Nobody understands quantum physics?

In summary, Feynman's statement that nobody understands quantum mechanics is often quoted as a witty remark, but it highlights the fact that quantum mechanics is not understood in the same way as classical mechanics. Quantum mechanics is a theory that does not assign values to all observables in the absence of measurement, unlike classical mechanics. However, it is still considered the most well-understood and rigorously tested theory ever. There have been many interpretations of quantum mechanics, and the "measurement problem" is still being debated. But for most practical applications, the minimal interpretation is sufficient. It is possible that new observational facts may one day lead to a major revision of quantum theory, as happened with the development of quantum electrodynamics. Despite these debates, quantum
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Listen just 1 minute, what does it mean when he said nobody understand quantum mechanics?
This sound like comedy

 
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  • #2
This is one of Feynman's witty remarks, quoted again and again. Feynman, of course, contradicted himself, delivering a beautiful exposition of QM in vol. 3 of his Feynman lectures (and even with the no-nonsense interpretation, called the "shut-up-and-calculate interpretation", which in my opinion is the best interpretation to be used in the introductory QM lecture ;-)).

I think, to the contrary, that QM is the best-understood and most rigidly tested theory ever, because of the apparent "weirdness" of this theory!
 
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  • #3
John Mcrain said:
what does it mean when he said nobody understand quantum mechanics?
It means that we do not understand it in the same sense in which we understand classical mechanics. Classical mechanics is a theory that assigns values to all observables, even in the absence of measurement. It usually assigns them in a deterministic manner, but there is also stochastic classical mechanics which assigns them in a probabilistic manner. Basic quantum mechanics (QM), on the other hand, does not assign such values in the absence of measurement, neither deterministically nor probabilistically. Some interpretations of QM go beyond basic QM and assign values for some variables even in the absence of measurement, but we don't know which of such interpretations, if any, is right.
 
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  • #4
It's simply that in QT the states have a different meaning than in classical physics, and this meaning is in better accordance with the phenomena than classical physics. That may be against our intuition, but that's no argument against theories which fit the phenomena better than others, which are more intuitive. So to the contrary of this claim, I think with QT we understand the phenomena better than with classical physics.
 
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  • #5
vanhees71 said:
QM is the best-understood and most rigidly tested theory ever,
For Feynman, in his famous and only half-joking remark, "to understand" certainly meant more than "being able to calculate". And in this sense, it can be argued that even Maxwell did not "understand" electrodynamics. Hertz could do the calculations and discover electromagnetic waves. But neither he nor Maxwell were fully aware of the theory's conflict with classical mechanics, and especially the traditional concept of time. One can argue that electrodynamics was fully understood only after 1905, after the work of Einstein and Minkowski.

Two physics revolutions are said to have occurred in the 20th century: relativity in 1905 and quantum theory in 1925. But while 1905 marked the end of a four decades long "gestation period", 1925 was the beginning of a long struggle to "understand" quantum theory and how it fits together with classical physics. How does the classical world arise from the quantum "fog"? The number of interpretations of quantum theory has grown, and the "measurement problem " is still being debated. Of course one can say that the theory is "finished" and not likely to need substantial revision (just as Maxwell's theory was "finished"), but I think that a deeper understanding of quantum theory awaits us too.
 
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  • #6
I'd rather say, how the classical fog arises from QT. You get from QT to classical physics by "forgetting enough details", i.e., by coarse graining over many microscopic degrees of freedom to describe collective macroscopic observables.

The number of interpretations of QT may have grown, but of this one only takes notice when reading the literature in a small community working on the boarder between physics and philosophy. For the application of QT within the physics (and also now in the engineering) community, one just needs the minimal interpretation. This holds even true for physicists working on the physics part of these questions like the 2022 Nobel laureats and their collaborators. Particularly there's no measurement problem but to the contrary more and more refined measurements testing more and more accurately generic quantum phenomena become possible with technological progress, and the physics part of these "foundational issues" is so solidly confirmed today that it becomes the basis of applied engineering research. Last but not least thus quantum-informatics curricula are developed for university-of-applied-science students. For all this the philsophical issues are pretty irrelevant.

All that does, of course, not imply that one day new observational facts occur, which enforce a major revision of QT. It's even likely when it comes to the big open physical (!!!) problem of quantum gravity.

To draw an analogy with Maxwell electrodynamics, of course QT enforced a revision, because one had to develop QED as a quantum extension of classical electrodynamics. In fact it started the entire discovery of quantum physics with Planck's paper on black-body radiation.
 
