Is the Many Worlds Interpretation Widely Accepted Among Physicists?

[secur]

I have a few questions regarding Many Worlds Interpretation (MWI)

Terminology: QM is just the math, doubted by none; MWI and CI (Copenhagen Interpretation) are ontological interpretations.

David Deutsch believes MWI with certainty. He even says that it's "proven" experimentally. How many theoretical physicists go along with that? How is Deutsch's attitude viewed, generally, in the community? Do any theoretical physicists disbelieve MWI with certainty?

Stephen Hawking has used MWI model for some theoretical work (right?). He's in situations where (arguably) no "collapse of the wave function" would occur, like Big Bang or Black Holes. In that case CI is equivalent to MWI anyway, isn't it? He puts special emphasis on reversibility of physical laws: no information is ever lost. MWI fits this requirement, but "collapsed" wave function violates it. Is that why he uses MWI? How prevalent is this approach in theoretical cosmology generally?

Does MWI facilitate the attempt to unify GR and QM?

I think that experimental physicists ignore MWI and work with CI by default. Is that true?

These questions are not about physics per se but the culture of physicists, so aren't ideal for this forum. In defense of them, let me make the following points.

- The overall issue is important in terms of the relation of physicists to society
- They concern mainstream physics or at least close to that
- They are not speculative or argumentative
- There's nowhere else to ask them, to get authoritative answers without impassioned argument (I have experimentally proven that, elsewhere on the net and at local universities)

Take these questions as merely guidelines, no need to go through them "bullet by bullet". Most important, please correct any misunderstandings evident in the questions.

 

[bhobba]

Its just an interpretation - there are tons. There is no way to tell the difference experimentally between any of them - at least no one has figured out a way.

I don't hold to MW but have read Wallace's book on it. Its legit IMHO - but some have issues with it.

Does MWI facilitate the attempt to unify GR and QM?.

The issue isn't quite what the popular press or even some professional literature would have you believe:
http://arxiv.org/pdf/1209.3511v1.pdf
'Effective field theory has added something important to the understanding of quantum gravity. One can find thousands of statements in the literature to the effect that “general relativity and quantum mechanics are incompatible”. These are completely outdated and no longer relevant. Effective field theory shows that general relativity and quantum mechanics work together perfectly normally over a range of scales and curvatures, including those relevant for the world that we see around us. However, effective field theories are only valid over some range of scales. General relativity certainly does have problematic issues at extreme scales. There are important problems which the effective field theory does not solve because they are beyond its range of validity. However, this means that the issue of quantum gravity is not what we thought it to be. Rather than a fundamental incompatibility of quantum mechanics and gravity, we are in the more familiar situation of needing a more complete theory beyond the range of their combined applicability. The usual marriage of general relativity and quantum mechanics is fine at ordinary energies, but we now seek to uncover the modifications that must be present in more extreme conditions. This is the modern view of the problem of quantum gravity, and it represents progress over the outdated view of the past.'

Thanks
Bill

 

[secur]

Thanks bhobba for your input.

"The Effective Field Theory of Quantum Gravity" looks good and you're right, it is a side of the question I was not familiar with. In fact, "am" not familiar with, because it will take a while to absorb the paper! If there are questions I'll ask in another thread, because this is of great interest to me, and others no doubt.

I'll have to check out Wallace's "The Emergent Multiverse". I've read a fair amount already (Deutsch) but there's always room for one more. Lev Vaidman has this to say: "... Wallace's description of the theory is exactly the many-worlds interpretation (MWI) I believe in." But MWI per se is not really my concern. I'm neither for nor against, awaiting some experimental relevance.

Now, I hate to bug you, but I'm still curious about the attitude of the overall physics community - as an "anthropological" question, not technical. I'd appreciate just a rough estimate of how many share your view, compared to how many "believe" MWI.

Perhaps you only want to comment on your own views, without "gossiping"; commendable; then ignore this request, and thanks again.

 

[bhobba]

Perhaps you only want to comment on your own views, without "gossiping"; commendable; then ignore this request, and thanks again.

Its not that. Its simply using Google you can find all you need to know about others views without me regugitating them second hand. I will probably need to refresh my memory about it anyway so you may as well do it.

Thanks
Bill

 

[vanhees71]

Well, I think that these debates on interpretation do not reflect in any way the real status of this issue in science. There's a scientific basis to it, and that are the marvelous experiments done nowadays to check the quantum behavior of nature in its extreme, particularly Bell tests with entangled states of photons, quantum dots, single molecules, etc. etc. All these tests confirm the predictions of minimally interpreted quantum theory. By minimally interpreted I mean to take the math of quantum theory (which is not doubted seriously by anybody) and apply it to real-world experiments using the usual probabilistic meaning of states that are defined by real-world preparation procedures on the investigated systems.

Everything else is metaphysics and make an interesting topic to be discussed on institute parties...

 

[secur]

Thanks for responses bhobba and vanhees71,

Apparently I didn't phrase my anthropological question well; however I've got my answer. The following gives my conclusions, with some detail for others who may be interested.

Here's the type of answer I was looking for: "I'd say maybe 50% of my colleages believe MWI; a few think it's science fiction; most don't care." With some elaboration, perhaps. Consider election pollsters. They want to know how many voters like candidate A, how many B. They don't care about the merits of these candidates. That's exactly the type of information I want.

I've found sketchy data on the net, but it's very unreliable; the date, usually not available.
David Raub, 1995: nearly 60% thought that the many-worlds interpretation was "true".
Max Tegmark poll, 1997: 16.67 voting for MWI, 46% against, 37.5 undecided.
Anton Zeilinger poll: approximately 18% pro-MWI, 50% against.
Sean Carroll off-hand statement, paraphrased: more than 50% of working physicists believe MWI
Peter Thiel: "Some 55-60% of theoretical physicists believe in many-worlds today."

