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When Classical Universes Collide, The Result is Quantum Mechanics, Say Physicists

One of the strangest ideas to emerge from 20th century physics is the many-worlds interpretation of quantum mechanics, which attempts to explain the puzzling, counter-intuitive effects of quantum mechanics.

Updated version of the previous article.

These puzzling phenomena are things like quantum interference in which a quantum particle passing through a double slit produces an interference pattern, something that can only happen the particle passes through both slits at the same time.

That’s not something that is easy to reconcile with our experience of the universe. So the many-worlds interpretation explains this with the idea that there are actually two universes involved in such an experiment.

In one of these, the particle passes through one slit while in the other universe, it passes through the second slit. In other respects these universes are identical. The interference pattern arises because these universes are in a quantum superposition.

That’s easier to digest because no particle is ever in two places at the same time. But it raises other issues. Big ones!

The many worlds interpretation holds that anything that could happen actually does happen in a parallel universe. So there must be an infinite number of parallel universes containing everything that could ever have happened.

In one of these you are president, in another an ageing Elvis is living happily in Graceland with his many grandchildren and in another the Soviets won the Cold War and so on.

That’s a difficult idea to swallow and there other problems with the many worlds interpretation too. One is that physicists can use quantum mechanics to predict that a particular outcome of a measurement is a hundred times more likely than another. And when they do the experiment, that’s exactly what happens.

By contrast, the many worlds interpretation predicts that all possible outcomes actually happen but does not explain why physicists see the most probable outcome a hundred times more often than others.

That’s why many physicists remain unconvinced and why quantum theorists are still searching for more believeable interpretations

Today, Michael Hall at Griffith University in Brisbane, Australia, and a few pals say they have an alternative interpretation that tackles the counter-intuitive problems of quantum mechanics but is still able to explain why experiments behave in the way they do.

On the face of it, Hall and co’s new interpretation has some similarities with the many worlds interpretation. They assume for example, that there are indeed many different parallel universes.

But there important differences. For start, each of these universes is entirely classical—in other words, each evolves according to classical Newtonian physics.

However, these universes can also interact by means of a repulsive force that prevents particles in different universes from approaching each other arbitrarily closely. “All quantum-like effects arise from the existence of this interaction,” say Hall and co. “In the absence of an interworld interaction, the worlds evolve independently under purely Newtonian dynamics.”

These guys call their new approach the “many interacting worlds” interpretation of quantum mechanics. And they say it immediately solves the probability problem because quantum probabilities play no role in any of the worlds. “Probabilities arise only because observers are ignorant of which world they actually occupy,” say Hall and co.

Here’s what happens. The worlds evolve in different ways, depending on what actually happens in each. And this allows them to be grouped into classes according to these macroscopic outcomes.

It’s the relative size of these classes that gives rise to the probabilistic nature of measurement. “The orthodox quantum probabilities will then be approximately proportional to the number of worlds in each class,” say Hall and co.

They go on to study a number of examples in which the repulsive interaction between different worlds gives rise to various quantum effects that physicists see. Quantum tunnelling, for example, is the phenomenon in which a particle with a certain energy passes through a barrier with a higher energy, which it could not pass classically.

In the many interacting worlds interpretation, Hall and co imagine two parallel worlds in each of which a particle heads towards a barrier. In one world, the particle bounces off the barrier but in the other, the repulsion between particles gives the second the boost it needs to overcome the barrier. “This boost can be sufficient for this world to pass through the barrier region, with the other world being reflected, in direct analogy to quantum tunnelling,” they say.

They also show how this repulsion leads to the effect known in conventional quantum mechanics as wave-packet spreading. And how this spreading causes quantum interference.

They go on to model how this interference occurs in a double slit experiment using a computer simulation of more than 40 interacting worlds. The results, at least in this simulation, are similar to those predicted by quantum mechanics.

And they say that as the number of worlds becomes larger, the results become closer to that predicted by quantum mechanics.

That’s an interesting idea that will certainly pique the interest of many physicists who worry about the interpretation of quantum mechanics.

There are still problems ahead, of course. Perhaps the most severe, and therefore the most interesting, is how the many interacting worlds interpretation handles entanglement, the strange phenomenon in which quantum particles share the same existence even though they are spatially separated.

Many physicists think that experiments have already ruled out the possibility that entanglement can be explained by the existence of additional parameters that are hidden in conventional quantum mechanics. If Hall and co’s idea falls into this class, it will get short shrift.

And therein lies the problem—experimental verification. Being a practical bunch, most physicists are uninterested in new interpretations unless they lead to predictions that can be experimentally tested.

That’s why the original Copenhagen interpretation of quantum mechanics is still so popular. It may have any number of philosophical deficiencies but it makes the same predictions as all the other interpretations and so physicists stick by it. (Hence its nickname: the “shut up and calculate” interpretation of quantum mechanics.)

But Hall and co leave open the possibility that their theory will lead to testable predictions. The key is in the number of worlds that actually make up the universe or multiverse.

The predictions from their theory become close to conventional quantum mechanics as the number of worlds tends to infinity. So if there are a finite number of worlds, there ought to be ways to experimentally measure this.

That’s for future work, of course. We’ll look be following it, and the reaction to it, with interest.

Ref: : Quantum Phenomena Modelled By Interactions Between Many Classical Worlds

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