## Why Many Worlds?

Welcome to the first in a series of articles about the infamous “Many-Worlds Interpretation” (MWI) of Quantum Mechanics. Many laymen will already have heard of this theory in popular science and sci-fi shows, but may be unsure of how plausible it really is. Is the MWI a serious academic proposal? Or is it just a fanciful interpretation that lends itself well to convoluted plot lines in TV shows?

Despite controversial elements within the MWI, what isn’t controversial is that this theory is taken seriously by eminent philosophers and physicists. It isn’t some pseudoscientific theory of spirituality or mentality, it’s a full-blooded reading of the mathematical framework of quantum theory. To understand why the MWI is the subject of copious scientific research, we need to take a look at the central problem of quantum theory – the Measurement Problem. Before we get to this however, it is pertinent to explain why we’re making so much fuss about quantum mechanics at all. This is especially important because, as we shall see later on, Many-World theorists claim that their interpretation is the only one that takes quantum theory seriously.

## Why Quantum Mechanics?

The overriding sentiment quantum mechanics conjures in the public’s consciousness is one of weirdness. People know that quantum mechanics is strange, so it is an understandable reaction to be cautious of how much trust one should have in it. Quantum theory tells us that the world behaves very differently at the microscopic level to how we’d expect. Particles no longer have definite locations, or definite velocities. Instead, the fundamental building blocks of the universe are spread out as waves of probability and uncertainty. Why should we take such a counter-intuitive theory of reality seriously at all?

Quantum mechanics is one of the central pillars of modern physics, and is an absolutely essential module in any university physics course. The reason for this is simply due to its successful and accurate predictions. The fact is that quantum mechanics has passed all sorts of experimental milestones and tests; however uncomfortable physicists are with the theory, nature keeps telling us that it describes reality better than our intuitions. The photoelectric effect teaches us that light sometimes behaves as particles, the double-slit experiment shows us that all particles have both wave-like and particle-like properties, the Aspect experiment showed us that quantum entanglement is real, and that it cannot be explained by “hidden-variables”. It is now beyond any dispute that quantum theory provides a very accurate description of the microscopic world. To put things on quantitative terms, Quantum Electrodynamics (QED), which is the special relativistic version of quantum theory is in agreement with experiment to an accuracy of 10^{−8} . This is a remarkable achievement and it’s the reason that QED is often called “the most accurate physical theory ever”.

A more tangible reason you should be willing to embrace the reliability of quantum theory is because you already rely on it every day, in the technology that powers your life! The electronic devices we use are built on quantum theory: lasers, transistors, microchips, MRI scanners, and even USB sticks. If quantum theory were not an accurate physical theory of the microscopic world, the computer I’m using to type this article would not function!

You may wonder why we still discuss quantum theory at all if there are more up-to-date versions of it compatible with relativity like QED. Why are we talking about interpretation of quantum theory, and not interpretations of QED? QED may be a more general version of quantum theory, but its mathematical framework is based on the same foundations as in quantum theory. We don’t have particles with defined positions or momenta, and what we calculate are still probabilities for outcomes in certain experiments. The logic of quantum theory is broadly unchanged in QED, but QED is technically much more fiddly and so it’s much easier to discuss quantum theory instead of QED.

At this point you should have ample reason to trust that quantum theory reflects the fundamental behaviour of microscopic systems. Now that we’re agreed that quantum theory describes the world, we can ask what it says about it and why what it says causes such difficulty. In the next article, we’ll take a look at the central problem of quantum theory – the measurement problem – and how it leads us to positing the existence of many worlds.