Photo by NASA/MAPs Team/Getty Images, Massimiliano Donati/Alamy Live News

Carlo Rovelli on his search for the theory of everything

Can quantum physics reveal the secrets of the universe—as well as humanity’s entangled nature?
March 27, 2021

My interactions with my surroundings define me. I do not exist in any meaningful sense independently of those interactions. Air molecules are bouncing off the atoms of my skin, photons of light are being absorbed by atoms in my eye, compressions and rarefactions of the air are jostling my eardrum. I also live in a complex web of social interactions. And, at this moment, via a transatlantic telephone line, I am interacting with Carlo Rovelli. 

Rovelli has just come indoors from shovelling snow outside his house in Canada. The Italian theoretical physicist has been in London, Ontario, working at the University of Western Ontario, hunkered down through the Covid crisis. “It has been sad to see what the rest of the world has been going through,” he says. “I have been very privileged to be here.” Not only does Rovelli have a forest nearby but he has had the luxury of time to think—and is no longer exhausted by his globetrotting tour as the author of Seven Brief Lessons on Physics. In the seven years since the book was published, it has been translated into 44 languages.

Rovelli has written a new book. Its title, Helgoland, refers to a barren island off the North Sea coast of Germany, where the 23-year-old physicist Werner Heisenberg (who would go on to work on the unrealised Nazi atomic bomb) retreated in June 1925. He was trying to make sense of recent atomic experiments, which had revealed an Alice in Wonderland submicroscopic realm where a single atom could be in two places at once; where events happened for no reason at all; and where atoms could influence each other instantaneously—even if on opposite sides of the universe.

Heisenberg’s breakthrough was to realise that, as far as atoms and their components are concerned, everything is interaction. Subatomic particles such as electrons and photons are not objects that exist independently of being prodded and poked, but merely the sum total of their interactions with the rest of the world. “Basically, physics confirmed what several philosophers over the centuries have suspected—that the world is a web of interactions and nothing exists independently of that web,” Rovelli tells me. “It is at the atomic and subatomic, or quantum, level that we confront this truth most dramatically.”

M97TWR Werner Heisenberg (1901-1976). Portrait of the German theoretical physicist, c.1927. M97TWR Werner Heisenberg (1901-1976). Portrait of the German theoretical physicist, c.1927.

Werner Heisenberg in 1927, two years after his breakthrough on the composition of atoms. Photo: IanDagnall Computing/Alamy Stock Photo 

Rovelli points out that the great Danish physicist Niels Bohr, a friend of Heisenberg, inadvertently muddied the waters for a century by drawing a distinction between the “quantum” world of atoms and the everyday, or “classical,” world of the measuring apparatus “observing” those atoms. “In reality, there is no such distinction,” says Rovelli. “We live in a quantum universe. It’s just that, when many atoms come together, the quantum effects are washed out and it is possible to think of ‘objects’ with an independent existence such as chairs and tables and people.” Normal physics can thus be successfully applied, while it fails at the subatomic level.

Perhaps the most mind-blowing feature of the quantum world—so mind-blowing that Einstein believed it could not possibly be true—is “non-locality.” If two subatomic particles are “born” together, their properties are intertwined, or “entangled.” Say, for instance, two electrons are born with opposite spins: if the first spins clockwise, the other spins anticlockwise, and vice-versa. The electrons actually have no properties independent of interaction so if they are not observed in any way at their origin—say in London, England—their spins will be undetermined. Now imagine one is sent in a sealed box to London, Ontario, where its spin is measured. If it is clockwise, the electron in England becomes anti-clockwise. This instantaneous transatlantic influence is an apparent violation of Einstein’s special theory of relativity, which forbids anything moving faster than light.

The seeming paradoxes here have consumed many great minds. But Rovelli provides the best explanation of non-locality that I have ever read. All that actually happened, he says, is that one electron was measured with respect to an apparatus in Ontario and the other with respect to an apparatus in England. There is no God-like perspective that sees both electrons at the same time so that their spins can be legitimately compared. How the spins relate to each other is undetermined until an experimenter in Ontario communicates information about their electron to their colleague in England. Regardless of whether the news bounces between the continents via satellite beam or through an internet cable, “this information necessarily travels at less than the speed of light,” says Rovelli. “Non-locality is therefore no puzzle after all.”

Rovelli’s passion for physics came by an unusual route: political activism. Born in 1956, he grew up in Verona, a provincial Italian town where people had strongly conservative views. Despite the disgracing of the Italian right over the preceding generation, “some of my school teachers did not conceal their sympathies for fascism,” says Rovelli.

