Why Philosophy of Quantum Mechanics Is More Important Than That of Poached Eggs
Interview by Richard Marshall.
‘I think the scientists who are unhappy with metaphysics generally have a rather narrow view of what metaphysics is – that it’s speculation, unconstrained by empirical findings, angels on the head of a pin stuff. I’m not saying that doesn’t go on. But there is such a thing as empirically informed metaphysics. If you want to find out about the nature of the physical world, then sure, look to physics. But don’t expect a physics textbook to provide all the answers.‘
‘Defenders of ontological vagueness typically associate vagueness with composition – for a composite object like a mountain, it can be vague which hunks of matter are parts of it and which are not. But on some understandings of the quantum world, even the location of a single particle can be a vague matter. ‘
Peter Lewis has been the recipient of an NSF Scholar’s Award, and a Visiting Fellow at Sydney University and the Australian National University. His research interests are in philosophy of science, especially philosophy of physics, scientific realism and scientific methodology. He has published articles on the foundations of quantum mechanics and on scientific realism. Here he discusses quantum mechanics, its metaphysical implications, underdetermination, why it might present a new kind of vagueness, why he’s not an Everettian, whether the 3-D world is an illusion and what he thinks about pessimism about scientific theories. Vavoom…
3:AM: What made you become a philosopher?
Peter Lewis: A bunch of things came together. I was a fairly directionless physics undergraduate. I certainly wasn’t cut out to be an experimentalist. In my third year, I was assigned Nick Huggett as a tutorial partner, and he was applying to philosophy graduate school, which seemed very cool. At the same time, I met Amie Thomasson – the love of my life. She was a philosophy student, and her work always seemed much more interesting than mine (and without all those pesky labs). After graduating, I got a job as a programmer and started applying to Ph.D. programs in philosophy.
3:AM: You’ve written about some of the philosophically interesting questions raised by quantum mechanics. So before we start, could you just sketch for us what quantum mechanics is, and what it is supposed to be explaining?
PL: Historically speaking, quantum mechanics was developed as a theory of matter at small scales – a theory that explains the behavior of atoms, electrons and so forth. The theory that was developed in the 1920s has a number of surprising features. Magnitudes that previously we took to be continuous – like the energy of an electron in an atom – turn out to only take certain discrete values. And any two distinct states of a system can be superposed – added together – to get a third state of the system. The resulting theory is highly successful — it accurately models the behavior of particles, atoms and molecules, and underwrites the operation of all our electronic devices.
But even though quantum mechanics was developed to explain the behavior of small things, it purports to be a perfectly general theory of matter, at all scales. And this is where difficulties start to arise. If you measure a particle that is in a superposition of distinct states, and you use quantum mechanics to model the measuring device as well as the particle, the theory entails that you should get a superposition of distinct measurement outcomes. But what we in fact observe is one outcome or the other. Why is that? Is it that we were somehow wrong to think that quantum mechanics applies to systems at all scales? If so, why? If not, why don’t we see superpositions everywhere? That is, even though quantum mechanics is a highly successful predictive theory, we don’t really have a firm grasp on the way it explains our observations.
3:AM: Now it’s a scientific theory but it raises metaphysical questions doesn’t it. Some scientists aren’t happy with metaphysics – so why aren’t they and do you think they ought to be?
PL: I think the scientists who are unhappy with metaphysics generally have a rather narrow view of what metaphysics is – that it’s speculation, unconstrained by empirical findings, angels on the head of a pin stuff. I’m not saying that doesn’t go on. But there is such a thing as empirically informed metaphysics. If you want to find out about the nature of the physical world, then sure, look to physics. But don’t expect a physics textbook to provide all the answers. In particular, quantum mechanics is highly metaphysically opaque. So there’s philosophical work to be done in unpacking the metaphysics of physical theories.
