I What is the current status of Many Worlds?

[Quantumental]

Despite its lackluster reception at its conception by Hugh Everett and subsequent advocacy by Bryce S. DeWitt the concept of "just take the theory seriously" and intriguing science-fiction concept of parallel worlds eventually gave it a major resurgence, much thanks to David Deutsch's pioneering work on the concept of quantum computation and militant advocacy of Everett. In the 90s the few people who refused to shut up and calculate started digging deeper into its intrinsic postulate and seductive simplicity and then in the mid-2000s the Everettian Relative State / Many Worlds Interpretation became mainstream as a consequence of the Oxford nucleus of proponents; David Deustch, Simon Saunders and David Wallace's work in the early to mid 2000s.

Now some 15 years later, hundreds of papers reference the interpretation yearly, tens of best selling books has gone into its depths and numerous high-profile sci-fi series has made it a pop-culture staple. Once left in the dustbin this controversial interpretation in 2021 seems only rivaled by orthodoxy and the more pragmatic "shut-up and calculate" crowd.

A lot of people have a visceral reaction, either favorably or negatively when confronted with the idea of wavefunction fundamentalism and its implied infinitely splitting worlds. This 'controversy' has just increased the frequency of discussion.

The promise of Everett was simple, in fact it was simplicity itself, in some sense it was a non-interpretation, at least that was its claim. Despite its elegant allure, now even ~70 years after its birth, the there are so many different Many Worlds Interpretations that they are hard to keep track of. Some proponents advocate for the view that the wavefunction is fundamental, other prominent advocates such as David Wallace and Christopher Timpson counter with more complicated views of Spacetime State Realism others yet argue over how to interpret probability and add axioms to justify their Born Rule derivation, then you have the more exotic perspectives of "Many Minds" (taken seriously by decoherence's father in Dieter Zeh and used as a counterpoint to Many Worlds by the likes of David Albert), and recently we've seen blends of Bohmian Mechanics and Many Worlds in the shape of several different "Many Interacting Worlds", then if you really want to go all-in you have prominent cosmologists like Leonard Susskind and others proposing that in fact Many Worlds is just another way to interpret the spatially separating Multiverse predicted by Eternal Inflation and String Theory.

As someone who's been following its developments for the past 17 years closely I am left feeling that "Many Worlds" is similar to Platonism. You have a lot of mathematicians who adhere to some form of Platonism, but when you inquire about specifics it turns out that they all disagree on what platonism even means. In this spirit of confusion I'd love to hear what the thoughts on Many Worlds are in 2021 by everyone here at PF.

 

[vanhees71]

I must admit, I have no clue what the "many-worlds interpretation" really says. When we measure, say the polarization of a single photon, assuming an ideal measurement setup (which you can achieve nowadays for photons most closely) we get one and only one result. Preparing an ensemble of single photons always in the same state these well-defined unique results follow the probabilistic predictions of QED and we are happy how accurate the theory predicts these probabilities. There's not the slightest hint of all the other outcomes in all the "many other worlds" or "parallel universes" being created in an "act of measurement/observation". So which, obviously only philosophical or at best metaphysical but not scientific/physical problem is "solved" by the MWI in comparison to the minimal/orthodox interpretation? I can't even say whether I should agree or disagree with the world view apparently implied by the MWI or not because of this lack of understanding of the implied meaning.

 

[Demystifier]

My point of view is this: MWI is right that the wave function splits, but wrong that the wave function is ontological. Instead, wave function is nomological, e.g. as in Bohmian mechanics.

 

[PeterDonis]

MWI is right that the wave function splits, but wrong that the wave function is ontological. Instead, wave function is nomological, e.g. as in Bohmian mechanics.

Isn't the wave function part of the ontology of Bohmian mechanics? Or are you saying that only the quantum potential (which is not exactly the same as the wave function) is ontological in BM?

 

[PeroK]

My point of view is this: MWI is right that the wave function splits, but wrong that the wave function is ontological. Instead, wave function is nomological, e.g. as in Bohmian mechanics.

Remind me - what does ontology mean?

 

[Demystifier]

Remind me - what does ontology mean?

See the recent thread entitled "Ontology"!

 

[WernerQH]

The promise of Everett was simple, in fact it was simplicity itself, in some sense it was a non-interpretation, at least that was its claim.

Yes. MWI declares a fragment of a theory to be the whole story. But there's more to quantum theory than Schrödinger's equation. MWI remains silent on how the all-encompassing "wave function" relates to the real world that we perceive. It just offers lots of hand waving.