An only child with a loving mother, Rovelli grew up in a happy and protected environment. The flip side of this was that he felt he could do nothing without his mother’s approval and therefore was in a prison from which he needed to break free. His parents actually encouraged his independence by sending him twice, aged seven and eight, and alone, to England to learn English. This led to the 15-year-old Rovelli hitch-hiking alone from Paris to Sofia in Bulgaria, sleeping outside in the countryside, a trip that horrified his parents but made him “very happy.” 

He rebelled against the close-mindedness of Verona, railing against the world of money and power and hierarchies. “I travelled the world a lot, wanting to learn and experience all I could,” he says. “And everywhere I found like-minded young people who believed that a better, kinder, more compassionate world was possible.” Rovelli was only 12 in 1968 but, like many of his generation, he was influenced by ideas lingering from that rebellion. Ultimately, however, the revolution he wanted failed. “Sadly, we were never able to convince the majority,” he says. “Most people did not want to change the world.”

When Rovelli became an “orphan of the revolution,” he was studying science at the University of Bologna. Fortuitously, his political dejection coincided with his discovery of the extraordinary magic of physics. A professor set him an essay on group theory and its applications to quantum theory. He confessed he knew nothing about quantum theory, and the professor answered: “Well, go and read about it.”

Rovelli spent a month reading, primarily the classic 1930 treatise of Paul Dirac. With mounting excitement, he realised that here was a window on the deep reality that underpinned the world. “It was mind-blowing—better than an LSD trip,” he says. “What is reality? And how does it work?” What also impressed Rovelli was that quantum theory had been a successful revolution that really had durably overturned all previous, or classical, physics. “We had not achieved a revolution in the human world,” he says. “But scientific revolutions were entirely possible, and this was a powerful realisation for me. I discovered also that I was good at physics.”

The two great revolutions of 20th-century physics were quantum theory—that describes the small-scale realm of atoms and their constituents (though actually it describes everything)—and Einstein’s theory of gravity (also known as “the general theory of relativity”), which describes the large-scale realm of stars and galaxies and the entire universe. But once upon a time—in the Big Bang, 13.82bn years ago—the universe was very very small. So in order to address ultimate questions concerning its origin, it is necessary to unite quantum theory and general relativity.

The problem is that the two theories appear incompatible. Whereas general relativity is a theory of certainty, predicting the exact path of a body such as a planet, quantum theory is a theory of uncertainty, predicting only probabilities of events such as the possible trajectories of an atom flying through space. And whereas Einstein’s theory of gravity views the world as continuous, quantum theory views it as grainy, like a newspaper photograph seen close-up, with everything from energy to spin and electric charge coming in tiny indivisible chunks, or “quanta.”

In the late 1980s, Rovelli roved around Italian, American and British universities. Along with other celebrated physicists such as Abhay Ashtekar at Syracuse and Lee Smolin at Yale, he attempted to show that space-time itself—the currency of general relativity—ultimately comes in such indivisible chunks. Their equations revealed that, down at the impossibly small “Planck scale,” space-time is made of finite “loops” woven together into a complex shifting network. “In principle, when we zoom out from this ultra-small, grainy scale, there emerges Einstein’s theory of continuous space-time,” says Rovelli.

“Quantum theory impressed Rovelli as a successful revolution that really had overturned all previous physics”

“Loop quantum gravity,” as Rovelli’s theory is called, reveals that the universe in the Big Bang had a minimum size and was not born from an infinitesimally small, infinitely dense “singularity,” as implied by general relativity. Instead, the theory hints that the universe may have contracted down in a “Big Crunch” before exploding in the Big Bang. “The hope is that it might one day be possible to spot the signature of such a contracting phase on the cosmic background radiation,” says Rovelli.

Rovelli is modest about quantum loop gravity. “It is not an overly ambitious theory,” he admits. Like everyone working at the frontier of physics, he knows he is groping in the dark. “For four days a week I am completely convinced the theory is right, for two days I have doubts and for one day I think it is completely wrong!” says Rovelli.

The best-known rival of quantum loop gravity is “superstring theory,” which views the fundamental building blocks of the world not as point-like particles but as tiny “strings” of mass-energy vibrating in space-time of 10 dimensions. Our current best picture of the fundamental world—the “Standard Model”—fails to explain why the fundamental subatomic particles have the masses they have and why the fundamental forces have the strengths they have. “The hope was that string theory… would predict the magnitude of all the unknown parameters,” says Rovelli. “Unfortunately, we have discovered there is not one string theory but an astronomical number of them.” Rovelli highlights another confidence-lowering problem: the failure to find particles predicted by “supersymmetry,” string theory’s indispensable concomitant, at the Large Hadron Collider near Geneva.