Seems to indicate more than 50% pro-MWI as of today.

To emphasize my anthropological slant, some quotes from my own posts above,
- How many theoretical physicists go along with that [Deutsch's MWI opinion]?
- I'd appreciate just a rough estimate of how many share your view, compared to how many "believe" MWI.
- These questions are not about physics per se but the culture of physicists ...

And responses to you:

bhobba: using Google you can find all you need to know about others views without me regugitating them second hand. I will probably need to refresh my memory about it anyway so you may as well do it.

- Google gave me only very sketchy data, shown above. I'm not asking for a super-accurate number, no need to refresh your memory; just - off the top of your head - how popular is MWI among your colleagues?

vanhees71: All these tests confirm the predictions of minimally interpreted quantum theory ... Everything else is metaphysics and makes an interesting topic to be discussed on institute parties...

- So, when discussing at institute parties: about how many would you say are strong MWI supporters?

Until 2014 I was able to attend some of those parties myself with an anti-MWI physicist friend (very opinionated; also anti-String Theory) who is, unfortunately, no longer with us. At that time this question hadn't occurrred to me, but I did notice increasing intolerance among MWI'ers. Now I no longer have "insider information" so must ask here.

Note well, I don't care about this a whole lot, it's just one more interesting question out of thousands.

To wrap it up: there's no need for any further responses. Notice no MWI supporters have responded. That's because they sensed I'm studying them as an anthropologist. Reading between the lines, this is my conclusion. You, bhobba and vanhees71, are in a minority who don't believe (or disbelieve) MWI. Strong supporters are more than 50%. Those who disagree are in for ridicule and would rather just avoid the whole thing. You two were brave to respond at all, and I won't bother you any further. Thanks very much!

 

[vanhees71]

Well, the point is that you have to distinguish between different levels of perception of science. As an anthropologist you know that better than me who is a theoretical physicist working in the field of relativistic heavy-ion collisions, applying many-body quantum field theory, kinetic theory, and relativistic hydrodynamics to describe phenomena related to the strongly interacting matter, undergoing most probably a transition from a quark-gluon plasma via a hot and dense hadron gas to the final observations of the created particles in the detectors of our experimental colleagues. As you can imagine in our daily work in this field the various interpretations of quantum theory do not play a lot of a role. We just use the theory as a tool to describe the evolution of these collisions in terms of statistical outcomes of measurements. These measurements themselves are about large ensembles of events and use statistics to evaluate these outcomes.

Then there is of course also another level of thinking, e.g., when you prepare a lecture about quantum theory. Here, I also favorize what I call the "no-nonsense approach". One should teach physics as physics first and concentrate on the mathematical formalism and its interpretation as far as physics is concerned. So you start with some heuristics, often with a short summary about the historical development of quantum theory, starting with Planck's discovery of the "quantum of action", i.e., the fundamental Planck constant, ##\hbar##, which is the new feature of QT compared to classical physics. Usually you end up with Schrödinger's wave mechanics, treat a few important examples (harmonic oscillator, rigid rotator, hydrogen atom) and then develop the full theory in terms of the abstract Hilbert space, the algebra of observables, and so on. The only thing you need to make sense of this formalism with regard to its relation to real experiments in the lab and observations in nature is the probabilistic interpretation of the notion of state (represented by a positive semidefinite trace-class operator, the Statistical operator). That's it as far as physics is concerned, and that's what I call "minimal interpretation" (it's also what's called the ensemble interpretation).

I guess most of my colleagues more or less have a similar point of view towards interpretation. There's a lot happening in the AMO (atomic, molecular, optical physics) community to experimentally test the predictions of quantum theory, including the very strong correlations that are impossible in a classical local realistic model according to Bell's inequality and related statistical theorems. With an overwhelming accuracy, i.e., statistical significance, the predictions of the minimally interpreted quantum theory are confirmed by these highly accurate experiments.

I also think that most of the interpretations (like Copenhagen a la Bohr, i.e., without collapse, many worlds, consistent histories) are not too far from this minimal interpretation, and I can't say that I dislike them in any way, but I think they are quite superfluous as far as physics is concerned. They introduce sometimes strange ideas about the existence of many parallel worlds, which are not observable. So the physicists doesn't need to care about it, because what's in principle unobservable is simply not part of physics, because it cannot be experimentally tested. Then there are interpretations I cannot make logical sense of (e.g., Copenhagen with collapse a la Heisenberg, introducing a quantum-classical cut of dynamics; de Broglie-Bohm with its notorious difficulties to by applied to relativistic quantum field theory, which is the most comprehensive model with the most accurate results ever; or solipsistic ideas a la von Neumann, where the consciousness of the observer enters the description of physical observations). The collapse a la Heisenberg contradicts the very foundation of all of physics which is the relativistic space-time structure and the related notion of causality. In my opinion it's also not necessary to make sense of the formalism with respect to observations in the real world, and that's why I find it very easy and unproblematic to just abandon it from the interpretation of the theory.

Then there's another level, where some people think science should provide more than just an accurate description of observations of nature, looking for some "deeper" metaphysical grounds. Then you start to come into a realm, where science even can get quasi-religious features, sometimes even esoterics. All this has nothing to do with science in my opinion. You can belief in it or not, it doesn't change anything concerning our understanding of nature in the sense of science.

When it comes to polls, you must be very careful. Already the choice of participants in the poll can give significantly different results from asking all physicists (which is practically impossible, I guess).

 

[secur]

Thanks! I'm surprised how close your "QM 101" is to what I learned 40+ years ago. I had the vague impression you started them off with QFT; I've met grad students who don't seem to recognize Hilbert space, they start with a Lagrangian with gauge fields, derive everything magically from symmetries, etc. I ask them "where's Dirac's equation in all this?" - and they often don't (apparently) know. Fortunately their professors do; the answer turns out to involve "spinors" - a.k.a. 4-component spin matrices.