 

[Demystifier]

Isn't the wave function part of the ontology of Bohmian mechanics? Or are you saying that only the quantum potential (which is not exactly the same as the wave function) is ontological in BM?

Such a view is rather obsolete. The quantum potential is as obsolete as e.g. relativistic mass in modern formulation of special relativity. Wave function is fundamental but not ontic, in the same sense in which Hamiltonian in classical mechanics is fundamental but not ontic.

 

[PeterDonis]

The quantum potential is as obsolete as e.g. relativistic mass in modern formulation of special relativity.

It is? Then what is considered the "modern" formulation of Bohmian mechanics? I thought the whole point of BM was to have a deterministic equation for the time evolution of particle positions, just like in classical mechanics. To do that you need the quantum potential.

 

[lukephysics]

why do physicists think the other worlds are real? since it’s impossible to ever interact with them It feels like they are imaginary potential worlds, based off a probability distribution (wave function) and once entangled, one outcome is solidified. Basically going from many probable worlds to one real one each time. Seems many worlds and many probable worlds are indistinguishable logically and mathematically. So why postulate an infinitely more complex interpretation over a simpler one?

 

[Demystifier]

It is? Then what is considered the "modern" formulation of Bohmian mechanics? I thought the whole point of BM was to have a deterministic equation for the time evolution of particle positions, just like in classical mechanics. To do that you need the quantum potential.

No, you don't need quantum potential for that. Wave function is enough. From the quantum potential (or its gradient to be more precise) you calculate the accelerations of particles. From the wave function (or more precisely from the conserved current expressed in terms of wave function and its gradient) you compute the velocities of particles. Knowing velocities is sufficient for finding the trajectories. In this sense, Bohmian mechanics in its modern form is really about velocities, not about accelerations. It is almost like pre-Newtonian physics, in the sense that the cause of motion is something that determines the velocity, not the acceleration.

 

[Demystifier]

why do physicists think the other worlds are real? since it’s impossible to ever interact with them It feels like they are imaginary potential worlds, based off a probability distribution (wave function) and once entangled, one outcome is solidified. Basically going from many probable worlds to one real one each time. Seems many worlds and many probable worlds are indistinguishable logically and mathematically. So why postulate an infinitely more complex interpretation over a simpler one?

This is one of the most frequent misconceptions about many worlds. The additional worlds are not postulated. They are derived, from the assumptions that the wave function is real (ontic) and that the unitary evolution is always right.

 

[lukephysics]

This is one of the most frequent misconceptions about many worlds. The additional worlds are not postulated. They are derived, from the assumptions that the wave function is real (ontic) and that the unitary evolution is always right

I understand it could be a misconception, but I am trying to reason through it. Am I correct to say a real inaccessible world is identical mathematically and logically to an imaginary or hypothetical branch from the reference point of our branch?

if that’s true then it follows that a probability interpretation of the wave function is equal in every way to a many worlds one and it’s choose the flavour you want?

 

[WernerQH]

This is one of the most frequent misconceptions about many worlds. The additional worlds are not postulated. They are derived, from the assumptions that the wave function is real (ontic) and that the unitary evolution is always right.

MWI itself is based on a misconception about the role of the wave function in the quantum formalism. Of course, using logical deduction, you can spread this nonsense into many other areas.

 

[CelHolo]

The quantum state acts like a probability distribution, enters experimental practice in terms of expectations and probabilities and obeys theorems that are either the exact same as or generalisations of those obeyed by standard probability distributions.

So to me there seems to be no motivation for considering it to be a "physical wave" like a classical EM field, so like vanhees71 above I really don't get this whole approach.

 

[Demystifier]

MWI itself is based on a misconception about the role of the wave function in the quantum formalism. Of course, using logical deduction, you can spread this nonsense into many other areas.

Agreed! For instance, if one assumes that the classical Hamiltonian $H(x,p)$ is ontic (where $x$ and $p$ are not evaluated along trajectories $x(t)$, $p(t)$), one can obtain a many-world interpretation of classical mechanics.

 

[CelHolo]

That's because you are one of those who does not get the notion of "ontology".

I understand the notion of ontology, it's a fairly simple concept, I just think the motivations of this MWI approach are very weak and not commensurate with either the practice or mathematical properties of the quantum formalism.

 

[Demystifier]

The quantum state acts like a probability distribution, enters experimental practice in terms of expectations and probabilities and obeys theorems that are either the exact same as or generalisations of those obeyed by standard probability distributions.

So to me there seems to be no motivation for considering it to be a "physical wave" like a classical EM field, so like vanhees71 above I really don't get this whole approach.

