Our Final Hour: A Scientist’s Warning

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Excerpt from Our Final Hour: A Scientist’s Warning, by Lord Martin Rees (Basic Books, 2003). Reprinted with permission from the author.

Chapter 10: The Doomsday Philosophers

Can pure thought tell us whether humanity’s years are numbered?

Philosophers sometimes advance ingenious arguments that may seem clinching to some, but to others seem mere wordplay, or an intellectual sleight of hand, though it is not easy to pinpoint the flaw. There is a modern philosophical argument that humanity’s future is bleak that may seem in this dubious category, but which (with provisos) has weathered a good deal of scrutiny. The argument was invented by my friend and colleague Brandon Carter, a pioneer in the use of the so-called anthropic principle in science, the idea that laws governing the universe must have been rather special in order for life and complexity to have emerged. He first presented this argument, to the bemusement of his academic audience, at a conference hosted by the Royal Society in London in 1983. The idea was actually just an afterthought in a lecture discussing the likelihood that life would evolve on planets orbiting other stars. It led Carter to conclude that intelligent life was rare elsewhere in the universe, and that even though the Sun would keep shining for billions of years, life’s long-term future was bleak.

This “Doomsday argument” depends on a kind of “Copernican principle” or “principle of mediocrity” applied to our position in time. Ever since Copernicus, we have denied ourselves a central location in the universe. Likewise, according to Carter, we shouldn’t assume that we are living at a special time in the history of humanity, neither among the very first nor among the very last of our species. Consider our place in the “roll call” of Homo sapiens. We know our place only very roughly: most estimates suggest that the number of human beings who have preceded us is around sixty billion, so our number in the roll call is in this range. A consequence of this figure is that ten percent of the people who have ever lived are alive today. At first sight this seems a remarkably high proportion, given that mankind can be traced back through thousands of generations. But for most of human history—the entire preagricultural era before (maybe) 8000 B.C.E.—there were probably fewer than ten million people in the world. By Roman times, world population was around three hundred million, and only in the nineteenth century did it rise above a billion. The dead outnumber the living, but only by a factor of ten.

Now consider two different scenarios for humanity’s future: a “pessimistic” one, where our species dies out within one or two centuries (or if it survives longer than that has a much diminished population), so that the total number of humans who will ever exist is one hundred billion; and an “optimistic” scenario, where humanity survives for many millennia with at least the present population (or perhaps even spreads far beyond Earth with an ever-enlarging population), so that trillions of people are destined to be born in future. Brandon Carter argues that the “principle of mediocrity” should lead us to bet on the “pessimistic” scenario. Our place in the roll call (about halfway through) is then entirely unsurprising and typical, whereas in the “optimistic” scenario, where a high population persists into the far future, those living in the twenty-first century would be early in the roll call of humanity.

A simple analogy brings out the essence of the argument. Suppose that you are shown two identical urns: you are told that one contains just ten tickets, numbered from 1 to 10, and the other contains a thousand tickets, numbered from 1 to 1000. Suppose you pick one of the urns, draw a ticket from it, and find that you have drawn the number 6. You would then surely guess that you had, very probably, picked from the urn containing only 10 tickets: it would be very surprising to draw a ticket number as small as 6 from the urn containing a thousand tickets. Indeed, if you were equally likely, a priori, to have picked either urn, a simple probability argument then shows that having drawn the number 6, the odds are now one hundred to one that you actually chose from the urn containing only ten tickets.

Carter argues, along the same lines as in the case of the two urns, that our known place in the roll call of humans (about sixty billion human beings have preceded us) tilts the argument in favour of the hypothesis that there will be only one hundred billion humans, and would disfavour an alternative supposition that there would be more than one hundred trillion. So the argument suggests that the world’s population cannot continue for many generations at its present level; either it must decline gradually, and be sustained at a far lower level than at present, or a catastrophe will overcome our species within a few generations.

An even simpler argument was used by Richard Gott, a professor at Princeton University with a thirty-year record of zany but original insights on faster-than-light travel, time machines, and the like. If we come upon some object or phenomenon, we are unlikely to be doing so very near the beginning of its life, nor very near its end. So it is a fair presumption that something that is already ancient will last for a long time in the future, and something that is of recent origin shouldn’t be expected to be so durable. Gott recalls, for instance, that in 1970 he visited the Berlin Wall (then twelve years old) and the pyramids (over four thousand years old); his argument would have predicted (correctly) that the pyramids would very probably be still standing in the twenty-first century; but it would be unsurprising if the Berlin Wall wasn’t (and of course, it has gone).

