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

There is no Copenhagen interpretation of QM

  1. Aug 24, 2009 #1

    Demystifier

    User Avatar
    Science Advisor

    Many physicists say that they prefer the "Copenhagen" interpretation of QM, but it does not mean that all these physicists prefer the same (or even a very similar) interpretation. There are at least 4 very different interpretations that are sometimes referred to as "Copenhagen":

    1. Shut up and calculate - this is actually the interpretation that most practical physicists adopt.

    2. Positivism - QM is only about the results of measurements, not about reality existing without measurements. (This is essentially the philosophy of Bohr.)

    3. Collapse interpretation - when the measurement is performed, then the wave function collapses. (von Neumann)

    4. Information interpretation - the wave function does not represent reality, but only the information about reality. (It is somewhat similar to 2., but still significantly different from it.)

    What do you think?
    I am not asking you to say which interpretation do you find most appealing (we have many other topics on that), but to say whether you agree there there is no SINGLE interpretation that may be called "Copenhagen".
     
  2. jcsd
  3. Aug 24, 2009 #2
    How is "shut up and calculate" an interpretation? What's the interpretation of the wavefunction in the case of #3, and how is it different from #4? If, according to #2, everything is about the measurements, then in what way is such a measurement process different from #3?

    No, I don't agree with you - all of the four points you mention are features of the same interpretation (which is a little outdated anyway). They do not exclude each other at all.
     
  4. Aug 24, 2009 #3

    Demystifier

    User Avatar
    Science Advisor

    Well, perhaps it should not be called an interpretation, but it certainly is one view of QM.

    In 3. one assumes that the wave function is real, i.e., that it exists even without measurements. Many physicists think that way.

    See my answer above.

    I agree that they do not exclude each other, still I don't agree that they are the same. Many physicists think of 1-4 as DIFFERENT, so in this sense they ARE different (for those physicists).
     
  5. Aug 24, 2009 #4

    Fra

    User Avatar

    I think I partly agree with Demystifier that there are versions of Copenhagenish type interpretations.

    I often consider myself to be close to this interpretation at least relative to some others, like bohm etc, but I am still more radical than many others so I count myself as different than the most common version.

    The shut up & calculate interpretation is more like no interpretation to me. It's what you use when you use QM to solve actual problems in calculations. That is not the place to question the theory.

    This is to me mainly the difference between the engineering perspective of a theory as a tool, and the fundamental scientific perspective that ponders how to make the tools better.

    From then engineering view, you don't need to understand why it works.

    But for the tool maker, understanding why it works, and ponder how it can be even better is the whole point.

    I'm probably 4, and in that view the collapse is simlpy an information update. Not really a mystery. By nature the collapses are thus also subjective (if that makes it unreal or real depends if you talk about objective or subjective reality - IMO there is only subjective reality), which leaves it as no contradiction if one observer sees a collapse and one sees a gradual transition. It's just different views.

    Wether the wavefunction exists without measurement, I think it kind of does, but it doesn't exist without a measurement-history, since I think of the wavefunction as a steady state structure. Which is why it's constantly evolving. Without measurement, there is the self-evolution (schrödinger), but this self-evolution is perturbed upon measurement.

    Except for radical views such as evolving law, copenhagen is IMO the "least speculative" and most in line with the scientific method of the more commona interpretations. That's not to say I don't see problems, but that is IMO problems with quantum mechanics structure, not problem of "interpretations".

    /Fredrik
     
  6. Aug 24, 2009 #5
    Any interpretation of qm would eventually involve 1., wouldn't it? 2. and 4. are definitely part of the CI. But not 3. -- at least not literally.

    The way I look at it, there's an approach to understanding what qm means and what can be said about Nature that might be called 'Copenhagenish' and included in a collection of statements called the CI.

    So, I suppose I disagree with you. 2. and 4. are part of an approach that seeks to define what qm does (objectively) mean (not what it might mean if we take certain constructions literally), and what sorts of limitations on what can be objectively demonstrated and communicated about Nature given the quantum of action and the requirements for objective communication.
     