Finding a “theory of everything” that unites quantum theory and Einstein’s theory remains the Holy Grail of physics. But might the days of Covid give us hope of a breakthrough? In 1665, Isaac Newton self-isolated on his family’s farm in Lincolnshire while bubonic plague raged across Britain. There, in lockdown, he discovered the universal law of gravity and changed the face of science. Is there a 21st-century Newton out there, perhaps, who will furnish us with the elusive theory of everything? “I wouldn’t totally exclude the possibility,” says Rovelli. “Many physicists are working without the normal distractions.”

“It is no coincidence that Einstein, who made the most discoveries, also made the greatest number of mistakes”

The problem of finding a “theory of everything” is discussed in Rovelli’s popular books, starting with Seven Brief Lessons on Physics, which he proudly tells me has sold more than a million copies. His writing, like his career in physics, came about unexpectedly. “I always recorded my thoughts in diaries and the things I learnt from my wide reading in notebooks,” says Rovelli. “It resulted in 2009 in a popular book on Anaximander, a 6th-century BC Greek philosopher who I believe was a proto-scientific thinker.”

WASHINGTON - FEBRUARY 10:  This image, showing the first all-sky microwave image of the universe soon after the Big Bang, was released by a team of astronomers from NASA and Princeton University on February 10, 2003 in Washington, DC. The image was captur WASHINGTON - FEBRUARY 10: This image, showing the first all-sky microwave image of the universe soon after the Big Bang, was released by a team of astronomers from NASA and Princeton University on February 10, 2003 in Washington, DC. The image was captur

A map of cosmic microwave background radiation released shortly after the Big Bang. Photo: NASA/MAPs Team/Getty Images

After the Anaximander book, Rovelli was asked to write a column for the Italian newspaper Il Sole 24 Ore. This led him to be poached by another, bigger-selling and more prestigious paper, the Corriere della Sera. The paper was in favour of Italian troops being sent to Iraq in the 2003 war, something to which the still-radical Rovelli was strongly opposed. He wrote his first column saying this, expecting his piece to be rejected. “But to my surprise the paper published it,” he says. “It was a pivotal moment. I saw that I was free to write what I wanted.”

The column led to an approach from the publisher Adelphi, which commissioned and published Seven Brief Lessons on Physics. And the rest is history. Rovelli’s books seamlessly interweave what he sees as the essence of physics with his personal views on subjects such as culture, society and politics. “I seem to have two distinct audiences,” he says. “People who know nothing about physics and whose eyes are opened to the wonder of it, and people who know a lot about physics.” Rovelli tells me how pleased he was when one of his scientific “enemies,” the Nobel laureate David Gross who is critical of loop quantum gravity, told him how much he had enjoyed one of his books. “We found we shared the same deep appreciation of the beauty of physics,” says Rovelli.

The worst thing about writing, says Rovelli, is the time it takes up, both in actually honing his thoughts into a succinct and captivating form and in publicising a finished book. But he also thinks there is something wonderful in it. “I no longer feel alone,” he says. “I used to think my political ideas about the world were different from mainstream society and I didn’t dare air them. But seeing many people take them seriously has re-connected me with humankind and stopped me feeling isolated.”

More generally, Rovelli is convinced we must work together. “Co-operation is better than competition,” he says. “Physics has reinforced the fact that we are all part of an interactive web and there are no solutions to our global problems without recognising and embracing that.”

There is no getting away from interdependence, and Rovelli also sees it as a positive virtue to be open to changing your mind. “Lots of people think they are smart, that they see things better than others,” he says. “But the scientific view has a lesson for the human world because it allows for changes. It is surely no accident that Einstein, who made the most discoveries, also made the greatest number of mistakes and changed his mind the most number of times.”

We have now been talking for two hours. I end our conversation with a big question. “What is the universe?” Rovelli’s answer is unexpected. “You are asking the wrong question.” There is no God-like perspective, he says, from which the universe can be observed. There is no universe “out there” because we are in it. So we need to think from within: “All we can ask is: what is our particular perspective from within and how does it relate to all the other possible internal perspectives?”

Those words, at once modest and profound, travel—at well within the speed of light—from London, England, to London, Ontario, conveying all the favourite themes of Rovelli’s work: interaction, contextuality, relationality. The things that make up the world at its deepest level are intertwined and work together. We too are intertwined and must work together. It is the only way ahead Rovelli sees for the human race.  

Carlo Rovelli’s new book “Helgoland” (Allen Lane) is out now