Of all the big ideas floating around — long-termism, degrowth, space colonisation and so on — is any quite as radical as quantum physics?
For a century, physicists have known that the classical assumptions about the universe are incorrect — electrons do not orbit the nucleus of an atom, but exist in waves around it. So an electron can be in more than one place at once, until we observe it. Schrodinger’s cat is alive or dead: we can only know which by looking, which itself determines the answer. Why this is true is unclear.
This realisation is “almost psychedelic”, argues Carlo Rovelli, the Italian physicist who specialises in making complex ideas seem simple. Quantum physics has been used to develop semiconductors and lasers. But the first thing to understand is that you cannot truly understand it, because no one does.
Rovelli tries to explain the problem in his book Helgoland, and ends up meditating on the nature of science and uncertainty. At a time of climate change and pandemics, his popular physics remains among the best-sellers. “Melting glaciers are important, but the most powerful aspect of science for me has been exactly the fact that it challenges our worldview,” he tells me, informal and relentlessly curious, on a trip to London from his home in Canada. The public thirsts to understand the universe, even though, as Rovelli writes in Helgoland: “Knowing that my girlfriend obeys Maxwell’s equations will not help me to make her happy.”
It helps that Rovelli is happy to push outside scientific boundaries, quoting Buddhist philosophy and organising letters for global disarmament. For him, physics inspires culture. “Good philosophers have always listened to physics. Kant wouldn’t have been Kant without Newton just before; half of what Kant does is making sense of Newton. Vice versa, Einstein wouldn’t have been Einstein without Kant. Einstein read Kant and Kant’s ruminations about space and time were the main ingredient for him to write equations that predicted black holes.”
Rovelli’s hope now is that popular culture absorbs the implications of quantum physics. Films such as Spider-Man: No Way Home and Everything Everywhere All at Once have tended to do so through the theory of many universes: that is, the solution to Schrodinger’s cat is that the cat is both alive and dead, as there are infinite universes, covering every possible outcome. Rovelli himself rejects the theory as “crazy”.
How is it possible for physicists to have such different interpretations? “Science is not just about writing equations and making predictions. It’s about reconceptualising the world. If you think what happened with Copernicus, the way we tell the story is that he understood that the Earth is spinning around the sun. But can you prove that the Earth is not the centre of the universe? No, you couldn’t make an experiment. The point is that, if you change your conceptual structure, everything starts making much more sense.” Similarly, black holes were predicted by Einstein’s equations, before they were detected.
Rovelli wants us to accept that certainty is often out of reach: “We’re never sure about anything.” Indeed, there is only one time in our conversation when he expresses certainty: when I ask whether we could be living in a simulation, as Elon Musk says is “most likely”. “No. Of that I’m sure. We’re not in a simulation. It’s a silly idea. Who is the simulator? Some guy?” The theory is that, with increased computing power, any species that could run a simulation of its ancestors would run many of them, so we cannot be sure we are not in a simulation. “Come on, have you played with AI? It’s completely different to real reality,” despairs Rovelli.
Uncertainty does not mean that all ideas are equally valid. “We don’t learn something more about reality by making absurd hypotheses and asking how they can be disproved. That’s stupid, because everything can happen and nothing can be disproved,” says Rovelli. “We learn more about reality by taking what we know and seeing what plausibly it implies.”
Fame came late to Rovelli, now 66. He started writing for a broad audience in his fifties; his 2014 book Seven Brief Lessons on Physics was a surprise hit. His path had been unusual. After a spell of hippie activism ended in failure, he applied to study physics at university partly because the queue was shorter than for other courses. “I went into science from the disappointment of the dream of changing the world.” But in science as in politics, he long felt outside the mainstream.
His particular contribution has been to loop quantum gravity theory, which posits that space itself is not continuous but granular and that the grains are woven together by loops (“the Financial Times was one of the first media that took it seriously”). It remains a theory. “I don’t know if I’m right or wrong. I’m more optimistic than 10 years ago . . . Most of our competitors are in trouble.”
