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Comment author: Jinoc 21 October 2014 12:22:57PM 0 points [-]

How do you choose the measure over Everett branches in the absence of interactions between branches?

Comment author: gjm 21 October 2014 02:34:59PM 2 points [-]

According to the Born rule.

Comment author: philh 21 October 2014 01:43:25PM 0 points [-]

How does this differ from increasing the expected value of your utility function under a collapse hypothesis?

Comment author: gjm 21 October 2014 02:34:43PM 4 points [-]

I don't think it either does or should, at least for typical utility functions that don't explicitly care about what interpretation of QM is correct.

In response to AI Tao
Comment author: gjm 21 October 2014 02:28:42PM 2 points [-]

An agent's optimization power does not equal the unlikelihood of the world it creates. At most it's the unlikelihood of the worlds it can creates, if creating them serves its goals. But since (so at least it seems plausible) most agents will have no limit to the properties they would like the world to have, most agents will not stop trying to optimize the world until they reach the limits of their abilities, which may make improbability a reasonable surrogate for power.

Perhaps your point is precisely that we shouldn't take "unlikelihood of world produced" as a reliable indication of optimization power, contra EY's proposal. If so, I suppose I agree, but I don't think the argument here is any good, because:

If "power" is taken to mean "improbability of world produced" then it is plainly not the case that doing nothing produces the most improbable (hence most evidential-of-power) world. Because the improbability that indicates power is improbability conditional on the agent not acting. So you're mixing up two completely different notions of "unlikely" and it's not surprising you get paradoxical-looking results.

Comment author: gjm 21 October 2014 02:18:44PM 3 points [-]

Another possible conclusion is that the "moral parliament" model either doesn't match how you actually think, or doesn't match how you "should" think.

In response to comment by gjm on Questions on Theism
Comment author: Unknowns 19 October 2014 04:15:56PM 0 points [-]

I suspect it was downvoted because it contains the words "it's perfectly possible for one religion to be right".

Comment author: gjm 19 October 2014 08:54:09PM 1 point [-]

That would be sad; I think I have a higher opinion of the LW readership than that. Still, I guess anyone can have a bad day.

Comment author: pragmatist 18 October 2014 05:37:43PM *  2 points [-]

Bohmian mechanics has developed quite a bit since Bohm. Its most significant contemporary defenders are Sheldon Goldstein, Nino Zanghi and Detlef Durr, and they advocate the ontology I described. See, for instance, this paper. From the abstract:

[...] the modern formulation of Bohm's quantum theory known as Bohmian mechanics is committed only to particles' positions and a law of motion... this view can avoid the open questions that the traditional view faces according to which Bohm's theory is committed to a wave function that is a physical entity over and above the particles...

Some other sources for this view.

Comment author: gjm 18 October 2014 08:13:05PM -1 points [-]

Interesting paper -- thanks!

It seems to me to argue not that the Bohmian stance is that the wavefunction isn't a thing, but that one good Bohmian stance is that the wavefunction isn't a thing. Of course that might suffice as a rebuttal to the common claim that Bohmian mechanics is basically a less honest version of MWI plus some extra unnecessary bits.

... Though the paper's approach doesn't seem perfectly satisfactory to me -- the proposal is that the universal wavefunction should be considered a law of nature, which seems to me about as reasonable as considering every fact about the universe a "law of nature" and doing away with contingency altogether. I confess that I don't have much in the way of actual arguments against doing this, though. It just seems to violate a general pattern I think I see, that it works best to put random-contingent-looking stuff in (so to speak) the data rather than the code.

(... And: even if we deny that the wavefunction is a thing, the usual argument still seems to me to have considerable force: Bohmian mechanics includes all the same stuff as Everettian, even if it reclassifies some bits as laws rather than things, plus extra stuff -- all those ontologically basic particles -- that seems to serve no purpose beyond making the theory feel a bit more natural to some physicists.)

It might turn out (I suspect only with a complete theory of quantum gravity in hand) that actually there's a really briefly specifiable universal wavefunction that naturally gets everything we see as one of its branches. In that case, my objection to treating the wavefunction as a law of nature rather than a part of nature would probably go away. I'm not sure it would really do much to make Bohmian QM look better than Everettian, though.

(Disclaimer: I'm not really a physicist or a philosopher of science, and my intuitions on this stuff aren't worth very much.)

