Is it possible to be both alive and dead at the same time? This is the question central to the famous Schrödinger’s Cat thought experiment. In the version posed by Erwin Schrödinger, a cat is placed in an opaque box for an hour with a small piece of radioactive material which has an equal probability of decaying or not in that time period. If some radioactivity is detected by a Geiger counter also placed in the box, a relay releases a hammer which breaks a flask of hydrocyanic acid, killing the cat. If no radioactivity is detected, the cat lives. Before we open the box at the end of the hour, we estimate the chance that the radioactive material will decay and the cat will be dead at 50/50, the same as that it will be alive. Before we open the box, however, is the cat alive (and we don’t know it yet), dead (and we don’t know it yet) or both alive and dead (until we open the box and find out).

Common sense would seem to indicate that it is either alive or dead, but we don’t know until we open the box. Traditional quantum theory suggests otherwise. The cat is both alive, with a certain probability, and dead, with a certain probability, until we open the box and find out, when it has to become one or the other with a probability of 100 per cent. In quantum terminology, the cat is in a superposition (two states at the same time) of being alive and dead, which only collapses into one state (dead or alive) when the cat is observed. This might seem absurd when applied to a cat. After all surely it was either alive or dead before we opened the box and found out. It was simply that we didn’t know which. That may be true, when applied to cats. But when applied to the microscopic quantum world, such common sense goes out the window as a description of reality. For example, photons (the smallest measure of light) can exist simultaneously in both wave and particle states, and travel in both clockwise and anti-clockwise directions at the same time. Each state exists in the same moment. As soon as the photon is observed, however, it must settle on one unique state. In other words, the common sense that we can apply to cats we cannot apply to photons or other particles at the quantum level.

So what is going on? The traditional explanation as to why the same quantum particle can exist in different states simultaneously is known as the Copenhagen Interpretation. First proposed by Niels Bohr in the early twentieth century, the Copenhagen interpretation states that a quantum particle does not exist in any one state but in all possible states at the same time, with various probabilities. It is only when we observe it that it must in effect choose which of these states it exists as. At the sub-atomic level, then, particles seem to exist in a state of what is called ‘coherent superposition’, in which they can be two things at the same time, and only become one when they are forced to do so by the act of being observed. The total of all possible states is known as the ‘wave function.’ When the quantum particle is observed, the superposition ‘collapses’ and the object is forced into one of the states that make up its wave function.

The problem with this explanation is that all these different states exist. By observing the object, it might be that it reduces down to one of these states, but what has happened to the others? Where have they disappeared to?

This question lies at the heart of the so-called ‘Quantum Suicide’ thought experiment.

It goes like this. A man (not a cat) sits down in front of a gun which is linked to a machine that measures the spin of a quantum particle (a quark). If it is measured as spinning clockwise, the gun will fire and kill the man. If it is measured as spinning anti-clockwise, it will not fire and the man will survive to undergo the same experiment again.

The question is – will the man survive, and how long will he survive for? This thought experiment, proposed by Max Tegmark, has been answered in different ways by quantum theorists depending on whether or not they adhere to the Copenhagen Interpretation. In that interpretation, the gun will go off with a certain probability, depending on which way the quark is spinning. Eventually, by the laws of chance, the man will be killed, probably sooner rather than later. A growing number of theorists believe something else, however. They see both states (the particle is spinning clockwise and spinning anti-clockwise) as equally real, so there are two real outcomes. In one world, the man dies and in the other he lives. The experiment repeats, and the same split occurs. In one world there will exist a man who survives an indefinite number of rounds. In the other worlds, he is dead.

The difference between these alternative approaches is critical. The Copenhagen approach is to propose that the simultaneously existing states (for example, the quark that is spinning both clockwise and anti-clockwise simultaneously) exist in one world, and collapse into one of these states when observed. Meanwhile, the other states mysteriously disappear. The other approach is to posit that these simultaneously existing states are real states, and neither magically disappears, but branch off into different realities when observed. What is happening is that in one world, the particle is observed spinning clockwise (in the Quantum Suicide thought experiment, the man dies) and in the other world the particle is observed spinning the other way (and the man lives). Crucially, according to this interpretation both worlds are real. In other words, they are not notional states of one world but alternative realities. This is the so-called ‘Many Worlds Theory.’

Where is the burden of proof in trying to determine which interpretation of reality is correct? This depends on whether we take the one world that we can observe as the default position or the wave function of all possible states as represented in the mathematics of the wave function as the reality. Adherents to the Many Worlds position argue that the default is to go with what is described in the mathematics underpinning quantum theory – that the wave function represents all of reality. According to this argument, the minimal mathematical structure needed to make sense of quantum mechanics is the existence of many worlds which branch off, each of which contains an alternative reality. Moreover, these worlds are real. To say that our world, the one that we are observing, is the only real one, despite all the other possible worlds or measurement outcomes, has been likened to when we believed that the Earth was at the centre of the universe. There is no real justification, according to this interpretation, for saying that our branch of all possible states is the only real one, and that all other branches are non-existent or are ‘disappeared worlds.’ Put another way, the mathematics of quantum mechanics describes these different worlds. Nothing in the maths says that this world that we observe is more real than another world. So the burden of proof is on those who say it is. The viewpoint of the Copenhagen school is diametrically opposite. They argue that the hard evidence is of the world we are in, and the burden of proof is on those positing other worlds containing other branches of reality.

Depending on which default position we choose to adopt will determine whether we are adherents of the Copenhagen or the ‘Many Worlds’ schools.

For me personally, the logic of the argument points to the Many Worlds school. But to believe that they are right, and the Copenhagen school is wrong, seems kind of crazy, and totally counter-intuitive. In another world, of course, I’m probably saying the exact opposite.