Editing Quantum suicide and immortality (section)

=== David Lewis' commentary and subsequent criticism ===

The philosopher [[David Lewis (philosopher)|David Lewis]] explored the possibility of quantum immortality in a 2001 lecture titled "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 afterwards. In the lecture, published posthumously in 2004, Lewis rejected the many-worlds interpretation, allowing that it offers initial theoretical attractions, but also arguing that it suffers from irremediable flaws, mainly regarding probabilities, and came to tentatively endorse the [[Ghirardi–Rimini–Weber theory]] instead. Lewis concluded the lecture by stating that the quantum suicide thought experiment, if applied to real-world causes of death, would entail what he deemed a "terrifying corollary": as all causes of death are ultimately quantum-mechanical in nature, if the many-worlds interpretation were true, in Lewis' view 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 he or she survives. If survival is guaranteed, however, this is not the case for good health or integrity. This would lead to a [[Tithonus]]-like deterioration of one's body that continues indefinitively, leaving the subject forever just short of death.<ref name=dlewis/><ref name=papineau/>

Interviewed for the 2004 book ''Schrödinger's Rabbits'', Tegmark rejected this scenario for the reason that "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 [...] [Tegmark] is 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<ref name="usc_edu">{{cite web|url=https://dornsife.usc.edu/david-wallace|title=About me|last=Wallace|first=David|access-date=24 May 2020|archive-date=2 October 2020|archive-url=https://web.archive.org/web/20201002212611/https://dornsife.usc.edu/david-wallace|url-status=dead}}</ref> undermines 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 argued by 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."<ref>{{cite book |last=Bruce|first=Colin|year=2004|chapter=The Terror of Many Worlds|title=Schrödinger's Rabbits: The Many Worlds of Quantum}}</ref>

Directly responding to Lewis' lecture, British philosopher and many-worlds proponent [[David Papineau]], while finding Lewis' 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: "[I]t is by no means obvious why Everettians should modify their intensity rule<ref group=note>By "intensity rule", Lewis and Papineau mean the [[Born rule]], the rule used to apportion probabilities in quantum mechanical events.</ref> 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."<ref name=papineau>{{cite web| url = https://www.davidpapineau.co.uk/uploads/1/8/5/5/18551740/lewis_and_schro%CC%88dinger._final_final.doc| title = David Papineau, "David Lewis and Schroedinger's Cat", 2004}}</ref>

On a similar note, quoting Lewis' position that death should not be expected as an experience, philosopher of science Charles Sebens concedes that, in a quantum suicide experiment, "[i]t is tempting to think you should expect survival with certainty." However, he remarks that expectation of survival could follow only if the quantum branching and death were absolutely simultaneous, otherwise normal chances of death apply: "[i]f death is indeed immediate on all branches but one, the thought has some plausibility. But if there is any delay it should be rejected. In such a case, there is a short period of time when there are multiple copies of you, each (effectively) causally isolated from the others and able to assign a credence to being the one who will live. Only one will survive. Surely rationality does not compel you to be maximally optimistic in such a scenario." Sebens also explores the possibility that death might not be simultaneous to branching, but still faster than a human can mentally realize the outcome of the experiment. Again, an agent should expect to die with normal probabilities: "[d]o the copies need to last long enough to have thoughts to cause trouble?<ref group=note>By "cause trouble," Sebens means spoiling the expectations of necessary subjective survival.</ref> I think not. If you survive, you can consider what credences you should have assigned during the short period after splitting when you coexisted with the other copies."<ref>{{Cite journal|title=Killer Collapse Empirically Probing the Philosophically Unsatisfactory Region of GRW|journal=Synthese|url=http://philsci-archive.pitt.edu/11350/7/Killer_Collapse_PhilSci_Archive_3.pdf |archive-url=https://web.archive.org/web/20170922225748/http://philsci-archive.pitt.edu/11350/7/Killer_Collapse_PhilSci_Archive_3.pdf |archive-date=2017-09-22 |url-status=live|last=Sebens|first=Charles|date=2015-01-29|volume=192|issue=8|pages=2599–2615|doi=10.1007/s11229-015-0680-x|s2cid=17259579}}</ref>

Writing in the journal ''[[Ratio (journal)|Ratio]]'', philosopher [[István Aranyosi]], while noting that "[the] tension between the idea of states being both actual and probable is taken as the chief weakness of the many-worlds interpretation of quantum mechanics," summarizes that most of the critical commentary of Lewis' immortality argument has revolved around its premises. But even if, for the sake of argument, one were willing to entirely accept Lewis' assumptions, Aranyosi strongly denies that the "terrifying corollary" would be the correct implication of said premises. Instead, the two scenarios that would most likely follow would be what Aranyosi describes as the "comforting corollary", in which an observer should never expect to get very sick in the first place, or the "momentary life" picture, in which an observer should expect "eternal life, spent almost entirely in an unconscious state", punctuated by extremely brief, amnesiac moments of consciousness. Thus, Aranyosi concludes that while "[w]e can't assess whether one or the other [of the two alternative scenarios] gets the lion's share of the total intensity associated with branches compatible with self-awareness, [...] we can be sure that they together (i.e. their disjunction) do indeed get the lion's share, which is much reassuring."<ref>{{Cite journal|title=Should we fear quantum torment?|journal=Ratio|url=http://istvanaranyosi.net/resources/Should%20we%20fear%20qt%20final.pdf |archive-url=https://web.archive.org/web/20121031051725/http://istvanaranyosi.net/resources/Should%20we%20fear%20qt%20final.pdf |archive-date=2012-10-31 |url-status=live|last=Aranyosi|first=Istvan|date=2012-08-15|volume=25|issue=3|pages=249–259|doi=10.1111/j.1467-9329.2012.00540.x|hdl=11693/21341|hdl-access=free}}</ref>