Carlo Rovelli’s Radical Perspective on Reality
There is no objective reality, according to Rovelli — only perspectives. “This is very radical, because you can no longer say, ‘This is a list of things in the world, and this is how they are.’”
Jan Jackle for Quanta Magazine
Introduction
Sitting outside a Catholic church on the French Riviera, Carlo Rovelli jutted his head forward and backward, imitating a pigeon trotting by. Pigeons bob their heads, he told me, not only to stabilize their vision but also to gauge distances to objects — compensating for their limited binocular vision. “It’s all perspectival,” he said.
A theoretical physicist affiliated with Aix-Marseille University, Rovelli studies how we perceive reality from our limited vantage point. His research is wide-ranging, running the gamut from quantum information to black holes, and often delves into the history and philosophy of science. In the late 1980s, he helped develop a theory called loop quantum gravity that aims to describe the quantum underpinnings of space and time. A decade later, he proposed a new “relational” interpretation of quantum mechanics, which goes so far as to suggest that there is no objective reality whatsoever, only perspectives on reality — be they a physicist’s or a pigeon’s.
More recently, he’s gained recognition as a best-selling author of popular science books, including Seven Brief Lessons on Physics, which has sold more than 2 million copies worldwide — placing him in a limelight he’s still adjusting to. “I’m very bad at being somewhat famous,” he said. “I’m always getting myself in trouble.” (During my visit, he was fending off criticism from the president of the Italian Physical Society, who accused him of defaming Enrico Fermi as a “bloodthirsty fascist/Nazi.”)
Rovelli’s own perspective on physics is heavily influenced by his rebellious, countercultural youth. A student protestor in an attempted political revolution in Bologna in 1977, Rovelli worked at a subversive left-wing radio station, drafted an illegal manifesto, and was later detained for refusing compulsory military service. Disillusioned by societal norms, “I had a sense that we were confused about how to think about reality around us,” he said. At 69, he remains politically engaged (and often enraged). “Part of me is still an old hippie.”
After the political unrest in Bologna petered out, Rovelli transferred his deep misgivings to the very fabric of reality. He used the same proclivity for challenging traditional ways of thinking to confront long-standing problems in the foundations of physics — not by rejecting established theories, but by embracing a new perspective on them. His approach centers around a radical openness to abandoning intuitions about how the world works.
To confront his own biases, whether about physics or society, Rovelli turns to philosophy. He often publishes on metaphysical topics and advocates for more dialogue between the disciplines. His newest book, published this month in Italian, is a deep dive into the intersection of philosophy and physics, a mash-up he sees as the key to understanding what our existing theories are really telling us.
Quanta visited Rovelli at his home overlooking the cliffs of Cassis. Over a 12-hour conversation, held while we lounged on his patio, strolled around town, and cruised on his 100-year-old sailboat, we discussed religion, war, consciousness, media, love, pigeons and, of course, physics. The interview has been condensed and edited for clarity.
What is your central question, and how did it lead you to study quantum gravity?
My central question has always been: How does the world work? We have two main theories that work incredibly well for different domains: general relativity and quantum mechanics. When I learned about these theories in school, I was impressed by how radical they were. They both challenge very foundational conceptions that we have about the world around us — of space as an empty stage where objects exist, and of time as a steady linear flow. They resonated with this idea I had that if you really want to understand reality, you have to be ready to be radical.
All attempts to disprove quantum mechanics and general relativity have failed. But nevertheless, in this picture, there’s clearly a crack. There are phenomena out there — like objects falling into a black hole — that fall outside the domain of both theories. When you try to put the two theories together, they appear to result in all sorts of contradictions and paradoxes. To me, the interface of these two theories — the problem of quantum gravity — was really this deep, profound gap in our fundamental physical picture of the world.
Jan Jackle for Quanta Magazine
Tell me about the approach you’ve taken to fill that gap: loop quantum gravity.
Loop quantum gravity is a very conservative approach with a very radical consequence. It’s an attempt to say: Let’s take seriously what we’ve learned from general relativity and quantum mechanics all the way through and see where they lead us. There are no extra fields, extra particles, modifications of the Einstein equations, or other hypotheses about nature. It’s just an effort to make coherent what we know so far.
Basically, loop quantum gravity implies that space is not infinitely divisible — it’s made of elementary chunks, which are linked together into loops. The theory is a very simple set of equations, but there’s no time variables and no space variables. Those concepts emerge from the way these quanta of gravity interact and transform. What we call space is the quantity of these loops, and what we call time is how the loops evolve continuously.
How do we account for our common experience of time if it’s not fundamental?
Our experience of time flowing forward is a product of the second law of thermodynamics — the tendency for physical systems to increase in disorder, or what we call entropy. But this only appears fundamental from our perspective. We happen to be beings that are connected to certain macroscopic variables with respect to which entropy is globally moving in one direction.
My intuition is that the overall flow of time really could be like the rotation of the sky every day. It’s a majestic, immense phenomenon, but it’s actually an illusion. This is a totally perspectival understanding of the second law of thermodynamics. It’s real in the same sense that the rotating sky is real, but it’s real only with respect to us.
Jan Jackle for Quanta Magazine
One critique of loop quantum gravity is that it contradicts certain predictions of Einstein, namely that the speed of light is constant for all wavelengths. What do you make of this critique?
The theory has evolved a lot over the last 20 years, and the current version is not incompatible with Einstein’s predictions — the speed of light is indeed constant at all physical wavelengths. That said, there are some things about loop quantum gravity that still need resolving. We’re not sure how the different versions of the theory are equivalent to one another. We have a problem in which particle scattering seems to generate infinite amounts of low-energy radiation. And solving the equations is still a very complicated task that we’re working to simplify.
