If quantum physics didn’t work so incredibly well, nobody would take it all that seriously. As Albert Einstein wrote in 1912, "The more success the quantum theory has, the sillier it looks." After all, quantum experiments tell us that particles can exist in two places at once and tunnel through insurmountable barriers.
Now, however, a trio of physicists is striving to take the silliness out of quantum theory by developing a model that can explain away its strangest effects—by invoking the existence of multiple parallel classical worlds. Each world, they posit, is filled with particles that can exert a group force on their nearby clones in our universe, creating quantum effects.
The theory of quantum mechanics has proved to be amazingly successful at describing events at the smallest scale—enough to fully revolutionize modern electronics and computers. In return, however, it has unsettled the conventional notions of physical reality held by scientists and laymen alike.
It has long seemed that physicists must just accept that the intuitive everyday "classical" laws of physics that govern macroscopic objects do not apply in the microscopic realm. But the new model of Many Interacting Worlds (MIW) developed by Howard Wiseman, a quantum physicist at Griffith University, in Brisbane, Australia, and colleagues may offer an explanation for these bizarre phenomena that relies only on common-sense classical rules. With the help of an FQXi grant of almost $104,000, they are now investigating how interference from these other universes may affect our own—and whether there is a way to find proof of the existence of parallel realities in the lab.
"We prefer to follow Einstein (and like-minded others) in seeking a more realistic theory," Wiseman says. "We’ve taken a fresh perspective involving a fundamental shift from previous quantum interpretations."
When Parallel Worlds Interact
Howard Wiseman describes the Many Interacting Worlds model to Zeeya Merali.
The standard interpretation of quantum theory eschews the idea that there is an objective reality at all. Instead, it says that before scientists scrutinize the properties of a quantum object by measuring them, those properties—such as its location—are not set. Quantum equations can predict with stunning accuracy the probability of getting certain results over many thousands of runs of the experiment; but they cannot tell you with certainty exactly which answer you will get in any single test.
Another famous interpretation, first laid out in the 1950s by American theorist Hugh Everett III suggests that when such measurements are carried out they force a split in reality, creating multiple parallel quantum universes—one for each possible experimental outcome that could be realized. (See "The Many Lives of Hugh Everett III.") The new theory of interacting classical worlds should not be confused with Everett’s theory, however. Where Everett’s universes arise as a result of the seemingly strange laws at play at the quantum level, Wiseman and his colleagues, Michael Hall, also at Griffith University, and Dirk-André Deckert of Ludwig-Maximilians-University in Munich, Germany are searching for a description of reality that rejects quantum weirdness entirely.
With that goal in mind, a few years back, Wiseman began to ponder what would happen if multiple worlds not only existed, but could influence each other. Within these worlds even objects on the smallest scales obey the plain old rules that Isaac Newton devised to explain force and motion. A classical law is also used to describe the forces that the parallel worlds exert on each other. "Ours is a new picture of reality at the atomic scale," Hall says, adding that they believe it to be "both elegant in principle, and useful for calculations in practice."
Otherworldly Influences A repulsive force between particles in different worlds could explain the odd trajectories taken in the double-slit experiment, and other effects.
The three theorists postulate that there are a huge but finite number of worlds, or universes. Each of these is categorized by their configurations—the list of the positions of all the particles in the world. Most worlds have very different configurations, they conjecture, but some are similar, with a few sharing almost identical configurations that feature only microscopic differences in the positions of some particles. It’s these almost identical worlds that exert the strongest influences over each other, creating a repulsive force between particles and their parallel clones (really, "almost-clones") that can influence the paths they take in experiments in one world.
Remarkably, in 2014, the team calculated that a repelling force of this type, acting between a large number of worlds, can be used to explain many puzzling effects of quantum physics (Michael J. W. Hall, Dirk-André Deckert, Howard M. Wiseman; Phys. Rev. X 4, 041013 (2014)).
Consider, for instance, how the MIW interpretation explains quantum tunneling—when a particle unaccountably surmounts an energy barrier that it should not be able to according to classical physics. "In our view, when a particle approaches a barrier that really means that there are parallel classical worlds with particles in each, each approaching the barrier from the other side," Hall explains. A repulsive interaction occurs between these particles; in one world, the particle gets pushed toward the barrier faster than average and so possesses enough energy to get over it. "Clones of the particle in other worlds are correspondingly slowed and reflected by the barrier," says Hall.
If you postulate these worlds, then you can reproduce the equations, the phenomena, the weirdness of quantum mechanics
- Michael Hall
Additionally, in the MIW view, quantum-related probabilities arise from our ignorance as to which specific world we actually occupy. This explains why scientists cannot predict the outcome of an experiment with certainty. "If you postulate these worlds, then you can reproduce the equations, the phenomena, the weirdness of quantum mechanics," Hall says. (Listen to Wiseman describing some of the many quantum phenomena explained by MIW, in detail, including quantum tunneling, indeterminism, double-slit interference, uncertainty, zero-point and vacuum energies, quantised energy levels, and entanglement, in an extended interview on the FQXi podcast.)
Shelley Goldstein, a mathematician and quantum theorist at Rutgers University in New Brunswick, New Jersey, shares the team’s desire for a deterministic alternative to standard quantum theory, but worries that the cost may be too dear in the MIW interpretation. "They want to seem as classical as possible," he says. "The price for doing that is having all these interacting worlds, and it’s a pretty big price." One big technical concern, Goldstein adds, is that the model assumes particles and their parallel clones zip about in a regular way, whereas in real life, even gas particles in a box (obeying classical physics) tend to have more variation in their speeds than the model allows. Despite his skepticism, however, Goldstein notes that the model deserves further investigation.
Michael Hall Griffith University.
With their FQXi grant, the team plans to do just that, with a hopeful eye to developing a way to test the model experimentally. It has been tough to think up experiments that can distinguish between many of the other proposed interpretations because they tend to make the same predictions for quantum experiments, by design. The MIW approach is different, however, because its predictions vary slightly depending on the overall number of classical parallel universes invoked by the model.
Ian Durham, a quantum physicist at St. Anselm College in Manchester, New Hampshire, is excited at the thought of potentially finding evidence of parallel universes. (Durham also discusses MIW on the FQXi podcast.) "Whether it’s true or not," Durham says, "I give them credit for formulating an interpretation that at least has the possibility of producing some testable predictions."
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