Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Why did they ignore Bohr?

  1. Dec 7, 2009 #1


    User Avatar
    Gold Member

    One of the curiosities of 20th century physics was that the complementarity principle was central to the QM revolution, yet physics only took heed of uncertainty. Why?

    Kote would certainly have an opinion. But here are some relevant snippets to remind others.

    Niels Bohr on the importance and universality of complementarity or conjugate pairs....

    [So it was not just about QM but a general philosophical fact - profound truths come in opposing pairs.]

    [The familiar statement of particle~wave and location~momentum duality.]

    [Now we can see that the dualities are not states but limits. So it is not a case of either/or when it comes to a question like is it looking like a wave or a particle? Instead it is about the separation towards limits, in which an electron becomes increasingly particle-like as it becomes increasingly less wave-like.]

    [Again, what is fundamental is not a binary division that just exist, but instead the separation of a particle's potential in one or other possible direction.]

    [Again the social perspective. Bohr thought he was on to something fundamental. But the physics community didn't want to go there.]
    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. Dec 7, 2009 #2
    Okay, you caught me; I studied Bohr for a little while :tongue:. Here are some more quotes you may find interesting. They are about all I came across when it comes to Bohr extending complementarity though (I think. I didn't record much from his Essays on Atomic Physics and Human Knowledge). I'm not familiar with any program he had to extend complementarity (not saying it didn't happen), but he certainly thought it was pretty important and had some wide applications. Bohr in Atomic Theory and the Description of Nature (1934, p96):
    I don't have the full quote handy for this next one... but here's me quoting Bohr (on qualia?) :wink::
    And another brief bit on free will...
  4. Dec 7, 2009 #3
    I think Bohr probably actually was an either-or guy. The type of measurement you take determines which property you get, and when you actually take a measurement there's nothing quantum, uncertain, or indeterminate about it. Bohr (showing his Kantian side):
    Wave/particle duality was a misleading representation of Bohr's views though, and he himself abandoned its use. Waves and particles are classical things with persistence, intrinsic objective properties, and certain causal implications, none of which Bohr fundamentally believed in. From http://plato.stanford.edu/entries/qm-copenhagen/#4 (worth reading):
    To respond to your original question, Bohr himself stopped using the term "complementarity," for similar reasons to why he gave up the wave-particle pair. From the same section by Jan Faye on SEP:
    But at the same time, I think one could make a case for Bohr being a modeler. What sets Bohr apart in my mind is his denial of intrinsic objective properties in real objects. For Bohr, real objects only exist in the context of their interactions with other things, and they have no context-independent, persistent properties. Momentum is not a persistent property of an electron. It is a property that manifests itself for the duration of an electron's collision with another object.

    One would expect Bohr to be an instrumentalist. He denied that we should be looking for properties that would meet Einstein's (well, EPR's) criterion for reality. He didn't believe that objects had any properties (or existence) in a metaphorical vacuum. (I find it helpful to consider Bohr with a dose of Berkeley's "common-sense" arguments for rejecting materialism :smile:.) Bohr, however, was not an instrumentalist. He was a strong realist. He was able to hold this view by realizing (correctly? incorrectly?) that physics had never been about any more than the model, and we had always been calling the model reality. Furthermore, he believed it was impossible to do any better. So either we can call the model reality, as we do naively, or we can never begin to know the first thing about what basic reality is (the second option is what Bohm actually settled on). So our model doesn't allow us to visualize causality like we wanted to - big deal - the experiments are telling us that this is an impossible goal, so let's let it go. Bohr again:
    I think Bohr had some troubles in that he changed his own terminology and spoke liberally about subjectivity and observation. To him, however, these had nothing to do with humans or minds, so there was no confusion. Also, he spoke in very realistic terms while maintaining that real objects had no intrinsic objective properties and that there was no "view from nowhere." These two views are difficult to reconcile, and Bohr didn't emphasize their reconciliation. It's also hard to understand how a "no view from nowhere" stance could not require one to fall back on minds as subjective observers. And finally, the "no view from nowhere" stance is opposed to the entire implicit goal of physics, which is to describe the world from the objective point of view.

