The relational interpretation of quantum physics

A reaction by a reader of my book mentioned that she didn’t find any mention of Carlo Rovelli’s relational quantum physics interpretation. Indeed. So I’ll repair that here. Rovelli sets his idea of relational quantum physics out in his recent book, Helgoland – Making sense of the Quantum Revolution. A book that should be read by everyone interested in the puzzling aspects of quantum physics. Rovelli is a good storyteller quite capable of keeping such a difficult subject interesting for the lay reader.

To me, his relational interpretation seems to say that all the material objects, that physics is about, only exist in relation to each other. They need each other to manifest their physical qualities. Without each other they are literally nothing.

The observer effect

With his relational interpretation Rovelli tries to explain the observer effect in quantum physics. That is the effect that the measurement of a quantum object gives the object its physical properties, such as location, speed en energy, which is the so-called quantum collapse. And also the effect that the way of observation determines in what way the objects will manifest itself. Before measurement the quantum objects has no physical properties. It does not exist in a material sense. It’s not there, not yet. Now you should realize that all objects in the world, including our own bodies, are in fact quantum objects.

That this is the case, has gradually become an inescapable conclusion for most physicists. The so-called delayed choice experiments have confirmed that before the measurement the measured particle does not yet exist physically. For a description of such an experiment carried out in 2007, I refer to my book, chapter 7 paragraph ‘The quantum eraser experiments’, or to the published article ‘Random Delayed-Choice Quantum Eraser via Two-PhotonImaging‘. Rovelli, in the introduction of his book, looking out at the sea, philosophizes with a colleague about this seeming absurd aspect of reality.

The hand in the interferometer

Rovelli describes in his book – chapter II around pages 45-46 – the surprising effect his hand evokes when he holds it in the path of a beam of photons in a Mach-Zehnder interferometer. This is a device where the incoming photons can travel along two different trajects that meet each other again at a beam splitter where interference occurs. If the device is properly configured and the photon can ‘travel’ unhindered along both roads, it turns out that interference happening at the second beam splitter causes the photons to be able only to leave the device in the direction of detector D1. It will only be detector D1 which detects photons. Detetector D2 detects nothing.

Mach-Zehnder interferometer, both trajects have equal lengths. All photons go to D1.

But when Rovelli blocks one of the two beam paths with his hand, and blocks thus half the photons, the unblocked photons, which have therefore traveled the lower path, suddenly arrive evenly distributed at both detectors.

Mach-Zehnder with hand blocking the upper path.

The photons that reach the second mirror apparently ‘know’ that Rovelli’s hand blocks the other path and can now freely choose between the two detectors. The big question is, how do they ‘know’ that?

Everything connected?

If you want to hold on to the image of a permanent objective world outside of us, then you can’t do much other than assuming that quantum objects are somehow connected to each other, that they have a relationship. Thus arrives Rovelli at his relational interpretation of quantum physics. But if you think a little further, then you will hopefully realize that this is a rather obfuscating technical term for the idea that everything is connected. And that is precisely the message that has reached us time and time again through Indian wisdom traditions, mystics, seers and – not unimportant – through reports of near death experiences.

The idea of a universe where objects only exist in relation to each other explains here the so-called quantum collapse, caused by the measuring instrument and also the undeniable observer effect that quantum physicists have been dealing with since the beginning of the last century. It is then not the awareness of the observer, but the fact that the observer is also a composition of quantum objects, which explains the observer effect. In my opinion, this is a variation of panpsychism, which says that everything is conscious. If you assume that everything is connected and ‘know’ where all the other objects are in the universe, which means awareness, then I do think the step is small to idealism, the idea that everything is actually happening within consciousness, a idea promoted these days strongly by – among many others – Bernardo Kastrup. Idealism is much simpler than making every separate object aware of all the others and therefore easier to understand than Rovelli’s panpsychism. Which does not mean that I even remotely understand what consciousness is, what it does and why, even though I experience it almost every moment.

Paul J. van Leeuwen graduated in applied physics in Delft TU in 1974. There was little attention to the significance of quantum physics for the view on reality at that time. However, much later in his life he discovered that there is an important and clear connection between quantum physics and consciousness. What he learned between then and today resulted in a post academic course in quantum physics for non-physicists. A little bit later he decided to put the contents of that course, and more, in a book published in Dutch: Kwantumfysica, Informatie en Bewustzijn - and started a website on the subject. You can also find a Dutch version of his website on He translated the Dutch version of his book in English: 'Quantum Physics is NOT Weird'.

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