Siegfried: Are there other changes in the Standard-Model numbers that would have such dramatic effects?

Donoghue: My own contribution here is about the Higgs field [the field that is responsible for the Higgs boson]. It has a much smaller value than its expected range within the Standard Model. But if you change it by a bit, then atoms don’t form and nuclei don’t form—again, the world changes dramatically. My collaborators and I were the ones that pointed that out.

There’s some maybe six or seven of these constraints—parameters of the Standard Model that have to be just so in order to satisfy the need for atoms, the need for stars, planets, et cetera. So about six combinations of the parameters are constrained anthropically.

Siegfried: By “anthropically,” you mean that these parameters are constrained to narrow values in order to have a universe where life can exist. That is an old idea known as the anthropic principle, which has historically been unpopular with many physicists.

Donoghue: Yes, I think almost anybody would prefer to have a well-developed theory that doesn’t have to invoke any anthropic reasoning. But nevertheless, it’s possible that these types of theories occur. To not consider them would also be unscientific. So you’re forced into looking at them because we have examples where it would occur.

Historically there’s a lot of resistance to anthropic reasoning, because at least the popular explanations of it seem to get causality backward. It was sort of saying that we [our existence] determine the parameters of the universe, and that didn’t feel right. The modern version of it, with the multiverse, is more physical in the sense that if you do have these differing domains with different parameters, we would only find ourselves in one that allows atoms and nuclei. So the causality is right. The parameters are such that we can be here. The modern view is more physical.

Siegfried: If there is a multiverse, then doesn’t that change some of the goals of physics, such as the search for a unified theory of everything, and require some sort of anthropic reasoning?

Donoghue: What we can know may depend on things that may end up being out of our reach to explore. The idea that we should be searching for a unified theory that explains all of nature may, in fact, be the wrong motivation. It’s certainly true that multiverse theories raise the possibility that we will never be able to answer these questions. And that’s disturbing.

Siegfried: Does that mean the multiverse changes some of the questions that physicists should be asking?

Donoghue: We certainly still should be trying to answer “how” questions about how does the W boson decay or the Higgs boson, how does it decay, to try to get our best description of nature. And we have to realize we may not be able to get the ultimate theory because we may not be able to probe enough of the universe to answer certain questions. That’s a discouraging feature. I have to admit when I first heard of anthropic reasoning in physics my stomach sank. It kills some of the things that you’d like to do.