- #1

- 435

- 28

This sound like comedy

You are using an out of date browser. It may not display this or other websites correctly.

You should upgrade or use an alternative browser.

You should upgrade or use an alternative browser.

- Thread starter John Mcrain
- Start date

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

- #1

- 435

- 28

This sound like comedy

Physics news on Phys.org

- #2

- 24,492

- 15,000

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!

- #3

- 14,108

- 6,608

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,John Mcrain said:what does it mean when he said nobody understand quantum mechanics?

- #4

- 24,492

- 15,000

- #5

- 500

- 341

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.vanhees71 said:QM is the best-understood and most rigidly tested theory ever,

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

- #6

- 24,492

- 15,000

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.

- #7

- 500

- 341

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 thevanhees71 said:All that does, of course, not imply that one day new observational facts occur, which enforce a major revision of QT.

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.vanhees71 said:It's even likely when it comes to the big open physical (!!!) problem of quantum gravity.

- #8

Mentor

- 44,916

- 22,251

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.WernerQH said:Should we really extend the "weirdness" of quantum theory to the field of gravity?

- #9

Gold Member

- 1,023

- 917

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:John Mcrain said:.....what does it mean when he said nobody understand quantum mechanics?

##P=probability##

##\phi= probability~amplitude##

##P=|\phi|^2##

##\phi=\phi_1+\phi_2##

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

##P=P_1+P_2##

Then he remarks, again in a simple and concise form: “

That’s all what he means.

- #10

- 24,492

- 15,000

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:It is reassuring to hear that at least one person reallyunderstandsquantum theoy. And can exclude reformulations of quantum theory of the kind that occurred with electrodynamics.

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 thetheoreticalpictures in different frames of reference.

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

Gold Member

- 1,508

- 452

- #12

- 6,838

- 2,278

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.

- #13

Staff Emeritus

Science Advisor

Education Advisor

2023 Award

- 32,780

- 19,174

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.

- #14

Science Advisor

Homework Helper

Gold Member

- 6,735

- 2,447

It kinda depends on what you mean by "understanding"

- #15

Gold Member

- 22,380

- 6,076

Certainly!Paul Colby said:

The primary purpose of a theory is to make testable predictions, and it does that extremely well.

Comprehensibility is a

.

- #16

Gold Member

- 1,508

- 452

- #17

Science Advisor

Homework Helper

- 6,445

- 5,560

my electrons are always blue.....?.......you tell me

- #18

Mentor

- 14,725

- 9,169

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".Paul Colby said:

- #19

Gold Member

- 22,380

- 6,076

There was a time when you likewise thought Santa Claus was real. But the more you learned, the more your understanding evolved.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.

- #20

Gold Member

- 1,508

- 452

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.DaveC426913 said:There was a time when you likewise thought Santa Claus was real. But the more you learned, the more your understanding evolved.

Last edited:

- #21

Science Advisor

Homework Helper

- 6,445

- 5,560

I find QM fundamentally more appealing and perhaps less vexing.

- #22

- 24,492

- 15,000

- #23

- 24,492

- 15,000

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).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.

- #24

Gold Member

- 1,508

- 452

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.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).

- #25

- 1,243

- 986

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.

- #26

- 24,492

- 15,000

- #27

Gold Member

- 1,023

- 917

"

- #28

- 24,492

- 15,000

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

- 133

- 55

Feynman fails to understand that idealists, even if very few, understand QM inside out.

- #30

Gold Member

2023 Award

- 167

- 301

That's all he meant. If one reads his essays he usually says that in the following sentence.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"

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.

- #31

- 24,492

- 15,000

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.

- #32

Science Advisor

Gold Member

- 821

- 690

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:That's all he meant. If one reads his essays he usually says that in the following sentence.

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.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.

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.

- #33

- 24,492

- 15,000

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.

- #34

- 4,078

- 593

I think mathematics is obviously the only way to make quantitative statements anyway as it will involves measures and numbers in some form.vanhees71 said:Bohr is right that the only adequate language to discuss physics is math.

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.

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

- #35

Gold Member

2023 Award

- 167

- 301

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.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

Share:

- Replies
- 175

- Views
- 5K

- Replies
- 460

- Views
- 16K

- Replies
- 2

- Views
- 519

- Replies
- 218

- Views
- 11K

- Replies
- 87

- Views
- 2K

- Replies
- 13

- Views
- 396

- Replies
- 8

- Views
- 299

- Replies
- 204

- Views
- 6K

- Replies
- 7

- Views
- 407

- Replies
- 27

- Views
- 729