Essay/Philosophy of Science

Too many worlds

Nobody knows what happens inside quantum experiments. So why are some so keen to believe in parallel universes?

Philip Ball

Illustration by Clayton Junior

is a British science writer, whose work appears in Nature, New Scientist and Prospect, among others. His latest book is Invisible: The Dangerous Allure of the Unseen (2014).

3,400 words

Edited by Ed Lake

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In July 2011, participants at a conference on the placid shore of Lake Traunsee in Austria were polled on what they thought the meeting was about. You might imagine that this question would have been settled in advance, but since the broad theme was quantum theory, perhaps a degree of uncertainty was to be expected. The title of the conference was ‘Quantum Physics and the Nature of Reality’. The poll, completed by 33 of the participating physicists, mathematicians and philosophers, posed a range of unresolved questions about the relationship between those two things, one of which was: ‘What is your favourite interpretation of quantum mechanics?’

The word ‘favourite’ speaks volumes. Isn’t science supposed to be decided by experiment and observation, free from personal preferences? But experiments in quantum physics have been obstinately silent on what it means. All we can do is develop hunches, intuitions and, yes, cherished ideas. Of these, the survey offered no fewer than 11 to choose from (as well as ‘other’ and ‘none’).

The most popular (supported by 42 per cent of the very small sample) was basically the view put forward by Niels Bohr, Werner Heisenberg and their colleagues in the early days of quantum theory. Today it is known as the Copenhagen Interpretation. More on that below. You might not recognise most of the other alternatives, such as Quantum Bayesianism, Relational Quantum Mechanics, and Objective Collapse (which is not, as you might suppose, just saying ‘what the hell’). Maybe you haven’t heard of the Copenhagen Interpretation either. But in third place (18 per cent) was the Many Worlds Interpretation (MWI), and I suspect you do know something about that, since the MWI is the one with all the glamour and publicity. It tells us that we have multiple selves, living other lives in other universes, quite possibly doing all the things that we dream of but will never achieve (or never dare). Who could resist such an idea?

Yet resist we should. We should resist not just because MWI is unlikely to be true, or even because, since no one knows how to test it, the idea is perhaps not truly scientific at all. Those are valid criticisms, but the main reason we should hold out is that it is incoherent, both philosophically and logically. There could be no better contender for Wolfgang Pauli’s famous put-down: it is not even wrong.

And yet, it attracts both publicity and extraordinarily confident endorsement. Why? To understand that, we need to see why, more than 100 years after quantum theory was first conceived, experts are still gathering to debate what it means.

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Despite its shaky foundations, quantum mechanics is extraordinarily successful. In fact you’d be hard pushed to find a more successful scientific theory. It can predict all kinds of phenomena with amazing precision, from the colours of grass and sky to the transparency of glass, the way enzymes work and how the Sun shines.

This is because it is largely a technique: a set of procedures for calculating what properties substances ought to have based on the positions and energies of their constituent subatomic particles. The calculations are hard. For anything more complicated than a hydrogen atom, it is necessary to make simplifications and approximations. But we can do that very reliably – and so the vast majority of physicists, chemists and engineers who use quantum theory today don’t need to go to conferences on the nature of reality. They can do their job perfectly well if, in the words of the physicist David Mermin, they just ‘shut up and calculate’.

It is true, though, that the equations seem to insist on some strange things. They imply that very small entities such as atoms and subatomic particles can be in several places at the same time. A single electron can seem to pass through two holes at once, interfering with its own motion as if it was a wave. What’s more, we can’t know everything about a particle at the same time: Heisenberg’s uncertainty principle forbids such perfect knowledge. And two particles can seem to affect one another instantly across immense tracts of space, in apparent (but not actual) violation of Albert Einstein’s theory of special relativity.

Before we look, there are only probabilities. When we open the box, they give way to a single actuality

Quantum scientists, for the most part, just accept such things. They are no longer especially controversial. What really divides opinion is the fact that the theory seems to do away with the idea of an objective reality that we can study ‘from the outside’. Such a notion has been central to science from its beginnings – and yet quantum mechanics insists that we can’t make a measurement without influencing what we measure. This isn’t a problem of acute sensitivity. It’s more fundamental than that. I’ll explain.

