Is the Universe Anthropic?

Has the universe been fine-tuned just for us?

Posted Nov 08, 2018

There is debate whether the “anthropic principle” is a scientific or a philosophical concept – or primarily a religious one. Either way, it might be the most challenging argument for human specialness. The anthropic principle is based on the suggestion that if any of an array of precise physical constants, such as the gravitational constant, the exact electric charge on the proton, the mass of electrons and neutrons and a number of other characteristics of the universe were any different, human life would be impossible.

There are many reasons, in any event, to doubt that the universe has been fine-tuned for our benefit. For one, if such tuning has happened, what’s the basis for assuming that it happened with us “in mind”? (Never mind the question of “Whose mind?”) It is worth noting that these various physical constants are not necessarily evidence that the universe is fine-tuned to produce human beings; it could have been generated to produce the hairy nosed wombats of Australia, or maybe the bacteria and viruses, which outnumber human beings by many orders of magnitude. If so, then the impact on Homo sapiens was merely an unanticipated side-effect.

In The Salmon of Doubt, Douglas Adams developed what has later become known as the “puddle theory,” as follows:

Imagine a puddle waking up one morning and thinking, 'This is an interesting world I find myself in, an interesting hole I find myself in, fits me rather neatly, doesn't it? In fact, it fits me staggeringly well, must have been made to have me in it!' This is such a powerful idea that as the sun rises in the sky and the air heats up and as, gradually, the puddle gets smaller and smaller, it's still frantically hanging on to the notion that everything's going to be all right, because this World was meant to have him in it, was built to have him in it; so the moment he disappears catches him rather by surprise. I think this may be something we need to be on the watch out for.

The anthropic principle was first introduced, it appears, by astrophysicist Brandon Carter at a conference in Krakow, Poland, celebrating the 500th anniversary of the birth of Copernicus. The venue is, in a sense, ironic, given that Copernicus helped evict the Earth – and thus, humanity – from its prior centrality, while the anthropic principle threatens (or promises) to re-establish this centrality. For Carter, “our location in the universe is necessarily privileged to the extent of being compatible with our existence as observers." Here, “location” means not just our physical coordinates in space but also our existence at particular intervals of time.

Before Brandon Carter, Alfred Russell Wallace (co-discoverer with Darwin of the principle of natural selection) seems to have anticipated the anthropic principle in 1904, when he wrote that “Such a vast and complex universe as that which we know exists around us may have been absolutely required ... in order to produce a world that should be precisely adapted in every detail for the orderly development of life culminating in man."

In A Brief History of Time, Stephen Hawking described a number of physical constants and astrophysical phenomena that seem at least consistent with the anthropic principle, including such questions as "Why did the universe start out with so nearly the critical rate of expansion that separates models that recollapse from those that go on expanding forever, such that even now, ten thousand million years later, it is still expanding at nearly the critical rate?" Hawking explains that "if the rate of expansion one second after the Big Bang had been smaller by even one part in a hundred thousand million million, the universe would have recollapsed before it ever reached its present size." In short, we would have been victimized by a kind of Big Crunch.

Time, now, for a brief excursion to the “cosmological constant,” introduced by Albert Einstein, and which he considered his “greatest blunder” – but which currently seems remarkably prescient. Einstein was troubled by the fact that gravity would cause the universe to collapse onto itself (that Big Crunch), so he introduced a “constant,” essentially out of thin air, that pulled in the opposite direction, causing the cosmos to remain stable.    

Bear in mind that Einstein had been working before Edwin Hubble discovered that the universe was actually expanding. Today, the cosmological constant is widely considered intimately connected to so-called dark energy, and physicists such as Steven Weinberg – not a religious believer — point out that if this constant were just a smidgeon larger, then instead of a Big Crunch, the universe would be vaporously insubstantial, expanding at a rate that precludes the formation of galaxies, never mind planets.

Devotees of the anthropic principle have yet more ammunition. Thus, after Wallace but before Carter, physicist Robert Dicke noted in 1961 that the age of the universe (currently estimated at 14.5 billion years) reflects a kind of Goldilocks Principle, a “golden interval” in which it is neither too young nor too old, but just right. If the universe were younger – i.e., if the Big Bang had occurred in the more recent past – it would not have allowed for enough time to accumulate elements heavier than hydrogen and helium via nucleosynthesis. There also wouldn’t be any medium-size, rocky planets and thus, no us. By the same token, if the universe were substantially older than it is, nearly all stars would be too elderly to remain part of what astrophysicists call the “main sequence,” having matured into white and red dwarfs. As a result, there wouldn’t be any stable planetary systems. And thus, once more, no us.

A similar argument can be raised concerning the four fundamental interactions connecting mass and energy: gravitation, electromagnetic attraction and repulsion, and the “strong” and “weak” nuclear forces. These can be seen as balanced in precisely the manner needed to produce matter, and thus, ultimately, the emergence of life. The strong interaction is what holds neutrons and protons together in an atomic nucleus, and that also binds quarks together to form the various subatomic particles. If this strong force were just a tiny bit stronger, then nuclear fusion would have converted the universe’s hydrogen into helium, and water – essential for life as we know it – wouldn’t exist.

There are other perspectives. For example, physicist Fred Adams maintains that the necessary conditions for a life-supporting universe aren’t so demanding after all. “The parameters of our universe,” he writes, “could have varied by large factors and still allowed for working stars and potentially habitable planets.” Sure enough, in February, 2017 NASA astronomers excitedly announced that they had discovered seven Earth-sized planets orbiting a dwarf star, three of which appear to be in the “habitable zone,” including a reasonable likelihood of liquid water. This system, known as Trappist-1, is about 40 light years from Earth, and there is every reason to believe that the basic laws of physics obtain there as they do here.

Nonetheless, Adams noted that

The force of gravity could have been 1,000 times stronger or 1 billion times weaker, and stars would still function as long-lived nuclear burning engines. The electromagnetic force could have been stronger or weaker by factors of 100. Nuclear reaction rates could have varied over many orders of magnitude. Alternative stellar physics could have produced the heavy elements that make up the basic raw material for planets and people. Clearly, the parameters that determine stellar structure and evolution are not overly fine-tuned.

What to believe? More coming up in my next post.

David P. Barash is professor of philosophy emeritus at the University of Washington. His most recent book relevant to this topic is Through a Glass Brightly: using science to see our species as we really are (2018, Oxford University Press).