[lisper]

> it's MWI in disguise

That's kind of like saying that GRW is Copenhagen in disguise. It's not wrong, but only because it's making the word "disguise" do some pretty heavy lifting.

> MWI is quantum realism

No, it isn't because it can't account for the Born rule. See:

https://blog.rongarret.info/2019/07/the-trouble-with-many-wo...

[GoblinSlayer]

It's a strange conclusion. You seemingly consider one measurement and expect to see Born rule, and when it doesn't manifest, then MWI is wrong? But Born rule doesn't manifest at sample size one in any interpretation, it manifests only in a long string of measurements. If you consider a long string of measurements, you will see Born rule as <Ψ|Born rule> = 1 - O(exp(-N)), which is basically a definition of empirical tendency.

Well, now I see that QIT isn't quite there. You say classical behavior emerges by tracing, mathematically, not as a physical process? In MWI classical behavior emerges as a physical process, not by tracing. That "look at part of the system (in which case you see classical behavior)" is provided by linear independence of different branches, so each observer naturally observes their branch from inside, and it looks isolated from other branches.

[lisper]

> You seemingly consider one measurement and expect to see Born rule

Huh??? No, of course not. The Born rule is about probabilities. It cannot manifest in a single measurement.

> classical behavior emerges by tracing, mathematically, not as a physical process?

No. The mathematical description of classical outcomes emerges by tracing, which is to say, by discarding information. The physical interpretation of that is left open.

> In MWI classical behavior emerges as a physical process

That's right. MWI commits to a physical interpretation of the math. But there is no scientific or philosophical justification for this, and in fact, when you dig into the details all kinds of problems emerge that are swept under the rug by its proponents. Nonetheless, many MWI proponents insist that it is the One True Interpretation, including some who really ought to know better.

> each observer naturally observes their branch from inside, and it looks isolated from other branches.

Yes, I know. But this doesn't solve the problem. In order to get a mathematical description of me I have to trace the wave function in my preferred basis, which is to say, I have to throw out all of the other branches. And this is not just a computational hack. It's mathematically necessary. Discarding information is the only way to get classical, irreversible processes (like measurement) out of the unitary dynamics of the wave function. So a reasonable interpretation of the math is that I exist only if parallel universes don't. And I'm pretty sure I exist.

I'm not telling you this because I expect you to accept it, merely to show you that the MWI is not self-evidently the One True Interpretation.

[GoblinSlayer]

In the blog post you say that tracing lets you consider a subset of an entangled system in isolation from the rest of the system. That was consistent with MWI where states are isolated from each other and not discarded. Mathematically discarding isn't necessary, isolation is sufficient.

[lisper]

Sure. So? MWI is not mathematically untenable, it's just incomplete (because it can't account for the Born rule) and IMHO philosophically untenable because it requires that no experiment can demonstrate the existence of a fully isolated universe, i.e. a universe whose macroscopic configuration is different from ours [1]. This feature of the MWI is what I call an IPU -- an Invisible Pink Unicorn -- something that the theory insists exists despite the fact that the theory also requires it to be unmeasurable even in principle. If you want to believe it exists, fine. Just don't call it science, and definitely don't insist that anyone who doesn't accept it is being irrational. It's attitudes like that that turn Rationalism (with a capital R) into a cult.

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[1] https://blog.rongarret.info/2009/04/on-shadow-photons-and-re...

(Note that I wrote this 16 years ago, so not everything is 100% accurate, but I stand by the central point.)

[GoblinSlayer]

That blog post associates for me with the indirect observation of branching that I mentioned. Double slit experiment with one detector has a seemingly impossible phenomenon when the interference pattern disappears without measurement in our branch, so without branching this phenomenon can't be explained as "pattern disappears due to measurement", because measurement didn't happen. In MWI in this case the pattern disappears due to measurement in the other branch, that branch splits from our with the measured part of the photon and the pattern disappears in our branch. Our branch isn't privileged in this case, actually it's affected by the other branch, and the situation is symmetric: when measurement happens in our branch, pattern disappears in the other branch without measurement there. Well, technically if the other branch is destroyed by measurement there, the result for our branch is the same I guess. And if this phenomenon can't be explained without branching, then it's almost direct evidence for branching.

[lisper]

> Double slit experiment with one detector has a seemingly impossible phenomenon when the interference pattern disappears without measurement in our branch

Then you don't understand quantum mechanics at all. You should read this:

https://flownet.com/ron/QM.pdf

The TL;DR is that measurement and entanglement are the same phenomenon. A particle can become entangled with a detector even if the detector doesn't register anything.

But that is neither here nor there. Why do you get interference with no detectors? Your theory is that a detector at one slit is somehow paired with a "virtual detector" in a parallel branch at the other slit. But why would that "virtual detector" go away when the real detector is removed? Why is it never the case that there is a "virtual detector" at either slit unless there is a real detector at one of them?

