Can we test for parallel worlds?

A bold new theory suggests many worlds have existed, side-by-side, since the beginning of time. Cathal O'Connell investigates.

A new theory suggests some parallel worlds are still home to dinosaurs, while others are almost identical to our own.
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Parallel worlds are standard fare for sci-fi, but the idea originates from quantum physics where the seemingly bizarre notion of alternative universes is taken very seriously.

Now an Australian team has taken the weirdness a step further. Their new theory, published in Physical Review X, is dubbed “Many interacting worlds”. It not only proposes that stable parallel worlds exist but suggests it might be possible to test for their existence.

“It’s kind of a radical idea, but it looks very interesting and promising” says Bill Poirier, a quantum dynamicist at Texas Tech University, who proposed an early version of the theory in 2010.

Parallel worlds were first evoked in the 1950s to explain quantum effects such as how a particle can appear to be in two places at once. Alas Hugh Everett, the American physicist who proposed them, suffered such ridicule he quit science. In his “many worlds” interpretation of quantum theory every quantum measurement causes the universe to “branch” into a bunch of new universes. It was as if, at the flip of a quantum coin, two universes would sprout into existence – one for heads, and one for tails.

The new theory, bravely proposed by Howard Wiseman, Director of the Centre of Quantum Dynamics at Griffith University, is different. No new universes are ever created. Instead many worlds have existed, side-by-side, since the beginning of time. Some follow the best sci-fi plots, for instance “worlds where the dinosaur-killing asteroid never hit” says Wiseman. Others are almost identical to our own, inhabited by versions of ourselves on alternative Earths, perhaps differing only in the shape of a cirrus cloud above parallel-Melbourne on a bright spring parallel-morning. It’s the interaction of these nearby worlds that gives rise to quantum effects, the theory says.

So what’s led Wiseman to stir yet another seriously weird theory into the mix?

Physicists have been trying to come to terms with the experimental findings in the quantum world for a century. “It’s still notoriously unfathomable”, he says. “This motivated us to look for a better description of what is really going on.”

One of the unfathomable experiments Wiseman is referring to is the “two-slit” experiment for electrons. It’s a version of the famous two-slit experiment first performed by Thomas Young in the 1800s with light. He found that a light beam passing through two closely spaced slits in a screen onto a wall behind did not, as one might expect, produce an image of two lines. Instead it formed an interference pattern rather like the overlapping wakes of two boats, leading Young to deduce that light is a type of wave.

In the early 20th century Niels Bohr and others began to find evidence that fundamental particles, like electrons, also had wave-like properties. The proof came in 1961 when the two-slit experiment was performed with electrons and a wave-like interference pattern, just like that of light, was found.

It was strange to find that particles could act like waves, but then things got stranger. Responding to a suggestion by Richard Feynman, in 1974 Giulio Pozzi and colleagues at the University of Bologna showed you still get an interference pattern when you fire one electron at a time – as if each individual electron passes through both slits and interferes with itself. Feynman said that all of quantum mechanics could be gleaned from carefully thinking about this one experiment. Physicists have been doing that for decades, yet are still tormented for lack of an intuitive understanding. “The question,” says Wiseman, “is how do you explain what’s going on?”

Einstein abhorred how quantum theory turned the electron from a particle into a cloud. He, like Newton, imagined a universe that ran like clockwork.

Niels Bohr’s quantum theory managed to predict the behaviours using some strange maths known as the “wave function” to pictures the electron neither as a particle nor a wave, but as a fuzzy “cloud of probability”. But what, exactly, that means in physical terms has never been well defined. “There has been a lot of debate and controversy for 100 years,” says Poirier.

Einstein abhorred how quantum theory turned the electron from a particle into a cloud. He, like Newton, imagined a universe that ran like clockwork. “God does not play dice,” said Einstein. “I think Einstein should stop telling God what to do,” responded Bohr.

The new theory proposed by the Griffith team is a lot closer to Einstein’s vision than Bohr’s. Gone are the probability clouds along with the other conundrums of wave-particle duality. In the new picture the electrons being fired at the slits are particles after all – tiny little spheres just as Newton would have imagined them. In our world the electron might pass through the bottom slit. But in a parallel world the electron passes through the top slit. As the two ghostly twins travel towards the detectors (one in our world, one in a parallel world), their paths could overlap. But according to the theory, a newly proposed repulsive force stops the electrons coming too close to one another. In effect, the electron in our world “collides” with its ghostly twin, like billiard balls knocking together as they roll across a pool table.

According to Wiseman and his team this interaction between parallel worlds leads to just the type of interference patterns observed – implying electrons are not waves after all. They have supported their theory by running computer simulations of the two-slit experiment using up to 41 interacting worlds. “It certainly captured the essential features of peaks and troughs in the right places,” says Wiseman.

Though he says it is in its early stages, Poirier is impressed with the theory and predicts it will generate huge interest in the physics community. “

By restricting the worlds to be discrete or finite, Poirier adds, the Griffith team has developed equations that are much easier for a computer to solve. Quantum mechanical calculations that would usually take minutes were completed “in a matter of seconds,” says Michael Hall, lead author of the study. Hall hopes that eventually this will lead to applications in predicting real world chemical reactions.

And if the number of worlds is finite – as modelled in the team’s computer simulations – rather than infinite, then the predictions made by the new theory will deviate from standard quantum theory. Though the deviations are likely to be only slight they could be testable in the lab using experiments similar to the double slit. Tantalisingly, as the size of the deviations depends on the number of parallel worlds, these experiments could provide an effective measure of how many worlds are out there.

But… parallel worlds? Is this not all too absurd to take seriously? Not for the physicists, it seems. And as David Wallace points out in The Emergent Multiverse, our sense of absurdity evolved to help us scratch a living on the savannahs of Africa. “The Universe is not obliged to conform to it.”

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Cathal O'Connell is a science writer based in Melbourne.