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torekp comments on Scott Aaronson's cautious optimism for the MWI - Less Wrong Discussion
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As far as I can tell (being a non-physicist), the Transactional Interpretation shares the mathematical simplicity of MWI. And here<PDF> Kastner and Cramer argue that TI can derive the Born probabilities naturally, whereas MWI is said to need a detour through "the application of social philosophy and decision theory to subjectively defined ‘rational’ observers". So maybe TI is simpler.
The "possibilities" they posit seem quite parallel (pardon the pun) to the multiple worlds or bifurcated observers of MWI, so I don't see the philosophical advantage there, that they tout. But integrating the Born probabilities more tightly into the physics is a plus, if true.
With respect to Born probabilities, TI is on the level of MWI, it has no derivation for them. Similarly, its ontology is rhetorical rather than rigorous.
A central issue for any zigzag-in-time or retrocausal theory of QM would be vacuum polarization, which was the stumbling block for the most serious effort, by Feynman and Wheeler. But Feynman-Wheeler theory is also where the path integral was born, so TI advocates could say, we just need to go back and finish it properly.
Stopped reading the linked paper when it made a mistake because of treating "worlds" as literal things being "split off." Gotta use quantum mechanics if you're going to talk about quantum mechanics. Maybe they corrected it later, but I didn't even want to wade through to find out.
Although they do not "split off" in the same envisioned early on by DeWitt, there is definitely some unanswered questions here. Alastair Wilson and Simon Saunders has raised this issue. Are all the worlds in the wavefunction from the beginning of time or do they somehow spring out from one world? This is called overlap vs non-overlap (first discussed by David Lewis).
Since you are the expert, by all means answer this for us.
So, by "world" in this post I'll mean "basis sate for the universe." The basis is arbitrary, so what "world" means will still depend on how I'm choosing what "worlds" are - there's the energy basis, for instance, where nothing ever changes if you look at just one of those "worlds." But you can have animals or computers in your basis states if you want - they aren't energy eigenstates, so they change with time.
Anyhow, currently the universe is spread out over a very wide variety of energy eigenstates, which is a fancy way of saying that lots of stuff changes. If we only allow quantum mechanics (that is, strictly follow MWI), this spread over "energy-worlds" is how the universe has been since the beginning of time. But if we look at the exact same state a different way, you could just call the initial state of the universe a basis state, and then, lo and behold, the universe would have sprung from one world, and the distribution of worlds then changed over time. This way of looking at things is probably pretty useful for cosmology. Or you could use worlds that change over time but don't include the original state of the universe, giving you overlap again. This is what we do unintentionally when we choose worlds that have humans in them, which is also pretty useful :)
For overlap vs. non-overlap to get more complicated than "both are valid pictures," you'd need some model where there weren't any static worlds to talk about - this would be a change to QM though. Also, this does raise the interesting question of how complicated that initial world (if we look at it that way) was. It doesn't have to be too complicated before we see interesting stuff.
Anyhow, it's pretty likely I was too hasty in my mistake-detection. But meh, I rarely regret putting off reading things. And I only occasionally regret putting my foot in my mouth :)
To be perfectly honest, I do not see an answer to my question here.
You do explain some, but it seems that you end up indirectly stating that it is "semantics" whether the worlds overlap or not overlap. From what you say here it all depends on how you look at it, but that there is no "truth" of the matter. But that cannot be, either the worlds are overlapping or they are not. That is just the very fact of objective reality.
So while "both pictures are valid" in terms of math, not both can be the same. Metaphysically they are not the same and they got very different effects on episteomology. Also in terms of for instance quantum suicide. In overlap, it's hard to argue against some sort of Quantum Immortality, whilst in non-overlap death is just as in a classical one world theory.
What I am saying is that if one person says "all the worlds have always existed" and another says "the worlds spread out from one world," it's possible that both of them are being consistent, but then they are using two different definitions of "world." I am also saying that there is no basis that is "more real" than the others - only that some are more useful, and it's okay that people use different definitions as long as they're clear about it.
And yes, both pictures can describe the same thing. Have you worked with Bell states at all? Or am I misinterpreting your name and you actually haven't taken a class on quantum mechanics before?
