Anthropic Principle

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In 1937 Paul Dirac noticed two enormous numbers agreeing: the electric force between a proton and an electron exceeds the gravitational by a factor near 10³⁹, and the age of the universe, in atomic time units, comes out near the same number. A coincidence holding only at the present epoch, he reasoned, is no coincidence a few billion years on; the agreement must be permanent — which required the gravitational constant to weaken as the universe ages.

The answer that founded a subject took one page of Nature in 1961. Robert Dicke pointed out that the present epoch is not a random draw: observers are made of carbon, carbon must be forged in stars, the forging takes time, and observers need stars still burning — so the coincidence holds precisely when anyone exists to notice it. The age of the universe, Dicke wrote, “is limited by the criteria for the possible existence of physicists.” No varying constant was needed; later work sided with Dicke.

Brandon Carter, a Cambridge postdoc, turned Dicke’s move into a method and — at John Wheeler’s prompting — gave it a name, at an International Astronomical Union symposium held in Kraków in September 1973 to honor the five-hundredth anniversary of Copernicus’s birth; the paper appeared the following year. The venue carried the meaning: Carter meant the “anthropic principle” as a correction to Copernican modesty pushed too far — not being central is not the same as being typical. He later cast it as the reasonable middle between two errors — the “autocentric” assumption that our situation is privileged by right, the “ubiquity” assumption that it fairly samples the universe — with expectation spread instead uniformly over comparable observers, human or otherwise. He regretted “anthropic” — observers, not humans, were the point — but judged it too late to change.

The Kraków paper distinguished two principles, mangled together ever since, so the original meanings matter. The weak anthropic principle states that observers must be prepared to find their location in the universe privileged — in time as much as in space — at least to the extent of being compatible with their own existence. This is Dicke’s logic generalized: an observation selection effect, method rather than metaphysics, sound even by its harshest critics’ admission. The strong anthropic principle states that “the Universe (and hence the fundamental parameters on which it depends) must be such as to admit the creation of observers within it at some stage.” Carter meant the much-misread “must” as inference, not destiny: since observers exist, restrictions on the fundamental parameters follow — the existence of physicists is data about physics. Promoting deduction to explanation was, he added, a last resort, requiring a world ensemble of differing universes from which observers sample a livable member. Aimed outward, the same logic led Carter in 1983 to argue that intelligence took so large a fraction of the Sun’s lifetime to evolve that observers should be rare, and the search for other civilizations slow to succeed.

The mangling arrived in 1986, with the book that taught most readers the subject. John Barrow and Frank Tipler’s The Anthropic Cosmological Principle — by one 2011 count cited 1,740 times to the Carter paper’s 226 — kept Carter’s wording for the strong principle and changed what it meant. Their strong principle is teleological — the universe must bring observers into being, necessity rather than deduction — and they canonized two more: Wheeler’s participatory principle, on which observers are required to bring the universe into being at all, and their own final anthropic principle, on which “intelligent information-processing must come into existence in the Universe, and, once it comes into existence, it will never die out.” Martin Gardner’s review rechristened the last the Completely Ridiculous Anthropic Principle; both additions have faded from serious discussion. The damage to the vocabulary has not. The Stanford Encyclopedia of Philosophy states flatly that Barrow and Tipler misinterpreted Carter’s strong principle along teleological lines; Carter protested that a universe designed for life is a “finality principle… quite different from, and even contradictory with, what I intended”; yet standard reference works still present the teleological version as the strong anthropic principle. Anyone meeting the term must check whose it is.

The principle’s material is the catalogue of apparent fine-tunings — established physics in the numbers, open dispute in the framing. The vacuum energy of empty space, naively estimated from quantum field theory, exceeds the measured value by between fifty and 123 orders of magnitude, depending on assumptions; the measured value, in Planck units, is about 10⁻¹²³, and a value a few orders of magnitude larger would have kept galaxies from forming. The neutron outweighs the proton by 1.29 MeV, about two and a half electron masses; more, and deuterium would not fuse, starving ordinary stars; less, and the early universe would have burned its hydrogen to helium. Martin Rees’s Just Six Numbers (2000) compressed the catalogue to six — among them the 0.007 efficiency of hydrogen fusion and the 10⁻⁵ amplitude of the primordial density ripples — each in a band outside which the universe goes sterile. The catalogue is genuine and also theory-dependent — model universes lacking the weak nuclear force entirely, adjusted elsewhere, appear habitable; “fine-tuned” is always relative to which dials one imagines turning.

