From its very inception quantum mechanics troubled physicists. It seemed to challenge our conception of reality and lead to apparent contradictions. One of the founders of quantum mechanics, Werner Heisenberg, questioned whether the theory offered a description of reality at all. Others, like Niels Bohr, claimed that somehow human consciousness played a role in the theory. In this interview, Carlo Rovelli explains Heisenberg’s anti-realist motivations, clarifies the role of the “observer” in quantum mechanics, and articulates his relational interpretation of the theory, according to which reality is a network of interactions.

The founders of quantum mechanics were very uncomfortable with its results – famously Einstein thought it an incomplete theory and quipped “God doesn’t play dice”, and Schrödinger abandoned physics altogether for biology. What was so radically different about quantum mechanics than classical physics that caused such discomfort to its own creators?

Physics used to describe what happens in a physical process. If you kick a ball and break a window, physics describes the full path of the ball from your feet to the window. Quantum theory doesn’t do so.  It only describes how your kicking the ball gives rise to the breaking of the window, without telling what happens in between, how the ball has been flying. When you try to fill-in a story of what happens in between, you get nonsense: like the ball being in two places at the same time.

Are physicists today still conscious of how radical the theory is and its wider implications for our picture of reality?  Or have we become so familiar with quantum mechanics, because of its technological applications and because it’s been around for a while now, that we don’t appreciate its radical nature?

Yes, I think that the majority of physicists are very conscious of how radical the theory is.  Some think about it, some just accept it, and use it to predict the outcome of processes, without bothering asking what the hell is really going on. But most realise that this is definitely strange. 

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Consciousness never played a role in quantum mechanics, except for some fringe speculations that I do not believe have any solid ground.

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Do you think there will ever come a time when quantum mechanics no longer puzzles us? Or will it always challenge our received picture of reality?

I think that a time will come when we have clearly understood the radical novelty of quantum theory and what it says about the world. I am optimistic that this time will be soon. But the everyday picture of reality will be challenged. This is what science has always done, like when we discovered that the Earth is round, that it spins fast, or that the chimpanzees are cousins, or that solid matter is made of atoms. It is just a process of continuously readjusting our understanding of the world. It’s not different from what happens when we see a distant forest as a uniform green and then get closer and start seeing the trees, the branches, the insects....  we learn more about the world, and in this process our previous pictures are challenged.  

SUGGESTED READING Consciousness is the collapse of the wave function By Stuart Hameroff The observer and consciousness seemed to play quite a central role in the early interpretations of quantum mechanics, but no longer. Why is that?

Consciousness never played a role in quantum mechanics, except for some fringe speculations that I do not believe have any solid ground. The notion of "observer" should not be misunderstood.  In quantum physics parlance an "observer" can be a detector, a screen, or even a stone. Anything that is affected by a process.  It does not need to be conscious, or human, or living, or anything of the sort...  

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The world is not a set of things with properties - it is a network of interactions.

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Does quantum mechanics still have an important place for the observer, more important than classical physics?

SUGGESTED READING We still don't understand the measurement problem By Sabine Hossenfelder In the example of the process where you kick a ball and break a window, the "observer" is the glass of the window. It is the physical thing that is affected by the process. In this general sense, the notion of "observer" plays a role. It is not a human observer, it is the physical system affected by a phenomenon. Quantum theory does not describe what happens during a process, but only the way the process affects physical systems, which are called "observers". 

You espouse what is known as the relational interpretation of quantum mechanics, can you give a brief account of what that is?

It is a modern refinement of the traditional interpretation developed by the founders of the theory. The idea is that what quantum theory is teaching us is that we should not think that the properties of something (for instance the kicked ball) are always defined. Rather, properties are just the way something affects something else. So, the ball has literally no properties --not even a position-- until it affects something, the glass of the windows, for instance.  This tells us that the world is not a set of things with properties. The world is a network of interactions. 

Heisenberg, one of the key pioneers of quantum mechanics, was very influenced by philosophy, and in particular an anti-realist philosophy. How did that affect his work on quantum mechanics, and the version of it that he put forward? 