Rovelli likens the process of scientific discovery to zooming in on a forest. At first you just see the forest, “you go on, and see that it’s different: there are trees. And then you go closer and you see the trunk, animals, insects. And then ever closer . . . You get layers and layers and layers of understanding . . . We accumulate knowledge. What Newton understood is correct; it just can be understood better.”
The task is not so much working out the answer, as working out what is wrong with the question. “You have to always find something in your assumptions to throw away, in order to make the next step.” His own process is, he imagines, “very similar to a novelist’s. If I think of the most cited papers I’ve done, the ideas came out when I was driving the car, walking in the forest, doing something else completely. You need to know the subject very well. So you keep walking around the boundary of the knowledge, and saying ‘this doesn’t work’. Then at some point, by thinking about something else, something clicks.”
His preferred form of quantum physics is the relational interpretation — the view, developed since the 1990s, that all objects affect each other, so that an object’s properties are only defined in the moments that it relates with another object. This brings him close to Buddhism. Where is the distinction between physics and philosophy? “The tools of the trade are completely different, but there is a large grey area in between.” Rovelli is an atheist, but says he has “students who are very devout . . . I do understand people for whom the idea of God plays a role, and they are great scientists.”
For Rovelli, it’s an easy jump from believing objects are in fact interactions to advocating a collectivist, leftist politics. Since achieving fame, he feels a political responsibility again. When we meet, Italy, where he still spends much of his time, looks set to elect a far-right coalition. “It’s a country that cannot find itself, that has a low opinion of itself,” he laments. “It’s a country that internationally has decided not to have a voice . . . It’s a very non-courageous country internationally, because of fascism and because it lost the war. Italians think that they are a third-world country, which is not true. They think that Portugal is more powerful than Italy, which is nonsense.”
Last year Rovelli organised a group of eminent scientists and public figures, including former Beatle Paul McCartney and experimental psychologist Steven Pinker, calling for military spending to be diverted to climate change and other causes, under the banner The Global Peace Dividend Initiative. Russia’s invasion of Ukraine a few months later seemed inconvenient for the cause. He suggests his move was misinterpreted: “The global peace dividend is not the idea: ‘Tell your government to stop spending on the military.’ It is the idea: ‘Tell your government to negotiate with your enemy so you both spend less on the military.’”
On the spot
Favourite science fiction?
Kim Stanley Robinson’s Mars trilogy.
Best things about Canada?
The kindness of people, and the natural world. We had wolves in our garden.
What’s the hardest thing about getting old?
No, it’s the other way round. I grew up thinking everybody over 30 was an idiot. To my incredible surprise, life gets better.
How worried are you by artificial intelligence? I don’t think it’s so dangerous. All my life I heard the marvellous things that will happen in artificial intelligence; most did not come true.
If military spending is a waste of resources, what about space travel? “It’s like asking a soccer fan if building a stadium is a good use of resources! I wish humankind went seriously back to space travel. I was 13 when man went to the Moon, and it was obvious to me that when I was an adult I would have my own spaceship and go round the solar system.”
Rovelli’s politics can be frustrating. On the day before Russia’s invasion of Ukraine, he tweeted: “Five words are sufficient to stop the Ukraine madness: ‘Nato won’t expand to Ukraine’. So hard to say them?” The next day, he doubled down: “Putin is disgusting: how can he dare invade a country and killing human beings? Only the Americans are entitled to regularly do so!” He defends his stance: “The west is making a mistake in the long term by not realising that we have to share power with the rest of the world.”
This misses quite a lot, but reveals something about his discipline. After the second world war, many nuclear physicists felt a guilt at the atomic bomb that their field had created. Rovelli instead feels a responsibility to avoid conflict. He is exasperated by UK prime minister Liz Truss’s insistence that she would launch nuclear weapons. “You have a new prime minister who said publicly that she was ready to push the button. I think humankind underestimates the catastrophe that is very possible.” Nuclear physics seems radically separate from everyday life, but there is only so much time you can spend thinking about it without worrying about nuclear war.
Additional reporting by Dylan Neri