    Of course this is all just the view that I have come to on Bohr. Someone please correct me :smile:.
    Last edited: Dec 7, 2009
  5. Dec 7, 2009 #4
    I do not know the history of why Bohr has been ignored but it might be useful to point out that the uncertainty principal is not a philosphical precept but rather a feature of quantum mechanics. Mathematically is a feature of the Fourier transform. Bohr's theory seems to me, and correct me when I err, to be a conceptual interpretation of theory rather than a feature of it.

    The Uncertainty Principal applies in all of quantum mechanics not only for position and momentum.

    To my naive mind, the philosophers have tried to objectify QM in terms of common everyday/classical ideas and fail to realize that QM objects such a electrons are neither waves or particles but something different. The wave nature comes from the the interference of wave functions, the particle nature from the calculation of discrete measurement events, a process that is still not understood. However, the wave function is not really a wave in the classical sense. It is a solution of a complex heat equation not of a wave equation. The wave function evolves not as a wave would but as a Markov like process where amplitudes replace probabilities. In fact, it is simple to derive the Shroedinger equation from such a Markov like process just as you can derive the heat equation from a continuous Brownian motion. For me, it helps to think of the Markov process as the actual quantum mechanical phenomenon and all else is a epiphenomenon of it (e.g. wave interference) -- except for measurement which is still not understood. A good example for me is the evolution of spin states according to the Shroedinger equation. I do not think of this in terms of wave particle duality
    Last edited: Dec 7, 2009
  6. Dec 7, 2009 #5
    We are talking about the conceptual parts of the theory, but there's no separating the mathematical portions from their interpretation. A physical theory without a conceptual basis is just pure meaningless math. The math itself never tells you which measurements to plug in for each variable - that's where the conceptual interpretation comes in. That's also why physics and philosophy are so entwined.

    For a theory to be correct there can't be any contradictions in either its math or its conceptual framework. For philosophers, it's not good enough to say that measurement is not yet understood. Measurement is the link between the conceptual and mathematical aspects of theories. It's how you find physical meaning for your variables. If you don't understand measurement, you can't show that your theory is self-consistent and lacks contradictions. Philosophers will leave the experiments and the development of more and more complete and accurate theories to the physicists. We'll help out, however, when a dose of logical consistency and conceptual analysis is required :smile:.

    I'm sure there are plenty of bad philosophers who don't understand the physics, but there are also a lot of good ones who do. Just as Bohr, Heisenberg, and Bohm wrote books on philosophy, contemporary philosophers publish in leading physics journals. http://www.princeton.edu/~hhalvors/" [Broken] is one of my favorite examples (yes, he taught Princeton's graduate course, "Foundations of Algebraic Quantum Theory"). No philosophers take anyone seriously if they get the physics wrong.

    See http://en.wikipedia.org/wiki/Duhem–Quine_thesis and http://en.wikipedia.org/wiki/Confirmation_holism.
    Last edited by a moderator: May 4, 2017
  7. Dec 7, 2009 #6
    I am not sure what your point is. The distinction that I made is correct.

    Suppose I interpret quantum mechanics to be a manifestation of the unity of chi in the universe. All things are connected through energy and spirit. Would you call this a feature of the theory?

    BTW the measurement problem is an active area of scientific research - Serious physicists - not philosophers - are investigating it. Currently there are competing theories of measurement all of which are considered unsatisfactory. A layman's treatise on this is Quantum Mechanics and experience by David Albert - a philosopher! My first exposrue to it was in a QM course given by Brian Greene. He did not discuss measurement philosophically but as a problem that needed a theoretical physics framework.
    Last edited by a moderator: May 4, 2017
  8. Dec 7, 2009 #7
    Of course it would be a feature of the theory. It would also invalidate your theory, unless you could somehow reinterpret reality in terms of your concept of chi - in which case it would just be silly and not philosophically or scientifically satisfying.

