The idea that an infinite number of parallel worlds could exist alongside our own is hard to wrap the mind around, but a version of this so-called Many Worlds theory could provide an answer to the controversial idea of quantum mechanics and its many different interpretations.
Bill Poirier, a professor of physics at Texas Tech University in Lubbock, proposed a theory that not only assumes parallel worlds exist, but also says their interaction can explain all the quantum mechanics "weirdness" in the observable universe.
Poirier first published the idea four years ago, but other physicists have recently started building on the idea and have demonstrated that it is mathematically possible. The latest research was published Oct. 23 in the journal Physical Review X.
Quantum mechanics is the branch of physics that describes the rules that govern the universe on the microscopic scale. It tries to explain how subatomic particles can behave as both particles and as waves. It also offers an explanation about why particles appear to exist in multiple positions at the same time. [The 9 Biggest Unsolved Mysteries in Physics]
This fuzzy clump of possible positions is described by a "wave function" — an equation that predicts the many possible spots a given particle can occupy. But the wave function collapses the second anyone measures the actual position of the particle. This is where the multiverse theory comes in.
Some physicists believe that once a particle's position is measured, the many other positions it could take according to its wave function split off and create separate, parallel worlds, each only slightly different from the original.
Hugh Everett was the first physicist to propose the possibility of a multiverse — an infinite number of parallel universes that exist alongside our own. He published his "Many Worlds" theory in the 1950s, but the idea was not well-received in the academic world.
Everett ended his career in physics shortly after getting his Ph.D., but many physicists now take the multiverse and parallel-worlds idea seriously. Poirier reworked the Many Worlds theory into the less abstract "Many Interacting Worlds" (MIW) theory, which could help explain the weird world of quantum mechanics.
Quantum mechanics has existed for more than a century, but its interpretation is just as controversial today as it was 100 years ago, Poirier wrote in his original paper.
Albert Einstein was not a fan of quantum mechanics. The idea that a particle could exist in a haze of probability instead of a definite location did not make sense to him, and he once famously said, "God does not play dice with the universe." However, this new MIW theory might have helped to put Einstein's mind at ease. In the MIW theory, quantum particles don't act like waves at all. Each parallel world has normal-behaving particles and physical objects. The wave-function equation doesn't have to exist at all.
In the new study, which builds on Poirier's idea, physicists from Griffith University in Australia and the University of California, Davis, demonstrate that it only takes two interacting parallel worlds — not an infinite number — to produce the weird quantum behavior that physicists have observed. Neighboring worlds repulse one another, the researchers wrote in the paper. This force of repulsion could explain bizarre quantum effects, such as particles that can tunnel through barriers.
But how can physicists prove we're living in just one of millions of other worlds, or that these worlds interact? Poirier thinks it will take some time to develop a way to test the idea.
"Experimental observations are the ultimate test of any theory," Poirier said in a statement. "So far, Many Interacting Worlds makes the same predictions as standard quantum theory, so all we can say for sure at present is that it might be correct."
The authors of the new paper hope that expanding the MIW theory will lead to ways to test for parallel worlds and further explain quantum mechanics.
Richard Feynman, a physicist who worked on the Manhattan Project, once said, "I think I can safely say that nobody understands quantum mechanics," but Poirier and his colleagues argue that physicists have much to gain from trying.