A Snapshot of Foundational Attitudes Toward Quantum MechanicsThey kind of repeated a 1997 poll organized by Max Tegmark (among 48 participants of a conference at that time) but among 33 participants of a recent quantum-foundations conference.

*Off-topic. Greatest snowfall in Jerusalem since 1992. President Peres teaches the Israeli how to keep snowmen from getting cold.*

The main result is that there is no consensus on anything. I was actually pleased because I was expecting that there would be a consensus on the wrong answers because that "subdiscipline" of physics is completely screwed; instead, the correct answers were mostly the strongest ones. What did they ask?

**Randomness of quantum processes (e.g. decay of nuclei) is:**

**64%: fundamental****48%: irreducible**- 9%: only apparent
- 0%: masking a hidden determinism

At any rate, in politics, 64% for "fundamental randomness" would be a solid majority.

**Do objects have well-defined properties before measurements?**

- 52%: yes, in some cases
**48%: no**- 9%: undecided
- 3%: always

At any rate, at least roughly 1/2 of the folks has an idea about this important point which is not too bad.

**Einstein's view on quantum mechanics is:**

**64%: wrong**- 12%: will be shown wrong
- 12%: we don't know
- 6%: will be shown right
- 0%: is right

**Bohr's view on quantum mechanics is:**

- 30%: we have to wait
- 27%: wrong
**21%: correct**- 9%: will be shown right
- 3%: will be shown wrong

**A similar split appears when it comes to the measurement problem:**

- 39%: solved (now or later) in a different way
**27%: a pseudoproblem**- 27%: none of the above
- 24%: a severe difficulty threatening QM
**15%: solved by decoherence**

But if you don't care where exactly this boundary lies, you may simply assume that the boundary is somewhere. The microscopic world clearly needs the full quantum mechanics; and for some large objects, the classical logic if not physics becomes acceptable. Bohr et al. knew that so they pretty much postulated this dichotomy – one that we may derive more clearly by decoherence today. If you don't care about the location of the boundary and you're OK with the previous answer "no objective reality prior to the measurement", I think that you must conclude that the measurement problem is a pseudoproblem.

**What is the message of the violation of Bell's inequalities?**

**64%: local realism is untenable**- 52%: unperformed experiments have no results
- 36%: some notion of nonlocality
- 12%: action at a distance in the physical world
- 6%: let's not jump the gun, take loopholes seriously

**Quantum information is:**

- 76%: fresh air in quantum foundations
- 27%: we need to wait
**6%: useful for applications but of no relevance to foundations**- 6%: neither useful nor relevant

The answer "fresh air" could also be fine in the sense that the quantum-foundation people may be stuck with rubbish and crackpot non-quantum theories trying to "explain" quantum mechanics – so if they do something that isn't wrong, e.g. quantum algorithms, it's a breath of fresh air. But I am not sure whether this was what was meant by the "fresh air".

We must also wait and see to decide whether they will be constructed and whether it will be soon etc. If something will stop us for a very long time, we could also say that they're not "useful". So all answers have some potential to be OK. Much of it depends on the next question:

**When will we have a working and useful quantum computer?**

- 9%: within 10 years
- 42%: 10-25 years
- 30%: 25-50 years
- 0%: 50-100 years
- 15%: never

Some unusual complaints against the quantum computation were recently discussed by Scott Aaronson: response to Dyakonov, Zork's blogorhythm

**Right interpretation of state vectors:**

**27%: epistemic/informational**- 24%: ontic
**33%: a mix of epistemic and ontic**- 3%: purely statistical as in ensemble interpretation
- 12%: other

I believe that all the people who answer "ontic" really mean that the wave function is conceptually a set of classical degrees of freedom. That's why this answer is just wrong. All other answers may be partly OK. Epistemic/informational is OK as a "negation" of the wrong "ontic" answer". However, "epistemic" could also lead one to believe that there exist other hidden variables not included in the wave function – i.e. that the wave function is an incomplete description. That's wrong as well. The wave function isn't "ontic" in the classical sense but it's as complete – and, in this sense, as close to "ontic" – as you can get. So you could vote for some "mix" or for "other" which means that you're not quite satisfied with any answer that was given.

