Quantum suicide

Quantum suicide is a thought experiment. It originally went as follows: Say I build a suicide device, such as a gun which will fire a bullet into my head, which is triggered by a quantum event - such as a device which measures whether a particle is spin-up or spin-down. If it measures spin-up, it doesn't fire; if spin-down, it fires. This is essentially Schrödinger's cat, with myself as the cat. If the many-worlds interpretation of quantum mechanics is true, then at the point the measurement is made, the universe splits into two — one universe in which it is spin-up and I live, and another in which it is spin-down and I die. I will cease to exist in one universe but not in another. So, the argument goes, although there will be other people who will exist in the universes in which I die, I will only ever exist in the universes in which I survive, so I will only ever observe the universes in which I survive. From my perspective, I don't die. This result is referred to as quantum immortality.

Mischaracterizations
Although originally referring explicitly to death from measuring a single quantum particle, the idea is often understood to include normal causes of death (which are caused by very many quantum particles). It thus posits that no one ever dies, they only appear to. Whenever I might die, there will be another universe in which I still live, some quantum event (even if astronomically unlikely) which saves me from death. Hence, it is claimed, I will never actually experience my own death, but from my own perspective will live forever, even as countless others will witness me die countless times. Life will, however, get very lonely, since everyone I know will eventually die from my perspective.

Dubiousness of quantum immortality
However, there are many problems if one wishes to take quantum immortality seriously. First and foremost, it is dependent on the many-worlds interpretation being correct. Despite what some people claim, though, it is far from established that many-worlds is actually right. One survey of attendees, mostly physicists, to a conference on "Quantum Physics and the Nature of Reality" had only 18% choose many-worlds as their preferred interpretation.

The interpretation problem underscores the fact that quantum mechanics cannot be physics' theory of everything. In particular, it cannot presently be reconciled with relativity and thus, gravity. Quantum mechanics is often thought of as saying literally anything is possible (even if with a very tiny probability), but this should be understood as meaning that anything allowable within the rules of quantum mechanics and the rest of physics is possible. Therefore, it is unclear how quantum mechanical effects work exactly on the macroscopic scale. For example, can random quantum events really preserve your brain through trauma from a high-speed car accident, or lack of oxygen from drowning? Or even bubble up a copy of you out of thin air somewhere else in the world, sufficiently far away?

Even if the requisite type of multiverse is real, and even if survival is physically possible in some universe, you will not experience anything like quantum immortality. The idea assumes that consciousness cannot be interrupted, or end, from its own viewpoint. This must be false. Every night when you go to sleep, your consciousness ends (temporarily in this case). If your consciousness only flowed into universes where it was preserved, you would expect to experience 'quantum insomnia,' which of course does not happen. Thus, if you were to perform quantum suicide, your consciousness would terminate with increasing probability the more times the experiment was performed, not experience immortality.

Another major flaw in the idea is the fact that consciousness is not fundamental and indivisible. Rather, it emerges from the interplay of neurons in the brain, and can be weakened gradually due to parts of the brain being inactivated (as when sleeping) or removed. As a person dies, it takes billions of Planck times (the quantum, or smallest measurement, of time) for their brain states to get more and more damaged, regardless of if it is slowly due to old age, or at light speed due to a vacuum decay. Now we have the problem of arbitrarily defining which of these successively more damaged states is the most damaged it can be and still be 'you.' In quantum suicide, why would your thread of identity follow only the brain states in the universe where the gun does not fire, and not the brain states in the universe where the bullet is slowly (from a quantum perspective) traveling through the air and then entering your brain, drastically altering its state? Since consciousness is emergent, there is no cutoff where the brain states suddenly cease to be 'you,' and the destruction of your brain causes your consciousness to flicker out.

This very problem was recognized by Max Tegmark, who first set out the thought experiment. He stated:

"However, I think there's a flaw. After all, dying isn't a binary thing where you're either dead or alive - rather, there's a whole continuum of states of progressively decreasing self-awareness....I suspect that when I get old, my brain cells will gradually give out (indeed, that's already started happening...) so that I keep feeling self-aware, but less and less so, the final "death" being quite anti-climactic, sort of like when an amoeba croaks."

- Max Tegmark

Lower degree of world observation after a potential quantum suicide
The final flaw in this idea, also pointed out by Tegmark, is that any world where you survive quantum suicide has a much lower measure than the world you lived in beforehand. In other words, your probability amplitude in the wavefunction decreases significantly, meaning you exist to a much lower 'degree' than you had before. Per the anthropic principle, you are less likely to find yourself in a world where you are less likely to exist, that is, a world with a lower measure has a lower probability of being observed. Therefore, you will have a lower probability of observing a world in which you survive after the experiment than the world in which you initially set up the experiment.

Considering all these factors, this should not be thought of as a serious theory of immortality. Nor should you attempt to verify or falsify it.