David Wallace was still in high school when he first encountered the notion of parallel universes. To a British teenager hooked on science fiction and physics, it sounded “weird but cool”, he recalls. Twenty years later, the topic still absorbs him.
A professor specialising in the philosophy of physics at the University of Southern California, he recently penned a book titled The Emergent Multiverse.
For some people, the idea there is more than one universe out there sounds absurd. Sure, it’s fair game for the writers of Star Trek, and perhaps even for philosophers, but how can it be a serious scientific idea?
In fact, it was first put forward some 60 years by a very serious physicist, Hugh Everett III, who proposed the idea of “Many Worlds” as a way of making sense of quantum mechanics – the strange science of the subatomic world.
Everett’s idea was not well received. But by the time Wallace was wrestling with quantum mechanics as a physics PhD student four decades later, the idea had been resurrected. “What was a revelation to me was realising that these parallel ‘worlds’ weren’t something extra that you added to quantum mechanics; they were there in the mathematics of the theory all along. And it was realising that that got me thinking, ‘Ok, this is probably right – or at least, this is the best route that we have at the moment to try to make sense of this’.”
Quantum mechanics isn’t the only theory that leads scientists to the idea of parallel universes. String theory – an attempt to stitch gravity into the equations that govern the quantum world by proposing the existence of 11-dimensional vibrating strings – also suggests our universe is just one of many in a vast cosmic landscape. Meanwhile, inflation theory holds that our universe inflated as a bubble of space-time shortly after the Big Bang. If it happened once, then perhaps it happened many times and is still happening.
Of these various paths to parallel worlds, the one that emerged from quantum mechanics, now referred to as “Many Worlds”, was the first out of the gate. It’s rooted in mathematics, and it wouldn’t be the first time that this esoteric language unveiled a hidden reality. The Big Bang, black holes and the concept of curved space, for instance, all first emerged from a scrawl of mathematical equations. Now evidence they exist is virtually bulletproof. We can hear the “echo” of the Big Bang – the cosmic microwave background radiation – with radio telescopes. Just last year, we detected the gravitational signatures of colliding black holes. And Uber drivers would be lost if Google Maps didn’t take account of Einstein’s curved space-time.
So can we find any evidence for the existence of Many Worlds? Probably not. But advocates say that’s not a deal breaker. They consider Many Worlds to be just one more of the predictions of quantum mechanics – a theory whose other predictions have been exhaustively proved. Because of that, a surprising number of respectable physicists are willing to entertain the reality of multiple worlds.
Clues to the weirdness of our universe began to emerge in the early 1900s when the founders of quantum physics, including Max Planck, Albert Einstein and Werner Heisenberg in Germany, along with Niels Bohr in Denmark, began to investigate the structure of the atom.
It was Planck who showed that energy came in discrete bundles or quanta. Einstein backed him up by showing that light radiates its energy as bundles that we now call photons. Yet for 100 years before that, light was understood to be a wave. Now scientists were forced to think of light as both a particle and a wave.
Then in the 1920s, a young Frenchman named Louis de Broglie argued this type of behaviour wasn’t restricted to light: any kind of particle should display this duality. He described the idea of “matter waves” in his doctoral thesis in 1923. The counterintuitive notion almost cost him his degree, until Einstein gave it a nod of approval.
Bizarre as the idea was, Austrian physicist Erwin Schrödinger nevertheless found he could describe matter waves with a mathematical equation called the “wave function”.
Schrödinger’s equation did not describe a world that we experience. Tracking a subatomic particle was nothing like describing the position and velocity of a fired cannonball. Rather, his wave function described particles as some sort of statistical entities. All one could do was describe the likelihood of where and when to find them. Schrödinger attempted a grasp on reality by imagining matter waves as being something like “smeared-out” particles.
Of all the tests of quantum mechanics conducted over the years, the one that sheds the most light on this weird science is the decidedly low-tech double slit experiment.
Back in the 19th century, British polymath Thomas Young first used it to show the wave nature of light. Aim a beam of light at a sheet with two thin, rectangular slits cut into it. The light passes through the slits and projects onto a screen. But what you see on the screen is not two bars of light – rather, you get an “interference pattern” – an array of light and dark patches. It’s the signature pattern produced by interfering waves, similar to that made by ripples on a pond. The peaks or bright patches are where the crests of light waves meet other crests; the dark patches are where the crests meet troughs, cancelling each other out.
