In comments to yesterday's post about my favorite Many-Worlds story, a couple of people mention "All the Myriad Ways," a Larry Niven short story. I don't think I've ever actually read the story, but it gets brought up all the time, so I'm familiar with the concept. It's an angle on Many-Worlds that I don't like, and has something in common with the central conceit of Inception, which is also not high on my list of literary tropes, though my reaction isn't anywhere near as negative as Scott's.
If you're not familiar with it, here's the summary from Wikipedia:
A police detective, pondering a rash of unexplained suicides and murder-suicides occurring since the discovery of travel to parallel universes, begins to realize that being if all possible choices that might be made are actually made in parallel universes, people will see their freedom of choice as meaningless. The choice not to commit suicide, or not to commit a crime, seems meaningless if one knows that in some other universe, the choice went the other way. They therefore kill themselves or commit the crime, because they abandon the sense of choice.
This is a dramatization of a philosophical idea that fails for me on such a fundamental level that I can't really take it seriously. I just don't understand how the existence of other universes has any effect on me. In the end, my personality is the result of a myriad of choices I've made in my life, and some alternate-universe version of me who made different choices is a different person. And the notion of committing suicide at all, let alone over something as daft as the existence of parallel universes, is sufficiently foreign to my mindset that I can't really consider anyone who would do that to be "me" in any meaningful sense.
This is true for the actual Many-Worlds interpretation, in which the alternate versions would be completely inaccessible, and it goes double for parallel universes that you can actually visit. In the first case, you could at least make the case that the alternates are using the same fundamental particles I am, just in a different branch of the wavefunction. If you can visit their home worlds, though, they are absolutely and unequivocally (to my mind) other people. And while other people who I've never met deciding to commit suicide is kind of a bummer, it's not something that's going to ruin my life.
This business of duplicates is a variant of the question "What if we don't have free will?", another philosophical question that I just can't take seriously. If we do have free will, then I have an obligation to choose to act in an ethical manner (however you define ethics). If we don't have free will, by definition, there's nothing I can do about it, so it's hardly worth expending any energy on (not that I could choose to expend energy on it or not).
The question at the heart of Inception-- "What if we're actually dreaming?"-- is in a similar place. If everything I seem to see around me is actually a flawless illusion, there's nothing I can do about it, so I might as well carry on as if what I see is actually real. There isn't really any sensible alternative course of action, absent some really convincing proof to the contrary.
This attitude probably renders me incapable of doing certain kinds of theory-- I find Boltzmann brains and the anthropic principle pretty worthless as well-- but, you know, I can live with that.
That's the reason why I really like "Divided by Infinity", as I said yesterday: the effectiveness of the story doesn't come from any abstract philosophy, but from a feature inherent in the notion of Many-Worlds style parallel universes. It's also got a sort of inevitability to it that "All the Myriad Ways" and similar stories lack-- there's nothing the narrator could do to escape the story. Most of the time I read alternate-universe stories, I'm left thinking "You know, in the universe where you're not a complete dope, this is a really boring story." Wilson avoids that issue, and the result is far and away the best story about these ideas I've ever seen.
I thought the idea behind All the Myriad Ways was not that alternate-universe versions of you make those other decisions in other universes. It's that you make all possible decisions and actions at every given moment, and each different decision or action instantly generates a new universe.
IOW, no matter how foreign the idea of suicide might seem to you, in fact you do decide to commit suicide at every moment where it's at all possible for you to do so. And every time you do, the result is (at least) two new parallel universes - one where you did decide to commit suicide, and one where you did not.
I don't necessarily accept the idea, but it's not the same as some 'other you' whom you've never met deciding to commit suicide.
Many-worlds and (some aspects of) Time Travel to me are different faces of the same thing, realistically speaking. And the utility of the concept of "Free Will" - whether for or against it - is bound up in information theory.
I probably do not have "Free Will" in the sense that, given the bounds of my knowledge and my skill in predicting the consequences of my actions, for any given brain state encompassing these my choice of action will always be the same. But, changing any aspect of those criteria will change the mentation and so may change my choice. It is very rare to be in a situation where there really is only a coin toss as the means for deciding which path to take. So I probably have free will to choose whether or not to obey my conscience or to ignore my knowledge base, and which way I go will be dependent on my ability to predict the consequences, but not free when it comes to the resultant action from those chosen bases.