Anyway, the only term I don't recognize is "trace-class operator, the Statistical operator". This must boil down, partly, to what we used to call a bounded (positive-semidefinite) Hermitian operator. We happily used unbounded operators, for instance, dirac delta functions for position whose Fourier transform gives infinite wave packets - silly, but it worked somehow. I imagine "trace-class" means you're making sure to use a bounded operator (finite trace) - commendably rigorous. And since you don't mention Projection operators I imagine the "Statistical operator" must play that role. In fact, I think I was taught "collapse a la Heisenberg", which I suppose I need to unlearn.

The problem with QFT in Minkowski space, you lose the intuitive connection to experiments where time is definitely not "just another dimension", and the experimenter lives in Euclidean space. Particles are particles, not excitations of the field. With an arbitrary basis, probabilities don't sum to 1 at a given time, at least not manifestly. My sense is that losing this feel for experiments allows some physicists to veer into science-fiction ontologies; it's all just math. So I'm glad that's not how you teach undergrads.

vanhees17: Then there's another level, where some people think science should provide more than just an accurate description of observations of nature, looking for some "deeper" metaphysical grounds. Then you start to come into a realm, where science even can get quasi-religious features ...

By the way, I like the soft sciences - took grad courses in anthropology and philosophy - but my degree is mathematics, and apart from a few years teaching, my career was mostly in software. Anyway, science should, only, accurately describe nature. But scientists must communicate that description to philosophers, and the general public. Unfortunately few philosophers can understand it. Fortunately many scientists (physicists in particular) are closet philosophers, happy to help out: in fact, they're willing to do the philosopher's job for them entirely! That's alright, but wear a different hat so we can tell what you're up to. When your colleagues stop providing an accurate description of nature and start telling us what it all means, quasi-religiously, I suggest they put on a bishop's mitre :-)

Thanks for taking the time to clarify your position. I'll be hanging around, trying to catch up on decades of amazing progress in physics, and look forward to helping out when you physicists veer into philosophy - where perhaps the shoe is on the other foot.

 

[bhobba]

Google gave me only very sketchy data, shown above. I'm not asking for a super-accurate number, no need to refresh your memory; just - off the top of your head than 50%.

http://www.preposterousuniverse.com/blog/2013/01/17/the-most-embarrassing-graph-in-modern-physics/

The issue with MW is the use of decision theory:
http://www.preposterousuniverse.com/blog/2012/04/16/quantum-mechanics-and-decision-theory/

MW is weird but that actually isn't the reason most physicists don't adhere to it - it's usually being unhappy with things like the above. Its why I don't adhere to it, ie for me its just too weird - but mathematically very beautiful.

If it appeals then have a cook at Consistent Histories which is often described as many worlds without the many worlds:
http://arxiv.org/abs/gr-qc/9407040

In Consistent Histories QM is the stochastic theory of what are called histories. In MW each history is a new world.

Thanks
Bill

 

[bhobba]

Thanks! I'm surprised how close your "QM 101" is to what I learned 40+ years ago.

Personally for me the following was a big leap forward in what QM is:
http://arxiv.org/pdf/quant-ph/0101012.pdf

This clarifies the mathematics. Physically the issue is its a theory about observations that appear in a common-sense classical world. How a theory that assumes such a world at the start explains that world is IMHO the central mystery of QM. Great progress particularly in the area of deoherence has been made in resoving it - but a few issues remain. Here is the book to get if you want the detail:
https://www.amazon.com/dp/3540357734/?tag=pfamazon01-20

Thanks
Bill

 

[vanhees71]

Thanks! I'm surprised how close your "QM 101" is to what I learned 40+ years ago. I had the vague impression you started them off with QFT; I've met grad students who don't seem to recognize Hilbert space, they start with a Lagrangian with gauge fields, derive everything magically from symmetries, etc. I ask them "where's Dirac's equation in all this?" - and they often don't (apparently) know. Fortunately their professors do; the answer turns out to involve "spinors" - a.k.a. 4-component spin matrices.

Well, then your grad students should learn a bit about the foundations. I don't think that it makes much sense to start the quantum-theory course with relativistic QFT. Without a solid foundation in non-relativistic quantum mechanics it's not really possible to understand relativistic QFT. As far as non-relativistic quantum mechanics is concerned, I guess there's little to be changed from a course given in the 1970ies. I think there will be more emphasis on symmetries and group-theoretical methods now than then, because these techniques indeed are the key of the whole topic. Also there has been a tremendous experimental progress concerning the foundations (Bell experiments) which were not yet possible then. Also this has an influence on how we teach QM nowadays. I'd also leave out the socalled "old quantum mechanics" which is more emphasized in the older textbooks than is healthy.

Another thing is the introduction to relativistic QT. Here I'm strongly against repeating the awful treatment of the very early days, where the Dirac equation was introduced as if you could have a single-particle wave-function representation a la Schrödinger (or Pauli for spin 1/2 for that matter) as in non-relativistic quantum theory. You have very confusing discussions about the paradoxes already in the most simple cases of motion of an electron in simple external-potential problems and then end up with the Dirac sea (which doesn't exist) and a single-particle description that is interpreted in terms of a many-body theory (Dirac's hole-theory formulation of QED). This is all avoidable by starting right away with relativistic QFT. This is no problem, if you have treated non-relativistic QFT as an elegant tool to describe many-body systems at the end of the non-relativistic QM lecture.

Dirac's equation of course occurs later in problems like the hydrogen atom after the appropriate soft-photon approximation, leading to a description where you do perturbation theory around the Dirac-Kepler problem with the proton substituted by a static Coulomb field. The perturbative corrections then lead to the Lamb shift in a very systematic way.

