The notion that our universe may be just one in a series of endless parallel universes—some very similar, some wildly different—has captivated the hearts and minds of many science fiction fans. A branching multiverse, objects that exist in two places at once, light that behaves as both particles and waves—these are but a few of the weird facets of reality brought to us by quantum mechanics.
But parallel universes hold a special place in this list. They may, in fact, be the root of all quantum weirdness. In a paper published last week in the journal Physical Review X, quantum physicist Howard Wiseman and colleagues lay the groundwork for their new, “many interacting worlds” theory.
In this novel approach to quantum physics, each individual world is ruled by classical Newtonian mechanics. All that quantum behavior? Actually the result of different worlds smashing into each other.
This approach is in stark contrast to the traditional “many worlds” interpretation of quantum mechanics, which goes something like this: There are a bunch of parallel realities out there, and any time an event is observed in any of them, that universe branches to spawn a whole slew of new realities, one for each possible outcome of the observation. This process of universes birthing universes repeats itself ad infinitum.
all other worlds are as real as our world, and they’ve all been around since the beginning of time
Several aspects of the many worlds theory are troubling. For one, it doesn’t precisely define when an observation occurs, making it impossible to say how many worlds actually exist. Secondly, different observational “outcomes” have different probabilities: All worlds are real, but some worlds are more real than others. If this sounds confusing to you, rest assured you’re in good company.
“The problem with the many worlds interpretation is that it’s fuzzy,” said Wiseman. “Simply put, we cannot count the number of worlds that exist at any point in time. This makes the whole notion very hard to reconcile with the claim that these worlds are real.”
Instead, “many interacting worlds” proposes a fixed, although gargantuan, number of universes.
“In our theory, all other worlds are as real as our world, and they’ve all been around since the beginning of time,” Wiseman said. “The only mystery is what particular world we occupy.”
Some of these universes are our cosmic neighbors—they’re nearly identical to ours in the position of every single particle. According to Wiseman’s theory, it’s a sort of repulsive force between these neighbors that’s responsible for all quantum phenomena.
This talk discusses a bit of the ramifications of universes bumping into each other.
“Quantum mechanics has always been a puzzle because of the subtle but deep ways it deviates from Newtonian mechanics,” Wiseman wrote last week. “That these deviations could be due to a delicate interaction of essentially Newtonian worlds with 'nearby' parallel worlds is an entirely new solution to the quantum puzzle.”
If that’s not enough to keep you up at night, add to it that the interactive force between universes is like nothing physicists have ever known. Most forces in our experience occur between two discrete bodies: The gravitational interaction between the Earth and the Moon, for instance. But in Wiseman’s theory, forces exist between clusters of universes, and they cannot be decomposed into individual, two-body interactions.
“This force only exists when universes are close together in…well, whatever space parallel universes exist in,” Wiseman told me.
If correct, the theory would mean that the onerous “wave function”—a complex mathematical formulation that quantum physicists developed to describe how tiny particles interacts—is not a fundamental component of reality. (In an interesting, related development, researchers at Brown recently shattered a quantum wave function.)
The idea of interactions with other universes is no longer pure fantasy
If the team's theory rings true, it would also mean that widely-accepted conclusions based on the Heisenberg uncertainty principle—that a particle’s speed and position can never be simultaneously known—have a fatal flaw.
“Operationally, the Heisenberg uncertainty principle still holds true in our theory. We can never know a particle’s speed and position at the same time," Wiseman said. "But it’s the deduction from there, that particles don’t have definite positions and velocities, that’s incorrect.”
We just can’t measure those definite positions and velocities, because lots of other universes are constantly bumping up against our own. Jerks.
There are many competing interpretations for quantum mechanics, and this new one is sure to elicit opposition from other factions in the scientific community. But Wiseman is optimistic that others can be convinced to take his team's concept seriously. And for good reason: Many interacting worlds has already been able to reproduce standard features of quantum mechanics, including the double slit experiment, which is usually taken to demonstrates that light can behave as either a wave or a particle.
So, the question that’s actually on everyone’s mind: If parallel universes are not so parallel after all, will we macroscopic beings one day be able to interact with the other universes in our neighborhood?
“It’s not a part of our theory,” said Wiseman. “But, if a force does exist between parallel worlds, you can start to wonder, what if that force is not exactly how we’ve written it down. The idea of interactions with other universes is no longer pure fantasy.”
It opens up doors, so to speak.