Actually, I'm talking of the many-worlds interpretation of quantum physics. You were fiscal enough to point at the difference with the multiverse, so now you're stuck with it. — Olivier5

"fiscal: relating to government revenue, especially taxes."

Hokay!

I just apply the definition of the oh-so-many-worlds scenario. It is a scenario that exhausts all quantum possibilities, by definition. So, assuming for the sake of the argument that tossing a coin is quantic, in the many-worlds interpretation there is one world where you get "head" — Olivier5

Alright!

and another world where you get "tail". — Olivier5

Sounds like I win either way!

If you toss the coin one million times, one of the world "created" by your tossing will have you get 1 million times "head" in a row, and in another world, another version of yourself got 1 million times "tail" in a row. And all the possible combinations in between those two extremes would also see the light of day in their own world. — Olivier5

Whateva.

It sounds rather absurd, I know, but that’s what the many-worlders are saying.

It sounds rather absurd, I know, but that’s what the many-worlders are saying. — Olivier5

You're right. For multiverse, it's not necessarily true that whatever can happen does happen. But for many-worlds, it does seem to be the case that the universe splits into every possible outcome. It's a pretty nutty idea. Sean Carroll is a big believer. For my part I'm holding out for better physics in the future. Maybe someone will eventually sort this out.

When they describe many-worlds they always talk about binary choices. The car turns left or the car turns right, the cat is dead or alive. But they never consider continuous choices. If the car is in the middle of an open field, it can turn in any direction. That's a continuous range of choices. Is there a world for each of these choices, uncountably many of them? Or perhaps only a finite collection for each Planck-length sized angle the car can turn? I don't know what the MWers say about that.

The multiverse is obscenely anti-ockhamist, it assumes a awful lot and for no good reason. — Olivier5

I think this is misguided. The MWI doesn't add any assumptions; in fact, its appeal is that it takes assumptions away.

The apparent premise here is that MWI is adding an unnecessary premise that there are many worlds. The problem with this is that MWI doesn't add this assumption; QM realism does. In the double slit experiment, the particle somehow goes through both slits... that's where the many worlds come from, and it's already there. What MWI does, versus the traditional interpretation, is remove an assumption... namely, it removes the assumption that there's a privileged viewpoint where collapse is "real"; we can loosely call this classical realism.

Take Schrodinger's cat experiment as an example. Before Schrodinger opens the box, Schrodinger is already compelled to model its contents as a superposition between two worlds. In one world, a cat observed a lump of material decay; that's the dead cat. In that world, the decay was detected, the detector broke a vial of poison, and the poison is what killed the cat. In the other world, a cat observed a lump of material not decay; that the living cat. In that world, no decay was detected, the vial contained the poison, and the cat just lived. From the point of view of the dead cat, the radioactive material is not in a superposition between decaying and not decaying; it has already collapsed into decaying. Similarly, from the point of view of the living cat, the radioactive material has already collapsed into not decaying. But Schrodinger's model has both of these terms in it; so the cats' points-of-view collapses are not "real" collapses. They are instead just entanglements; the cat's state is entangled with the state of decay of the material. So if the living cat's sees apparent collapse, what makes Schrodinger special when he opens the box? How come Schrodinger's collapse is ontic when the cat's collapse is just perspective?

So to emphasize, MWI doesn't add the assumption of multiple worlds; the assumption leading to multiple worlds is already there. The traditional interpretation has a world with a dead cat and a living cat in it; those are worlds. MWI instead removes the assumption that some collapses are ontic whereas others are just apparent.

When they describe many-worlds they always talk about binary choices. The car turns left or the car turns right, the cat is dead or alive. — fishfry

Same response to fishfry... answering this question from an MWI point of view is easy. Don't look at the many worlds, because that's not the assumption; the worlds are just perspectives. They're descriptions to classical beings like us. The worlds are emergent; it's the wavefunction that's real.

So just look at the wavefunction. Is there continuity there? If so, since the universal wavefunction is ontic (under MWI; that is the assumption), then they must be real. If you're looking at a human choosing to steer, it's not entirely clear that correlates to a wavefunction continuous along all paths... the math leading up to that could get insanely complicated. But there are continuities in the wavefunction; if you start with the double slit, but you cut "infinite slits" into the barrier, then the photon goes everywhere. In that case, that wavefunction being ontic (under an MWI perspective), all of those are real (so long as you can come up with "worlds" based on each of those, each such world would be real).

the particle somehow goes through both slits — InPitzotl

You can't understand something in this one world, so you need to assume gazillions of worlds.

