Fine-tuning, alternative physical laws, and the multiverse

How many times can this hero nearly die? Bullets miss, explosions happen just far enough away, and falling objects land somewhere else. Think of the joke about the stormtroopers in Star Wars: Luke and Han run through danger while their enemies seem able to hit everything except them. Change one shot, and the story could end very differently. Keep that string of improbable escapes in mind. We do not yet know why the author brought it up.

Then comes the surprise: this is about the universe, not movie criticism. Physical laws take the place of the bullets and explosions. Change a law slightly, the suggestion goes, and the conditions that allow life might disappear. The strong nuclear force is one example. Slightly different strength would have meant very little carbon, which seems necessary for planets and living things. We do not need to understand how that force works. We need the relationship: a small change to a condition could prevent the successful outcome.

But are these really the only conditions that could allow life? Or are they the conditions that allowed life to develop in the form we know? Those are different claims. Showing that a change would interfere with our familiar universe does not yet show that every different arrangement would make all forms of life impossible. That is a useful objection to keep in mind as the argument develops.

The third paragraph offers one explanation for the apparent luck: perhaps there are many universes with different physical laws. Most might not support matter and life as we know them, but a huge number of possibilities gives nature a good chance to get a suitable combination at least once. We happen to live in a successful one. The explanation is not that another universe reaches over and helps ours. It is that an unlikely success becomes less surprising when there are many opportunities for it to occur.

Now the author challenges the assumption that our laws are so exceptional. Why do scientists think there are so few alternatives compatible with life? Largely because they have had trouble identifying alternatives. That difficulty matters, but it is not the same as proving alternatives cannot exist. Perhaps the way scientists are looking limits what they can find.

Here is the problem with the conventional search. Scientists change one physical constant while leaving all the others fixed, then calculate what happens. Imagine a panel of ten adjustable settings. They move the first control and leave the other nine where they were. If the resulting universe cannot support life, they conclude that moving that control causes a problem. But what if moving the first and second controls together produced a workable arrangement? The one-control experiment has not tested that combination. We do not need actual values for gravity or nuclear forces to understand the missing possibility.

The author and a colleague tried changing multiple constants together. They found numerous hypothetical universes whose very different laws would still permit complex structures and perhaps intelligent life. That gives the earlier objection substance: the familiar arrangement may not be nearly as special as the restricted search suggested. These are models of possibilities, not observations of inhabited alternate universes. And “perhaps” intelligent life is a possibility, not a guarantee.

What follows for the multiverse? If life does not require such an extraordinarily narrow arrangement, the fine-tuning argument loses some of its force. But weakening that argument does not settle whether the multiverse exists. The author gives two other reasons to keep the idea open: some models of the universe's origin suggest something like a multiverse, and the concept may help solve other puzzles in cosmology.

So separate the finding from its implications. More arrangements of physical laws may permit life than scientists commonly think. That challenges one reason offered for believing in a multiverse; it does not eliminate every possible reason. The author questions an argument without closing the door on the idea it supports.