Physicist Sean Carroll and the biggest ideas in the universe

Sean Carroll, a physicist at Johns Hopkins University, spoke at the Bell House in Brooklyn, New York, in an event presented by New York City's Secret Science Club. He talked about quantum field theory, which is now considered the definitive explanation of what reality is made of. So, pretty important stuff.

Photo credit: Ruben Bolling

His new book The Biggest Ideas in the Universe: Quanta and Fields was released this week. It's the second in a three-book series in which he goes through the important ideas of Physics for non-academics, but actually using and carefully explaining the equations that physicists use.

Carroll is one of the most important communicators of Science that we have, both because he is so talented at explaining complicated concepts carefully, thoughtfully, clearly, and inspiringly, and because he so clearly prioritizes that goal and has devoted much of his career to it.

I also saw Carroll speak in September, when he spoke with fellow physicist Brian Greene at an event presented by The World Science Festival, on the question: Does Quantum Mechanics Imply Multiple Universes?

I think we all have Multiple-Universes fatigue from its over-saturation as a science fiction plot device in popular culture. But Carroll is a leading proponent of the idea that quantum mechanics does in fact imply multiple universes, and I'll bet people will be surprised to learn that while there is far from unanimous consensus on the subject, that idea is a very mainstream, respected position in physics today.

I recommend this video of that half-hour conversation, in which Carroll clearly and entertainingly lays out the case for the Multiple Universe theory: that every time there are several possibilities of what might happen in the universe, the universe branches into several versions of itself, each of which contains the realization of one of those possibilities.

The Multiple Universe theory is Carroll's favored solution to that quantum measurement problem of quantum mechanics. As Greene explains in his introduction of Carroll:

[Q]uantum mechanics … predicts the probability that the world will be one way or another tomorrow. In a sense, then, reality kind of hovers in an unfamiliar, ghostly mixture of many possible dispositions: particle here and particle there; particle spinning this way and spinning that way. And so on. Possibilities captured by the so-called quantum wave function. And only upon some notion of observation or interaction does the world transition into the single, definite reality of common experience.

The previous range of possible outcomes, each provided by quantum mechanics with the particular probability for happening, seemingly collapses to one and only definite reality. Now, how that transition actually happens — the so-called quantum measurement problem — that remains an unsettled question that to this day inspires creative theorizing and heated debate.

So Carroll believes the way that transition happens is that the universe splits into multiple universes in which each outcome is in each universe. Until "measured" a particle may said to be spinning this way and that way. When measured, the universe suddenly splits into two universes, one in which it is spinning this way, and one in which it is spinning that way.

I found this exchange about the Physics academic community's efforts to solve this problem interesting:

Greene: What do you think the timescale is to get these answers? … It's on the order of a century that we've been using this theory to make these most precise predictions in the history of Science, and they are confirmed by exquisite experiments — they all work. And yet here we are, still arguing about what it all really means for the nature of reality. Is this ultimately soluble, or is it that our brains are searching for a kind of intuition that the Universe isn't set up to provide?

Carroll: … We haven't been sending our best. We haven't putting the effort into it. I joke about the fact that if you told an outsider that physicists have this really important theory that is at the foundation of everything they do, but there is this looming question at the heart of it, they would say: Oh, well, the people who work on that theory must be the superstars of physics, who are fought over by the best universities, with superstar salaries thrown at them. And you and I know: No, you're actually fired if you work on that.

Greene said that the prevailing attitude at the effort to understand the foundations of quantum mechanics is "Shut up and calculate."

Carroll responded, "I notice that these folks are actually very good at calculating; less good at shutting up. But we can all do both. I think there's room for all of it. To me, it's this wonderful, delicious puzzle that nature has given us."

I met up with Carroll after that event, and he told me that it's difficult for him to do research into these fundamental questions, because if he collaborates with his graduate students on papers on that subject, he feels that he's actually damaging their future employment prospects.

And I can't recommend highly enough Carroll's amazing and very popular podcast, Mindscape. In it, he interviews not just physicists, but scientists of all types. Through incisive questions, Carroll presents the work of these scientists in the clearest, most understandable framework possible. I find it astounding that a physicist of his stature has the intellectual bandwidth to be so deeply curious about and conversant in so many fields of science.

The Biggest Ideas in the Universe: Quanta and Fields