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.
> 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.
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.
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.
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.
> 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:
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?
Before measurement it's one detector, then it interacts with half photon, which is superposition |photon>+|no photon>, and the detector's state splits, the first state interacts with the |photon> state and measures it, nothing happens to the second state, because there's nothing to measure in the |no photon> state, but the first state splits away from the second state and leaves it alone, that's how the second state ends up alone with pure |no photon> state, but this really was done to it by the first state, the second state can't do it by itself. For this to happen in the second branch you need measurement to happen in the first branch, so when you are in the second branch, the first branch still needs to exist even though you don't see it, otherwise your branch won't be able to be as it is.
When you remove detector and start next measurement, you start with your one branch, branches from previous measurements don't affect it, the phenomenon happens during decoherence, nothing happens after it.
Your article explains this with branching without saying the word.
It's a part of photon's state at the slit with detector, the other part is at another slit. It's superposition of photon near detector and no photon near detector.
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?
> 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.
Again, huh??? Where in that blog post do I try to "get this [sic] statistics from one measurement"?
> I provided 3 ways to demonstrate it experimentally, even in principle, not sure what problem you have.
No, you didn't. You apparently don't understand what is meant by "branches with different macroscopic configurations than our own" and I don't have time to explain it to you. Sorry. Go read up on decoherence, and then come back and describe an experiment that can demonstrate the existence of a fully decohered branch. You can't, because if you could it would by definition not be fully docohered.
>Where in that blog post do I try to "get this [sic] statistics from one measurement"?
In the discussion how different people place bets on A and B outcomes of experiment. Well, you didn't state clearly why you believe that MWI doesn't account for Born rule. MWI accounts for Born rule as statistics of measurements, and the discussion of bets is the closest this in that blog post to consideration of statistics of measurements, but that discussion seemingly considers one measurement, that's why it doesn't see statistics.
>Go read up on decoherence, and then come back and describe an experiment that can demonstrate the existence of a fully decohered branch.
It looks like a logical problem to me. You suggest that decoherence both produces and doesn't produce fully decohered branches? Violation of the law of excluded middle? If the law of excluded middle doesn't work, I don't think experiments can demonstrate anything.
> you didn't state clearly why you believe that MWI doesn't account for Born rule
That's what the whole post was about. The MWI doesn't account for the Born rule unless you add additional, questionable assumptions like branch indifference to the SE.
> You suggest that decoherence both produces and doesn't produce fully decohered branches?
No, that is not even remotely what I am saying. You are beginning to sound like a troll.
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.