I Sean Carroll podcast on many worlds interpretation

[pinball1970]

TL;DR Summary

A Guardian podcast where Sean Carroll discusses the 'Quantum from the start approach,' his new book and points on the many worlds.

Interpretation of quantum mechanics is something that is discussed at length on pf so it would be interesting to get views of the quantum guys on this short (30 mins) podcast. Thanks.
https://www.theguardian.com/science...ew-theory-of-spacetime-science-weekly-podcast

 

[DennisN]

Thanks, I will listen to it! I like Carroll as a communicator.

 

[A. Neumaier]

Summary: A Guardian podcast where Sean Carroll discusses the 'Quantum from the start approach,' his new book and points on the many worlds.

Interpretation of quantum mechanics is something that is discussed at length on pf so it would be interesting to get views of the quantum guys on this short (30 mins) podcast. Thanks.
https://www.theguardian.com/science...ew-theory-of-spacetime-science-weekly-podcast

See
https://www.math.columbia.edu/~woit/wordpress/?p=11277

 

[pinball1970]

See
https://www.math.columbia.edu/~woit/wordpress/?p=11277

The links suggests the book is mostly ok. The many worlds parts not so much. 'woo' in fact.

 

[vanhees71]

I couldn't agree more with Woit's clear analysis. I'm however worried about his final paragraph.

What I've seen in the public press about the book (mostly just commercials rather than critical reviews), made me hope that it'll not appear here on PF. Since I've not seen the book myself yet, I can't say what's commercial propaganda and what's really written in the book, I can't comment on it. The one thing I find very disturbing is that it seems, according to the short statement by Woit cited above, Carroll himself uses this dubious propaganda machine to sell his book. This does not serve the intention of outreach of scientists to the public but is the opposite to it. The public, as the main investors in fundamental research through the use of tax money, has the right to be informed about science in an as undistorted way as possible, i.e., it has to be simplified enough to make it understandable but not simpler (Einstein) and to use esoterics to sell QT to the general audience is even worse than explaining something wrong through too much simplification. It's simply "anti-science" and should not be tolerated by any scientist!

Now at the end of his short article Woit writes:

[Woit] Those in the physics and science communication communities who care about the public understanding of quantum mechanics should think hard about what they can do to deal with this situation. They may however come to the same conclusion I’ve just reached: best to ignore him, which I’ll try to do from now on.

Indeed, the first sentence is right and an important statement to be thought indeed by all physics and science communication communities (I suppose he refers to outreach to the public). But the 2nd sentence is the worst reaction one can think of! This would mean that when confronted with the abuse of science by using anti-science just for commercial reasons to sell a book supposed to be a popularization of science rather than nonsensical science fiction, as a scientist you should hide back in the "ivory tower".

In times, where worldwide the voice of scientists is villified as "fake news", it is the more important that serious scientists contradict such propaganda and tell the truth according to the best of our contemporary knowledge. It's not easy, particularly as there's no absolute certainty about anything in science but everything is subject to critical revision as soon as new facts become known through the thorough scientific method, but if science should have a chance to be taken seriously as a means to provide objective knowledge, which is also important to make informed decisions in politics and society, one must contradict utter nonsense whenever it's brought to the public, for which mediocre reason ever. It's the more important if the utter nonsense comes from a serious scientist who for sure knows better and uses the utter nonsense just as bad advertisement to sell some popular-science book. Even if it's a good popular-science book, it's the worst thing one can do in public outreach: One should always have in mind that the sensationalism in the public media (including the WWW) reaches orders of magnitude more people than the few who really read the book! Just for not to reward the author with the money from buying a copy, I for sure won't read it myself!

 

[pinball1970]

I couldn't agree more with Woit's clear analysis. I'm however worried about his final paragraph.

What I've seen in the public press about the book (mostly just commercials rather than critical reviews), made me hope that it'll not appear here on PF. Since I've not seen the book myself yet, I can't say what's commercial propaganda and what's really written in the book, I can't comment on it. The one thing I find very disturbing is that it seems, according to the short statement by Woit cited above, Carroll himself uses this dubious propaganda machine to sell his book. This does not serve the intention of outreach of scientists to the public but is the opposite to it. The public, as the main investors in fundamental research through the use of tax money, has the right to be informed about science in an as undistorted way as possible, i.e., it has to be simplified enough to make it understandable but not simpler (Einstein) and to use esoterics to sell QT to the general audience is even worse than explaining something wrong through too much simplification. It's simply "anti-science" and should not be tolerated by any scientist!

Now at the end of his short article Woit writes:
Indeed, the first sentence is right and an important statement to be thought indeed by all physics and science communication communities (I suppose he refers to outreach to the public). But the 2nd sentence is the worst reaction one can think of! This would mean that when confronted with the abuse of science by using anti-science just for commercial reasons to sell a book supposed to be a popularization of science rather than nonsensical science fiction, as a scientist you should hide back in the "ivory tower".

In times, where worldwide the voice of scientists is villified as "fake news", it is the more important that serious scientists contradict such propaganda and tell the truth according to the best of our contemporary knowledge. It's not easy, particularly as there's no absolute certainty about anything in science but everything is subject to critical revision as soon as new facts become known through the thorough scientific method, but if science should have a chance to be taken seriously as a means to provide objective knowledge, which is also important to make informed decisions in politics and society, one must contradict utter nonsense whenever it's brought to the public, for which mediocre reason ever. It's the more important if the utter nonsense comes from a serious scientist who for sure knows better and uses the utter nonsense just as bad advertisement to sell some popular-science book. Even if it's a good popular-science book, it's the worst thing one can do in public outreach: One should always have in mind that the sensationalism in the public media (including the WWW) reaches orders of magnitude more people than the few who really read the book! Just for not to reward the author with the money from buying a copy, I for sure won't read it myself!