  7. Aug 24, 2009 #6

    DrChinese

    User Avatar
    Science Advisor
    Gold Member

    I personally see all of the above as Copenhagen, especially the first 2. The third starts getting into the interpretation side of things a little, although I think the fuzziness of the collapse concept (when does it occur? is it physical?) is attached to Copenhagen.
     
  8. Aug 24, 2009 #7
    The problem with these versions of CI is that CI proponents switch from one to another (even unconsciously) making them very difficult to "catch"

    They can discuss, for example, subject "are virtual particles real?" claiming that contrary to the virtual ones the real particles DO exist. And at the same time, when asked about the mysterious collapse they can hide behind "wavefunction is just an information, just an knowledge, it is not real"
     
  9. Aug 24, 2009 #8
    Every theory that pretends to be about physics must have an ontology and an interpretation that connects the math with the assumed ontology. Different QM interpretations have different choices about ontology. Having said that these are my comments to your 1-4 choices:

    1 equals 2. At a minimum, the instrument readings must be a part of the ontology. Without them there is nothing to calculate. The difference between 1 and 2 is only one of the form, 1 being less explicit.

    3 must be a constituent of 1 and 2 because the collapse changes the way the calculations are performed. If the wave-function is interpreted as a calculation tool to relate experimental preparations and instrument readings, 3 does not qualify as a distinct interpretation.

    About 4, I cannot quite understand what new ontology the word "information" brings.

    I think that characteristic of CI is that it only accepts macroscopic objects as a part of its ontology. Various flavors differ in what the fundamental properties of these objects are (instrument readings, brain-information, or knowledge, etc).
     
  10. Aug 24, 2009 #9

    Fredrik

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    If I understand 4 correctly, it's the "interpretation" I've been supporting in a large number of posts here. There are many different ways to say the same thing, for example:

    4'. The wavefunction doesn't represent the properties of the physical system. It represents the properties of an ensemble of systems that have been identically prepared.

    4''. QM is a set of rules that tell us how to compute the probabilities of possible results of experiments. Nothing more, nothing less. It doesn't include, and doesn't need, an ontological interpretation.

    The first is the way Einstein said it. The books by Ballentine and Isham use similar language. (Note that the ensemble in question consists of the systems that are being measured in all the similar experiments that we can perform, and that we don't have to perform all of them. We just need a large enough number to get good statistics).

    The second is the way I've been saying it. I don't consider the "ontology" in 4' an ontological interpretation of the wavefunction, since it doesn't describe what the system "is" on its own.

    Edit: I'm too lazy to rewrite the above, so I'll just add this comment. What Demystifier actually had in mind is probably the view that the wavefunction represents our knowledge of the system. I just find that one weird and kind of dumb. At the very least, it would require some technical definition of "knowledge" to make sense.
     
    Last edited: Aug 24, 2009
  11. Aug 24, 2009 #10

    Fredrik

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    I agree that people don't seem to mean the same thing. Most seem to mean an extreme version of 3, in which it is postulated that measurement devices are exactly classical, and the "collapse" is a physical process that changes the superposition exactly into an eigenstate. Niels Bohr is probably spinning in his grave.

    As I said in the other recent thread...
    What about the intepretation proffered by Ballentine (described in my previous post)? Should that be another item on your list, or do you think it's covered by 2 or 4, or "isn't Copenhagen at all"?
     
  12. Aug 24, 2009 #11
    Are you referring to Zeilinger's interpretation? :
    http://homepage.univie.ac.at/johannes.kofler/Files/Publications/Kofler,%20Zeilinger%20(2006)%20-%20Information%20and%20the%20Schroedinger%20cat%20paradox.pdf [Broken]
     
    Last edited by a moderator: May 4, 2017
  13. Aug 24, 2009 #12

    I think this is the only interpretation that attempts at making some sense. Though, every one of all the propositions(interpretations) of how the world might be outside of our perception, makes me want to throw up.
     
