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How do we know that Quantum Mechanics is right?

  1. Jan 21, 2005 #1
    What I mean is, how do we know Quantum Mechanics is not some real good approximation to an even better theory about micro interactions? Just like Newtons's Law of Gravitation is just a good approximation to General Relativity. I do not have a problem with Qauntum Mechanics but in order for physicists to unite the two pillars dont we need the right theories? I just want to know what makes physicists so confident that Quantum Mechanics is not just an approximation. Thank you for your time.


    And about the succesful predictions that we experience today, Newtons Law of Gravitation predicted many things that General Relativity predicts, such as black holes.A theory can be approximate and still make true predictions.
     
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  3. Jan 21, 2005 #2

    dextercioby

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    We hope that QM (and its pupil QFT) are just low-energy approximations of another theory,a unifying theory...This is the thought that drove theorists over the last 30 yrs...

    I told u,they are sure of the opposite...QFT is not the ultimate theory...

    WHAT???? :surprised :surprised Please explain,or better show calculations...

    Daniel.
     
  4. Jan 21, 2005 #3
    well there is always the shwartzchild radius of a black hole which you can calculate using conservation of energy and gravitational potential energy:

    r=2G*M/c^2

    What I was trying to get at was that Newtons Law of Gravitation was just an approximation to General Relativity and that although Newton's Law was not ultimatly correct it could still predict orbits of planets, radius of a black holes, escape velocity,ect. I just wanted to know if Quantum Mechanics was ultimatly the correct theory, for like you said, low energy approximations. If Quantum Mechanics is not quite right and a better approximation is needed to create these TOE's then physicists should have a good level of confidence that QM is the correct approximation.
     
  5. Jan 21, 2005 #4
    I think what is holding us up in trying to create a more complete theory is precisely the matter that caused all the rumpus back in 1935, namely the incompatibility of QM with local realism or (as Einstein himself realised later, and Bell's theorem confirms) with the existence of local hidden variables.

    If the quantum physics community were to let go of their present belief that experiment has confirmed that hidden variables are impossible, there would be a much better chance of progress. Another conviction that may have to be relaxed is Planck's rule relating energy to frequency. It has some meaning in certain contexts but has been assumed to apply everywhere, at all frequencies and for all sources of radiation.

    Caroline
    http://freespace.virgin.net/ch.thompson1/
     
  6. Jan 21, 2005 #5

    dextercioby

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    Sure,but from Schwarzschild (sic) radius computed via deviuos reasoning (equating a nonrelativistic formula to a relativistic one) to black holes and their property is a long way...

    Daniel...
     
  7. Jan 21, 2005 #6

    vanesch

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    This is an often reoccuring misunderstanding of what science is all about. Scientists NEVER know for sure that their theories are right. The only thing they know for sure is a number of theories which cannot be right, which have been disproven by experiment.
    Scientists are not confident that QM is not just an approximation ; in fact, some theorists spend a lot of time trying to figure out other theories that could do just as well. The only constraints are that they have, in the right limits, to be compatible with those parts of QM where it is known to work well, within the accuracies of the experiments that have been testing QM.
    And know what ? It is damn hard to do so !
    So, the state is that *as long as no other alternative is available* we take as a working hypothesis that QM is right. This is the working hypothesis of 99% of all physicists, except those 1% theorists which try to find other ways. And they are courageous but have a hard time coming up with something else.

    cheers,
    Patrick.

    PS: 38.3% of all statistics are made up.
     
  8. Jan 21, 2005 #7

    ahrkron

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    QM works extremelly well in all contexts we have been able to test it. As vanesh said, it is always possible that the development of detectors with higher accuracies unveil disagreements, previously undiscernible because of larger experimental errors. However, since the development of QM, when disagreements have been found between theory and experiment, it has always been the case that they are better explained by other effects (sometimes unveiling new properties of matter, that can later on be confirmed and used) than by a change in QM.

    There are a few people trying to find ways to do away with QM,... but bear in mind that those are much, much fewer than when the theory was first developed. Back then, a lot of intellectual effort went into trying to find a way out of the strange properties of QM, but both its agreement with experiment and its internal consistency helped to convince most people.
     
  9. Jan 21, 2005 #8
    Okay, how do you know GR is not just a really good aproximation to an ever better theory called X Theory? How do you know X theory is not just a really good aproximation to an ever better theory called X' Theory? etc., ad infinitum. QM It's the best we can do for now I guess.

    In my opinion what really is important is to make sure data is treated properly, and that if something wierd is observed, those who see it are sufficiently well versed in the current scientific paradigm to be aware of the divergence.
     
    Last edited: Jan 21, 2005
  10. Jan 21, 2005 #9
    Right! It is, or ought to be, the results that don't fit prediction that lead to the real improvements, but in practice there are almost always social and political pressures to publish results that, as far as possible, do agree with accepted theory. In the Bell test experiments (an area I've put quite a lot of work into) I should be very surprised if the vast majority of experimental runs (the "preliminary" ones) did not in fact agree with the local realist prediction. The experimenters, though, aware of the fact that so many others have produced results "violating a Bell inequality" (and hence reported as in conflict with local realism), naturally tend to think they've done something wrong. They fiddle around with the apparatus, persuading themselves that they are getting nearer to producing a true "quantum state", until they too obtain a Bell test violation. But they may have unknowingly caused the widening of one of the possible loopholes! They may have destroyed the validity of the test, so that their result is still compatible with local realism. Without careful analysis of the actual apparatus used, who is to tell?

