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gjm comments on Do Earths with slower economic growth have a better chance at FAI? - Less Wrong Discussion
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It unfortunately also explains
All of which are true in other possible worlds, which for all we know may have a greater amplitude than ours. That we are alive does not give us any information on how probable we are, because we can't observe the reference class. For all we know, we're one of those worlds that skate very, very close to the edge of disaster, and the two recessions of the aughts are the only thing that have kept us alive; but those recessions were actually extremely unlikely, and the "mainline" branches of humanity, the most probable ones, are alive because the Cuban War of 1963 set the economy back to steam and horses. (To be sure, they have their problems, but UFAI isn't among them.)
Note that, if you take many-worlds seriously, then in branches where UFAI is developed, there will still be some probability of survival due to five cosmic rays with exactly the right energies hitting the central CPU at just the right times and places, causing SkyNet to divide by zero instead of three. But the ones who survive due to that event won't be very probable humans. :)
If most copies of me died in the shooting but I survived, I should expect to find that I survived for only one reason, not for multiple independent reasons. Perhaps the killer's gun jammed at the crucial moment, or perhaps I found a good place to hide, but not both.
On the other hand, if you are being shot at repeatedly and survive a long time, you should expect there to be lots of reasons (or one reason with very broad scope -- maybe everyone's guns were sabotaged in a single operation, or maybe they've been told to let you live, or a god is looking out for you). And it's only in that sort of situation that anthropic "explanations" would be in any way sensible.
It's always true enough to say "well, of course I find myself still alive because if I weren't I wouldn't be contemplating the fact that I'm still alive". But most of the time this is really uninteresting. Perhaps it always is.
The examples given in this thread seem to me to call out for anthropic explanations to much the same extent as does the fact that I'm over 40 years old and not dead yet.
This just prompted me to try to set a subjective probability that quantum immortality works, so e.g. if I remember concluding that it was 5% likely at 35 and find myself still alive at 95, I will believe in quantum immortality (going by SSA tables).
I'm currently finding this subjective probability too creepy to actually calculate.
I suggest giving some thought first to exactly what "believing in quantum immortality" really amounts to.
To me, it means expecting to experience the highest-weighted factorization of the hamiltonian that contains a conscious instantiation of me, no matter how worse-than-death that branch may be.
I think you should analyse further. Expecting conditional on still being alive? Surely you expect that even without "quantum immortality". Expecting to find yourself still alive, and experience that? Again, what exactly do you mean by that? What does it mean to expect to find yourself still alive? (Presumably not that others will expect to find you still alive in any useful sense, because with that definition you don't get q.i.)
I expect there are Everett branches in which you live to 120 as a result of a lot of good luck (or, depending on what state you're in, bad luck). Almost equivalently, I expect there's a small but nonzero probability that you live to 120 as a result of a lot of luck. { If you live to 120 / In those branches where you live to 120 } you will probably have experienced a lot of surprising things that enabled your survival. None of this is in any way dependent on quantum mechanics, still less on the many-worlds interpretation.
It seems to me that "believing in quantum immortality" is a matter of one's own values and interpretive choices, much more than of any actual beliefs about how the world is. But I may be missing something.
I should perhaps be more clear that I'm not distinguishing between "MWI and functionalism are true" and "quantum immortality works." That is, if "I" consciously experience dying, and my consciouness ceases, but "I" go on experiencing things in other everett branches, I'm counting that as QI.
I'm currently making observations consistent with my own existence. If I stop making that kind of observation, I consider that no longer being alive.
Going again with the example of a 35 year old: Conditional on having been born, I have a 96% chance of still being alive. So whatever my prior on QI, that's far less than a decibel of evidence in favor of it. Still, ceteris paribus, it's more likely than it was at age 5.
Sure. But I'm not sure I made the point I was trying to make as clearly as I hoped, so I'll try again.
Imagine two possible worlds. In one of them, QM works basically as currently believed, and the way it does this is exactly as described by MWI. In the other, there is at every time a single kinda-classical-ish state of the world, with Copenhagen-style collapses or something happening as required.
In either universe it is possible that you will find yourself still alive at 120 (or much more) despite having had plenty of opportunities to be killed off by accident, illness, etc. In either universe, the probability of this is very low (which in the former case means most of the measure of where we are now ends up with you dead earlier, and in the latter means whatever exactly probability means in a non-MWI world). In either universe, every observation you make will show yourself alive, however improbable that may seem.
How does observing yourself still alive at 150 count as evidence for MWI, given all that?
What you mustn't say (so it seems to me): "The probability of finding myself alive is very low on collapse theories and high on MWI, so seeing myself still alive at 150 is evidence for MWI over collapse theories". If you mean the probability conditional on you making the observation at age 150, it's 1 in both cases. If you mean the probability not conditional on that, it's tiny in both cases. (Assuming arguendo that Pr(nanotech etc. makes lots of people live to be very old by then) is negligible.) The same applies if you try to go halfway and take the probability simply conditional on you making the observation: MWI or no MWI, only a tiny fraction of observations you make will be at age 150.
In the MWI-universe, it is probable at near unity that I will find myself still alive at 120. In the objective collapse universe, there's only a small fraction of a percent chance that I'll find myself alive at 120. In the objective collapse universe, every observation I make will show myself alive--but there's only a fraction of a percent of a chance that I'll make an observation that shows my age as 120.
The probability of my making the observation "I am 150 years old," given objective collapse, is one of those probabilities so small it's dominated by "stark raving mad" type scenarios. Nobody you've ever known has made that observation; neither has anybody they know. How can this not be evidence?
You are correct.