I chose not to label the "ensemble interpretation" as correct because the ensemble interpretation makes the claim that only the statistics of the huge repetition of the very same experiment may be predicted by quantum mechanics. This is a very "restricted" or "modest" claim about the powers of quantum mechanics and this modesty is actually wrong. Even if I make 1 million completely different experiments, quantum physics may predict things with a great accuracy.

Imagine that you have 1 million different unstable nuclei (OK, I know that there are not this many isotopes: think about molecules if it's a problem for you) with the lifetime of 10 seconds (for each of them). You observe them for 1 second. Quantum mechanics predicts that 905,000 plus minus 1,000 or so nuclei will remain undecayed (it's not exactly 900,000 because the decrease is exponential, not linear). The relatively small error margin is possible despite the fact that no pair of the nuclei consisted of the same species!

So it's just wrong to say that you need to repeat exactly the same experiment many times. If you want to construct a "nearly certain" proposition – e.g. the proposition that the number of undecayed nuclei in the experiment above is between 900,000 and 910,000 – you may combine the probabilistically known propositions in many creative ways. That's why one shouldn't reduce the probabilistic knowledge just to some particular non-probabilistic one. You could think it's a "safe thing to do". However, you implicitly make statements that quantum mechanics can't achieve certain things – even though it can.

**The observer is:**

**39%: a complex quantum system****21%: should play no fundamental role whatsoever****55%: plays a fundamental role in the application of the formalism but plays no distinguished physical role**- 6%: plays a distinct physical role (soul collapses wave function...)

While the observer is clearly just another complex physical system and plays no distinguished physical role (so the third answer is also OK), it may play a role in the application of the formalism. However, in synergy e.g. with the consistent-histories approach, we may attribute the "subjective choices" to the consistent histories themselves (a framework listing the questions) rather than the observer as an object. For this reason, it looks legitimate to me if someone says that the observer plays no fundamental role whatsoever.

The denial of the "complex quantum system" by 61% of the participants is the most shocking feature of the answers here.

**Reconstruction of quantum theory:**

- 15%: gives useful insights and has/will supersede the interpretation program
- 45%: gives useful insights but we still need an interpretation
**30%: cannot solve the quantum foundations**- 27%: will lead to a deeper theory than QM
- 12%: don't know

I also think that the very hypothetical strategy of "reconstruction" is a misunderstanding of the scientific method in general. The right wisdom about Nature can't be directly "reconstructed" by any well-defined procedure. In science, one has to formulate hypotheses at the beginning, and then test them. The "guesswork" (which can't be mechanical) is a necessary part of the process; the creativity and ingenuity of the physicists matters exactly because the "guesswork" matters. This is related to various logical arrows: the evolution of the Universe is a consequence of the laws of physics, not vice versa, so to determine the laws of physics from the observations of the evolution is an "inverse problem" that simply can't have a straightforward solution. It has to be "inferred" by Bayesian inference. That's why the answers always depend on some priors, too, although the dependence may become weak enough once a sufficient amount of evidence is collected and processed.

**Favorite interpretation:**

**0%: consistent histories****42%: Copenhagen**- 0%: de Broglie-Bohm pilot wave
- 18%: Everett many worlds/minds
- 24%: information-based
- 0%: modal
- 9%: objective collapse, GRW or Penrose
- 6%: quantum Bayesianism
- 6%: relational quantum mechanics
- 0%: ensemble interpretation
- 0%: transactional interpretation
- 12%: other
- 12%: no preferred one

**How often have you switched interpretation?**

- 33%: never
- 21%: once
- 21%: several times
- 21%: no preferred interpretation

**Does the choice of interpretation depend on philosophical prejudices?**

**58%: a lot**- 27%: a little
- 15%: not at all

**Superpositions of macro- different states are:**

**67%: possible in principle**- 36%: will eventually be realized
- 12%: in principle impossible
- 6%: impossible because of collapse theory

**In 50 years, conferences on quantum foundations:**

- 48%: will still be organized
- 15%: probably no
**24%: who knows**- 12%: I organize one no matter what

The authors also discuss some correlations between the answers, probably kind of obvious ones... TRF readers voted overwhelmingly "NO", just like I did.