 

[secur]

response to bhobba:

bhobba, I hope you're not a fan of Sean Carroll. Turns out he's a poster boy for the "closet philosopher" physicist I complained of above. In fact, he's out of the closet ...

Carroll: As we see, among this expert collection of physicists, philosophers, and mathematicians, there is not much of a consensus. ... The embarrassing thing is that we don’t have agreement.

- Ontology is philosophy, not physics. It's clear from this comment he's a true believer in something - turns out to be MWI. If you have faith in MWI, get some experiment to prove it; until then you're doing philosophy. If you're on company time, request that the bursar's office reduce your pay accordingly. Actually, leave off entirely: physicists make poor philosophers (apparently).

Carroll: For quantum mechanics, by contrast, all we really have to do (most people believe) is think about it in the right way.

- Thinking the right way: it's nowhere near as easy as it looks.

It's encouraging that every comment to this blog agrees with me:

Romain: The embarrassing part is that we are actually trying to find a correct interpretation.
Eric Liefer: I also disagree with you that it is just a matter of “thinking about it in the right way”. Experiments should play a major role as much as they do in any other area of physics.
Nicholas Malaya: As they say: ‘shut up and calculate!’
doc c: This post goes to the heart of the conflict between science and religion.

I googled Dr. Carroll and this video lecture came up: "God is not a good". 'Nuff said.

Carroll: But Deutsch argues — and on this I think he’s correct — that this result implies we should act as if the Born Rule is true if we are rational decision-makers. ... As someone who is sympathetic to pragmatism, I think that “we should always act as if A is true” is the same as “A is true.” So the Born Rule emerges from the MWI plus some seemingly-innocent axioms of decision theory.

- William James is rolling over in his grave

Consistent Histories: Great! This is the source of many terms and ideas that have been baffling me the last couple of days reading posts on the QM forum. Very sane: no sci-fi here. Only one red flag: denial of counterfactual, evidently to brush off Bell's theorem.

Quantum Theory From Five Reasonable Axioms by Lucien Hardy:

- This is very cool. Not my main interest, and a bit over my head, but it's hard to stop reading: it's like a magician pulling QM out of a hat.

Hardy: If a 19th century ancestor of Schroedinger had complained about “dammed classical jumps” then he might have attempted to derive a continuous theory of probability and arrived at quantum theory.

- I try to imagine Lord Kelvin or Maxwell doing this ... It's true they had the ability and knowledge. They, unlike me, could master it at one reading - at least, in Thomson's case, when he was younger. But they would have no idea why they should. After Planck, and Einstein's photoelectic effect, the motivation might be there; but it really makes elegant sense only in the light of QC.

Note, this approach can't give you h. Nor, as Hardy knows, any quantum properties not accessible via correspondence principle. I see the requirement of continuity (in reversible transformations) getting you to Lie groups. He gets the complex coefficients (heart of QM) by showing the Lie dimension must be 2 ... tortured logic, but valid no doubt.

Dirac's "Principles of Quantum Mechanics":

By the way, Dirac does some "magic tricks" too. For instance he shows where the "i" in QM comes from in three different ways. When a normalized vector moves, it can only be rotating - i.e., the derivative must be multiplication by i. And his hands-on applications of Noether's theorem are clever. He gets momentum conservation by ... sliding the experimental apparatus sideways! Perhaps everyone knows these tricks, but I never saw them elsewhere. You have to dig to get the most out of that book, he's pathologically diffident; in fact I suspect Dirac (a wonderful person) was mildly autistic. But if you listen carefully to his mumbling, it's pure gold.

 

[bhobba]

Perhaps everyone knows these tricks, but I never saw them elsewhere.

Read Chapter 3 of Ballentine:
https://www.amazon.com/gp/product/9814578584/?tag=pfamazon01-20

Combine it with Hardy and QM is pulled out of a hat. Interesting exercise seeing what physical assumptions are being made.

Thanks
Bill

 

[.Scott]

Now, I hate to bug you, but I'm still curious about the attitude of the overall physics community - as an "anthropological" question, not technical. I'd appreciate just a rough estimate of how many share your view, compared to how many "believe" MWI.

Since you asked ...

To be clear, MWI is more than just superpositioning, because superpositioning resolves back to a single "world", whereas MWI allows these worlds to ever increase in number.

Personally, I doubt the "I" in MWI. If it's an interpretation, there should be no way to differentiate it from other "interpretations". But if there are many worlds - and an ever increasing number of them - then there is an ever-increasing amount of information in anyone of those worlds.

Consider what I call "addressing information". Without MWI, you can fix yourself in the universe by four coordinates - X, Y, Z, and time. With only those four numbers (perhaps mapped to accommodate black holes and whatever), will put you at a spot in the universe that is different - an in theory, recognizably different.

With MWI, you need an ever-increasing amount of information to designate the exact branch that you followed. Each new coordinate would put you at a new location that is, in theory, recognizably different from any other spot. That implies a huge ever-present flow of information into each "world" of the universe.

So you can put me down as an MWI doubter - or perhaps, in more contemporary terms, an MWI-denier.

 

[StevieTNZ]

You might find this article of interest: http://www.bbc.com/earth/story/20160318-why-there-might-be-many-more-universes-besides-our-own

 

[secur]

Thanks vanhees71, .Scott, StevieTNZ

vanhees71, all your comments make sense but a couple slight misunderstandings:

vanhees71: Well, then your grad students should learn a bit about the foundations.

- I need to clarify, they're not "my" grad students. After a stint as a junior faculty member in Math around 40 years ago, I left to work on Navy software, TD'ing on some huge projects. This century, 2010 - 2014, I hung around the local physics dep't, desperately boring grad students at a few get-togethers by asking "So, young man / my dear, what are you studying?" These are the grad students I was referring to. So though I'd love to be one of the academic crowd, I'm not. At best, an "honorary member".

