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What the state of Quantum mechanics (QM) is?

  1. Jul 17, 2008 #1
    "Anyone who has not been shocked by Quantum physics has not understood it," said N. Bohr.
    Now I am confused by the interpretation of the state of Quantum mechanics (QM).

    I still remember my teachers told me in class that the prediction of QM is valid only for large enough experimental result. That is to say, QM doesn’t discuss the individual level of the experiment. For example, we can get the interferential pattern in the double-slit experiment only after enough particles have gotten the screen. And it is consistent with the result of QM. But it is meaningless to ask how one particle goes through the slits.
    Today, I read a paper which discusses the individual experiment in QM.

    So I am confused.

    Can we discuss what the state of Quantum mechanics (QM) is?

    Thank you!
  2. jcsd
  3. Jul 17, 2008 #2
    In the double slit experiment, you are kind of right, because QM predicts a interference pattern, that is a propabillity distribution on the detector, only sending one particle though the slits would only yield one click in the detector, so it would be hard to say if the predicted pattern is correct.

    But you can talk about states of a single particle, but meassuring in the lab if this particle is in a given state can be difficult, and because QM is propabillity you will often need many measurements, to comfirm your result. But theoretically there is nothing wrong in talking about a single state. I'm not very good in a lab, but is guess if one want to measure that a particle is in a given state, one would need to do the experiment over and over until you have good enough data. Maybe some clever techniques have been developed so one can peform these kind of things in a single experiment, in some special cases?

    Maybe if you elaborate on 'the individual experiment in QM' it would be easier to give an answer?
  4. Jul 17, 2008 #3


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    What QM says about the individual experiment, is the probability of its outcome. Of course, to actually check the accuracy of such a theory you would have to perform multiple experiments and see if the probability distribution matches the predicted one.
    For example, suppose I put 10 blue and 10 red balls in a vase. Then obviously, the chances of me picking a red (or blue) ball are 1/2. But this does not actually tell me anything useful, if all I'm going to do is pick out one ball. Even if the distribution was red 99.99999999% against blue 0.00000001% I could still pick out a blue ball when taking just one. Only when doing this experiment over and over again could I distinguish between the two.

    That was also probably what you mean by "QM doesn’t discuss the individual level of the experiment".

    So now for your question: it is hard for us to comment on "a paper" without knowing anything about it. Could you post a reference to that paper?
  5. Jul 17, 2008 #4
    That's all true of "classical" QM but quantum field theory, quantum electrodynamics (electromagnetism) and quantum chronodynamics (nuclear force) have made astonishingly accurate predictions that have led to the standard model and basically all of particle physics. We've come a long, long way from the double slit! :)

    There are several areas where quantum theory has stalled. One is, obviously, gravity - quantum field theory just cannot be applied to gravity in the same way it's applied to the other forces and no one is yet sure why. Hopefully the Large Hadron Collider (the so-called black hole factory) will change things there. We are also still not sure exactly what the wavefunction represents physically, if anything, and may not be for some time. And quantum computers, while promising, are feared by some to be vaporware, i.e., never going to happen. (Although I suspect this one, like alternative energy sources, will start gaining traction as soon as Big Silicon realizes that they've reached the limit of what they can do with transistors).
  6. Jul 17, 2008 #5
    Please Peter, refrain from calling LHC a BH factory. That is misleading because it distract attention from the point of LHC, and most probably wrong.
  7. Jul 17, 2008 #6
    I always resort to Heisenberg's paragraph in his book "The Physicist's Conception of Nature" Chapter: The Idea of Nature in Contemporary Physics, whenever I get confused over quantum mechanics, or come across to pseudo scientific interpretations of it.

    I will write the exact paragraph here:

    Although he talks about elementary particles, his arguments on quantum mechanics is valid for every physical process. Quantum Mechanics talks about the interaction of two physical systems, you cannot isolate any physical object and talk about it. You cannot put a particle in a box, measure where it is and tell me it is at that exact position. You can only tell me that you interacted your measuring device with the particle and the measuring device gave you a readible, audible... data of interaction at point x. And the particle may not be at position x, before or after the interaction.

