Many Interacting Worlds: The Parallel Universe Theory Explained

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Many Interacting Worlds: A 2014 Proposal About Parallel Universes
Universe Today

Short answer: “Many Interacting Worlds” (MIW) is a proposal, published in October 2014 by Michael Hall and Howard Wiseman of Griffith University in Australia and Dirk-André Deckert of the University of California, that tries to explain quantum behaviour as the result of a huge number of ordinary, classical worlds gently pushing against one another. It is a mathematical model and an interpretation of quantum mechanics. It has not been confirmed by any experiment, and it does not show that we could ever visit or signal another universe.

That distinction matters, because the story spread online under headlines saying that parallel worlds “exist” and “interact”. What the physicists actually did is more modest and, in some ways, more interesting: they showed that a set of rules for many interacting classical worlds can reproduce several signature effects of quantum physics.

Why physicists keep reaching for other worlds

Quantum mechanics is the most precisely tested theory in science. It predicts the results of laboratory experiments with astonishing accuracy, and it underpins lasers, transistors and MRI scanners. What it does not do is tell us, in plain terms, what is going on between measurements. A particle seems to travel through two slits at once, a trapped electron can “tunnel” through a wall it lacks the energy to climb, and the outcome of a single measurement can only be given as a probability.

Physicists have offered several competing stories, called interpretations, to make sense of this. They all agree on the numbers that come out of experiments; they disagree about what reality is like underneath. Richard Feynman’s much-quoted line, “I think I can safely say that nobody understands quantum mechanics,” is still a fair summary of the mood.

Ideas involving many worlds are attractive because they remove some of the strangeness from the single world we see. If every possibility is realised somewhere, the odd behaviour of one particle might be a side effect of a larger, perfectly orderly picture.

Everett’s many worlds: branches that never meet

The best-known version comes from Hugh Everett III, who set it out in 1957 while working on his doctorate under John Archibald Wheeler. Everett called it the “relative state” formulation. The catchier name “many worlds” was popularised by Bryce DeWitt in the 1970s, after the idea had been largely ignored for about a decade.

In Everett’s picture there is one universal wave function that never collapses. When a quantum system interacts with its surroundings, the wave function splits into branches, one for each possible outcome, and each branch contains its own version of the observer. Crucially, once the branches separate they evolve independently and cannot influence one another. That is also the main criticism of the approach: if the other branches can never be detected, some scientists argue the idea cannot be tested at all.

What the Many Interacting Worlds model proposes

Hall, Deckert and Wiseman turned Everett’s picture on its head. Their paper in Physical Review X (volume 4, article 041013), titled “Quantum phenomena modelled by interactions between many classical worlds”, starts not from a wave function but from a very large, though finite, collection of worlds. According to Griffith University’s summary of the work, the model rests on three assumptions:

  • Our universe is one of an enormous number of worlds. Most are very different from ours, while some are almost identical.
  • All of these worlds are equally real. Each one persists through time and has precisely defined properties, so particles in each world have definite positions.
  • Worlds that are very similar to each other feel a mutual repulsion, a kind of universal force that pushes nearby worlds to become less alike.

Each individual world follows deterministic, Newton-style rules. Probability enters only because an observer does not know which of the many worlds they happen to be in. In the limit of infinitely many worlds, the authors show that the familiar wave function can be recovered as a secondary, derived quantity rather than as the basic ingredient of reality.

The Griffith announcement quotes Michael Hall on the most appealing feature: “The beauty of our approach is that if there is just one world our theory reduces to Newtonian mechanics.” With a gigantic number of worlds, the model reproduces quantum mechanics; in between, it predicts behaviour that is neither classical nor fully quantum. Some 2014 articles attributed that remark to Wiseman, but the university’s own release credits Hall.

What the model can already reproduce

The arXiv version of the paper lists several quantum effects that emerge from the repulsion between worlds in a simple test model:

  • Ehrenfest’s theorem, the rule that average quantum values follow classical laws of motion.
  • Wave-packet spreading, the way a localised particle’s position becomes less certain over time.
  • Barrier tunnelling, where particles cross an energy barrier that classical physics says they cannot.
  • Zero-point energy, the minimum energy a quantum system keeps even in its lowest state.

Using computer simulations, the team also calculated quantum ground states and reproduced the interference pattern of the famous double-slit experiment in a qualitative way. That is a respectable result for a new framework, but it is a proof of concept, not a full replacement for quantum theory.

Readers who know the subject may notice a family resemblance to Bohmian mechanics, another interpretation in which particles always have definite positions. MIW shares that feature but replaces Bohm’s guiding wave with the push and pull of other worlds.

Can the idea be tested?

This is the question that decides whether MIW becomes physics or stays philosophy. Because the model predicts slightly different behaviour when the number of worlds is finite, the authors suggested it might one day be possible to test for the existence of other worlds, and that the new mental picture could help in planning experiments that probe or exploit quantum effects. The Griffith release does not describe a specific experiment, and none has confirmed the model since.

There may also be a practical spin-off that does not depend on the worlds being real. Approximating quantum evolution with a finite set of interacting trajectories could be a useful calculation tool in molecular dynamics, for example when modelling chemical reactions or the way drugs act on the body.

Reaction from other physicists was mixed. Some welcomed a fresh approach to an old puzzle, while others were openly dismissive; the Czech theoretical physicist Luboš Motl, for one, reportedly called it a waste of time on his blog. Sharp disagreement of this kind is normal for interpretations of quantum mechanics, which are notoriously hard to separate experimentally.

Common misunderstandings

“Scientists proved parallel universes exist”

No. MIW is a hypothesis about how quantum behaviour might arise. Showing that a model reproduces known effects is not the same as showing that its ingredients are real.

“We could contact or travel to another world”

Nothing in the paper describes communication or travel. The interaction is a subtle statistical repulsion between whole worlds, not a doorway. Any talk of contact, like the two Earths of the TV series Fringe, belongs to science fiction.

“Quantum physics breaks cause and effect”

Quantum events are probabilistic and sometimes counter-intuitive, but standard quantum mechanics does not overturn cause and effect, and it does not allow signals to travel faster than light. MIW goes further in the opposite direction by making every individual world fully deterministic.

“This replaces Everett’s many worlds”

Not yet. It is one of several competing pictures. Everett’s branches never interact; in MIW, interaction is the whole point. Which, if either, describes nature is still open.

Why the idea still matters

Even if Many Interacting Worlds never wins acceptance, it is a good example of how theoretical physics moves forward. Researchers take a puzzle everyone agrees on, propose a precise model with clear assumptions, show what it can and cannot reproduce, and then look for a place where its predictions differ from the standard theory. That last step is where the real test lies.

For anyone curious about parallel universes, the honest takeaway is a balanced one: serious physicists do take the idea of many worlds seriously as a way to understand quantum mechanics, but every version so far remains an interpretation, not an observed fact. Watch for experiments that can tell the interpretations apart, because that is where the story will be decided.

Sources: Griffith University News: New quantum theory is out of this parallel world; Hall, Deckert and Wiseman, “Quantum phenomena modelled by interactions between many classical worlds”, arXiv:1402.6144 (Physical Review X 4, 041013); Physical Review X journal page; Wikipedia: Many-worlds interpretation; earlier coverage by Anna LeMind at The Mind Unleashed; header image via Universe Today.

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