So discovering things about how the future was affected by my choices before the events unfold has to affect things. And in that vein may I recommend what was for me the essential Time Travel story: "Thrice Upon a Time" by James P. Hogan.
The issue of whether to care about committing suicide in a universe (multiverse?) where there is splitting into multiple worlds is actually central to the question of how to make sense of probability in the many worlds interpretation. From previous posts I know that you don't see the point of this question, but I do think it is the basis of the current main technical objections to the many worlds interpretation (technical as opposed to objections to the basic assumptions that lead to taking many-worlds seriously that is).
Let's put it this way. Suppose you lived in an essentially classical universe in which there is a God and he tells you that every time you measure the polarization of a photon he is going to destroy the current universe and create two almost identical copies of it, the only difference between them being which outcome of the measurement you saw. The question is, how much should you *care* about your successors in the new universes? In one sense, you shouldn't care at all because the original you has been destroyed. On the other hand, if you are a materialist and believe that your consciousness is completely captured by your physical state then you should believe that the *you* in both copies of the universe have equal claim to be the same you as you. It is clear that there is a great deal of ambiguity in how you should act in such a world. Classical decision theory, which is the basis of probability, does not apply since it assumes that, for every action, one and only one outcome actually occurs.
There is not much difference between this and what is actually claimed to happen in the many-worlds interpretation. The question then becomes: Since all branches of the wavefunction have an equal claim to represent reality, why should we use the amplitude of a branch to weight how much we care about it? If you die in a branch that has a very small amplitude then that is still a copy of you that is dying, so what does the arbitrary number attached to the branch have to do with anything?
It seems to me that Niven's story is getting at something relevant. However, it does represent a rather singular solution to the "caring measure" problem, i.e. you should only care about the copies of you that are successful. You could equally well argue for the opposite conclusion, i.e. since each copy of you has a valid claim to be the real you, you should act in such a way to cause none of them any harm. This would lead to paralyzing inaction, e.g. not daring to cross the road, rather than the nihilism in Niven's story, but I suppose it would make for a much less interesting story.
In any case, neither of these strategies leads to the Born rule, so the question still remains. For that you have to buy the decision theoretic arguments of Deutsch, Wallace, et. al. (or possibly Zurek's somewhat similar envariance idea), which is probably as close as you can get to a sensible answer to this question.
"I just don't understand how the existence of other universes has any effect on me. In the end, my personality is the result of a myriad of choices I've made in my life, and some alternate-universe version of me who made different choices is a different person"
This is a common misunderstanding of the story, but I believe it is a misunderstanding - people in the story are not just being confronted with the idea that people just like them in almost every way are making hugely different choices; they are also confronted with the fact that the choices of other people are having dramatic effects on them. The superstar athelete learns that in another universe, someone with just as many skills is a pathetic slob, because of a chance injury; the miserable single man learns that but for the lack of a chance encounter, he might be a happily married father of four. In both cases, no choice by the person was involved - just good or bad luck. That certainly wouldn't affect everyone, but there are people who would resent the chance success of people just like them, or who would be unnerved by how much their (oh so apparently inevitable) success was due to chance.
You might also be interested in Greg Egan's "Singleton" in which someone, upset by the idea of quantum chance affecting his decisions, decides to do something about it. It's online at the author's site here gregegan.customer.netspace.net.au/MISC/SINGLETON/Singleton.html
For good literature related to the Many-Worlds interpretation, try "The Garden of Forking Paths" by Jorge Luis Borges. In fact, the entirety of "Labyrinths" is pretty much essential reading, and rather incredible.
I know it's probably too late, but these things really shouldn't be called 'universes', as that's bad metaphysics. If these theories are true, then it turns out THE universe actually has these compartment thingies in it.
It reminds me of once reading that "Scientists discovered the Greeks were wrong about atoms, which are divisible after all." Well, no, what they discovered is that they had called the wrong things atoms!
Inception actually answers that question in the same way as you do. Cobb does not care if the top spins or falls, whether he is dreaming or not is completely irrelevant to him.
Inception was not so much about "Is it a dream or not?", but rather it was about the process of creating experiences and a depiction of hyperreality. It did not try to specifically ask any question from the viewers, it just presented a story with some deep underlying themes.
On an unrelated note, I have to agree with #5. Borges' stories are amazing pieces of literature and everyone should read them.