I will get a copy still I think.
I am not in a position to separate Science from woo and reading other threads from the last couple of years on pf regarding many worlds has not cleared that up.

 

[Elias1960]

but if science should have a chance to be taken seriously as a means to provide objective knowledge, which is also important to make informed decisions in politics and society, one must contradict utter nonsense whenever it's brought to the public, for which mediocre reason ever. It's the more important if the utter nonsense comes from a serious scientist who for sure knows better and uses the utter nonsense just as bad advertisement to sell some popular-science book.

A good intention, but I have already given up on this. There is too much support for such nonsense like many worlds in the scientific community itself.

While lumo is often wrong and in general uses language inappropriate for a scientific discussion, in this particular question I agree with his scientific position: https://motls.blogspot.com/2019/09/qm-it-is-very-dumb-to-deny-need-for.html

 

[Swamp Thing]

Is this a better way to put things in the right perspective for a general audience? Or are there any flaws in her presentation as well?



And here's a question: Assume you have a photon in a superposition of energy values spread out smoothly over a continuous range. Let it go through a prism and fall on a screen, where the possible spread of detection positions forms a continuous region across the screen.

Does the MWI say that there is a continuum of universes, each differing from another in terms of the spot position on the screen, with "adjacent" universes differing only in an infinitesimal change in spot position? And what about the various different possible microstates of the screen that map into the same measurement outcome? Do we end up with one universe for each microstate of the detector? And if the same experiment is going on on another table across the lab, are there that many more universes to span the product space of outcomes of Experiment 1 and Experiment 2?

 

[vanhees71]

A good intention, but I have already given up on this. There is too much support for such nonsense like many worlds in the scientific community itself.

While lumo is often wrong and in general uses language inappropriate for a scientific discussion, in this particular question I agree with his scientific position: https://motls.blogspot.com/2019/09/qm-it-is-very-dumb-to-deny-need-for.html

I think, if you rename "many-worlds interpretation" back to its original name "relative-state interpretation", it's fine, but then of course you cannot discuss this in a popular-science book at all. I never understood the point of Everett's paper ever. I can't tell you what this interpretation means in a scientific sense.

Here, I also agree with Motl except with his claim that "collapse" is necessary. For me it's either the description of the realization of a measurement outcome by an observer. Then usually nothing happens to the systems measured at all. It's just looking at the result of the measurement of a human being realizing what comes out, which can be months or years after the experiment has been done reading out a measurements protocol. Nobody in his right mind will claim that only then something collapses and the state of the measured system is collapsed to some other state. This is in no way different than getting knowledge about the outcome of a random experiment within deterministic classical physics (like throughing dice). That's a completely epistemic interpretation.

Another point is the case of "subsequent measurements", i.e., you have a system and make some measurement without distroying the system and then do another measurement on it. Then you have to describe the interaction of the first measurement with the measured object to know its (pure or mixed state) after the first measurement. At least in principle this is governed by unitary time evolution of the complete system, including the first measurement device.

E.g., you put a first Stern-Gerlach magnet to measure $s_z$ and then use only the one beam with definite $s_z=+\hbar/2$ to subsequently use a second Stern-Gerlach magnet to measure $s_x$. Then you know the partial beam entering the 2nd magnet is (at least to a very good approximation) in the pure state $|s_z=+\hbar/2 \rangle$, and the outcome of measurements at the 2nd is governed by Born's rule. Here no collapse concerning the 1st measurement is needed at all, only relative simple unitary time evolution of the system+1st magnet system, where the 1st magnet is a simple term in the complete Hamiltonian of the measured particle.

 

[DarMM]

I never understood the point of Everett's paper ever

I think it was that some people had a problem with QM having two different time evolutions. They saw unitary evolution $U(t)$ and then when you make an observation you use the Von Neumann projection postulate (or generalizations there of). The "problem" being that you can't derive the projection from unitary evolution.

Of course from a statistical point of view this combination of time evolution and projection isn't surprising. If one models a mote of dust in a glass of water via Brownian motion, then if the mote starts in some distribution of positions $\rho(q,0)$ it then evolves via a stochastic operator $S(t)$ to another distribution $\rho(q,t)$. However if you then make a measurement and determine the mote of dust is within some region $A$ your probability distribution would be projected as:
$$\rho(q,t) \rightarrow \rho^{'}(q,t) = \frac{\chi_{A}(q)\rho(q,t)\chi_{A}(q)}{\int{\chi_{A}(q)\rho(q,t)\chi_{A}(q)dq}}$$
with $\chi$ the characteristic function of $A$.

 

[Heikki Tuuri]

I do not understand why Peter Woit, Sabine Hossenfelder, and Lubos Motl protest against the Many Worlds interpretation. MWI is one of the established and quite popular interpretations of quantum mechanics. Have these three people done new research which refutes earlier work on MWI? No.

David Bohm devised a method where an observing subject can "choose" a branch where he lives, assuming the Schrödinger equation. The other branches still exist and one may call them "parallel universes".

One of the fundamental principles of quantum mechanics is that there is not just one, Newtonian, deterministic universe. We have to assume alternative paths of the universe exist and calculate the interference pattern from the probability amplitudes. That is, assuming "many worlds" is at the heart of quantum mechanics.