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  • #7
It is reassuring to hear that at least one person really understands quantum theoy. And can exclude reformulations of quantum theory of the kind that occurred with electrodynamics. :smile:
vanhees71 said:
All that does, of course, not imply that one day new observational facts occur, which enforce a major revision of QT.
I suppose you meant to write "rule out" instead of "imply". I don't envision a major revision of QT either. It is interesting to note though, that Einstein's paper does not so much focus on the Michelson-Morley experiment, as on the consistency of the theoretical pictures in different frames of reference.

vanhees71 said:
It's even likely when it comes to the big open physical (!!!) problem of quantum gravity.
I'm not sure a theory of quantum gravity needs to exist. Should we really extend the "weirdness" of quantum theory to the field of gravity? But this is drifting off topic.
 
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  • #8
WernerQH said:
Should we really extend the "weirdness" of quantum theory to the field of gravity?
The standard argument for this is that, if you have matter in a superposition of being in different positions, then the gravitational field/spacetime geometry would need to be in a superposition as well, meaning we need a quantum theory of gravity.
 
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  • #9
John Mcrain said:
.....what does it mean when he said nobody understand quantum mechanics?
In his Feynman Lectures on Physics, Volume I, Chapter 37, “Quantum Behavior”, Feynman presents in section 37–7 the first principles of quantum mechanics in a simple and concise form:

1. The probability of an event in an ideal experiment is given by the square of the absolute value of a complex number ##\phi## which is called the probability amplitude:

##P=probability##
##\phi= probability~amplitude##
##P=|\phi|^2##

2. When an event can occur in several alternative ways, the probability amplitude for the event is the sum of the probability amplitudes for each way considered separately. There is interference:

##\phi=\phi_1+\phi_2##
##P=|\phi_1+\phi_2|^2##

3. If an experiment is performed which is capable of determining whether one or another alternative is actually taken, the probability of the event is the sum of the probabilities for each alternative. The interference is lost:

##P=P_1+P_2##

Then he remarks, again in a simple and concise form: “One might still like to ask: ‘How does it work? What is the machinery behind the law?’ No one has found any machinery behind the law. No one can ‘explain’ any more than we have just ‘explained.’ No one will give you any deeper representation of the situation. We have no ideas about a more basic mechanism from which these results can be deduced.

That’s all what he means.
 
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  • #10
WernerQH said:
It is reassuring to hear that at least one person really understands quantum theoy. And can exclude reformulations of quantum theory of the kind that occurred with electrodynamics. :smile:

I suppose you meant to write "rule out" instead of "imply". I don't envision a major revision of QT either. It is interesting to note though, that Einstein's paper does not so much focus on the Michelson-Morley experiment, as on the consistency of the theoretical pictures in different frames of reference.
I wanted to say that of course QT may at some time be revised when new observations prove it wrong, but there's not the slightest hint yet.
WernerQH said:
I'm not sure a theory of quantum gravity needs to exist. Should we really extend the "weirdness" of quantum theory to the field of gravity? But this is drifting off topic.
 
  • #11
IMO saying no one understands QT is more a statement about how our minds are structured than a statement about the theory.
 
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  • #12
Richard Feynman - The Character of Physical Law -
Part 6 Probability and Uncertainty - The Quantum Mechanical View of Nature
Messenger Lectures, Cornell U, Nov 18, 1964
t=7m40s

Feynman said:
There was a time when the newspaper said that only 12 men understood the theory of relativity.
I don't believe there ever was such a time.
There might have been a time when only one man did
because he's the only guy who
caught on when he---before he wrote his paper.
But after people read the paper a lot of people kind of understood
the theory of relativity in some way or other, but more than twelve.
On the other hand, I think I can safely say that nobody understands quantum mechanics.
 
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  • #13
I think this statement has a realized potential for mischief.

It is hard to read Feynman's mind, especially since asking him is no longer an option, but it is likely that a good portion of what he meant is that our intuition, which has been developed over macroscopic objects, is ill-suited to QM. And once you get NRQM, there is relativistic QM, QFT, etc. all progressively further from our experience..

However, the fact that nobody has a complete understanding does not mean nobody understand anything.

And that's where the mischief comes in:: "You scientists with your white coats and your numbers and equations - you don't understand it any better than I do!" Which is of course nonsense.
 
