[mettamage]

> If you get down to the quantum level there is no such thing as objective reality.

What do you mean by that? It still exists doesn't it? Albeit in a probabilistic sense that becomes non-probabilistic at larger scales.

I don't know much about quantum other than the high level conceptual stuff.

[lisper]

> It still exists doesn't it?

It's controversial, but here is the argument that the answer is "no": See https://flownet.com/ron/QM.pdf

Or if you prefer a video: https://www.youtube.com/watch?v=dEaecUuEqfc

[GoblinSlayer]

That's non sequitur.

>Under QIT, a measurement is just the propagation of a mutually entangled state to a large number of particles.

eyeroll so it's MWI in disguise, but MWI is quantum realism. Illusion they talk about is that the observed macroscopic state is a part of the bigger superposition (incomplete observation). But that's dumb, even if it's a part of a bigger state, it's still real, because it's not made up, but observed.

[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.

[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.

[mensetmanusman]

It’s probabilistic at all length scales. For example our solar system may suddenly come undone according to simulations.