[jerf]

"If quantum immortality is true, and I, knowing that, step in front of a car, how is it going to prevent my bones from breaking?"

Quantum immortality doesn't promise health. It promises consciousness.

It is in fact a horrible idea and you better damned well hope it is false, because it looks a lot less like "I'm going to live forever in at least some fraction of the multiverse and be healthy and happy!" and rather a lot more like SCP-2718 [1]. Probably without the pain in question, but certainly with your consciousness stuck in a body that is literally minimally capable of being conscious and nothing else, past the heat-death of the universe, until the point where even quantum randomness isn't enough to keep you conscious, assuming that such a point can even arrive. QI doesn't protect your mobility, your senses, your health or happiness... just your consciousness. On the plus side, "minimal consciousness" doesn't necessarily entail a great deal of awareness of time passing.

[1]: http://www.scp-wiki.net/scp-2718 , for those not familiar with the format of the site, be sure to click the "► Play" on the bottom for the text I'm referring to. On the topic of SCP, the "End of Death" canon rather resembles your story, albeit moreso: http://www.scp-wiki.net/end-of-death-hub

[red75prime]

It also doesn't necessarily entail remembering who you are. So I think it's not that horrible: you lose more and more of your memories and your capabilities of forming thoughts and intentions, until it's just generic consciousness aware of itself doing nothing and of (what's left from) input stimuli.

[dhimes]

So, in the limit it's not observably different from death? Then it's death.

[jerf]

It's all the things. QI is basically just an consciousness-centric view of the already-understood truth that in the quantum multiverse, everything that can possibly happen does. It just means that there is some way in which you may actually experience some aspect of that, because your future conscious experience excludes all the universes in which you are not conscious. You can draw a subset of future universes in which you are minimally conscious, in which you're just a wee bit more conscious than that, in which you're conscious enough to be aware of your plight indefinitely, in which as I say in another message you're healthy indefinitely. You're not causing anything to be or not to be, you're just carving various subsets of radically differing, but non-zero, size out of the near-infinity of the full quantum multiverse, which exist whether or not you choose to regard them.

So, you know, in a fraction of the universes in which you remain conscious that would require something like 1 over a number in arrow notation to describe, you'll be conscious enough to be perturbed. This is utterly, utterly dominated by the universes in which you're just sort of there. However, if you then choose to exclude those latter universes on the grounds that you consider that to be "dead", then that brings that tiny fraction back to the fore by virtue of eliminating everything else.

It's a lot of definition chopping and manipulating rather enormous exponentials in the probabilities. However, there is still an underlying truth there, if the multiverse is true. It's just more subtle than the casual human view of "survival" or a brief gloss of the argument might entail. You have to think in quantum terms about whether or not this spin goes that way or the other, not in human terms like whether or not the gun fires. The latter is made up of an incomprehensibly large number of the former sort of things, and as you start talking about the fringe cases the quantum events require staggeringly enormous exponential probabilities to describe anything even remotely human-visible.

[red75prime]

Branching of multiverse on every measurement event is an approximation. The branches aren't fully isolated. I suspect that extremely low amplitude branches undergo merger events, which makes it impossible to have coherent timeline in them. But I'm completely out of my depth here.

[jerf]

I don't think that's an official part of the theory. It may be true, but I don't think anyone's proposed it.

Defining "low amplitude" branch meaningfully would be a real challenge. As I like to say sometimes, the probability of anything happening is indistinguishable from zero. 15-16 billion years after the Big Bang, we've already got a pretty low amplitude, one that would require something like arrow notation (as I reference in another comment) to describe where we are now relative to where the universe started. Heck, it takes arrow notation just to describe how much probability mass we're shedding every second.

(It actually occurs to me after a discussion of this that I can tweak an argument I've been growing over the years to prove that you can't have all three of "a universe that never ends", "the quantum multiverse", and "a coherent conscious experience".)

[red75prime]

A world where each and every person still lives would be pretty bizarre. That's around 107 billion people.

[dhimes]

Great explanation. Thank you. And, I got to look up "arrow notation." I had never heard of it.