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Home , Anthropic principle

409 .N..::.:A:.:..::TU:.:.=RE:....:.V.::.:OL.:.:.:. J07~2~ff=B=RU=AR:.:..._Y.:.:.:1984..:______NEWS AND VIEWS.------=

New twist for anthropic The originator of the doctrine that our observations of the are biased because they are made by us now concludes that there have been at most two critical steps in biological . THE is Dr Brandon whenever geophysical circumstances are by the lifespan of the Sun as a ­ Carter's choice of a philosophical middle favourable, and thus on a denial of the burning . So the observation, after a ground between what he calls the contradictory hypothesis that the mere 4,000 million years, that there are "primitive " of the pre­ of life is improbable. But, says sentient beings able to contemplate their Copernicans and "its equally unjustifiable Carter, the only evidence bearing on the own emergence is a sign that, in the event, antithesis" of extreme representations of question is the observation (by ourselves) the course of evolution on the has the relativistic , the that life has indeed emerged on the surface been exceptionally short. assertion that, "apart from local inhomo­ of the Earth, whence there is no way of dis­ Carter takes this argument one argu­ geneities", no place (or time) in the Uni­ tinguishing between the two hypotheses. mentative step further. Suppose, he says, verse can be privileged in any way. When Dirac's large-numbers hypothesis is logi­ that the evolution of sentient beings Carter introduced his principle a decade cally identical. Dirac at the outset argued depends on n critical steps (defined as ago (/AU Symp. 63, 291; 1974), his that coincidence between the age of the unlikely evolutionary steps, which are thus objective was modestly to point out that Universe (strictly, the Hubble time) and the "slow", compared with the majority of the mere fact that our observations of the is so remarkable that evolutionary steps). Then it is possible to Universe are made by people who are the one must expect the gravitational constant show that the chance of all these n steps products of some 3,000 million years of to decrease with time so as to preserve the being completed by time t is proportional biological evolution implies an inescapable coincidence. (Numerically, the Hubble to tn and also to show that evolution (the bias. Carter's latest version of the time is I 058 in units in which the velocity of Sun's time as a hydrogen-burning star or argument, now published as part of the light, Planck's constant and the gravi­ T0) exceeds that occupied by evolution so report of last year's meeting at the Royal tational coupling strength are determined far by a quantity of the order of (1/n)t. But Society on the constants of physics (Phil. by the three-halves power of the square of this time is itself of the order of T 0, whence Trans. R. Soc. A 310, 347-363; 1983) is the mass, of the order of 10 38 in the Carter concludes that n must be a whole more radical, and a challenge to biologists same units.) Carter's view is that the number greater than zero and less than or as well as astronomers. coincidence is not a coincidence at all, equal to two - in short, either I or 2. The reasons why the anthropic principle but a consequence of the Carter insists that the general character of should figure in a symposium on the occasioned because the observations are this result does not depend on the details of constants of physics are not as surprising as necessarily made by people whose existence the calculation but acknowledges that it they may seem. The luminosity of a star of requires a Hubble time large enough so that poses a dilemma: how can the fossil record given mass, and thus the speed of its evo­ some can have completed their evo­ be squared with the conclusion that there lution from a young star to, say, a red lution and have generated the chemical have been at most two critical steps in evo­ giant, depends linearly on the value of the elements of which people are made, but not lution so far? His own preference is to face gravitational constant. If this value were so long that all stars have run their course. the dilemma head on; he recommends the only a little greater than it is now thought to This is the stuff of lunch-room evolution of the genetic code and the be, the Sun would have run through its arguments about the anthropic principle in organization of the nervous system as evolution faster, so that biological the past decade. Carter's new departure candidates for the two critical steps. evolution might have got no further than, begins with another coincidence - that What weight should be given to this say, the dinosaurs before the Earth was between the time-span of biological conclusion? In his paper, Carter does not engulfed by a red giant. And a smaller evolution on the surface of the Earth seek to mollify his critics, dismissing the gravitational constant might never have (say, 0.4 X 10 10 years) and the length of attention being paid to the "inflationary allowed for the particular circumstances time the Sun is likely to spend as a universe'' in all its varieties as an attempt to that led to the emergence of living things in hydrogen-burning star (say 10 10 years) resurrect the "beguiling notion" of the the first place. So there is a chance of before becoming a red giant with perfect cosmological principle, and learning something about the value of the engulfing the Earth's . complaining that those who hold that only gravitational constant simply by exploiting What connection can there be between falsifiable hypotheses are admissible are in the circumstance that its evaluation two such apparently independent effect saying that science has accomplished depends on observations made by people. quantities as the timescale of biological nothing so far. Carter now says that if he had known that evolution, presumably determined by the The Popper school will be further the name "anthropic principle" would be complexity of living organisms, and the offended that Carter not merely uses the so widely used, he would have looked timescale of stellar evolution, determined verb "to induce" but that he openly uses harder for a different way of referring to (in Carter's words) by "the weakness of the language of Bayesian probability, the the bias caused by self-selection. gravitation"? None, he says. The apparent calculus of informed guesswork as some In passing, Carter directs well-aimed coincidence is merely a consequence of the consider it. Even his friends may say that he darts at two fashionable preoccupations, "habitual mistake" of overlooking the has too lightly discarded the possibility that speculation about extraterrestrial life and anthropic principle. his coincidence may be explained if evo­ Dirac's large-numbers hypothesis, each of The most plausible way of accounting lution is a rapid process which happens on which he says is sustained by neglect of the for it is to suppose that the timescale of bio­ the Earth to have been slow. But Carter is anthropic principle. Telescoped, the logical evolution, itself a Markovian plainly not afraid to stir up trouble and has argument is this. The search for extra­ process (one thing leading to another) also provided a tangible example of how terrestrial life is based on the hypothesis whose course must be uncertain, is on the the anthropic principle may be made that the emergence of living things is likely average much longer than the time allowed quantitative. John Maddox

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