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Interpritations of Quantum Mechanics

  1. Jun 5, 2004 #1
    :smile: Hello ppl. I was wondering how many different interpritations of quantum mechanics there are? I was told there was one called the "many worlds" interpretation. What other ones are there and what do they mean...and what does this one really mean. I wanted to ask this in this Quantum physics forum b/c I was hoping there are ppl that know quantum mechanics very well here.
     
  2. jcsd
  3. Jun 5, 2004 #2
    MAN!!! Is this a hard, stupid, or just ignorant question? NO one seems to be replying...whats up with that?
     
  4. Jun 5, 2004 #3

    turin

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    Alem2000,
    I don't think most people hang out here all day on saturday. My experience is that you need to give the post, on average, about a day to get a satisfactory response. Your question is not at all stupid. You have heard something about QM and you are hungry for more info, clarification, and such. That is exactly what most serious posters around like, I think.

    I'll get you started, but I'm sure you will get a lot better post from some of the others.

    I've found the Schroedinger picture with the Copenhagen interpretation to be the paradigm for traditional physics instruction. What this means is that the Schroedinger equation is taken for granted, that the wave function is the physical thing that undergoes dynamics and operators simply do the telling, and that it doesn't even make sense to consider the wave function's effect on anything else without collapsing.

    There is also the Bohm-deBroglie interpretation that says there is actually a little dot flying around, but, since it is so tiny, the effects of its wave come into play. The dot itself only goes through one slit of the two-slit experiment, for example, but the wave is difracted so that the dot still seems to follow a wave-like behavior.

    The many worlds interpretation is also known as the Everett interpretation (or something like that). I don't know too much about it.
     
  5. Jun 5, 2004 #4
    Well lets take something from Relativity and ask how QM deals with the same thing?..In Relativity, "Acceleration and Gravity" give out the same effect for something in freefall, they are termed as being Equivilent.

    How does QM deal with the same effects?.in QM what is Gravity and what is Acceleration.
     
  6. Jun 5, 2004 #5

    selfAdjoint

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    QM does not deal with gravity. The reason often given is that gravity is a much weaker forces than the ones dealt with in quantum physics: the electromagnetic force, the weak force that is responsible for radioactivity, and the strong force that holds the nucleus together.

    Acceleration, as rate of change of speed, is treated in QM, although physicists usually prefer to work with rate of change of momentum.

    I see your point about acceleraation and gravity in relativity (I assume you mean general relativity), but as it could give some confusion to others I'll restate it: it is impossible to tell (locally) whether a given acceleration is due to an imposed force or to gravity.
     
    Last edited: Jun 5, 2004
  7. Jun 6, 2004 #6
    Thanks for the clarity SelfAdjoint. My post was meant to instigate thought into the process involved, and how there exists a Macro Force with definate paramiters in GR, and as one approaches the Quantum Domain the Force and effects are reduces to non definate paramiters for local events.

    Is it possible to define (locally) whether a given momentum is produced by a Macro or Micro force?..interesting!
     
  8. Jun 6, 2004 #7
    There are also the collapse theories, this approach is also called Dynamical reduction program. These theories modify the Schroedinger equation, adding to it stochastic and nonlinear terms. Seems that these theories are also able to explain why macroscopic objects exhibit definite characteristics, while microscopic objects do not.
     
  9. Jun 7, 2004 #8
    A not-so-short list of interpretations of QM
    -----------------------------------------

    There are almost as many intepretations of QM as there are physicists/philosophers who work on the foundations of the theory. I have developed my own classification, which is bond to be slightly controversial because not everyone will agree that it is correct. I have also indicated a few names associated with the interpretations. A quick google should get you some more info on them. Note, the list is far from complete.

    The classification:
    -----------------

    Conservative: Takes the standard formalism of QM without modification and seeks to find a language to talk about it that avoids all the paradoxes and conceptual difficulties. Although conservative in that it doesn't do any damage to the maths of QM, it is likely to be radical from a philosophical point of view (so perhaps conservative is not the best word in this sense).

    Realist: Takes something in the QM formalism to represent things that really exist in the world. Usual choices are things like position of particles or the wavefunction itself.

    Partial-realist: Sometimes takes something in the QM formalism to represent things that really exist, but there are some extra rules that must be satisfied for this to be the case.

    Beyond QM: These are really different theories rather than interpretations because they make some predictions that differ from QM. These predictions are likely to be in areas of the theory that have not been adequately tested as of yet.