That’s fine for light – already known to behave like a wave – but what about bits of matter? The work of de Broglie hinted that tiny particles such as electrons would behave in an identical fashion. That’s exactly what American physicists Clinton Davisson and Lester Germer, working at Bell labs, found when they accidentally performed a version of the double slit experiment.
Their original intention had been to figure out the structure of a nickel crystal by firing electrons at it and measuring the angles at which they scattered off its surface. Instead of revealing something about the nature of the crystal, they revealed something startling about the nature of electrons. The crystal had provided the equivalent of tiny slits. And travelling through them, the electron beam produced an interference pattern – just as light waves do.
The experiment was undeniable proof of the bizarre nature of reality that has continued to shake physics ever since. “Anyone who is not shocked by quantum theory has not understood a single word,” Bohr said.
How can an electron, a particle of matter, be a wave? Stanford physicist Leonard Susskind expressed his discomfort this way on YouTube: “A rock is an example of a particle; an ocean wave is an example of a wave. Now someone’s telling you a rock is like an ocean wave. What?!”
Things get even more bizarre. More sophisticated versions of the double slit experiment were carried out using an electron gun, with the rate of firing slowed so that only one electron was released at a time, passing through one of the two slits. Yet over time, an interference pattern emerged on the phosphor-coated screen behind it. It was as if the single electron was passing through both slits at the same time and interfering with itself, so to speak. This ability to be in two places at the same time is termed superposition.
Somehow, this was just one more of the amazing properties of the wave function. Not only was an electron to be thought of as a haze of probabilities (that nobody really understood), it could exist in two places at the same time.
But what happened to the electron’s haze of probabilities when it hit the phosphor screen? Suddenly all those probabilities collapsed into one point. It’s as if an ocean wave, at the moment of wetting the shore, suddenly shrank to wet only one grain of sand. Something about the very act of detection led the wave function to collapse and behave like a particle.
Israeli scientists in 1998 vividly demonstrated this “observer effect”. When a device akin to a Geiger counter was positioned to detect electrons as they approach the slits, the screen on the other side no longer recorded an interference pattern. The act of observing them reduced them to behaving as mere particles.
How did physicists explain this assault to our understanding of reality?
One response was not to try. As Bohr put it, “there is no quantum world. There is only an abstract quantum physical description”, while Heisenberg offered, “what we observe is not nature itself, but nature exposed to our method of questioning”.
In a sense they rejected the reality of quantum world while accepting that the mathematics of the wave function accurately predicted its behaviour. And in fact, the wave function has proved to be uncannily accurate, enabling us to predict the nature of chemical reactions, the development of lasers, electronics, computing and quantum encryption methods – technologies that provide 25% of the US gross national product. For Bohr and Heisenberg, the fact that the mathematics worked was the end of the story. They represented the so-called “Copenhagen interpretation”, also known, somewhat snarkily, as the “shut up and calculate” school of thought.
But other physicists, including Einstein and Schrödinger, felt something was missing. They remained troubled about what quantum mechanics said about the nature of reality. The two camps were reprising an age-old philosophical debate about the meaning of mathematics. Is mathematics merely a useful abstraction, as the Copenhagen-ists argued? Or does it point to a hidden reality?
For decades, the Copenhagen interpretation was the only game in town. Then in the 1950s, Hugh Everett, working on his PhD at Princeton University, issued a bold challenge. He had been wondering about the critical concept at the heart of quantum mechanics, the collapse of the wave function when a measurement is made. What exactly causes it to collapse, eliminating all possibilities but one?
Apparently after a night of drinking with friends, he came up with a radical answer: the wave function doesn’t collapse. Instead, all possibilities occur, but in different universes. When we make a measurement, the universe branches to accommodate those different outcomes. And since just about any physical interaction involves quantum processes – the world is, after all, made up of atoms. This splitting, then, is happening all the time, creating an infinity of universes, each being a slightly altered doppelgänger of our own.
Everett’s thesis advisor John Wheeler, excited by his student’s theory, travelled to Bohr’s institute for theoretical physics in Copenhagen with a copy of the Many Worlds thesis. Neither Bohr nor his colleagues were impressed. Alexander Stern, an American scientist working at Bohr’s institute, described it as “theology”.
