Is The Doomsday Argument Easily Refuted

Already embattled on other fronts, Carter has presented the doomsday argument only in lectures and seminars, never in print. However, I published it on p. 214 of Universes in a long foot-note. Since then I have investigated it in several articles. The argument is certainly controversial. So far, however, I have managed to find only one good ground for doubting it. Suppose that the cosmos is radically indeterministic, perhaps for reasons of quantum physics. Suppose also that the indeterminism is likely to influence how long the human race will survive. There then isn't yet any relevant firm fact, 'out there in the world' and in theory predictable by anybody who knew enough about the present arrangement of the world's particles, concerning how long it will survive—like the fact that hidden cards include a definite number of aces, a number you are trying to estimate, or like the fact that exactly nine or exactly sixty names remain to be drawn from a lottery urn, after your own name has been drawn from it. Yet in order to run really smoothly, the doomsday argument does need the existence of a firm fact of this kind, I believe.10

Even if it ran only rather roughly, though, the doomsday argument could have considerable importance. In particular, it might throw severe doubt on the theory that the human race will very probably survive for many thousand years. For anyone who believes in radically indeterministic factors yet says that something 'will very probably occur' must mean that even those factors are unlikely to prevent its occurrence.

People have suggested many reasons for distrusting the doomsday argument. At least a dozen times, I too dreamed up what seemed a crushing refutation of it. Be suspicious of such refutations, no matter how proud you may be of them! Probability theory is full of traps. Don't put complete trust in the first 'blindingly obvious objection' that springs to mind. The doomsday argument has now been thought about rather hard by some rather good brains. What seems to have emerged is that it doesn't fall victim to any simple counter-argument.

If it did, then almost all 'anthropic' reasoning—reasoning which draws attention to when and where an observer could at all expect to be—would be in severe trouble. As we have seen, the preconditions of observership are seldom entirely firm. Observers might just possibly exist early in the Big Bang, as patterns of particles emitted randomly from black holes. Users of the anthropic principle therefore ask about an observer's probable location in space and in time. Now, most criticisms of the doomsday argument treat probable location in time in a way in which nobody would dream of treating probable location in space. This goes against the entire tradition of anthropic reasoning, which looks on time and space as equally grist to its mill. And it is hard to see any justification for going against tradition here—once, that is to say, we have made the important concession that the future might be radically indeterministic, in which case the doomsday argument would be weakened although not destroyed.

Look at one very common criticism. Any people of a heavily populated far future are not alive yet. Hence we certainly cannot find ourselves among them, in the way that we could find ourselves in some heavily populated city rather than in a tiny village. We are considering the doomsday argument now, which means at around AD 2000. We know we are at around AD 2000. We'd be just as sure of it, no matter what our theory was about how many humans would exist later. It's because we live near the year 2000 that we can say that the human race got as far as this safely, but cannot say how much further it will get! The evidence we possess of the risks facing humankind is evidence from around AD 2000, not evidence collected many thousand years later.

Well, those remarks are all of them correct, yet how could they invalidate the doomsday argument? Brandon Carter doesn't doubt that the neighbourhood of AD 2000 is now—that he really is in that neighbourhood, with 100 per cent probability. What he asks himself is the following. As a human observer, how likely would one have been to find oneself there, if the lives of all but a very small proportion of all humans were to be lived later? Of course, the lives of you and me are not particularly early among those of all humans alive now, let alone among those ofall humans born so far, but to keep insisting on this is to miss Carter's point.

The uselessness of protesting that later humans aren't alive yet can be shown, it seems, with a simple story. Imagine an experiment planned as follows. At some point in time, three humans would each be given an emerald. Several centuries afterwards, when a completely different set of humans was alive, five thousand humans would again each be given an emerald. Imagine next that you have yourself been given an emerald in the experiment. You have no knowledge, however, of whether your century is the earlier century in which just three people were to be in this situation, or the later century in which five thousand were to be in it. Do you say to yourself that if yours were the earlier century then the five thousand people wouldn't be alive yet, and that therefore you'd have no chance of being among them? On this basis, do you conclude that you might just as well bet that you lived in the earlier century?

Suppose you in fact betted that you lived there. If every emeraldgetter in the experiment betted in this way, there would be five thousand losers and only three winners. The sensible bet, therefore, is that yours is instead the later century of the two. INTRODUCTION

What if you were somewhat unsure whether the experimental plan called for more people to get emeralds in the later century than in the earlier? Getting an emerald would now give somewhat weaker grounds for betting that you lived in the later century. If you next came to know you were in fact in the earlier century, then your new knowledge would strengthen your reasons for doubting that many more emeralds—or any—would be distributed in the later century. Throughout, it would of course be true that people who hadn't yet been born weren't yet observing anything. But this truth would be utterly irrelevant.

Continue reading here: Some Further Examples Of Attempted Refutations

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