Quantum suicide and immortality
Quantum suicide is a thought experiment in quantum mechanics and the philosophy of physics. Purportedly, it can 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.
Most experts believe that the experiment would not work in the real world.:371
Hugh Everett did not mention quantum suicide or quantum immortality in writing; his work was intended as a solution to the paradoxes of quantum mechanics. Keith Lynch's biography states that "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".
The quantum suicide thought experiment involves a similar apparatus to Schrödinger's cat – a box which kills the occupant in a given time frame with probability one-half due to quantum uncertainty.[note 1] 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 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 the Copenhagen interpretation is true, the wave function has already collapsed; thus, if 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, 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. Due to the many-worlds interpretation, the above scenario has the opposite property: the probability of a version of the experimenter living is necessarily one for any number of iterations.
In response to questions about "subjective immortality", Max 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 states 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.
Tegmark now believes experimenters should only expect a normal probability of survival, not immortality. The experimenter's probability amplitude in the wavefunction decreases significantly, meaning they exist with a much lower measure than they had before. Per the anthropic principle, a person is less likely to find themselves in a world where they are less likely to exist, that is, a world with a lower measure has a lower probability of being observed by them. Therefore, the experimenter will have a lower probability of observing the world in which they survive than the earlier world in which they set up the experiment. This same problem of reduced measure was pointed out by Lev Vaidman in the Stanford Encyclopedia of Philosophy.
Physicist David Deutsch, though a proponent of the many-worlds interpretation, states regarding quantum suicide that "that way of applying probabilities does not follow directly from quantum theory, as the usual one does. It requires an additional assumption, namely that when making decisions one should ignore the histories in which the decision-maker is absent....[M]y guess is that the assumption is false." Physicist David Wallace suggests that a decision-theoretic analysis shows that "an agent who prefers certain life to certain death is rationally compelled to prefer life in high-weight branches and death in low-weight branches to the opposite."
Physicist Sean M. Carroll, another proponent of the many-worlds interpretation, states regarding quantum suicide that neither experiences nor rewards should be thought of as being shared between future versions of oneself, as they become distinct persons when the world splits. He further states that one cannot pick out some future versions of oneself as "really you" over others, and that quantum suicide still cuts off the existence of some of these future selves, which would be worth objecting to just as if there were a single world.
- Tegmark, Max (November 1998). "Quantum immortality". Retrieved 25 October 2010.
- Wallace, David (2012). The Emergent Multiverse: Quantum Theory According to the Everett Interpretation. Oxford University Press. pp. 369–372. ISBN 978-0-19-954696-1.
- See Eugene Shikhovtsev's Biography of Everett: Keith Lynch remembers 1979–1980
- Moravec, Hans (1988). "The Doomsday Device". Mind Children: The Future of Robot and Human Intelligence. Harvard: Harvard University Press. p. 188. ISBN 978-0-674-57618-6.
- Marchal, Bruno (1988). "Informatique théorique et philosophie de l'esprit" [Theoretical Computer Science and Philosophy of Mind]. Acte du 3ème colloque international Cognition et Connaissance [Proceedings of the 3rd International Conference Cognition and Knowledge]. Toulouse: 193–227.
- Marchal, Bruno (1991). De Glas, M.; Gabbay, D. (eds.). "Mechanism and personal identity" (PDF). Proceedings of WOCFAI 91. Paris. Angkor.: 335–345.
- Tegmark, Max The Interpretation of Quantum Mechanics: Many Worlds or Many Words?, 1998
- Tegmark, Max (2014). "Is Time An Illusion?". Our Mathematical Universe. Vintage Books.
- Vaidman, Lev (2018). "Many-Worlds Interpretation of Quantum Mechanics". The Stanford Encyclopedia of Philosophy. Metaphysics Research Lab, Stanford University.
- Deutsch, David (2011). "The Beginning". The Beginning of Infinity. Penguin Group.
- Carroll, Sean (2019). "The Human Side - Living and Thinking in a Quantum Universe". Something Deeply Hidden: Quantum Worlds and the Emergence of Spacetime. Penguin. ISBN 9781524743024. At Google Books.
- Mallah, J. Many-Worlds Interpretations Can Not Imply 'Quantum Immortality'
- Higgo, J., "Does the 'many-worlds' interpretation of quantum mechanics imply immortality?", archived from the original on 27 August 2007.