*The following is an extended version of the article found in the programme for "Constellations".*

In “Constellations”, we explore the idea that there could exist other universes besides our own – as Marianne describes it, “the possibility that we’re part of a multiverse”. This seems like science fiction – a parallel universe, of course, is a tried and tested sci-fi trope, explored in Doctor Who, Star Trek and Buffy the Vampire Slayer, for example.

The idea of parallel universes, however, is grounded in quantum mechanics.

Quantum mechanics deals with the physics of very small objects like atoms and molecules, the opposite of classical mechanics, which deals with physics of bigger objects i.e. objects we can see with the naked eye.

With classical mechanics, most things can be explained by just knowing a few things – for example, if we want to measure the movement of a toy car rolling down a hill, we might want to know about position, velocity, gravity, but once we know all these things, we can predict exactly what will happen to the car in the future.

Quantum mechanics is not so straightforward. Quantum mechanics relies on all this basic information (position, velocity, etc. which we call a “quantum state”) but we can only gain a basic notion of the outcome – we calculate it in terms of probabilities. In order to say anything concrete about the quantum state, we need an observer to take measurements and actually look at what is happening.

This causes physicists a lot of problems, because it means we cannot correctly predict what is going to happen to quantum particles. These issues have spawned many possible explanations, which we refer to as the “interpretations of quantum mechanics”.

**The Copenhagen Interpretation**

The Copenhagen Interpretation was first posed by physicist Niels Bohr in 1920. This follows the idea that a quantum particle doesn’t exist in one state or another, but actually exists as a combination of both states until an observation is made. Once an observation is made, the quantum particle just exists in one state – the state that has been observed. This interpretation makes mathematicians very happy – it is relatively straightforward to model the quantum particle as a wave oscillating between two states which then collapses to a straight line when an observation is made.

But, whilst mathematicians are happy, the Copenhagen Interpretation still encounters a few problems. A major problem is that it is difficult to define exactly what we mean by observation.

Suppose, for example, we throw a ball, and the ball behaves like a quantum particle, so that, under the exact same conditions, it is possible for the ball to land at either point A or point B. We cannot say exactly which point the ball is at, so we say that it lies in a probabilistic haze of both points A and B together. The ball is literally in both places at once (or neither place) until we go and look and say “it’s definitely in position A”. But does this mean that human observation forces the ball to decide where to be? What if it’s seen by a dog first? Or a grasshopper? Or a molecule of air?

It’s a similar idea to the old saying “If a tree falls in a forest but no one is there to hear it fall, does it make a sound?” What happens to the quantum particles that no one can measure? Do they exist in a weird combination of two states forever and ever?

**The Many Worlds Interpretation**

Physicists weren’t happy with this idea, and in 1957 a student at Princeton University, Hugh Everett, came up with a new interpretation, which is known today as the “many worlds interpretation”. This interpretation deals with the fact that every possible state of a quantum particle exists in some universe or other, and that each time we make an observation, the universe takes one path or another.

So this means that, when a scientist looks at a ball, she literally splits into two alternate versions of herself, one that sees that ball at position A, and another that sees the ball at position B. From her perspective, it seems that she has influenced the position of the ball just by looking at it, when in fact, she has divided into two different people living in two different parallel worlds.

But the problem is that these observations are happening all the time. So, if the ball is thrown again, and again, and again, there are not just two different parallel worlds, but many, many parallel worlds, all stacking up on top of each other. All events that can occur will occur, and this is detrimental to quantum theory. What is the point of having a theory where the final answer is “maybe this happens”?

**Cosmic inflation and bubble universes**

In Constellations, Marianne is interested in early universe cosmology. This is basically the theory of how the universe began, looking at what happened in the first nanoseconds following the Big Bang. During this period, the universe expanded very very quickly, in a process which we call “cosmic inflation”. Recent theories suggest that, during cosmic inflation, some areas of the universe would stop expanding whilst others would continue, and this causes what is known as “bubble universes”.

One way to think about this is by considering a loaf of bread expanding in the oven. At various points in the bread, we get gas pockets. These gas pockets are basically our bubble universes. Each bubble universe is infinite in size, and they all coexist alongside each other, like a big cosmic bubble bath.

The bubbles, however, do not interact in any way – they are all totally separate. This means that the laws of physics in our universe do not have to be the same as the laws of physics in other universes, and again, this makes physicists worried.

A recent idea, put forward in 2011 by Yasunori Nomura of the University of California, states that these infinitely many bubble universes are mathematically equivalent to the many worlds predicted by quantum mechanics. This would mean that a new universe is not created every time a measurement is made, but that these universes already exist – have existed since the start of time.

Perhaps this is why Constellations is structured the way it is – there is no common starting point for Roland and Marianne. There is no specific universe that spawns all the other universes that we see them in. The universes have always existed, forever and ever, since the Big Bang.

Tom Morley, 2019

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