In the latest poll of our Science fiction, science fact project you told us that you wanted to know if there are parallel universes. We went to speak to physicist Adrian Kent and philosopher of physics David Wallace to find out. Click here to see other articles exploring this question.

Are there parallel universes? Universes in which, rather than reading this article, you are still asleep; in which you are happier, unhappier, richer, poorer, or even dead? The answer is "possibly". It's a controversial claim but one that has won more and more followers over the last few decades.

Fuzzy reality

Are there parallel universes?

The origin of this parallelism lies in the physics of the very small. At the beginning of the twentieth century physicists developed quantum mechanics to understand the world at the smallest scales. The theory suggests that in this tiny world reality is fuzzy. Little particles, for example electrons, don't need to be either here or there, they can be in several places at once. And they can also simultaneously possess other properties we would normally deem mutually exclusive. When this happens physicists say that the particles are in a superposition of several different states. (You can read a more detailed account of this in our introduction to Schrödinger's equation.)

Experiments have confirmed that superposition is real. Even molecules as large as buckyballs, which consist of 60 carbon atoms, can indeed be in several places at once.

It may well take you a few deep breaths to take this in and you'll immediately ask the obvious question. Why, when I look for a particle, do I only ever find it in one place? This is the famous measurement problem of quantum mechanics. More dramatically, since we are all made up of particles, why are we ourselves (apparently) only ever in one place?

Quantum mechanics itself does not give an answer to this question. One possibility is that the theory doesn't give you the full picture. Perhaps there is another mechanism in nature, one we don't yet understand, which forces reality to snap to exactly one of all the superposed states when we make a measurement. Reality might be fuzzy at the tiniest scales, but as soon as something larger interferes, an experimenter or a measurement device, it is forced down one route only. "If you think there is something extra, then you have the problem of describing what that extra thing could be," says Adrian Kent, a quantum physicist at the University of Cambridge. "How do we describe it mathematically, how can we test it empirically? That's a great big ongoing research programme." (You can read more about this in this Plus article.)

Taking the maths literally

The other possibility is one that requires another deep breath. Perhaps all the possible outcomes of a measurement are equally real: when you make the measurement, to see where a particle is for example, the world splits into different branches. In each branch a copy of you sees the particle in one of the possible locations.

This many-worlds idea was first proposed by the physicist Hugh Everett in his PhD thesis published in 1957. It might seem crazy, but it is rooted in the maths that underlies quantum mechanics. The equations of quantum mechanics don't indicate that something special should happen at the point of measurement, so why not let them run their course and see what happens? "What the mathematics then tells us is that if a particle is [in superposition of two states A and B], the person doing the measurement goes into a superposition of seeing the particle in [state A] and seeing it in [state B]," explains David Wallace, a philosopher of physics at the University of Oxford. So microscopic superposition turns into macroscopic superposition.

But while the maths does not decide between A or B, neither does it blend the two beyond recognition. The mathematical expression describing the situation can be split into two parts each describing a world in which the experimenter sees exactly one of the two possibilities. If you take this literally, then you have to admit that reality has split.

The branching Universe

But is a physicist making a measurement the only thing that can cause reality to split? The answer is no. A measurement of a system in superposition is an interaction with that system and there are other physical processes that can interact with it too. For example, cosmic rays can be in a superposition of going in lots of different directions at once. If one of these directions is homing in on a crystal on Earth, then the impact of the ray will leave a track on the crystal. The crystal is effectively measuring where the ray is. And since the ray is in superposition of homing in on the crystal and not homing in on it, the crystal goes into superposition of having a track and not having a track. And thus, so goes the Everett interpretation, reality splits.

The physicist Erwin Schrödinger devised a famous thought experiment in which a cat in a box is in superposition of two states: dead and alive. According to the Everett interpretation, when you open the box and observe the cat the world splits into two branches: in one of them the cat is dead and in the other alive. Image: Dc987.

