Quantum mechanics started out by saying access to fundamental reality was impossible. And from the outset, some imagined that quantum mechanics would eventually eliminate the importance of human measurement in determining reality. Yet a century on, the measurement problem still refuses to disappear, and the new generation of quantum technologies, from quantum computers to quantum sensors, only makes it harder to ignore. As theory becomes engineering, Paul Davies argues that every attempt to “save” traditional views of realism fail, and that physics can no longer avoid confronting the role of observation in the structure of reality itself.

Omari Edwards: Quantum mechanics is a century old this year, and you have been working in the field your whole career. Why did it feel like the right moment now to write a book explicitly about quantum mechanics and call it Quantum 2.0?

Paul Davies: I’ve worked on quantum mechanics my entire career, but I’d never actually sat down and written a book about it. This year marks one hundred years since the birth of quantum mechanics, and that seemed like a good enough reason in itself. But on top of that, over the last few years it has become very clear that the entire field is about to undergo a stupendous transformation.

The transition from classical physics to quantum mechanics a century ago was a dramatic change in how we understood the world, and it reshaped the technologies that followed. Now we are on the cusp of what I call “Quantum 2.0”. It promises to be just as transformative, not only in our understanding of reality, but in the commercial and technological consequences that will flow from it.

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Quantum mechanics does not uncover facts that were already there. Measurement brings reality into being.

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For a reader coming to this cold, what does quantum mechanics really change about our picture of reality, compared with the common-sense view of solid tables and chairs that exist whether we look at them or not?

The thing that will probably surprise people most is what quantum physics tells us about the nature of reality. We tend to think that objects like tables and chairs really exist out there in the world, whether anyone looks at them or not. But if you keep zooming in, that picture dissolves. At the atomic level, solid objects turn into vibrating patterns of energy. Everything becomes fuzzy and uncertain.

The uncertainty is built into the laws of physics themselves. You can know everything there is to know about a system and still not be able to predict reliably what will happen next. Quantum mechanics does not tell you what will happen, it gives you the betting odds.

People often think this is no different from a coin toss or the weather, where we also deal in probabilities. But in those cases, the uncertainty is only due to ignorance. If you knew every detail about a tossed coin, you could work out which way it would land. Quantum uncertainty is different. It cannot be eliminated, no matter how much information you have. It is irreducible.

That means reality itself is not fully settled until something is observed. If you ask where an atom is and you measure it, you will find it in a definite place. But quantum mechanics says it was not already there waiting to be discovered. Before the measurement, the very idea of position has no meaning. The atom existed in a blurred state containing many possible outcomes at once, and the act of observation brings one of them into being. This is the Measurement Problem.