No doubt they were taught the foundations, but weren't on top of them a few years later. In math, we had a saying: "You don't really understand even calculus, until you teach it". You need to get to teaching before you really appreciate the fundamentals.

Finally, it seems to me that in QFT spinor fields can be considered the Dirac equation in disguise. The book QFT, Lewis H. Ryder, refers to the "Dirac Lagrangian" when using them. But to you, the single-particle eqn (well, two particles considering antiparticle solution) is "the" Dirac eqn - the one he originally wrote down (sans spinors of course). Just different terminology, I believe.

.Scott, unique view on MW "interpretation": the "huge flow of info" should have consequences. Sounds like it's almost falsifiable in your opinion. That's much better than "not even wrong".

.Scott: So you can put me down as an MWI doubter - or perhaps, in more contemporary terms, an MWI-denier.

- Yes: doubt = denial, when minds become closed. Scary!

StevieTNZ, that article is not bad at all, but I have a problem with it: while this is being presented to the public, what other fascinating real physics isn't? Non-linear crystals, EIT, exoplanets, plenty of cool stuff I'm here to learn about. People have limited science info bandwidth (understatement of the year), science community has responsibility to feed it wisely. Consider this statement from the article:

article: The trouble is, unless you have some other explanation for fine-tuning, someone will assert that God must have set things up this way. The astrophysicist Bernard Carr has put it bluntly: "If you don't want God, you'd better have a multiverse".

- Many people - rightly, judging by Dr. Carr's "warning" - suspect a deliberate social-engineering misinformation campaign. This is a frightening statement, in what it says about some popularizing physicists' true motivations - Dr. Carroll (MWI-er, and proselytizing atheist) being an excellent case in point.

But let's not get into that! I'm researching questions on LWI, colliding Kerr BHs, and other topics to catch up on: real physics.

 

[f95toli]

Here's the type of answer I was looking for: "I'd say maybe 50% of my colleages believe MWI; a few think it's science fiction; most don't care." With some elaboration, perhaps. Consider election pollsters. They want to know how many voters like candidate A, how many B. They don't care about the merits of these candidates. That's exactly the type of information I want.

I think you are missing the point a bit. Most scientists who work with quantum mechanics know very little about the various interpretations, meaning they would have no way of answering your poll. This mainly because it has no relevance at all to the work they do and it is not something they really teach you about in courses. Note that this includes many people who work on experiments that are related to the "foundations" of QM; i.e. things like Bell tests etc. The reason is simply that unless there is -at least in principle- a way to learn more about what is correct from an experiment (which when you -by definition- can't when it comes to different interpretations) it is metaphysics and most physicists are therefore not interested (which doesn't mean that it is not something that is occasionally discussed during coffee breaks). I've spent most of my career working on things like macroscopic quantum tunneling, sources of decoherence, quantum computing, non-classical states of light etc but the different interpretations have simply never come up in a "technical setting".

The problem with pop-sci QM is that it gives the general public the idea that this is something physicist worry about, and this is simply not correct expect for a small minority of -mainly- theorists.

 

[secur]

Thanks f95toli,

One of the questions I asked in the first post was: "I think that experimental physicists ignore MWI and work with CI by default. Is that true?"

You're answering "yes"; even more: you don't "work with CI by default", rather the issue doesn't even come up. And, you're including not only experimental, but most theoretical, physicists. Good, that's what I wanted to know.

f95toli: The problem with pop-sci QM is that it gives the general public the idea that this is something physicist worry about, and this is simply not correct expect for a small minority of -mainly- theorists.

- Fine, but extend this reasoning. If pop-sci only made people think physicists worry a lot about QM ontology, well, no great harm done. But in fact, pop-sci gives the wrong view about many important things. For instance, the public thinks, today, that multiverses really do exist; that "Quantum Suicide" is worth considering; that soon they'll be uploaded to computers and achieve immortality; a long litany of weird ideas. Pop-sci has the full intellectual authority of institutions like Caltech, MIT, University of Tel Aviv, and Cambridge behind it. Don't you agree this is not good?

Compared to books about aliens, astrology and Ramses' tomb, pop-sci is much more dangerous, given its intellectual authority. In fact pop-sci is part of the reason much of the public has given up on science and turned to mysticism.

BTW let me put in a brief "plug" for Brian Clegg, almost the only pop-sci author today who presents a more balanced, sane view.

Throughout his career, Michael Faraday gave lectures at the Royal Institute in London to inform the public about real science; many others did so, such as Humphrey Davy, but Faraday was the best of them all. These lectures drew many young people into the field; in fact without one of Davy's lectures, Faraday himself would have been a printer all his life. They had a great positive effect on society.

We need to get back to that type of public information, not mis-information. The fact that most physicists don't agree with pop-sci is very encouraging; now if you'd just say that, en masse, to your local media outlets ... that would be great!

Anyway, I see this issue is more important than I thought at first. Perhaps I should find another forum where this discussion fits better; it doesn't really belong here, does it?

 

[A. Neumaier]

Pop-sci has the full intellectual authority of institutions like Caltech, MIT, University of Tel Aviv, and Cambridge behind it. Don't you agree this is not good?

See my post in another thread, the link there, and the subsequent discussion.

 

[vanhees71]

Fine, but extend this reasoning. If pop-sci only made people think physicists worry a lot about QM ontology, well, no great harm done. But in fact, pop-sci gives the wrong view about many important things. For instance, the public thinks, today, that multiverses really do exist; that "Quantum Suicide" is worth considering; that soon they'll be uploaded to computers and achieve immortality; a long litany of weird ideas. Pop-sci has the full intellectual authority of institutions like Caltech, MIT, University of Tel Aviv, and Cambridge behind it. Don't you agree this is not good?

