‘see’ things that are not apparent using ordinary visible light. This should be no surprise: for example military snipers can ‘see in the dark’ using infra-red waves – detecting things by the heat they emit. The technology in those days wasn’t terribly slick, and the radio signals were recorded on long rolls of paper using automatic pens that drew wiggly curves in good old-fashioned ink. Bell was given the task of looking for interesting things on the paper charts – carefully scanning about 400 feet of chart per week. What she found was very strange – a signal that pulsated about thirty times per second. Hewish was sceptical, suspecting that the signal was somehow generated by their measuring instruments, but Bell was convinced it was genuine. She searched through three miles of previous charts and found several earlier instances of the same signal, which proved she was right. Something out there was emitting the radio equivalent of a reverberating whistle. The object responsible was named a ‘pulsar’ – a pulsating starlike object.
    What could these strange things be? Some people suggested they were radio signals from an alien civilization, but all attempts to extract the alien equivalent of
The Jerry Springer Show
failed (which was possibly just as well). There seemed to be no structured messages hidden in the signals. In fact, what they are now believed to be is even stranger than an alien TV programme. Pulsars are thought to be