Help: Is there a quick and dirty way to explain quantum immortality?

You think of your life as a branching tree of possibilities rather than a single line in time. And the immortality is supposed to happen because according to MWI, every possibility, no matter how improbable, actually happens, so there's always a branch of the tree that keeps growing - a possibility where the cancer goes away by itself, the bullet goes astray, and so on.

One possible attitude to quantum immortality would be that no matter what happens to you, there's always another copy of you who survives. But it seems that people who are really into quantum immortality want to identify with their copies - they have decided to think of the whole tree of possibilities as themselves, and not just the current branch. Wikipedia puts it like this: "Over time, the experimenter would therefore never perceive his or her own death." That's a little retarded, because if you're going to identify with the whole tree of possibilities, then you should identify with every death. So "quantum immortality" actually means that you die infinitely many times and in every possible way, but there's no final death.

Here's my explanation of quantum immortality:

Say you want to do something humans regard as important, like factoring large numbers. Lock yourself in a chamber that draws on a source of true quantum randomness to guess factors. Set it to kill you whenever it guesses incorrectly.

A short moment later, you will be alive, and have the correct factorization. This is because the only being you will identify with will be the being that decohered into the wavefunction branch that guessed correctly.

Seriously, this is the best way for humans to factor integers and solve other computationally-difficult problems. Give it a try.

Generally with this sort of thing it helps to be concrete.

Ask him to imagine that he's James Bond retiring at 60 years old. He's survived more lucky occasions then he can count. Why? Because he's the remaining lucky one.

As to continuity of identity, this is actually a tricky part. Ask him if he identifies with himself from 1 millisecond ago. Ok, but another version of him that had had that 1 millisecond go slightly differently would also identify with the one from 1 millisecond ago. And there identification is just as reasonable. So you should identify with the one who did the last millisecond slightly differently.

This doesn't quite work out (because presumably there's some sufficiently different version of me that doesn't have a goatee and thus is a goody-two shoes, and I'm not going to identify with that one.) But this detail isn't too relevant because the real notion is observers.

I'd skip the issue of continuity completely if you can and simply explain that the idea is that at any given point if you accept MWI then there will always be an observer who shares an identical history up to you at the current time.

Frankly, unless both you and your interlocutor have a firm mathematical understanding of quantum theory, discussing topics like these is like a debate about subtleties of engine tuning between people who have never opened a car hood. It's simply impossible for anyone to reach any real understanding or insight in such a situation.

The first question is whether you understand the point you were trying to make. Maybe your argument doesn't hold any water, and so difficult to profess. You didn't clearly explain your position in the post. (What about "continuity of identity"? What did you try to explain about "quantum immortality"?)

I'm a MWI cynic*, so here is my approach.

There is an interpretation of quantum mechanics called the many-world interpretation that rejects the wave-function collapse of the classical interpretation.

This interpretation leads some people to the belief that all possible alternative histories are real. Everything that can happen does happen. They assume that if you die in this world, you would also continue living on in another world. Since there must always be a chance you won't die, then there must be a world in which you live forever.

The problem with this belief is that at best it applies only for simple quantum systems, and generally not to events as large and complex as a person's death. To avoid death, a very large number of quantum level alternatives have to be simultaneously selected for. The probability of this simultaneous selection will be effectively zero.

In MWI terms this means that you die in all worlds, except the impossible ones.

Value the life you have and don't depend on quantum immortality.

* I like the MWI on lack of wave-function collapse and on quantum decoherence, but I don't think that the idea of separate worlds is necessary.

The (very brief) explanation I normally use:

So, according to quantum physics, any time something can happen more than one way, it actually happens all those ways, but in different universes. So let's say you flip a coin. In one universe, it comes up heads, but in another it comes up tails. However, it's not really that simple, because there are all kinds of crazy things that can happen but are really unlikely. So it's more like, in almost half the universes, the coin lands heads, and in the other almost half of the universes, the coin lands tails, but in a tiny fraction of the universes the coin actually lands on its side. Anyway, quantum immortality holds that, since you can't perceive universes in which your perceptive abilities no longer exist, you'll always end up in a universe where you're still alive. So imagine you push a button that has a 50% chance of killing you. From an observer's perspective, you will die 50% of the time. However, from your perspective, you'll only perceive those universes in which you don't die, so you'll never actually die, no matter how many times you push the button. This applies to everything, since there's never a true 100% chance of death, no matter how crazy the situation.

I don't have any answers to you question, but I have my own questions. Maybe someone will answer them here.

Just yesterday I had a long discussion with wedrifid about quantum suicide. It came to a halt when wedrifid got very offended because I claimed that he misrepresented my position. Or something like that. I am still not sure. Anyway, in spite of this, I think it was an interesting discussion mostly relevant here. There I enumerated several reasons why committing quantum suicide is a bad idea. But I am still very confused about the real question: How would it feel to commit quantum suicide? I wrote:

My common sense tells me that if I put a (quantum or other) gun in my mouth right now, and pull the trigger many times, then the next thing I will feel is not that I am very lucky. Rather, I will not feel anything at all because I will be dead. I am quite sure about this instinct, and let us assume for a minute that it is indeed correct. This can mean two things. One possible conclusion is that MWI must be wrong. Another possible conclusion is that MWI is right but we make some error when we try to apply MWI to this situation. I give high probability to both of this possibilities, and I am very interested in any new insights.

Can I ask a related question? Is there a physical model available that allows for immortality (eternally stable structure) in a cyclic model of the universe only (limited space with finite time between cycles)?

MWI and other parallel universe models seem to allow for suitable ways of replication and escape - but I never found anything related for a cyclic model. There is talk of surviving the Heat death (superconductor based computers) and Big Crunch/Big bang (using suitable black holes, etc..) - but there is one specific problem I haven't seen addressed: Particle decay.

If everything else works as planned and a future stable structure is created - in a cyclic model - Is there any way to prevent it from catastrophically desintegrating through particle decay (which is bound to happen in enough finite time)?

The best way to get at the idea behind quantum immortality might be to explain that if the universe is infinite there must be in infinite number of yous because there are only so many ways of arranging the atoms in a galaxy so all possible galaxies exist in infinite number. No matter what happens there must always be galaxies in which a you somehow escapes death so there will always be a you that experiences life.

Pretend you undergo a dangerous operation in which you have a 99% chance of "dying". Before the operation you can correctly say to yourself that "I will wake up from this operation" because some versions of you will survive. Also, in some galaxies death will be cured before you age too much and therefore yous will be alive in 1,000 years.