Gott even showed how the argument applied to Broadway shows. He made a list of all the plays and musicals that were running on Broadway on a particular day (May 27, 1993) and found out how long each show had been open. On that basis, he predicted that those that had been running longest would survive furthest into the future. Cats had already been running for 10.6 years, and it kept going for more than seven years more. Most of the others, which had been running less than a month, closed within a few more weeks.

Of course, most of us could have made all Gott’s predictions without using his line of argument at all, from our familiarity with basic history, the general robustness and durability of artefacts of different types, and so forth. We also know about American tastes, and the economics of the theatre. The more background information we have, the more confident can our predictions be. But even a newly landed alien devoid of such background knowledge, who knew nothing except how long these various phenomena had existed, could have used Gott’s argument to make some crude but correct predictions. And of course the future duration of humanity is something about which we are as ignorant as any Martian would be about the sociology of Broadway shows. Gott therefore argues, following Carter, that this line of reasoning can tell us something—indeed, something far from cheerful—about the likely longevity of our species.

Obviously humankind’s future cannot be stripped down to a simple mathematical model. Our destiny depends on multitudinous factors, above all—a main theme of this book—on choices that we ourselves make during the present century. The Canadian philosopher John Leslie takes the line that the Doomsday argument nonetheless tilts the odds: it should make you less optimistic about humanity’s long-range future than you would otherwise be. If you thought, a priori, that it was overwhelmingly probable that humanity would continue, with a high population, for millennia, then the Doomsday argument would reduce your confidence, though you might still end up favouring that scenario. This can be understood by generalising the urn example. Suppose that instead of just two urns, there were millions of urns that each contained a thousand tickets and only one that contained just ten. Then if you pick an urn at random, you would be surprised to draw a 6. But if there were millions of “thousand-ticket” urns, then it would be less surprising that you had drawn an unusually low number from one of them than that you had picked the unique urn with only ten tickets in it. Likewise, if the a priori probability strongly favours a prolonged future for humanity, then “doom soon” might be less likely than finding ourselves coming very early in the roll call of humanity.

Leslie can thereby resolve another conundrum that at first sight seems to discredit the entire line of argument. Suppose that we had a fateful decision that would determine whether the species might soon be extinguished, or else whether it would survive almost indefinitely. For instance, this might be the choice of whether to foster the first community away from Earth, which, once established, would spawn so many others that one would be guaranteed to survive. If such a community were indeed established and flourished, we would currently find ourselves exceedingly early in the roll call. Does the Doomsday argument somehow constrain us towards the choice that leads to a truncated human future? Leslie argues that we are free to choose, but that the choice we make affects the prior probability of the two scenarios.

Another ambiguity concerns who or what should be counted: How do we define humanity? If the entire biosphere were to be wiped out in some global catastrophe, then there is no doubt about when the roll call ends. But if our species were to morph into something else, would this amount to the end of humanity? If so, the Carter-Gott argument might be telling us something different: it could offer support for Kurzweil, Moravec, and others who predict a “takeover” by machines within this crucial century. Or suppose there are other beings on other worlds. Then perhaps all intelligent beings, not just humans, should be in the “reference class.” There is then no clear way to order the roll call, and the argument collapses. (Gott and Leslie have used similar reasoning to argue against there being other worlds with much higher populations than ours. If there were, they claim, we should be surprised not to be in one of them.)

When I first heard Carter’s Doomsday argument, it reminded me of George Orwell’s robust comment in a different context: “You must be a real intellectual to believe that—no ordinary person could be so foolish.” But pinpointing an explicit flaw is not a trivial exercise. It is worth doing so, however, since none of us welcomes a new argument that humanity’s days may be numbered.

Excerpted from Our Final Hour by Lord Martin Rees. Copyright © Martin Rees, 2003. All rights reserved.

Martin Rees is one of the world’s most distinguished scientists and an internationally-renowned lecturer. He holds the positions of Astronomer Royal and Master of Trinity College Cambridge, and is a member of the British House of Lords. He was President of The Royal Society during its landmark 350th anniversary. Author of more than 500 research papers on cosmological topics ranging from black holes to quantum physics to the Big Bang, Rees has received countless awards for his scientific contributions. But equally significant has been his devotion to explaining the complexities of science for a general audience, in books like Before the Beginning and Our Cosmic Habitat.

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