    Last edited by a moderator: May 4, 2017
  14. Aug 24, 2009 #13
    In fact Bohr and Heisenberg never totally agreed on how to understand the mathematical formalism of quantum mechanics, and none of them ever used the term “the Copenhagen interpretation” as a joint name for their ideas. In fact, Bohr once distanced himself from what he considered to be Heisenberg's more subjective interpretation (APHK, p.51). The term is rather a label introduced by people opposing Bohr's idea of complementarity, to identify what they saw as the common features behind the Bohr-Heisenberg interpretation as it emerged in the late 1920s.

    http://plato.stanford.edu/entries/qm-copenhagen/

    It's easy for a view to sound confused and inconsistent when it's presented by its opponents and never its supposed founders.
     
  15. Aug 24, 2009 #14
    An interpretation of quantum mechanics is 'a statement which attempts to explain how quantum mechanics informs our understanding of nature'. The above formulation attempts to do no such thing, so it's not an interpretation. To state otherwise simply means you're not interested in interpretations and you don't understand them, because you're an engineer.

    Positivism as a concept was effectively 'disproved' (if one can do such a thing in philosophy) by the late 1960s.

    Truth begins in sense experience, but does not end there. Positivism fails to prove that there are not abstract ideas, laws, and principles, beyond particular observable facts and relationships and necessary principles, or that we cannot know them. Nor does it prove that material and corporeal things constitute the whole order of existing beings, and that our knowledge is limited to them.

    Positivism ignores all humanly significant and interesting problems, citing its refusal to engage in reflection; it gives to a particular methodology an absolutist status and can do this only because it has partly forgotten, partly repressed its knowledge of the roots of this methodology in human concerns.

    As we all know (not!), one can conceive of QM describing objectively existing real waves and particles (Bohm interpretation) in a perfectly straightforward way, so the Bohrian positivistic rhetoric of finality and inevitability of CI ('We see that it cannot be otherwise', 'This is something there is no way round', 'The situation is an unavoidable one', the 'most direct expression of a fact..as the only rational interpretation of quantum mechanics' etc.) is simply incorrect. He uses circular demonstrations of consistency disguised as compelling arguments of inevitability.

    This is usually taken to require that the wave function represents an objectively-existing physically real wave field which collapses (instantaneously, at infinite speed across the whole universe, if you make your experiment big enough). Which kind of implies something like the GRW viewpoint (with all of its well-known problems, including the Schroedinger equation not being correct).

    Recall that in the Bohm view, things are made of particles guided by the wave so even though the objectively-existing real field (represented mathematically by the wave function) never actually collapses (the Schroedinger equation is correct) particular branches are picked out by whichever one the particles deterministically end up in, so it effectively collapses. To me this is the obvious way around all the usual weirdness measurement bollocks.

    Or some people mean that 'knowledge' or 'information' instantaneously collapses, in which case you mean option (4):

    Quite simply cannot be correct: as I have argued in this forum before, for anyone who keeps up with modern developments in experimental physics, the evidence for the fact that 'the wave field exists' is unequivocal:

    In matter-wave optics experiments for example - we find that it is possible to diffract, reflect, focus, interfere, do stimulated emission with the wave field (the thing that is mathematically represented by the wave function). This is clear experimental evidence for the objective existence of the wave. If the wave can be subject to and utilized in such a process, it logically follows that the wave field must exist in order to act and be acted upon.

    Just thinking about the two-slit experiment, it is not possible for a field representing 'information' or 'knowledge' to interfere with itself, and to behave like it satisfies a wave equation, if it does not in fact represent a real wave. It just isn't. I'm sorry, but it's true.