Good of you Lubos to share your thoughts

ReplyDeleteand feelings about these (as well as may other) topics or aspects of What Is going on.

I experienced this overview to be both interesting and satisfying!

As if anybody would be interested in my opinion: I find that the answer with a "No" to "Do objects have well-defined properties before measurements?" to be most obviously required (for most trivial and almost boringly realistic and obvious reasons that are built into the question) to be answered so.

Lastly, I have a strong sense that scientists' striving to build Quantum Computers will never bear fruit in any other way that they might generate amazingly useful spin-offs along the way (as if to the pot of computing gold at the end of a rainbow).

ReplyDelete"Bohr's pronouncement[s] often lacked the rigor and accuracy to be completely certain that he meant the right thing."Dear Lubos, this shocks me. I had been under the impression from reading TRF that Niels Bohr's language was always crystal clear for two reasons, namely that his understanding of nature was the most profound. This is why he won every argument with Einstein, hands down. And also, that he was a meticulous man, a real stickler about precision in language, even more so than other physicists. Even if quantum mechanics is not the hardest thing to wrap your head around (that distinction probably should go to mathematical theorems that only a few humans can understand), it may be the most counter-intuitive. Hence I had privately concluded that Bohr, not Newton, may have been the smartest man who ever lived. (Even if I sound sarcastic or satirical, I am not; I am being serious.)

So in your opinion, which 20th century physicist was (1) the most unambiguous in his or her expression and (2) the best communicator of his or her research?

Minor typo: and it always asks answers "because we derived it"

Well, Eugene, I think that the "most profound" understanding and "crystal clear communication skills" are two totally different things and I would prefer to associated Bohr with the former, not the latter.

ReplyDeleteI think he won the debates with Einstein because his physics was right, not because he was a great speaker.

I did not get (what the word "irreducible" means in the context of the first question either ... :-/. Reading this term I am rather thinking about group theory...

ReplyDeleteWith the quantum measurements I have no problem :-P

I always thought that quantum information is just an application of QM

The interpratation of state vector question I did not understand (to many confusing probably philosophical terms in the answers) before reading the TRF explanation, why do the state vectors have to be interpreted ...?

About the reconstruction of quantum theory I have never heared before ...

Why do they dont like the consistent histories? I like this since I have read a certain nice TRF article about it :-)

I enjoyed reading this poll and the explanations :-)

Dear Eugene, someone knowledgeable with debates between Bohr and Einstein wrote the following: ``Bohr was inconsistent, unclear, willfully obscure and right. Einstein was consistent, clear, down-to-earth and wrong.''

ReplyDeleteDear Lubos, thank you so much for this blog. I am a regular reader who enjoys your explanations. One question on quantum foundations: Though I adhere to the "no objective reality prior to the measurement" view, the fact that weak measurements (Aharonov) are possible has caused me to begin to doubt it a bit. What is your take on this issue? Maybe you can enlighten me, thank you :)

ReplyDeleteI am not clear how more than 100% can arise for answers on, for example, whether objects have well defined properties before measurement.

ReplyDeleteYes, no, sometimes and undecided appear mutually exclusive. Same for other questions. Why is this?

Dear Martin, thanks for your compliments. Weak measurements aren't real measurements - what they measure depends on the precise protocol, not only on the "state of the system". Some extra comments on weak measurements and what's wrong with their usage as arguments against postulates of QM such as the uncertainty principle:

ReplyDeletehttp://motls.blogspot.cz/2012/09/pseudoscience-hiding-behind-weak.html?m=1

I was also puzzled. Maybe the respondents couldn't even realize that their answers were exclusive and they got the freedom to check many boxes.

ReplyDeleteWhat if there is some observer who doesn't know anything about you. Maybe it's an observer in the past who died before you were born. That observer couldn't have been able to measure you. Subjectively, are you nonexistent to that observer?