The most widely used form of quantum maths, devised by Erwin Schrödinger in the 1920s, involves an abstract object called a wavefunction. This wavefunction expresses all that can be known about a quantum object, such as a particle. But it doesn’t tell you what properties the object has. Instead, it enumerates all the possible properties it could have, along with their relative probabilities. Which of these possibilities is real? Is an electron here or there? We can find out by looking. But here’s the thing: quantum mechanics seems to be telling us that the very act of looking – of making a measurement – forces the universe to make that decision, at random. Before we look, there are only probabilities. When we open the box, those probabilities give way to a single, determinate actuality: something conventionally called collapse of the wavefunction. But wavefunction collapse isn’t actually part of the theory: it has to be put in by hand, as it were. That’s rightly considered to be most unsatisfactory.

We are left with what’s called the Measurement Problem, which really comes down to this: between the rainbow-smear of probabilities in our equations and the matter-of-fact determinacy of everything we can actually measure, what on Earth is going on?

Hence the menu of options at the Traunsee conference. The dominant view, the Copenhagen Interpretation, just shrugs and accepts wavefunction collapse as an additional ingredient of the theory, a clumsy fudge that we don’t understand but which we seem forced to make do with, at least for now. Another view is that the transition from probability to actuality isn’t just a mathematical sleight-of-hand but is in fact a concrete physical process, a little like the radioactive decay of an atom. That’s the Objective Collapse interpretation, and among its advocates is Roger Penrose, who suspects that it might involve gravity.

And then there’s the Many Worlds option – though its proponents, who include heavyweights such as Stephen Hawking and the Nobel laureate Frank Wilczek, are oddly reluctant to concede that their preferred view admits of any rivals. As far as they are concerned, the MWI is the only way of taking quantum theory seriously. It ‘should be (but is not) uncontroversial’, according to Wilczek.

The idea first appeared in the 1957 doctoral thesis of the US physicist Hugh Everett. He asked why, instead of fretting about the cumbersome nature of wavefunction collapse, we don’t just do away with it. What if this collapse is just an illusion, and all the possibilities announced in the wavefunction have a physical reality? Perhaps when we make a measurement we see only one of those realities, yet the others have a separate existence, too.

An existence where? This is where the many worlds come in. Everett himself never used that term, but in the 1970s the physicist Bryce DeWitt started championing his proposals, and it was DeWitt who argued that the alternative outcomes of the experiment must exist in a parallel reality: another world. You measure the path of an electron, and in this world it seems to go this way, but in another world it went that way.

That requires a parallel, identical apparatus for the electron to traverse. More – it requires a parallel you to measure it. Once begun, this process of fabrication has no end: you have to build an entire parallel universe around that one electron, identical in all respects except where the electron went. You avoid the complication of wavefunction collapse, but at the expense of making another universe.

This picture really gets extravagant when you appreciate what a measurement is. In one view, any interaction between one quantum entity and another – a photon of light bouncing off an atom – can produce alternative outcomes, and so demands parallel universes. As DeWitt put it: ‘every quantum transition taking place on every star, in every galaxy, in every remote corner of the universe is splitting our local world on earth into myriads of copies’. Recall that this profusion is deemed necessary only because we don’t yet understand wavefunction collapse. It’s a way of avoiding the mathematical ungainliness of that lacuna. ‘If you prefer a simple and purely mathematical theory, then you – like me – are stuck with the many-worlds interpretation,’ claims one of the view’s most prominent popularisers, the MIT physicist Max Tegmark.

That would be easier to swallow if the ‘mathematical simplicity’ were not so cheaply bought. The corollary of Everett’s proposal is that there is in fact just a single wavefunction for the entire universe. The ‘simple maths’ comes from representing this universal wavefunction as the symbol Ψ: allegedly a complete description of everything that is or ever was, including the stuff we don’t yet understand. And Many Worlders are oddly evasive about specifying exactly what constitutes a ‘measurement’ or ‘experiment’ that induces the splitting of Ψ into multiple worlds. You might sense some issues being swept under the carpet here.