The problem with this view is the following. If the wave function is not a "physical thing" (like a classical EM field), then what is the "physical thing"? The many-world interpretation arises from two requirements:
1) Ontology: The theory must say what the "physical thing" is.
2) Formal minimality: The theory should not contain any new equations except those that are already present in standard QM.

 

[Demystifier]

I understand it could be a misconception, but I am trying to reason through it. Am I correct to say a real inaccessible world is identical mathematically and logically to an imaginary or hypothetical branch from the reference point of our branch?

if that’s true then it follows that a probability interpretation of the wave function is equal in every way to a many worlds one and it’s choose the flavour you want?

First question: yes.
Second question: I don't understand the question.

 

[CelHolo]

To be clear I know that those are it's motivations, but I don't understand persuing an approach where one's conception of the quantum state is so divorced from its use and mathematical properties.

 

[vanhees71]

The problem with this view is the following. If the wave function is not a "physical thing" (like a classical EM field), then what is the "physical thing"? The many-world interpretation arises from two requirements:
1) Ontology: The theory must say what the "physical thing" is.
2) Formal minimality: The theory should not contain any new equations except those that are already present in standard QM.

I thought ontology is about the "Ding an sich", i.e., about the objects in nature and not about theories. Theories describe the objects but are not the objects themselves. At least that's what I got from the other thread, though still I've not seen a clear definition of what it means when one says, a mathematical construct like the wave function, the electromagnetic field, the configuration or phase space of a point-particle system, the classical or quantum mechanical Hamiltonian etc. etc. are "ontic".

 

[Demystifier]

I thought ontology is about the "Ding an sich", i.e., about the objects in nature and not about theories. Theories describe the objects but are not the objects themselves. At least that's what I got from the other thread, though still I've not seen a clear definition of what it means when one says, a mathematical construct like the wave function, the electromagnetic field, the configuration or phase space of a point-particle system, the classical or quantum mechanical Hamiltonian etc. etc. are "ontic".

Ontology in theoretical physics is about objects in nature according to theories. For instance, according to classical mechanics ontology is pointlike particles, while according to classical EM ontology is the EM field. Of course, there is no precise definition of "ontic" (except in the context of PBR theorem, where the meaning is somewhat different).

 

[CelHolo]

I thought ontology is about the "Ding an sich", i.e., about the objects in nature and not about theories. Theories describe the objects but are not the objects themselves. At least that's what I got from the other thread, though still I've not seen a clear definition of what it means when one says, a mathematical construct like the wave function, the electromagnetic field, the configuration or phase space of a point-particle system, the classical or quantum mechanical Hamiltonian etc. etc. are "ontic".

I would think of it in terms of the following. If I say "the stone is heavy" the sentence and words are not literally the things themselves, but in some sense "directly refer to them". Where as if I said "the chance of somebody voting for party X is 67%" is a more indirect statement, since a probability is not a simple property of an individual object.

Similar in classical EM field strengths and magnetic fluxes are simple/direct properties of the EM field. Where as the quantum state, which concerns outcome statistics of experiments, is more indirect.

Now I don't think any of this matters or is a problem with QM, but it's often the source of some people's issues with the theory. They want direct statements about measurement independent properties of things.

 

[vanhees71]

But the probabilities of QT describe precisely what's observed: If I measure a quantity that has not a determined value according to the state the system is prepared in on an ensemble of such prepared systems I really observe that I "randomly" get measurement results with "frequencies" approaching the probabilities predicted by QT. So QT describes (a) that the outcomes of my measurements are random and (b) even quantifies the probabilities for the occurance of these outcomes correctly.

So what's qualitatively different between QT predicting the randomness of outcomes quantitatively correct and classical electrodynamics describing precisely the intereference pattern of light going through a double slit?

 

[CelHolo]

I completely agree vanhees71, I don't really get the issue either, but the above is how others have expressed it to me in person. They also tend to have a problem with measurement being a primitive concept in the theory, again not really an issue to me.

 

[lukephysics]

To me the wave function is just a pdf describing a random layer of reality we are not privy too. In finance why don’t we see statistical analysis called many worlds of finance? Where does the difference lie that makes QM treated as a unity(ontic) as opposed to a statistical system?

 

[Demystifier]

To me the wave function is just a pdf describing a random layer of reality we are not privy too. In finance why don’t we see statistical analysis called many worlds of finance? Where does the difference lie that makes QM treated as a unity(ontic) as opposed to a statistical system?

In finance there is a clear ontology, e.g. the actual numbers on your account which are there even when you don't look. In QM (in its orthodox form) there is no clear ontology, that's the difference.