 

[vanhees71]

Can you experimentally verify the claim that there is not just one Newtonian determinstic universe? First of all that claim is disproven anyway with very high significance, because for sure the observed universe is very very very... close to a relativistic universe. Since we have so far no consistent deterministic model for all phenomena, which given the many Bell tests disproving the validity of local deterministic models must necessarily be nonlocal, I tend to believe the relativistic QFT is the closest we have today as a theory. It's incomplete in not describing gravity quantum theoretically but otherwise there's not a single hint at its invalidity whatsoever.

I'd rather claim standard QT in its minimal statistical interpretation is all you need to describe nature in a no-nonsense way. I've not seen the slightest evidence for parallel universes whatsoever. Using Occam's razor I simply don't need them!

 

[Heikki Tuuri]

@vanhees71,

one can, of course, declare that the other branches, or alternative paths, of the universe do not exist "really", and only my branch exists. Then you suddenly have a Copenhagen-style version of MWI.

Now I realize that a version of the Many Worlds dispute dates back to Plato. Do imagined objects, like the centaur, exist?

What if we need the concept of a centaur when we calculate some real physical process? Does the centaur exist in that case? One can argue endlessly about the existence of abstract concepts.

 

[akvadrako]

I do not understand why Peter Woit, Sabine Hossenfelder, and Lubos Motl protest against the Many Worlds interpretation. MWI is one of the established and quite popular interpretations of quantum mechanics. Have these three people done new research which refutes earlier work on MWI? No.

I don't know why they do it, but it's clear they still don't really understand what it's saying, at least in the most modern and charitable versions.

For example in Woit's recent post he says:

My problem here is that the whole splitting thing seems to me to lead to all sorts of trouble (how does the splitting occur? what counts as a separate world? what characterizes separate worlds?), so if I’m told I don’t need to invoke multiple worlds, why do so? According to Carroll, they’re “enormously convenient”, but for what (other than for papering over rather than solving a hard problem)?

Lubos often makes the same argument: that many worlds is useless because worlds can't be precisely defined. But of course they can't and they don't need to be, because many worlds is not really about worlds — that's just a rough way to describe how the evolution looks. It doesn't matter if it's convenient.

My best guess is that they (Lubos and Woit) think of QM as a calculation aid. Then it actually matters if MWI is less convienent or less clear to do calculations in.

 

[Heikki Tuuri]

@akvadrako,

the exact definition of the splitting or branching is a problem in MWI, Peter Woit is right in that.

Suppose that I shoot a single photon at a screen and a spot appears. In MWI we say that the universe has split into branches according to the observed location of the spot.

However, it is very hard to define precisely when and how the splitting happens. The wave function develops smoothly. There is no definite point of splitting. One might say that the full wave function of the system is the continuum of the Many Worlds, and only in some special cases it is convenient to classify sets of different paths of the system as branches.

David Bohm introduced a way to pick a single branch from the wave function of the Schrödinger equation. That was a major step forward. However, it is hard to define what is splitting in the Bohm framework. Each set of initial values of hidden variables specifies a single branch. There is no splitting.

 

[Elias1960]

I do not understand why Peter Woit, Sabine Hossenfelder, and Lubos Motl protest against the Many Worlds interpretation. MWI is one of the established and quite popular interpretations of quantum mechanics. Have these three people done new research which refutes earlier work on MWI? No.

They simply say that this is not even an interpretation, because it is not well-defined at all. Probability does not make sense in many worlds, to start with. They claim to be able to derive the Born rule, but are completely unable to explain what could be the meaning of probability. And to define the real world, you need much more structure than one vector in a quite unspecified Hilbert space. This additional structure is not defined at all. In the Copenhagen interpretation, all the necessary additional structure (say, what is position measurement, momentum measurement, and so on) comes from the classical part. In MWI it is simply undefined. They like to apply decoherence, but decoherence presupposes additional structure, namely a subdivision into a system and its environment. In everyday life, such subdivisions obviously exist, so that decoherence works. But its role in MWI should be fundamental, thus, the presuppositions for using it have to be fundamental too. Thus, MWI proponents would have to define them, and have to do this before applying decoherence.

David Bohm devised a method where an observing subject can "choose" a branch where he lives, assuming the Schrödinger equation. The other branches still exist and one may call them "parallel universes".

That makes no sense.

One of the fundamental principles of quantum mechanics is that there is not just one, Newtonian, deterministic universe. We have to assume alternative paths of the universe exist and calculate the interference pattern from the probability amplitudes. That is, assuming "many worlds" is at the heart of quantum mechanics.

No. Randomness is something completely different. It means, it is not well-defined now what will be the state of the universe tomorrow. But there will be a single state. Else, it makes no sense to compute probabilities for what will happen tomorrow.

 

[akvadrako]

@akvadrako,

the exact definition of the splitting or branching is a problem in MWI, Peter Woit is right in that.

I mean it isn’t a problem in the sense of something that needs fixing; it doesn’t effect the ontology or the predictions.

You could say it’s a open problem in the sense it hasn’t been done satisfactorily. But it’s just an issue of definition.

 

[Elias1960]

I mean it isn’t a problem in the sense of something that needs fixing; it doesn’t effect the ontology or the predictions.

It is a problem, namely the problem that MWI is not a well-defined interpretation. There exists a nice name, but no appropriate definition of this interpretation. Are branches well-defined objects or not? If not, every paper about MWI containing the word "branch" goes into the dustbin. Else, give the precise definition.

(This problem does, indeed, not need fixing. One can simply throw away MWI, problem solved, SCNR.)

