A universe made by kisses. Interview with Carlo Rovelli

In June 1925, a then twenty-three-year-old physicist named Werner Heisenberg, who suffered from hay fever, retreated to a desolate rocky island in the North Sea called Helgoland.

It was on Helgoland, in a sudden moment of eureka, that the young German physicist came up with one of the most transformative scientific concepts: quantum theory. Heisenberg’s scientific breakthrough ushered in a revolutionary chapter for physics — paving the way for countless discoveries. Quantum theory allowed scientists to peer into an “interior of strange beauty” — as Heisenberg described. At the time of its discovery, however, quantum theory’s scientific and philosophical implications were rudimentary and still today remain only partially explored.

Almost a century later, our understanding of the world around us remains firmly rooted in Heisenberg’s theorisations — countless experiments have led to practical applications that shape our daily lives. And yet, the way particles behave is still profoundly mysterious.

Carlo Rovelli, renowned Italian physicist, discoverer of loop quantum gravity theory and bestselling author of the acclaimed science books Seven Brief Lessons on Physics (2007) and The Order of Time (2014), narrates in his new book Helgoland, the adventure of quantum physics. Helgoland begins with quantum theory’s pioneering thinkers — Erwin Schrödinger, Max Born, Paul Dirac, Pascual Jordan, Albert Einstein and none other than Werner Heisenberg — to its contemporary interpretations.

DUST met Rovelli to discuss how our vision of a universe made of matter with its peculiar properties should be supplanted by that of a universe made of relations, and how each particle responds to another in an infinite succession of correspondences.

michele fossi – “Werner Heisenberg”, you write in the first pages of Helgoland, “ lifted a veil between us and reality; behind that veil an abyss appeared”. What was it that seemed so disturbing and dizzying to the young physicist on that windy German island?

carlo rovelli – It wasn’t even clear to him. That’s what characterises abysses. Just like in an abyss when we look down and see that there is nothing under our feet supporting us, so Heisenberg realised that the conventional metaphysics of classical physics no longer functioned. These are the words he used later to recall the anguished excitement he felt at that discovery. “I was deeply alarmed… I had the sensation that I was looking through the surface of the phenomena into an interior of strange beauty; the thought stunned me that now I would have to investigate this new richness of mathematical structure that nature had so generously unfolded for me…” These are words that bring shivers. Through the surface of things, “an interior of strange beauty”. They resonate with the words written by Galileo, when he saw the emergence of a mathematical regularity in his spatial measurements of balls rolling down an inclined plane, the first mathematical law discovered by humanity to describe the movement of objects on the earth: “there is no emotion like glimpsing the mathematical law behind the disorder of appearances”.

m.f. – Can you describe, in a few words, the scientific leap Heisenberg made?

c.r. – Heisenberg’s leap is as reckless as it is simple. No one had managed to find the force able to guide electrons in their bizarre behaviour? Well, let’s forget about a new force… Let’s use one we already know: the electrical force that attracts the electron to the nucleus. We cannot come up with new laws of motion that justify Bohr’s leaps? Let’s stick to the laws of motion we already know without changing them. Instead, let’s change our way of thinking about the electron. Let’s give up the idea that an electron is an object that moves along a trajectory. Let’s stop describing the electron’s motion. Let’s describe, only, what we observe of the electron from the outside: the intensity and frequency of the light it emits. Let’s base everything only on the quantities that are observable. This is the principle idea. The electron emits light when jumping from one orbit to another. Its observable quantities depend on two discrete variables: the orbit of departure and the orbit of arrival. Then, they can be written in the form of tables in which the orbits of departure make up the rows, and the orbits of arriva the columns. These tables are the matrices with which Heisenberg replaced the old physical variables. His new mechanics is called the “mechanics of matrices”.

m.f. – One of the key protagonists of Helgoland is the libertine, non-conformist Erwin Schrödinger. During a romantic tryst with a secret lover in a chalet in the Swiss Alps, he, like Heisenberg, also had a formidable intuition; marking a turning point in studies on quantum physics. The electron must be treated mathematically — like a wave.

c.r. – Yes, but it was a misleading intuition. It appeared to clarify things, but instead confused them even further. Schrödinger hoped that his wave mechanics would avoid the discontinuity of the quanta. But the debate immediately became very heated. Heisenberg is cutting: “the more I think of the physical aspects of Schrödinger’s theory, the more I find them repugnant. What Schrödinger writes about the ‘visualisability of his theory’ is probably not quite exact. In other words, they’re nonsense”. Schr.dinger, for his part, tried to refute these concepts with irony: “I can’t imagine that an electron jumps like a flea”.

m.f. – But Heisenberg was right?