 

[PeterDonis]

Bohmian mechanics in its modern form is really about velocities

Do you have a reference for this formulation?

 

[lukephysics]

In finance there is a clear ontology, e.g. the actual numbers on your account which are there even when you don't look. In QM (in its orthodox form) there is no clear ontology, that's the difference.

Not saying I’m not wrong but still trying to think through it.

I’m talking about financial markets where you are predicting the position of a market. You only have a PDF of the position of the market, and when you measure it it collapses to 1. Same as the wave function. Why isn’t predicting market prices also talked about as many worlds?

 

[PeterDonis]

Why isn’t predicting market prices also many worlds?

In the financial case, only one of the possible outcomes in the PDF actually occurs. In many worlds, all of the possible outcomes occur.

 

[CelHolo]

I’m talking about financial markets where you are predicting the position of a market. You only have a PDF of the position of the market, and when you measure it it collapses to 1. Same as the wave function. Why isn’t predicting market prices also talked about as many worlds?

You could do it, i.e. consider a classical probability distribution as a "real wave" and thus that each price "occurs in some world", it's just historically very few people have done so.

 

[bob012345]

Many worlds is the kind of thinking that happens when physicists don't get out enough. I mean how can we laugh at cranks trying to make energy out of nothing while postulating that the universe does the same by infinite amounts at every instant?

 

[PeterDonis]

how can we laugh at cranks trying to make energy out of nothing while postulating that the universe does the same by infinite amounts at every instant?

There is no "creation of energy" in many worlds. The term "many worlds" is in fact misleading since no "worlds" are created; the wave function just evolves unitarily all the time, so nothing is ever created or destroyed.

 

[bob012345]

There is no "creation of energy" in many worlds. The term "many worlds" is in fact misleading since no "worlds" are created; the wave function just evolves unitarily all the time, so nothing is ever created or destroyed.

Thanks. Does the theory then actually say there are many 'worlds' however that is defined in existence already, thus other copies of each of us?

 

[lukephysics]

Peterdonis I’ve never before heard anyone say the MWI branches aren’t real universes. Quite the opposite. Can you please provide links for more information about what you meant?

 

[PeterDonis]

Does the theory then actually say there are many 'worlds'

The term "worlds" refers to branches of the wave function. Heuristically, if two systems, $S$ and $M$, where $M$ is some kind of "measuring device" or "observer" and is assumed to be macroscopic, interact, they become entangled; the overall wave function might evolve from something like this:

$$
\Psi_\text{initial} = S_\text{prepared} M_\text{ready}
$$

To something like this:

$$
\Psi_\text{final} = S_\text{result A} M_\text{measured result A} + S_\text{result B} M_\text{measured result B}
$$

Each of the terms on the RHS of $\Psi_\text{final}$ is referred to as a "world". But the evolution from $\Psi_\text{initial}$ to $\Psi_\text{final}$ is unitary, so nothing is "created" or "destroyed"; all that happens is that two subsystems get entangled by a unitary interaction between them.

 

[PeterDonis]

I’ve never before heard anyone say the MWI branches aren’t real universes.

I didn't say that either.

Can you please provide links for more information about what you meant?

See post #40.

 

[bob012345]

The term "worlds" refers to branches of the wave function. Heuristically, if two systems, $S$ and $M$, where $M$ is some kind of "measuring device" or "observer" and is assumed to be macroscopic, interact, they become entangled; the overall wave function might evolve from something like this:

$$
\Psi_\text{initial} = S_\text{prepared} M_\text{ready}
$$

To something like this:

$$
\Psi_\text{final} = S_\text{result A} M_\text{measured result A} + S_\text{result B} M_\text{measured result B}
$$

Each of the terms on the RHS of $\Psi_\text{final}$ is referred to as a "world". But the evolution from $\Psi_\text{initial}$ to $\Psi_\text{final}$ is unitary, so nothing is "created" or "destroyed"; all that happens is that two subsystems get entangled by a unitary interaction between them.

Great but what is the relationship between $S_\text{prepared}$ and $S_\text{result A}, S_\text{result B}$? in terms of real systems? In other words, does each of $S_\text{result A}, S_\text{result B}$ contain all that was in $S_\text{prepared}$?

 

[PeterDonis]

Great but what is the relationship between $S_\text{prepared}$ and $S_\text{result A}, S_\text{result B}$? in terms of real systems? In other words, does each of $S_\text{result A}, S_\text{result B}$ contain all that was in $S_\text{prepared}$?

What do you mean by the phrase "all that was in"? What in the math does that phrase correspond to?

