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gjm comments on The map of quantum (big world) immortality - Less Wrong Discussion
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What exactly do you mean by "you" here? (I think maybe different things in different cases.)
Maybe I should try to get rid of that word. So let's suppose we have a conscious observer in a situation like that, so that they have a very, very large probability to die soon and a small but non-zero probability to survive. Now, if there is only one world that doesn't split and there are no copies of that observer, i.e. other observers who have a conscious experience identical or very similar to that of our original observer, then that observer should expect that i) the only outcome that they may experience is one in which they survive, but that ii) most likely they will not experience any outcome.
Whereas given MWI and QI, there will be an observer (numerous such observers, actually) who will rememeber being the original observer and feel like they are the same observer, with a certainty.
So "you" kind of means "someone who feels like he/she/it is you".
But if you hold "you X" to be true merely because someone who feels like they're you does X, without regard for how plentiful those someones are across the multiverse (or perhaps just that part of it that can be considered the future of the-you-I'm-talking-to, or something) then you're going to have trouble preferring a 1% chance of death (or pain or poverty or whatever) to a 99% chance. I think this indicates that that's a bad way to use the language.
I'm not sure I entirely get what you're saying; but basically, yes, I can see trouble there.
But I think that, at its core, the point of QI is just to say that given MWI, conscious observers should expect to subjectively exist forever, and in that it differs from our normal intuition which is that without extra effort like signing up for cryonics, we should be pretty certain that we'll die at some point and no longer exist after that. I'm not sure that all this talk about identity exactly hits the mark, although it's relevant in the sense that I'm hopeful that somebody manages to show me why QI isn't as bad as it seems to be.
QI or no QI, we should believe the following two things.
In every outcome I will ever get to experience, I will still be alive.
In the vast majority of outcomes 200 years from now (assuming no big medical breakthroughs etc.), measured in any terms that aren't defined by my experiences, I will be dead.
What QI mostly seems to add to this is some (questionable) definitions of words like "you", and really not much else.
I agree with qmotus that something is being added, not so much by QI, as by the many worlds interpretation. There is certainly a difference between "there will be only one outcome" and "all possible outcomes will happen."
If we think all possible outcomes will happen, and if you assume that "200 years from now, I will still be alive," is a possible outcome, it follows from your #1 that I will experience being alive 200 years from now. This isn't a question of how we define "I" - it is true on any definition, given that the premises use the same definition. (This is not to deny that I will also be dead -- that follows as well.)
If only one possible outcome will happen, then very likely 200 years from now, I will not experience being alive.
So if QI adds anything to MWI, it would be that "200 years from now, I will still be alive," and the like, are possible outcomes.
There's no observable difference between them. In particular, "happen" here has to include "happen on branches inaccessible to us", which means that a lot of the intuitions we've developed for how we should feel about something "happening" or not "happening" need to be treated with extreme caution.
OK. But the plausibility -- even on MWI -- of (1) "all possible outcomes will happen" plus (2) "it is possible that 200 years from now, I will still be alive" depends on either an unusual meaning for "will happen" or an unusual meaning for "I" (or of course both).
Maybe the right way to put it is this. MWI turns "ordinary" uncertainty (not knowing how the world is or will be) into indexical uncertainty (not knowing where in the world "I" will be). If you accept MWI, then you can take something like "X will happen" to mean "I will be in a branch where X happens" (in which case you're only entitled to say it when X happens on all branches, or at least a good enough approximation to that) or to mean "there will be a branch where X happens" (in which case you shouldn't feel about that in the same way as you feel about things definitely happening in the usual sense).
So: yes, on some branch I will experience being alive 200 years from now; this indeed follows from MWI. But to go from there to saying flatly "I will experience being alive 200 years from now" you need to be using "I will ..." locutions in a very nonstandard manner. If your employer asks "Will you embezzle all our money?" and your intentions are honest, you will probably not answer "yes" even though presumably there's some very low-measure portion of the multiverse where for some reason you set out to do so and succeed.
Whether that nonstandard usage is a matter of redefining "I" (so it applies equally to every possible continuation of present-you, however low its measure) or "will" (so it applies equally to every possible future, however low its measure) is up to you. But as soon as you say "I will experience being alive 200 years from now" you are speaking a different language from the one you speak when you say "I will not embezzle all your money". The latter is still a useful thing to be able to say, and I suggest that it's better not to redefine our language so that "I will" stops being usable to distinguish large-measure futures from tiny-measure futures.
Unless they were already possible outcomes without MWI, they are not possible outcomes with MWI (whether QI or no QI).
What MWI adds is that in a particular sense they are not merely possible outcomes but certain outcomes. But note that the thing that MWI makes (so far as we know) a certain outcome is not what we normally express by "in 200 years I will still be alive".
You raise a valid point, which makes me think that our language may simply be inadequate to describe living in many worlds. Because both "yes" and "no" seem to me to be valid answers to the question "will you embezzle all our money".
I still don't think that it refutes QI, though. Take an observer at some moment: looking towards the future and ignoring the branches where they don't exist, they will see that every branch will lead to them living to be infinitely old; but every branch doesn't lead to them embezzling their employer's money.
Do you mean that it's not certain because of the identity considerations presented, or that MWI doesn't even say that it's necessarily true in some branch?
I don't think refuting is what QI needs. It is, actually, true (on MWI) that despite the train rushing towards you while you're tied to the tracks, or your multiply-metastatic inoperable cancer, or whatever other horrors, there are teeny-tiny bits of wavefunction (and hence of reality) in which you somehow survive those horrors.
What QI says that isn't just restating MWI is as much a matter of attitude to that fact as anything else.
I wasn't claiming that QI and inevitable embezzlement are exactly analogous; the former involves an anthropic(ish) element absent from the latter.
The "so far as we know" was because of the possibility that there are catastrophes MWI gives you no way to survive (though I think that can only be true in so far as QM-as-presently-understood is incomplete or incorrect). The "not what we normally express by ..." was because of what I'd been saying in the rest of my comment.
I see. But I fail to understand, then, how this is uninteresting, as you said in your original comment. Let's say you find yourself on those rain tracks: what do you expect to happen, then? What if a family member or other important person comes to see you for (what they believe to be) a final time? Do you simply say goodbye to them, fully aware that from your point of view, it won't be a final time? What if we repeat this for a hundred times in a row?
I would say that QI (actually, MWI) adds a third thing, which is that "I will experience every outcome where I'm alive", but it seems that I'm not able to communicate my points very effectively here.
How does MWI do that? On the face of it, MWI says nothing about experience, so how do you get that third thing from MWI? (I think you'll need to do it by adding questionable word definitions, assumptions about personal identity, etc. But I'm willing to be shown I'm wrong!)
I think this post by entirelyuseless answers your question quite well, so if you're still puzzled by this, we can continue there. Also, I don't see how QI depends on any additional weird assumptions. After all, you're using the word "experience" in your list of two points without defining it exactly. I don't see why it's necessary to define it either: a conscious experience is most likely simply a computational thing with a physical basis, and MWI and these other big world scenarios essentially say that all physical states (that are not prohibited by the laws of physics) happen somewhere.
As you can see, I've replied at some length to entirelyuseless's comment.