[lisper]

If you get down to the quantum level there is no such thing as objective reality. Our perception that the world is made of classical objects that actually exist at particular places at particular times and have continuity of identity is an illusion. But it's a really compelling illusion, and you won't go far wrong treating it as if it were the truth in 99% of real-world situations. Likewise, free will is an illusion, nothing more than a reflection of our ignorance of how our brains work. But it is a really compelling illusion, and you won't go far wrong treating it as if it were the truth, at least some of the time.

[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.

[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.

[mensetmanusman]

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

[TheAncientGeek]

There is no local realism. That doesn't at all add up to all-in-the-head idealism.

[lisper]

That's true. There is a metaphysical reality "out there", but it is radically different from what we perceive. Hence: an illusion. Note that an illusion is emphatically NOT the same thing as a delusion. Illusions are real sensory experiences common to nearly all humans. They just happen not to correspond to reality.

[lo_zamoyski]

How would you know? If all that's known is either known through the senses or drawn out by reason from what is known through the senses, then by declaring that sense data do not reflect reality, you've cut yourself off form the possibility of knowing reality altogether.

[lisper]

> How would you know?

Because that is the best explanation for what I observe.

> by declaring that sense data do not reflect reality, you've cut yourself off form the possibility of knowing reality altogether

That is true, but only in the uninteresting sense that I can never completely eliminate the possibility that I am living in the Matrix. So yes, it's possible that I'm wrong about the existence of objective reality. But if objective reality is itself an illusion, it's a sufficiently compelling illusion that I'm not going to go far wrong by acting as if it were real.

[lo_zamoyski]

> That is true, but only in the uninteresting sense that I can never completely eliminate the possibility that I am living in the Matrix. [...] But if objective reality is itself an illusion, it's a sufficiently compelling illusion that I'm not going to go far wrong by acting as if it were real.

That seems squishy, as what constitutes "going far wrong" is not meaningful under skeptical assumptions.

A better stance is one of cognitive optimism that avoids the irrationality of skepticism. Skepticism is irrational, because it leads to incoherence, and because there is no rational warrant to categorically doubt the senses. For doubt to be rational, there must be a reason for it. To doubt without reason is not to be rational, but to be willful, and willful beliefs cannot be reasoned with; they are not the product of evidence or inference — and they certainly aren't self-evident — but rather the product of arbitrary choice. The logical possibility of living in the Matrix is no reason for doubting the senses, just as the logical possibility of there being poison in your sandwich is no reason for doubting you'll survive eating it.

The difference between our positions is that I begin from a position of natural trust toward the senses and toward reason as the only rational possibility and default. I have no choice but to reason well or to reason poorly. I recognize that my senses and my inferences can err, but it does not follow that they always err. Indeed, the very claim that they can err presumes I can tell when they do.

So, if my inferences lead me to a position that undermines their own coherence, then I must conclude that my inferences are wrong (including those that led me to adopt a certain interpretation of, say, scientific measurements).

> Because that is the best explanation for what I observe.

But if your explanation involves contradiction of what you observe, then that is not only not the best explanation, but no explanation at all! An explanation cannot deny the thing it seeks to explain. Thus, by denying the objective reality of what you perceive, you are barred from inferring that denial.

[lisper]

> what constitutes "going far wrong" is not meaningful under skeptical assumptions.

I can be more precise about this. It means that the predictions I make on the basis of this assumption are very likely to be correct.

> Skepticism is irrational

No, it isn't. The vast majority of my beliefs about the world are not a result of direct observations, but nth-hand accounts. I believe, for example, that the orbit of Mercury precesses, but not because I've ever measured it myself, but rather because I heard it from a source that I consider credible. But assessing the credibility of a source is hard and error-prone, especially nowadays. There is always the possibility that a source is mistaken or actively trying to deceive you. And even for things you observe first-hand there are all kinds of cognitive biases you have to take into account. So skepticism is warranted.

> I begin from a position of natural trust toward the senses

That will lead you astray because your senses are unreliable.

> if your explanation involves contradiction of what you observe

But it doesn't. At worst it involves a contradiction of what I think I observe.

[TheAncientGeek]

It can't be that different, either, or our senses would be of no practical use.

[lisper]

That's not true. What our senses perceive (classical reality) is an emergent phenomenon of the underlying metaphysical truth (quantum mechanics). Those two things are about as radically different as you can get. That's why the measurement problem is a thing. But that doesn't mean our senses are of no practical use.