Why does the Quantum Immortality Theory imply that "I" will live forever

Question

I don't know if this is more a question for physics stackexchange or philosophy stackexchange, but if the theories behind quantum immortality are true (eg. Many worlds interpretation), why does it imply we live forever.

I understand the idea that in most near death situations, there is a reality where you live, and a reality you will die. And I accept that one cannot experience their own death because it implies the death of their consciousness. But I do not understand how these two facts combine to create "we live forever!"

It seems the "you's" in all realities ( if the many-worlds interpretation is true) are separate entities with their own subjective experience. So what guarantees, that if you are in fact in the reality where you die, that version of you will not just come to an end, and the consciousness will just be gone forever, and that version will just experience oblivion? Why would it instead start experiencing the reality of a whole other version of itself, if it seems all versions of oneself are separate entities?

In other words, if "I" die, as in the version of me who is writing this, my individual consciousness, separate from all the other consciousnesses out there that are other versions of "me," what reason do I have to believe that I would not just experience oblivion, as my consciousness disappears, but somehow have me, my personal consciousness, start experiencing the universe of a whole different consciousness?

I have a feeling we cannot make the assumption that "we" will live forever, seeing as we don't really know if the other you's are the same consciousness as us, and if we could really experience what they experience. That's making too many assumptions about personal identity that we have not proven, philosophically or scientifically.

What's more, what guarantees that there really is always a reality where you live. What about situations where the universe as we know it ends? There is no way you could not die. Unless, of course, we are implying that that situation will never even arise in the first place?

Answer 1

Quantum immortality requires a few (dubious) metaphysical assumptions, in addition to the widely-accepted current models of physics:

1. Our modern understanding of quantum physics is fundamentally true, and we just haven't noticed that there are no contradictions in it.
2. The entire universe is governed by the evolution of a universal wavefunction.
3. The entirety of this universal wavefunction is wholly real.
4. Identity is purely a function of the physical.
5. Experience is (classically) finite – it ends. (“Ends upon death” is the usual formulation, but you can substitute this for “ends upon sleep” or whatever else you like.)
6. A man can cross the same river twice.

• Assumption 4 causes assumption 2 to imply that the “you”s in different “universes” (actually “branches” of the universal wavefunction) diverge – essentially, there are (numerically) more “you”s as time goes on.
• Assumption 3 states that all of those “you”s exist – even the really unlikely ones, like the one who has been lifted into the air by winds every time they tried to fly.
• Assumption 5 means that some branches containing “you” can evolve into branches not containing “you”. We can ignore those, since you aren't in them.
• All the branches that are left contain “you”. Since they are all real, even the really unlikely ones, that means (by the laws of conditional probability) that “you” will always experience some unlikely circumstance that prevents “you” from ceasing to experience.
• Assumption 1 means the actual universe actually works this way – this is the most dubious assumption of them all. We're aware of many existing contradictions in our understanding of physics, so there's a lot of evidence pointing to assumption 1 being completely incorrect.

Thus, Quantum Immortality means you can't die. There is zero reason to believe that the universe actually works that way.

Also note prevents you from dying. Quantum Immortality only claims that your experience won't ever end. Jump off a cliff? Quantum Immortality doesn't protect you from breaking your legs, torso, spine, rupturing your lungs, suffering various degrees of brain damage… It only “protects” you from cessation of experience, even if only by amazingly unlikely events like ATP molecules spontaneously assembling themselves in your neurons. It only “kicks in” when you are a fraction of an instant away from death.

Fortunately, there is zero reason to believe the universe actually works that way.