thesis Anthropic attitudes

Is the universe the way it is to ensure making it with life in mind not do the best the emergence of life — and, ultimately, The best clarification job possible? Moreover, those who invoke a conscious, human life? This assertion, of what the anthropic Creator would almost certainly do so even known as the anthropic principle, has if life turned out to be consistent with any been supported by a number of prominent principle means choice of the parameters of the universe. scientists and philosophers. They see it as might take the form of Hence, fine-tuning seems to have little to perhaps the only conclusion to draw from do with this belief. the fact that life, at least as we know it, could mild satire. On the multiverse idea, Landsman not possibly exist if fundamental aspects of argues that it actually only explains fine- the universe were just a tiny bit different. restores the religious perspective to tuning if one makes a host of further The uncharged neutron, for example, science, seeing evidence of God and the strong assumptions. One might add hat is heavier than the charged proton — and centrality of human life in the universe. the science behind the multiverse notion is thank goodness. If it weren’t, the instability Tipler even wrote a book entitled rather extravagantly speculative. Put string of the proton would destroy chemistry based The Physics of Christianity. theory and inflationary cosmology together on the electromagnetic interaction. Yet the A second common response, perhaps and you have a theory claiming to predict neutron–proton mass difference can’t be too just as radical, supposes that our Universe almost anything one might possibly see, large, for otherwise nuclear fusion would not is just one among innumerably many which seems like a step away from science. fuel the stars. Stars, and our Sun, require a others in a ‘multiverse’. If we happen to If we try to remain within science, fine-tuning of neutron and proton masses to exist in an unusual universe where life Landsman concludes, we’re left with blind within about 10%. exists, this idea asserts, that’s no surprise. necessity, a kind of negative interpretation. The smoothness of the energy There are so many universes that intelligent “We don’t really know” if there is any fine- distribution in the early universe appears to observers must exist in some of them. tuning, because we just don’t know enough have been tuned even more precisely. If the And any intelligent observer will, by about the flexibility with which the universe intensity of variations were less than a tenth necessity, experience living in just such an came into creation. “The present state of of current estimates, galaxies would never unusual universe. science,” as he writes, “does not allow us have formed, while if it had been more than The third possibility Landsman lists isn’t to make such a choice now (at least not ten times larger, matter would have been quite so cosmic. Maybe it’s all just blind rationally), and the question even arises if too lumpy and there would be no stars, only chance and pure luck that the constants of science will ever be able to make it [except] black holes. The allowed variation is of the Nature and initial conditions took the rare perhaps philosophically.” order of about one part in 100,000. set of values that has enabled life. Accepting But Landsman finishes by proposing one Still more evidence comes from the this is, in some sense, turning away from further possibility — that “the fine-tuning expansion of the Universe. Whether the desire for natural explanations and problem is misguided” and the result of the universe expands forever, or instead essentially accepting a miracle. In this, confusion. That it might best be resolved by ultimately collapses, is determined by I would say, it also shares a spirit with “some appropriate therapy”. matter density, which currently seems to the first two explanations, by design and The by-design argument, especially, be very close to the critical threshold. For by multiverse. seems a little too convenient: a species in this to be true now, cosmologists believe, it But the fourth common idea on a perplexing universe looks around itself must have been tuned even more precisely Landsman’s list is a little different. He refers and — Lo! — finds comforting evidence in the distant past — right on the boundary to it as the position of ‘blind necessity’. that its very existence must have been part to an accuracy of some 55 decimal places. Perhaps there was little or no wiggle room of the original plan. Rather, Landsman Without all this incredible calibration, in how the Universe got created, and our asks, might it not be that ‘fine-tuning’ there would be no Sun, solar system, Earth, thinking that there was simply reflects our doesn’t require any explanation at all? and certainly no thinking, wondering ignorance. In a universe so vast in both Maybe it’s not that our Universe has been human beings. So, things do seem to time and space, shouldn’t we be a little fine-tuned for life, but merely that life, our be extraordinarily well designed for more humble in thinking we’ve worked particular form of life, has been fine-tuned our existence, right? Well, not so fast. everything out in a few hundred years of to our Universe? There are other possibilities, indeed, lots modern science? Could the parameters of He quotes the philosopher John Earman of them, as physicist Klaas Landsman the universe possibly have been different? (Am. Philos. Quart. 24, 307–317; 1987), of the Radboud University Nijmegen We have no idea. This position essentially who suggested that the best clarification of in the Netherlands notes in a review dismisses the mystery as something we what the anthropic principle means might (http://arxiv.org/abs/1505.05359v2; I’ve shouldn’t expect to be able to handle take the form of mild satire: “Imagine, if you drawn on his paper in the preceding just yet. will, the wonderment of a species of mud discussion). What are the four most In his article, Landsman surveys some worms who discover that if the constant of common explanations? of the arguments going against the first two thermometric conductivity of mud were The first — advanced by people such positions. For example, on the argument for different by a small percentage they would as physicist Frank Tipler — simply design, the observed apparent ‘fine-tuning’ not be able to survive”. ❐ assumes that if there’s tuning, there must certainly doesn’t imply that the universe is have been a Tuner. This ‘by design’ view optimal for life. But why would a Designer MARK BUCHANAN

520 NATURE PHYSICS | VOL 11 | JULY 2015 | www.nature.com/naturephysics

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