Quantum weirdness is a sign of many ordinary but invisible universes jostling to share the same space as ours, according to a bold new idea
THE wave function has collapsed – permanently. A new approach to quantum mechanics eliminates some of its most famous oddities, including the concept of quantum objects being both a wave and a particle, and existing in multiple states at once.
In short, the approach removes the wave function and demotes the equation that describes it. In its place are a huge but finite number of ordinary, parallel worlds, whose jostling explains the weird effects normally ascribed to quantum mechanics.
Quantum theory was dreamed up to describe the strange behaviour of particles like atoms and electrons. For nearly a century, physicists have explained the peculiarities of their quantum properties – such as wave-particle duality and indeterminism – by invoking an entity called the wave function, which exists in a superposition of all possible states at once right up until someone observes it, at which point it is said to "collapse" into a single state.
Physicist Erwin Schrödinger famously illustrated this idea by imagining a cat in a box that is both dead and alive until someone opens the box to check on it. The probability that the cat will survive is given by the Schrödinger equation, which describes all the possible states that the wave function can take.
The Schrödinger equation predicts the outcomes of experiments perfectly. But many physicists are uncomfortable with seeing the wave function as a fundamental aspect of reality, preferring to treat its companion equation as a calculating device and seeking a deeper theory to explain what is really going on.
"You can't think of the wave function as a real thing," says Howard Wiseman of Griffith University in Queensland, Australia. But if the wave function is not real, what is?
Now, Wiseman and colleagues have come up with an answer. Our universe, they claim, shares space with a large number of other universes, each of which follows the classical, Newtonian laws of physics. In this view, particles in our universe feel a subtle push from corresponding particles in all the other universes. Everything we think of as quantum weirdness is the result of these worlds bumping into each other (Physical Review X, doi.org/wtw).
"One way to think about it is that they coexist in the same space as our universe, like ghost universes," Wiseman says. These other worlds are mostly invisible because they only interact with ours under very strict conditions, and only in very minute ways, he says, via a force acting between similar particles in different universes. But this interaction could be enough to explain quantum mechanics.
To demonstrate that the idea has legs, Wiseman and his team showed mathematically that the many interacting worlds theory can explain specific effects. "The first thing you have to do is show that it can reproduce results, because quantum mechanics has been tested to incredible accuracy," Wiseman says. "If you can't actually reproduce quantum mechanics, you're sunk from the beginning."
They chose a classic test called the double-slit experiment, which is usually read as evidence that photons act like both a wave and a particle. The effects of phantom photons in as few as 41 other worlds could give qualitatively the same result, the team says (see "Phantom photons").
They show that the theory can account for other effects as well, including the stability of matter: other worlds pressing in on our own stop electrons from falling into the nucleus of atoms, as they would in a purely Newtonian world.
"If those interactions were turned off, or if there was only one world, then all such effects would vanish, and Newtonian physics would reign supreme," says Wiseman's colleague Michael Hall, who is also at Griffith.
The approach is still in its infancy, and the theorists have a lot of work to do to flesh it out. For example, they haven't modelled how a finite number of worlds can explain entanglement, a long-distance relationship between quantum particles that Einstein called "spooky action at a distance". But it could work if the number of worlds is infinite.
Many worlds
The many interacting worlds idea echoes an earlier many-worlds interpretation of quantum mechanics, thought up by theorist Hugh Everett in the 1950s, in which the universe splits into pairs of parallel universes every time a wave function collapses. The cat is both dead and alive, according to Everett – it just depends which universe you look in.
But there are some important differences. The Everettian many-worlds interpretation treats the wave function as a fundamental part of reality, for one. And once two worlds split, they almost never interact with each other.
In the new theory, the parallel worlds have always been there, and interact via repulsive forces between corresponding particles as soon as they diverge a tiny bit.
The approach has some advantages over the standard many-worlds interpretation, says Lev Vaidman at Tel Aviv University in Israel, who has worked extensively on the Everettian approach. For example, the many-worlds view struggles to explain probability in a world where everything that could possibly happen does happen. With many interacting worlds, probability falls easily out of the mathematics.
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