c.r. – Yes. Gradually, it became obvious that wave mechanics isn’t any clearer than Heisenberg’s matrix mechanics. It is another instrument of calculation that produces the right numbers, which perhaps are easier to use, however by itself, it does not give us a clear and immediate image of what is going on as Schrödinger hoped. If every time we see an electron, we see it in only one point, how can the electron be a wave diffused in space?

m.f. – Did Schrödinger ever admit his defeat?

c.r. – Yes, many years later in the 1950s, he wrote, “There was a time when the creators of wave mechanics consoled themselves with the illusion that they had eliminated the discontinuities of quantum theory. But the discontinuities eliminated by the equation of the theory reappear as soon as we compare the theory with what we observe”.

m.f. – Another milestone, in understanding quantum phenomena, came with Max Born. You write: “with his air of a modest engineer, he was the least flamboyant and least known among the creators of quantum theory, but perhaps he was its real craftsman, besides being, as Americans like to say, “the only adult in the room”.

c.r. – I believe that the role played by Max Born, in the birth of quantum mechanics, has been widely underestimated. When we begin to think of the many scientists to attribute quantum theory to, many names do indeed come up. Think: Bohr, Planck, Schrödinger, De Broglie, Einstein, Heisenberg, Dirac… rarely is Max Born ever mentioned. Born had a profound influence on the field. Heisenberg may have come up with the ideas, however he was Born’s assistant. It was Born who recognised that a radical leap was needed. It was Born who realised that a theory could be constructed from Heisenberg’s confusing calculations. I believe it was Born who wrote the true new fundamental equation: XP-PX=iħ. The first articles of the complete theory bear his name, and it was Born who clarified what the heck Schrödinger’s wave meant. You see, there are the creative personalities, the front-men, the old sages… And then there is the guy who does everything behind the scenes. That was Max Born.

m.f. – One of quantum physics’ most challenging concepts to digest is known as “quantum superposition”. This being the idea that an object can have contradictory properties — to be in one place while also in another.

c.r. – It’s hard to digest because it’s objectively confusing. I think it’s been poorly formulated. It isn’t that objects have contradictory properties. It’s that they don’t always have properties. An object’s properties are the way it interacts with other objects. When it doesn’t act, it doesn’t necessarily have properties. In classical physics, we assume that objects always have properties, even when they don’t interact; that properties are something more than the way objects interact. However, a careful analysis of the world shows this assumption doesn’t hold up.

m.f. – Within quantum physics, the “many worlds” interpretation feels closer to science fiction than reality. According to its supporters, the function of the ψ wave should not be interpreted as a probability, but rather as a real entity that describes the world as it effectively is. The cat that is both awake while asleep at the same time? There’s no contradiction — the supporters of this interpretation argue that you just need to assume, like in the film The Matrix, the existence of a parallel world. For every Carlo Rovelli in this world who sees a cat that’s awake, there’s another Carlo Rovelli in a parallel world who sees the cat asleep. And that’s how they solve the problem. The cat is both asleep and awake.

c.r. – It doesn’t convince me.

m.f. – How come?

c.r. – Partly for technical reasons: it seems to be that the “many worlds” interpretation doesn’t clarify the role of the usual physical variables we use to describe the world. Reality isn’t just an abstract wave. And partly for philosophical reasons. I think that postulating the existence of other worlds in order to free ourselves from the conceptual confusion induced by quantum theory is not the right attitude. We have to learn from the theory, not adapt the theory to our prejudices about reality. Quantum theory isn’t only Schrödinger’s wave equation. Its mathematical structure is far greater than this mere assumption.

m.f. – Now we come to the main thesis of your book: the relational interpretation of the quantum theory. In your book you write, “We have to abandon something that seemed to us very, very natural: the idea of a world made up of things. We have to recognise this as an old prejudice, an old cart that is no longer useful”. Can you explain the essence of the theory?

c.r. – The discovery of quantum theory, I believe, is the discovery that the properties of everything are nothing more than how that thing influences other things. They only exist in interaction with other things. Quantum theory teaches us how things influence each other, and that is the best description of nature that we have today. Bohr speaks of the impossibility of clearly separating the behaviour of atomic systems from interacting with the measuring apparatus that serves to define the conditions under which the phenomenon appears. Appropriately revised, Bohr’s remarks encapsulate the discovery that underlies the theory: the impossibility of separating an object’s properties from the interactions where these properties manifest themselves, and from the objects to which they manifest themselves. An object’s properties are the way in which it interacts with any other object.

m.f. – In a previous interview, you have declared: “A thing is something which remains equal to itself. A stone is a thing because I can ask where the stone is tomorrow, while a happening is something that is limited in space and time. A kiss is not a thing, because I cannot ask, Where is a kiss? Where will it be tomorrow? It just happened, now. That’s how we should look at the world: not as made by things, like stones, but by happenings, like kisses.”