The catalogue’s most storied entry is its one famous prediction. In 1953 Fred Hoyle, working out how stars could synthesize carbon through the improbable triple-alpha process, concluded that the carbon-12 nucleus must possess an excited state near the combined energy of three helium nuclei — his figure was 7.68 MeV; the modern one is 7.65 — and Willy Fowler’s group promptly found it. The “Hoyle state” is solid physics, and the sequel sharpens it: the next capture, carbon plus helium into oxygen, just misses resonance, which is why the carbon survives. Shift the underlying nuclear force by 0.4 percent, later calculation showed, and stars make almost solely carbon or almost solely oxygen, not both. As the anthropic showcase, though, the story is contested twice over. Helge Kragh’s history finds that Hoyle and his contemporaries never connected the level with life — the anthropic gloss arrived in the 1980s, decades late. And Smolin observed that the logic runs from carbon’s observed abundance, not from carbon-based observers; it would have gone through for neon, which no living thing requires. The physics is solid; the parable was retrofitted.

The logic underneath is easily stated and easily misplaced. Evidence gathered by observers is conditioned on whatever observers require; a sample taken from inside a selection effect cannot be read as fair. No one is surprised, Weinberg observed, that Earth orbits where water is liquid: planets are plentiful, and only the watered ones grow astronomers. But the correction explains something only when there is an ensemble to select from — many planets, many epochs, or, at the limit, many universes. At that limit the reasoning earned its keep — Weinberg’s 1987 anthropic bound on the cosmological constant, borne out by the 1998 discovery of cosmic acceleration, remains its best and still-disputed success — and acquired its patron, the string landscape, whose profusion of vacua, Susskind concluded, “gives credence to the Anthropic Principle.” Both stories are the multiverse’s to tell; what matters here is the dependence. The anthropic principle is not a claim about what exists but a rule for reasoning from biased evidence — it neither requires nor supplies other worlds, yet without it other worlds would explain nothing.

The criticisms divide by temperament. The oldest is the tautology charge: “life exists only where life can exist” informs no one. The standard rebuttal: selection effects are anything but trivial — deciding which parameter changes forbid observers is hard physics, and the Dirac episode shows the reasoning bites. Smolin pressed the harder objection in 2004: the principle “cannot yield any falsifiable predictions, and therefore cannot be a part of science,” its celebrated successes being either one-universe selection effects (Dicke’s) or arguments from observed facts that never needed life as a premise (Hoyle’s). Penrose’s complaint concerns character rather than logic — the principle “tends to be invoked by theorists whenever they do not have a good enough theory to explain the observed facts” — a move that, to many physicists, feels like giving up. The most quoted criticism is a comic’s: in a 1998 Cambridge talk Douglas Adams conjured a sentient puddle admiring its hole — “fits me rather neatly, doesn’t it? In fact it fits me staggeringly well, must have been made to have me in it!” — clinging on as the sun climbs and the water goes. The puddle’s error is not in noticing the fit but in reading it backwards — which is, compressed to a fable, the weak anthropic principle itself.

Fine-tuning arguments now occupy the place in natural theology that Paley’s eyes and wings held before Darwin, and the anthropic principle is their constant rival. Both begin from the same datum — a universe startlingly fitted to observers — and the difference, as this entry weighs it, lies in what each needs the fit to be. The design argument needs the fit to be intended: the explanatory arrow runs from a purpose to a world. Anthropic reasoning needs the fit only to be conditioned on: the arrow reverses, and apparent tailoring becomes a fact about sampling rather than intent. Even partisans concede the neutrality: an evangelical science body grants the principle “is compatible with either design or non-design so it doesn’t favor either possibility” — the theological work falls to a design inference the principle does not contain — while the standard theistic counter, William Lane Craig’s per the secondary literature, attacks the dependency itself: no actual world ensemble, no anthropic deflation. Three things constantly blur together and must be kept apart: fine-tuning, a datum of physics; the fine-tuning argument, an inference from that datum to an intender; and the anthropic principle, which is neither — a rule of evidence, named at the five-hundredth birthday of the man who moved the observer out of the center of things. Observers hold no central place in the universe. They hold, necessarily, a possible one — and everything they will ever measure is measured from there.

→ Related: Multiverse · String Theory · Fermi Paradox · Best Of All Possible Worlds