Quantum suicide and immortality

In quantum mechanics, quantum suicide was a thought experiment. It was independently proposed in 1987 by Hans Moravec and in 1988 by Bruno Marchal, and further developed by Max Tegmark in 1998, [Tegmark, Max [http://www.arxiv.org/abs/quant-ph/9709032/ The Interpretation of Quantum Mechanics: Many Worlds or Many Words?] , 1998] . It 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 thought experiment. The experiment 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

An experimenter sits in front of a loaded gun which is triggered or not triggered depending on the decay of some radioactive atoms.

Repeated runs of the experiment

With each run of the experiment there is a 50-50 chance that the gun will be triggered and the experimenter will die. According to the Copenhagen interpretation, the gun will (in all likelihood) eventually be triggered and the experimenter will die. If the many-worlds interpretation is correct then at each run of the experiment, the experimenter 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 experimenter dies, he will cease to exist.

The experimenter's point of view

However, from the point of view of the non-dead copies of the experimenter, 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 experimenter will notice that he never seems to die, therefore "proving" himself to be invulnerable to the gun mechanism in question, at least from his own point of view.

Nuclear bomb

Another example is where a experimenter detonates a nuclear bomb beside himself. In almost all parallel universes, the nuclear explosion will vaporize the experimenter. However, there should be a small set of alternative universes in which the experimenter somehow survives (i.e. the set of universes which support a "miraculous" survival scenario, or some extremely unlikely, but technically possible event occurs saving the experimenter).

Quantum immortality

The idea behind quantum immortality is that the experimenter 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 experimenter would therefore never perceive his or her own death.Fact|date=April 2008

Required assumptions and controversy

Proponents point out that while it is highly speculative, quantum immortality (QI) violates no known laws of physics as long as certain controversial assumptions are true:Fact|date=April 2008

# 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 involve the existence of parallel universes. "(Parallel universes may be possible through other mechanisms in the Copenhagen interpretation.)"
# All of the possible scenarios in which the proposed experimenter (or any entity being argued about in the thought experiment) can die support at least a small subset of survival scenarios.
# Not dying some finite number of times (perhaps in parallel universes) constitutes immortality.
# Permanent cessation of the consciousness, and the ability to observe, occurs when the gun is fired or bomb is triggered.

Further explanation

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 experimenter will decrease by 50% with each run of the experiment, but will remain non-zero.

Rareness of survival

The small-probability remaining branches are in effect though, unlikely to be experienced by most of the copies of the experimenter 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.

Counter-argument

It has been counter-argued that this is equivalent to a single-world situation in which one starts off with many copies of the experimenter, 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 experimenter; 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.

It is countered that this fails to take into account that in a M.W.I. the amplitude of being the living experimenter can be halved repeatedly without ever reaching zero. There is always some non-zero probability amplitude of being the surviving, observing experimenter. It has been claimed that the very fact that the amplitude becomes so small 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.

This "counter counter-argument" misses the central point of the original counter-argument: at each run the waveform of the experimenter decreases by 50%. What interpretation applies to the 50% of the waveform that is lost? The only satisfactory explanation is to state that it has propagated into the universe where the bomb goes off, and become the waveform of a pile of nuclear ash that was once an experimenter. Thus the original waveform can be considered as being two waveforms: one for the experimenter that survives and one for the experimenter that does not. Under this interpretation, there is a 50-50 chance the experimenter will never experience anything past the Quantum Suicide experiment, as there is a 50% chance that his waveform is the one that is destined to become nuclear ash.Fact|date=April 2008

Max Tegmark

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 in this world to an alternate one in which the person ceases to exist.Fact|date=April 2008

This seems to be similar to the logic behind Greg Egan's Dust Theory.

Chad T. Wood

Chad T. Wood also postulated on the theoretical immortality afforded by quantum physics. He labeled the theory Quantum Zen as a psychological tool to help students, first at Oswego State University and then elsewhere, cope with the pressures of the modern world. Over the past 10 years this science as self-help has caught the attention of people in various parts of the United States.Fact|date=April 2008

David Lewis

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.Fact|date=April 2008

Quantum suicide could fail

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 [Page, Don [http://www.arxiv.org/abs/quant-ph/9904004/ Observational Consequences of Many-Worlds Quantum Theory] , 1999] .

Consciousness causes collapse

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

Controversy

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 suicide and quantum immortality in fiction

Authors of Science fiction have used themes involving both quantum suicide and quantum immortality. The basic idea is that a person who dies in one world may survive in another world or parallel universe.

ee also

* Final anthropic principle
* Multiverse

References

External links

* [http://www.higgo.com/quantum/qti.htm "Does the 'many-worlds' interpretation of quantum mechanics imply immortality?"] by James Higgo
* [http://www.weidai.com/everything.html The 'Everything' mailing list (and archives), A "discussion of the idea that all possible universes exist".]
* [http://science.howstuffworks.com/quantum-suicide.htm How Quantum Suicide Works]