Wheeler wrote to Everett, saying: “Your beautiful wave function formalism of course remains unshaken: but all of us feel that the real issue is the words that are to be attached to the quantities of the formalism.” In other words, Wheeler was worried that people might think Everett was serious about those other worlds actually existing. (In fact, Everett was serious about them.)
Wheeler talked Everett into removing references to split worlds; his thesis was eventually whittled down to a quarter its original length. Everett acquiesced because he needed his PhD to land a job. He ended up leaving theoretical physics to take a position at the Pentagon as a weapons strategist. During his years there, his mathematical models looked at scenarios such as the catastrophic death rate from radioactive fallout in a nuclear war, helping to develop the concept of mutually assured destruction. Perhaps we have Everett to thank for having any world at all.
For Wallace, the Cold War atmosphere helps explain the intolerance for Everett’s ideas. Many labs emphasised applied science over abstract theorising with an emphasis on building weapons. “You had a longish period where theoretical and conceptual questions were subordinated to a very practical attitude to physics,” says Wallace.
But beginning in the 1980s, a new openness for bold, unconventional thinking took hold and Many Worlds made a comeback. This was partly because the older generation of physicists – the ones who had worked alongside Bohr, Heisenberg and the rest of the gang – had retired or died, but also because the need to understand the quantum world grew ever more compelling. The boundary between the quantum and classical worlds had become hopelessly blurred. In 1991, a team at the University of Vienna carried out a double-slit experiment to demonstrate the wave-particle duality of some very big particles – “buckyballs” composed of 60 carbon atoms. In 2013, the university did it again with a molecule composed of 800 atoms.
Quantum weirdness has also entered our world with some down-to-earth applications – most notably, the drive to develop a quantum computer. Their bits are composed of atoms that have a spin state that can be either up or down. But it turns out they can also be in a state of superposition – simultaneously up and down. Quantum computers take advantage of these superpositions to perform calculations that some claim will be billions of times faster than conventional computers in some cases.
The more that quantum weirdness leaps out of the textbooks and into the laboratory, the less adequate the Copenhagen interpretation seems – and the more willing physicists are to consider the alternatives. “All of these things, I think, brought the question of what the [Many Worlds] theory actually means back onto the table,” Wallace says.
The idea of Many Worlds suffers from an obvious flaw. We may never be able to find a way to either prove or disprove their existence. And as 20th-century philosopher Karl Popper argued, a theory has to be falsifiable to even qualify as science.
But Wallace says this does not disqualify Many Worlds. He and other proponents are quick to argue that Many Worlds is a prediction of quantum mechanics – itself probably the most exhaustively tested theory in history. “There’s no general principle in science or in physics that says, ‘theories that postulate lots of stuff are bad’,” says Wallace. “There’s just a principle that says, ‘theories that postulate stuff that don’t pull their weight, that don’t have explanatory power, are bad’.”
And many respectable physicists say that as far as explanatory power, Many Worlds is just as good as wave functions that collapse as soon as they feel they are being watched. “All physicists agree the maths of quantum mechanics tells us we have to fundamentally change our picture of reality in some way,” says Michael Hall, a mathematical physicist at Griffith University in Queensland. “The Everettian Many Worlds interpretation is no better or worse than others in this regard.”
Howard Wiseman, a theoretical physicist and Hall’s colleague at Griffith University, puts it more strongly. “Many Worlds is certainly an obvious interpretation of the theory; it is even arguable that it is the simplest. Until, or unless, we get new experimental evidence on the matter, I think we should take all well-formulated interpretations seriously and see where they lead, scientifically and philosophically.” The two Australian physicists recently published their own variation on a Many Worlds theory – see box below: Modern many-worlds theories.
Sean Carroll, a theoretical physicist at California Institute of Technology, agrees. “Our job as physicists is to construct a theory that does the best job of providing an accurate account of the observations – and then to take that theory seriously.” Carroll also finds Many Worlds offers the most succinct way of making sense of quantum theory. “There’s nothing that you have to put into quantum mechanics to make Many Worlds happen. To stop Many Worlds from happening, you have to change quantum mechanics somehow.”
‘to stop many worlds from happening, you have to change quantum mechanics somehow.’
The fact that we can’t test for other universes doesn’t bother Many Worlds enthusiasts. After all, this isn’t the first time that a theory of physics has predicted something that’s beyond our ability to investigate. Einstein’s theory of general relativity, for example, makes predictions about the interior of black holes – a realm that no human will ever enter (or at least, return from). We accept these ideas even if we can’t test them, because general relativity has been extraordinarily successful in many different realms.