The states I wrote down are using perfectly straightforward notation from standard QM. There is nothing mysterious about them. $S$ and $M$ refer to different subsystems (i.e., different degrees of freedom), and the different subscripts refer to different possible states of those subsystems, considered in isolation. Which states those are, and what their intended physical interpretation is, should be obvious from the subscripts. $\Psi_\text{initial}$ is obviously a product state (i.e., separable), and $\Psi_\text{final}$ is obviously an entangled state; but $S$ and $M$ in both states refer to the same subsystems.

If what I wrote above does not answer your question, then there is an issue with your question; you are asking something that seems to be meaningful in vague ordinary language, but actually isn't when you try to pin it down to precise math.

 

[lukephysics]

You can write that binary formula for simple beam splitters with two outcomes but in reality there are so many splits every time step everywhere in the universe, and measurements are also happening everywhere constantly. It seems really hard to imagine this is a good way to erase the measurement problem like this even though it’s said to be the simplest interpretation of the math.

 

[bob012345]

What do you mean by the phrase "all that was in"? What in the math does that phrase correspond to?

I mean if $S_{prepared}$ was let's say a Uranium atom in some state and $M$ was a measurement system in some lab on some planet then are $S_{result A} , S_{result B}$ literally two different Uranium atoms in different 'worlds' or is this just a mathematical statement of a final wave-function over different outcomes. I'm trying to understand if this concept of different worlds co-existing is real or just a mathematical construct with no physical meaning.

 

[PeterDonis]

I mean if $S_{prepared}$ was let's say a Uranium atom in some state and $M$ was a measurement system in some lab on some planet then are $S_{result A} , S_{result B}$ literally two different Uranium atoms in different 'worlds' or is this just a mathematical statement of a final wave-function over different outcomes. I'm trying to understand if this concept of different worlds co-existing is real or just a mathematical construct with no physical meaning.

Go read my post #43 again, particularly the last paragraph. $S$ refers to the degrees of freedom that correspond to the Uranium atom. That's all there is to it as far as the math is concerned. Everything else is just standard QM applied to entangled states. If that doesn't answer your question, that means there is an issue with the question; it seems to you to be asking something meaningful, but it actually isn't.

 

[PeterDonis]

It seems really hard to imagine this is a good way to erase the measurement problem like this even though it’s said to be the simplest interpretation of the math.

Many physicists would agree with you, since many physicists do not agree with the MWI.

 

[bob012345]

Go read my post #43 again, particularly the last paragraph. $S$ refers to the degrees of freedom that correspond to the Uranium atom. That's all there is to it as far as the math is concerned. Everything else is just standard QM applied to entangled states. If that doesn't answer your question, that means there is an issue with the question; it seems to you to be asking something meaningful, but it actually isn't.

I have spent quite a bit of time trying to figure out how to respond to that third paragraph. I think I asked a reasonable question. I think you understand it.

 

[PeterDonis]

I think I asked a reasonable question.

Then you are wrong. See below.

I think you understand it.

You think incorrectly. Your question can't be understood because it is not well-defined. I know it seems to you that it is, but it isn't. That is why physicists don't use vague ordinary language; they use math. Again, you need to look at the math, not try to reason about the MWI in vague ordinary language. It won't work.

 

[PeterDonis]

you need to look at the math, not try to reason about the MWI in vague ordinary language. It won't work.

Btw, the fact that I am insisting on correct descriptions of what the MWI actually says does not mean I agree with the MWI. As a matter of personal opinion, I don't. As I think I remarked in another thread a while ago about interpretations, one needs to be even more careful about correctly stating what an interpretation says if one disagrees with it.

 

[PeterDonis]

I have spent quite a bit of time trying to figure out how to respond to that third paragraph.

If you're still having trouble with it, try taking a step back and asking yourself this question: in the state $\Psi_\text{final}$ that I wrote down, the $S$ and $M$ subsystems are entangled. What does standard QM say about the states of individual subsystems that are entangled? (The "subsystems" could just be two electrons, instead of an electron and a measuring device.)

 

[bob012345]

Then you are wrong. See below.You think incorrectly. Your question can't be understood because it is not well-defined. I know it seems to you that it is, but it isn't. That is why physicists don't use vague ordinary language; they use math. Again, you need to look at the math, not try to reason about the MWI in vague ordinary language. It won't work.

Then help me ask the correct question! You are the mentor not me!

 

[PeterDonis]

Then help me ask the correct question! You are the mentor not me!

I've already given you a couple of suggestions.

 

[Demystifier]

Why isn’t predicting market prices also talked about as many worlds?

Because we know what's the ontology of market prices, which is not the many world ontology. We know that the price PDF is just a map, not the territory.