In 'quantum immortality', what is "survival" meant to entail?

Question

I'm aware that it's meant to be a reductio, but have read - I think - someone saying that quantum immortality (QI) is not much of a panacea just because we will often be horrifically disabled by the near death. So presumably QI does not mean that whoever dies in this world goes on to experience life as if nothing at all happened, even that the cat was never put in a box. So what exactly "survives" in quantum immortality? Could it even "prove" that the dead (in this world) experience nothingness?

Answer 1

"Quantum Immortality" is a controversial extension of the already controversial "many worlds" interpretation of quantum mechanics. The problem being addressed is that the world seems nondeterministic at the subatomic level, and to be ruled entirely by chance. (At the larger level, we see the most probable outcomes in aggregate, which yields a world that is much more predictable.)

Worse (in the sense of being counter to common sense) than being unpredictable, subatomic particles also sometimes seem to be in multiple different states at once, and to "collapse" into a single state only when observed. One interpretation of this is the many-worlds hypothesis, which claims that when we observe a particle, we aren't "collapsing" its possibilities, we're actually splitting the universe into two alternate universes, one in which the particle goes one way, and one in which it goes another.

The idea of quantum immortality is:

• Somewhere in the branching of the universes there's a place where you live in one universe and die in another. If you were able to select the branches carefully enough, always picking the branch where you live, you'd be able to find a path through the multiverse where you were alive forever.

• There's a version of the theory that goes on to state that not only is this true, but also that your individual consciousness will always adhere to the branch where you survive. So subjectively, no conscious being will ever experience death. In alternate worlds they will appear to die, but in the world they experience, they will always manage to cheat death. To answer your question, what "survives" is your individual consciousness. (As you indicated, however, it in no way guarantees a continuing good quality of life. So it's somewhat cold comfort for those seeking a non-religious version of immortality.)

• Alternately (!) your consciousness doesn't choose branches. This way of conceptualizing QI starts from the observation that if you are conscious then you haven't previously died in the universe you are experiencing. As time progresses forward, more and more versions of you will die, but supposedly, there will always be at least one version of your consciousness surviving somewhere out there in the multitude of universes, even as all the others eventually wink out. By focusing on the surviving consciousness we're in effect exploiting survivorship bias.

Supposedly the proposition is "falsifiable." If you contrived a situation where death was almost certain, and you managed to escape, it would serve as evidence in favor of the theory. (Only the experimenter themselves could convincingly observe the result, and only if it succeeded. If the hypothesis was false, they would just be dead.) That claim is controversial, to say the least. It should also go without saying that this is pure metaphysical speculation with a veneer of science, and that it cannot be said to "prove" anything.

Answer 2

I should confess at the outset that I have no time for the pop-science version of the many worlds interpretation. That said, I will try to answer your question in accordance with the principle of charity.

In quantum theory, quantum objects have associated mathematical functions called 'wave functions' which play a role in quantifying certain observable properties of the object, such as its energy, momentum etc. In the original 'Copenhagen' interpretation of quantum theory, the wave function of an object was taken to undergo abrupt changes whenever the object was the subject of a 'measurement'. (I have put the word measurement in quotes to signify that it used in a very specific way in quantum theory.) Wave functions come in sets, or families, associated with different measurables. For example, there is one family associated with the energy of the object, and another associated with the position of the object. The idea is that if you measure the position of an object, its wave function immediately after the measurement has to be one of the members of the position family of wave functions. If you then measure the energy of an object, its wave function changes to become one of the members of the energy family of wave functions.

These families of wave functions have the property that you can take any member of a particular family and express it as a blend of wave functions from another family, rather as the sound of a chord is a blend of individual notes and overtones. According to the Copenhagen interpretation, if a particle with some arbitrary wave function, which is a blend of energy wave functions, undergoes a measurement of the particle's energy, the measurement picks out a particular energy wave function from the blend, and makes that the new wave function for the particle.

Many physicist rightly think these abrupt changes associated with measurements are very suspect, which has generated a lot of interest in alternative interpretations.

The MWI takes the view that when a 'measurement' is performed, instead of the wave function abruptly changing, there is some kind of splitting of reality, so that all of the components of the wave function before the measurement continue to be present after the measurement but in different worlds that are isolated from each other, so there is no way for what happens in one world to influence what happens in another.

Now we need to unpack the word 'measurement'. Really, a measurement is just an interaction between the quantum object being measured and the (possibly large number of) quantum objects that form the detecting part of the measuring device, so a measurement is not really any different from any other kind of interaction, and since interactions are happening all the time at the quantum level, the splitting of wave functions into different worlds is also happening all the time.

In the MWI, the splitting means that all of the possible outcomes of interactions exist, but in a countless number of different worlds. This means that there are countless versions of you, each version representing a different evolution of the configuration of the particles of which you are comprised. So quantum immorality does not mean that you die and are reborn in some way- it means that any of your countless parallel copies can die but the remainder continue to live.

There are many conceptual difficulties with the pop-science version of MWI, not the least being the question of how you experience being a unique version of you if there are countless different versions of you existing in parallel.