It’s ironic that in The Grand Design, after declaring philosophy dead, Stephen Hawking goes on to tell us what the world is like according to recent scientific theories – to engage in empirically-informed metaphysics. That’s fine – philosophers don’t have a monopoly over metaphysics. But neither do physicists have a monopoly over the interpretation of physical theories.
3:AM: Why are the metaphysical implications of quantum mechanics more important than the metaphysical implications of poached eggs? Is it because it’s a revisionist theory of physics, so its importance lies in the way it supplants classical mechanics?
PL: That’s right. It’s easy to presuppose classical mechanics in our metaphysics – to picture fundamental physical reality in terms of arrangements of particles. Quantum mechanics suggests that that kind of picture might be radically mistaken – there might be no particles, or even a straightforward sense in which stuff is arranged in space. Poached eggs are nice, but rarely surprising.
3:AM: What are the metaphysical implications of interference and entanglement? On the face of it they seem to suggest a very wild ontological picture like something out of Dr Strange – so wouldn’t it be best to shut down any talk of ontology and stick to predictive powers only?
PL: Interference suggests that matter is (at least in part) a spread-out wave-like thing, as opposed to being made only of discrete particles. Entanglement suggests a kind of holism – that there are properties of collections of things that aren’t reducible to the properties of their parts. Both of these implications are surprising, but is seems a bit extreme to give up on ontology altogether at this point. What fun is that?
3:AM: You insist on a realistic account of the world falling out of quantum mechanics. You suggest three incompatible ways of understanding that reality – how can we make a choice between them if the empirical evidence underdetermines the theory?
PL: My hope is that the underdetermination is just a temporary thing. Collapse and no-collapse understandings can in principle be empirically distinguished – and hopefully eventually in practice. The two most prominent no-collapse approaches – hidden variable theories and many worlds theories — are trickier: there’s a sense in which they make the same predictions. But even here there’s hope of an empirical answer, I think. It’s hard to see how to reconcile hidden variable theories with special relativity – and if we can’t do that, then hidden variable theories are ruled out on the empirical grounds that they can’t be applied to fast-moving objects. On the other hand, it’s hard to see how to generate probabilistic predictions in a many worlds theory, because such theories say that every outcome of an experiment actually occurs. If we can’t do that, then many worlds theories are ruled out on the empirical grounds that they don’t generate the observed probabilities of measurement outcomes. If we’re lucky, at the end of the day we’ll be left with precisely one understanding left standing.
3:AM: Does quantum mechanics mean that the world is vague? Usually vagueness is approached as a semantic matter – but should we be discussing it in terms of ontology instead/as well?
PL: I’m not sure that quantum mechanics gives you any new arguments for ontological vagueness. But if you were inclined to see vagueness in terms of ontology anyway, then quantum mechanics gives you a new kind of vagueness to think about. Defenders of ontological vagueness typically associate vagueness with composition – for a composite object like a mountain, it can be vague which hunks of matter are parts of it and which are not. But on some understandings of the quantum world, even the location of a single particle can be a vague matter. Unless you accept a hidden variable theory, then the particle is represented by a wave packet, and a wave packet has fuzzy edges. (if you do accept a hidden variable theory like Bohm’s, then particles have precise positions, but their other properties will typically have this wave-like vagueness.)
3:AM: What does follow from quantum mechanics in terms of determinism and probability?
PL: Not a lot! This is where the underdetermination you brought up above has bite. Bohm’s hidden variable theory is completely deterministic, just like classical mechanics. The GRW spontaneous collapse theory is fundamentally indeterministic – there are irreducible probabilities in the laws themselves. And Everettian many-worlds theories are hard to categorize: on the one hand, the laws governing the physical state are completely deterministic, but on the other hand, the “branching” of the quantum state into multiple worlds seems to be a source of some kind of indeterminism, although the precise sense of indeterminism is hard to put your finger on. It certainly wouldn’t be right to say that quantum mechanics entails indeterminism.
3:AM: You think there’s no good reason to treat probability differently when trying to understand quantum sleeping beauty do you? What is this, why is probability treated differently and what happens when you don’t? And are you an Everettian?