 

[Minnesota Joe]

It is a problem, namely the problem that MWI is not a well-defined interpretation. There exists a nice name, but no appropriate definition of this interpretation. Are branches well-defined objects or not? If not, every paper about MWI containing the word "branch" goes into the dustbin. Else, give the precise definition.

(This problem does, indeed, not need fixing. One can simply throw away MWI, problem solved, SCNR.)

In defense of Sean Carroll, he acknowledges that this (probability/branch issue) is one of the most important problems with MWI. On his podcast Mindscape he is quite explicit that his project could all end in disaster and in his interview with David Albert they discuss Albert's objections which are similar to yours. I guess about the only thing to say, then, is that the MWI proponents do seem to be trying to find a precise definition of "branch". Whether or not they will be successful is another matter. I'm very skeptical about that.

I bought his book anyway, because I enjoy reading him, but in the OP's podcast and others he makes a certain claim about other interpretations that I don't really understand. He claims that other interpretations "work hard" to get rid of the many worlds. That seems to be an over-reach to me.

Carroll's claim seems a little question-begging, almost like something you would believe only if you first accepted that the Schrodinger equation describes many-worlds (which is precisely what is at issue).

Where are the other "worlds" in, say, the de Broglie-Bohm (dBB)? The empty packets?As I understand it, dBB has real particles and a real wave-function (whatever that is...?) that guides the particles and it is the particle that shows up on the detector screen. It is true that, if the particle had been in the empty wave-packet then the particle would have been detected somewhere else. But it wasn't in that wave-packet. So it seems like the counter-factual makes sense, but there is only ever the world, not many worlds--the empty wave-packet is still around somewhere in that world too. Does this sound right?

(After writing this I realized that maybe "work hard" to get rid of the worlds means a combination of the empty wave packets and having to have the initial $|\psi|^2$ distribution. But I still can't make sense of the claim. It still seems like it is only necessary to talk of the world, not worlds. Guess I'll have to read his book.)

 

[PeterDonis]

in this particular question I agree with his scientific position

He makes one interesting claim in that post that I have not seen discussion of in the literature: he claims that interpretations that say the wave function is real (his "R2" category) predict that heat capacities of quantum systems should be huge, many orders of magnitude larger than we actually measure them to be, because the wave function even for a single molecule would include so many physical degrees of freedom (for example, the wave function of a single qubit has to encode enough information to specify a point on the Bloch sphere). Does anyone know of any more rigorous treatment of such a claim in the literature?

 

[DarMM]

Adán Cabello has a good paper on the topic of interpretations and thermodynamics here:
https://arxiv.org/abs/1509.03641
Published in Phys Rev A as well:
https://journals.aps.org/pra/abstract/10.1103/PhysRevA.94.052127

 

[vanhees71]

It is a problem, namely the problem that MWI is not a well-defined interpretation. There exists a nice name, but no appropriate definition of this interpretation. Are branches well-defined objects or not? If not, every paper about MWI containing the word "branch" goes into the dustbin. Else, give the precise definition.

(This problem does, indeed, not need fixing. One can simply throw away MWI, problem solved, SCNR.)

I've also no clue what these many worlds are good for, but for a much simpler reason: If they can't be observed, is the claim of their existence still science or just some science-fiction fantasy? It's simply superfluous, if you ask me.

 

[vanhees71]

He makes one interesting claim in that post that I have not seen discussion of in the literature: he claims that interpretations that say the wave function is real (his "R2" category) predict that heat capacities of quantum systems should be huge, many orders of magnitude larger than we actually measure them to be, because the wave function even for a single molecule would include so many physical degrees of freedom (for example, the wave function of a single qubit has to encode enough information to specify a point on the Bloch sphere). Does anyone know of any more rigorous treatment of such a claim in the literature?

Hm? In the standard definition of entropy a la von Neumann the maximum entropy of 1 qu-bit is $k_{\text{B}} \ln 2$, and this has recently been measured in a fancy realization of a quantum Maxwell demon in CQED:

https://doi.org/10.1073/pnas.1704827114

 

[vanhees71]

Adán Cabello has a good paper on the topic of interpretations and thermodynamics here:
https://arxiv.org/abs/1509.03641
Published in Phys Rev A as well:
https://journals.aps.org/pra/abstract/10.1103/PhysRevA.94.052127

Very interesting paper. I only don't understand, how possibility (a) (which is the standard assumption if I understand it right since a measurement always extends the system by coupling it to the measurement device + "environment") prevents the heat from being infinite given the arguments before. Of course in this most physical solution, the heat is dissipated to "the environment", and this heat is finite. It should just be the quoted Landauer heat $k_{\text{B}} T \ln 2$ at each measurement of a spin component in an arbitrary direction. In the mean time the Landauer principle has been verified:

https://doi.org/10.1038/nature10872

 

[Heikki Tuuri]

The thought experiment of a photon forming a spot on a screen might clarify what is splitting in MWI.

We initialize the Bohm model by assigning the wave function. We keep the wave function fixed in this thought experiment.

We then give initial values to the hidden variables.

The trajectories of the hidden variables (or particles) for a set of initial values constitute a branch.

The thought experiment is like defining the full wave pattern of a lake, and then letting little toy boats sail on those waves.

A scientist in a single branch prepares the photon experiment. The result is deterministic in the Bohm model, but the scientist cannot know hidden variable values and cannot predict the outcome.

The scientist then measures the photon forming a spot at a location P.

Where was the splitting? It was a metaphor used by the scientist in his mind. He did not know precisely in which branch he was. After the experiment, he knows more. There was no physical splitting.