The champions of Many Worlds theories ask that we treat the predictions of quantum theory with equal seriousness.
Of course there are legions of detractors. Massimo Pigliucci, a philosopher of science at the City University of New York, is often to be found immersed in discussions – perhaps arguments is a better word – with Carroll, over the merits of the Many Worlds view (both of them are prolific bloggers, and neither is particularly shy). “Sean keeps telling me, ‘the mathematics are perfectly clear.’ But it’s not a question of the mathematics being clear – it’s a question of the metaphysics being clouded,” Pigliucci says. On several occasions he’s asked Carroll where, exactly, the “parallel Massimos” are located – and he’s never been satisfied with the answer. “It seems like the whole thing is predicated on a confusion between mathematical and physical reality,” says Pigliucci.
But others are prepared to follow the equations wherever they lead. And if that means there are parallel versions of each of us “out there”, so be it. The cosmos is a weird place. But then, as Wallace points out, nature has no obligation to conform to our expectations.
Modern many-worlds theories
In February 2012, Howard Wiseman at Griffith University in Queensland was hosting a colleague, Dirk-Andre Deckert from the University of California, Davis. As quantum physicists are apt to do, the conversation soon turned to the “wave function” that describes the behaviour of particles in the subatomic world. Ninety-one years after its mathematical description, just what is actually being described remains open to interpretation.
“We thought perhaps we could come up with something that would offer better simulation methods for chemical reactions,” Wiseman says. “But we ended up being radical; we threw away the wave function.”
The duo focused on two interpretations. One was Everett’s Many Worlds proposal, which states that the wave function never collapses. Instead, at the moment of measurement, the different outcomes are realised in different worlds that never interact with each other. The other interpretation, known as de Broglie-Bohm, was worked out by Louis de Broglie in 1927, and again, independently, by David Bohm in 1952. In this view, every particle is viewed as a real physical entity, surfing atop a probability wave.
Both interpretations have problems, Wiseman says. Everett’s theory was vague on when world splitting took place and whether the worlds were equivalent. De Broglie-Bohm, meanwhile, had all the structures of Everett’s theory but labelled only one world as being real. “I was excited by the idea of getting rid of these difficulties by melding the two interpretations,” Wiseman explains.
Wiseman turned to his colleague Michael Hall, a mathematical physicist at Griffith University, to help model an alternate multiple world scenario.
They were not the first to do so. Many Worlds scenarios have been making a comeback in the last few years. What’s different about the latest models is that unlike Everett’s worlds, these interact.
For instance, William Poirier, a physical chemist at Texas Tech University, published a version in 2010. In his model, the quantum effects arise from stresses and strains within a fluid-like continuum of worlds. The version crafted by Hall, Deckert and Wiseman was published in 2014 in Physical Review X. Dubbed “Many Interacting Worlds” (MIW), it holds that there is no wave function; rather its job is done by other worlds just as real as our own. They are numerous, but finite. Every particle in every world has a definite position and speed. The more similar these worlds are to our own, the more strongly their particles interact with ours.
So does their model make testable predictions? Wiseman says they are still at the stage of validating their model to see if it does as good a job as the wave function at describing quantum effects. So far it has succeeded in modelling the results of the double-slit experiment (diagram above).
Wiseman says they can also use it to calculate the ground states of electrons, but not yet their excited states. “As I see it, the theory doesn’t have drawbacks. That’s not to say it’s easy.” Significantly, if the number of universes is finite, the MIW’s predictions turn out to be ever-so-slightly different from those of standard quantum mechanics. “This does bring some hope that perhaps our interpretation is directly testable,” says Hall.
The MIW model has received plaudits from colleagues. According to Charles Sebens, a philosopher of physics at the University of California, San Diego, who also came up with a theory of interacting worlds, MIW “gets you back to this picture that we haven’t had in physics for a long time – of particles interacting with each other through forces”.
And then there’s the question of whether the Queensland physicists think their model has anything to with reality. Hall is sceptical. “You need an awful lot of reality in MIW.” But he adds, “I’m sceptical about all interpretations.” Wiseman, on the other hand, says he is “open-minded” and finds the MIW approach “quite compelling”.