Quantum suicide is a thought experiment in quantum mechanics and the philosophy of physics. It was originally claimed that it can distinguish between the Copenhagen interpretation of quantum mechanics and the many-worlds interpretation of quantum mechanics. It is based on imagining oneself as the cat in the Schrödinger's cat thought experiment. **Quantum immortality** refers to the experience of surviving quantum suicide.

The thought experiment was created by Max  In the thought experiment, a person stands in front of a gun which fires if it detects a subatomic particle as having an upward spin, or does not fire if the gun detects a downward spin. The gun does this repeatedly. From an outside perspective, both outcomes are equally likely. However, according to the thought experiment, if the many-worlds interpretation is true, the person in front of the gun finds that the gun never fires, even though this is very, very unlikely. In the many-worlds interpretation, the gun fires in one world and does not fire in another, but the person can only continue being conscious in a world in which the gun does not fire. According to the thought experiment, such a person would find themselves to be immortal.

In response to questions about whether people should generally expect to be immortal, Max Tegmark stated that is flawed reasoning because dying is not an instantaneous event, and "fully dead" and "fully alive" are not the only possible outcomes, as in the thought experiment. Rather, it is a progressive process, with a continuous series of states of decreasing consciousness. In most real causes of death, one's self-awareness fades out gradually. It is only within this specific imaginary scenario that a person finds themselves surviving.

Most experts believe that the experiment would not actually work in the real world.

Max Tegmark now believes that from their own point of view, the person in the thought experiment should not expect immortality. Since they die in some worlds, they afterwards exist in much fewer worlds than they had before. People are less likely to find themselves in a world where their own existence is less likely. Therefore, it is only a possibility, not a certainty, that the person who does the experiment then goes on to feel like they survived. This same problem, of not existing as much afterwards, was pointed out by Lev Vaidman in the *Stanford Encyclopedia of Philosophy*.

Physicist David Deutsch, though in favor of the many-worlds interpretation, states regarding quantum suicide that it would not work under the normal probability rules of quantum mechanics. Instead, one would need to add an additional assumption of ignoring worlds where the experimenter is not there. He believes that assumption is false. Physicist David Wallace argues that a decision theory analysis shows that a person who prefers certain life to certain death must prefer to keep themselves alive in worlds that are more likely outcomes, not just in less likely ones.

Physicist Sean M. Carroll, though also in favor of the many-worlds interpretation, states about quantum suicide that neither experiences nor rewards should be thought of as being shared between future versions of oneself, because these future versions become distinct persons when the world splits. He then states that a person cannot pick out some future versions of oneself as really being oneself and not the others. He concludes that quantum suicide kills some of these future selves, which is a bad thing the same as if there were no other worlds.

Related pages
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* Multiverse

Other websites
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* 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.