Physical splitting, actually, would require non-determinism. The Bohm model is deterministic.

The Bohm model requires the entire wave function in the calculation how the hidden variables develop in time. In that it differs from a Newtonian model.

What are the Many Worlds in the Bohm model? They are the different branches which we get with different initial values of the hidden variables.

The scientist did not know the initial values of the hidden variables. In his mind he assumes that all the branches do exist. He gradually gains knowledge in which branch he is.

Can we discard all the other branches except the one where the scientist lives? Yes, but we need to keep the wave function. In a sense, the wave function makes all branches "existing".

 

[bhobba]

I think, if you rename "many-worlds interpretation" back to its original name "relative-state interpretation", it's fine, but then of course you cannot discuss this in a popular-science book at all. I never understood the point of Everett's paper ever. I can't tell you what this interpretation means in a scientific sense.

Exactly. I will post the link again but many worlds, rightly viewed, is quite reasonable


It's now a lot like decoherent histories but without considering the other histories as 'real' or if like Gell- Mann you take real to mean one equal footing, there is actually not much if any difference..

But some legitimate scientists who really should know better like to sensationalize things and even introduce consciousness which of course there is no reason to. Of course like all interpretations it may be true but I fear Sean has succumbed to populism.

These days I only recommend two popularizations - Feynman - QED and Susskind's book

Thanks
Bill

 

[Minnesota Joe]

What are the Many Worlds in the Bohm model? They are the different branches which we get with different initial values of the hidden variables.

The scientist did not know the initial values of the hidden variables. In his mind he assumes that all the branches do exist. He gradually gains knowledge in which branch he is.

Can we discard all the other branches except the one where the scientist lives? Yes, but we need to keep the wave function. In a sense, the wave function makes all branches "existing".

I don't know if you were responding to my questions but...

Keeping it to two worlds per wave-function for simplicity, it is my understanding that in Sean Carroll's MWI a "branch" is one of two fully realized worlds. Else why would he talk about different versions of Sean Carroll and such? I don't think this is a metaphor.

When someone applies "worlds" to Bohmian mechanics or other interpretations, they are assuming the Many World Interpretation and that is confusing and question-begging. They ought not smuggle in "worlds" talk and apply it to other interpretations if they want to convince people. That is precisely what they have to demonstrate.

 

[PeterDonis]

In the standard definition of entropy a la von Neumann the maximum entropy of 1 qu-bit is $k_{\text{B}} \ln 2$,

The maximum entropy of one bit is $k_B \ln 2$ in the standard definition. But a qubit is not a bit. A bit can only have two values, $0$ or $1$. A qubit's wave function can have any value on the Bloch sphere. Measuring a qubit can only result in one of two values, but a single measurement on a qubit is not sufficient to tell you its exact wave function. Strictly speaking, it takes an infinite number of measurements (on an ensemble of identically prepared qubits) to do that.

That, at least, is how I understand Motl's argument, and it seems at least worth enough consideration for somebody to have written a paper on it at some point; that's why I asked if anyone knows of such a paper.

 

[Heikki Tuuri]

I invoke the Bohm model because I do not know of any other way of defining what a branch is in MWI.

A branch is a continuous function of its initial value settings. There are a continuum many almost exact copies of Sean Carroll in different branches, if we like to think in that way.

The Copenhagen interpretation speaks of "a collapse of the wave function". If the scientist works in an isolated laboratory, no collapse can happen. We cannot discard parts of the whole wave function of an isolated system. Why does the scientist observe a collapse?

In the Bohm model he does measure one exact outcome from a quantum experiment. He did not know the values of the hidden variables and could not predict the outcome, only a probability distribution. If he is in the Copenhagen camp, then in his mind he has a metaphor that a wave function has collapsed.

Thus, both the branching and the collapse are just metaphors in the mind of a scientist.

 

[bhobba]

When someone applies "worlds" to Bohmian mechanics or other interpretations, they are assuming the Many World Interpretation and that is confusing and question-begging. They ought not smuggle in "worlds" talk and apply it to other interpretations if they want to convince people. That is precisely what they have to demonstrate.

I was responding to Vanhees. I used to participate a lot in these interpretation discussions. But nowadays I think a lot of it is pointless. I was known to argue a lot that collapse is not required by QM. I spent a lot of posts discussing what collapse was. But then I realized really it doesn't matter. We know from QM that when you observe a system you get an eigenvalue of the observable. That's beyond doubt and its a semantic issue if you call it collapse. I don't think collapse is appropriate - but really I am just making a semantic choice. Its pointless constantly arguing semantics. The second part of QM about wave-functions and probability follows from Gleason's Theorem using the observable axiom (with the caveat of non-contextuality) so really the observable axiom is the primary one. Formally, because you have to hold one even if you are not aware of it, I hold to the Ignorance Ensemble interpretation. Its similar to when using probability you are likely subconsciously using some kind of interpretation - most likely the frequentest interpretation. In fact John Baez believes there is a connection between them:
http://math.ucr.edu/home/baez/bayes.htmlill

I am always struck by how we have all this discussion about QM but there is not the same angst about probability.

In fact its now known that QM is just a generalization of probability theory:
https://arxiv.org/abs/1402.6562
Looked at that way I say - why worry, except of course to correct misconceptions such that consciousnesses must be involved - it doesn't. Of course it may do, but its very much a backwater these days, mostly confined to sensationalist popularizations.