    There are no theories without interpretations, only meaningless math.

    The physicists working on the measurement problem are working with the purely conceptual aspects of quantum mechanics. The mathematical framework for nonrelativistic QM has been established for decades. It is also just incorrect that philosophers don't work on the issue professionally.

    I will also point out that neither Bohr nor Bohm, nor most of the other physicists involved in the development of QM, saw any measurement problem. The measurement problem is a conceptual issue with collapse interpretations of QM. Bohr's interpretation is no-collapse, so there is no problem.

    See http://plato.stanford.edu/entries/qt-measurement/ for an academic philosophical overview of the topic.
  9. Dec 7, 2009 #8
    thanks for the references. Show me in the postulates of quantum mechanics how you derive wave particle duality. Lets do this with spin 1/2.

    If you agree that some philosophical interpretations are less useful than others then you should admit in theory that the uncertainty principal which is language that describes a structural feature of QM may be different qualitatively than an interpetation which does not.

    I certainly was not saying that physics is just math. My point was that the uncertainty principal can be derived from quantum mechanics. Wave particle duality can not.

    I do not see why Bohm's theory of QM is purely conceptual - whatever that really means. Perhaps you could tell me. Bohm's theory is seen by some as a path to solving the measurement problem.

    You have misread and I think trivialized my points.
    Last edited: Dec 7, 2009
  10. Dec 7, 2009 #9


    User Avatar
    Gold Member

    But this is the subjective~objective duality of epistemology~ontology, or the modelling relation. It is about the way minds know worlds. Definitely a dichotomy, but not the more challenging one I meant.

    What I was citing was ontological complementarity. What is actually case "out there" when it comes to objects like electrons.

    And Bohr's shock at first being forced towards a principle - that objects like electrons and photons are particles~waves. Then realising this was deeply logical - in the old dialogic, yin-yang, thesis~antithesis way. And then...well the physics community seemed not to get the importance of this?

    Instead, the fact was accepted - enshrined as an uncertainty principle and an apparatus of bra-kets, operators, hilbert spaces, etc - and the unconventional strangeness of the fact pushed aside.
  11. Dec 7, 2009 #10


    User Avatar
    Gold Member

    Thanks. And I would entirely agree with this progression. It is a move towards a systems or process philosophy view of reality. Instead of talking about rival states or kinds of objects, he is now talking about different kinds of relationshops that weave located "objectness" within the constraints of contexts.

    Note how this matches the similar progression in foundations of maths from set theory (containers and their contents - static existence both) to category theory (processes~structures to match Bohr's kinematic-dynamic).

    I agree - in my limited reading of Bohr. He knew that QM focuses attention on the issue of observers. Classical physics had been able to ignore the creative process of reality by simply treating reality as a static existent object. A bunch of atoms in a void. Then QM introduced a model of the observed side of reality, and failed to deliver a model of the observation.

    So again, physics was taking something to just exist (but now it was wavefunctions, uncertainty, complementary states and entangled probabilities), cutting away the relationships that make creation a process. But in doing so, QM also most clearly created the observer problem.

    Bohr could see that with no epistemic cut in the model - no definition of the complementary act of observation - the finger could get pointed back in endless regress to consciousness itself. However, this is not a natural view. And if he were around today, no doubt he would favour decoherence as the right way to go in introducing "observation" back into the physical picture.

    I see decoherence as the view from everywhere. The universe is a vast decohering system. It decoheres across all possible scale. It is a self-organising network of relationships. Which would make observation pretty objective - the universe is the observing system constituted by what it observes.

    So Bohr would seem to be recognising first the essential process which is mind-like - all about an interaction between observers and the observed. Then he was trying to put this familiar epistemological~ontological or subjective~objective dichotomy "out there" in the universe, completing the revolution half-begun by QM.
  12. Dec 7, 2009 #11


    User Avatar
    Gold Member

    Maybe this is indeed one reason why Bohr's insight was ignored. It does seem more a qualitative distinction than a quantitative one. To actually do quantum mechanics, the big view was broken down into simpler devices. So you can make practical calculations which treat the wavefunction as a success of existing states, further removing the need to treat things as being about a continuous system formed of relationships.