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  • #14
It kinda depends on what you mean by "understanding"
 
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  • #15
Paul Colby said:
IMO saying no one understands QT is more a statement about how our minds are structured than a statement about the theory.
Certainly!
The primary purpose of a theory is to make testable predictions, and it does that extremely well.
Comprehensibility is a secondary function. :wink:
.
 
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  • #16
Well, when ever I contemplate a ball I get a mental image of a ball. Can’t help it. This is just how I’m wired. Same thing happens when I consider an electron in a box. Can’t help it. No matter how wrong I know this mental picture is, still get a mental image of a small sphere in a box. Fortunately, it’s always a black and white image because I know electrons have no color.
 
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  • #17
my electrons are always blue.....?.......you tell me
 
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  • #18
Paul Colby said:
IMO saying no one understands QT is more a statement about how our minds are structured than a statement about the theory.
I think that it might be even less than that. It's not a so much a statement as an invitation to a rather sterile debate about the meaning of the word "understanding".
 
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  • #19
Paul Colby said:
Well, when ever I contemplate a ball I get a mental image of a ball. Can’t help it. This is just how I’m wired. Same thing happens when I consider an electron in a box. Can’t help it.
There was a time when you likewise thought Santa Claus was real. But the more you learned, the more your understanding evolved.
 
  • #20
DaveC426913 said:
There was a time when you likewise thought Santa Claus was real. But the more you learned, the more your understanding evolved.
Well, it’s more than that. These mental pictures are quite involuntary. No matter how hard I try, I will always picture the spin as having a value even when we must concede based on experiments that no such prior assignments are possible.
 
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  • #21
There are equally vexing problems in classical physics. For instance, identical particles: How identical is identical enough? Quantum mechanics neatly solves them. I won't make a list here.
I find QM fundamentally more appealing and perhaps less vexing.
 
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  • #22
Another problem is the radiation reaction of point particles in classical electromagnetics. There's no consistent description. The best approximation seems to be the Landau-Lifshitz approximation to the Lorentz-Abraham-Dirac equation. QED solves this problem.
 
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  • #23
Paul Colby said:
Well, it’s more than that. These mental pictures are quite involuntary. No matter how hard I try, I will always picture the spin as having a value even when we must concede based on experiments that no such prior assignments are possible.
But then you can get misleading intuitive ideas, which you must then correct when doing the correct calculations! It's better to forget wrong intuitions in learning natural sciences. In fact, it's the job description of the natural scientist to exorce wrong intuitions by doing research (and hopefully also teaching his or her findings to the following generations of STEM students).
 
  • #24
vanhees71 said:
But then you can get misleading intuitive ideas, which you must then correct when doing the correct calculations! It's better to forget wrong intuitions in learning natural sciences. In fact, it's the job description of the natural scientist to exorce wrong intuitions by doing research (and hopefully also teaching his or her findings to the following generations of STEM students).
Yes, I understand and accept QM and it’s formalism. What I’m suggesting is that flawed mental images and concepts persist even when one is ignoring them. This has less effect on people formally trained. Still, as humans, the underlying behavior of macroscopic systems is very much hard wired like it or not.
 
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  • #25
We should probably think about this separately for quantum mechanics (the theory), versus the actual "quantum" stuff (the reality).

In an absolute sense, the former can be argued to be trivial, and the latter can be argued to be impossible.

But maybe an absolute sense isn't really that helpful. To advance physics, maybe we need to have some ideas about what the theory implies about reality that go beyond its predictions so that we know what kind of things to try next. I.e., maybe there can be things which are not absolute predictions of a theory, but which are hinted at by a theory. Some of those things might be testable and some not, and we might not be able to tell yet. Our ideas about these things are not random. That kind of understanding is probably fundamentally limited (cannot be absolute) but probably isn't completely absent or impossible. Understanding isn't binary.
 
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  • #26
The "actual quantum stuff" in fact IS not impossible but it describes how Nature behaves. It's the classical description that is flawed outside the realm of its applicability. After all, that's why QT has been discovered 98 years ago by Born and Jordan, following an idea by Heisenberg!
 
  • #27
In “BOOJUMS ALL THE WAY THROUGH: Communicating Science in a Prosaic Age” (first published 1990), N. David Mermin remarks in chapter 14 “The philosophical writings of Niels Bohr”:

"Typical quantum effects, Bohr notes, resist pictorial representations. Nor should this surprise us, since our ability to construct such representations or explanations in ordinary language was entirely developed in the course of coping with classical phenomena. Bohr repeatedly insists that we must therefore be content with a purely symbolic mathematical algorithm, which connects one classically specified set of conditions to another. This formalism offers no explanation in the customary sense, but by embracing all possible experimental arrangements it demonstrates the logical consistency of the entire scheme, which is all we can demand of it.
 