BTW - What The Bleep Do We Know Anyway is rot of the first order. We had a post here from some poor schoolkid forced to watch it. He was really confused and it took a while to explain the truth to him. That teacher needs a good talking to.

Thanks
Bill

 

[Minnesota Joe]

Exactly. I will post the link again but many worlds, rightly viewed, is quite reasonable

It's now a lot like decoherent histories but without considering the other histories as 'real' or if like Gell- Mann you take real to mean one equal footing, there is actually not much if any difference..

Interesting, but Gell-Mann's doesn't really dig very deeply in the video. He just asserts that which branch is real "is determined only probabilistically".

The MWI people might agree that the two branches are indeed on "equal footing" to the point of being indistinguishable. The wave function doesn't collapse. So it is only bias to call one real (the one I'm in now) and the other fake. They might invoke a sort of Copernican principle. I don't know for sure how the would respond though and I could be wrong. And I don't think MWI has a good explanation for probabilities either however.

But some legitimate scientists who really should know better like to sensationalize things and even introduce consciousness which of course there is no reason to. Of course like all interpretations it may be true but I fear Sean has succumbed to populism.

That's the thing. I don't think Carroll is sensationalizing anything. I think he is dead serious about MWI as a research project and seeing how far he can take it. He agrees it is radical relative the physics community. He is quite up front about the possibility it will crash and burn, even though he believes it is the "correct" interpretation in the sense of the most probably true. His book is supposed to explain why (I haven't read it yet). It is also--again, according to him--the most popular interpretation his field of cosmology.

These days I only recommend two popularizations - Feynman - QED and Susskind's book

Hey, thanks for the reminder. I'd forgotten about QED and need to read it. Which Susskind book is that?

 

[bhobba]

I invoke the Bohm model because I do not know of any other way of defining what a branch is in MWI.

Formally a branch is simply a series of projection operators. Its similar in Consistent Histories:
http://quantum.phys.cmu.edu/CHS/histories.html
Thanks
Bill

 

[Minnesota Joe]

I was responding to Vanhees.

Oh, ha, my comment wasn't aimed at you or anyone here! I was originally complaining about something Sean Carroll claimed. He claims interpretations other than MWI have to "work hard" to get rid of the many worlds.

 

[bhobba]

Which Susskind book is that?

https://www.amazon.com/dp/0465062903/?tag=pfamazon01-20

Thanks
Bill

 

[Minnesota Joe]

https://www.amazon.com/dp/0465062903/?tag=pfamazon01-20

Ok thanks, I read that one. It is good, but it seems mostly like standard "shut up and calculate" QM to me. Susskind takes the collapse as a postulate and doesn't give it enough discussion in my opinion. He's very dismissive or indifferent about quantum foundations too (not in the book where I can't remember him even mentioning it, but elsewhere). But it does contain an interesting discussion of EPR and locality that I need to reread.

 

[DarMM]

I was known to argue a lot that collapse is not required by QM. I spent a lot of posts discussing what collapse was. But then I realized really it doesn't matter. We know from QM that when you observe a system you get an eigenvalue of the observable

If you measure a PVM, for a POVM in general you do not. I don't think that affects your main point much, just a technicality.

I'm not sure what you mean by QM not needing collapse. I mean after an observation you update the state, right?

 

[Minnesota Joe]

I invoke the Bohm model because I do not know of any other way of defining what a branch is in MWI.

I think it means something more radical to MWI proponents. Ones like Carroll and the late DeWitt I mean.

A branch is a continuous function of its initial value settings. There are a continuum many almost exact copies of Sean Carroll in different branches, if we like to think in that way.

The Copenhagen interpretation speaks of "a collapse of the wave function". If the scientist works in an isolated laboratory, no collapse can happen. We cannot discard parts of the whole wave function of an isolated system. Why does the scientist observe a collapse?

In the Bohm model he does measure one exact outcome from a quantum experiment. He did not know the values of the hidden variables and could not predict the outcome, only a probability distribution. If he is in the Copenhagen camp, then in his mind he has a metaphor that a wave function has collapsed.

Thus, both the branching and the collapse are just metaphors in the mind of a scientist.

You are using it more metaphorically I guess, which I wasn't complaining about. I was complaining about Sean Carroll's position which--I think--are that worlds are real. The are actual. Whatever you want to call it. And then he wants to argue that all the other interpretations have them too, but they just work hard to get rid of them.

 

[Michael Price]

@akvadrako,
There is no splitting [in MWI].

It depends on how you define splitting, of course. At the end of the day you have the evolution of the universal wavefunction, and all possible worlds or histories are embedded within it. There are histories with advanced non-avian dinosaurs and victorious Nazis. There are none of these in our world, so they must have split off somehow, by any sensible definition of splitting.

 

[DarMM]

Very interesting paper. I only don't understand, how possibility (a) (which is the standard assumption if I understand it right since a measurement always extends the system by coupling it to the measurement device + "environment") prevents the heat from being infinite given the arguments before

First, yes (a) is the standard assumption, i.e. Copenhagen/Minimal.

As for why, Cabello summarizes it well on page 3:

In contrast, if probabilities are not determined by intrinsic properties of the system, then measurement out-comes are created randomly when the observables are measured, without any need to overwrite information in the system and therefore without the system dissipating heat due to Landauer’s principle

Basically the extra heat cost is from overriding the information stored in pre-existent determined variables. No such variables means no extra heat cost. That's how Copenhagen/Minimal gets out of it.

Now since this cost comes from overriding pre-existent determined information one can also avoid it by supposing that the amount of information in the system is infinitely large, since then you never need to override the information. This is what happens in Bohmian Mechanics and Many-Worlds where even a qubit has an infinite amount of deterministic information in it.