    The list:
    --------

    Consistent Histories/Decoherent Histories (Conservative, Partial-Realist)
    Griffith, Omnes, Gellman, Hartle

    Conciousness based (Conservative)
    Wigner

    Copenhagen (Conservative)
    Bohr, Heisenberg et. al.

    de-Broglie Bohm (Realist, can be beyond QM if you generalize the allowed initial probability densities)
    de-Broglie, Bohm, Valentini, people from southern Europe (for some reason)

    Decoherence solves everything (Conservative)
    Zurek

    Everett Relative State Interpretation (Conservative)
    Everett, Wallace

    Everything is Information (Conservative)
    Wheeler, Fuchs, Zeilinger

    Ithica Interpretation (Conservative)
    Mermin

    Many-worlds (Conservative)
    Deutsch, Vaidman (maybe Everett and Wallace depending on your reading)

    Modal (Conservative, Partial-Realist)
    Mainly philosophers: Bub, Clifton et. al.

    Orthodox (Axiomatic) QM (Conservative)
    von Neumann

    Quantum Logic Interpretations (Conservative, Partial-Realist)
    Putnam

    Relational Interpretations (Conservative)
    Rovelli

    The Shut-Up-and-Calculate Interpreatiaion (also known as FAPP) (Conservative)
    Most of the physics community

    Spontaneous Collapse models, e.g. GRW theory (Realist, Beyond QM)
    Gihradi, Rimini, Weber, Pearle

    Transactional Interpreatation (Conservative)
    Cramer (see also Huw Price)
     
  10. Jun 7, 2004 #9
    "The Shut-Up-and-Calculate Interpreatiaion " - certainly this is the most prevalent in normal physics curricula across the nation.

    copenhagen seems to still be the preferred general interpretation for theoretical physics, though interpretation in itself is hotly debated everwhere all the time. i still cling to the concept that the simplest explanation is usually the best, and while there isnt any particluar way to "understand" CI logically, it still strikes me as fundamentally the most sound and accurate.
     
  11. Jun 8, 2004 #10
    I disagree. Hardly any theoretical physicists subscribe to the Copenhagen interpretation and those that do usually misunderstand what it means. This is not too surprising because it is difficult to find a clear statement of what Niels Bohr's interpretation actually was.

    Most physicists believe that QM applies equally well to all systems in the universe, including the whole universe itself. This goes directly against the grain of Copenhagen, which postulates that there must always be a "classical" component of any system, although it is deliberately vague on where this cut between classical and quantum should be placed. In fact, Heisenberg argued that the cut may be placed anywhere that is convenient for describing a particular experiment, but that such a cut must be placed somewhere. Thus, Copenhagen has no place for a "wavefunction of the universe" unless there is somewhere outside of the universe from which to observe it, which is basically a contradiction.

    Note also that "textbook quantum mechanics" does not usually give the Copenhagen interpretation. It is actually much closer to von-Neuman's approach.

    I think there is no way that the Copenhagen (mis)interpretation should be taken seriously today. Issues from the foundations of QM are becoming of practiacal importance for quantum computing and quantum communication. Also, quantum gravity is difficult to construct partly because we don't have a clear answer to the problems of quantum theory. Thus, the interpretation of QM is no longer a problem with no practical consequences for physics.
     
  12. Jun 8, 2004 #11
    in that there are only two basic tenets of CI (there is no underlying reality, and observation creates reality), i dont know exactly what you are finding there to disparage as a "(mis)interpretation." it was early detractors, such as einstein, who calimed there must be a classical component, not bohr and the solvay congress, though Bohr and Heisenberg never totally agreed on how to understand the mathematical formalism of quantum mechanics. and i certainly dont think you can support the comment that it should not be, or isnt taken seriously today, or that it somehow limits research on quantum computing or communication. eric weisstein writes "In the years since its formulation, it (CI) has come to be regarded by many as the "standard" philosophical interpretation of quantum mechanics." even the wikipedia states "The Copenhagen interpretation is the mainstream interpretation of quantum mechanics." quantum gravity is a myth.
     
  13. Jun 9, 2004 #12
    You are confusing two different meanings of "classical component". Einstein thought that the QM formalism was incomplete in that there should be some underlying reality to which the probabilities predicted by QM refer. You might call this reality a "classical component".

    Bohr certainly did not believe this, but he did believe that classical concepts were necessary to describe quantum experiments themselves, i.e. the description of the experimental setup and the fact that a particular outcome is obtained is necessarily described in terms of our everyday classical experience.

