Early Human Interpretations of Natural Disasters

CAN WE PREDICT THE FUTURE? I n ancient times, the world must have seemed pretty arbitrary. Disasters such as floods, plagues, earthquakes or volcanoes must have seemed to happen without warning or apparent reason. Primitive people attributed such natural phenomena to a pantheon of gods and goddesses, who behaved in a capricious and whimsical way. There was no way to predict what they would do, and the only hope was to win favour by gifts or actions. Many people still partially subscribe to this belief and try to make a pact with fortune. They offer to behave better or be kinder if only they can get an Agrade for a course or pass their driving test. Gradually however, people must have noticed certain regularities in the behaviour of nature. These regularities were most obvious in the motion of the heavenly bodies across the sky. So astronomy was the first science to be developed. It was put on a firm mathematical basis by Newton more than 300 years ago, and we still use his theory of gravity to predict the motion of almost all celestial bodies. Following the example of astronomy, it was found that other natural phenomena also obeyed definite scientific laws. This led to the idea of scientific determinism, which seems first to have been publicly expressed by the French scientist Pierre-Simon Laplace. I would like to quote to you Laplace’s actual words, but Laplace was rather like Proust in that he wrote sentences of inordinate length and complexity. So I have decided to paraphrase the quotation. In effect what he said was that if at one time we knew the positions and speeds of all the particles in the universe, then we would be able to calculate their behaviour at any other time in the past or future. There is a probably apocryphal story that when Laplace was asked by Napoleon how God fitted into this system, he replied, ‘Sire, I have not needed that hypothesis.’ I don’t think that Laplace was claiming that God didn’t exist. It is just that God doesn’t intervene to break the laws of science. That must be the position of every scientist. A scientific law is not a scientific law if it only holds when some supernatural being decides to let things run and not intervene. The idea that the state of the universe at one time determines the state at all other times has been a central tenet of science ever since Laplace’s time. It implies that we can predict the future, in principle at least. In practice, however, our ability to predict the future is severely limited by the complexity of the equations, and the fact that they often have a property called chaos. As those who have seen Jurassic Park will know, this means a tiny disturbance in one place can cause a major change in another. A butterfly flapping its wings in Australia can cause rain in Central Park, New York. The trouble is, it is not repeatable. The next time the butterfly flaps its wings a host of other things will be different, which will also influence the weather. This chaos factor is why weather forecasts can be so unreliable. Despite these practical difficulties, scientific determinism remained the official dogma throughout the nineteenth century. However, in the twentieth century there were two developments that show that Laplace’s vision, of a complete prediction of the future, cannot be realised. The first of these developments was what is called quantum mechanics. This was put forward in 1900 by the German physicist Max Planck as an ad hoc hypothesis, to solve an outstanding paradox. According to the classical nineteenth-century ideas dating back to Laplace, a hot body, like a piece of red-hot metal, should give off radiation. It would lose energy in radio waves, the infra-red, visible light, ultra-violet, X-rays and gamma rays, all at the same rate. This would mean not only that we would all die of skin cancer, but also that everything in the universe would be at the same temperature, which clearly it isn’t. However, Planck showed one could avoid this disaster if one gave up the idea that the amount of radiation could have just any value, and said instead that radiation came only in packets or quanta of a certain size. It is a bit like saying that you can’t buy sugar loose in the supermarket, it has to be in kilogram bags. The energy in the packets or quanta is higher for ultra-violet and X-rays than for infra-red or visible light. It means that unless a body is very hot, like the Sun, it will not have enough energy to give off even a single quantum of ultra-violet or X-rays. That is why we don’t get sunburn from a cup of coffee. Planck regarded the idea of quanta as just a mathematical trick, and not as having any physical reality, whatever that might mean. However, physicists began to find other behaviour that could be explained only in terms of quantities having discrete or quantised values rather than continuously variable ones. For example, it was found that elementary particles behaved rather like little tops, spinning about an axis. But the amount of spin couldn’t have just any value. It had to be some multiple of a basic unit. Because this unit is very small, one does not notice that a normal top really slows down in a rapid sequence of discrete steps, rather than as a continuous process. But, for tops as small as atoms, the discrete nature of spin is very important. It was some time before people realised the implications of this quantum behaviour for determinism. It was not until 1927 that Werner Heisenberg, another German physicist, pointed out that you couldn’t measure simultaneously both the position and speed of a particle exactly. To see where a particle is, one has to shine light on it. But by Planck’s work one can’t use an arbitrarily small amount of light. One has to use at least one quantum. This will disturb the particle and change its speed in a way that can’t be predicted. To measure the position of the particle accurately, you will have to use light of short wavelength, like ultra-violet, X-rays or gamma rays. But again, by Planck’s work, quanta of these forms of light have higher energies than those of visible light. So they will disturb the speed of the particle more. It is a no-win situation: the more accurately you try to measure the position of the particle, the less accurately you can know the speed, and vice versa. This is summed up in the Uncertainty Principle that Heisenberg formulated; the uncertainty in the position of a particle