I'd even go further and say, it's very bad and harmful for science! First of all it gives the wrong impression about what science is about in the public and what to expect from science as an accurate quantitative description (not explanation) of the world and possible applications in engineering it might imply later.

Second young people get a wrong impression about what's science about and may be disappointed when starting to learn what's really behind science at high school (it's another topic that nowadays the high schools do not fulfill this expectation in a satisfactory way, at least not in Germany) and get distracted from their interested in science, engineering, math, and informatics. That's a pitty and even dangerous for our society since I think it's pretty evident that we need good talented offspring in these subject to solve the problems of modern life (most of all the problem to find a clean source of energy).

 

[atyy]

Then there is of course also another level of thinking, e.g., when you prepare a lecture about quantum theory. Here, I also favorize what I call the "no-nonsense approach". One should teach physics as physics first and concentrate on the mathematical formalism and its interpretation as far as physics is concerned. So you start with some heuristics, often with a short summary about the historical development of quantum theory, starting with Planck's discovery of the "quantum of action", i.e., the fundamental Planck constant, ##\hbar##, which is the new feature of QT compared to classical physics. Usually you end up with Schrödinger's wave mechanics, treat a few important examples (harmonic oscillator, rigid rotator, hydrogen atom) and then develop the full theory in terms of the abstract Hilbert space, the algebra of observables, and so on. The only thing you need to make sense of this formalism with regard to its relation to real experiments in the lab and observations in nature is the probabilistic interpretation of the notion of state (represented by a positive semidefinite trace-class operator, the Statistical operator). That's it as far as physics is concerned, and that's what I call "minimal interpretation" (it's also what's called the ensemble interpretation).

Is there any evidence that Heisenberg thought the classical/quantum cut was absolute?

 

[craigi]

I have a few questions regarding Many Worlds Interpretation (MWI)

Terminology: QM is just the math, doubted by none; MWI and CI (Copenhagen Interpretation) are ontological interpretations.

David Deutsch believes MWI with certainty. He even says that it's "proven" experimentally. How many theoretical physicists go along with that? How is Deutsch's attitude viewed, generally, in the community? Do any theoretical physicists disbelieve MWI with certainty?

Stephen Hawking has used MWI model for some theoretical work (right?). He's in situations where (arguably) no "collapse of the wave function" would occur, like Big Bang or Black Holes. In that case CI is equivalent to MWI anyway, isn't it? He puts special emphasis on reversibility of physical laws: no information is ever lost. MWI fits this requirement, but "collapsed" wave function violates it. Is that why he uses MWI? How prevalent is this approach in theoretical cosmology generally?

Does MWI facilitate the attempt to unify GR and QM?

I think that experimental physicists ignore MWI and work with CI by default. Is that true?

These questions are not about physics per se but the culture of physicists, so aren't ideal for this forum. In defense of them, let me make the following points.

- The overall issue is important in terms of the relation of physicists to society
- They concern mainstream physics or at least close to that
- They are not speculative or argumentative
- There's nowhere else to ask them, to get authoritative answers without impassioned argument (I have experimentally proven that, elsewhere on the net and at local universities)

Take these questions as merely guidelines, no need to go through them "bullet by bullet". Most important, please correct any misunderstandings evident in the questions.

I don't want to get involved in this in depth again because some on this sub-forum are wedded to their views and tend get upset about the interpretations of QM, but since you mention Hawking and Deutsch in the context of the MWI, I want to add that Hawking has described the MWI as "trivially true" and Deutsch has published on how the MWI could be tested. He argues that in all likelihood the test could be performed within the next 100 years. Regarding how the interpretations of QM are perceived amongst theoretical physicists, Tegmark has conducted impromptu surveys of the attendees of his talks and has found that the MWI is better received. I'll add one comment of my own. Be careful when discussing an interpretation of QM, that you're not discussing with someone, their own interpretation of the interpretation which you'd like to discuss.

 

[secur]

Thanks A. Neumaier, vanhees71,

Although I pride myself on quickly scanning a lot of material to get the gist of it, the referenced thread, "Quantum mechanics is not weird, unless presented as such" is too much! 28 pages, then blocked! This is a pretty hot topic.

I hope your book with the "thermal interpretation" is doing well. No doubt it's saner than others.

Drakkith posted this there: "Several argumentative and off-topic posts, and the responses to them, have been removed. I remind all members to please stay on topic and civil in your discussions."

Thumbs up to that.

My position on "quantum weirdness": it's only weird if, for some silly reason, you expect our intuitions - evolved in the macro world, for the purpose of catching prey and mates - to apply to sub-atomic particles. That view is, indeed, weird - not QM! QM behaves however the experiments demonstrate it behaves.

My position on QM ontology: a fascinating, significant puzzle, and - in other threads - I may ask questions, even put forth opinions. (Luckily, I don't have to "shut up and calculate" to earn my daily bread.) But that debate has nothing to do with the current topic, where my interest is anthropological, sociological. From this slant, QM may be less important than cosmology, where "magical" presentations threaten to turn the public off science entirely.

vanhees71: I'd even go further and say, it's very bad and harmful for science! ... young people get a wrong impression about what's science about ...

Well, that's my concern. Re. MWI (which may be right, or not, or undecidable) I sense a movement afoot to make it the establishment position. Some of you who want to ignore the whole thing may conceivably find getting tenure (or whatever) someday requires placing your left hand on "The Beginning of Infinity" and swearing allegiance to the great and holy Multiverse Quantum Field. Exaggeration, sure; but mark my words. Thus the "magic" hard-sell can affect you guys professionally, not just the public. Remember how String Theory orthodoxy shut down string alternatives (not that there's anything wrong with Strings!)

On this forum I see many posts say something like this: "forget that stuff you heard in that book. Here's the facts: ..." It would be better if they came here "knowing" nothing at all - no unlearning to do.