[GoblinSlayer]

In MWI the Born rule is a tendency of statistics of a long string of measurements. You tried to get this statistics from one measurement, which didn't work. If you want to see how MWI produces the Born rule, you should calculate statistics of a long string of measurements and see that this statistics is asymptotically close to Born statistics.

Other branches can be demonstrated indirectly by 1) quantitatively verifying unitary dynamics, 2) indirectly observing branching, 3) demonstrating that other theories are wrong. Branches are just superposition, if you want to eliminate branches, you should eliminate superposition with pilot wave or superdeterminism or something like that. This kind of unobservability isn't unique to MWI, in general theory of relativity we observe only a part of the universe, the rest being beyond event horizon and is unobservable. Do you believe only observable part of the universe exists and beyond it nothing exists?

[lisper]

> You tried to get this statistics from one measurement

Huh??? When?

> Other branches can be demonstrated

Sure, that's just QM 101. What you cannot demonstrate experimentally, not even in principle, is the existence of other branches with different macroscopic configurations than our own. Such branches are IPUs.

[Viliam1234]

If you insist that MWI must mean "a discrete number of clearly separated worlds", then yes, such interpretation would have a problem with the Born rule.

(That is apparently the definition the author of the linked article uses, guessing by his reaction: "Wait, what??? There is no 'well defined notion of how many branches there are?'")

I can only say that I have never met a proponent of MWI who meant this.

[lisper]

I am the author.

> I can only say that I have never met a proponent of MWI who meant this.

What can I say? There are a lot of MWI proponents who profess to believe this. Here, for example, is Sean Carroll answering the question, "How many parallel universes are there?"

https://www.youtube.com/watch?v=7tQiy5iCX4o

Of course, he doesn't actually give a concrete answer, but he very strongly implies that the question has an answer, i.e. that the question is a meaningful one to ask, and that implies that the MWI does in fact mean that there is a discrete number of clearly separated worlds.

In fact, I challenge you find a single example of a prominent MWI proponent saying something in public (which is to say, in a public forum or a publication whose target audience is the general public) that even implies that the many-worlds of the MWI are not distinct, countable entities. I only know of one example, and it is very well hidden.

There is a more fundamental problem: if the MWI does not mean "a discrete number of clearly separated worlds" then it fails as an interpretation of QM, i.e. as a solution to the measurement problem. The whole point is that measurements appear to produce discrete outcomes despite the fact that the math says that everything is one big quantum superposition. If all you have to say about this is, "Yeah, it's all one big quantum superposition" then you have failed to solve the problem. You have simply swept the hard part under the rug.

[Viliam1234]

> Of course, he doesn't actually give a concrete answer, but he very strongly implies that the question has an answer, i.e. that the question is a meaningful one to ask, and that implies that the MWI does in fact mean that there is a discrete number of clearly separated worlds.

In the video, Sean Carroll talks to a non-expert audience, so he must simplify some things, and then it is your or my guess about what the unsimplified version was supposed to be. He says something like: "we don't know, even whether it is finite or infinite, but if it is finite it is a very large number such as 10^10^123". But notice that he also uses as an analogy an interval from 0 to 1, which can be split to half as many times as you need.

You see this as him believing in discrete separated universes, of which there is a definite number (potentially infinite). Yes, that makes sense.

I see another possible understanding, that he is talking about "meaningfully different" universes, because that is what we care about on the macro level. To explain what I mean, imagine that we observe two particles. Any of them can be in a huge number of possible positions, moving in a huge number of possible directions, at a huge number of possible speed. But if we ask whether those two particles hit each other and transformed into another particle, that kinda collapses this huge possibility space into a "yes / no" question. Out of practically infinity, two meaningfully different options.

On a macro level, either the cat is alive or it is dead. Those are two meaningfully different states. If we focus on one particle in the cat's body, there is a continuum of where precisely that particle could be, and what momentum it has. So from the particle's perspective, there is a continuum of options. But from the cat's perspective, and the cat's owner's perspective, this continuum does not matter; unless it changes the macro state, i.e. the particle kills the cat, or at least maybe hits its neuron and makes it do something differently. So it seems possible to me that Sean Carroll talks about the number of worlds that are different from human perspective.

Then there is another problem in physics that we don't know how/whether the very space and time are quantized. We use the mathematical abstraction of a "real number" that has an infinite number of digits after the decimal dot, but of course that infinite number of digits can never be observed experimentally. We don't know. Maybe it is something like what Wolfram says, that on a deep level, spacetime is a discrete graph evolving according to some rules. If something like that would be the case, that would reduce the possible number of states in the universe, even on the micro level, to a huge but finite number. And the mixed state of the multiverse would consist of this finite number of branches, each of them assigned a tiny complex amplitude. So that's another way how things could get finite.

