Here is the pre-edited version of my article for Aeon on the Many Worlds Interpretation of quantum theory. I’m putting it here not because it is any better than the published version (Aeon’s editing was as excellent and improving as ever), but because it gives me a bit more room to go into some of the issues.
In my article I stood up for philosophy. But that doesn’t mean philosophers necessarily get it right either. In the ensuing discussion I have been directed to a talk by philosopher of science David Wallace. Here he criticizes the Copenhagen view that theories are there to make predictions, not to tell us how the world works. He gets a laugh from his audience for suggesting that, if this were so, scientists would have been forced to ask for funding for the LHC not because of what we’d learn from it but so that we could test the predictions made for it.
This is wrong on so many levels. Contrasting “finding out about the world” against “testing predictions of theories” is a totally false opposition. We obviously test predictions of theories to find out if they do a good job of helping us to explain and understand the world. The hope is that the theories, which are obviously idealizations, will get better and better at predicting the fine details of what we see around us, and thereby enable us to tell ever more complete and satisfying stories about why things are this way (and, of course, to allow us to do some useful stuff for “the relief of man’s estate). So there is a sense in which the justification for the LHC derided by Wallace is in fact completely the right one, although that would have been a very poor way of putting it. Almost no one in science (give or take the [very] odd Nobel laureate who capitalizes Truth like some religious crank) talks about “truth” – they recognize that our theories are simply meant to be good working descriptions of what we see, with predictive value. That makes them “true” not in some eternal Platonic sense but as ways of explaining the world that have more validity than the alternatives. No one considers Newtonian mechanics to be “untrue” because of general relativity. So in this regard, Wallace’s attack on the Copenhagen view is trivial. (I don’t doubt that he could put the case better – it’s just that he didn’t do so here.)
What I really object to is the idea, which Wallace repeats, that Many Worlds is simply “what the theory tells you”. To my mind, a theory tells you something if it predicts the corresponding states – say, the electrical current flowing through a circuit, or the reaction rate of an enzymatic process. Wallace asserts that quantum theory “predicts” a you seeing a live Schrödinger’s cat and a you seeing a dead one. I say, show me the equation where those “yous” appear (along with the universes they are in). The best the MWers can do is to say, well, let’s just denote those things as Ψ(live cat) and Ψ(dead cat), with Ψ representing the corresponding universes. Oh please.
Some objectors to my article have been keen to insist that the MWI really isn’t that bizarre: that the other “yous” don’t do peculiar things but are pretty much just like the you-you. I can see how some, indeed many, of them would be. But there is nothing to exclude those that are not, unless you do so by hand: “Oh, the mind doesn’t work that way, they are still rational beings.” What extraordinary confidence this shows in our ability to understand the rules governing human behaviour and consciousness in more parallel worlds than we can possibly imagine: as if the very laws of physics will make sure we behave properly. Collapsing the wavefunction seems a fairly minor sleight of hand (and moreover one we can actually continue to investigate) compared to that. The truth is that we no nothing about the full range of possibilities that the MWI insists on, and nor can we ever do so.
One of the comments underneath my article – and others will doubtless repeat this – makes the remark that Many Worlds is not really about “many universes branching off” at all. Well, I guess you could choose to believe Anonymous Pete instead of Brian Greene and Max Tegmark, if you wish. Or you could follow his link to Sean Carroll’s article, which is one of the examples I cite in my piece of why MWers simple evade the “self” issue altogether.
But you know, my real motivation for writing my article is not to try to bury the MWI (the day I start imagining I am capable of such things, intellectually or otherwise, is the day to put me out to grass), but to provoke its supporters into actually addressing these issues rather than blithely ignoring them while bleating about the (undoubted) problems with the alternatives. Who knows if it will work.
In 2011, participants at a conference on the placid shore of Lake Traunsee in Austria were polled on what the conference was about. You might imagine that this question would have been settled before the meeting was convened – but since the subject was quantum theory, it’s not surprising that there was still much uncertainty. The conference was called “Quantum Physics and the Nature of Reality”, and it grappled with what the theory actually means. The poll, completed by 33 of the participating physicists, mathematicians and philosophers, posed a range of unresolved questions, one of which was “What is your favourite interpretation of quantum mechanics?”
The mere question 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, favourite ideas.
Which interpretations did these experts favour? There were no fewer than 11 answers to choose from (as well as “other” and “none”). The most popular (42%) was the view put forward by Niels Bohr, Werner Heisenberg and their colleagues in the early days of quantum theory, now known as the Copenhagen Interpretation. In third place (18%) was the Many Worlds Interpretation (MWI).
You might not have heard of most of the alternatives, such as Quantum Bayesianism, Relational Quantum Mechanics, and Objective Collapse (which is not, as you might suppose, saying “what the hell”). Maybe you’ve not heard of the Copenhagen Interpretation either. But the MWI is the one with all the glamour and publicity. Why? Because 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 that idea?