Don't look at the many worlds, because that's not the assumption; the worlds are just perspectives. They're descriptions to classical beings like us. — InPitzotl

Why are we classic? Isn't that a contradiction to MWI, where everything is quantum mechanical?

Is there a world for each of these choices, uncountably many of them? Or perhaps only a finite collection for each Planck-length sized angle the car can turn? I don't know what the MWers say about that. — fishfry

I don't know either, but I just had this thought that whenever I dribble a bit when peeing, I create a few thousands universes (each with all these galaxies and black holes and stuff in it) just to account for where the drops of my urine may or may not fall. I feel like Zeus with thunder in my hand now.

You can't understand something in this one world, so you need to assume gazillions of worlds. — Olivier5

So you're not against MWI, but QM?

Why are we classic? Isn't that a contradiction to MWI, where everything is quantum mechanical? — SolarWind

No, it's in the wavefunction. When Schrodinger models the state of the box, he would model a superposition between two classical states. There's a cat that died, because the vial broke, because a detector detected decay, because there was decay. And there's a cat that didn't die, because the vial didn't break, because the detector didn't detect decay, because there wasn't any. The cat's states are entangled with the state of radioactive decay. And when Schrodinger opens the box, Schrodinger's states become entangled with the radioactive decay.

I don't know either, but I just had this thought that whenever I dribble a bit when peeing, I create a few thousands universes (each with all these galaxies and black holes and stuff in it) just to account for where the drops of my urine may or may not fall. I feel like Zeus with thunder in my hand now. — Olivier5

LOL. Now I can't unsee that.

I did some Googling around (ie "research") and found this interesting thread.

https://physics.stackexchange.com/questions/536522/how-does-many-worlds-interpretation-work-for-non-50-50-probabilities

What's the problem with having an infinite number of branches? David Deutsch, a leading modern proponent of the Many Worlds interpretation, proposes that scenario in his popular book, The Fabric of Reality. In this picture, the universe started with an infinite number of parallel branches or strands, and at each quantum decision various subsets of those strands diverge, with all of the strands in any given bunch being 100% identical.

This was in response to someone asking if there can be uncountably many branches. So the either/or binary choices are just a popularization. The hard core many-worlders are perfectly fine with uncountably infinite branching at every instant.

So you're not against MWI, but QM? — InPitzotl

QM is science. I am pro science, always. The MWI is an attempt to stick to the metaphysics of Galileo and Newton, i.e. to strict determinism, in an era where this idea is obsolete precisely because of QM.

I am pro QM, but anti MWI. I don't buy the observer's magical powers to collapse a wave function either. My interpretation is that interactions with stuff collapse or at least restrict the wave function, allowing stable, predictable macrostructures to emerge from highly unstable and unpredictable micro elements.

Thales said that all things were made of water. This was his way to say that the world was not made of several elements (fire, water, earth, air for instance) as people believed in his time but made of just ONE substance. QM is telling us that this universal substance -- the water of Thales -- is in fact a probability wave.

"Why are we classic? Isn't that a contradiction to MWI, where everything is quantum mechanical?"
— SolarWind
No, it's in the wavefunction. — InPitzotl

I don't understand a word, can't it be more detailed?

The hard core many-worlders are perfectly fine with uncountably infinite branching at every instant. — fishfry

As I said, in this interpretation, everything that can happen does happen. Including flying unicorns, I think.

QM is science. I am pro science, always. The MWI is an attempt to stick to the metaphysics of Galileo and Newton, i.e. to strict determinism, in an era where this idea is obsolete precisely because of QM. — Olivier5

The MWI is just an interpretation. It embraces quantum realism, giving up classical realism. It hasn't been demonstrated true.

All I'm arguing is that it's naive to argue that MWI is making more assumptions; the core of MWI, explained in terms of Schrodinger's cat, is that there's nothing privileged about Schrodinger opening the box versus the cat.

