Nope, nowhere in MW is the claim made that measuring the spin of an electron means "you have to build an entire parallel universe around that one electron, identical in all respects except where the electron went". — tom

And how does that connect with what Orzel (or I) have argued?

The point is that even if you step back from many actual worlds - Tegmark's parallelism - MWI proponents still seem to believe in crisply real branches (and branching points). But it makes more ontological sense to treat that too as a mathematical idealisation.

So the argument is that the universe, as a whole, could never have the definiteness required to create itself as some entangled mass of matchingly definite world branches. You can get something like that occasionally - with a sufficiently isolated system. But even ordinary quantum experiments have a bit of jiggery-pokery going on in that they don't control for the fact that actual environmental isolation is physically impossible.

And physical reality ought to trump mathematical idealisation in this regard.

And how does that connect with what Orzel (or I) have argued? — apokrisis

My apologies, I misread where Wayfarer's quote came from. It came from an article that Orzel did NOT like apparently.

Text-book QM claims that Schrödinger's cat is in a macroscopically indefinite state - a superposition of being alive and being dead.

Everett claims that the cat is in a superposition of macroscopically definite states - a superposition of an alive cat and a dead cat.

Text-book QM claims the act of observation transforms the indefinite cat into a definite cat, by a process indistinguishable from magic.

Everett (actually bare quantum formalism) claims that any environment that interacts with the cat in superposition will itself enter a superposed state, whose components correspond to entanglement with a macroscopically definite cat. Decoherence guarantees that very rapidly, the components of the environmental superposition cease to interfere with each other.

the question still stands - you said 'under Everett you accept multiplicity' - multiplicity of what?

Macroscopically definite states, just as the formalism indicates.

In non-technical terminology, what does a 'macroscopically definite state' consist of?

Here is an excerpt from the Phillip Ball essay:

Everett asked why, instead of fretting about the cumbersome nature of wavefunction collapse, we don’t just do away with it. What if this collapse is just an illusion, and all the possibilities announced in the wavefunction have a physical reality? Perhaps when we make a measurement we see only one of those realities, yet the others have a separate existence, too.

An existence where? This is where the many worlds come in. Everett himself never used that term, but in the 1970s the physicist Bryce DeWitt started championing his proposals, and it was DeWitt who argued that the alternative outcomes of the experiment must exist in a parallel reality: another world. You measure the path of an electron, and in this world it seems to go this way, but in another world it went that way.

So, for the umpteenth time, 'many worlds' means 'many worlds'? Yes or no?

Everett (actually bare quantum formalism) claims that any environment that interacts with the cat in superposition will itself enter a superposed state, — tom

Not sure why entering a macro superposition state is any less magical than exiting it.

The bare quantum formalism still requires its "observer", even if the tacking on of the further formalism of statistical mechanics - in the guise of the decohering environment - is certainly the way to deflate the notion of the "observer".

So collapse folk have the problem of getting rid of entanglement. No-collapse folk have the problem of initiating it. Sure the wavefunction evolves in the required fashion. But observers are then the necessary element to create the context that results in some actually specified wavefunction.

Meanwhile decoherence as a general machinery helps out both in making it clear that "observation" is not about conscious human experimenters but about the concrete existence of a thermalising environment. The Universe has the means to "observe itself" in that it has a definite past that acts as a general constraint on the indefiniteness of its future.

My apologies, I misread where Wayfarer's quote came from. It came from an article that Orzel did NOT like apparently. — tom

So I'm guessing you didn't even read that Orzel link you posted in rebuttal?

I actually asked a serious question. You might have had some worthwhile points to make about Orzel's angle.

So, for the umpteenth time, 'many worlds' means 'many worlds'? Yes or no? — Wayfarer

Yes. In the Schrodinger's Cat thought experiment, there are two cats, both of them equally real. One is alive (and wondering why it is in a box) and one is dead.

In non-technical terminology, what does a 'macroscopically definite state' consist of? — Wayfarer

I refer you to my post about macroscopically definite cats above.

So, for the umpteenth time, 'many worlds' means 'many worlds'? Yes or no? — Wayfarer

I reckon 'many worlds' must mean 'many worlds', what else could it mean?

Frankly, you can call decohered macroscopic superpositions whatever you like, it makes no difference.

Yes, there are parallel universes and we can find out about them.

The many worlds interpretation exists to preserve determinism.
Many experts hope this interpretation is true because it can be mathematically modeled.

If the universe is truly non-deterministic then that could mean there will never be a theory of everything that describes all of the universe's forces and natural laws.

Yes, there are parallel universes and we can find out about them.

Well, glad we got to the bottom of that, although it directly contradicts and answer you gave just above it.

If the universe is truly non-deterministic then that could mean there will never be a theory of everything that describes all of the universe's forces and natural laws.

The other explanation might be that physics only sees part of what is real, or that what is physical and what is real are not synonymous. It will be interesting to see how far people will go to avoid that.

Well, glad we got to the bottom of that, although it directly contradicts and answer you gave just above it. — Wayfarer

That was another Tom in another world breaking through. By his own logic, his every possible state of belief is a real macrostate. So it hardly matters if he is contradictory. That's going to be the case no matter what. :)

beat me to it.. X-)

Word is that the MWI is a religion. All hail the possible worlds that exist!

