## Introduction

The field of quantum theory first emerged in the early 20th century, with astounding scientific breakthroughs driven by the preeminent physicists of the period. Revelations about the behaviour of subatomic particles not only shattered the assumptions of classical mechanics, but also prompted many to ponder an existential question: What is the fundamental nature of reality?

Since then, dozens of theories have been proposed, each offering a view on how the peculiar world of quantum mechanics should be interpreted. This article provides a brief overview of some of the most dominant schools of thought that have emerged over the last hundred years, and the reasoning behind them.

Before you go any further ensure you have read our article explaining the basics of quantum mechanics.

## Copenhagen

First put forward by some of the founding fathers of quantum mechanics such as Niels Bohr and Werner Heisenberg, the Copenhagen interpretation was the first attempt to explain the seemingly inexplicable, and was widely viewed as the prevailing theory until the 1950’s.

The basic principle is that a quantum particle exists in all possible states (known as coherent superposition) up until the point it is observed. It was theorised that this happens because the rules of quantum mechanics only apply to subatomic systems, and therefore when interaction occurs with a macroscopic measuring instrument, the wave function collapses, and the characteristics of a classical system are assumed.

It is grounded in the notion that Physics is fundamentally the science of measurement and that a quantity only really exists once it has been measured, a concept which is supported by the famous thought experiment ‘Schrödinger’s cat’.

It is still supported by many physicists today, though some take issue with the inclusion of classical concepts in the interpretation and particularly the unexplained discrepancies in the behaviour of macroscopic objects and sub-microscopic particles. However, Bohr and his associates believed they were essential in helping describe experimental observations.

## Many Worlds

The many worlds interpretation attempts to remove this distinction, asserting that if the wave function is present at the quantum level, then it should extend to the entire universe rather than collapsing upon interaction with a measuring instrument.

Instead, this theory asserts that once a measurement occurs, the universe splits into many different universes. Essentially, when a quantum experiment is conducted, every possible result is in fact obtained, each with its own world, even though we are only aware of the world in which the result we have observed, occurred. The number of worlds in which a certain outcome occurs depends on the probability that that result is obtained.

It was first put forward by Hugh Everett in 1957 and is regarded by many physicists as preferable to the Copenhagen interpretation because there is no unexplained wave function collapse. It is undoubtedly an alluring prospect to consider the infinite possible universes that might exist in parallel each with its own version of you and every other person.

However, there are also criticisms of this interpretation. Firstly, what causes universes to split is just as enigmatic as the wave function collapse. Secondly, given that a measurement is defined as every interaction between two quantum entities, the number of worlds created would be staggering. For example, if a single photon hits a planet in some far corner of the universe, another copy of Earth would be produced. This has led some to state that the interpretation gives disproportionate importance to every single quantum event.

## Pilot-Wave Theory

Pilot-wave theory, also known as Bohmian mechanics or de Broglie-Bohm theory, is possibly the easiest of the quantum interpretations to comprehend. First put forward by Louis de Broglie in 1927, and later expanded upon by David Bohm in the 1950s, posits that particles always have definable properties and don’t also exist as waves as in other interpretations.

Instead, the theory argues that there are also real waves that guide how the particles move. The behaviour of a particle is described partially by its wave function; however, a “guiding equation” completes the description of the particles, determining the actual position of a particle and its velocity in terms of the wave function.

In the famous double-slit experiment, firing single photons creates an interference pattern, despite the lack of anything to actually interfere with those photons. Pilot wave theory states that physical waves also exist in addition to the particle and act as a sort of ‘track’ for it to follow, and that if we knew all the properties of the particle, then we would be able to predict its final position.

Despite the seemingly logical nature of this interpretation, there are several criticisms of the theory that have prevented it from becoming widely accepted. Primarily, it necessitates the existence of hidden variables which either cannot or currently are not recognised in order to remove the probabilistic behaviour of particles present in other interpretations. This has led many to call the concept inelegant and unnecessarily complicated. De Broglie himself later rejected his own theory in favour of more conventional interpretations.

## Quantum Bayesianism

Quantum Bayesianism or QBism has been labelled by many as one of the most radical interpretations of quantum mechanics. Its use of subjective Bayesian probability differentiates it from other theories. Essentially, instead of viewing probability as the frequency of an occurrence, it is regarded as a quantification of personal knowledge. Therefore, while the other interpretations listed in this article all define the wave function as something which describes an objective reality common to all observers, QBism instead suggests that it only describes one observer’s subjective knowledge of a system. The system itself does not have a wave function, but rather the observer has one which may be different to another observer’s wave function. For example, the collapse of the wave function is explained as the process of the observer gaining knowledge of a system via a measurement. While numerous of physicists agree that QBism is a coherent and well-defined interpretation, its strongly subjective nature means it is undoubtedly philosophically challenging, with some even going so far as to accuse it of solipsism and instrumentalism. Therefore, many ultimately prefer more conventional interpretations.

## Conclusion

There are many different interpretations of quantum theory, all with their own unique take on the curious behaviour of particles at the subatomic level. From the parallel universes of many worlds to the subjective realities of QBism, the combination of science and philosophy makes this an undeniably thought-provoking subject; and there are plenty more interpretations out there for you to explore, along with variations on those included in this article. With continued research and the promise of further scientific breakthroughs, it remains to be seen how our perception of quantum mechanics might evolve in the future, and who knows? We may soon get one step closer to finally understanding the nature of reality itself.