ETA:

My interpretation is that interactions with stuff collapse or at least restrict the wave function, allowing stable, predictable macrostructures to emerge from highly unstable and unpredictable micro elements. — Olivier5

MWI just gets rid of that collapse (at least ontically, in the sense that the other terms disappear from the universal wavefunction and "become unreal"). The apparent collapse is explained by observers themselves entangling with quantum systems. That explanation isn't new; it's how Schrodinger would explain the cat. MWI is just saying as with the cat, so with Schrodinger.

can't it be more detailed — SolarWind

See edit.

MWI is just saying as with the cat, so with Schrodinger. — InPitzotl

I cannot see myself as a quantum object. What determines in which of the many worlds I am? It makes a difference to me whether I win the jackpot or one of my many copies.

A fun novel exploring MWI here, in case you were interested...

All I'm arguing is that it's naive to argue that MWI is making more assumptions; the core of MWI, explained in terms of Schrodinger's cat, is that there's nothing privileged about Schrodinger opening the box versus the cat. — InPitzotl

MWI says that there are infinite worlds, while Schrödinger assumes his cat can't be dead and alive at the same time. Can you spot which assumes less and which assumes more?

What determines in which of the many worlds I am? — SolarWind

I think that gets outside of MWI proper and into philosophy of identity. Personally, I think personal identity constructed anyway.

Let's say S1 is Schrodinger before opening the box, SA2 is Schrodinger who opens the box seeing a living cat; SB2 is Schrodinger who opens the box seeing a dead cat. Then neither SA2 or SB2 are the "genuine" Schrodinger; they both have equal claims of being the same as S1. But SA2 and SB2 do not affect each other; those "terms" in the wavefunction are now decohered. So SA2 and SB2 are different from each other.

What determines in which of the many worlds I am? It makes a difference to me whether I win the jackpot or one of my many copies. — SolarWind

Given the model above, there's no genuine version of Schrodinger to ask about.

MWI says that there are infinite worlds, while Schrödinger assumes his cat can't be dead and alive at the same time. Can you spot which assumes less and which assumes more? — Olivier5

There's a problem with your phrasing. "Schrodinger's cat" isn't an interpretation of QM; it's a thought experiment in it. What you're comparing is something akin to MWI and a traditional interpretation.

The traditional interpretation assumes there are two fundamental processes; Schrodinger's equation and Born Rule. MWI sees the assumption of the Born Rule as a fundamental process as unnecessary, so doesn't assume it. Since one is less than two, MWI is making fewer assumptions. MWI is saying there are an infinite number of worlds because it makes less assumptions; getting rid of those worlds requires more assumptions than MWI makes (that BR is fundamental).

There's a problem with your phrasing. "Schrodinger's cat" isn't an interpretation of QM; it's a thought experiment in it. What you're comparing is something akin to MWI and a traditional interpretation. — InPitzotl

I am actually talking of Erwin Schrödinger's own interpretation of QM, which he tried to argue for in his famous thought experiment about a cat in a box. In his view, the cat had to be either alive or dead, hence the Copenhagen interpretation was impossible to hold. Read about the history of the thought experiment.

I am actually talking of Schrödinger's own interpretation of QM, which he tried to argue for in his famous thought experiment about a cat in a box. In his view, the cat had to be either alive or dead, hence the Copenhagen interpretation was impossible to hold. Read about the history of the thought experiment. — Olivier5

Okay, history. Regarding the history of Schrodinger's cat per se, that was introduced in 1935 by Schrodinger's "Die gegenwärtige Situation in der Quantenmechanik". I don't speak German, but there are translations of "The Present Situation in Quantum Mechanics" in English. In this paper, Schrodinger introduced the cat thought experiment, and discussed his ideas on quantum mechanics. I have no idea what you're referring to here in particular that's any different from what I have said, but I have an English translation, so if you want to refer to parts of this paper go ahead.

