In their 1990 book The Early Universe, cosmologists Rocky Kolb and Mike Turner write: “It is unclear to one of the authors how a concept as lame as the ‘anthropic idea’ was ever elevated to the status of a principle.” But much to that author’s chagrin, we are now seeing a renaissance in discussions of that idea, lame or otherwise; a case in point being Paul Davies’ new book The Goldilocks Enigma: Why is the Universe Just Right for Life?.

The anthropic principle, as originally expressed by Brandon Carter in 1974, states that: “What we can expect to observe must be restricted by the conditions necessary for our presence as observers.” That is, given that we are here, we already know that the universe must have had properties – values of coupling constants, number of space–time dimensions, magnitude of the cosmological constant, and so on – such that life could evolve. Is this a vacuous statement, a tautology? Is it at least descriptive? Or could it even be predictive – can we get a better handle on fundamental theories armed with this knowledge?

Lost in the multiverse

Davies, a cosmologist who has just moved to Arizona State University in the US, starts by inverting the question to ask what it is about the properties of the universe that make it so perfectly suited to life. In the canonical example, Fred Hoyle pointed out that the formation of heavy elements in stars depends exquisitely on thermonuclear reaction rates. Tiny deviations in either direction would make stars unable to turn helium into the carbon of which all life (with the usual “as we know it” caveat that lurks behind these arguments) is made. But these rates are not written in any obvious way into the fabric of the Standard Model of particle physics. They just emerge in an as-yet incalculable way from the more fundamental parameters of the model.

Such questions have come to the fore over the last decade as string theory has morphed into “M-theory” with its “landscape” of 10⁵⁰⁰ possible vacua, each with its own fundamental parameters. Is the world as we know it somehow uniquely determined, or are we simply a random sample from that vast set of possibilities? Or could every one of those vacua be realized somewhere in a gargantuan “multiverse”?

The first half of The Goldilocks Enigma is a workmanlike introduction to the standard models of particle physics and cosmology. This material has been treated at least as well elsewhere, but is necessary background for the discussion of the anthropic argument itself. But it is when Davies moves into these murkier waters that he starts to get into trouble. The problem with anthropic arguments is that they are entirely devoted to distinctions without differences: choosing between theories that, by definition, have identical observational consequences; that is, the universe as we are currently able to see it. Hence, the upshot of the ongoing discussions of the anthropic principle is that we are still too ignorant to make any definitive statements.

Still, Davies himself is convinced that these arguments have force. He describes two extreme possibilities: a unique, “just-so” universe that will only be understandable in the light of a truly final theory; and an “absurd” universe that just happens to have its properties randomly dialled-in for no underlying reason at all. But he prefers what he proudly refers to as a teleological explanation: a so-called life principle arising out of physical laws that only become definitely determined as the universe evolves.

“Nobody would deny that atoms, stars and galaxies are fundamental features of the universe,” says Davies. But he should be more careful with his use of the word “fundamental”. Clearly these things are emergent properties of the universe, and it is exactly this emergence out of quarks, strings or whatever lies at the bottom that gives force to anthropic arguments at all. He goes on: “It seems clear that life (and mind and culture too) is an equally significant step on the path of cosmic evolution.” Perhaps that can seem obvious to one of those living beings, with a mind, inside the only culture we know of.

But the universe does not need an “atom principle” to ensure that atoms are formed, or even a “carbon principle” to enforce the details of stellar nucleosynthesis that Hoyle was worried about. So it is not at all clear that we should elevate the existence of life to something in need of a “life principle” separate from the laws of physics themselves – the same laws responsible for those atoms and nuclei. Indeed, this is just the criticism correctly levelled by Davies himself against the proponents of intelligent design: just because we are not yet able to understand the evolution of the mechanism of the flagellum, that does not mean it did not evolve.

Still, these ideas are entertaining, if only barely coherent; and along the way, Davies highlights some of their weirder repercussions. Chief among these is something that philosopher Nick Bostrom at Oxford University has called the “presumptuous philosopher problem”. If you take anthropic arguments at face value, then when considering models of the universe, you should give more weight to theories that predict a greater number of intelligent beings. A priori, you as an individual are more likely to exist if there are many intelligent beings rather than few. If you buy into this, then you should prefer models that predict vast universes to tiny ones, perhaps by odds that no observational evidence could possibly overcome. In particular, Bostrom (and Davies) point out that we should take seriously the possibility that we are living, Matrix-like, in a computer simulation, since simulated minds could be much more common than real ones.

In getting to this point, however, we realize that we have circled round to something philosophers have been worrying about for longer than physicists: how can I be sure I am not just a “brain in a vat”? This strain of scepticism started with Descartes and was only quashed by Ludwig Wittgenstein and his followers in the last century. Wittgenstein famously said, “Whereof one cannot speak, thereof one must be silent”, and perhaps, in the absence of better theories and data to support them, that is how physicists should treat anthropic questions.