With the need for an observer eliminated you can imagine the branching game as having gone on since the beginning of time. "[According to the many-worlds view] this has been going on right from the Big Bang," says Kent. "The Universe started out perhaps in a simple quantum state, but it very quickly became a superposition of lots of different descriptions of the Universe, lots of configurations of galaxies. In some of these branches the Earth would have formed and in some of them it wouldn't. And in some of those where the Earth formed we would have evolved and in some we wouldn't."

Pleased to meet me?

But why are we never aware of those other versions of ourselves? Why do we never see big objects like fridges or people in more than one place at once? Intriguingly, Everett's original formulation couldn't rule that out. In principle reality could split in the wrong way so that an experimenter would see an electron, say, in some weird indeterminate location. But Everett hadn't reckoned with the outside world. As soon as the electron interacts with the outside world, with photons or cosmic rays whizzing past, any perceptible interference between the states "the electron is in location A" and "the electron is in location B" essentially leaks out into the wider world and dissipates. Just as the ripples caused by a stone thrown into a lake peter out as they spread, so the interference becomes so small as to be imperceptible — and the observer sees only one definite outcome when looking at the electron. This process, called decoherence, happens incredibly fast, within a fraction of a second, so we're never aware of it.

Since people and fridges interact with zillions of particles all the time, decoherence grounds them firmly in a one-track world: they are either here or there. If you're dealing with something microscopic, like an electron, then you may be able to isolate it from the outside world sufficiently well to observe superposition. Not by looking at it directly, but by leaving it well alone and then looking for tell tale signs that superposition must have taken place — that's exactly how scientists have been able to confirm that it exists. "But the bigger a system gets the harder it is to isolate it from the external environment," explains Wallace. "So then it becomes harder and harder to detect that what we have is two interacting goings-on rather than just one goings-on."

Which me is real?

What does all of this mean for us? "In Everett's view [all the different branches] are there in reality," says Kent. "There are many copies of you and it makes no sense to ask which ones are real. There is a democracy among those copies, they are all equally valid. You would have the same memories [as all the other copies of you] until the point that you split. After that there could be very slight differences between the copies of you, or [after some time] there could be huge ones."

Will you ever meet your other selves?

Kent, who is not a proponent of the many-worlds view, also points out that it should have profound consequences for your attitude to life. "It could make you very nihilistic, because after all what's the point of making a big effort, in some branches whatever you try to achieve will work out anyway. You could also be overwhelmed with grief: every time you're driving down the motorway little branches of you will peel off as they crash and die. Or you could devote your life to being as risk averse as possible in order to look after future copies of yourself as much as you can. People who take an altruistic view of society, leaning to socialism or egalitarianism, ought to take the same kind of view of their successors. Even more so because they are all you, so you have an even stronger moral responsibility [to look after them]."

The many-worlds view may not appeal to common sense but proponents praise its scientific elegance: it relies on the existing maths of quantum mechanics. You don't have to introduce that mysterious "extra something" that makes reality snap from a state of superposition to a definite, single state. "There isn't a new postulate or a new physical principle," says Wallace. "It's just what emerges from taking the theory we have and taking it seriously."

Not everyone is a believer — and this isn't down to the apparent craziness of the theory. After all, many theories seemed crazy when they were first conceived and today no physicist worth their salt would be deterred by that. It's down mostly to two problems. Some people, including Kent, don't quite believe that decoherence is enough to explain the neat splitting of the world into branches in which the world looks like it does. They worry that, after all, some extra assumptions are necessary to make decoherence work.

The other problem is that the maths equips different branches of the world with what looks like probabilities. But if all branches are equally real, how can it make sense to say that one is more likely than another? In the 1990s the physicist David Deutsch came up with an intriguing way to make sense of this: imagine that you're betting on your future selves. This is what we'll look at in the next article.

About this article

Marianne Freiberger is Editor of Plus. She interviewed Adrian Kent and David Wallace in December 2012. Kent and Wallace are co-editors of the book Many Worlds?: Everett, Quantum Theory & Reality.