c.r. – The object itself is nothing but the sum of its interactions with other objects. Reality is constructed by a network of interactions beyond which it is not even possible to understand what we are talking about. Instead of viewing the world as a set of objects with definite properties, quantum theory invites us to see the physical world as a network of relationships — of which objects are hubs. There are no special entities that are “observers”.

m.f. – Let’s go back to our cat. Can the relational interpretation help us understand the mysterious coexistence of waking and sleeping in a less eccentric way than the “many worlds” interpretation?

c.r. – If you are the cat, the sleeping pill was released or it wasn’t. Consequently, you are either awake or asleep. For me, a physical system outside its box — you are neither awake nor asleep. I can say that for me, “there is a quantum superposition between different states”, and this only means that facts can occur relative to me that would not happen either if you, the cat, were awake, or if you were asleep. The relational interpretation allows both perspectives to be true, yours, in relation to which there is a determined state, and mine, in relation to which there isn’t one. Each concerns interactions that involve two different observers: you and me.

m.f. – So, what you are saying, in essence, is that there can be properties in an object that are real with respect to one subject, but not to another. That is also not a concept that is easy to digest. Can you give an example?

c.r. – Exactly! It’s not really a strange concept. An object has a velocity of 3m/s with respect to one thing and 5m/s with respect to another. Now I have a velocity of zero with respect to the Earth and a high velocity with respect to the sun.

m.f. – Velocity, in effect, has a relative nature. It doesn’t make sense to speak of velocity without specifying in relation to what. But please explain, are all physical sizes to be considered relative? For example, what about mass? Not long ago, I read that the mass of the electron was recently measured with new precision. It would appear to be a constant that doesn’t vary with the environment which the electron is interacting with (for example, passing from one element of the periodic table to another).

c.r. – The physical quantities I am speaking of are variable ones, those that change in time. The mass you are speaking about is determined by a constant that enters into the fundamental equations.

m.f. – Clear! Let’s move on to another quantum phenomenon that is simply mind-boggling: entanglement. You suggest that “entanglement is the phenomenon according to which two objects distant from each other” for example, two particles that had met in the past, retain a sort of strange connection — as if they could continue to speak to each other… Like two distant lovers who guess each other’s thoughts. They remain, as it were, “entangled”, tied to each other. This is a phenomenon that has been clearly proven in the laboratory. Recently, Chinese scientists were able to maintain in an entangled state two photons that were thousands of kilometres apart. Please explain how quanta and particles are able to speak over such incredible distances?

c.r. – They don’t speak to each other over a distance. The strange relationships at a distance are not exchanges of signals. They are much subtler than that.

m.f. – Does the relational interpretation help give meaning to this curious phenomenon?

c.r. – Even if we know everything there is to know in a particular situation about a single object if that object has interacted with others, we don’t know everything about it. We don’t know its relationships with the other objects of the universe. The relationship between two objects is not something that is contained in one or the other — it’s something more. This interconnection between all the components of the universe is disturbing but it’s real.

m.f. – I agree. It is.

c.r. – A key is understanding that entanglement is not a dance of two partners — it’s a dance of three. The apparent incongruence raised by what appears to be communication over a distance between two entangled objects is due to forgetting this fact: the existence of a third object that interacts with both systems is necessary to reveal this reality to the correlations.

m.f. – Are there explanations as to what that third entity could be in allowing two entangled particles to “ hear” each other?

c.r. – Yes, of course. There’s no mysterious entity! It’s simply that to “see” these correlations, it’s necessary that there be something that interacts with both systems in entanglement. As long as they remain separate, and do not communicate with the same system, their correlations cannot manifest themselves.

m.f. – There is a fascinating section in the book where you draw references between your relational interpretation and philosophy. In Plato’s dialogue The Sophist, the Eleatic Stranger provides us with an exquisitely “relational” definition of reality: “I say, therefore, that whatever which by its nature can act on something else or be subject to the slightest action by something else, however insignificant it may be, and even if only once, only that can be said to be truly real. Therefore, I propose this definition of being: that it is nothing other than action (δύναμις, dunamis)”. You then go on to quote Nagarjuna, one of the most important Buddhist philosophers and certainly one of the most radical: you attributed to him the merit of having intuited, already in the third century BCE, the relational nature of things.

c.r. – The central thesis in Nagarjuna’s book is simply that there are no things that have an existence of their own, independently of other things. The resonance with quantum mechanics is immediate. Obviously, Nagarjuna didn’t know, and couldn’t have known, about quanta. But like the good philosopher he was, he proposed an original way of thinking about the world; which we can use — if it helps us.