    I find that what Heisenberg tells us here is the most important thing quantum mechanics has ever done.

    I do not know what knowledge about the field theory or QED, QCD. QM, QFT, QED, QCD are working well, and what you exactly talk about when dealing with a QM system is also awesome and true. As long as one does not forget what QM talks about s/he should be fine and not confused, and not find oneself dealing with things magical and mysterious (there really are many people doing this believe me). QM is only hard and demanding because it is physics in its absolute form, but it is not magical.

    "xylai", think about the paragraph in Heisenberg's book, I am sure that you will hardly get confused with QM (and about its interpretations) anymore.
  8. Jul 17, 2008 #7
    Sorry I was trying to be sarcastic!
  9. Jul 17, 2008 #8

    All the answers that have preceeded mine to yours are the "mainstream" interpretations of QM, including your professors. In fact, you'll find that when it comes to the meaning of quantum mechanics, 99% of your professors are just as confused about it as you are, though they won't admit it for various reasons.

    However, I should let you know that there are alternatives to these mainstream ideas that are just as empirically correct. The most notable one is the de Broglie-Bohm (or Bohmian mechanics) version of quantum theory. If you would like to understand how you can in fact say that a single electron or photon definitely goes through one slit or another, and yet produces an apparent interference pattern, then you should read about this formulation of quantum theory:



    Also, I would strongly recommend being very skeptical of the very brash statements that the others have made about what quantum mechanics is actually about. There really is no consensus on this issue in the mainstream literature, contrary to what the others are trying to tell you. And the only way you will get unconfused is if you study the interpretational part of QM on your own and keep an objective and critical eye about anyone who says that this is the "ONLY" way to interpret or do quantum mechanics.
  10. Jul 17, 2008 #9
    I should first say that none of my professor of QM told me
    nor do I see that stated above. As for
    it has never been told to me before I was told that I must first study the mathematics and be able to do the calculations myself. I should say for instance that IMHO you should postpone the study of the interpretation of QM until you know what the Wigner function is, the analog of the probability density in phase space. Wigner function can be constructed to be positive-definite. This is just an example, it is not so crucial, but a fair technical understanding is generally admitted to be preferable before embarking on the various interpretations. Another possibility would be to get up to the formulations of Dirac and von Neumann, just to make sure you understand what you are talking about before trying to interpret it.

    To counterbalance Maaneli's suggestion to study the de Broglie-Bohm interpretation, I suggest you to study the relational interpretation of Rovelli. Instead of trying to pull back to classical physics, some people try to push forward, deeper into "quantum confusion" :tongue2:

    Once you have studied quantum field theory, you can look back, say "wait a minute, what do I do with the de Broglie-Bohm interpretation ?". Note that I do not even mention gauge field theory, which is essential to the standard model and even harder for Broglie-Bohm proponents. And then you can go back to Rovelli to study the thermal time hypothesis he developed with Connes.
  11. Jul 17, 2008 #10

    You see Xylai? Humanino is clearly a dogmatist about his own views. He has not even understood de Broglie-Bohm theory to accurately criticize it!

    To you Humanino, I wasn't talking to you, I was talking to the OP and respond to what her Prof says. Also, I find it a sickening joke that you would call the pilot wave theory "quantum confusion", but then promote the half-baked relational interpretation of Carlo Rovelli. (smiley face with tongue!)

    Also, there is no contradiction between studying the interpretational issues on your own and learning the math of the textbook formalism. So I don't know why on earth you were implying that there is.