ReplyDeleteOf course, everything that doesn't exist *now* should be considered non-existent by an observer now. Well, there are different kinds of existence, some of them allow past tense, some of them only allow the present tense, and those that allow the future tense are all speculative and can't be taken seriously.

ReplyDeleteIf I say that children born in the year 2200 don't exist, is it really so controversial? We don't know whether it makes sense to talk about such children - whether there will be mankind. At any rate, they don't exist now. In the same way, we don't exist for people who lived in the past.

Taking QM seriously, presentism is the only possible choice, eternalism is not. The free will theorem makes this point extremely robust.

But this is a lot of convention-dependent babbling, anyway. It's not physics. To make the proposition "A exists" well-defined, one has to have an operational procedure how to measure. Be sure that whatever sensible choice you determine, it will agree that not-yet-born children don't exist.

The problem with presentism is the problem of now. Is now 2013, or 1900, or 2200?

ReplyDeleteDear Quantum, now is 2013. Thank me very much, you are welcome. Are you joking? ;-)

ReplyDeleteAt different moments, people could or will be allowed to say that they ear 1900 or 2200. But those are not us now. It's the very point of presentism that the existence depends on who and when is talking about the existence.

Tell me, if existence is subjective, can nonexistent beings subjectively say that they exist?

ReplyDeleteQuite on the contrary, Quantum, your question makes a perfect sense and I have repeatedly answered this question, most specifically here:

ReplyDeletehttp://motls.blogspot.com/2012/11/why-subjective-quantum-mechanics-allows.html?m=1

The answer is that the contradiction you suggest - that non-existence entities may behave as existing ones - can't occur. But the right proof within quantum mechanics that such contradictions can't occur does *not* boil to the assumption about the existence of objective reality, an assumption that is wrong. But science or physics didn't collapse when this assumption was shown to be wrong almost 90 years ago. Everything that worked still works.

Tell me, what is existence?

ReplyDeleteIt's either a trivial noun (derived from a verb) that everyone understands, or it's something that is ill-defined physics and I don't need this concept for anything in physics.

ReplyDeleteCould you please stop wasting my time with similar idiotic comments? Thanks.

Is Quantum the size of your brain ?

ReplyDeleteOh Shannon,

ReplyDeleteooouuch, LOL :-D !!!

"The most correct interpretation, consistent histories, was picked by 0

ReplyDeletepercent of the participants. What a pack of assholes, politely speaking.

;-)" Ah, Motl's mellowing :-)

:-D

ReplyDeleteEqually strange is that 64% accept that " local realism" is excluded but none accept consistent histories.

ReplyDeleteI may be incapable of the details but it certainly seems that if one is willing to abandon locality or better yet objective reality that consistent histories provides the best "picture". Certainly it. Is what I thought made sense but I leave it to those with better brains than I. I was about to make a wisecrack about the utter stupidity of many worlds but in this universe that would add nothing. You recently had an excellent post totally demolishing the many worlds interpretation so I assume your readers fully appreciate that among experts it is strange that many worlds would get any support regardless whether consistent histories is or not the correct approach.

Lubos, what's your opinion on observers being classical souls outside of our universe in Heaven collapsing the wave function?

ReplyDeleteLubos, are you in a quantum superposition right now? Pretend I'm Wigner and you're my friend. ;-) ;-0

ReplyDeleteThere are no exactly classical objects in our Universe and there is no role for "souls" and "Heaven" or "things outside the Universe" [oxymoron] in physics so "classical souls" is the doubly or triply idiotic "hypothesis".

ReplyDeleteIn my logical framework or consistent histories, the macroscopic quantities describing my body are in rather well-defined, quasiclassical states. So no superpositions.

ReplyDeleteIt is a trivial mistake for me to answer questions "how things are from the viewpoint of a different logical framework than mine". Some metaobserver may observe me and describe me by superpositions of macroscopically distinct states but I have measured myself so from my viewpoint, none of these large uncertainties exists.

The price that I pay for making these possibly premature conclusions about well-defined values of classical observables is that if and when the wave functions for these "seemingly almost decohered" degrees of freedom recohere, the resulting surprising "miracles" will look like inconsistencies in my logic because these "miracles" follow from (extremely unlikely) violations of the "consistency of the histories" that I assumed to be consistent.