    So, personally I think the above considerations show that any claims that the Copenhagen Interpretation must be the logically preferred interpretation of right-thinking physicists (which one still hears quite often) cannot be correct. Such claims often have their basis in a misunderstanding of what Copenhagen means (as Demystifier points out), but also from 'not thinking very hard about the alternatives', or from uncritical hero-worship of [insert name of favourite 1920s scientists here]. Today it is simply untenable to regard the views of Bohr and Heisenberg as in any sense standard or canonical. They are more 'smoke and mirrors' than a unique compelling world view forced on us by experiment. The meaning of quantum theory is today an open question.

    Today it is also clear (to bang on about my favourite topic) that the rejection of Bohmian hidden variables theories by Bohr, Heisenberg, Pauli et al. - not merely as hopeless but as downright meaningless - was ultimately irrational (particularly if we use their reasoning).
     
  16. Aug 24, 2009 #15
    You must never become a police detective. When trying to solve a case, vomiting on anyone who attempts to present you with reasoned arguments based on suggestive evidence is not usually helpful.
     
  17. Aug 24, 2009 #16

    This view is close to imposing our classical view of reality(the human baggage) on the quantum realm.
    I think the whole point in not being able to deduce a meaningful description of reality from quantum theory is because qunatum theory demands that the Universe be an ON-OFF phenomenon. The continuity and solidity of the world exists only in the imagination(whatever that really is), fed by senses that cannot discern the waves of energy and information that make up the quantum level of existence.
    If QM is right, we are all flickering in and out of existence all the time at 10^-43sec. intervals. If we could fine-tune our senses, we could actually see the gaps in our existence. We are here, and then not here, and then here again. The sense of continuity(and continuous motion) is an illusion and is held only by our memories in the elusive concept of Time(this isn't too different from how we are fooled to believe in continuous motion by looking at 50Hz or 100Hz tv screens).
    This poses serious challenges to the notion that space, matter and time are fundamental concepts. If these are to go, there is only information left as a fundamental concept(0's and 1's) but we are still far from reaching a convincing information interpretation of qm, IMO.
     
    Last edited: Aug 24, 2009
  18. Aug 24, 2009 #17
    I agree with original poster that different people have different things in mind by 'copenhagen' interpretation. But I'm not sure Bohr held the kind of extreme positivism you ascribe to him. It's also noteworthy that complementarity, which played a big role in writings and thought, doesn't appear at all in your list. However, I find him an interesting and difficult writer and I don't really understand his thought.

    yossell
     
  19. Aug 24, 2009 #18
    A realm which these days we can photograph, video, and manipulate at the atomic level with repeatable results. Remember 'the quantum realm' just means 'quite small things'.
    Look, we're just trying to understand the Schroedinger equation here..
    Oh God.
    And your reason for not taking the obvious answer (as is done in every other theory of physics, and indeed science) is what.. I mean, are you trying to impress some woman, or something? Sometimes being mysterious can be cool, right!

    Pressure is due to atoms repeatedly banging on the sides of a container, no? Not in The Mysterious World of Wavejumper, because reality winks out of existence every 10^-43 seconds. Hmmm..

    OK - why don't we say nature 'acts as if' atoms bang on the side of the container. Then if we teach it that way, people will understand it, even though it's just umm.. a metaphor, or whatever.
     
    Last edited: Aug 24, 2009
  20. Aug 24, 2009 #19
    Yet, after 80 years of study its ontology is even more mysterious(which happens to be what this thread is about).


    Obvious answer to what?


    That's how it looks from the classical perspective(and it oddly obeys the Schroedinger's equation). The "mysterious world of WaveJumper" is the same as saying "the mysterious ontology of the quantum world of WaveJumper".


    That's how it is. And the quantum realm cannot be locally deterministic. Yet our 'classical' logic happens to be just that - local deterministic.
     