Suppose 100, or 1000, working-stiff physicists signed some sort of proclamation against the "magicians". Publicized it, made themselves available for interviews with local media, promoted this great web-site as a place to get the real low-down. You'd need some sexy headlines, similar to the "einstein was wrong" sort, but "Deutsch, Greene, Hawking (etc) are wrong" instead. I hate to single them out - in fact I hate to single anyone out - and you do too; but it would catch attention. Obviously you don't want any part of such a "movement". You've got work and life keeping you busy, like all of us (except, admittedly, retirees). But - what do you think?

 

[secur]

thanks for response craigi, here are some of your comments and my responses,

craigi: some on this sub-forum are wedded to their views and tend to get upset about the interpretations of QM,

- I'm dreaming of a more civilized approach which would turn down the heat

I want to add that Hawking has described the MWI as "trivially true"

- looked that up; Hawking is having his cake and eating it too. He means it's true because calculations give correct results. But of course that applies to any interpretation. The ontological q. - are the split-off universes "really real"? - is dodged entirely by Hawking's positivism. He wants to say they're real, because some of his colleagues think so; also, he doesn't want to say they're real, because some of his other colleagues don't think so.

... and Deutsch has published on how the MWI could be tested. He argues that in all likelihood the test could be performed within the next 100 years.

- looked that up too, it depends on technology that doesn't exist and may never. Also, if such tests are performed, CI'ers can still fit the results into their view. Admittedly if Quantum Computing, or "Conscious AGI", ever realized all its promises (or, lies, depending on your view), and it turned out 100% as Deutsch envisions, MWI would win. For now, however, his test is too hypothetical to change anyone's opinion (in my opinion).

Be careful when discussing an interpretation of QM, that you're not discussing with someone, their own interpretation of the interpretation which you'd like to discuss.

- very apt comment, I see that happen on other threads. But at the moment I'm trying to discuss not one interpretation (contra the title of the thread, "MWI questions") but interpretations in general.

I'm becoming convinced I can add nothing new to this topic! As I got into it a number of ideas popped up. For instance, ideas about the mind which must split in MWI; about counterfactuality in "Coherent Histories"; about what causes the "collapse" in collapse theories. But when I googled, or started to ask a question here on PF (a little list of previous q's pops up, great feature), I found someone else had beat me to it - usually by decades - and beaten it to death.

I finally thought of something I "knew" was new: the anthropological approach. Examine these divergent beliefs as belief systems, just as anthro studies religious beliefs. But turns out A. Neumaier (for instance) has had similar thoughts, and Max Tegmark, and half the population of Denmark (I don't doubt).

secur's theorem: All possible QM interpretations, and ideas about them, exist already in some universe: viz., this one.

 

[vanhees71]

Is there any evidence that Heisenberg thought the classical/quantum cut was absolute?

Well, I've my trouble to understand Heisenberg's papers, starting with his famous Helgoland paper. Maybe, I'm wrong, but as far as I understood, Heisenberg was a strong advocate of the necessity of a classical/quantum cut, but he never could come up with an objective way to define it.

 

[bhobba]

Well, I've my trouble to understand Heisenberg's papers, starting with his famous Helgoland paper. .

Same here - and its rather common:
http://www.mat.unimi.it/users/galgani/arch/heisenberg25amer_j_phys.pdf

Evidently Dirac did as well and led him to his much better formulation:
https://en.wikipedia.org/wiki/Paul_Dirac
'Dirac's attention was drawn to a mysterious mathematical relationship, at first sight unintelligible, that Heisenberg had reached. Several weeks later, back in Cambridge, Dirac suddenly recognised that this mathematical form had the same structure as the Poisson Brackets that occur in the classical dynamics of particle motion. From this thought he quickly developed a quantum theory that was based on non-commuting dynamical variables. This led him to a more profound and significant general formulation of quantum mechanics than was achieved by any other worker in this field.'

Maybe it was because:
'Dirac was famously not bothered by issues of interpretation in quantum theory. In fact, in a paper published in a book in his honor, he wrote: "The interpretation of quantum mechanics has been dealt with by many authors, and I do not want to discuss it here. I want to deal with more fundamental things."'

Thanks
Bill

 

[vanhees71]

Dirac was a very wise guy ;-).

 

[secur]

bhobba, thanks for the ref. to the excellent Aitchison et al paper. Although, can't help but mention, I can do without the hubris:

"Weinberg: Theoretical physicists in their most successful work tend to play one of two roles: they are either sages or magicians"
"Of course, it may not be possible to render completely comprehensible the mysterious processes whereby physicists ‘jump over all intermediate steps to a new insight about nature.’"

- If physicists were as smart as they think they are, they'd be smarter than ... well, than they are.

Re. Heisenberg: vanhees71, you're right about Heisenberg. I can't find a good quote to prove it, but he was definitely a "collapse"-ist. He put the cut between apparatus and system; if forced to, he'd move it to the observer. Farther than that he wouldn't go.

Re. Dirac: he wasn't the greatest physicist ever (that would be Newton) but he was the most sensible. bhobba, your Dirac quote is correct; but here's a little-known fact: towards the end of his life he changed his mind and came up with his own unique QM ontological interpretation. Read the excellent biography "The Strangest Man, Life of Paul Adrienne Maurice Dirac" (which I consider authoritative)

As long as we're at it let me correct an egregious error in Wikipedia's account of Dirac (which overall is alright).

Wikipedia: Another story told of Dirac is that when he first met the young Richard Feynman at a conference, he said after a long silence, "I have an equation. Do you have one too?". From Zee 2010.