If the wave function describing you recoheres, you would have forgotten your experiences, so how could you be surprised? ;-)

ReplyDeleteHow can you be so sure there are no classical souls outside of the universe looking in?

ReplyDeleteNo, I wouldn't have "forgotten" anything. I would just remember a wrong thing that is inconsistent with other facts.

ReplyDeleteBecause I followed, studied, and largely reproduced the accumulated results of at least 400 years of physics that have excluded the existence of souls, and 90 years of physics that have excluded the existence of classical objects. That's why I am "doubly sure" about the absence of "classical souls" in the physical architecture of Nature.

ReplyDeleteThere are more things in heaven and earth, Horatio,

ReplyDeleteThan are dreamt of in your philosophy.

They have only ruled out blatant forms of parapsychology. An "epiphenomenal" soul isn't ruled out.

ReplyDeleteSo, would Schroedinger's cat subjectively collapse its own superposition?

ReplyDeleteIs there any objective reality to the nonzero yet tiny off-diagonal elements of the density matrix?

ReplyDeleteThis is starting to sound dangerously close to MWI.

Dear KPM, physics isn't philosophy. It's a natural science. Could you please reduce the number of comments you are posting on this blog at least by a factor of ten? I see that you must believe that they're intelligent but it isn't necessarily so.

ReplyDeleteWhat does it mean to measure yourself? Is that the same thing as introspection?

ReplyDeleteWhat about the hypothesis that the whole universe is a dream by some soul?

ReplyDeleteThe

ReplyDeleteclaim that the ensemble interpretation gives the wrong answer for the many atom

decay example is not correct. The EI gives the same result as the CI in this

example. The assumption that is made for the erroneous claim here, is based on

a false premise as to what is the meaning of the EI. It is not correct to state

that the EI requires “ …to repeat exactly the same experiment many times” . The EI states

that the ensemble is based on “… similarly prepared systems…” that are to be performed

many times. However, this does not dictate that such similarly prepared systems

can not be done simultaneously in parallel if the systems are equivalent from a

probabilistic view. It is also not fruitful to imply that all calculations in

QM, necessarily involve the use of the logic of QM. Standard classical probability

theory must also be applied at the appropriate point. It is this standard probability

theory that allows for the many atom quantum probability to be calculated with

such precision, and this theory may equally well be applied to the EI. For

example:

Consider

throwing a dice 10 times. An estimate can then be made as to how accurate the relevant

probabilities of obtaining a specific number are. Now consider throwing 10 dice

at once, ten times. So long as the dice are non interacting, each throw can be

considered as performing 10 “similarly prepared systems” trials at once, thereby

producing an experiment that is identically equivalent to 100 “similarly prepared

systems”, but with what might be naively, incorrectly, stated as only 10 trials.

In

the millions of atom case, it is clear that each atom observation is effectively

being performed simultaneously. All atoms, statistically are the same, so this constitutes

millions of ensembles of measurements in one go. It would be quite daft to

suggest that the EI requires that one locate the very exact same electron and

pass that specific one through the detector over and over again. All that

matters is that the equivalent statistical ensembles are identified, so that

they can all be counted, whether one after the other, or all together, or a mix

of them.

In

my view, the EI is essentially the same as the CI, excepting for removal of a redundant,

experimental non provable metaphysical statement, i.e. that the state vector

represents a single system. It is a “shut up and calculate” simple number

cruncher that avoids daft ideas like two places at once.

www.kevinaylward.co.uk

The

ReplyDeleteclaim that the ensemble interpretation gives the wrong answer for the many atom

decay example is not correct. The EI gives the same result as the CI in this

example. The assumption that is made for the erroneous claim here, is based on

a false premise as to what is the meaning of the EI. It is not correct to state

that the EI requires “ …to repeat exactly the same experiment many times” . The EI states

that the ensemble is based on “… similarly prepared systems…” that are to be performed

many times. However, this does not dictate that such similarly prepared systems

can not be done simultaneously in parallel if the systems are equivalent from a

probabilistic view. It is also not fruitful to imply that all calculations in

QM, necessarily involve the use of the logic of QM. Standard classical probability

theory must also be applied at the appropriate point. It is this standard probability

theory that allows for the many atom quantum probability to be calculated with

such precision, and this theory may equally well be applied to the EI. For

example:

Consider

throwing a dice 10 times. An estimate can then be made as to how accurate the relevant

probabilities of obtaining a specific number are. Now consider throwing 10 dice

at once, ten times. So long as the dice are non interacting, each throw can be

considered as performing 10 “similarly prepared systems” trials at once, thereby

producing an experiment that is identically equivalent to 100 “similarly prepared

systems”, but with what might be naively, incorrectly, stated as only 10 trials.

In

the millions of atom case, it is clear that each atom observation is effectively

being performed simultaneously. All atoms, statistically are the same, so this constitutes

millions of ensembles of measurements in one go. It would be quite daft to

suggest that the EI requires that one locate the very exact same electron and

pass that specific one through the detector over and over again. All that

matters is that the equivalent statistical ensembles are identified, so that

they can all be counted, whether one after the other, or all together, or a mix

of them.

In

my view, the EI is essentially the same as the CI, excepting for removal of a redundant,

experimental non provable metaphysical statement, i.e. that the state vector

represents a single system. It is a “shut up and calculate” simple number

cruncher that avoids daft ideas like two places at once.

www.kevinaylward.co.uk

Dear Kevin, your comment makes no sense because the phrase "similarly prepared systems" is completely ill-defined. If I prepare a uranium-235 nucleus and a carbon-14 nucleus, they're clearly different nuclei. There may be analogies in the way how they were prepared but there are also differences. If you neglect all these differences between uranium-235 and carbon-14, you will surely get wrong physical predictions.

ReplyDeleteThe only relevant analogy here is that QM predicts probabilities of decay for both of these nuclei. If I count the number of decayed nuclei of *any* kind - each of them may be different - and the number is large, quantum mechanics predicts the number of decayed atoms within a small relative error margin. To do so, one has to use the full information "the probability of decay of nucleus #j is p_j" for each of the nuclei separately and one has to do a calculation based on these probabilities. That's why one *needs* to acknowledge that the state vector probabilistically represent a single system.

Vague phrases such as "similarly prepared atoms" can't possibly replace the calculation.

I

ReplyDeletewill have to think about your comments on the different atoms.

I

just don’t see how “probabilistically represent a single system” makes any

logical sense at all. The very definition of probability is (number of actual occurrences)

/ (total of possible occurrences). If QM makes a probability prediction, there

is no way to know if that probability is correct without effectively “repeating

the experiment”. One needs to be able to actually get a ratio of numbers. One

throw of a dice can not confirm a probability. In this way QM must use ensembles of experiments.

I did like your “the plus sign is an OR not AND”. Trivial but brilliant

observation that many seem to miss.

Aah, I real-ize it now.

ReplyDeleteLubos is a god! By the act of self-measurement, he brings himself into existence by a process of self-creation. Then, Lubos-god manifests everything else into existence by observing them, making him the Creator.

As he himself confessed, "nothing is real until it has been measured and the results reach me"..

two important points in physics: ANTIMATTER DOESN'T

ReplyDeleteEXIST IN THE UNIVERSE.THE ANTIPARTICLES ARE "EXOTICS" PHENOMENONS,IT IS ARE ENERGY GENERATED BY THE ASYMETRY OF SPACE AND TIME,COMJUGATED BY THE VIOLATION OF CP.THE RELATIVISTICS TRANSFORMATIONS OF ENERGY INTO MASS AND VICEVERSA,DERIVED OF RELATIVITY OF MOTIONS.THEN THE SPACETIME ARE SYMETRIES

THAT GENERATE THE ANTIPARTICLES AS BUNDLELED LOCALLY ENERGY IN THE QUANTIC QUANTUM.THESE ASYMETRIES IN THE SPACE AND TIME ARE "STRINGS" IN THE SPACETIME.