  21. Aug 24, 2009 #20
    It's only mysterious because you choose to make it so. As has been known for more than half a century, one can interpret quantum mechanics very simply in terms of motions of particles obeying a non-classical dynamics (because they are pushed around by the wave field). [see de Broglie-Bohm hidden variables theories]. This is just like how you can explain classical statistical mechanics in terms of the classical dynamics of particles, and it is in agreement with modern experimental evidence for the existence of both particles and waves. And yet you insist in trotting out all this out-there new-age babble in preference? Well, I don't understand it.
    Oddly, he says. :uhh:
    You're just willfully misunderstanding the whole thing - as you were last time we argued. Just because non-local connections between particles (mediated by the wave field) are possible does not mean that local interactions do not occur (i.e. that particles do not exert a force on things they bump into). I mean, what?
     
    Last edited: Aug 24, 2009
  22. Aug 24, 2009 #21

    How does the existence of particles and waves point to continuous movement of the electrons, say in the Hydrogen atom(without radiating energy)? Is the de Broglie-Bohm view of the atom different from Bohr's?





    How? David Bohm's notion was that the universe was an unbroken wholeness, right? The CI does not need an explanation for local effects in a non-local universe, because these local effects don't exist prior to measurement.



    Can you give me a link that says that there is continuous movement of particles in the context of De Broglie-Bohm's or any other interpretation?



    How is it not odd that an interpretation would assert that a quantum system does not have any properties before measurement, yet the Schroedinger equation gives us the exact probability of where an electron might land on the screen of the double slit experiment?
     
    Last edited: Aug 24, 2009
  23. Aug 24, 2009 #22
    In standard quantum mechanics, can you explain why the electron doesn't radiate energy?

    For example, in Helium atom, in the parts nearer to the spin-up electron or the spin-down electron, the magnetic fields are theoretically produced even in the standard QM.
    (Because the two electrons are apart.)

    While the two electrons are moving, the magnetic fields are changing, and the electromagnetic waves are emitted?
     
  24. Aug 25, 2009 #23
    What do you mean by "system"?

    As I pointed out before there can be no physical theory that does not have some kind of ontology. What you probably have in mind is that CI does not ascribe beable status to the wavefunction. But the theory need to have some beables, the "system" you are referring to, the instrument readings, etc.?
     
  25. Aug 25, 2009 #24

    It doesn't continuously radiate energy because it does not circle the nucleus in orbits. The HUP prohibits it from having definite position and momentum at the same time.


    I'd tentatively say that from our derived logical point of view(not according to the mathematical formalism), 'particles' appear to move continuously and in trajectories(there would not have been a classical world had it not been so).
    On a more formalised mathematical level however, the whole notion of classical continuous movement is an illusion. A bound electron should be understood as a standing wave spread over an area of space according to probability distributions. If we are to assign a classical trajectory for the electron, the closest i can think of is Feynman's path integral but it is a sum of an infinity of possible trajectories.

    In the words of Oppenheimer:

    "If we ask, for instance, whether the position of the electron remains the same, we must say ‘no’; if we ask whether the electron’s position changes with time, we must say ‘no’; if we ask whether the electron is at rest, we must say ‘no’; if we ask whether it is in motion, we must say ‘no’."
     
    Last edited: Aug 25, 2009
  26. Aug 25, 2009 #25
    the whole notion of classical continuous movement is an illusion. OK.
    So you say that the electron's motions are an illusion, they don't radiate enegy in standard QM?

    But How do you explain about the relativistic mass change of the electron?

    Please see the link http://en.wikipedia.org/wiki/Relativistic_quantum_chemistry
    .....................................................
    It is written as follows,
    A nucleus with a large charge will cause an electron to have a high velocity. A higher electron velocity means an increased electron relativistic mass, as a result the electrons will be near the nucleus more of the time and thereby contract the radius for small principal quantum numbers.
    ........................................................

    The relativistic mass change are bigger in Li++ He+ than hydrogen atom.
    If you say the electrons are not actually moving, what cause the relativic mass change?
    Or you are saying that even if the electrons are not actually moving, the relativistic mass change will occur?

    I think your idea is contrary to the standard relativistic theory.
    How do you think about it?
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook