Emily Adlam & David Wallace: The Quantum Interpretation That Divides Physicists

**SPEAKER_1** (0:01)
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**David Wallace** (0:25)
You disagree about whether we're living in one of the vast number of palagins.

**Emily Adlam** (0:29)
It doesn't make sense to believe in interpretation of quantum mechanics, which says we shouldn't believe quantum mechanics.

**Curt Jaimungal** (0:34)
This is David Wallace, Professor of Philosophy of Science at the University of Pittsburgh and one of the leading philosophical defenders of the many worlds interpretation. Years prior, David supervised Emily's undergraduate thesis at Oxford.

**David Wallace** (0:48)
I think it's much more likely we'll learn something about epistemology from resolving that than learn something about physics.

**Curt Jaimungal** (0:53)
Professor Emily Adlam, who's also been on the podcast before, argued that no one inside many world could ever confirm many worlds. Both believe the philosophy of physics is just as important as the physics itself.

**Emily Adlam** (1:05)
I don't know which interpretation is right.

**Curt Jaimungal** (1:07)
On this channel, I, Curt Jaimungal, interview researchers regarding their theories of reality with rigor and technical depth. Today, I bring you Emily Adlam and David Wallace's first joint interview starting with one of the deceptively simplest questions in quantum mechanics.
What is the wave function? We'll start with you, David.

**David Wallace** (1:25)
It's the representational tool in quantum mechanics that encodes the physical features systems have. So, there's a take on the wave function that just says it's epistemic, but the take I want to say is it's not a thing in itself, that's a category error, but it encodes the properties things have and the relations they stand in.
For a given system, the wave function of that system just encodes all the properties that system has.

**Curt Jaimungal** (1:56)
Emily, do you broadly agree?

**Emily Adlam** (1:59)
To some extent. I think there are two types of roles that states tends to play in physics, and one is to represent the current properties of the system at a time, and another is to encode predictions about how the system is going to behave in the future, and at least in classical physics, we typically expect those things to come together, that the predictions are just given by the current properties at a given time.
I think there are reasons to think that in quantum mechanics, that might not be the case, and so we might want to make more of a distinction between the properties that a system currently has and the prediction we want to make about its future. For that reason, I'm slightly sceptical about whether we should assume that the wave function represents properties or whether I just rather represent the state of predictions.

**Curt Jaimungal** (2:44)
People ask a question like, what is the wave function ontologically? Does it exist? Is there such a thing as a universal wave function? Is that all that there is? I know there are some views that all that there is, is this universal wave function evolving forward, but there are two different uses of the word universal wave function.

**David Wallace** (3:00)
There's a bunch of things mixed together there, and I should say instantly just to clarify where I'm coming from. I'm answering these questions flat out from the Everettian perspective, and I think it's probably right. If I was teaching a general philosophy of quantum mechanics class, I'd be a lot more ecumenical about what I think the wave function is.
I'm a bit more dogmatic as to what I think the right interpretation is there, Emily, so I'm probably willing to be a bit more committal there. So I would say that to ask what the wave function is ontologically, for everyone, is kind of the wrong question to ask. It's not a thing, or at least to suppose it's a thing, is a big substantive extra step. There's nothing in even very realist approaches to quantum mechanics to say that the wave function itself is a thing. If a representational reading of it's right, then it encodes properties things have.
To go to Emily's comment, if some more influential reading of it's right, then it encodes something else. But the point is, it's a tool we use in our theories either to represent or to predict or to infer or whatever. So then the question is, the universal wave function is, can we talk about the universe as a whole, as a quantum system with properties that can be represented? Then I think the answer is probably yes, in the sense that we do so fairly effectively when we're doing quantum cosmology. It's not what we normally do, but we can do it if we want to.