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  • #28
Bohr is right that the only adequate language to discuss physics is math. Only math is precise enough to communicate what we know by observation and from building mathematical theories ordering these observations in terms of fundamental general laws of nature. That's true not only for QT but also classical physics.

I think it's, however, wrong to expect "explanations" of phenomena from the natural sciences. What natural science does and is restricted to are descriptions of what is observed in Nature and the attempt to build theories and models which enable the derivation of these descriptions from an as few a number of "fundamental laws". So "explaining" some phenomenon "by physics" simply means that we understand the phenomenon from using the current theory of "fundamental laws" we have extracted in a complicated interplay between making ever more accurate observations and find ever more detailed theories.

With progress of science thus necessarily these "explanations" must change. Indeed, it has been discovered by Planck that despite great struggles one cannot explain the observed black-body radiation spectrum using the known classical theories of mechanics and electrodynamics but has to make the "quantum assumption" about the emission and absorption of electromagnetic energy by matter in "energy quanta" ##h \nu=\hbar \omega##, and from this in just 25 more years a new theory, "modern quantum theory", has been found, which now delivers much more "explanations" for the observed behavior of matter, and not only at the atomar and subatomar scale but also for the ordinary everyday macroscopic matter around us. In fact one should be aware that given the discovery of the atomistic structure of matter from classical physics is completely "unexplanable" why matter can be stable at all and why there are the accurately defined elements consisting of completely indistinguishable atoms, which build competely indistinguishable molecules and so on up to the "macroscopic matter" surrounding us and finally we ourselves are made of.

It is not clear to me, what's "more symbolical" or "more abstract" in the mathematical formalism of quantum theory than in the mathematical formalism of classical mechanics. All math is abstract. Maybe Bohr means that Newton's postulates are easier to understand since we are (usually pretty unconciously) using the definition of reference frames (i.e., a spacetime model) to temporally and spatially order "events", i.e., when we want to meet at a certain time at a certain place we use already some fundamental laws of nature as discovered first by Galilei and Newton, but that's not less abstract a description than using QT to describe the hydrogen atom, it's only less familiar to us to deal with single hydrogen atoms than to deal with macroscopic matter around us, and it shouldn't be too surprising that at such tiny scales needed to resolve the "inner workings" of atoms needs another description than to understand a macroscopic amount of hydrogen gas, where the atomistic scale is pretty irrelevant, and we use a macroscopic description in terms of thermodynamics (temperature, pressure, volume of the container) to describe hydrogen gas. So that "classical physics theories" seem to be "less abstract" than QT is just due to the simple fact that we are more experienced in dealing with macroscopic objects in our everyday life than in dealing with single particles, where the "quantum phenomena" are more pronounced.
 
  • #29
Feynman fails to understand that idealists, even if very few, understand QM inside out.
 
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  • #30
vanhees71 said:
Maybe Bohr means that Newton's postulates are easier to understand since we are (usually pretty unconciously) using the definition of reference frames (i.e., a spacetime model) to temporally and spatially order "events"
That's all he meant. If one reads his essays he usually says that in the following sentence.

Similarly here if you listen to the talk, all Feynman means is that the probability calculus of QM is not reducible to some "visualisable" classical events playing out like you said.

At least for me when you actually read these people it's pretty clear what is meant, but it gets over analysed like it was novel by Proust. This one in particular is an off-hand joke fairly clear in context. There would be similar "mysterious" remarks in works by Hamilton, Einstein on Relativity and so on if they were over-analysed in this way.
 
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  • #31
The problem with Bohr is that he is so unclear in his writing that it invites such "philosophing" about "what might the author have wanted to say", and that's why QT till today is often displayed as something mystic. I find Bohr' and Heisenberg's writings did a bad job in "interpreting QT", because they were too philosophical rather than to wrap off all the dust of the unclear quarter of a century of "old quantum theory", where you had no clear picture of how to describe "quantum phenomena", leading to indeed vague and inconsistent pictures like "wave-particle duality", which was substituted by Bohr just by the even more obscure "complemantarity principle". I think with modern QT there's no need for such philosophical distortions of a very elegant and clear mathematical formulation which just does what all deep physical theories do, i.e., summarizing many empirical facts into a scheme of a few generally valid basic principles, with which all these and (hopefully) many to be discovered phenomena in the future can be described.

