Australasian Science: Australia's authority on science since 1938

## When Parallel Worlds Collide

By Michael Hall

**Bizarre quantum phenomena, such as particles tunnelling through barriers and behaving like waves, can be explained by subtle interactions between our world and others. **

Quantum mechanics is so weird that one of its founders, Niels Bohr, said: “Those who are not shocked when they first come across quantum theory cannot possibly have understood it”.

It’s not shocking because of the maths – which is used to describe the workings of the chips in your computer and the laser in your DVD player – but because of something unexpected that the maths tells us: we can only make sense of the world, at small scales, if we fundamentally change our picture of reality in some way.

Indeed, some physicists, including Bohr himself, have seriously suggested giving up reality altogether, asserting that atoms and electrons are just fuzzy clouds of probability that cannot be further analysed. Albert Einstein famously hated this idea, declaring: “God does not play dice!”

Other physicists have come up with alternative suggestions, such as faster-than-light communication, a lack of free will to choose experimental settings, and the splitting of our world into many new independent worlds each time an experiment is performed.

All of these are sufficiently shocking to be able to explain quantum effects. However, my colleagues and I have recently proposed a new picture of reality at the atomic scale that is both elegant in principle and useful for calculations in practice. It is based on the concept of many interacting worlds.

### Many Interacting Worlds

In Newtonian physics, one can imagine a list of the positions of all the particles in our world. This list is called the “configuration” of the world. Because the particles move about and interact with each other, the configuration changes in time. Newton’s laws of motion tell us how to predict the configuration at future times.

The curved lines in Figure 1 depict the changing configurations of four different possible worlds. If these worlds were described by Newtonian physics, each configuration would change independently of the others. However, the worlds in the figure are not independent: when two worlds approach each other they are repelled.

Remarkably, a repelling force of this type, acting between a large number of worlds, can explain all the effects of quantum physics.

### Quantum Tunnelling

As a simple example, consider the phenomenon of quantum tunnelling, where a particle such as an electron has a non-zero probability of escaping through a barrier despite having insufficient energy to penetrate it by any classical means. Such tunnelling is critical to technologies such as radiocarbon dating and solar cells, and is used in scanning tunnelling microscopes to image individual atoms.

To understand the physics of quantum tunnelling in the “many interacting worlds” approach, consider Figure 2 in which two particles, one in each of two worlds, approach an energy barrier in each world.

In Newtonian physics, neither particle has enough energy to penetrate the barrier, and will simply bounce off from it. But in the “many interacting worlds” approach there is an interaction between the two worlds that causes the particles to repel each other. This mutual repulsion pushes the particles apart, giving the leading particle enough energy and speed to pass through the barrier. The other particle will correspondingly lose energy and speed, and bounce off the barrier.

Thus there is a very simple explanation of quantum tunnelling in our approach, based on a repelling force between worlds and the conservation of energy. Of course, to obtain accurate predictions of observed quantum tunnelling, many more worlds than just two must be considered, but the basic principle is the same.

Similarly, all of the other bizarre effects predicted by quantum mechanics, such as the Heisenberg uncertainty principle and wave-particle duality, can be explained in terms of mutual repulsion between neighbouring configurations of a large number of parallel worlds.

The hard work in coming up with such an explanation is in finding a suitable form for the repulsive force acting between these worlds. After arguing back and forth for nearly a year over the details, my colleagues – Prof Howard Wiseman at Griffith University and Dr Dirk-André Deckert at Ludwig-Maximilian University in Munich – and I determined that the force was very different from other types of forces in physics, such as gravitation and electromagnetism.

Indeed, the forces operating between a group of worlds are only significant when every particle in one of the worlds is very close to its corresponding particle in the other worlds. This is why quantum effects are typically small and only seen at short distances.

### Where Do Quantum Probabilities Come From?

Our “many interacting worlds” approach is very different from previous notions of “parallel worlds” in quantum mechanics and movies. For example, unlike a well-known idea put forward by Hugh Everett in 1957, our worlds interact and they never split into new worlds.

Our approach is more similar in spirit to a 2010 proposal by Bill Poirier from Texas Tech University, who suggested that quantum effects arise from stresses and strains within a fluid-like continuum of worlds. In our approach, however, there are only a finite (but enormous) number of worlds that directly interact with each other while conserving the total energy.

Having a finite number of worlds leads to a major advantage of the “many interacting worlds” approach: the very natural manner in which probabilities appear. Unlike standard quantum mechanics, no mysterious “wave function” is needed to determine these probabilities. Instead, they arise simply from ignorance as to which one of the possible worlds we actually occupy.

In particular, if there are N worlds that are compatible with the knowledge of an observer, then the observer does not know which one of these worlds he or she actually occupies, and so assigns an equal probability of 1/N to each of them. This is similar to assigning a probability of 1/6 to each of the six possible outcomes of a die before it is thrown. This simple idea is all that’s needed to make correct predictions about the statistics of quantum experiments, within the “many interacting worlds”picture.

### Wave–Particle Duality

It’s interesting to put the concepts of a repelling force and equal probabilities together to see how many interacting worlds explain the famous double-slit experiment, in which a quantum particle appears to act like a wave.

If a water wave or a sound wave is sent through a screen with two slits, then part of the wave will go through each slit and recombine to form an interference pattern on a screen on the other side. What happens if quantum particles such as neutrons are sent one-by-one through such slits? Amazingly, the locations of the successive particles build up into a similar interference pattern on the other side. It is as though each particle somehow went through both slits at the same time, just like a wave.

This wave–particle duality is easy to understand in the “many interacting worlds” approach. Each line in Figure 3 represents the trajectory of a particle from a different world. The mutual force of repulsion between the worlds causes these trajectories to deviate from straight lines, and the result is an interference pattern just like what we see in experiments.

The bright regions in Figure 3 correspond to regions where the predicted quantum probability of finding a particle is high. It can be seen that there are correspondingly more worlds in these bright regions. Thus, since each world is equally likely (1/N), there is a greater probability of being in a world in which the particle occupies a bright region, as is predicted by quantum mechanics.

### What Use Is It?

The “many interacting worlds” approach is not only an elegant new way of understanding quantum phenomena – it may actually be of further value in several ways.

The most immediate likely application is as a useful tool for quantum calculations. Such calculations are needed to understand molecular reactions (e.g. for drug design) but are notoriously difficult to carry out. We believe it may be possible to model molecular quantum dynamics more efficiently and accurately by using a relatively small number of interacting worlds for calculations.

Having a different way of thinking about the atomic scale may also provide a starting point for developing new physics. For example, the problem of how to incorporate gravity into quantum mechanics remains unsolved. However, it is very hard to change the maths of quantum mechanics without running into problems such as negative probabilities or closed loops in time. The “many interacting worlds” approach allows consistent tweaking of the maths. For example, changing the number of worlds slightly changes predictions. This gives us hope that our approach will help solve existing problems in quantum theory.

Finally, it may be possible to experimentally find direct evidence of other worlds. This would require new predictions that follow from the existence of a finite number of interacting worlds, and is purely speculation at this stage. In the meantime it is nice to think that they may be out there, ensuring the existence of those subtle quantum effects that keep our computer chips and DVD players in operation!

*Michael Hall is a Senior Research Fellow in the Centre for Quantum Dynamics at Griffith University.*