Many-world interpretation: an interpretation of quantum theory according to which there is a multitude of parallel universes in addition to our own. This is not to be confused with the potentially infinite number of parallel worlds within our own universe.

Quantum theory describes objects by means of wave functions that combine all of their possible states. Only an external factor such as an observation or measurement event can reduce the object's wave function to a single state. What state that is depends on both chance and the probabilities of the possible states.

A well-known thought experiment illustrating this is Schrödinger's cat. Here, a cat is locked into a closed box together with an unstable atom that has a 50% probability of decaying. If the decay occurs, a detector in the box will notice and release poison gas, killing the cat.*

According to the Copenhagen interpretation of quantum theory — named after the Copenhagen physicist Niels Bohr — the cat remains in a mixed state of life and death for as long as it is locked up in the box. Its wave function consists in an overlap of both states. Only when the box is opened the cat will cross over into either a fully alive or a fully dead state, depending on the gas's decaying or not.

According to the many-worlds interpretation developed by Hugh Everett, the world is split up into two parallel worlds as soon as the box is opened. In one of them, the cat is alive; in the other, it is not. Otherwise, the worlds are entirely identical; in particular, unlike the universes posited in Andrej Linde'smultiverse theory, they obey the same laws of nature. The observer does not notice anything of this split since she continues to live in both worlds. Since there many processes in nature that are very similar under observation, there is now a finite but extremely large number of parallel worlds in the universe.

Mathematically, the two theories are identical and support the same observations. Physically, however, the many-worlds theory seems easier to understand than the Copenhagen interpretation, for the former does not involve the special role of measurement events and chance that is typical of the latter. Still, physicists do not feel comfortable with the idea that the observer's consciousness "decides" for or against a given world, nor with the enormous number of postulated worlds the many-worlds interpretation seems to require. Therefore, the Copenhagen interpretation is more widely accepted in physics.

Some recent efforts have aimed at developing experiments that would enable us to determine which of the two theories is superior. Most physicists, however, think that no such experiments are possible. The following proposal was especially controversial: We set up an isolated box with an unstable atom and a poison gas capsule. A physicist sufficiently dedicated to the cause is locked into the box in the role of Schrödinger's cat. Now, if the physicist's consciousness will always "decide" in favor of the world in which he will continue to live, theoretically nothing bad could happen to him. However, to date no physicist has been willing to take part in this experiment.

* We might be tempted to think that the physicist Erwin Schrödinger, who devised this scenario, was a cat hater. In reality, however, he owned two pet cats.

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