Quantum suicide

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In quantum mechanics, quantum suicide is a thought experiment which was independently proposed in 1987 by Hans Moravec and in 1988 by Bruno Marchal, and further developed by Max Tegmark in 1998^[1], that attempts to distinguish between the Copenhagen interpretation of quantum mechanics and the Everett many-worlds interpretation by means of a variation of the Schrödinger's cat experiment. The experiment essentially involves looking at the Schrödinger's cat experiment from the point of view of the cat.

In this experiment, a physicist sits in front of a gun which is triggered or not triggered depending on the decay of some radioactive atom. With each run of the experiment there is a 50-50 chance that the gun will be triggered and the physicist will die. If the Copenhagen interpretation is correct, then the gun will eventually be triggered and the physicist will die. If the many-worlds interpretation is correct then at each run of the experiment the physicist will be split into one world in which he lives and another world in which he dies. After many runs of the experiment, there will be many worlds. In the worlds where the physicist dies, he will cease to exist. However, from the point of view of the non-dead copies of the physicist, the experiment will continue running without his ceasing to exist, because at each branch, he will only be able to observe the result in the world in which he survives, and if many-worlds is correct, the surviving copies of the physicist will notice that he never seems to die.

If the many-worlds interpretation is correct, the measure (given in MWI by the squared norm of the wavefunction) of the surviving copies of the physicist will decrease by 50% with each run of the experiment, but will remain non-zero.

It has been counter-argued this is equivalent to a single-world situation in which one starts off with many copies of the physicist, and the number of surviving copies is decreased by 50% with each run. Therefore, the quantum nature of the experiment provides no benefit to the physicist; in terms of his life expectancy or rational decision making, or even in terms of his trying to decide whether the many-worlds interpretation is correct, the many-worlds interpretation gives results that are the same as that of a single-world interpretation.

Some people think this fails to take into account that in a MWI the amplitude of being the living physicist can be halved repeatedly without ever reaching zero. There is always some non-zero probability amplitude of being the surviving, observing physicist. It has been claimed that the very fact that the amplitude becomes so small that lends weight to the credence the physicist should give the MWI, since such a small probability would be highly unlikely to be experienced if wave-function collapse were true instead of the Many Worlds.

However that is false - because even if the MWI is true, the small-probability remaining branches are, in effect, unlikely to be experienced by most of the copies of the physicist that started out. Most of the observer-moments in the universe will not be in such low measure situations because measure is proportional to the number of copies and therefore the number of that type of observer-moment.

Critics contend quantum suicide fails as a thought experiment to achieve its intended purpose, but there are arguments involving anthropic considerations among entire universes which do provide evidence for the many-worlds interpretation, such as that of Page^[2].

Quantum suicide remains controversial because a number of thinkers disagree on its success or failure and particularly its irrelevance to life expectancy and decision making. A variation of this thought experiment allegedly suggests a controversial outcome known as quantum immortality, which is the argument that if the many-worlds interpretation of quantum mechanics is correct then a conscious observer can never cease to exist.

[edit] In fiction

Similar themes have been explored in the following works:

• Larry Niven's short story "All the Myriad Ways", collected in a collection of the same name (1971)

• Dan Simmons's novel The Hollow Man (1992). Simmons also describes a quantum execution mechanism in his Hyperion Cantos series.

• Greg Egan's novel Quarantine (1992)

• Greg Egan's novel Permutation City (1994), in which one character repeatedly had his mind uploaded and his copy eventually terminated, but found out that he always "ended up" in another world, where his survival was explained by increasingly improbable circumstances.

• Robert Charles Wilson's short story Divided by Infinity (1998)

• Denis Johnson's novel Already Dead (A California Gothic) (1998)

• Jason Shiga's book Meanwhile (2004)

• Greg Bear's short Story "Schrodinger's Plague" found in his book Tangents deals with a doomsday version of this experiment in which instead of a single scientist dying, a deadly virus is released into the populace.

• In "The One" Jet Li portrays a man that "jumps" between all these different worlds in order to kill himself.

[edit] References