In other words, you just need to broaden your mind beyond your parochial idea of what ‘you’ means

But let’s stick with it. What are these parallel worlds like? In the ‘multiverse’ of the Many Worlds view, says Tegmark, ‘all possible states exist at every instant’. That’s quite an ambiguous statement, since it might either mean all states that could evolve from some initial configuration, or all imaginable arrangements of all particles. But, either way, we face some nonsensical implications. You see, the MWI does some radical stuff to you and me.

‘The act of making a decision,’ says Tegmark – a ‘decision’ here being interchangeable with an experiment or measurement – ‘causes a person to split into multiple copies.’ Brian Greene, another prominent MWI advocate, tells us gleefully that ‘each copy is you’. In other words, you just need to broaden your mind beyond your parochial idea of what ‘you’ means. Each of these individuals has its own consciousness, and so each believes he or she is ‘you’ – but the real ‘you’ is their sum total. This means that Greene and Tegmark don’t support the MWI at all – it’s only these particular copies (and presumably some others) who do. ‘Listen to me, not them!’ Tegmark might reply. But don’t they all say that?

The Russian-Israeli physicist Lev Vaidman is one of the few supporters of the MWI to have attempted to think this through more carefully. ‘“I” is defined at a particular time by a complete (classical) description of the state of my body and of my brain,’ he explains. ‘At the present moment there are many different “Levs” in different worlds, but it is meaningless to say that now there is another “I”.’ This presumably rescues moral autonomy in the face of all those other Levs doing all the wicked deeds imaginable (and probably some that are not).

Yet it is also scientifically and, I think, logically meaningless to say that there is an ‘I’ at all in his definition, given that we must assume that any ‘I’ is generating copies faster than the speed of thought. In this view, a ‘complete description’ of the state of Lev’s body and brain never exists. Now it’s the ‘I’, rather than the collapse of a wavefunction, that is being put in by hand.

The difficulties don’t end there. It is extraordinary how attached the MWI advocates are to themselves, as if all the Many Worlds contain Xeroxed copies leading other lives. That image isn’t, however, what the idea is really about – it’s a sci-fi scenario derived from it. As Tegmark explains, the MWI is really about all possible states existing at every instant. Some of these, it’s true, must contain essentially indistinguishable Maxes doing and seeing different things. Tegmark waxes lyrical about these: ‘I feel a strong kinship with parallel Maxes, even though I never get to meet them. They share my values, my feelings, my memories – they’re closer to me than brothers.’

Most MWI popularizers think they are blowing our minds with this stuff, whereas in fact they are flattering them. They delve into the implications for personhood just far enough to lull us with the seductive uncanniness of the centuries-old Doppelgänger trope, and then flit off again. The result sounds transgressively exciting while familiar enough to be persuasive. You see, for some reason, Tegmark doesn’t trouble his mind about the many, many more almost-Maxes, near-copies with perhaps a gene or two mutated – not to mention the not-much-like Maxes, and so on into a continuum of utterly different beings. Why not? Because you can’t make neat ontological statements about them, or embrace them as brothers. They spoil the story, the rotters. They turn it into a story that doesn’t make sense, that can’t even be told. So they become the mad relatives in the attic. The conceit of ‘multiple selves’ isn’t at all what the MWI, taken at face value, is proposing. On the contrary, it is dismantling the whole notion of selfhood – it is denying any real meaning of ‘you’ at all.

In the Borgesian library of Many Worlds, it seems there can be no fact of the matter about what is or isn’t you, and what you did or didn’t do

Is that really so different from what we keep hearing from neuroscientists and psychologists – that our comforting notions of selfhood are all just an illusion concocted by the brain to allow us to function? I think it is. There is a gulf between a useful but fragile cognitive construct based on measurable sensory phenomena, and a claim to dissolve all personhood and autonomy because it makes the maths neater. In the Borgesian library of Many Worlds, it seems there can be no fact of the matter about what is or isn’t you, and what you did or didn’t do.

Compared with these problems, the difficulty of testing the MWI experimentally (which would seem necessary if it is to be considered truly scientific) is a small matter. ‘It’s trivial to falsify [MWI],’ boasts the Caltech cosmologist Sean Carroll, another supporter: ‘just do an experiment that violates the Schrödinger equation or the principle of superposition, which are the only things the theory assumes.’ But most other interpretations of quantum theory assume them (at least) too – so such an experiment would rule them all out, and say nothing about the special status of the MWI. No, we’d quite like to see some evidence for those other universes that this particular interpretation uniquely predicts. That’s just what the hypothesis forbids, you say? What a nuisance.

