The dials were set for us

In 1953 Fred Hoyle, a Yorkshire astronomer with no patience for anybody, made a prediction nobody asked for. Stars make carbon by fusing three helium nuclei, but that reaction should be far too slow to produce the amount of carbon that exists, and carbon exists, because Hoyle was made of it. So, he reasoned, the carbon nucleus must have an excited state at one very particular energy, about 7.65 million electron volts, that acts as a stepping stone and speeds the whole thing up. He walked into a nuclear physics lab at Caltech and told them to go look. They looked, mostly to get rid of him. It was there, within a hair of where he said.

That is the fine tuning problem in one story. The universe makes life. Look closely and it makes life using numbers that did not have to be what they are and could very easily have been something else.

The numbers

Martin Rees counts six that matter most. The strength of gravity against the other forces: a bit stronger and stars burn out in a few million years, before anything evolves. The fraction of mass released when hydrogen fuses: a bit different and you get a universe of pure hydrogen, or no hydrogen at all. The amount of matter in the universe, which sits almost exactly on the line between flying apart too fast for galaxies to form and collapsing before they can. The size of the ripples in the early universe that grew into galaxies: smaller and you get a smooth dead gas, bigger and you get a universe of black holes. The number of spatial dimensions. And the cosmological constant, the energy of empty space, which comes out smaller than theory predicts by a factor of about ten to the one hundred and twentieth, and which if it did not would have blown everything apart before the first star lit.

Not one of those is explained by current physics. They are measured and typed into the equations by hand.

The anthropic principle

In 1973, at a conference in Krakow marking five hundred years since Copernicus, the physicist Brandon Carter gave the problem its name. The anthropic principle in its weak form is almost a tautology: do not be surprised to find the universe compatible with observers, because if it were not, you would not be here to be surprised. Carter meant it as a correction to Copernican humility. We are not in a typical spot in the universe, he said, because typical spots are empty. We are where observers can be, and that filters what we see.

The strong form, which Carter also stated and most physicists reject, says the universe must have the properties that let observers show up. The weak form explains why we find the dials set for life. The strong form says somebody set them.

Three answers, and you have met all three

Physicists have mostly sorted into three camps. The first says a deeper theory will one day explain the numbers and the coincidence will dissolve, the way Kepler's musical ratios dissolved into Newton's gravity. The second says there are a vast number of universes, a multiverse, each with different numbers, and we naturally find ourselves in one of the rare ones where living is possible: a lottery with so many tickets somebody had to win. Steven Weinberg made that argument rigorous for the cosmological constant in 1987, ten years before anyone measured it, and the measurement landed where his argument said it would. The third camp says the dials were set, and declines to say by whom.

Now line them up against this shelf. The deeper theory answer is Pythagoras: all is number, and the numbers will turn out to have had no choice. The multiverse answer is the Stoics in reverse: not one body with one breath, but every possible body, and we are the one that happened to breathe. The third answer is the Timaeus, a craftsman arranging the mess according to the best pattern. Fine tuning is as above, so below in the language of nuclear physics: the pattern of the fundamental constants and the pattern of a living body are so tightly tied that one cannot exist unless the other is very nearly exactly what it is.

Nobody knows which is right. That three of the oldest positions in this library are the three live options in modern cosmology is either a coincidence or, as Jung would put it, a meaningful one. Charles, you said it was scary how much sense it made. This is the essay where the physicists quietly agree with you.