The quantum world is like a diagonal line. One person comes up to it and says "Ah! Here is a diagonal line! It has just as much horizontal as it does vertical, therefore it is a mixture between horizontal and vertical." Another person comes up to it and says "Ah! Here is a diagonal line! It is a perfect rising diagonal, and is not even a little biased towards the falling diagonal." Will these two people argue over whether the line is made of two components or one?
I understand what you are saying, which I think my last post showed quite clearly, but this still does not answer the actual question at hand. What you are saying really amounts to saying that "realism and solipsism are the same", because we cannot really distinguish either through science, all we can do is use logic and metaphysical "reasoning".
Obviously both overlap and non-overlap cannot be true, they are ontologically different, yet you seem to say that "because the equations doesn't decide, reality isn't decided" which is some sort of extreme positivism.
Have you read any of the papers that outline this? Alastair Wilson have written several: http://www.alastairwilson.org/
Maybe you're just used to talking with people who are better at interpreting you, or people who are more similar to you. Clearly understandable to people you talk with every day isn't always clearly understandable to me, as we've seen.
Could you explain this? Is this a metaphor, or are have you interpreted my statements about vectors to actually bear on realism vs. solipsism? Perhaps we have been talking about two different things.
Ah. See, this is the sort of thing I was trying to illustrate with the example of the diagonal line. A line being made of one component is ontologically different from a line being made of two components. Does this matter?
What happens if a one-componenter runs into a two-componenter? Do they argue? Does the first say "because of [insert convincing component-ist argument here], it's ONE component!" Are there valid component-ist arguments? How can the two-componenter respond?
I think it would go more like this: the first one says "hey, if you describe lines in terms of plus and minus diagonals, this one is clearly just a plus diagonal, so why say it has two components?" And the second says "Oh, huh, you're right. But there are lots of horizontal and vertical lines out there, so two-components is more useful." And the first says "yeah, that makes sense, unless you were building a ramp or something." "Well then, cheerio." "Toodles."
The reason this was so anticlimactic is because each participant could frame their ontology in a universal language (vectors!), and the ontologies were lossless transformations of each other - in this case the transformation was as simple as tilting your head. This clarity of the situation leaves no room for appeals to componentism. Arguments are for when both people are uncertain. When people know what's going on, there's simply a difference.
Could you point me to an example? Similar to how we are potentially talking about two different things, Alastair Wilson seemed to be talking about something other than physics in the papers I skimmed. The phrase "the most appropriate metaphysics to underwrite the semantics renders Everettian quantum mechanics a theory of non-overlapping worlds" exemplifies this for me.
Sure I can accept that I might have overestimated how well you should've been able to interpret my post.
Solipsism vs Realism is indeed a metaphor. If you are saying what I think you are saying, then it is quite equivalent.
I do not think that your example of a diagonal line is the same as overlap vs non-overlap at all. In overlap vs non-overlap the ontological differences matter. In a overlapping world, if you are shot, you are guaranteed to survive in another branch, so QI has to be true. In non-overlap, if you get shot, you just die. There is no consciousness that continue on in another branch that it was never connected to...
Also it makes away with the incoherence problem, which is HUGE if you are in the "Born Rule can be derived from decision-theoretic camp".
It is metaphysics, I've already said this in the first post. There is no experiment that can ever distinguish either, just like no experiment can ever tell us if solipsism or realism is true. But obviously (assuming MWI is right) one of them are, only one, not both.
I think 5 of those papers are directly about non-overlap vs overlap, and I can't remember which makes the point best right now, so read any of them you'd like. Or you can read Simon Saunders paper which was in a chapter of the Many Worlds? 2010 book here: http://users.ox.ac.uk/~lina0174/chance.pdf
Ah, I see. "Metaphysics."
By which you mean "taking human morality and decision-making, which evolved in a classical world, and figuring out what decisions you should make in a quantum universe."
Would you agree that overlap vs. non-overlap cannot be answered without looking inside humans, and in fact has little to do with the universe apart from a few postulates of quantum mechanics? For some reason I thought we were talking about the universe.
Anyhow, I think Shane Legg had a nice paper on porting utility functions, though of course humans are inconsistent and you immediately run into problems of how to idealize them. The basic idea being that you split up changes into "new things to care about" and "new ways to express old things." Quantum suicide is probably the easiest thing to deal with via this method.