Originally published 7 June 1993
Associated Press news report: “Repent, The End of the World May be Nearer Than You Think, Astrophysicist Calculates.”
“Limits Seen on Human Existence,” said the New York Times.
In the May 27 [1993] issue of the journal Nature, Princeton University astrophysicist J. Richard Gott calculates that there is a 95 percent chance that the human race could perish between 5,100 and 7.8 million years from now.
Nature is a respected scientific journal, perhaps the best known in the world; anything published there should be taken seriously. The reported limits on our future existence are convincingly precise — not 5,000 years, but 5,100, not 8 million but 7.8 million; clearly, we are not dealing here with wild guesses. And if you glance at the Nature article, you will find it dense with impressive equations.
Which explains, I suppose, the hype in the popular press. The AP tag line says it all by juxtaposing “repent” and “astrophysicist.” Science confirms the Apocalypse. Mathematics proves the Millennium is nigh.
Yeah.
There is less here than meets the eye.
First, let’s get behind those equations in Nature. The gist of Gott’s article can be stated simply:
Assume your local phone book has 100 pages, and that there is nothing special about your name.
The odds of finding your name on any particular page is 1 percent.
There’s a 95 percent chance of finding your name somewhere between pages 2.5 and 97.5, because that’s how many pages are in the interval.
Now assume that the lifetime of our species is the phone book. The human species has been around for 200,000 years.
If we are now at page 2.5 of human existence, then each pages consists of 80,000 years (200,000 divided by 2.5) and we have 7.8 million years to go.
If we are at page 97.5 of human existence, then each page consists of 2,050 years (200,000 divided by 97.5) and we have 5,100 years to go.
Therefore we can say with 95 percent confidence that our species has a future of between 5,100 and 7.8 million years.
Simple. And essentially meaningless.
I could equally well say with 99 percent confidence that the future of our species lies between 1,005 years and 39.8 million years.
Or I could say with 100 percent confidence that the Apocalypse will arrive sometime between today and the infinite future.
Clearly, the precise numbers reported in the popular press don’t mean a heck of a lot.
At the heart of Gott’s argument is the so-called Copernican Principle, which states that there is nothing special about our position in time or space. In my summary of Gott’s calculation, this is equivalent to saying there is an equal probability that your name could be on any page of the phone book.
Astronomers generally accept the Copernican Principle because of experience. Every time we thought we were central or special we discovered otherwise. We inhabit a typical planet of a typical star in a typical neighborhood in a typical galaxy in a typical part of the observable universe. On the cosmic scale, it seems, we are utterly mediocre.
I fully accept the Copernican Principle and think it is probably a excellent guide for research. But it is not science in the same way that the Law of Gravity, say, is science. There is no experiment we can perform to verify or falsify the principle, nor can we derive it mathematically from other principles. It is philosophical, not scientific.
Copernicus himself didn’t accept the Copernican Principle as defined by Gott. Most people who read the news reports of Gott’s calculation almost certainly reject the Copernican Principle, and prefer to think, for religious or other reasons, that we are indeed special.
In particular, those people who respond to the apocalyptic overtones of the story are probably the very people who most strongly reject the principle upon which the calculation is based.
There’s no real need for invoking the Copernican Principle to estimate the lifetime of our species. The fossil record of life on Earth indicates that a typical species survives for something between 1 million and 10 million years. For mammals, the lifetime is between .5 million and 5 million, depending on the geologic epoch. We’ve known this for a long time. So what’s the big deal about Gott’s calculation?
There’s also the question of how long the human species has been around so far. Gott uses a figure of 200,000 years, but there is no agreement on this figure by paleontologists. As Gott himself points out, recent estimates range from 100,000 years to 250,000 years. Which also makes the precise numbers of the news reports meaningless.
Neither the Copernican Principle nor the fossil record is likely to be an adequate guide to the future of our particular species. The new factor is technology, which may have the effect of wiping us out prematurely or resetting the Copernican clock. I would guess that within a time span far less than 5,100 years, and maybe less than 100, we will have created forms of artificial intelligence in advance of our own, either organic or electronic. It is not clear what that will mean for the future of our species, or for the evolution of intelligence in the cosmos.
And finally, consider this. If we do not occupy a special point in time, then even our scientific and philosophic understanding of the world is destined to change dramatically. What, we might ask, will have replaced the Copernican Principle 7.8 million years from now?
