MINCE pie or another slice of Christmas cake? If you’re having trouble deciding, perhaps you need the Universe Splitter. Type the choices confronting you and it promises to contact “a laboratory in Geneva” that will conduct an experiment to tell you which decision to make.
The app’s guarantee is that, whatever the best choice might be, you will get to enjoy it for sure – if not in this world, then as another you in a parallel one. Sink back into your armchair, pour a brandy (or sherry?) and prepare to have your mind split apart. We are about to enter the quantum realm – home, perhaps, to many worlds where you can have your cake, and pie, and eat both. Or can you?
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The origins of this agreeable suggestion lie in a city famed for its sweetmeats: Vienna, Austria. In the early 1920s, physicist Erwin Schrödinger was seeking an equation that could explain the workings of quantum particles – things like photons and the tiniest building blocks of the matter that makes up you, me, your armchair and that plate of Christmas cake.
In 1925, Schrödinger found his equation, which indicated that everything there is to know about a quantum particle was described by a rather mysterious mathematical entity called a wave function. A year later, German physicist Max Born argued that this wave function supplied you with the probability of finding a quantum particle at a point in space, if you performed some experiment to measure it. As long as you didn’t measure it, this position was somehow indefinable, characterised only by this spread-out probability wave.
“The Universe Splitter app contacts a lab in Geneva to make your decision”
This was odd. The quantities in the equations of classical physics represent measurable properties of the world – the charge of an electron, the location of a brandy glass, the mass of a mince pie. Properties, not just probabilities; the thing itself, not our experience of it when we seek to observe it. Born’s interpretation seemed to imply that, until we measure a quantum particle, “collapsing” its wave function into a certain and unique outcome, we can’t really speak about it at all.
This raises all sorts of hairy questions. For a start, what counts as a measurement that takes us from probability to certainty? Quantum experiments have shown that it seems to involve not just doing something with a measuring instrument, but also consciously noticing the result. So what is “real” before we look? And what happens to other possible outcomes encoded in the wave function when we force it to collapse into just one?
Multiplying yous
The Universe Splitter emerges from one attempt to solve these puzzles. In the cold war US of the 1950s, a PhD student at Princeton University named Hugh Everett III produced perhaps the most mind-boggling doctoral thesis of all time. He proposed that Schrödinger’s wave function doesn’t collapse at all, and the other possibilities encoded in it don’t actually disappear when we make a quantum measurement. Instead, every time anyone makes a measurement, the universe splits into a number of universes equivalent to the number of possible outcomes – along with a copy of whoever is doing the measuring, and everything else in that universe, too.
Multiplying universes and selves in this way might sound implausible, not to mention highly profligate. But that’s an aesthetic, not a scientific objection, and for adherents of the “many-worlds” interpretation of the quantum realm, this is serious science. at the University of Oxford, one of its most committed advocates, once began a talk at a meeting honouring Everett’s work by saying, “I’ll start with a simple fact: in this room, in some nearby universes, Hugh Everett is here with us, celebrating.” Everett had in fact died several years earlier, in 1982 at the age of just 51, his end hastened by excessive eating, heavy drinking and chain smoking – at least in the branch of the quantum multiverse where we all ended up.
This many-worlds interpretation implies that if we attach our fates – cake or pie? – in the macroscopic classical world to the outcome of a universe-splitting quantum measurement in the quantum world, we can ensure all outcomes occur somewhere in the quantum multiverse. This means we enjoy the best (and also the worst) of all possible worlds, whatever choice we make. Which is what the Universe Splitter app purports to do.
You type into the app the two possible outcomes of the decision you have to make, and press the button. According to the blurb from its Californian maker, Aerfish, it connects “to a quantum device, which fires single photons at a partially-silvered mirror. Each photon will simultaneously bounce off the mirror and pass through it – but in separate universes”.
The device will inform you which outcome was measured in the world you entered and the associated decision. But because the universe has just split, it will be telling you the opposite in a parallel world. Follow through with the decision it gives you, and you can rest assured you are also experiencing the other elsewhere.
Physicist at Tel Aviv University in Israel is unequivocal that this sort of self-splitting really occurs. “Based on what I know from physics, this is the only reasonable option,” he says. “At the present moment, there are many different Levs in different worlds.” He says his “I” corresponds not just to a particular Lev in the moment, but also to many Levs in the future.
