Sean M. Carroll, physicist: ‘Understanding just 5% of the universe already seems like a lot’

‘Equations are like poems,’ says the researcher, who feels very comfortable at the intersection of science and culture and has a popular podcast where he discusses fundamental ideas of physics

Sean M. Carroll fisica
Sean M. Carroll poses in his office at Johns Hopkins University in Baltimore, in May 2023.LENIN NOLLY (Lenin Nolly/El Pais)

Sean M. Carroll’s latest book “contains complex formulas!”

It doesn’t seem like the best selling point for a work that aspires to conquer the masses. But the theoretical physicist – who has also mastered the art of the scientific best-seller – perfected it in the United States with The Biggest Ideas in the Universe: Space, Time and Movement (2022). In it, he reviews the history of fundamental physics in nine concepts: conservation, change, dynamics, space, time, space-time, geometry, gravity and black holes. He wrote the book after being encouraged by the success of the talks he posted on YouTube during the pandemic. It’s the first in a trilogy that – he promises – will “increase in complexity,” until they reach the abstract debates of the present and the future of the discipline, which are the focus of his popular podcast, Mindscape.

“Equations are like poems,” he says, in an interview with EL PAIS in May at his office in Johns Hopkins University, in Baltimore. Across the tree-lined street from campus is the institute from which the James Webb telescope is controlled.

The Philadelphia-born Carroll, 56, is comfortable moving between science and culture: this past semester, he taught a course called Natural Philosophy, which he designed. “Like a good poem, an equation forces you to think, to interpret,” he muses. “It’s short, intense… nothing is left over. And it’s impossible to paraphrase it, in the same way that it’s impossible to tell the plot of The Waste Land (by T. S. Eliot). The popularization of physics uses analogies, metaphors and anecdotes. I wanted to go further, show readers what’s behind the curtain. Although it’s not a textbook: it’s not necessary to know how to solve [the formulas] – it’s enough to understand them.”

Question. What’s natural philosophy?

Answer. It’s a kind of knowledge that reminds us that philosophy and science used to be the same. What we now call science was a subset of philosophy. Around 1800, they separated. Today, knowledge is much broader than in Aristotle’s time – it’s impossible to be an expert in everything.

Q. You’re a leading theoretical physicist who also theorizes – like Aristotle – about the dramatic arts. Is it no longer possible to be a polymath?

A. There’s extraordinary access to knowledge in many different areas, but I find it very difficult to be a cutting-edge researcher in more than one field. Edward Witten comes to mind: [he’s] one of today’s leading theoretical physicists, who’s also the best in mathematical physics. But [this doesn’t happen much] in very different disciplines, such as biology or history. Today, everything is more compartmentalized, partly because of how universities are organized. I advocate for more [fusion]. There’s practical knowledge, in which you don’t need philosophy… for example, if you want to locate the farthest galaxy. [But] if you’re interested in why the Big Bang happened, [philosophy] will be useful to you. There’s a joke that, when philosophy begins to answer questions, it becomes something else: psychology, physics, biology...

Newton – who we consider to have essentially been a mathematician, or a physicist – would have called himself a philosopher. Nowadays, no one would call me that, even though some of my questions are quite philosophical in nature: What is quantum mechanics? Where does the universe come from?

Q. Why is there something instead of nothing?

A. The short answer to that question is that – I think – it’s an unanswered question. Surely, it belongs more to the field of philosophy than to physics. When you ask why there’s something rather than nothing, you’re assuming that there’s a reason, a fundamental fact, [as to] why the universe exists. Well, I believe that there’s no such fundamental fact. I believe that the universe, simply, exists.

Q. In the book, you explain that, in an expanding universe, energy isn’t conserved.

A. It depends on what you understand by energy. In physics – if you lived in the time of Isaac Newton – there were ideas that made perfect sense. But then came general relativity and quantum mechanics, [where the concept] of energy changes in meaning. If you take the energy of this table, of this chair, of the building and of each planet and add up all the photons in the universe – and take time into account – then, the answer is that energy isn’t conserved. Because [time and space are] changing. It’s like if you put a cup of coffee on top of this table: energy is conserved, but if you throw it into the ocean, it’s not. It’s as if the waves were the space-time that pushes the cup.

Q. Do you share the idea that philosophy is somewhat stagnant?

A. No. It seems to me that scientists [treat] philosophy unfairly, by asking for tangible results. We’re so obsessed with getting the right answers. Philosophers are very patient and insightful. They need to make sure that all their words mean something. And they definitely help us illuminate the foundations on which science is built.

Q. The book begins with the formulation of a wish: to live in a world where, after work, people discuss dark matter in the pub. What would have to change in order for that to happen?

