In a In a cubist painting, reality is more than a single perspective can capture. wikipedia, CC BY-SA

According to a school of thought known as QBism, quantum mechanics is a guide to action.

What does quantum mechanics, the most successful theory ever proposed by physics, teach us about reality? The starting point for most philosophers of physics is that quantum mechanics must somehow provide a description of the world as it is independently of us, the users of the theory.

This has led to a large number of incompatible worldviews. Some believe the implication of quantum mechanics is that there are parallel worlds as in the Marvel Comic universe; some believe it implies signals that travel faster than light, contradicting all that Einstein taught us. Some say it implies that the future affects the past.

According to QBism, an approach developed by Christopher Fuchs and me, the great lesson of quantum mechanics is that the usual starting point of the philosophers is simply wrong. Quantum mechanics does not describe reality as it is by itself. Instead, it is a tool that helps guide agents immersed in the world when they contemplate taking actions on parts of it external to themselves.

The use of the word “agent” rather than the familiar “observer” highlights that quantum mechanics is about actions that participate in creating reality, rather than observations of a reality that exists independently of the agent.

QBism and its homophone, the art movement Cubism, share the understanding that reality is more than what a single agent’s perspective can capture. However, unlike the art movement, QBism does not attempt to represent reality. It does not attempt to bring the different perspectives together in one “third-person” view. QBism is fundamentally anti-representational and first person.

Rescuing free will

This puts QBism in direct contradiction with the two pillars of the 19th-century conception of a mechanistic universe. One is that nature is governed by physical laws in the same way that a mechanical toy is governed by its mechanism. The other is that it is, in principle, possible to have an objective view of the universe from the outside – from a God’s eye or third-person standpoint.

This mechanistic vision is still dominant among 21st-century scientists. For instance, in their 2010 book The Grand Design, Stephen Hawking and Leonard Mlodinow write: “It is hard to imagine how free will can operate if our behaviour is determined by physical law, so it seems that we are no more than biological machines and that free will is just an illusion.”

Instead, the QBist vision is that of an unfinished universe, of a world that allows for genuine freedom, a world in which agents matter and participate in the making of reality. […]

Continue reading: ‘QBism’: quantum mechanics is not a description of objective reality – it reveals a world of genuine free will

Ruediger Schack

Professor of mathematics, Royal Holloway University of London