Might this all simply be a habit of a certain sort of mind? The MWI has a striking parallel in analytic philosophy that goes by the name of modal realism. Ever since Gottfried Leibniz argued that the problem of good and evil can be resolved by postulating that ours is the best of all possible worlds, the notion of ‘possible worlds’ has supplied philosophers with a scheme for debating the issue of the necessity or contingency of truths.

The US philosopher David Lewis pushed this line of thought to its limits by asserting that all worlds that are possible have a genuine physical existence, albeit isolated causally and spatiotemporally from ours. Such worlds, he realised, would give a clear meaning to sentences about whether or not such-and-such a thing might have been different, or gone differently. Could Neil Armstrong have become a bus driver instead of an astronaut, for example? On the face of it, such sentences are meaningful – it seems like claims about alternative possibilities ought to be true or false in some straightforward sense. But in this world, we see only actualities, not alternative possibilities. What, then, makes statements about ‘what might have been’ true? Odd as it sounds, this has been a vexing puzzle in philosophy. But according to Lewis’ framework, something is possible if it happens in at least one world, and impossible if it happens in no worlds. A tidy solution – so tidy, indeed, that he concluded that the other worlds were real.

It allows worlds where gods, magic and miracles exist and where science is violated by chance breakdowns of the statistical regularities

Many philosophers regard this as legerdemain, and yet the similarities with the MWI of quantum theory are clear: the proposition stems not from any empirical motive but simply because it allegedly simplifies matters. (And Lewis’ modal realism does at least have the virtue that he thought in some detail about the issues of personal identity it throws up.) In what might be a case of convergent intellectual evolution, Tegmark’s so-called Ultimate Ensemble theory – a many-worlds picture not explicitly predicated on quantum principles but still including them – has been interpreted as a mathematical expression of modal realism, since it proposes that all mathematical entities that can be calculated in principle (that is, which are possible in the sense of being ‘computable’) must be real. In any event, both ideas display a discomfort with arbitrariness in the universe, and both stem from the same human impulse that invents fictional fantasies about parallel worlds and that enjoys speculating about counterfactual histories.

Which is why, if I call these ideas fantasies, it is not to deride or dismiss them but to keep in view the fact that, beneath their apparel of scientific equations or symbolic logic, they are acts of imagination, of ‘just supposing’. But when taken to the extreme, they become a kind of nihilism: if you believe everything then you believe nothing. The MWI allows – perhaps insists – not just on our having cosily familial ‘quantum brothers’ but on worlds where gods, magic and miracles exist and where science is inevitably (if rarely) violated by chance breakdowns of the usual statistical regularities of physics.

Certainly, to say that the world(s) surely can’t be that weird is no objection at all; Many Worlders harp on about this complaint precisely because it is so easily dismissed. MWI doesn’t, though, imply that things really are weirder than we thought; it denies us any way of saying anything, because it entails saying (and doing) everything else too, while at the same time removing the ‘you’ who says it. This does not demand broad-mindedness, but rather a blind acceptance of ontological incoherence.

That its supporters refuse to engage in any depth with the questions the MWI poses about the ontology and autonomy of self is lamentable. But this is (speaking as an ex-physicist) very much a physicist’s blind spot: a failure to recognise – or perhaps to care – that problems arising at a level beyond that of the fundamental, abstract theory can be anything more than a minor inconvenience.

If the MWI were supported by some sound science, we would have to deal with it – and to do so with more seriousness than the merry invention of Doppelgängers to measure both quantum states of a photon. But it is not. It is grounded in a half-baked philosophical argument about a preference to simplify the axioms. Until Many Worlders can take seriously the philosophical implications of their vision, it’s not clear why their colleagues, or the rest of us, should demur from the judgment of the philosopher of science Robert Crease that the MWI is ‘one of the most implausible and unrealistic ideas in the history of science’. Here, after all, is a theory that seems to allow everything conceivable to happen. To pretend that its only conceptual challenge is that it leads to scenarios like the plot of Sliding Doors (1998) shows a puzzling lacuna in the formidable minds of its advocates. Perhaps they should stop trying to tell us that philosophy is dead.

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