ONE OF THESE DAYS COPERNICUS WILL BE RIGHT ABOUT SOMETHING:

A GRIM
RECKONING (J. Richard Gott III, New Scientist)

In 1969, after graduating from Harvard but before starting further study in astro-physics at Princeton University, I took a summer holiday in Europe and visited the Berlin Wall. It was the height of the Cold War, and the wall was then eight years old. Standing in it ominous shadow, I began to wonder how long it would last. Having no special knowledge of East-West relations, I hadn't much to go on. But I hit on a curious way to estimate the wall's likely lifetime knowing only its age.

I reasoned, first of all, that there was nothing special about my visit. That is, I didn't come to see the wall being erected or demolished--I just happened to have a holiday, and came to stand there at some random moment during the wall's existence. So, I thought, there was a 50 per cent chance that I was seeing the wall during the middle two quarters of its lifetime (see Diagram, below). If I was at the beginning of this interval, then one-quarter of the wall's life had passed and three-quarters remained. On the other hand, if I was at the end of of this interval, then three-quarters had passed and only one-quarter lay in the future. In this way I reckoned that there was a 50 per cent chance the wall would last from 1/3 to 3 times as long as it had already.

Before leaving the wall, I predicted to a friend, that it would with 50 per cent likelihood, last more than two and two-thirds years but less than 24. I then returned from holiday and went on to other things. But my prediction, and the peculiar line of reasoning that lay behind it, stayed with me. Twenty years later, in November 1989 the Berlin Wall cam down--unexpectedly, but in line with my prediction.

Intrigued that the approach seemed to work, I eventually set out its logic in Nature(vol 363, p315, 1993). There, instead of using the 50 percent mark, I adopted the more standards scientific criterion that the prediction should have at least a 95 per cent chance of being correct. This makes the numbers in the formula come out a bit different, but the argument remains the same. If there is nothing special about your observation of something, then there is a 95 per cent chance that you are seeing it during the middle 95 per cent of its observable lifetime, rather than during the first or last 2.5 per cent (see Diagram, p 38). At one extreme the future is only 1/39 as long as the past. At the other, it is 39 times as long. With 95 per cent certainty, this fixes the future longevity of whatever you observe as being between 1/39 and 39 times as long as its past. [...]

As another test, I used my formula on the day my Nature paper was published to predict the future longevities of the 44 Broadway and off-Broadway plays and musicals in New York; 36 have now closed--all in a agreement with the predictions. The Will Rogers Follies, which had been open for 757 days, closed after another 101 days, and the Kiss of the Spider Woman open for 24 days, closed in another 765 days. In each case the future longevity was within a factor of 39 of the past longevity, as predicted.

This is all good fun. You can predict approximately how long something will last without knowing anything that its current age. But in the past few months, in the light of the spectacular success of NASA's Mars Pathfinder mission, I've been reminded of of a far more serious implication of this way of thinking. Applying it to the human race forces mt to conclude that our extinction as a species is a very real possibility, and that we had better take steps to improve our survival prospects before it's too late. Let me explain why I have such a sense of urgency, wand why we had better begin colonising space--and very soon.

In the 16th century, Nicolaus Copernicus pointed out that the Earth revolved about the Sun, rather than vice versa, and in one swift move, displaced humanity from its privileged place at the centre of the Universe. We now see the Earth as circling an unexceptional star among thousands of millions of others in our unexceptional Galaxy. This perspective is summed up more generally in the "Copernican Principle", which is the position that one's location is unlikely to be special.

Early this century, when astronomer Edwin Hubble observed approximately the same number of galaxies receding from Earth in all directions, it looked as if our Galaxy was at the exact centre of a great explosion. But reasoning with the Copernican principle, scientists concluded instead that the Universe must look that way to observers in every galaxy--it would be presumptuous to think that out galaxy is special. As a working hypothesis, the Copernican principle has been enormously successful because, out of all the places intelligent observers could be, there are only a few special places and many nonspecial places. A person is simply more likely to be in one of the many nonspecial places. but the Copernican principle doesn't apply only to placement of galaxies in space-- it works for placement of moments of time as well. Inset 1

What does it imply for Homo sapiens? We have been around for about 200 000 years. If there is nothing special about the present moment, then it is 95 per cent certain that the future duration of our species is between 1/39 and 39 times 200 000 years. That is, we should last for at least another 5100 years but less than 7.8 million years.

Since we have no actuarial data on other intelligent species, this Copernican estimate may be the best we can find. It gives our species a likely longevity of between 0.205 million and 8 million years, which is quite in line with those for other hominids and mammals. The Earth is littered with the bones of extinct species and it doesn't take much to see that we could meet the same fate. Our ancestor H. erectus last 1.6 million years, while H. neanderthalensis lasted 0.3 million years. The mean duration of mammal species is 2 million years, and even the great Tyrannosaurus rex lasted only 2.5 million years.

For us, the end might come from a drastic climate change, nuclear war, a wandering asteroid or comet, or some other catastrophe that catches us by surprise, such as a bad epidemic. If remain a one-planet species, we are exposed to the same risk as other species, and are likely to perish on the same timescale.