Comment author: pragmatist 18 October 2014 11:51:29AM *  2 points [-]

The Bohmian stance is that the "pilot wave" isn't a real thing, it's a mathematical tool. The stuff that actually exists in the universe is the particles. The pilot wave is just a construct we use to predict how the particles move. So it's a little misleading to say that the particles are epiphenomenal. Ordinarily, when we say that X is epiphenomenal in some theory, we mean that X is causally affected by all the other stuff in the universe but does not itself have any causal effect on any other stuff. The Bohmian position is that there is no other stuff in the universe besides the particles, so it doesn't make really sense to say the particles are epiphenomenal.

Similarly, saying that all observers will find themselves somewhere in the pilot wave is also a bit misleading. It's true that there are mathematical structures within the pilot wave (including in those parts of it that do not carry particles) that correspond to observers. However, since the pilot wave isn't a real thing, those observers don't actually exist. The only observers that exist are the ones made out of particles.

MWI, on the other hand, interprets the wave function as representing a real physical object, so any structures within the wave function correspond to stuff that actually exists in the universe.

Comment author: gjm 18 October 2014 03:34:19PM 1 point [-]

The Bohmian stance is that the "pilot wave" isn't a real thing [...] The stuff that actually exists in the universe is the particles.

That isn't how I've heard it. E.g., Wikipedia: "The onyology [...] consists of a configuration [...] and a pilot wave." Bohm's book "The undivided universe" says (I'm going off the Amazon look-inside feature so it's possible that this would be invalidated by more context): "Let us now discuss this ontology in a more systematic way. Its key points are: [...] 2. This particle [sc. an electron -- gjm] is never separate from a new type of quantum field that fundamentally affects it." (The "new type of quantum field" is the wavefunction.) This seems to say in so many words that the wavefunction is as real as the particles in Bohmian mechanics, which seems to me enough to (e.g.) say that "observers" encoded therein are real.

Comment author: pragmatist 18 October 2014 04:48:18AM 1 point [-]

In the pilot wave theory, the probability that you will witness yourself surviving the experiment after it is performed say 1000 times is really really small. In MWI that probability is close to 1 (provided you consider all future versions of yourself to be "yourself"). So if you witness yourself surviving the experiment after it is performed 1000 times, you should update in favor of MWI over pilot wave theory (if those are the two contenders).

Comment author: gjm 18 October 2014 03:16:38PM 5 points [-]

I am skeptical of the existence of any clearly definable sense of "the probability that you will witness yourself surviving the experiment" that (1) yields different answers for Everett and for Bohm, and (2) doesn't have excessively counterintuitive properties (e.g., probabilities not adding up to 1).

Probability that any you looking at the outcome of the experiment after 1000 runs sees you alive? 1, either way. Probability that someone looking from outside sees you alive after 1000 runs? Pretty much indistinguishable from 0, either way.

You only get the "probability 1 of survival" thing out of MWI by effectively conditionalizing on your survival. But you can do that just as well whatever interpretation of QM you happen to be using.

If I find myself alive after 1000 runs of the experiment ... well, what I actually conclude, regardless of preferred interpretation of QM, is that the experiment was set up wrong, or someone sabotaged it, or some hitherto-unsuspected superbeing is messing with things. But if such possibilities are ruled out somehow, I conclude that something staggeringly improbable happened, and I conclude that whether I am using Everett or Bohm. I don't expect to go on living for ever under MWI; the vast majority of my measure doesn't. What I expect is that whatever bits of my wavefunction survive, survive. Which is entirely tautological, and is equivalent to "if I survive, I survive" in a collapse-y interpretation.

Comment author: cameroncowan 17 October 2014 11:51:39PM 1 point [-]

Earthsea by Ursula Laguin (sp?) I am also a fan of promoting classic literature whenever possible. I recommend "On the Road" by Jack Kerouac and The Outsiders.

Comment author: gjm 18 October 2014 12:01:56AM 1 point [-]

Laguin (sp?)

Le Guin.

Comment author: travisrm89 17 October 2014 06:40:38PM 1 point [-]

There is at least one situation in which you might expect something different under MWI than under pilot-wave: quantum suicide. If you rig a gun so that it kills you if a photon passes through a half-silvered mirror, then under MWI (and some possibly reasonable assumptions about consciousness) you would expect the photon to never pass through the mirror no matter how many experiments you perform, but under pilot-wave you would expect to be dead after the first few experiments.

Comment author: gjm 17 October 2014 09:01:11PM 3 points [-]

I'm not convinced there's a real difference there.

In both cases you expect that in no experiment you observe (and survive) will the gun fire and kill you. In both cases you expect that an independent observer will see the gun fire and kill you about half the time. In both cases you expect that there is some chance that you survive through many experiments (and, I repeat, that in all those you will find that the gun didn't fire or fired in some unintended way or something) -- what actual observable difference is there here?

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