    It is also far from clear that there are only two basic tenents of CI. In fact, each physicist who supposedly subscribed to CI had a different reading of the interpretation. This seems to apply equally well today. One of my reasons for rejecting Copenhagen is this vagueness. Perhaps you should look at http://plato.stanford.edu/entries/qm-copenhagen/

    Hmm, I think it would be wrong to attriute the Copenhagen Interpretation to the Solvay Congress. This was a meeting of most of the leading physicists of the day, including Einstein, and it is far from clear that the majority subscribed to Bohr or Heisenberg's interpretation of the theory.

    To support the "isn't" part, here are the results from an informal poll of physicists taken by Tegmark in 1999 (http://xxx.soton.ac.uk/abs/quant-ph/0101077).

    For the "should not be" part, I have to admit that it depends on what you mean by the Copenhagen Interpretation. If you mean some sort of interpretation that is operational in nature, i.e. makes no claims to an underlying reality beyond the probabilities predicted by QM, then I think that good arguments can be made for taking it seriously. However, if you mean the explicit positions of Bohr, Heisenberg or other Copenhagenish pioneers of quantum theory, then I do not think they are appropriate for modern physics.

    I don't think either of these sources are definitive authorities on the subject.

    I agree that current theories of quantum gravity are likely to be wrong. In my view, this is because we need to sort out the foundations of quantum theory before the correct path becomes clear. However, that is probably a minority opinion amongst physicists.

    What exactly do you mean by "quantum gravity is a myth"? Do you think that a reconcilliation of the two most important theories in physics is impossible? Do you think that there can be no physical theory that accurately describes the early universe at times when both gravity and quantum mechanical effects are equally important? If so, then I would like to know how you can support this position.
     
  14. Jun 9, 2004 #13
    "I don't think either of these sources are definitive authorities on the subject." no duh. and no argument about that, but it does point out that for general acceptance, i dont think you can wholesale ignore it as a foundation from which to begin understanding its implications.

    i personally have never been impressed by the many worlds interpretation - it just seems so ludicrous, specious, and irrelevant, especially in light of how mind-numbingly incomprehensible observable reality already is. just because an interpretation is "allowed" via certain solutions to equations does not mean that they are reasonable or logically feasible. i also cannot buy into bohm's semi-pilot wave ontological morass - his book, wholeness and the implicate order, was a difficult read at best, and i could not agree with many of his lines of thought - though i admit he is an outstanding thinker. modified dynamics does not seem to offer anything new or valuable, IMHO. which only leaves CI and "undecided" - i can greatly support "undecided" as a safe position :-)

    "Do you think that a reconcilliation of the two most important theories in physics is impossible?" hmmm - tough question. surely the extrapolation of energy curves indicates unification of forces, including gravity, at extreme levels may be reasonable. however, quantum mechanics does not appear reconcilable with GR at any fundamental level that i am able to grasp. my long suffering issues with GR lead me to think the whole idea of particles is a primary misconception as entities unto themselves (ie that have some specific reality), and that the solution will only come with a far deeper understanding of fields and their interactions. i know this is contrary to the standard model, which serves us so well for real-world applications, but for an overarching GUT, it may be that we will need to build it on a foundation of field theory, rather than particle/quantum basis. it seems far too common for theorists to speak of particles and fields as separate aspects, even though we are all taught early on that particles are merely the manifestation of fields. this is supported by the simple understanding that as you delve down into the makeup of any particle, you at some point recognize that there is no "thing" there - leptons are point particles, they occupy no volume - what is that? baryons are made up of quarks, which are point particles. the concepts of virtual photons and gluons, and the near-complete non-understanding of the nature of force-carriers in general, do not fit into any frame of comprehension, and as competing theories, do not come anywhere near the elegance and beauty provided by GR. field theory is where it is at. ooops, i am starting to reveal some of my personal bias... you must keep in mind that i am just an old hack who long ago admitted that i shall forever remain confused about the nature of reality. OTOH, it is with great pity that i regard the vast number of humans who never even learn enough to realize how incomprehensible reality actually is - "it is not only stranger than we conceive, it is stranger than we CAN conceive..."
     
  15. Jun 10, 2004 #14
    I couldn't agree more with you about the many-worlds interpretation. It seems to be fairly contentless and to offer little insight into the real problems, since many-worlds would also be "allowed" by classical physics. I also agree that "undecided" is probably the safest position.