PL: No, I’m not an Everettian, mainly because I still can’t see my way to finding probabilities in a many-worlds theory. If every outcome of a quantum measurement actually occurs, each in a different branch of reality, then probability must somehow be grounded in self-location: after a measurement, each of my successors can be uncertain about which branch they occupy, and probabilities can measure this uncertainty. The point of my quantum Sleeping Beauty paper was to explore the analogy between the branch-location uncertainty in the quantum case and the temporal location uncertainty induced my memory loss in the Sleeping Beauty case. My claim was that if you treat the two kinds of uncertainty in a parallel way, then an Everettian has to be a halfer about sleeping beauty (the unpopular position!). This particular argument has been successfully refuted (by Groisman, Hallakoun and Vaidman, Analysis 2013). But I’m still not convinced that self-location uncertainty can generate the probabilities we need to do quantum mechanics.
3:AM: Is the three dimensional world an illusion?
PL: No. It’s true that one of the standard representations of the quantum state – the wave function – is a function of 3N coordinates, where N is the number of particles in the system concerned. David Albert uses straightforward realism about the quantum state of the universe to infer that the world is 3N-dimensional, where N is the number of particles in the universe. Albert concludes that the three-dimensional world is an illusion – but this is a rhetorical flourish, and at the end of the day he doesn’t really mean it. After all, just because a theory is most easily expressed in a 3N-dimensional space, it doesn’t follow that the theory represents a 3N-dimensional world. The parameters in the representation might be doing something else.
3:AM: So does quantum mechanics require holism, the idea that wholes can’t be reduced to the properties of their parts?
PL: I’m generally skeptical of “quantum mechanics requires X” claims, because of underdetermination. But this one is quite plausible. Each of the three major ways of understanding quantum mechanics appeals to irreducible properties of wholes in its explanation of the behavior of entangled systems. So this looks like a fairly robust conclusion. Still, it can be resisted. For example, if you accept a retrocausal interpretation of quantum mechanics, according to which entanglement phenomena are explained by information traveling backwards in time, then no appeal to holistic properties is required.
3:AM: What does that tell us about the ontological priorities of parts and wholes and about the existence of localised individuals?
PL: Not much, I think. Jonathan Schaffer shows how to avoid irreducible properties by turning ontology on its head – if the whole is ontologically prior to its parts, then you reduce the properties of the parts to the properties of the whole, and you can do this even for entangled systems. But I’m not sure what the motivation is for such a radical move. What’s wrong with irreducible properties?
As for localized individuals, it’s underdetermined again. Bohm’s hidden variable theory has localized particles, but spontaneous collapse theories and many-worlds theories do not. In the latter cases, sometimes the wave-like underlying reality bunches up into a well-defined entity we can follow for a while, and in this sense particles exist as emergent entities. But they’re not fundamental.
3:AM: Putnam and Lauden both argue for a pessimistic view that because most scientific theories in the past have been false we shouldn’t be confident that today’s won’t be false either. And that would include quantum mechanics. Why do you think this is a fallacy?
PL: Well, my original argument was that the pessimistic induction embodies the base rate fallacy. Even if Laudan is right that most successful past theories turned out to be false, it doesn’t follow that success is an unreliable guide to truth – it all depends on the rate of false theories in the past and today. Put another way, how would we expect things to look if science were in fact converging on the truth? We’d expect a bunch of false starts, many of which were successful in a limited way – in other words, exactly how the history actually looks.
So as far as realism about the quantum world goes, I’m not worried by the pessimistic induction. Underdetermination, on the other hand, is a worry. Hence my hope that it will go away. But at least with underdetermination we have a range of possible realist world pictures.
3:AM: And finally, are there five books you could recommend (other than your own of course which we’ll all be dashing away to read) that will take us further into your philosophical world?
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First published in 3:AM Magazine: Saturday, February 11th, 2017.