    Also, if you're going to so coyly critcize pilot wave theory, you should be aware that there are fully self-consistent field and abelian gauge theory versions of pilot wave theory which are empirically equivalent to textbook QFT. Once you study that, you can then look back and say

    "wait a minute, what do I do with the relational interpretation ?"
  12. Jul 17, 2008 #11
    I should say I have not chosen any specific interpretation. I think some are more interesting than others, and I prefer them on this basis, but I certainly would not claim that one is more accurate than another since they are a matter of philosophical choice. Please keep in mind that among scientists, "dogmatism" is an insult.
    I have and have read Bohm's textbook among other things. It is interesting, very much I should say, and I spent several weeks reading this book you know. I certainly do not reject it.
    It's a bit aggressive. Do you feel unsecure ? :tongue2:
    I was calling Rovelli's work "quantum confusion". There is no way to call Bohm's interpretation like this, it would be non-sense.
    I was suggesting that one should study the math first to a decent level, and then only look at the various interpretations. As far as I can tell, this is what the majority of people do. I studied a bit of interpretation on my own as I was studying the formalism on my own. After I had completed university lectures on QM, I found it much easier to understand the interpretations and the subtleties between them. If you can't do the math, you can only contemplate the interpretations. If you can do the maths, you can critically compare them.
    So you have QED, "bravo !". That's not the end of the standard model. There is no doubt that
    • Bohm's interpretation is more developed
    • Rovelli's interpretation receives more attention today, since it needs to be developed !
  13. Jul 17, 2008 #12


    My bad, sorry. For some reason I misread your words (in reference to calling Rovelli's interpretation "quantum confusion" as opposed to deBB). That's the third time I did that today! Very unusual for me.

    I didn't mean to imply anything personal by saying "I wasn't talking to you", I was just saying that I wasn't talking to you about your prof, but rather the OP's. BTW, you don't agree that undergraduate QM courses frequently give the impression that there is no interpretation of QM or that it's all been settled?

    So you have studied Bohm's books and pilot wave theory? That' refreshing around here. I should point out again that actually, the pilot wave theories, stochastic mechanics, and GRW collapse theories are empirically distinguishable from the others. So it's not just a matter of philosophical choice. If you like, I can give you the references to the new (but NOT controversial) work that demonstrates this.

    I agree you have to study the math to understand the details of the interpretational issues. But again that is not in conflict with thinking about interpretational issues on your own. In fact, you don't really need very much math to identify the problems of measurement, or to understand the EPRB paradox and start asking all the right interpretational questions. You just need to be very alert!
  14. Jul 18, 2008 #13
    No offense taken. I've been hard on other people somewhere else on PF today :rofl:

    It's hard for me to say, what I can tell you is that my teacher clearly stated in her first lecture "I'll teach you the formalism of QM within Copenhagen's interpretation. In the list of books, I suggest you consult those on interpretation only after we completed this series of lecture." From hindsight, I agree with her that it makes things easier, basically because Copenhagen's interpretation does not really address the interpretation issue :rolleyes: (this is a bit caricatural, even maybe cynical, that's what I have been doing lately) But then, I have had since quite a few teacher friends or acquaintance who give the same advice in their first lecture. I could not say exactly how it was stated to the students of course. I do not teach myself, but I think I would also do it like that : make a firm statement in the first lecture that QM is fascinating but unsettling, that there is no consensus on the interpretation, and therefore to understand the subtleties it is better to first be able to do the calculations right. I am aware that this is controversial by itself, and I would only choose this because I think it is the easiest way.
    I guess Bohm's interpretation should be one of the very first to begin with ! If my first post here suggested it was not a good idea to study it, then I miswrote it. To be honest, I took a renewed interest in QM's interpretation rather long after finishing QM. Once I had completed QM, I plunged eagerly into QFT :smile: I came back seriously to the interpretation only after I finished my graduation program.

    That's probably true, as I said, it is also a matter of personal choice.

