Quantum mechanics is a branch of physics which describes the behaviour of matter at the atomic and sub-atomic level. In the quantum world, matter behaves in a completely differently manner to the objects we see around us. A striking feature is that quantum systems can exist in more than one state simultaneously; in the macroscopic world, this is like saying a radio can be on and off at the same time.. This contradicts our everyday experiences, leading to some very interesting consequences which cannot be fully explained by our current quantum theories.

Two famous experiments illustrate some of the unusual characteristics of the quantum world. The first is the double slit experiment, originally performed by Thomas Young in the early 19th century. It demonstrates the dual nature of matter—the fact that matter sometimes behaves like a collection of particles, and sometimes behaves as a wave. The experiment involves firing a single beam of electrons at two slits with a photographic plate beyond. Electrons have mass and momentum, which are properties of particles, so we might expect them to exhibit particle-like behaviour by producing a fairly even pattern over the whole of the photographic plate: imagine throwing tennis balls at a wall through two holes. But instead we observe a series of light and dark bands, much like the pattern seen when two pebbles are dropped into a pool of water and the resulting waves interact. This suggests the electrons are behaving like waves; each slit acts as a single wave source, and the wave’s peaks and troughs interfere constructively (combine) or destructively (cancel out) to produce a series of light and dark fringes. Mathematically, these waves are treated as combinations of various different possible states of the electrons, each characterised by a construction called a ‘wave function’. To describe the nature of matter fully all of these wave functions must be gathered together, undergoing what is called ‘superposition’. It’s this process of combining wave functions which allows interference to occur.

So far, quantum theory can cope. But there’s more.

Our second experiment is affectionately entitled Schrödinger’s Cat. It should be stressed at this point that this is a thought experiment, and no cats have actually been mistreated by quantum physicists! It involves a cat, sealed in a box, with only a Geiger counter, a bottle of cyanide and an atom of a radioactive material for company. The half-life of the radioactive material is sixty minutes, which means that after one hour there is a fifty-fifty chance that the atom has radioactively decayed. The system is rigged so that if the Geiger counter detects the atom decaying, the cyanide bottle is smashed and the cat dies. So after one hour, what has happened to the cat? The mathematics of quantum mechanics would have it that the poor animal is in a superposition of states— it is in some way both dead and alive as both outcomes are equally likely. But this just doesn’t make sense. After all, when we open up the box to check, we will definitely see either a living cat, or a dead cat—a single result. It seems that at some stage, the superposition of states must be reduced to a single state which decides the fate of the cat – a process referred to as the collapse of the wave function. Various ways of understanding the collapse have been proposed and some of these hypotheses throw up more questions than answers.

Decoherence

An important result of quantum mechanics is decoherence theory. This reminds us that when we carry out a measurement on a system which is in a superposition of states, the system is not isolated: the states all interact separately with the environment, so that the environment itself becomes part of the superposition.

Imagine you are looking at a rainbow—you will see the end of the rainbow hovering over a particular spot. However your friend on the other side of town will see the end of the same rainbow above a different location. In other words, where you stand dramatically affects what you see. Since we, as observers, are now included in the system, our experiments will give determinate results as if the wave function has collapsed to a single state i.e. the end of the rainbow appears to be in one place as a direct result of our observation although there are many spots it could be in.

However, decoherence only produces the appearance of wave function collapse, not a real collapse- there are still many possible ends to the rainbow, we just don’t have any way to observe this superposition. Thus decoherence helps with the mathematics, but as an interpretation of what is really going on it remains incomplete.

Copenhagen Interpretation

The oldest way of understanding quantum mechanics is called the ‘Copenhagen Interpretation’, This approach, developed in part by Danish physicist Niels Bohr in the early history of quantum mechanics, involves using the theory to make predictions, but reserving judgement as to the deeper meaning of the mathematics. That means ignoring the idea of Schrödinger’s cat being dead or not, and satisfy ourselves that there are two possible outcomes to the experiment with certain probabilities.

Though it’s easy to dismiss this as an attempt to sidestep the really hard questions in quantum mechanics, proponents of the Copenhagen Interpretation have sound philosophical reasons for their position. After all, we cannot be sure that there is any deeper reality underlying quantum mechanics—and even if we could, we would have no real reason to think that it resembles our everyday experience to such a degree that it is necessary for us to have any conceptual grasp of it.

The likelihood of deciding conclusively on a correct interpretation of quantum mechanics any time soon appears small. Many scientists are content to use the theory as a predictive tool without trying to understand what it really means, but for those who dislike this approach, there are a multitude of theories aiming to account for the strange behaviour of quantum physics. Physics alone cannot decide between the interpretations: ultimately this is an area where individuals must come to their own conclusion, guided as much by philosophical principles as by physical facts.

Many-Worlds Interpretation

Despite the name, this interpretation doesn’t really postulate distinct worlds separate from ours in space and time. In fact, this is perhaps the simplest interpretation: it merely claims that there is no wave-function collapse at all. When a measurement is made, rather than multiple states collapsing to one, the observer comes to exist in multiple states. This is not obvious to us as we are only ever aware of being in one of these states, but simultaneously, other versions of us are in other states and are aware of those states instead. Schrödinger’s poor cat is indeed both alive and dead—the reason we only ever observe it one way or the other is that one version of us observes the living cat while a different version of us observes the dead one.

This hypothesis seems rather implausible. It is hard to believe that there are multiple versions of the same person existing and having different experiences in different branches of the world. It is nonetheless gaining popularity, mainly because it doesn’t require us to postulate wave-function collapse as an additional feature of the theory. However, all the other methods of introducing the collapse appear rather artificial, which gives us reason to think that perhaps no such thing really occurs.

Consciousness

One suggestion is that the wave function collapse is caused by consciousness: macroscopic systems can indeed be in several states at once, but as soon as a conscious observer makes a measurement of the system, the wave function collapses as part of this interaction. This theory would claim that Schrödinger’s cat can indeed be both dead and alive, but only until we open the box. Once we check to see the state of the cat, the wave function collapses, and the cat is once again in a single determinate state.

The main problem with this view is that there is little evidence to suggest that consciousness plays any special role in quantum theory. Rather, it shows that there’s something distinctive about the measurement process—we have no reason to think that it is specifically consciousness that plays the pivotal role.

Emily Adlam is a second year student reading physics and philosophy.

Art by Maria Demidova.