    However, the options available in the poll do not represent the only possible choices and they are colored by Tegmark's own opinion on the subject. My personal view is that quantum states should be regarded as states of knowledge/information/belief (pick your favourite). This raises the question of what they are states of knowledge about. I haven't discounted the posibility that they are about what might be called "hidden variables", but not of the Bohmian sort. I do not like the guidance role that the wavefunction plays in that theory. I would also prefer the "hidden variables" to respect more of the symmetry that is present in quantum theory, i.e. not to pick out position as a special variable and not to have a preferred reference frame. I am open to the idea that the "hidden variables" might not have a naive realist interpretation, which is why I have put them in quotes.

    If such an interpretation proves to be impossible, then I would retreat to the position that quantum states represent knowledge about our possible interventions in the world, which has a much more Copenhagenish flavour.

    My own opinion on quantum gravity is that we probably have to go beyond both quantum theory and general relativity. It should probably be based on some sort of discrete structure, in order to have a minimum length scale. However, I don't think it is a good idea to take a discrete structure and then simply attach a Hilbert space to it and say that you then have a quantum theory. Many approaches seem to be basically doing this and it always seems fairly arbitrary to me. Hopefully, investigations into quantum foundations will eventually reveal a better account of what it means to be "quantum" than this.
     
  16. Jun 10, 2004 #15

    ZapperZ

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    We need to be a bit more careful in adopting this view of "states" in QM. If we consider them to be just as what you said, i.e. the state of OUR knowledge of the system (i.e. it is not actually a direct representation of the actual system), then QM is nothing more than ordinary, classical statistical probability. It would be equivalent to saying that we assign the probability of 1/2 in getting a head or a tail when tossing a coin, not because of the inherent randomness in the process, but rather because of our ignorance of the intricate details of the dynamics. But is this really an accurate depiction of QM also?

    I used to believe that line of thought until I looked even closer to what exactly can be see that is a direct manifestation of "non-classical, non-statistical" effects of QM. Superposition of states is the clearest example. Is this merely a reflection of our ignorance of the system, or is the system REALLY in a simultaneous combination of states. I would say that there are ample evidence of the latter being the true case. Example: the existence of an energy gap between bonding and antibonding states of H2 molecule. An electron is REALLY spreading itself over BOTH H atom site simultaneously (i.e. the schrodinger cat IS really both dead AND alive), even though upon measurement, it assumes a definite postion. Now you would never see such unusual effects in classical statistics that merely reflect our ignorance or knowledge of the system.

    Zz.
     
  17. Jun 10, 2004 #16

    selfAdjoint

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    But doesn't "reallly" become problematical itself in QM? Some physicists don't believe virtual particles "really" exist, although like your molecular consequences of superposition, they may have "real" effects.
     
  18. Jun 10, 2004 #17

    ZapperZ

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    I used the word "really" in the pedestrian sense, not in any deep, philosophical connotation. I hate to think that this string would deterorate into a discussion of the meaning of the word "real".

    If you think carefully, ALL of what you accept to be "real" are based on EFFECTS, and the manifestation of the various properties of the "real stuff" via those effects. So to differentiate between "real" and "real effects" is rather strange from my perpespective. I've seen such arguments being used to argue that "electrons" aren't "real".

    Zz.
     
  19. Jun 10, 2004 #18
    Predictive outcomes in a three particle entanglement?

    (abc +a'b'c')

    That's real :)
     
  20. Jun 11, 2004 #19
    Coming from a quantum information makes viewing quantum states as states of knowledge an attractive position for me. It seems to make some of the quantum information protocols slightly less mysterious than if you take the quantum state to be a state of reality. In my opinion, the two best arguments for this position from a modern perspective are:

    http://xxx.arxiv.org/abs/quant-ph/0205039
    http://xxx.arxiv.org/abs/quant-ph/0401052

    There are some key differences between the points of view in these papers and, in particular, the first takes the quantum state to be far more subjective than the second. The second paper is closer to my view onthe subject.
     
  21. Jun 16, 2004 #20
    From the issue of "Glast" and the understanding of Quantum Gravity, I am looking for a way in which to describe "geometical realities over a vast distance in space.

    One of the ways that I am looking at , putting aside FTL and Vsl characteristics, the issue of high energy photons travelling through space a lot slower then low energy photons. We do understand that the speed of light remains consistent in a absolute vacuum. So for this measure I am looking to the early universe.

    Now one of the issue brought up has to do with the interaction with the graviton(theoretcally if such action was to take place how would this action effect the photon) We understand gravity wells may have an effect on the differences of the photon, so if such was the case then, how would you view "entanglement" after the photon has interacted?

    Gravitational Lensing :)

    Avenues to quantum geometry?
     
    Last edited: Jun 16, 2004
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