Since I imagine you'll find this is kind of strange it's easiest to illustrate with a dice. Obviously for a dice we have $\Omega = \left\{1,2,3,4,5,6\right\}$ as the sample space and some function $p\left(\omega\right)$ as the probability distribution satisfying:
$$\sum_{i}p\left(\omega_{i}\right) = 1$$
(All this is known to you of course, just to set it up)

Many Worlds and Bohmian mechanics essentially correspond to saying $p\left(\omega\right)$ is a physical degree of freedom, rather than just being a set of probabilities. If you claim that, then the dice goes from having a finite set of physical states to an infinite amount. As each $p\left(\omega\right)$ is a physical configuration.

 

[Michael Price]

[Lubos] makes one interesting claim in that post that I have not seen discussion of in the literature: he claims that interpretations that say the wave function is real (his "R2" category) predict that heat capacities of quantum systems should be huge, many orders of magnitude larger than we actually measure them to be, because the wave function even for a single molecule would include so many physical degrees of freedom (for example, the wave function of a single qubit has to encode enough information to specify a point on the Bloch sphere). Does anyone know of any more rigorous treatment of such a claim in the literature?

Lubos' heat capacity claim is just so bonkers it is hard to see why he pushes it. The problem is that Lubos is so far gone into confirmation bias against MWI that even the most nutty statements seem reasonable to him if they are anti-MWI. MWI is an interpretation and interpretations don't change physical results, such as specific heat capacities.

 

[PeterDonis]

Lubos' heat capacity claim is just so bonkers it is hard to see why he pushes it.

Why do you think it is bonkers?

MWI is an interpretation and interpretations don't change physical results

But Motl's argument is basically that, if "MWI" includes the claim that "the wave function is physically real", then it can't be just an interpretation; it must actually make a different prediction from standard QM, the one about heat capacities. His argument might be wrong, but it can't be rebutted by just saying "MWI is an interpretation", because whether or not the MWI actually is just an interpretation is one of the points at issue.

 

[akvadrako]

But some legitimate scientists who really should know better like to sensationalize things and even introduce consciousness which of course there is no reason to. Of course like all interpretations it may be true but I fear Sean has succumbed to populism.

I'm not sure where Sean invokes consciousness (I would be happy to get a link), but I do think it unfortunately does have some role to play in MWI. That's because while the objective ontology of MWI is clear, conscious agents are just subsystems embedded in a larger system. So if you care about connecting the objective description to what is measurable, what some would call science, you need to understand the dynamics of consciousness and how it relates to the environment.

The big question for me is how much of a role self-locating uncertainty plays.

 

[Heikki Tuuri]

It depends on how you define splitting, of course. At the end of the day you have the evolution of the universal wavefunction, and all possible worlds or histories are embedded within it. There are histories with advanced non-avian dinosaurs and victorious Nazis. There are none of these in our world, so they must have split off somehow, by any sensible definition of splitting.

Let us think about that using the Bohm model. We first pick a wave function for the > 10^80 particles that explode in the Big Bang. The wave function stays fixed in this thought experiment.

The wave function allows also extremely improbable developments.

Each branch is distinct. It is a deterministic function of the initial values. Is there a branch where Earth develops almost exactly like our own, but then 100 million years ago dinosaurs start building railroads?

If the movement of particles were truly random, then, of course, we could shuffle the particles as we like and conjure up such dinosaurs.

But we picked a fixed wave function at the Big Bang. It is not clear at all that we can choose initial values such that intelligent dinosaurs arise.

We can construct a suitable wave function by making one for the dinosaur civilization and then calculating back to the Big Bang. But we may have picked a different wave function at the start.

I had thought that MWI allows all kinds of strange worlds, but that might not be the case.

 

[Michael Price]

Exactly. I will post the link again but many worlds, rightly viewed, is quite reasonable


It's now a lot like decoherent histories but without considering the other histories as 'real' or if like Gell- Mann you take real to mean one equal footing, there is actually not much if any difference..

But some legitimate scientists who really should know better like to sensationalize things and even introduce consciousness which of course there is no reason to. Of course like all interpretations it may be true but I fear Sean has succumbed to populism.

These days I only recommend two popularizations - Feynman - QED and Susskind's book

Thanks
Bill

Murray GM is just playing with words and evading the issue: are the Everett world's real? MWI says they are. It is not popularization to state this. Indeed it is condescending not to say this - or else you are saying the public need protection from the truth because they can't handle it?

Why do you think it is bonkers?
But Motl's argument is basically that, if "MWI" includes the claim that "the wave function is physically real", then it can't be just an interpretation; it must actually make a different prediction from standard QM, the one about heat capacities. His argument might be wrong, but it can't be rebutted by just saying "MWI is an interpretation", because whether or not the MWI actually is just an interpretation is one of the points at issue.

The putative reality of the wavefunction does not change any measurements made - it won't move a needle on a gauge, for example. It changes whether the needle is ghostly or real, but not its dynamics, and is the dynamics that determines the needle's position, on how it is correlated with whatever it is measuring.

There is universal agreement in the literature that interpretations do not differ about empirical results. Our experiences are embedded in the correlations within the universal wavefunction. These correlations do not depend on ontological status of the UW. (I do not think there is any published claim to the contrary.)