    Physics ignored complementarity because it wanted to move on to lower-level technical applications. And this could be done using the old classical mechanical mindset quite adequately.

    But the essential point is still that uncertainty reduces to complementarity. You may start with a state of naked QM potential where anything seems possible, but when you want to reduce this what can actually emerge, you end up with "only complementaries".

    And Bohr's point was that physics expected reality to reduce to "only one thing". Not always two things. And two things related in this exact, mutually-defined, or complementary way.

    And hey, it was what philosophers had always found. Fundamental concepts are also aways dual. So plainly something was going on.

    Here is Bohr right in the centre of the most important scientific revolution of all time. He comes to a realisation. His peers all agree he is onto something key. And then physics just shrugs it shoulders, encodes the duality as a necessary quantitative correction and ignores any suggestion that a conceptual correction might also be in order.

    Why has physics been so resistant to the idea? The fact that modern physics is largely about practical technical application - shut up and calculate - certainly seems a large part of the story.

    It is not as if they proved the logic of the concept wrong?
  13. Dec 7, 2009 #12
    Wilczek mentions Bohr and profound truths and mistakes in his book. Anyone can make a mistake. But only a genius can make a profound mistake. He was referring to Newton. Kind of like what Picasso said about art, art is a lie that makes us realize truth.

    I always liked this Bohr quote:

    "Everything we call real is made of things that cannot be regarded as real."
  14. Dec 7, 2009 #13


    User Avatar
    Gold Member

    Exactly. This is the paradox at the heart of human modelling. And the problem is that the modelled then becomes "the real" for the vast majority.

    Newton's "profound mistake" I presume you mean was the idea of action at a distance. All his contemporaries, were trying to model gravity as a process. Descartes, for example, wanted the actions transmitted via the swirling motions of tiny jostling atoms, a motile corpuscular aether.

    But Newton said no, I take the limit as a being an actual state and throw away any idea of a process that might get us there. His letters to Hooke revealed his doubts about the reality of this. But it was the bold act of reduction - from what seemed real to what was clearly unreal - that proved to be the most efficient approach to modelling the situation.
  15. Dec 7, 2009 #14
    I still don't see what the duality really is. Maybe this is what I need to understand. The ideas of particle and wave both fail in quantum mechanics and it seems to me that we are using these old pictures to explain something different. The mathematical models are also entirely different.

    I think this is why in QM the idea of force is de-emphasized and ultimately replaced. Energy takes the high seat.

    But you have in several threads postulated the idea of necessary duality. Perhaps you could elaborate on it and explain why you think it is important.

    The measurement problem seems on the surface to fit your idea of necessary duality since you have the duality of an indefinite continuous thing - a wave function - that becomes definite and discrete at the moment of measurement.
  16. Dec 7, 2009 #15


    User Avatar
    Gold Member

    My actual model of duality is now based on symmetry and symmetry-breaking. So we start with the unbroken, a pure symmetry potential of some kind, and then we break it as far as it can be broken. Thus we end up with an asymmetry. And in fact a scale asymmetry - the local~global symmetry breaking that is modelled in hierarchy theory and systems science.

    So I come to this postulate from another branch of science - the modelling of complexity. Yet now I am interested in how it has cropped up also in fundamental physics - the modelling of simplicity. And why it has then been ignored when it does crop up. Bohr being a good example (though we have string dualities as a current issue of course).

    Discussing Bohr, note that Kote has pointed out that particle~wave is the "beginner's" framing of the duality. As I said, it is still talking in terms of objective states rather than active processes. Kinematic-dynamic complementarity moves us towards the active view.

    And then there Bohr's point about profound truths being recognized by the fact that the opposite is also a profound truth. This is the history of Greek philosophy in a nutshell.