It's like with Newton's principia: There were a lot of empirical facts about the motion of planets, moons, and the Sun, including very accurate ones like Kepler's Laws, but it could be simplified by building a theory which systematically reduced the necessary basic principles to a minimum set of "fundamental laws", i.e., Newton's postulates/axioms, clear definitions of the relevant quantities like time, position, mass, force, etc. as well as the general law of the gravitational interaction, which could be formulated based on the clear definitions due to the postulates and then proven to be indeed generally valid (until GR refined the description even more). Also this "reduction" of many empirical findings to a few fundamental laws, from which "the phenomena could be derived" went along with a higher level of abstraction, although in Newton's case it's not so obvious, because the use of Euclidean geometry is very familiar to us from elementary school on. The even more abstract math of "infinitesimals" and analytical geometry was again a step to the use of more abstract descriptions, but making the whole description even more powerful.

You can go on in the history of physics and find that the more general and the more comprehensive you get with the theories the more abstraction is needed. Today we use pretty abstract concepts like group theory and topology to describe Nature with more and more accuracy and larger and larger realms of validity, and I'm pretty sure that the solution of the remaining puzzles (on the most fundamental level, that's for sure a fully self-consistent quantum theory including gravitation) will enforce the use of ever more abstract ways of thinking.
 
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  • #32
LittleSchwinger said:
That's all he meant. If one reads his essays he usually says that in the following sentence.
Maybe? Who knows! My impression is that it is Bohr's own fault that his writtings are misunderstood. My impression is that von Neumann's writtings are misunderstood too, but that it is much harder to blame von Neumann himself for that.

LittleSchwinger said:
Similarly here if you listen to the talk, all Feynman means is that the probability calculus of QM is not reducible to some "visualisable" classical events playing out like you said.
Feynman speaks very clear, and he is normally understood correctly. And if people "quote" him wrong or out of context (like "shut up and calculate (Feynman)"), then they often do it intentionally to promote some sort of agenda.

Just saying. It is nice to try to defend Bohr, because if you are able to understand what he was trying to say, then you see that he had a very good understanding of how nature works. But to imply that understanding Bohr would in any way be similar to understanding Feynman, that is frankly ridiculous.
 
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  • #33
I think the following nicely describes the issue with Bohr:

https://physicsworld.com/a/the-bohr-paradox/

Feynman for sure is another caliber. Here's always very clear and a role model of a "no-nonsense physicists" and also obviously a very diligent teacher as long as you restrict yourself to his scientific writings (research papers but also real textbooks). Of course, one should not take his popular-science writings at phase value, because you cannot expect even a genius like Feynman to get the science write without the use of the only adequate language to communicate it, i.e., math. Obscurity comes usually always due to the avoidance of the adequate mathematical language.
 
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  • #34
vanhees71 said:
Bohr is right that the only adequate language to discuss physics is math.
I think mathematics is obviously the only way to make quantitative statements anyway as it will involves measures and numbers in some form.

My own emphasis on Bohrs legacy is not the math. It is needed always, you cant get around that. Its just as natural that trying to find a way to speak quantitativley about rational "degress of belief" leads to some sort of probabality theory.

"purely symbolic mathematical algorithm, which connects one classically specified set of conditions to another."

For me the main insight from Bohr is that the quantitative description of even the subatomic domain (only indirectly observable via perturbation and monitoring the response) must be defined in terms of relations between the classical variables, which we at least in principle can know objectively by weak interations that does not alter the variable in question.

This is also the problem of CH - you need a "macroscopic/classic" context to even define QM as it stands. We have this in subatomic physics so its right on as long as we stay away from gravity cosmology. This should be an insight to that entertain the idea about a qantum state of the the whole universe.

I always interpreted Bohr as clear and honest. I always thought that as beeing the first generation into QM, he perhaps got the relation to classical mechanics best.

/Fredrik
 
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  • #35
vanhees71 said:
The problem with Bohr is that he is so unclear in his writing that it invites such "philosophing" about "what might the author have wanted to say", and that's why QT till today is often displayed as something mystic
Nice post. It probably just comes down to what styles of writing fit with one personally. One of my personal favourite essays on quantum theory is Schwinger's at the start of his "Symbolism of Atomic Measurement" book.
 
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