 

[Minnesota Joe]

I'm not sure where Sean invokes consciousness (I would be happy to get a link), but I do think it unfortunately does have some role to play in MWI. That's because while the objective ontology of MWI is clear, conscious agents are just subsystems embedded in a larger system. So if you care about connecting the objective description to what is measurable, what some would call science, you need to understand the dynamics of consciousness and how it relates to the environment.

To my knowledge Carroll doesn't invoke consciousness. Nor does he endorse a special role for consciousness. I think he has argued that consciousness emerges from physics, not the other way around. (I don't want to speak for him but I seem to remember him talking along those lines.)

 

[Michael Price]

Let us think about that using the Bohm model. We first pick a wave function for the > 10^80 particles that explode in the Big Bang. The wave function stays fixed in this thought experiment.

The wave function allows also extremely improbable developments.

Each branch is distinct. It is a deterministic function of the initial values. Is there a branch where Earth develops almost exactly like our own, but then 100 million years ago dinosaurs start building railroads?

If the movement of particles were truly random, then, of course, we could shuffle the particles as we like and conjure up such dinosaurs.

But we picked a fixed wave function at the Big Bang. It is not clear at all that we can choose initial values such that intelligent dinosaurs arise.

We can construct a suitable wave function by making one for the dinosaur civilization and then calculating back to the Big Bang. But we may have picked a different wave function at the start.

I had thought that MWI allows all kinds of strange worlds, but that might not be the case.

Personally I find the Schrödinger picture, where the wavefunction evolves with time, easier to use. It does not really matter, but I was thinking of worlds where dinosaurs survived to the modern day because the asteroid missed the Earth. Whether such a scenario is possible, nay inevitable, reminds of the discussions we had about Dr.Chinese being president. I say yes, but the discussion became very weird and heated and I will leave it at that.

 

[PeterDonis]

The putative reality of the wavefunction does not change any measurements made

It depends on what "reality of the wavefunction" implies. But phrasing it that way might be misleading; let me try rephrasing what I take to be Motl's argument (or at least a "steelman" version of it that appears to me to be worth considering) without using that terminology, at least at the start.

Consider a single qubit. If we don't know its state, the usual assumption in quantum statistical mechanics is that its entropy is of order unity (in "natural" units where "unity" means $k_B$). This means it has approximately 1 bit's worth of information in its unknown state.

However, specifying the exact quantum state of a qubit takes much more than 1 bit of information, since it is equivalent to specifying an exact point on the Bloch sphere. So the question is, does that exact quantum state refer to real, physical degrees of freedom? In other words, does it refer to real, physical information which, if we don't know the qubit's state, should be reflected in the entropy?

Motl's argument is basically that the MWI, or indeed any interpretation that treats the wave function as real, must answer "yes" to the above question. If that is true, then, whether the proponents of such interpretations realize it or not, or have even considered it or not, their interpretation actually isn't just an interpretation; it makes a testable physical prediction about entropy that is different from standard quantum statistical mechanics.

There is universal agreement in the literature that interpretations do not differ about empirical results.

There is agreement that that is the definition of an interpretation. But you can't establish that a particular thing that claims to be an interpretation, actually is one, by just quoting the definition.

 

[Minnesota Joe]

Just to put money to my claim that Sean Carroll takes MWI very seriously, I looked up a paper he mentions in on of his podcasts.

Mad-Dog Everettianism: Quantum Mechanics at Its Most Minimal, Sean M. Carroll, Ashmeet Singh

 

[Quanundrum]

The main annoyance I have (besides the obvious problem of probability) is how people like Sean Carroll make these extravagant claims in vivid language. Hyperbole is littered throughout the book and interviews he has done in the press. He adamantly claim that you live in multiple worlds and extrapolates wild claims that just confuses the general public. Even if you make the assumption that the Born Rule will one day be derived and MWI is correct, you don't know whether you are living in a divergent (all worlds exist from big bang in a separated fashion) multiverse or a splitting one (where you actually branch off from overlapping worlds). At first this may seem like a technical nuance, but it sits at the heart of all these radical claims. Quantum Immortality automatically goes out the window in the former, as does virtually all talk of you going into multiple futures at every instant.

Furthermore, the claim that "MWI is simply taking the equations seriously man" is lazy. To quote Mitchell Porter from Sean's latest blog post comment section

Mitchell Porter says:
September 21, 2019 at 6:04 pm
So when an observable has two eigenvalues, how many worlds are there – two? Two million? A continuum? Does the number of worlds relate to the probability of an observation? If yes, please explicitly exhibit the decomposition of the wavefunction into the appropriate worlds. And if the probabilities don’t come from the number of worlds, please explain where they do come from.

This is never answered by any of these dogmatic Everettians

 

[Michael Price]

The main annoyance I have (besides the obvious problem of probability) is how people like Sean Carroll make these extravagant claims in vivid language. Hyperbole is littered throughout the book and interviews he has done in the press. He adamantly claim that you live in multiple worlds and extrapolates wild claims that just confuses the general public. Even if you make the assumption that the Born Rule will one day be derived and MWI is correct, you don't know whether you are living in a divergent (all worlds exist from big bang in a separated fashion) multiverse or a splitting one (where you actually branch off from overlapping worlds). At first this may seem like a technical nuance, but it sits at the heart of all these radical claims. Quantum Immortality automatically goes out the window in the former, as does virtually all talk of you going into multiple futures at every instant.

Furthermore, the claim that "MWI is simply taking the equations seriously man" is lazy. To quote Mitchell Porter from Sean's latest blog post comment section
This is never answered by any of these dogmatic Everettians

https://www.quora.com/How-does-the-...ding-to-the-Born-rule/answer/Michael-Price-29