A. A lot. [In my books], I say that people need to be given more details. Physics is normally reduced to a set of facts [that need to be] memorized. But the process of science is completely different: it’s about formulating hypotheses – which are often wrong – and collecting data, which is very rarely definitive. Science is the simultaneous sum of the willingness to change your beliefs based on new evidence and the certainty that some of those beliefs are very difficult to change. People are usually willing to believe one of two ideas… almost never both.

Sean M. Carroll, during his interview with EL PAÍS, in Baltimore, on May 24, 2023.
Sean M. Carroll, during his interview with EL PAÍS, in Baltimore, on May 24, 2023.LENIN NOLLY (Lenin Nolly/El Pais)

Q. Your book can also be read as a treatise on the history of physics, with all its turning points. Where is discipline now?

A. As far as fundamental physics is concerned, we’re not at a turning point. Some people complain that we don’t have revolutionary ideas like those of a century ago. This seems unfair to me: that was a very, very special time. We discovered relativity and quantum mechanics, particle physics and the Big Bang theory. You can’t expect that to happen every 50 years! But, at the same time, there are parts of high-level physics – complex systems, thermodynamics, statistical mechanics, fluctuations, biophysics – where there are still fruits within reach, because we’re still at a very basic level. That’s what my third book will be about.

Q. How is it possible that, a century later, we still can’t explain more than 5% of the universe?

A. It’s true that the particles that we’ve detected in experiments constitute only 5%. There’s 25% that is [made up of] dark matter. And 70% is dark energy. We haven’t detected them in the laboratory, but we know that they’re there and we know some of their properties. We actually know quite a bit about [these particles], although we’re not quite done figuring out what they are. It seems to me that understanding 5% of the universe is already a lot.

Q. Everyone remembers what they were doing on 9/11, or when Michael Jackson died. In your profession, the question must be: What were you doing when the Higgs boson was discovered in 2012?

A. [Laughs]. I was at the CERN (the European Organization for Nuclear Research) press conference. I even wrote a book [about this]. It was very exciting. But, to be fair – and without wanting to detract from it – we expected to find something else, a lot of other things [instead of the Higgs particle]. We were awaiting a revolution, the dawn of a new golden age. It didn’t happen.

Q. Did you enjoy the Oscar-winning Everything Everywhere All at Once?

A. I loved it. The idea of the multiverse arises in physics from string theory, [specifically] the 10 dimensions and how they can interact with each other. They would be different universes, with different physical laws.

Q. The big question from humanism is: where does that theory leave identity? The main character in the film not only discovers that the multiverse exists, but also that she lives in the worst of its versions...

A. The moral of the film is that it’s not so after all. But yes, the thought of having twins out there making slightly different decisions than you certainly raises some deep questions. It’s convenient to differentiate between multiverses. There is the cosmological, which implies that there are regions of the universe very far away in space-time, in which the conditions are very different. [Then there’s the] concept derived from string theory: that the world isn’t made of particles, but of loops of strings, or what we call “cosmic inflation” (the rapid expansion of the universe). Both [ideas] belong to speculative physics and are far from proven, but they’re very popular. Then, there are the many worlds of quantum mechanics – [the discipline] which is actually much more likely to be true. It’s a pretty deep issue – we’re still struggling with how to deal with it as physicists and as philosophers.

Q. Was the recent announcement – that the United States is close to achieving nuclear fusion – exaggerated?

A. I’m always frustrated by these press releases, because there’s clearly a vested interest in communicating a big discovery. In reality, [the researchers involved] haven’t achieved a reaction that generates more energy than it takes to make the reaction. What they do is quick bookkeeping: they don’t include some of the energy they invest. [Nuclear] fusion could be a breakthrough… but it doesn’t seem to me that it will be a matter of a couple of years, but rather of decades.

Q. Are you concerned about the advancement of artificial intelligence?

A. I’m not alarmist about it. I don’t think there’s a reasonable chance that it will kill millions of people, or make the human race extinct. What’s much more likely is that [AI] will pave the way for human rights violations, attacks on privacy and misinformation in political campaigns.

Q. Do you allow your students to use ChatGPT?

A. I haven’t had a chance [to make a decision]. The software was released right after my classes ended last semester. It seems to me that there’s no other [option]… it’s as if a math teacher doesn’t allow his students to use a calculator at home to multiply five-digit numbers. I’ve used ChatGPT: I know it doesn’t write very good essays, it lies all the time, invents things and misinterprets others… but it knows a lot and can be very useful, even inspiring. The trick is to treat it as a tool.

Q. Let’s finish with another of your obsessions: free will.

A. I’m in favor of free will. Now, as with energy, it depends on what you mean by free will. The most positive way to think of other human beings is as agents who make decisions for given reasons. Sometimes, they’ll [make decisions] for irrational reasons. That’s how it is, even if we admit that the underlying laws of physics are perfectly deterministic – which they are not, due to quantum mechanics. But even if they were, it’s too complicated to predict exactly what a human being is going to do. For me, that’s free will.

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