m.f. – Towards the end of Helgoland, you express the hope that the relational interpretation can reach the wider public. “I think the time has come to look at this theory squarely in the face, and to discuss its nature outside the narrow circles of theoretical physicists and philosophers, to bring down its distilled honey, very sweet and a little intoxicating into the meshwork of the entire contemporary culture”. Are you sure that the impact of viewing the world this way would be positive? It occurs to me that the core idea of the relational interpretation — that every vision is partial — might end up dealing a final blow to the already shaky wall that separates truth from falsehood in our society — casting us into an extreme relativism.

c.r. – The properties of objects are relative, not arbitrary. To say “that wall is hard for my head” is not the same thing as saying that the wall is subjectively and arbitrarily hard. If I hit my head against it will I hurt myself, precisely because it’s hard for my head. The fear of relativism comes from not recognising that there is an immense space between total certainty and total uncertainty. It is in this space that our thoughts always move. I have no absolute certainty as to how I should drive from Vienna to Berlin. This is not to suggest that I am in total darkness. We are not interested in absolute certainty in our lives: we are interested in sufficiently reliable knowledge. And we are interested in knowing that we can always exchange ideas and improve this knowledge. We know that our point of view is always partial; that is precisely why we want to compare ideas and improve it.

m.f. – Distant objects that seem magically connected with each other, matter replaced with phantasmagoric probability waves — there is certainly something magic about quantum physics. Among the many disconcerting revelations you have had to come to terms with in your career, is there a specific revelation that continues to dazzle you, one that seems too incredible to be true?

c.r. – Quantum mechanics is the most disconcerting discovery I have encountered. When I studied it in the 1970s, it blew me away. Over the decades that followed, there have been many miraculous announcements of scientific findings that have stunned everyone. For instance, the correlation at very large distances between “entangled” particles that seem to be able to communicate with each other instantaneously. In reality, all these new phenomena had already been described in the textbook I used. The amazing thing is that until now, everything predicted by the theory, even the strangest things, have turned out to be true.

m.f. – It would be great if you could give us a more specific example of a physical phenomenon that you find particularly mysterious.

c.r. – In the book, I describe a very simple experiment which I was directly involved with: there are two bands of particles: all the particles that pass through one band have a certain property, all the particles that pass through another band have the same property, but if I take the one and the other together, there are some that don’t have that property. It’s as if all the men I see going through the right side door into a room with two doors are blond, and all the men entering through the left side door are also blond. But if I leave both doors open, inside the room, I find brown-haired men. It doesn’t seem logical, but that’s the way the world is.

m.f. – I read that your research’s main goal in theoretical physics has been to “understand the quantum nature of space and time” and make the quantum theory compatible with Einstein’s discoveries about space and time. Please explain, do time and space also have a quantum nature? Are there “quanta” of time and “quanta” of space?

c.r. – Yes, of course. I devoted my book Reality Is Not What It Seems to explain the quanta of space and time.

m.f. – How small do intervals of time have to be for them to be defined as “quanta”? And, above all, what did you observe that is “strange” about such small intervals? Could you elaborate on this?

c.r. – They’re very small. Much smaller than what we are currently capable of measuring. At that scale space and time are “discreet”, that is granular. These minimum quantities of space and time appear like a spatial and temporal continuum only because they are very small. This is the main prediction of the theory of quantum gravity which I am working on: loop quantum gravity.

m.f. – In that regard, I can’t help quoting from another book of yours, The Order of Time. Just reading the first pages of that book, I discovered that in the mountains, time runs faster than at sea level; that if things fall from up to down, it’s precisely by virtue of this slowing down of time. I even discovered that, following a gravitational wave, the flow of time — or perhaps it would better to say, “of times” since different times flow in different points in space — can “orient itself in such a way that, proceeding towards the future, as in a circle, one can find oneself at the point of departure”. In essence, at least on paper, it’s possible to travel in time. Perhaps the thing that surprised me the most was that if we descend to particles’ level, the past becomes one with the future: speaking in terms of cause and effect no longer has any meaning. Is that really true?

c.r. – Yes, that’s really so. It’s an old discovery of physics. The big difference between past and future that we all see around us depends on the fact that we don’t see the minute details of the physical processes. Every time there is a difference between past and future, there is a production of heat. For example, a pencil falls to the floor and stops. The opposite phenomenon in time doesn’t happen: we never see a pencil jump up by itself when it’s resting on the ground. Now when the pencil falls on the ground its energy is transmitted to the molecules of the floor which begin to move: the floor heats up. Heat is produced. Heat is energy diffused among microscopic variables whose individual movements we don’t see. It’s only when we get to these macroscopic descriptions of phenomena that disregard the microscopic movement of single molecules that a difference emerges between past and future. It’s surprising, but that’s the way it is!

By Michele Fossi

Published in DUST #18, “LOVE MORE”, 2021


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