    If I say chance, you say necessity. If I say stasis, you say flux. If I say discrete, you say continuous. If I say substance, you say form.

    And the reason is that to be able to go in some clear and definite direction, you have to by logical definition, be clearly and definitely leaving some place behind. Every crisp event must have a crisp context.

    If I am paddling a boat in a fog, am I really going anywhere or standing still? I can't actually say. But if the fog lifts and I can see I am paddling out to sea, then there is a context which makes my motion, or lack of it, meaningful.

    So this is the deep reason for dualities. You cannot have one definite direction without have the complementarity that actually makes it "definite". Every action needs a reaction to be an action (and reactions need actions to be a reaction).

    Name a key insight in physics that does not hinge on such dichotomies? Newton's third law is the one that makes sense of the others. Einstein saw the symmetry, the duality, that lurked in gravity and acceleration. That led to GR. Particle~wave was the key that unlocked QM.

    This seems to be the deep way the world works. It begins in symmetry. And when symmetry breaks, you are left with dualities. Or asymmetric dichotomies would be my own preferred technical jargon. When symmetries are completely broken, they take a particle~wave form where you have just the fundamental "twoness" or a local limit and a global limit, which makes a fundamental threeness if you include then the third thing of the hierarchical interactions of what gets split apart.

    Bohr was saying QM seems to be a particular example of something already known to be much more general in philosophy. So hey guys, why not stop to consider this?

    Yet are today's physicists being trained to think in these terms? Clearly not. So why not?

    Maybe it's why we have become great at churning out technology - just shut up and calculate - but there seems to be a grave shortage of Einsteins, Newtons and Bohrs around these days. The very source of their discoveries is being denied.
  17. Dec 7, 2009 #16
    I've typically interpreted complementarity as more of an epistemological than ontological view... although Bohr brings the two together very closely. From Faye (not that he's infallible):
    I can't speculate on whether Bohr would have changed his stance to include decoherence (well, I guess I could, but not well). But I don't think the interpretation Bohr did adopt is compatible with decoherence. Bohr was big on the wave function not having any physical meaning and collapse not representing anything in reality. He had no problem with not giving a meaning to collapse and went as far as to say that QM was complete and there was no further room to explain any underlying mechanisms. The mechanism, for Bohr, was inherently incomprehensible and unvisualizable. Only the classical results of interactions were comprehensible, hence the completeness of QM in statistical form. It's also possible that I'm just not familiar enough with decoherence.

    When Bohr writes, he emphasizes the subjectivity of relativity theory. He doesn't seem to find any of his inspiration in mind-body duality (but who knows what he actually thought). He derives all of his "subjectivity" and relativism from special relativity's notion that the properties of velocity and length etc were meaningless unless placed in the context of a certain reference frame (experimental setup). I don't know if this disagrees with what you said, but I'm reminded of it :smile:.

    The progression seems to me to be an extension out of relativity. Classical physics denies the objective intrinsicality of colors and meanings. Relativity denies the objective intrinsicality of position, velocity, size, and mass. In relativity, of course, you can often talk about inertial reference frames and rest mass etc. QM puts the nail in that coffin by saying that these properties don't just depend on reference frames but are mutually exclusive and therefore lack persistence. (Neglecting Bohm for the moment - I'll save arguments that he doesn't solve much ontologically :wink: - though philosophically I think he still solves a lot.) I think that Bohr realized this progression. I think it is a strength of his view that it restores an ontological equality to our naive conceptions of properties.
  18. Dec 7, 2009 #17


    User Avatar
    Gold Member

    It was the accounts of Bohr that stressed the idea he was of the "shut up and calculate" school that put me off reading him more closely in the past. But I am now thinking that this is a case of Bohr being interpreted in ways that suit the wider physics community - history is being rewritten to normalise his dangerous views. I have come across this same thing in ancient greek philosophy - history is told by the winners after all.

    I now think that Bohr had a more subtle understanding that would repay my research.