In fact, Vaidman merrily creates his own Levs using his own free – the Universe Splitter will set you back $1.99 – which also uses a quantum optical measurement to make a decision with up to six outcomes for you. You may need to choose between eating strudel, tiramisu, chocolate mousse or cheesecake, he suggests. If you “have a strong will and fulfil what the device tells you, your parallel selves will fulfil the other options”. Delicious.
But before you tuck in, there’s a problem. If, in the many worlds, every possible outcome of a quantum measurement happens with 100 per cent certainty, then what are those probabilities encoded in Schrödinger’s wave function all about? This is the biggest stumbling block for Everett’s idea: explaining why quantum mechanics looks to be governed by probability, if it is actually fully deterministic because everything that can happen does.
In the late 1990s, Deutsch offered an explanation. Imagine you fancy Christmas cake rather more than the mince pie, but still hanker after both. You could set up a quantum decider experiment to reflect your leanings – in the normal quantum formulation involving probabilities, with say 90 per cent chance of the “cake” outcome, and 10 per cent “mince pie”.
In the many-worlds formulation, you say that both branches are reached with 100 per cent probability, with what used to be probabilities becoming “branch weights”: essentially, how biased you have made the experiment towards or against a particular outcome. Deutsch showed that, if you know that each of your split selves will experience only one universe after the event, the only rational thing to do is to treat these branch weights as if they really are probabilities in the pre-split universe. “If Everett’s view is true, branch weight is an objective physical quantity with the right formal properties to be probability,” says David Wallace at the University of Southern California.
“You have split innumerable times in reading this article – faster than thought itself”
Not everyone is convinced. For one thing, this take seems to depend on saying that the apparent probabilities are nothing more than how we, if we are rational about it, ought to regard branch weights. An explanation that demands we assume a certain attitude towards the world doesn’t seem particularly compelling.
Vaidman advocates a different tack to overcome the many-worlds probability problem. His explanation is known as “self-locating uncertainty”. He illustrates it with a thought experiment. Imagine you agree to take a pill that puts you to sleep, and are placed in a room containing a chest. You know that, while you are asleep, someone else will perform a quantum experiment – fire a photon at a half-silvered mirror, say – and, according to the outcome, either place $1 million in the chest or nothing at all. In this scenario, when you wake up, it is perfectly reasonable for you to say before you look that there is a 50 per cent chance of the chest containing the fortune or not – a subjective probability that is set to be equal to the objective branching weight in the quantum experiment. In this scenario, apparent probability comes from the way that, after a quantum splitting, we are unsure which branch of the quantum multiverse we have ended up on.
at the University of Cologne, Germany, is unsure this solves the problem, either. “Usually we talk about the probabilities of a future experiment, not one that has already been made,” he says. “If we’re thinking about probabilities of future outcomes, the question of which branch I will end up on doesn’t even make sense, because the notion of ‘I’ changes. You know that there will be future versions of you in all branches, all of which see a single outcome and remember being you now. There is no uncertainty.”
Behind all this talk about what probability means in the many-worlds interpretation lurks a more fundamental question: what does “I” mean when worlds divide? Any quantum interaction, anywhere in the universe, that ends up with a classical outcome – a protein channel in a brain cell contributing to a thought that becomes an action, for example – should split the universe. It follows that I have split countless times while writing this article, as you have while reading it – faster, indeed, than thought itself. There’s never a moment when you can be aware of a unique you.
Who exactly are you?
So in that sleeping-pill thought experiment, is the “I” that goes to sleep the same as the I that wakes up? If we are not defined by a unique thread of conscious awareness, how exactly are we defined? Many-worlders deny that identity is a problem, saying that the “I” is just a well-defined point in – or perhaps trajectory through – the branching multiverse. But it isn’t easy to see what that means, if the branching is more fine-grained than thought.
Besides, the implication of an app like the Universe Splitter seems to be that, ultimately, our intentional acts can split the universe. The experiment is just the tool we do it with. But wasn’t many worlds supposed to do away with the observer’s apparent ability to shape reality in the conventional view of quantum mechanics?
We haven’t heard the last word yet – probably. And if you are still having difficulty deciding between Christmas cake and mince pie, bear in mind one caveat mentioned on the Universe Splitter website, should you choose to use it. You won’t be able to chase your other selves down their paths through the multiverse. “Universes cannot contact one another.”
This article appeared in print under the headline “The best of all possible worlds”
