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Quantum immortality is the controversial speculation deriving from the quantum suicide thought experiment that states the many-worlds interpretation of quantum mechanics implies that conscious beings are immortal.
Explanation of the thought experiment
Imagine that a physicist detonates a nuclear bomb beside him- or herself. 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 (ie. 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 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.
There are some parallels with this concept in the anthropic principle - one restatement of the anthropic principle in this context would be that any experience which contains a perception of a universe, reality, or existence must include also the idea of the entity which is perceiving said universe, reality, or existence.
Another example is one provided by quantum suicide, where a physicist sits in front of a gun which is triggered, or not triggered, by radioactive decay. With each run of the experiment there is a fifty-fifty 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 a world in which he lives and one in which he dies. In the worlds where the physicist dies, he will cease to exist. However, from the point of view 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 physicist will notice that he never seems to die, therefore "proving" himself to be immortal, at least from his own point of view in probability.
Required assumptions and controversy
Proponents point out that while it is highly speculative, quantum immortality (QI) violates no known laws of physics assuming two controversial assumptions are true:
- The many-worlds interpretation of quantum mechanics is the correct one, as opposed to the Copenhagen interpretation, which does not necessarily indicate the existence of parallel universes.
- 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.
A potential criticism of the theory is that the second assumption is not a necessary consequence of the many-worlds interpretation and may require the violation of laws that are still thought to be conserved across all possible realities. The many-worlds interpretation of quantum physics does not necessarily imply that "everything is possible," only that all outcomes that are possible will branch off from any given instant in time. Most physical laws of the universe still cannot be broken - for example, the first law of thermodynamics is still considered to be conserved in all probabilities, theoretically preventing a parallel universe in which this law is violated from ever branching off. This has implications that, from the point of view of the physicist, it is possible to reach a particular configuration of reality where the physicist's survival actually becomes impossible, because a survival scenario in that reality would at that point require a violation of a law of the universe that is not considered to be transgressible in any possible reality.
For example, in the nuclear-bomb scenario above, once the physicist has perceived the flash of light from the bomb's detonation, it is difficult to effectively describe a scenario in which the physicist continues living that does not violate basic biological principles. Living cells simply cannot remain alive at the temperatures found at the core of a nuclear reaction under any known subsets of modern science. For quantum immortality to be true, either the bomb would have to misfire (or otherwise not detonate) or an event would have to take place which made use of scientific principles that are not yet proven or discovered. Another example is natural biological death from old age, which may not be escapable in any parallel universe (at least without more advanced technology than is currently known).
Another potentially problematic area is that quantum immortality would also suggest that any conscious being could "cause" itself to experience highly improbable events in its own quantum probability simply by repeatedly placing itself in situations in which it is highly likely that the being will die (see Death instinct). Even though in most parallel universes the being would die, the only ones that the being could possibly subjectively experience would be the ones in which it experiences the unlikely survival scenario. This may turn out to be a violation of some sort of property of causality, the nature of which is still not well understood in quantum physics.
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 late philosopher David Lewis, in his final publication, "How Many Lives Has Schrödinger's Cat?", remarked that in the vast majority of the worlds in which an immortal observer might find herself (i.e. the subset of quantum-possible worlds in which the observer does not die), she 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.
Fictional depictions
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.