Most, if not all, major physicists in history were knowledgeable about philosophy and were influenced by philosophy. Heisenberg was under the influence of Ernst Mach empirio-criticism, and thanks to that he was able to jump out from the intuitive naive realism of things with properties. I think that there are many intermediate possibilities between naive realism and radical anti-realism, and missing these is what confuses us.  

Is it true that a lot of the interpretation problems that arise in quantum mechanics stem from a realist interpretation of physics – the view that our theories are actually describing what is taking place in reality? The conundrum of the double slit experiment, for example, or Schrödinger’s Cat, both arise by trying to reconstruct a picture of what a particle is doing at any one point. If realism is the problem, why aren’t more quantum physicists espousing anti-realism, like Heisenberg?

Heisenberg’s anti-realism was limited.  At his time, quantum theory was applied only exclusively to phenomena at very small scale, and in the laboratory, like atoms and molecules. To be anti-realist about those objects for him was a way to be careful about not overextending our prejudices about reality.  He was right. Today we have learned that quantum theory applies to everything, not just small things.  It applies to stars, to the universe, to ourselves... It is harder to be anti-realist about everything. Can one be an antirealist about her of himself?  

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What we need is to correct our intuitive notion of reality from excessive rigidity, not to give it up entirely.

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The notion of reality is a powerful tool and giving it up entirely is to weaken science. But the notion of reality can be constructed in ways compatible with our current science, and in this sense there are intermediate possibilities between naive realism and anti -realism.  Missing these is what confuses us. What we need is to correct our intuitive notion of reality from excessive rigidity, not to give it up entirely. Reality is out there and we describe it.  Renouncing this is renouncing our desire to understand.  Or, to think that there is a reality out there but it is in principle inaccessible is not very interesting, in my opinion, because "reality" is the name we give to whatever we have access to, or we think we may have access to in some sense; the rest... who cares? 

The notion of reality compatible with quantum theory is a reality that it is formed by systems that do not have always properties. It is formed by systems that interact, and their properties describe what happens at interactions. We better understand the world if we assume that things happens when things interact among themselves, not only when they interact with us. We are interested in more than just our direct observations.  We are interested in a global story about reality which includes our observations as a special case.   

You have said before that philosophy can be an inspiration for physics – many of the greats, Einstein, Heisenberg, Dirac, all engaged with philosophy. But can philosophy be more than mere inspiration for physics? Can physicists really, properly engage with the claims of philosophical theories, or are the two disciplines too different from one another to have a direct impact?

I think that the impact of philosophy on physics has always been much more than a vague inspiration.  Critical analysis, reflection about methodology, alternative ways of thinking, all this has repeatedly been changing the way we do science.  Even the scientists that today disparage philosophy are repeating recent philosophical theories, without realising. Those who claim that science is a clear and self-contained method based on experiments, falsification and paradigm changes, are just repeating simplified versions of Popper and Kuhn, not even realizing that in the meanwhile philosophy of science has got to a better understanding of scientific development. The two disciplines are completely different in their tools and methods, but they have traditionally worked at best in tandem.  By the way, the opposite is true as well: the best philosophers, from Hume to Kant to Witggenstein have always been well aware and strongly affected by the scientific developments of their times. 

Philosophy is difficult to interpret, and often takes years to get one’s head around a particular thinker. Is there a danger that physicists can also be misguided by a bad reading of a philosopher? I’ve heard you say, for example, that many physicists misunderstand the claims of Thomas Kuhn about the structure of scientific revolutions.

Everything requires efforts to be understood, and can be discussed forever, whether it is Kant's philosophy, biochemistry, Beethoven’s last quartets, King Lear or the Copernican revolution. But one should not confuse the endless, and sometimes sterile, discussions about the details of something, with the core content and the power of an idea, or an art piece.  We can appreciate something and let it modify our life and thinking, without worrying about the specialists' discussions on the number of angels that can dance on the point of a pin...