‘THE PARTICLE WORLD IS NOT A TOTALLY DIFFERENT REALITY’ According to quantum mechanics a particle is no longer in one single place, and entangled particles seem to know what state the other particle is in. Incomprehensible? Not if you accept that such particles are entirely defined by their interactions, argues Italian physicist Carlo Rovelli.
Text: Martijn van Calmthout Illustration: Pepijn Barnard
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n his latest book Helgoland, Italian physicist Carlo Rovelli takes the reader back to the eponymous birthplace of quantum physics: a treeless German North Sea island where young physics student Werner Heisenberg took refuge from hay fever in 1925. He came back with a theory of the atom that we still enjoy today. But that pleasure comes at a price, Rovelli says from his study in Toronto, Canada. ‘Quantum physics has not only brought us microelectronics, computers, molecular biology, nanochemistry and much more. It has also brought the realisation that our intuition does not work at all in the world of the very small. Quantum reality is incomprehensible. In my book I try to show that we are beginning to understand why.’ Carlo Rovelli. Italian by birth. Theoretical physicist. A friendly sixty-something with twinkling eyes and curls. About five years ago, he became world famous with his bestseller Seven Brief Lessons on Physics. A reassuringly thin, somewhat philosophical work on what we know about the foundations of reality.
16 | New Scientist | Leiden2022
Heligoland is thicker. Even more philoso phical. But it is also meant to be reassuring, Rovelli says. ‘The key message is: the particle world is no crazier than the everyday world and we can deal with it like adults. It is like the modern solar system. We see the sun moving across the sky every day, rising and setting. Yet we know: it is not the sun that moves, it is us. And no one is upset about that.’ What makes quantum physics so elusive?
‘It is that we think classically about the world of particles and atoms. You cannot do that. It goes back to Heisenberg on Helgoland. He tried to find order in the energy jumps of electrons in atoms, which we see thanks to the spectral lines in their light. How could electrons make only those steps? Heisenberg found the formulas that precisely describe the process. Provided, and there the incomprehensible begins, that numbers are replaced by matrices – by grids of numbers.’ What did that insight change?
‘Everything. Quantities are no longer fixed, as with Newton; there are only transitions between states and probabilities. This leads
mathematically to particles that are no longer in one place, but everywhere. To entangled particles that seem to know what state the other is in. To a cat in a box that is both awake and asleep – I don’t like to kill Schrödinger’s cat. In short, the famous quantum magic. And it’s very different from what objects do in everyday reality.’ Particles that are also waves.
‘It is often put that way, yes. But an electron is not one or the other. It manifests itself as one or the other, depending on the perspective; in the interaction with the researcher.’ There are quite a lot of interpretations in physics that aim to contain the craziness of the quantum world. You don’t like most of them. Why?
‘Take the popular many-worlds interpretation, in which reality continually splits
HOW MANY
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