Quantum suicide is a thought experiment in quantum mechanics and the philosophy of physics. Purportedly, it can falsify any interpretation of quantum mechanics other than the 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. It is sometimes conjectured to be applicable to real-world causes of death as well. As a thought experiment, quantum suicide is an intellectual exercise in which an abstract setup is followed through to its logical consequences merely to prove a theoretical point. Virtually all physicists and philosophers of science who have described it, especially in popularized treatments, underscore that it relies on contrived, idealized circumstances that may be impossible or exceedingly difficult to realize, and that its theoretical premises are controversial even among supporters of the many-worlds interpretation. Thus, as cosmologist Anthony Aguirre warns, "it would be foolish (and selfish) in the extreme to let this possibility guide one's actions in any life-and-death question."
History Hugh Everett III did not mention quantum suicide or quantum immortality in writing; his work was intended as a solution to the paradoxes of quantum mechanics. According to Eugene Shikhovtsev's biography of Everett, "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". Peter Byrne, another biographer of Everett, reports that Everett also privately discussed quantum suicide (such as playing high-stakes Russian roulette and surviving in the winning branch), but adds, "It is unlikely, however, that Everett subscribed to this [quantum immortality] view, as the only sure thing it guarantees is that the majority of your copies will die, hardly a rational goal." Among scientists, the thought experiment was introduced by Euan Squires in 1986. It was published independently by Hans Moravec in 1987 and Bruno Marchal in 1988; it was also described by Huw Price in 1997, who credited it to Dieter Zeh, and independently presented formally by Max Tegmark in 1998. It was discussed by philosophers Peter J. Lewis in 2000 and David Lewis in 2001.
Thought experiment The quantum suicide thought experiment involves a similar apparatus to Schrödinger's cat—a box that kills the occupant in a given time frame with probability one-half due to quantum measurement. The only difference is that the experimenter recording observations is inside the box. The significance of this thought experiment is that someone whose life or death depends on a qubit could possibly 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 a single-world interpretation of quantum mechanics (like the widely held Copenhagen interpretation), the wave function has already collapsed; thus, if the experimenter is already dead, there is a 0% chance of survival for any further iterations. But on the many-worlds interpretation, a superposition of the live experimenter necessarily exists (as 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—can have conscious experience. Putting aside the philosophical problems associated with individual identity and its persistence, under the many-worlds interpretation, the experimenter, or at least a version of them, continues to exist through all of their superpositions where the outcome of the experiment is that they live. In other words, a version of the experimenter survives all iterations of the experiment. Since the superpositions where a version of the experimenter lives occur by quantum necessity (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. A version of the experimenter surviving 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. According to the many-worlds interpretation, the scenario has the opposite property: the probability of a version of the experimenter living is necessarily one for any number of iterations. In the book Our Mathematical Universe, Max Tegmark lays out three criteria that, in abstract, a quantum suicide experiment must fulfill:
The random number generator must be quantum, not deterministic, so that the experimenter enters a state of superposition of being dead and alive. The experimenter must be rendered dead (or at least unconscious) on a time scale shorter than that on which they can become aware of the outcome of the quantum measurement. The experiment must be virtually certain to kill the experimenter, and not merely injure them.
Analysis of real-world feasibility In response to questions about "subjective immortality" from normal causes of death, Tegmark suggested that the flaw in that reasoning is that dying is not a binary event as in the thought experiment; it is a progressive process, with a continuum of states of decreasing consciousness. He says that in most real causes of death, one experiences such a gradual loss of self-awareness. It is only within the confines of an abstract scenario that an observer finds they defy all odds. Referring to the above criteria, he elaborates as follows: "Most accidents and common causes of death clearly don't satisfy all three criteria, suggesting you won't feel immortal after all. In particular, regarding criterion 2, under normal circumstances dying isn't a binary thing where you're either alive or dead [...] What makes the quantum suicide work is that it forces an abrupt transition."
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