Hugh Everett III, an American physicist who first proposed the “many worlds” interpretation of quantum physics, apparently believed that he was guaranteed a form of immortality due to the implications of the many worlds interpretation that he championed,
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, 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, we may say that a version of 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.