Carlo Rovelli stands in front of an exploding shed. Fragments of its walls and shattered contents – parts of a child’s tricycle, a record player, a shredded Wellington boot – hang in mid-air behind him. I have come to meet the physicist and bestselling author at an exhibition at the Tate Britain art gallery in London. The scattered objects are the work of Cornelia Parker, one of the UK’s most acclaimed contemporary artists, known for her large-scale installations that reconfigure everyday objects.
For Rovelli, based at Aix-Marseille University in France, Parker’s work is meaningful because it mirrors his take on the nature of reality. “I connect with the process: of her coming up with the idea, producing the idea, telling us about the idea and of us reacting to it,” he tells me. “We don’t understand Cornelia Parker’s work just by looking at it, and we don’t understand reality just by looking at objects.”
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Rovelli is an advocate of an idea known as relational quantum mechanics, the upshot of which is that objects don’t exist independently of each other. It is a concept that defies easy understanding, so Parker’s reality-challenging exhibition seemed like it might be a helpful setting for a conversation about it – and about what else Rovelli is up to. It is a happy coincidence that Parker’s shed is called Cold Dark Matter, a reference to the unidentified stuff that is thought to make up most of the universe. Because Rovelli now thinks he knows how we might finally pin down the true nature of dark matter.
The problems physicists such as Rovelli are trying to tackle are some of the deepest in science. Take the riddles related to quantum theory, our best guide to the subatomic world. It paints particles as occupying multiple possible states at once before they interact with other things. How to make sense of that? It is also difficult to see how quantum theory fits with general relativity, Albert Einstein’s theory of gravity, because the two theories require mutually incompatible mathematical frameworks to describe reality.
Rovelli formed his latest insights into how we might solve such problems when he began to dissect theoretical physicist Werner Heisenberg’s 1925 trip to the small, treeless island of Helgoland in the North Sea. Here, Heisenberg dedicated himself to resolving the confusion created by his contemporary Erwin Schrödinger’s description of quantum objects – electrons, neutrons, photons of light and so on – as waves that somehow manifest themselves to us as particles. Heisenberg’s approach was purely mathematical and he refrained from attributing a physical description to any individual entities.
Relational quantum mechanics
Rovelli admires this approach and builds on it in his recent book Helgoland, eventually drawing a stark conclusion: quantum objects, he says, have no independent reality. Instead, they exist only as relationships.
As we stand in the gallery, with Parker’s exploded objects looming over us, Rovelli explains it further. “We have discovered that, at the core of the physical reality, it’s not particles, it’s relational connections,” he says. “Each object is defined by the way it interacts with something else. So when it’s not interacting, it’s just not existing. An object is the ensemble of the ways in which it affects other objects around itself – an object exists reflected in everything else.”
It isn’t a testable claim, but that doesn’t bother Rovelli. “Science is not just about directly testable things,” he says. “It’s also about finding the right conceptual structure, one that works.” In other words, Rovelli believes that, while you may not be able to look at a key and determine whether it will turn a lock, if it is the right key, the door will open. And in his mind, the relational way of thinking can unlock the secrets of the universe, perhaps ultimately uniting quantum theory with gravity. “My main work is to understand quantum gravity: quantum aspects of space and time. And I think this relational way of understanding quantum mechanics is going to work much better.”
So where does dark matter come in? It will be possible to shed light on that by applying the relational ideas to black holes, he says. Stephen Hawking showed in the 1970s that a black hole slowly emits radiation, losing mass and eventually disappearing from the universe. The upshot of this is that everything it has ever swallowed must disappear too, including all the information carried in those objects. Since quantum laws don’t allow information to disappear from the universe, physicists have named this the black hole information paradox. They have fiery arguments about what exactly is going on and what the paradox tells us about the deep laws that govern physics.
Relational quantum mechanics could lead us to an answer, Rovelli says. “My hope is that, thinking about quantum mechanics in this relational way, we can get an understanding of what happens to black holes,” he says. His suspicion is that remnants of a black hole thought to remain after the process of Hawking evaporation has played out could evolve into something that resembles dark matter. He is far from certain that it will work out, but that’s OK too. “A good scientist is never sure,” he says.
Progress in quantum physics
If Rovelli is unruffled by the possibility of being wrong, he is also calmly optimistic about the future of physics, despite the many cries that the discipline has hit the rocks. It is true that it has had almost a century of failure in the hunt for dark matter, more than a century of doubts over the meaning of quantum physics and several decades of fruitless searching for a workable quantum gravity theory. But physics isn’t “stuck”, he says. “We’re making enormous progress. We have learned plenty of new experimental facts – for instance that there is no supersymmetry.”
Supersymmetry – the hypothesis that an ensemble of particles that mirror the ones we already know about remains to be detected – would have been a problem for some quantum gravity theories, says Rovelli. For him, the fact that supersymmetric particles didn’t turn up, as predicted by some, at the Large Hadron Collider was a cause for celebration, not dismay. “It is a good sign from nature saying, ‘good, good. You had the right intuition. Go!'”
Further through the gallery, we encounter another of Parker’s works: a ring of brass instruments suspended in mid-air on fine wires. Each has been flattened by a steamroller. The way they are lit, however, casts familiar-looking shadows on the walls: look only at these and you wouldn’t know that the instruments are broken. This room of unsettling, misleading perspectives seems like the right place to discuss the broader view of what we can and can’t know about our universe, so I ask Rovelli whether we will live to see a full description of reality’s core.
He takes a dim view of the question. “I don’t think that it makes sense to think there is an ultimate reality,” he says. “A forest seen from a distance is just a velvety green, but then you go closer, and you see the trees: they’re real. Then you see the trunk is real, and you see the atoms in the trunks are real. Reality is the ensemble of all these things: they’re all real, it’s just about understanding things better and better and better. We should get out of this mindset that ultimate reality is matter or language or God or mind or spirit.”
Rovelli is similarly dismissive of the search for a “final” theory. “We shouldn’t look for the ultimate theory; we should look to solve one problem at a time,” he says. “We don’t understand the quantum properties of space-time? Let’s study this. We don’t understand black holes? Let’s study this.”
It is in each of these searches, he says, that we find fulfilment and each provides further layers of understanding. “Of course we don’t understand things well yet: we don’t understand how our mind works, for instance. But whether we think of ourselves as social, psychological or chemical beings, these are all interesting perspectives, which are not in contradiction with one another.”
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