Advent calendar door #13: Are there parallel universes?
Are there parallel universes? Universes in which, rather than reading this, 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.
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.)
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. Intrigued? Then find out more in this article.