    From a point of view of mere schedule (very bad reason, but reality hits hard) which leads (some?) teachers to advise to postpone the interpretation : they want their students to be able to complete the tests, interpretation not being included. This is sad but somehow, to be honest, intellectually one will certainly have many interpretation question raised constantly while studying QM ! From the positive side, there are many good review books, in France we have d'Espagnat and Omnes, I'm not sure whether they are popular in the US. Those kind of books contain a lot of material and do require a fair understanding of QM. That may be another reason, strategically it is just more efficient to concentrate on one aspect (formalism) and then another afterwards (interpretation).
    I would certainly like to take a look at it. All references always welcome :smile:
  15. Jul 18, 2008 #14
    Your Prof sounds pretty reasonable. Though, I agree you should become really knowledgeable about the interpretational issues so that when you do teach QM, you can emphatically mention what the state of affairs is, as I find that to be very motivating for students to learn QM and then think about those fundamental questions. I myself became terribily interested in the interpretational questions at the very beginning (like high school), and it did not harm my acquisition of the mathematical tools.

    The books of Omnes and d'Espagnat you mention are well known, but almost never do you have Profs even recommending that students learn about the interpretational issues in QM. At Stony Brook University, Profs are hardcore positivists and operationalists. It's really unfortunate for the students.

    Here are the references:

    Generalizations of Quantum Mechanics
    Philip Pearle and Antony Valentini

    Inflationary Cosmology as a Probe of Primordial Quantum Mechanics
    Antony Valentini

    De Broglie-Bohm Prediction of Quantum Violations for Cosmological Super-Hubble Modes
    Antony Valentini

    Astrophysical and Cosmological Tests of Quantum Theory
    Antony Valentini

    Subquantum Information and Computation
    Antony Valentini

    The empirical predictions of Bohmian mechanics and GRW theory
    This talk was given on October 8, 2007, at the session on "Quantum Reality: Ontology, Probability, Relativity" of the "Shellyfest: A conference in honor of Shelly Goldstein on the occasion of his 60th birthday" at Rutgers University.

    The Quantum Formalism and the GRW Formalism
    Authors: Sheldon Goldstein, Roderich Tumulka, Nino Zanghi

    Hope this excites you!
  16. Jul 18, 2008 #15
    Thanks for those many references :bugeye:
    The names ring a bell, so it's likely that I'll have to bookmark those. It is indeed quite interesting.
  17. Jul 18, 2008 #16


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    Way back, people thought that they were the end all and be all and lived in the center of the Universe. The birth of modern science put those notions to rest. Rather, in response to Newton, the idea of the Universe as a machine came to be -- a causal, deterministic machine.This idea worked pretty well until 1900 or so, when those pesky atoms didn't behave correctly, line spectra and all that. All this happened around the time Einstein showed how to show the reality of atoms.

    Now, it turns out, that probability and statistics dominate most efforts in physics. This is, of course no indication that an alternate approach could not work. My pragmatist perspective is let's go with probability, it's the best game in town -- this forum is possible because of QM, etc. When someone comes up with something better, physicists will flock to the new, improved ..... But this is becoming the Age of Probability, which follows the Age of Determinism. My own take is: where is it written that Nature must conform to our ways of thinking and understanding? Things change; ideas change, understanding changes..... Yesterday, classical physics, today, quantum physics, tomorrow, who knows?

    One final. If you look through the physics literature, you'll find precious little discussion, let alone mention, of the Copenhagen interpretation -- it's just taken for granted. Or, actually what is taken for granted is what I've called many times in this forum, Practical Copenhagen --Schrodinger and Born. Its value is demonstrated by the extraordinary success of physics over the last century. Where are the problems?

    So like in many other fields, we have a method for computing probabilities. That's it-- in the physics case, the events of probability theory are measurements -- until someone comes up with a better way, a better theory. As things stand, as Heisenberg points out, there's stuff we cannot know now. As in, how did that particle get through a slit(s). Today, it's "We don't know" Room for research....

    If you add the idea (Peierls) that these QM probabilities refer to our states of knowledge, then physics begins to conform to the use and understanding of probabilities and statistics practiced in many other fields, over many years. (Including the very sophisticated worlds of Control Engineering and Communications Engineering.)

    Reilly Atkinson
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