The idea that the universe constantly splits into alternate realities has had a surprisingly warm reception in science, philosophy and popular literature. But this baroque solution to the unique problems of quantum mechanics is simply not good science. Not only should we resist its strange appeal, it’s time to find another way, writes Philip Ball.
Alternative realities hold an irresistible allure. Whether it’s Dickens’ A Christmas Carol, Frank Capra’s It’s a Wonderful Life, or the quantum-computed parallel universes of Alex Garland’s recent TV series Devs, the possible lives that we can imagine having led but did not lead offer a stage for acting out our fears and fantasies. Like Robert Frost’s traveller confronted with diverging paths in a wood, we often wonder where the road not taken might have borne us.
It is no surprise, then, that the Many Worlds interpretation (MWI) of quantum mechanics seems to hold such attraction. Even though most physicists dismiss or even deride it, it is often eagerly embraced by physics popularizers and their audiences. Yet it can be hard to figure out how seriously some of its advocates really take it. I believe some physicists genuinely see it as an elegant solution to deep conundrums of the notoriously mind-bending quantum theory, and I sympathize with some of their reasoning. But when they start talking about “quantum brothers” (and presumably sisters, though Many Worlds has curiously few female advocates), or using “quantum apps” to make difficult decisions by triggering some quantum measurement in the conviction that they are thereby splitting off a world where they made the other choice, I have to wonder whether, indifferent to the philosophical complications, they are just enjoying the fantasy.
I have to wonder whether, indifferent to the philosophical complications, physicists are just enjoying the fantasy of the Many Worlds interpretation.
Why, though, would anyone decide that the theory of quantum mechanics reveals an unimaginably vast, possibly infinite, series of other worlds constantly disentangling from our own? In his recent piece for IAI News, Daniel Nolan explains that the notion arises from the probabilistic nature of the theory. As German physicist Max Born (who is too rarely given due credit) argued in the 1920s, quantum mechanics seems to have a curious property unlike any other theory in science. It seems incapable, in general, of predicting what the observed outcome of a quantum event will be, but only the probabilities of all the possible outcomes. For a classical object like a tennis ball or a space rocket, Newton’s equations of motions can tell us exactly what path it will follow under a given set of circumstances. But fire a photon of light at two slit-like openings in a screen, and quantum mechanics can only offer probabilities of the road taken.
Only when we measure the outcome of a quantum event can we get a definitive answer. A 50:50 probability of outcomes A and B before the event, say, somehow turns into one or the other (a 100% certainty) when we look at the event once it has happened. It’s like flipping a coin: there’s a 50:50 chance of heads or tails before we flip, but once we’ve flipped and looked, it’s either 100% heads or 100% tails. Yet we can confirm that the initial 50:50 probability was right by repeating the experiment – making many tosses, whereupon we'll find that we get the same number of heads, on average, as tails.
[NHY1]In most of science, probabilistic predictions reflect a lack of knowledge about the details. If we knew, at the outset, the positions and movements of every atom in the coin, the flipping hand, and the air, we should be able to predict the outcome of the coin toss with certainty. (That’s impossible in practice, but not in principle.) Yet in quantum mechanics the probabilities are fundamental: the theory seems to insist that there is nothing we can possibly measure that will enable us to do better than a probabilistic prediction of outcomes.
Quantum mechanics seems to insist that there is nothing we can possibly measure that will enable us to do better than a probabilistic prediction of outcomes.
The question is then what changes a probability before the event to a certainty after it. There seems to be some abrupt change that quantum theory itself can’t account for. The equation devised by Erwin Schrödinger in 1924 to describe how a quantum particle behaves contains no such transformation. It describes particles and their behaviour in terms of a mathematical entity called a wave function (because it resembles the equation for a classical wave). As Born showed, the wave function can be used to calculate the probability that we will measure a particular value of the particle’s properties – its position in space, say. But Schrödinger’s equation seems to insist that these properties always behave smoothly, whereas a measurement seems to induce an abrupt jump to a particular value: what Nolan refers to as a “collapse process” of the wave function.
Quantum mechanics doesn’t itself have any prescription for the “collapse of the wave function” – it has to be added by hand in an ad hoc fashion, as first done by the Hungarian mathematical physicist John von Neumann in the 1930s. That’s awkward and unsatisfactory – sure, the maths works okay, but we’re none the wiser about what actually happens in the real world to convert smooth quantum probability waves into unique observations. The Many Worlds interpretation was proposed by Hugh Everett in his doctoral dissertation at Princeton in 1957 as a way to avoid this problem. What if, Everett said, there simply is no wave function collapse that selects one possible outcome from all the possibilities? What if, instead, they all occur – but in alternative realities? In this view, as the photon reaches the double slits, it passes through one slit in one universe and the other slit in the other.
But then there must be an observer in each universe that sees the two outcomes. In other words, everything splits: the apparatus, the experimenter, the lab, the universe. Everett was discouraged from saying much about this multiplication of actual people (as opposed just to particles) in his thesis, but the notion was there. It was expanded on in the 1970s, and the proliferation of quantum selves is now the typical “Wow!” element of popular presentations of the MWI.