What Is Quantum Mechanics Really Telling Us? | World Science Festival

Deutsch, echoing Bryce DeWitt, asserts that what are commonly called 'interpretations' of quantum mechanics are actually different theories. Some, like the Copenhagen interpretation, are incomplete, while others, like Everett's, are full theories about reality. This distinction is crucial for scientific progress, as treating them as mere interpretations lowers the standard of argument.

The host and Deutsch discuss how the term 'interpretation' is a misnomer, with Deutsch stating he got this view from Bryce DeWitt who was 'angry about this terminology'.

Deutsch argues against the instrumentalist view that physics is solely about making predictions. He contends that the primary purpose of physics, like all science, is to understand and control the world by explaining the 'seen in terms of the unseen,' generating new ideas about what is actually happening.

Deutsch states, 'I think that is giving up on the purpose of physics. The idea, the purpose of physics is to track down the sixth and seventh decimal place of predictions. It's just absurd.'

Deutsch favors the Heisenberg picture over the Schrodinger picture for understanding quantum theory. While both are mathematically equivalent, the Heisenberg picture treats observables as fundamental and time-evolving (q-numbers), with the state being constant. This avoids the conceptual difficulties of a 'wave function collapse' and offers a more intuitive description of how outcomes arise, similar to classical physics.

Deutsch states, 'I favor using the Heisenberg picture, which is the other way around. It takes as fundamental, the observables of a physical system, exactly like classical physics does.'

Deutsch argues that theories proposing wave function collapse (like GRW) are undermined by quantum computation. These theories require a parameter for the speed or complexity of collapse, but the dividing line for this parameter would constantly change as quantum computers become more powerful, making the theory unstable and impractical.

Deutsch explains that collapse 'has to take place at a certain speed, or at a certain complexity... And the bigger the quantum computer you're contemplating... this dividing line where you've got to set the parameter is going to change every time somebody makes a better quantum computer.'

In the Everettian multiverse, where all possible outcomes occur, Deutsch posits that probability is not a fundamental feature of reality. Instead, it's a concept that emerges from decision theory. When stripped of its probability axiom, conventional decision theory, combined with quantum mechanics, dictates how rational agents should 'bet' on outcomes, yielding the same probabilities as the Born rule.

Deutsch states, 'There is no such thing as probability at a fundamental level. The world according to Everett and quantum theory is completely deterministic.' He adds, 'We only want it for making decisions. And if we apply it with conventional decision theory... then we get the answer we want.'

Deutsch's updated view on free will separates it from the multiverse or quantum indeterminism. He argues that free will is the ability to introduce genuinely new knowledge or ideas into the world (inspiration), which are not reducible to prior physical states. This emergent layer of 'explanatory ideas' operates under its own laws, distinct from the deterministic physics of particles.

Deutsch explains, 'there really is something new that you are bringing into the world when you make a decision. Not any decision... But we do sometimes bring something new into the world, new knowledge, basically.' He contrasts this with 'grunt work' that can be automated.