    As to the episto~ontic issue, I feel Bohr would be taking the right approach if what he meant was that the wave-iness and particle-iness are not properties inherent in unseen objects but instead aspects of the questioning process - constraints exerted by the observer (whatever that may turn out to be) that shapes up the nature of the observables.

    So it could be that wave or particle is just the only questions that a human mind could imagine asking of a naked QM potential. Or it could be that wave~particle are more objective in being the way that the symmetry of reality must break when any questions start getting asked.

    My own belief (and it was a big surprise) is that epistemology and ontology do work the same way, they do have the same self-organising logic. So there is a reason why epistemology can seem to be ontology, even if we ought to keep them separated.

    Our understanding is vague. And this symmetry state can only be broken completely by dichotomies. Reality also starts vague. And likewise, its symmetry can only be completely broken by dichotomisations.

    OK, now I've introduced another good reason why Bohr was ignored. This way of understanding reality is just too self-referential and complicated for most people.

    Possibly also it is to inter-disciplinary. We are talking about a generalisation that emerges from across many different fields, all divided by their own jargons and customs. So to teach dichotomies 101, you would have to draw from physics, biology, thermodynamics, greek philosophy, tao, category theory....and who has time to draw all that knowledge together, boil it down?
  19. Dec 7, 2009 #18


    User Avatar
    Gold Member

    This is a good way of putting it. Nothing exists at points except what we create at points. And then we even create the points themselves!

    This is what I call the systems view. Top-down global constraint restricts naked freedoms to create what we find at a systems locales. But then - and it is a big but then - what gets created at locales is then the "stuff" that can additively and collectively construct the system.

    And then - another big then - it has to be the case that it is the right kind of stuff building the right kind of system, other wise the system won't have the right kind of constraints to be creating the local stuff. (Hofstadter sort of wrote about this as strange loops - http://en.wikipedia.org/wiki/Gödel,_Escher,_Bach)

    But anyway this is the logic of synergy and mutuality and other modern accounts of how two things interact to form a complementary whole.

    See for instance...
    Synergy - http://en.wikipedia.org/wiki/Synergy
    Synergetics - http://www.center-for-synergetics.de/
  20. Dec 7, 2009 #19
    Maybe we are in a culture that does not support the expensive experiments needed to test theories further. Many physicists have left for other things. As have many mathematicians. A mathematician I know who started a hedge fund said that we have a lot of Physics but we can't do the experiments to test any of it.

    In my Wall Street days physicists were flocking to the trading floor. In my group I hired a guy who specialized in predicting chaos and was tired of the researcher's life and living in New Mexico.

    Recently I attended a lecture on quantum computers. The researcher is part of a group at Microsoft. There are many theoretical questions that they need to answer to build this thing and they are testing their theories with real machines as well as mathematical models. He talked a lot about topological quantum field theories and Chern-Simons theories which seem to model aspects of quantum Hall fluids. I don't understand the stuff but it seems like many of the purely theoretical developments of the past half century are getting a chance to be tested.

    It is also possible that Physics has become so mathematical that is has needed to take a rest and focus on mathematical frameworks for a while. This seems to have happened in the 19'th century with the development of modern mathematics prior to the physical theories that relied on it.
    Last edited: Dec 7, 2009
  21. Dec 7, 2009 #20
    I wonder what his conversations with Wheeler were like on this subject. I remember reading in Wheeler's obituary:

    "You can talk about people like Buddha, Jesus, Moses, Confucius, but the thing that convinced me that such people existed were the conversations with Bohr,” Dr. Wheeler said.

    He thought Bohr was one of those transcendent figures.

    I also thought Dyson proposed an interesting dichotomy about scientists. Says there are two kinds. Unifiers and diversifiers. The unifiers are happy to leave the world a little less complicated than they found it. Diversifiers are happy leave the world a little more complicated than they found it. Says most physicists are unifiers but gave Wheeler as an example of an exception who was a diversifier.

    Dyson says the universe is built on the principle of maximum diversity.

Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Similar Discussions: Why did they ignore Bohr?