Yet you should. You should resist it not because it is unlikely to be true, or even because, since no one knows how to test it, the idea is not truly scientific at all. Those are valid criticisms, but the main reason you should resist it is that it is not a coherent idea, philosophically or logically. There could be no better contender for Wolfgang Pauli’s famous put-down: it is not even wrong.
Or to put it another way: the MWI is a triumph of canny marketing. That’s not some wicked ploy: no one stands to gain from its success. Rather, its adherents are like giddy lovers, blinded to the flaws beneath the superficial allure.
The measurement problem
To understand how this could happen, we need to see why, more than a hundred years after quantum theory was first conceived, experts are still gathering to debate what it means. Despite such apparently shaky foundations, it 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 quantum mechanics, the mathematical formulation of the theory, is largely a technique: a set of procedures for calculating what properties substances have based on the positions and energies of their constituent subatomic particles. The calculations are hard, and for anything more complicated than a hydrogen atom it’s necessary to make simplifications and approximations. But we can do that very reliably. 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 famous words of physicist David Mermin, they “shut up and calculate”, and don’t think too hard about what the equations mean.
It’s true that the equations seem to insist on some strange things. They imply that very small entities like 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 Einstein’s theory of special relativity.
But quantum scientists just accept such things. What really divides opinion is that quantum theory seems to do away with the notion, central to science from its beginnings, of an objective reality that we can study “from the outside”, as it were. 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. The most widespread form of quantum maths, devised by Erwin Schrodinger in the 1920s, describes a quantum entity using an abstract concept called a wavefunction. The wavefunction expresses all that can be known about the object. But a wavefunction doesn’t tell you what properties the object has; rather, 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? Is Schrödinger’s cat alive or dead? We can find out by looking – but quantum mechanics seems to be telling us that the very act of looking forces the universe to make that decision, at random. Before we looked, there were only probabilities.
The Copenhagen Interpretation insists that that’s all there is to it. To ask what state a quantum entity is in before we looked is meaningless. That was what provoked Einstein to complain about God playing dice. He couldn’t abandon the belief that quantum objects, like larger ones we can see and touch, have well defined properties at all times, even if we don’t know what they are. We believe that a cricket ball is red even if we don’t look at it; surely electrons should be no different? This “measurement problem” is at the root of the arguments.
Avoiding the collapse
The way the problem is conventionally expressed is to say that measurement – which really means any interaction of a particle with another system that could be used to deduce its state – “collapses” the wavefunction, extracting a single outcome from the range of probabilities that the wavefunction encodes. But the quantum mechanics offers no prescription for how this collapse occurs; it has to be put in by hand. That’s highly unsatisfactory.
There are various ways of looking at this. A Copenhagenist view might be simply to accept that wavefunction collapse is an additional ingredient of the theory, which we don’t understand. Another view is to suppose that wavefunction collapse isn’t just a mathematical sleight-of-hand but an actual, physical process, a little like radioactive decay of an atom, which could in principle be observed if only we had an experimental technique fast and sensitive enough. That’s the Objective Collapse interpretation, and among its advocates is Roger Penrose, who suspects that the collapse process might involve gravity.
Proponents of the Many Worlds Interpretation are oddly reluctant to admit that their preferred view is simply another option. They often like to insist that There Is No Alternative – that the MWI is the only way of taking quantum theory seriously. It’s surprising, then, that in fact Many Worlders don’t even take their own view seriously enough.
That view was presented in the 1957 doctoral thesis of the American 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 only see 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 his proposal was championed in the 1970s by the physicist Bryce De Witt, 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. The theory doesn’t exactly predict the other universe in the way that scientific theories usually make predictions. It’s just a deduction from the hypothesis that the other electron path is real too.
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 MIT physicist Max Tegmark, one of the most prominent MWI popularizers. 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 a symbol Ψ: allegedly a complete description of everything that is or ever was, including the stuff we don’t yet understand. You might sense some issues being swept under the carpet here.
What about us?
But let’s stick with it. What are these parallel worlds like? This hinges on what exactly the “experiments” that produce or differentiate them are. So you’d think that the Many Worlders would take care to get that straight. But they’re oddly evasive, or maybe just relaxed, about it. Even one of the theory’s most thoughtful supporters, Russian-Israeli physicist Lev Vaidman, seems to dodge the issue in his entry on the MWI in the Stanford Encyclopedia of Philosophy:
“Quantum experiments take place everywhere and very often, not just in physics laboratories: even the irregular blinking of an old fluorescent bulb is a quantum experiment.”
Vaidman stresses that every world has to be formally accessible from the others: it has to be derived from one of the alternatives encoded in the wavefunction of one of the particles. You could say that the universes are in this sense all connected, like stations on the London Underground. So what does this exclude? Nobody knows, and there is no obvious way of finding out.
