Quantum suicide and immortality

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, 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.

Quantum immortality is a metaphysical speculation derived from the quantum suicide thought experiment. It states that the many-worlds interpretation of quantum mechanics implies that conscious beings are immortal.

Explanation of the thought 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, therefore "proving" himself to be immortal, at least from his own point of view.

Another example is where a physicist detonates a nuclear bomb beside himself. In almost all parallel universes, the nuclear explosion will vaporize the physicist. However, there should be a small set of alternative universes in which the physicist somehow survives (i.e. the set of universes which support a "miraculous" survival scenario). The idea behind quantum immortality is that the physicist will remain alive in, and thus remain able to experience, at least one of the universes in this set, even though these universes form a tiny subset of all possible universes. Over time the physicist would therefore never perceive his or her own death.

Required assumptions and controversy
Proponents point out that while it is highly speculative, quantum immortality (QI) violates no known laws of physics, if certain controversial assumptions are true:


 * 1) That the many-worlds interpretation (MWI) is the correct interpretation of quantum mechanics, as opposed to the Copenhagen interpretation, the latter of which does not permit the existence of parallel universes.
 * 2) All of the possible scenarios in which the proposed physicist (or any entity being argued about in the thought experiment) can die support at least a small subset of survival scenarios.
 * 3) Not dying some finite number of times (perhaps in parallel universes) constitutes immortality.
 * 4) Cessation of the consciousness, and the ability to observe, occurs when the gun is fired or bomb is triggered.

If the many-worlds interpretation is correct, the measure (given in M.W.I. 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. The small-probability remaining branches are in effect though, 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.

However the rareness of an observer moment has no relation to presence or absence of experience; if the M.W.I. is true, all non-zero observer moments are experienced, even rare ones. Quantum suicide gives a recipe for entering into rare observer moments. The experimenter indeed knows that this type of observer moment is rare, which is why it would be unlikely to occur in interpretations of quantum physics that don't have many worlds.

It has been counter-argued that 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.

However, this fails to take into account that in a M.W.I. 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 it lends weight to the credibility of M.W.I., since such a small probability would be highly unlikely to be experienced if wave-function collapse were true instead of the Many Worlds.

Although quantum immortality is motivated by the quantum suicide thought experiment, Max Tegmark, one of the inventors of this experiment, has stated that he does not believe that quantum immortality is a consequence of his work. He argues that under any sort of normal conditions, before someone dies they undergo a period of diminishment of consciousness, a non-quantum decline (which can be anywhere from seconds to minutes to years), and hence there is no way of establishing a continuous existence from this world to an alternate one in which the person continues to exist.

Also, the philosopher David Lewis, in "How Many Lives Has Schrödinger's Cat?", remarked that in the vast majority of the worlds in which an immortal observer might find himself (i.e. the subset of quantum-possible worlds in which the observer does not die), he will survive, but will be terribly maimed. This is because in each of the scenarios typically given in thought experiments (nuclear bombing, Russian roulette, etc.), for every world in which the observer survives unscathed, there are likely to be far more worlds in which the observer survives terribly disfigured, badly disabled, and so on. It is for this reason, Lewis concludes, that we ought to hope that the many-worlds interpretation is false.

Interestingly, there is another route to quantum immortality that does not require the many-worlds interpretation; see Consciousness causes collapse.

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.

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.

Quantum Insomnia
This thought experiment leads many to suspect that a conscious observer in quantum mechanics will suffer from Quantum Insomnia, since, when trying to fall asleep, there will be other copies in parallel universe who are still awake. Although others doubt this is relevant, since they will later awaken.

In fiction
Similar Quantum Suicide 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.

Quantum Immortality themes have been explored in:


 * The Greg Egan novel Quarantine explores topics related to quantum immortality.
 * Other science fiction stories exploring these and related ideas include All the Myriad Ways by Larry Niven, and Divided by Infinity by Robert Charles Wilson.
 * Terry Pratchett's short story Death and What Comes Next has a philosopher arguing the principle with Death, who has come for him.
 * The episode Perfect Circles in the third season of Six Feet Under contains references and allusions to quantum immortality as a major character observes several possible outcomes of his life.
 * Steven Hall's novel The Raw Shark Texts contains references to Max Tegmark and The Quantum Machine Gun (an alternate name for Quantum suicide thought experiment) suggesting a possible Quantum immortality reading of the story.