Which interpretation of quantum mechanics is the best?

Rowan Berkley took on Iain's question with the help of quantum physicist Maria Violaris...

Rowan - Thanks, Iain. Quantum mechanics, or quantum physics, is the branch of physics that describes the behaviour of matter and energy at the smallest scales, atoms and subatomic particles, like electrons. Interpretations of quantum mechanics are the big-picture takeaways from the things it theoretically tells us. Dr Maria Violaris joins us today. She is a quantum research scientist, science communicator, and content creator on her YouTube channel.

Maria - Superposition is one of the really fundamental and interesting phenomena. So that's this phenomenon that a system described by quantum mechanics can be in two states at the same time. What happens when you observe a system is that you just see one of those states. So you collapse the superposition. That's the terminology that we use. The other one that I'd highlight is entanglement. Let's say you've got two people. One's called Alice, and she's got one of these particles. The other person is called Bob, and he's got another one of these particles. If Alice and Bob each individually take measurements of their particle, they can get correlations between their measurements that are much stronger than anything that you can get with classical physics. It's much more powerful than, say, how a pair of socks is correlated when you move them far apart.

Rowan - Interpretations of quantum mechanics are the theories of what these phenomena mean for reality.

Maria - So what happens in the Schrödinger's cat thought experiment, you've got a cat that is in a superposition of being dead and alive, but then when you observe it, when you open the box, which has this cat inside, you just see it dead or you just see it alive. You only see a single state. How did that collapse actually happen? I think it's most useful to divide them into two categories. One theory which says that when you observe the cat, it kind of irreversibly collapses into just one state, like dead or alive. That's known as the Copenhagen interpretation. And the other set of interpretations is the idea that the measurement of the cat or the measurement of a superposition is actually a reversible process. The many-worlds interpretation of quantum mechanics says that because you're applying quantum mechanics to the observer that looks at the cat as well, the observer is actually joining in that superposition and the observer has one version of themselves that sees the cat dead and one version that sees the cat alive. Really, these are kind of two categories of entire families of interpretations.

Rowan - What about the recent experiments or advances that Iain is referring to in this question?

Maria - I know lots of people were asking something similar recently when the Google Willow chip was announced. It essentially showed that something called quantum error correction is possible. And they showed that you can do certain computations on this chip that would take longer than the age of the universe to do on a classical computer. If you've done a computation on this chip that would have taken a classical computer longer than the age of the universe, even if it was using all the atoms in the universe, then physically, where did this computation take place? Like what was physically doing it? As a response, you could say, well, this shows that the multiverse interpretation is true because - as opposed to an interpretation where there's a collapse into a single world or these different versions of reality don't actually exist - you say, well, they must exist because otherwise, where did that power come from? Where was it happening? I think that the progress in our interpretations and in physics is never finished. So I don't really see a final, true interpretation as coming out because I think that there's a theory beyond quantum theory that we're going to discover at some point and that might change our conception of the world again.

Rowan - So Iain, recent quantum experiments and discoveries do not lead us to a definitive correct interpretation, but recent advances in computing power could be an argument for the many-worlds or multiverse interpretation.