Well, glad we got to the bottom of that, although it directly contradicts and answer you gave just above it. — Wayfarer

I challenge you to find anywhere in the formalism of Everettian Quantum Mechanics mention of parallel universes. Go ahead!

The axioms of quantum mechanics say NOTHING about parallel universes.

Right! So 'many worlds' is ok, but 'parallel universes' is not? Is there a difference?

I challenge you to find anywhere in the formalism of Everettian Quantum mechanics mention of 'many worlds'. Go ahead!

The axioms of quantum mechanics say NOTHING about 'many worlds'.

According to what I've read, the term 'many worlds' was introduced by Bryce DeWitt when he began to champion Everett's ideas some years later.

And you have already agreed that MWI stands for Many Worlds Interpretation.

So, what incentive to I have to look for those particular terms in the 'formalism of Everettian QM'? I think the ideas of 'many worlds' or 'parallel universes' are generally associated with Everett.

The old argument from consensus. — tom

If I was implying anything by the question, it was only that if anyone were to claim that most physicists buy MWI as an ontological commitment, the person making that claim about what most physicsts believe is probably mistaken in that. As you noted, and I agree with, " the nature of reality doesn't trouble most physicists . . . Most shut-up-and-calculate."

If we were to survey physicists, what percetage do you think would say that they buy MWI instrumentally versus buying it as making a realist ontological commitment? — Terrapin Station

How does one "buy MWI instrumentally"? If you are a shut-up-and-calculate instrumentalist, then ostensibly at least you have no use for interpretations, MWI included.

The old argument from consensus. — tom

I am curious, Tom, seeing as you so stridently promote a position held by a small minority of physicists, if not by Deutsch alone - a minority among a minority of MWI proponents, most of whom, I believe, do not hold that MWI is the only interpretation version of QM that can account for all known observations - I am curious, are you a physicist yourself? Do you have a thorough understanding of quantum physics? Is this position your own?

If you are a shut-up-and-calculate instrumentalist [with respect to MWI]... — SophistiCat

...then you buy MWI instrumentally.

What does it mean to be a "a shut-up-and-calculate instrumentalist with respect to MWI"? How would you use MWI in calculations or why would you even need to?

"With respect to MWI" doesn't imply "Using MWI in calculations."

MWI works as an instrumental interpretation of the calucations, as do other interpretations.

Can you explain what you mean by "instrumental interpretation"? To me it sounds like an oxymoron.

The many worlds interpretation exists to preserve determinism.
Many experts hope this interpretation is true because it can be mathematically modeled.

If the universe is truly non-deterministic then that could mean there will never be a theory of everything that describes all of the universe's forces and natural laws. — m-theory

This is archaic thinking to say the least. A shadow is non-deterministic, yet we can still calculate its impact and origins. In fact, you can't have a perfect shadow because virtual particles will always appear out of nowhere. What it requires is a systems logic that can express everything both causally and acausally. A Theory of Everything and Nothing.

Using determinism as a yard stick for the value of non-deterministic theories is absurd.

(Just fyi, by the way, if you want a better chance of me seeing something and responding, use the "reply" function)

Interpretations can be instrumental or they can be taken as ontological commitments.

I'm using "interpretation" in a broader sense than something like the "Many Worlds Interpretation," where that's referring to a "story" being explicitly set forth, by the way. I'm using "interpretation" in the sense of any meaning and/or implicational assignment with respect to an explanation or account of how something works.

Instrumental interpretations don't care about ontological commitments. It's a matter of simply approaching the explanation or account as something that works for what it is, where it doesn't matter if it's a fiction or not.

Ontological commmitment interpretations are the opposite, obviously. One takes the explanation or account to be literally picking out things in the world, just as they are.

Ontological commmitment interpretations are the opposite, obviously. One takes the explanation or account to be literally picking out things in the world, just as they are. — Terrapin Station

So, being an instrumentalist, you would class dinosaurs as just an 'interpretation' of fossils, rather than actually having existed? Perhaps you think fossils only come into existence when they are consciously observed?

I'm an instrumentalist on some things, and not on other things. I particularly tend to be an instrumentalist with respect to explanations/theories that are mathematical-only (or primarily), or that are more abstract in received view interpretations.

I would have thought that the 'many paths' are not phenomena. They're inferences. But one cannot see 'the other paths', by definition - they're what's in the 'other worlds'. We only see one path - so that is the only 'phenomenon' being observed, the rest is inference. — Wayfarer

This is where the realist metaphysics kicks in. What explains the interference pattern in a double-slit experiment? What is actually interfering? What we are seeing is the interference of many paths. It is like we are standing on a road and seeing the road fork in front of us. But instead of the two roads being side-by-side, the roads are superimposed on each other like a superimposed photograph.

Consider the bent-stick illusion. We seem to see a bent stick when it is partly submerged in water. But we seem to see a straight stick when it is out of the water. Are we seeing a stick that has a mysterious straight-bentness nature that depends on how we observe it? Is the scientist's job just to record the observables and shut-up-and-calculate or do we expect that there is a natural causal explanation for why the stick appears differently under different circumstances?

Similarly, are we seeing phenomena that have a mysterious wave-particle nature that depends on how we observe it? The natural causal explanation is that we are seeing multiple phenomena that exhibit an interference pattern when they are in superposition.