Over the last several decades, scientists have uncovered a startling fact about the laws of nature. The laws appear fine-tuned to permit life. Physicist Paul Davies observes that this fact has gained “broad agreement among physicists and cosmologists.”

Philosopher John Leslie lays out dozens of examples of fine-tuning in his 1989 book Universes. For life to exist, many numbers in nature need to be almost exactly what they are, to the precision of one part in big numbers like 10^40 (that’s a 1 with 40 zeros, just as a million is 1 with 6 zeros), 10^100, or in one example from physicist Sir Roger Penrose, even 10^10^123.

(To comprehend that last number’s size, imagine the following: you throw all the matter in our universe together, and by random chance, all that matter spontaneously forms identical copies of Michael Jordan. You then repeat this feat once per second, every second, for a hundred million billion billion billion years. You’ll have a total of something like 10^95 Michael Jordans. The odds of such a streak are about 1 in 10^10^123.)

Fine-tuning is required all over physics: in the strengths of forces, the masses of particles, the geometry of space, the smoothness of the universe, and so on.

Fine-tuning is also required at nearly every stage in the universe’s progress toward life: the expansion after the Big Bang, the appearance of matter, the feasibility of atoms, the stability of stars, the production of elements like carbon and oxygen, and so on.

The overall picture is overwhelming.

One explanation for fine-tuning is that the goal of life is built into reality. Religious folks have been talking about this for a while, calling it the work of God. This also explains why anything exists at all, and why existence obeys rational, mathematical laws.

Another explanation for fine-tuning is that we live in a multiverse. The idea is that many universes exist in parallel to our own, each with random laws of physics, and most of them devoid of life because they aren’t fine-tuned. But with enough universes, some will be fine-tuned by chance. Since people can only exist in universes that permit life, we shouldn’t be surprised to find fine-tuning in our own universe.

The suggested multiverses take many forms. Some, like the bubble universes of Andrei Linde’s chaotic inflation, or the landscape of string theory, or the Many Worlds interpretation of quantum mechanics, are popular because they address problems in physics aside from fine-tuning. Other multiverses, like the mathematical structures of Max Tegmark, or the modal realism of David Lewis, or the principle of fecundity of Robert Nozick, are pure metaphysical fancy.

These speculations all claim that with enough random universes, we should expect to find at least one with fine-tuning.


John Leslie points out a feature of the fine-tuning that most of these multiverses cannot explain.

When I first read his argument five years ago on page 64 of Universes, I jumped out of my chair and couldn’t sit back down for several minutes.

Here it is: to permit life, many values in nature need to be what they are not just for one reason, but for a whole list of reasons. For example, the strength of the electromagnetic force, which governs our interactions with everyday objects, needs to be very precisely as we find it so that (a) matter could form in the early universe, (b) quarks could be stable, (c) protons could be stable, (d) atoms could be stable, (e) stars could produce life-friendly radiation, and so on for (f), (g), (h)… until we reach you reading these words off your screen.

But a universe gets to pick just one value for the electromagnetic force. That one value has to fulfill all those strict requirements at once. And so, too, do other values in physics meet multiple requirements. How miraculous that each of the fine-tuning requirements lines up with all the others, without a single conflict, so that they can all be fulfilled simultaneously!

It’s a miracle that fine-tuning is possible at all, not just that it happened.

Here’s an analogy. Imagine Odysseus shooting an arrow through a line of twelve axes, as Homer describes in Book 21 of The Odyssey. How did he do it? You might conclude that Odysseus has impressive aim. Or, maybe Odysseus shot gazillions of arrows at those axes, and eventually an arrow got through them by chance. That’s reasonable enough. But Leslie would ask a further question. How were all those axes lined up in the first place? If any axe was a fraction of an inch to the left or right, no arrow could have ever made it through, no matter how lucky the shot. Someone lined up those axes with purpose and skill, so that a successful shot could even be possible.

I haven’t seen anyone respond to this argument’s implications. I was recently excited to see that Jim Holt’s bestselling 2012 book Why Does the World Exist? devotes page 208 to presenting Leslie’s argument. But Holt doesn’t elaborate on its implications either.

If Leslie is right, a multiverse could only attempt to explain fine-tuning if it shakes up the fine-tuning requirements themselves. Or, in my Odyssey analogy, we need a gazillion sets of axes, each set strewn randomly together, until one of the sets happens to form a precise line. Only then could we start shooting our gazillions of arrows and hope that one passes through the axes.

All but the most fanciful of the multiverses shoot the arrows but don’t adjust the axes. They fail to explain why successful fine-tuning for life is possible at all. And the very possibility cries out for explanation.