The main shortcoming is the lack of experiments supporting it. However, there’s hope on the horizon. There are some proposals to use loop quantum gravity to make sense of signatures in the cosmic microwave background radiation that’s left over from the Big Bang. And there’s another new idea I’m very excited about: If loop quantum gravity is right, there should exist tiny black holes weighing around 10 micrograms that are long-living and that interact only gravitationally. We’re thinking about ways to detect a background “wind” of these particles. And perhaps these tiny black holes are actually what we call dark matter, a mysterious widespread astronomical phenomenon that we have not yet understood.
Detection will be difficult, but it’s not out of the game. I’m hopeful there will be some experiment that will make the larger community see loop quantum gravity as the natural explanation. It’s far from clear that we cannot account for all of these phenomena using the existing theories that have worked so well for 100 years.
If we are to hold on to our existing theories, what picture do they paint about the nature of reality when taken together?
Rethinking space and time pushed me to view reality in a completely different way — not as a universe made of objects with defined properties, but as a network of interactions. This is the “relational” interpretation of quantum mechanics. In some sense, it’s a continuation of the trend in modern physics that we have seen with general relativity and quantum mechanics — a strong push toward perspectivalism.
We’re used to velocity being relative: The velocity of this table is different with respect to me, with respect to [that pigeon flying] outside, or with respect to the sun. Einstein showed us that time and length are also relative to different observers. Relational quantum mechanics takes this idea a step further. It argues that all properties of an object — its color, location, size, etc. — are in principle only definable in relation to another system. We need to give up the idea that there are material things which we’re describing from the outside. The best way of conceptualizing reality in light of modern science is in terms of the relative information that pieces of nature have about one another.
We can only say how the world looks from our limited, biased perspective. This is very radical, because you can no longer say, “This is a list of things in the world, and this is how they are.” We have to live with this lack of total description over reality.
Jan Jackle for Quanta Magazine
There’s something unsettling about this argument. It seems to undermine the ultimate goal of physics to describe the “true” nature of reality, does it not?
It very much does, but if you look at the history of science, the ultimate goal has been changing constantly. It went from describing the rotation of heavenly bodies to tracking the forces that guide particles to following the evolution of fields in space-time. I think that the problem of science is to figure out the right conceptual scheme to best understand nature as we see it. The relational perspective is rooted in a deep awareness that our knowledge about the world is fundamentally limited and that everything we see is partial. We have a much stronger and more honest way of approaching reality without being attached to this misleading idea of there being an ultimate truth. We must not confuse the knowledge we have with the reality of the world.
If this leaves you with a sense of emptiness about reality, that’s fair. But it’s precisely by knowing that our knowledge is limited that we are able to learn. Between absolute certainty and ignorance there’s all this interesting space in which we live.
You’ve written about how your change in worldview has been guided by philosophers. How do you view the relationship between philosophy and physics?
The disciplines desperately need one another. A philosopher who doesn’t think about science is not willing to engage with the knowledge we have, and that’s just silly. And a scientist who refuses to look at philosophy is trapped in ways of thinking from which there may be an escape. Historically, the relationship between physicists and philosophers has been very strong. All scientific revolutions have been strongly influenced by philosophical ideas. Copernicus, Galileo and Newton were all philosophers themselves. Einstein very explicitly credited his insights to philosophers like Immanuel Kant, Ernst Mach and others. And Erwin Schrödinger was likely influenced by his reading of the Upanishads, the sacred Hindu texts, when he came up with wave mechanics.
Jan Jackle for Quanta Magazine
But lately, the relationship between physicists and philosophers has been at an all-time low. Stephen Hawking famously pronounced that “philosophy is dead,” and Richard Feynman said things like “Philosophers are as good for science as ornithologists are good for birds.” What they don’t realize is that, first, they are doing philosophy by commenting on what it means to do science; and second, their whole view of science is already under the influence of American pragmatism thinking and philosophers like Karl Popper and Thomas Kuhn. What the physics community took away from these philosophers was that science is about picking new ideas out of thin air, developing a theory, and testing whether it’s right or wrong. This gives the false impression that scientific progress comes only in paradigm-shifting insights that overturn previous thinking, and that all new hypotheses are equally probable until falsified. But science is so much more than that. It’s a continuous process of building on past knowledge to refine our perspective.
In my opinion, this closed-mindedness is precisely the problem with modern theoretical physics. We’re undergoing a colossal jump in knowledge that’s forcing us to rethink notions of reality, information, time and space. Our community has wasted a lot of time searching after speculative ideas. What we need instead is to digest the knowledge we already have. And to do that, we need philosophy. Philosophers help us not to find the right answers to given questions, but to find the right questions to better conceptualize reality.
Jan Jackle for Quanta Magazine
In your book Helgoland, you talk about how the Buddhist philosopher Nagarjuna shaped your work. In what way did his texts open your mind?
The core idea of relational quantum mechanics is that when we talk about an object — be it an atom, a person or a galaxy — we are never just referring to the system alone. Rather, we are always referring to the interactions between this system and something else. We can only describe — and in fact understand — a thing as it relates to ourselves, or to our measuring devices.
Nagarjuna expresses a very similar idea: that no entity has a proper independent existence — things only exist depending on one another. By renouncing “primary” entities or any “ultimate absolute reality,” we can better make sense of the world in terms of how things manifest themselves to other things.
Relational quantum mechanics uses similar ideas to make sense of all quantum paradoxes in a precise mathematical way. The main idea is to give up questions about how things really are, in absolute terms. It’s just like how Galileo taught us that asking “Is this object really moving?” is meaningless, and Einstein taught us that asking “Are these two events really simultaneous?” is meaningless. The confusion about quantum mechanics, I believe, is generated by asking questions that have no meaning. The answer to the riddle is that there is no riddle.
