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".See Eugene Shikhovtsev's Biography of Everett: Keith Lynch remembers 1979–1980 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."How Many Lives Has Schrödinger's Cat?," Australasian Journal of Philosophy, Vol. 82, No. 1, pp. 3–22; March 2004.
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 simplest example of this is a weapon triggered by a two-level system. Schrödinger described his as a radioactive decay detector, while Moravec's was a device that measured the spin value of protons. 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.This is not unanimously agreed upon. Sebens argues, as will be detailed infra, that death must be instantaneous, not merely faster than the brain can process the result of the experiment. 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."
David Lewis' commentary and subsequent criticism The philosopher David Lewis explored the possibility of quantum immortality in his 2001 lecture "How Many Lives Has Schrödinger's Cat?", his first academic foray into the field of the interpretation of quantum mechanics—and his last, due to his death less than four months later. In the lecture, published posthumously in 2004, Lewis rejected the many-worlds interpretation, allowing that it offers initial theoretical attractions but arguing that it has irremediable flaws, mainly regarding probabilities, and came to tentatively endorse the Ghirardi–Rimini–Weber theory instead. Lewis concluded the lecture by saying that the quantum suicide thought experiment, if applied to real causes of death, entails what he deemed a "terrifying corollary": as all causes of death are ultimately quantum-mechanical in nature, on the many-worlds interpretation, an observer should subjectively "expect with certainty to go on forever surviving whatever dangers [he or she] may encounter", as there will always be possibilities of survival, no matter how unlikely; faced with branching events of survival and death, an observer should not "equally expect to experience life and death", as there is no such thing as experiencing death, and should thus divide his or her expectations only among branches where they survive. But though survival is guaranteed, good health and integrity are not. This leads to a Tithonus-like deterioration of one's body that continues indefinitely, leaving the subject forever just short of death.
Interviewed for the 2004 book Schrödinger's Rabbits, Tegmark rejected this scenario because "the fading of consciousness is a continuous process. Although I cannot experience a world line in which I am altogether absent, I can enter one in which my speed of thought is diminishing, my memories and other faculties fading [...] I am confident that even if he cannot die all at once, he can gently fade away." In the same book, philosopher of science and many-worlds proponent David Wallace attacks the case for real-world quantum immortality on the basis that death can be understood as a continuum of decreasing states of consciousness not only in time, as per Tegmark, but also in space: "our consciousness is not located at one unique point in the brain, but is presumably a kind of emergent or holistic property of a sufficiently large group of neurons [...] our consciousness might not be able to go out like a light, but it can dwindle exponentially until it is, for all practical purposes, gone."
Directly responding to Lewis's lecture, British philosopher and many-worlds proponent David Papineau, while finding Lewis's other objections to the many-worlds interpretation lacking, strongly denies that any modification to the usual probability rules is warranted in death situations. Assured subjective survival can follow from the quantum suicide idea only if an agent reasons in terms of "what will be experienced next" instead of the more obvious "what will happen next, whether will be experienced or not". He writes, "it is by no means obvious why Everettians should modify their intensity ruleBy "intensity rule", Lewis and Papineau mean the Born rule, the rule used to apportion probabilities in quantum mechanical events. in this way. For it seems perfectly open for them to apply the unmodified intensity rule in life-or-death situations, just as elsewhere. If they do this, then they can expect all futures in proportion to their intensities, whether or not those futures contain any of their live successors. For example, even when you know you are about to be the subject in a fifty-fifty Schrödinger’s experiment, you should expect a future branch where you perish, to just the same degree as you expect a future branch where you survive."