But regarding the history of what I'm discussing, MWI was introduced in 1956 by Hugh Everett's thesis, "The Theory of the Universal Wavefunction". In this paper's introduction, Hugh discussed the two processes I'm talking about and discusses problems with the consistency of entertaining both processes. In that introduction, wavefunction collapse via the Born Rule is what Everett calls Process 1. The evolution of the wavefunction is what Everett calls Process 2. The particular inconsistency Everett points out is the inconsistency between Process 1 and Process 2; the way it's supposed to work, you use Process 2 until you observe, then you take out Process 1 and use that. Everett discusses in particular the inconsistency that is introduced when one observer (B) tries to describe an observer (A) as part of a quantum system (S); in that case, B is supposed to use process 2 to model A+S, but A uses Process 1 to model S. Everett then discusses five alternatives for how to resolve this: (1) A solipsist approach (e.g., B gets to apply Process 1, A is wrong to apply Process 2), (2) A "realms" (my term) approach, where QM is simply held invalid to any observer or macroscopic system, (3) An "isolationist" (my term) approach, where we suppose that B cannot posses a model A+S for some reason, (4) An incompleteness approach, where we surmise that there's some other hidden theory explaining QM, (5) A universal approach, where we simply discard Process 1. Everett's theory presumes alternative 5; he is discarding Process 1 as being real.

My use of Schrodinger's cat is as a device to talk about Everett's model... alternative 5. In this device, Schrodinger is B, the cat is A, and the quantum system S is the radioactive material.

So all of that is before you. Now, what different thing than I said are you talking about?

Now, what different thing than I said are you talking about? — InPitzotl

I'm talking of Erwin Schrödinger's interpretation of QM.

I'm talking of Erwin Schrödinger's interpretation of QM. — Olivier5

Okay, I'll try this way. What about it?

Let's say S1 is Schrodinger before opening the box, SA2 is Schrodinger who opens the box seeing a living cat; SB2 is Schrodinger who opens the box seeing a dead cat. — InPitzotl

This may be true if the probability is 0.5 vs 0.5. What if we wait shorter for the radioactive element to decay and the ratio is 0.58 (living cat) vs 0.42 (dead cat)? How many SA2 and SB2 are there then?

This may be true if the probability is 0.5 vs 0.5. What if we wait shorter for the radioactive element to decay and the ratio is 0.58 (living cat) vs 0.42 (dead cat)? How many SA2 and SB2 are there then? — SolarWind

Still two, or many. It depends on how you resolve the fact that the BR appears to work in MWI, and that's something I'm not sure how to do... possibly that's a good reason not to buy into it, or maybe it's just something beyond my scope.

All I'm arguing is that it's naive to argue that MWI is making more assumptions; the core of MWI, explained in terms of Schrodinger's cat, is that there's nothing privileged about Schrodinger opening the box versus the cat.
— InPitzotl

MWI says that there are infinite worlds, while Schrödinger assumes his cat can't be dead and alive at the same time. Can you spot which assumes less and which assumes more? — Olivier5

In the mathematical formulation of quantum mechanics there are usually six postulates, listed here.

However it is possible to simplify the formulation by omitting (at least) the collapse postulate - postulate V in the link. That is the sense in which MWI requires less assumptions (read: formal postulates) than other interpretations.

The consequence is that every physical event is described by the Schrödinger equation - measurement is not singled out for special treatment.

What about it? — InPitzotl

It does assume an infinity of worlds.

The consequence is that every physical event is described by the Schrödinger equation - measurement is not singled out for special treatment. — Andrew M

Another consequence is that, everytime I pee, I create thousands of universes, just to account for where the droplets may fall.

It does assume an infinity of worlds. — Olivier5

Sean Carroll says nobody knows if MWI requires infinitely many worlds or not.

You have a source?

Another consequence is that, everytime I pee, I create thousands of universes, just to account for where the droplets may fall. — Olivier5

Yep. As David Wallace puts it, the alternatives are to change the physics or to change the philosophy.

What’s the advantage of the Everett interpretation in particular? Here’s one way to put it. In trying to interpret quantum mechanics, you’ve got two yes/no choices to make. Choice one: are you going to change the physics? Are you going to stick with the Schrodinger equation and the quantum state, or are you going to add dynamical collapse processes or hidden variables or backwards-in-time interactions or something? Choice two: are you going to change the philosophy? Are you going to stick with the straightforward way of reading a scientific theory as just telling us what the world is like, or are you going to start saying “a scientific theory is just a predictive algorithm for experiments” or “observers can’t just be modelled as physical systems” or “ordinary logic is wrong” or something? If you answer “no” to both questions, you’re stuck with the Everett interpretation, because the Everett interpretation is just the “take quantum mechanics completely literally” interpretation. — Interview with David Wallace