    It might help if there was some genuinely independent system for monitoring their procedures, but how many people not trained in quantum theory feel qualified to judge? The system is conditioned so as to become progressively more "inbred", with the difficulties of making a breakthrough into a radically different paradigm greater by the day.

    Caroline
    http://freespace.virgin.net/ch.thompson1/
     
  11. Jan 22, 2005 #10
    Caroline, I suggest you read about the Aharanov-Bohm effect. It is another example of non-locality in QM. If the Bell inequalities were proven false, and therefore local theories could exist, then you would still have the Aharanov-Bohm effect wich has been thoroughly tested.
     
  12. Jan 22, 2005 #11

    Haelfix

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    The Aharanov-Bohm effect is perfectly well understood by Quantum field theory, indeed nondetection of that would falsify the theory.

    Quantum mechanics seems theoretically at least to be valid at all scales, theoretically at least.

    Assume the opposite, you would have to have to devise some hidden variable or stochastic theory that merges smoothly into a probabalistic interpretation that then further merges smoothly back into classical mechanics. Very difficult! It has never been done, and there seems to be strong indications that in fact thats just mathematically excluded.

    Personally i've always been a little dismayed that that seems to be the case, I would like nothing more than to have generalized Copenhagen falsified, I simply loathe the theory from a conceptual point of view. But experiment has constrained us, we seem to be stuck with it from nearly every resource available to us.

    Even String theory and the quantum gravity people more or less work with the same old laws.
     
  13. Jan 22, 2005 #12
    Its quite obvious that QM is only an approximation. If it were a complete theory it would have to be the TOE. And what physicists are trying to do is not unite QM and GR as they are currently formulated, but to modify both of them or one of them to make them compatible with each other. You say , " how do we know QM is not some real good approximatin to an even better theory of micro interactions". You seem to think that there are different theories for micro and macro phenomenon. There obviously is a better theory of micro interactions and also all other pehnomenon in the universe, which is the TEO, which we dont have yet.
     
  14. Jan 22, 2005 #13

    Hans de Vries

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    The Aharonov-Bohm effect has nothing to do with non-localilty.

    from: http://rugth30.phys.rug.nl/quantummechanics/ab.htm :

    "The Aharonov-Bohm effect demonstrates that the electromagnetic
    potentials, rather than the electric and magnetic fields, are the
    fundamental quantities in quantum mechanics."

    See the link for some very nice computer simulations.
    The magnetic field is zero but the vector potential isn't


    Regards, Hans.
     
  15. Jan 22, 2005 #14
    You can view the Aharanov-Bohm effect as particles coupling to a vector potential, or as magnetic fields acting nonlocally on particles. Both views are equivalent.

    The whole effect can be put in terms of the magnetic flux that goes through the circuit formed by the two paths.
     
    Last edited: Jan 22, 2005
  16. Jan 22, 2005 #15

    Hans de Vries

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    No. It's the potentials which go in the QM equation, not the fields.
    The field is only zero because the potential is constant.

    What you suggest would mean "instantaneous" action by magnetic fields.
    This does not exists. The electromagnetic potentials and fields
    propagate with the speed of light ( see Lienard Wiechert potentials,
    in Jackson, chapter 14, for a formal treatment)

    Regards, Hans
     
    Last edited: Jan 22, 2005
  17. Jan 22, 2005 #16
    Hmmm ... No, I challenge this. Surely the Aharonov-Bohm effect merely proves that Maxwell's equations are slightly misleading? There is some kind of field there even when the equations there is none. This field presumably, like all others, propagates out from its sources at speed c.

    Caroline
     
  18. Jan 22, 2005 #17

    Hans de Vries

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    Indeed,

    This Field is the magnetic vector potential A. The way it propagates
    out from its sources at speed c
    is handled by the Lienard-Wiechert potentials.

    For nice 3D computer simulations of the Aharanov-Bohm effect.
    http://rugth30.phys.rug.nl/quantummechanics/ab.htm


    Regards, Hans
     
    Last edited: Jan 22, 2005
  19. Jan 22, 2005 #18
    Im not talking about TOE's. Im talking about the proper theory of micro interactions in order to get the TOE right!
     
  20. Jan 22, 2005 #19

    ahrkron

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    There is quite a lot of experimental evidence for every aspect of QM by itself and, also, have you seen the "review of particle properties" published by the PDG (Particle Data Group) every two years? It is a 1000+ page compilation of all numbers found in serious experiments all over the world. Each one of those numbers basically corresponds to a PhD thesis, reviewed by an experimental collaboration (similar to Fermilab's CDF, D0 or CERN's LEP), and pretty much ALL OF THEM are correctly predicted by QM. This includes the mass of mesons and hadrons (many more than the quarks, the mass of which we use as input, so we know that we are getting from the theory more than we are putting in, in a sense), their lifetimes, the frequency with which each decays to quite a big number of other particles, the shape of the distribution of their momenta, the way in which some of them exhibit the violation of the CP symmetry, how some oscillate into other particles (including how often and by how much), and many more quantities.

    All these numbers are well described by the Standard Model of particle physics, which is a Quantum Field Theory based on Quantum Mechanics and Special Relativity.

    Current efforts on tehoretical physics are dealing with much subtler effects than those you would expect if QM was a wrong starting point.
     
    Last edited: Jan 22, 2005
  21. Jan 23, 2005 #20
    See for example J.J. Sakurai, Modern Quantum Mechanics, page 472.
     
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