I put the question directly to Lev: what exactly counts as an experiment? An event qualifies, he replied “if it leads to more than one ‘story’”. He added: “If you toss a coin from your pocket, does it split the world? Say you see tails – is there parallel world with heads?” Well, that was certainly my question. But I was kind of hoping for an answer.
Most popularizers of the MWI are less reticent. In the “multiverse” of the Many Worlds view, says Tegmark, “all possible states exist at every instant”. One can argue about whether that’s the quite same as DeWitt’s version, but either way the result seems to accord with the popular view that everything that is physically possible is realized in one of the parallel universes.
The real problem, however, is that Many Worlders don’t seem keen to think about what this means. No, that’s too kind. They love to think about what it means – but only insofar as it lets them tell us wonderful, lurid and beguiling stories. The MWI seduces us by multiplying our selves beyond measure, giving us fantasy lives in which there is no obvious limit to what we can do. “The act of making a decision”, says Tegmark – a decision here counting as an experiment – “causes a person to split into multiple copies.”
That must be a pretty big deal, right? Not for theoretical physicist Sean Carroll of the California Institute of Technology, whose article “Why the Many-Worlds formulation of quantum mechanics is probably correct” on his popular blog Preposterous Universe makes no mention of these alter egos. Oh, they are there in the background all right – the “copies” of the human observer of a quantum event are casually mentioned in the midst of the 40-page paper by Carroll that his blog cites. But they are nothing compared with the relief of having to fret about wavefunction collapse. It’s as though the burning question about the existence of ghosts is whether they observe the normal laws of mechanics, rather than whether they would radically change our view of our own existence.
But if some Many Worlders are remarkably determined to avert their eyes, others delight in this multiplicity of self. They will contemplate it, however, only insofar as it lets them tell us wonderful, lurid and beguiling stories about fantasy lives in which there is no obvious limit to what we can do, because indeed in some world we’ve already done it.
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 uncanniness of the centuries-old Doppelgänger trope, and then flit off again. The result sounds transgressively exciting while familiar enough to be persuasive.
In what sense are those other copies actually “us”? Brian Greene, another prominent MW 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. Vaidman puts the issue more carefully: all the copies of himself are “Lev Vaidman”, but there’s only one that he can call “me”.
““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”.” 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. A “complete description” of the state of his body and brain never exists.
What’s more, this half-baked stitching together of quantum wavefunctions and the notion of mind leads to a reductio ad absurdum. It makes Lev Vaidman a terrible liar. He is actually a very decent fellow and I don’t want to impugn him, but by his own admission it seems virtually inevitable that “Lev Vaidman” has in other worlds denounced the MWI as a ridiculous fantasy, and has won a Nobel prize for showing, in the face of prevailing opinion, that it is false. (If these scenarios strike you as silly or frivolous, you’re getting the point.) “Lev Vaidman” is probably also a felon, for there is no prescription in the MWI for ruling out a world in which he has killed every physicist who believes in the MWI, or alternatively, every physicist who doesn’t. “OK, those Levs exist – but you should believe me, not them!” he might reply – except that this very belief denies the riposte any meaning.
The difficulties don’t end there. It is extraordinary how attached the MWI advocates are to themselves, as if all the Many Worlds simply have “copies” leading other lives. Vaidman’s neat categorization of “I” and “Lev” works because it sticks to the tidy conceit that the grown-up "I" is being split into ever more "copies" that do different things thereafter. (Not all MWI descriptions will call this copying of selves "splitting" - they say that the copies existed all along - but that doesn't alter the point.)
That isn't, however, what the MWI is really about – it's just 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.”
He 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.
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.
State of mind
Compared with these problems, the difficulty of testing the MWI experimentally (which would seem a requirement of it being truly scientific) is a small matter. ‘It’s trivial to falsify [MWI]’, boasts Carroll: ‘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 an experiment like that 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 American philosopher David Lewis pushed this line of thought to its limits by asserting, in the position called model realism, that all worlds that are possible have a genuine physical existence, albeit isolated causally and spatiotemporally from ours. On what grounds? Largely on the basis that there is no logical reason to deny their existence, but also because accepting this leads to an economy of axioms: you don’t have to explain away their non-existence. Many philosophers regard this as legerdemain, but 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 (after all, it takes only four words to say “everything possible is real”, right?). 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. Lewis’s modal realism does, however, at least have the virtue that he thought in some detail about the issues of personal identity it raises.
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”. Who can object to imagination? Not me. But when taken to the extreme, parallel universes 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 “we” 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 recognize, 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 judgement of the philosopher of science Robert Crease that the MWI is ‘one of the most implausible and unrealistic ideas in the history of science’ [see The Quantum Moment, 2014]. To pretend that the only conceptual challenge for a theory that allows everything conceivable to happen (or at best fails to provide any prescription for precluding the possibilities) is to accommodate Sliding Doors scenarios shows a puzzling lacuna in the formidable minds of its advocates. Perhaps they should stop trying to tell us that philosophy is dead.