In quantum mechanics, quantum suicide is a thought experiment, originally published independently by Hans Moravec in 1987 and Bruno Marchal in 1988, and independently developed further by Max Tegmark in 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, from the cat's point of view. Quantum immortality refers to the subjective experience of surviving quantum suicide regardless of the odds.
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Keith Lynch recalls that Hugh Everett took great delight in paradoxes such as the unexpected hanging. Everett did not mention quantum suicide or quantum immortality in writing, but his work was intended as a solution to the paradoxes of quantum mechanics. Lynch said "Everett firmly believed that his many-worlds theory guaranteed him immortality: his consciousness, he argued, is bound at each branching to follow whatever path does not lead to death"; Tegmark explains, however, that life and death situations do not normally hinge upon a sequence of binary quantum events like those in the thought experiment.
Thought experiment
The quantum suicide thought experiment involves the same apparatus as Schrödinger's cat — a box which kills the occupant in a given time frame with probability one half due to quantum uncertainty. The only difference is to have the experimenter recording observations be the one inside the box. The significance of this is that someone whose life or death depends on a qubit can distinguish between interpretations of quantum mechanics. By definition, fixed observers cannot.
At the start of the first iteration, under both interpretations, the probability of surviving the experiment is 50 %, as given by the squared norm of the wave function. At the start of the second iteration, assuming the Copenhagen interpretation is true, the wave function has already collapsed; thus, since the experimenter is already dead, there is a 0 % chance of survival for any further iterations. However, if the many-worlds interpretation is true, a superposition of the live experimenter necessarily exists (as also does the one who dies). Now, barring the possibility of life after death, after every iteration only one of the two experimenter superpositions – the live one – is capable of having any sort of conscious experience. Putting aside the philosophical problems associated with individual identity and its persistence, we may assert that, under the many-worlds interpretation, the experimenter continues to exist through all of their superpositions where the outcome of the experiment is that they live. In other words, we may say that the experimenter survives all iterations of the experiment, whichever its number. Since the superpositions where the experimenter lives occur by quantum necessity (again, under the many-worlds interpretation), it follows that their survival, after any realizable number of iterations, is physically necessary; hence, the notion of quantum immortality.
This stands in stark contrast to the implications of the Copenhagen interpretation, according to which, although the survival outcome is possible in every iteration, its probability tends towards zero as the number of iterations increases. Due to the many-worlds interpretation, the above scenario has the opposite property: the probability of the experimenter living is necessarily one for any number of iterations.
Max Tegmark's work
In response to questions about "subjective immortality", Max Tegmark made some brief comments: He acknowledged the argument that "everyone will be immortal" should follow if a survivor outcome is possible for all life-threatening events. The flaw in that argument, he suggests, is that dying is rarely a binary event; it is a progressive process. The quantum suicide thought experiment attempts to isolate all possible outcomes for the duration of the thought experiment. That isolation delays decoherence in such a way that the subjective experience of the superposition is illustrated. It is only within the confines of such an abstract quantum scenario that an observer finds they defy all odds.