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18 October 2023

Carlo Rovelli: “Einstein made more mistakes than anyone”

The theoretical physicist on the mysteries of the universe.

By Pippa Bailey

What happens to the matter that falls into a black hole? This, the rockstar theoretical physicist Carlo Rovelli told me, is “a child’s question”. It is also the question that has preoccupied him for the past eight years. “We trace the matter falling and entering, we see it disappearing. Where does it go? There’s this huge universe around us, full of holes. The Italians have this thing for the pasta with all the holes: scolapasta, we call it. Maybe there is no word in English.” A colander? Rovelli did not seem to think much of this poor approximation. “Colan–? Whatever,” he laughed. “The universe is like that.”

Rovelli, 67, in a polo shirt and jumper, his hair almost white but his eyebrows dark, spoke to me from his home in Canada. In 2019 he moved from Marseille to London, Ontario, where his girlfriend, a professor of philosophy and physics, had been offered a position.

[See also: Ludovic Slimak: “We cannot imagine another humanity”]

As the author of seven books, including Seven Brief Lessons on Physics (published in English in 2015), which sold more than a million copies, Rovelli came late to popular science writing. He long resisted the idea because he “wanted to do science, not to waste time writing books… But then I grew old.” In his late forties, he found himself motivated by “wonder in the science that I was developing. I was thinking, this is beautiful – why is it not shared with more people?”

Writing has given him “a sense of communication with the public which has changed my life. I was intellectually quite solitary.” Now, readers write to him with their questions or thanks. His third book, The Order of Time (2017), was read for audiobook by one such fan, Benedict Cumberbatch, and inspired an art installation by Es Devlin. In his latest, White Holes (published on 26 October), Rovelli takes the reader “on a mental journey in which one gets near a black hole, to the horizon, one gets into it, all the way to the bottom, and then after…” It is, he said, “a travelogue”.

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A black hole is created when a large star, its mass formerly sustained by the burning of hydrogen, dies and is crushed by gravity. The gravitational field around it is so strong that nothing – not even light – can escape. There are billions of black holes, some a few kilometres wide, others the size of solar systems. Albert Einstein’s equations of general relativity predicted their existence (though he thought the concept absurd) and in 2019 an image of one was captured for the first time. In it, a glowing red ring – material swirling around the black hole – surrounds a black disc: its entrance, known as the horizon. In White Holes, Rovelli, in lyrical prose and quoting extensively from Dante’s Inferno (he believes the divide between science and art “closes our minds”), takes us inside a black hole, a tunnel that narrows as it lengthens. At the bottom is the star that created it.

There is a paradox at the heart of modern physics. Einstein’s theory of general relativity describes a gravitational field that is space itself. The mass of the Sun bends the space around it, causing planets to circle it like marbles in a funnel, and time bends with it: clocks run more slowly where gravity is stronger. Then there is quantum mechanics, which describes the behaviour and interaction of subatomic particles. One deals with the very big, the other the very small; one is continuous, the other granular. Both are correct, but appear to contradict each other – and so the pursuit of a single, unifying “quantum gravity” has taken up much of Rovelli’s career. “The bet which has been the centre of my scientific work is that they can be reconciled by properly adjusting, by trying to find a way to fit them and write a single theory.” One of the solutions, posited by Rovelli and his colleagues, is “quantum loop gravity”, in which space is not a “uniform, continuous thing” but “granular, like a bag of sand… As light is made by photons, which are quanta of light, so space is made by these grains, which are quanta of space.” (The other prime contender is called M-theory.)

[See also: Thomas Hertog’s Q&A: “Stephen Hawking was the freest man I’ve known”]

There are two environments that force the physicist to try to reconcile these conceptual frameworks, where mass is so compressed that quantum effects are expected: the beginning of the universe, and black holes. Eventually, the star at the bottom of a black hole becomes so small that Einstein’s equations can take us no further. Then, Rovelli writes, it bounces and, in its rebound, becomes a white hole. Whereas matter can never leave a black hole, it can only leave a white one. A white-hole horizon is much smaller than the one we entered through – “like a floating speck of dust”, writes Rovelli. It releases the matter, and dissipates entirely.

The final twist in this hallucinatory journey is that because gravity bends time, what appears from inside the black hole to take seconds, like an explosion, for those outside is incredibly slow; the black hole appears static. “The star is collapsing, going very, very small, and very rapidly coming out in a matter of seconds. But outside, much more time is passing, not just hours – years, millions of years, billions of years.”

Carlo Rovelli was born in 1956 in Verona, Italy. He obtained his PhD in physics from the University of Padua, then did postdoctoral work in the US and France. The Italy of his youth was, he told me, experiencing great cultural and economic expansion. “There was a sort of idealism that had come out of the tragedy of the Second World War, which was: we’re not going to make wars any more, we’re going to… live in peace.” But there was also “a lot of nostalgia for fascism”, and “Verona was very much part of that. It was probably one of the most right-wing places in Italy.” Meanwhile, Rovelli “was in love with Che Guevara… I was totally taken with this idea that we can make the world better. I was very radical.”

Rovelli has spoken before of his experiences of taking LSD when younger. Theoretical physics can at times seem psychedelic – did the experience change how he sees the universe? “[It led] me to a sense that reality is more complex than it looks at first sight. If the simple organisation of the world… can be altered so profoundly with little chemicals in our brain, it means it is not so sure and definitive after all.” In White Holes, Rovelli writes that “the difficulty lies not in learning but in unlearning”: making great conceptual leaps requires you to leave behind old assumptions. He sometimes observes a colleague struggling to do so and thinks, “Oh, come on, just take a little bit of LSD.”

His rebellious side was what led him to physics. There was a moment, he told me, when he and his fellow radicals realised “we’re not going to change the world. And that’s exactly the moment in which I fell in love with science.” He looked to the great scientific discoveries and thought: “These are spectacular revolutions, which change our understanding of the world. There are people who are capable of doing these things, and I wanted to be one of them.”

For Rovelli, contrary to the idea that science is about certainties and absolutes, “there’s really no rule of the game. We’re just grasping, groping in the dark.” Sometimes your ideas are proved wrong. A willingness to make mistakes, he said, is “the core of the scientific method”. He once set about listing every mistake Einstein made: mathematical errors and conceptual failings; those he later saw and corrected, and those he never noticed. “Einstein is, among the good physicists of the last century, the one who made more mistakes than anybody else. [But] the list of things he got right is just astonishing. Obviously, the two are related. Mistakes are good, and part of not being afraid.”

No white hole has yet been observed in the cosmos, and their existence is not widely accepted among scientists. “All this is a hypothesis,” said Carlo Rovelli. “I don’t know if this is really what is going on. Deep down, I hope so, but I have doubts.” Even the greatest scientists get it wrong.

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This article appears in the 18 Oct 2023 issue of the New Statesman, War on Three Fronts