An alternative to the Copenhagen Interpretation was proposed in 1957 by Hugh Everett in what he initially called the Paradox of Waveform Collapse or, alternatively, Relative State Formulation. His theory was later renamed the Many Worlds Interpretation by Bryce De Witt and R. Neill Graham. In his theory, Everett supposed that instead of only one possible observable outcome to the state of a quantum particle, as the Copenhagen Interpretation proposes, that there are in fact infinitely many observable states, however, we are only consciously aware of one possible observable state. The other observable states exist in alternate worlds, of which Bryce De Witt proposed there are an infinite number. Everett himself was agnostic as to the actual reality of said alternate worlds beyond a mathematical level, however, his proposal indirectly led rise to the notion that there could be a infinite number of simultaneously existing realities (e.g., alternate worlds or dimensions).

We finally come to the actual thought experiment of Quantum Suicide. In this hypothetical experiment, an experimenter rigs the trigger of a lethal weapon to the measurement of the state of a subatomic particle, in most variations a proton released from an atom. Depending on the outcome of measurement, the experimenter either lives or dies. If the Relative State Formulation is accurate, then for every experiment there are two outcomes--one where the experimenter dies and one where the experimenter lives. According to Hans Moravec, the developer of this thought experiment, the experimenter is only consciously aware of the trials in which he or she survives. Thus, if the experimenter continually undergoes trials at Quantum Suicide and never dies, he or she has proven the Relative State Formulation accurate, even if they can only ever prove this to him or herself. This notion led rise to the idea of Quantum Immortality, which stipulates that as it is not possible for the experimenter to observe their own death, the only possible experience for the experimenter is their indefinite survival. Max Tegmark has critiqued the notion of Quantum Immortality, noting that the entire thought experiment of Quantum Suicide hinges on an experiment in which all variables are controlled, and it supposes a singular event rather than the gradual process of related events that make up most of human experience. Thus, he concludes, that even if it were possible to perform a real Quantum Suicide experiment and prove the Relative State Formulation accurate by continued survival, it does not necessarily follow that one has proven the existence of Quantum Immortality, alternate dimensions, or life-after-death, although the idea that there is a real scientific theory that can be used to validate the notion of alternate histories, time travel, and immortality has tickled the interest of historians, authors, philosophers, and spiritualists alike.