Maybe that story is told, but it's false. What actually happened, (paraphrasing from memory, don't have the book handy):

Feynman: "How did you feel when you discovered your great equation?"
Dirac: "Hmm .. ha ... hmmm"
Feynman: {talks enthusiastically for a while about his latest work}
Dirac: Don't know much about that. {leaves}

 

[zonde]

My position on "quantum weirdness": it's only weird if, for some silly reason, you expect our intuitions - evolved in the macro world, for the purpose of catching prey and mates - to apply to sub-atomic particles.

You have to be very careful with your position if don't want to find yourself in the camp of magicians. The reason is that a lot of that macro evolved intuition goes into foundations of science. So you have to precisely identify these bits of macro evolved intuition that you consider unreliable and show that discarding them do not break scientific approach in general.

 

[secur]

zonde, depends what you mean by "magicians".

Quoted above, Steven Weinberg says "some of the most successful physicists" are magicians, e.g. Werner Heisenberg. Another example is Satyendra Nath Bose (his first paper at least), and by the way Srinivasa Ramanujan the mathematician would belong in this camp, for sure. I would love to find myself in such company!

But you don't mean that I'm sure ...

So you have to precisely identify these bits of macro evolved intuition that you consider unreliable and show that discarding them do not break scientific approach in general.

- No, I don't consider any of my macro-evolved intuitions unreliable - in the proper arena. When setting up apparatus, running experiments, reasoning logically about them, writing papers, dealing with colleagues - the tasks of the scientific approach - our intuitions work great. We just have to scrupulously avoid applying them where they don't belong. For instance don't imagine particles as little baseballs. Don't imagine Black Holes as big baseballs, either.

Even more important (and, to put the discussion in context): the rule applies to some macro objects as well: new products of science which our intuitions didn't evolve for. Lasers, purified radium, nuclear bombs, carbon pollution ... dozens of examples.

I have a spot on my retina because some idiot applied his "intuition" to a green pointing laser. Thinking it was just a cool little flashlight he shone it in people's eyes when they were unaware, leaving us with these dead retinal cells. Marie Curie gave her life for science, you might say, by purifying radium and carrying it around in her pocket, like any other inert rock the human race had ever seen. Harry Truman's intuition told him the first nukes were just real big bombs, and bombs were made to be used, right? We all know how that turned out. When the industrial revolution hit England 200 years ago, people intuitively knew that fires are harmless when the ash vents into the atmosphere. Look how big the sky is, how little our smokestacks! What a mess they made.

You could write a book on this topic: modern products mishandled by incorrect application of "hunter-gatherer" instincts. QM is just one in a long list, but in a unique way: there's no danger per se, but you can't understand QM if you don't keep those instincts in their proper place.

However: I reject your idea that therefore these instincts are generally "unreliable". They're still reliable, and necessary, to actually do science, not to mention function day-to-day.

 

[A. Neumaier]

Pop-sci has the full intellectual authority of institutions like Caltech, MIT, University of Tel Aviv, and Cambridge behind it. Don't you agree this is not good?

I'd even go further and say, it's very bad and harmful for science! First of all it gives the wrong impression about what science is about in the public and what to expect from science as an accurate quantitative description (not explanation) of the world and possible applications in engineering it might imply later.

The problem is that both scientists and the general public at large enjoy this game too much to let it die.
See also https://www.physicsforums.com/posts/5445587 and the post after it.

 

[vanhees71]

Well, but I'd think that science itself is so exciting that you do not need to make it more exciting by rather telling science-fiction stories rather than the science itself. Science fiction is nice, and from time to time I like to read an SF story too, but then I read it as an SF story and not as a popularized explanation of science. The latter is a very difficult business, and of course you must make it somehow interesting to the general audience, simplify it as much as possible to make it understandable etc. but I think it's harmful for pure science and the legitimation of the large costs covered by tax-payer's money to a large extent if you sell science as science fiction or some sensational story of not yet explored results (like the "faster-than-light neutrinos" (->technical trouble with glas fibers and time-keeping), BICEP2's B-modes (->dust), the claim of having found some new particle way below the discovery "threshold" of ##5 \sigma## confidence level...). In the long term this destroys the credibility of science and scientists in the public, and imho the public has a right to be correctly informed about what science has to really say and not some science fiction. If you want to have science fiction you pay for watching a movie or an SF book but not billions for pure science!

 

[atyy]

Same here - and its rather common:
http://www.mat.unimi.it/users/galgani/arch/heisenberg25amer_j_phys.pdf

Evidently Dirac did as well and led him to his much better formulation:
https://en.wikipedia.org/wiki/Paul_Dirac
'Dirac's attention was drawn to a mysterious mathematical relationship, at first sight unintelligible, that Heisenberg had reached. Several weeks later, back in Cambridge, Dirac suddenly recognised that this mathematical form had the same structure as the Poisson Brackets that occur in the classical dynamics of particle motion. From this thought he quickly developed a quantum theory that was based on non-commuting dynamical variables. This led him to a more profound and significant general formulation of quantum mechanics than was achieved by any other worker in this field.'

Maybe it was because:
'Dirac was famously not bothered by issues of interpretation in quantum theory. In fact, in a paper published in a book in his honor, he wrote: "The interpretation of quantum mechanics has been dealt with by many authors, and I do not want to discuss it here. I want to deal with more fundamental things."'

Thanks
Bill

In fact Dirac was very bothered by issues of interpretation in quantum theory. He believed they would be solved by the incompleteness of quantum theory.

 

[atyy]

Dirac was a very wise guy ;-).

Yes, he was. Like most great physicists, he was concerned about the interpretational problems of quantum theory.

 

[atyy]

Well, I've my trouble to understand Heisenberg's papers, starting with his famous Helgoland paper. Maybe, I'm wrong, but as far as I understood, Heisenberg was a strong advocate of the necessity of a classical/quantum cut, but he never could come up with an objective way to define it.

Of course, because the cut is subjective.