And I am saying this just as a random guy who never studied these things, I just sometimes read something on the topic, and some ideas feel to me like obvious consequences of the stuff that is "in the water supply". So I believe that if I see a solution to a problem, then if it makes sense, someone like Sean Carroll is 10000x more likely to notice the problem and the solution, and develop it much further than I ever could. Or when you make a survey, and a half or a third of people who study quantum physics for living say that some version of MWI seems like the correct interpretation to them, I don't believe there is a simple devastating argument against it that all of these people have simply missed.

[lisper]

> I am saying this just as a random guy who never studied these things

OK, well, let me tell you as a non-random guy who has studied these things extensively that the MWI is very commonly misrepresented. It is not a case of simplification for a lay audience, it is flat-out lying, at least most of the time. The math does not say that there are parallel universes. All the math tells you is that in order to recover the results of experiments you have to throw away some of the information contained in the wave function. MWI proponents interpret this by saying that the discarded information has to correspond to something real, and they call that thing "parallel universes". But there are three problems with this. First, the MWI does not explain the Born rule. Second, the math doesn't tell you whether or not the discarded parts of the wave function describe something real or not. It is possible that mathematical operation of discarding parts of the wave function actually corresponds to real physical phenomenon, i.e. that whatever is described by the discarded parts of the wave function actually ceases to exist. This is a tenable scientific hypothesis. It's not easy to actually make it work, but it can be done and has been done. It's called GRW collapse [1]. So anyone who tells you that the MWI is the only possible scientifically tenable interpretation of QM is lying. And anyone who leaves open even the possibility that the "parallel universes" contained in the wave function are discrete is also lying. The only MWI proponent I've ever seen being intellectually honest about this.David Deutsch in his book "The Beginning of Infinity" chapter 11.

The third problem with the MWI is something called the "preferred basis problem". This one is harder to describe succinctly, and some people claim it has been solved, but I don't agree with them. In a nutshell, all two-state QM experiments rely on some macroscopic apparatus to split a particle into a superposition of two states. But if you model the entire universe as a quantum system, this apparatus is itself a quantum system that can be in a superposition of states, so you can't say, "The polarizing beam splitter is aligned vertically or it is aligned horizontally" any more than you can say "the cat is alive or it is dead" without begging the question.

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[1] https://en.wikipedia.org/wiki/Ghirardi%E2%80%93Rimini%E2%80%...

[GoblinSlayer]

You got the preferred basis problem backward. It's one branch interpretations that have this problem, since the extant branch is absolute reality and its basis is absolute reality basis, hence the problem of privileged basis. In MWI all branches are relative and none is absolute reality, so their basis can't be thought of as absolute reality basis, branches and basis exist, but they are equal to others. This produces other criticism: universal wave function thus becomes spaghetti in which classical behavior is difficult to see when you look at it from outside, because the basis of classical behavior isn't privileged.

[lisper]

No, MWI has this problem too because it still has to account for the branch I am in.

The only interpretation that does not have this problem is the NCI because in that interpretation I am part of the fundamental ontology, at least some of the time.

[GoblinSlayer]

>Second, the math doesn't tell you whether or not the discarded parts of the wave function describe something real or not.

The math tells that there are no privileged parts of wave function.

>So anyone who tells you that the MWI is the only possible scientifically tenable interpretation of QM is lying.

Didn't you admit yourself that if MWI works it's a big deal and will kick the chair from under other interpretations?

[lisper]

> The math tells that there are no privileged parts of wave function.

That's true. But my senses tell me that there is a privileged part of the wave function, namely, the branch that I'm in.

The way I think about it nowadays is that QM is like a Necker cube. You can look at it in two different ways. You can take the God's-eye view and look at the entire wave function, or you can take the mortal's eye view and look at only a proper subset of the wave function (which is necessary in order to recover classical reality). But you can't do both at the same time. For my day-to-day life, I have no choice but to take the mortal's-eye view because I am a mortal. All of the things that matter to me depend on classical reality, and so depend on my suspension of disbelief and acting as if my branch of the multiverse is privileged, even if I can intellectually jump out of the system momentarily and recognize that the mortal's eye view is necessarily incomplete.

> Didn't you admit yourself that if MWI works it's a big deal and will kick the chair from under other interpretations?

That depends on what you mean by "works". If someone can derive the Born rule from the Schrodinger equation that will be a big deal, a slam-dunk Nobel prize. But no one has done it, and I'm pretty sure it can't be done. I'm pretty sure that the Born rule is an emergent property of our branch of the multiverse. I believe the same is true of the Second Law and even three-dimensional space. You can slice-and-dice the wave function to give you physical spaces with any number of dimensions, but three is the magic number that gives you atoms and stars and planets with stable orbits [1] and so on. So I'm pretty sure the Born rule can only be explained by the anthropic principle. There's probably a Nobel prize waiting for the person who turns that intuition into a theorem.

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[1] https://physics.stackexchange.com/questions/50142/gravity-in...