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

I Quantum mechanics is not weird, unless presented as such

  1. Jan 5, 2016 #1

    A. Neumaier

    User Avatar
    Science Advisor
    2016 Award

    Does quantum mechanics have to be weird?

    It sells much better to the general public if it is presented that way, and there is a long history of proceeding that way.

    But in fact it is an obstacle for everyone who wants to truly understand quantum mechanics, and to physics students who have to unlearn what they were told as laypersons.
     
    Last edited by a moderator: Jan 6, 2016
  2. jcsd
  3. Jan 5, 2016 #2

    A. Neumaier

    User Avatar
    Science Advisor
    2016 Award

    It is only the author's view, not ''the modern view''. It cannot be the truth because quantum mechanics was in operation on Earth (or the universe) long before the existence of preparation and measuring devices (which is assumed by Hardy at the end of p.1) - a true derivation must explain why certain multi-particle systems called measurement devices work as postulated! Also the number N of degrees of freedom, which he takes to be finite throughout, is infinite already for the harmonic oscillator, which makes his ''derivation'' invalid for any real system except those considered in quantum information theory.

    Those who want to see that quantum mechanics is not at all weird (when presented in the right way) but very close to classical mechanics should read instead my online book Classical and Quantum Mechanics via Lie algebras. (At least I tried to ensure that nothing weird entered the book.)
     
  4. Jan 5, 2016 #3

    bhobba

    User Avatar
    Science Advisor
    Gold Member

    Well that's the question isn't it. I don't think so - but likely for different reasons than you.

    My view of the fundamental basis of QM is as per Chapter 2 of Ballentine.

    There the same diagram as in Hardy's paper is used to define a quantum state. That's its definition. Hardy shows that and a few other reasonable assumptions leads to the two axioms as found in Ballentine.

    Your position is since states must exist independent of such an arrangement it cant be the basis of QM? Have I got that correct? If so then the ensemble interpretation is kaput and I think many would argue that one.

    Thanks
    Bill
     
    Last edited: Jan 5, 2016
  5. Jan 5, 2016 #4

    A. Neumaier

    User Avatar
    Science Advisor
    2016 Award

    Why? There is a difference between an interpretation and a derivation.

    An interpretation of quantum mechanics relates the formalism to the actual informal practice of using quantum mechanics in our scientific culture.
    Thus it may use objects familiar from our culture without having to explain their working. It must only show that there is a consistent relation between theory and practice.
    ** The minimal statistical interpretation (which you call the ensemble interpretation) does this for predicting the outcome of experiments. It is silent about the interpretation of quantum mechanics in the absence of measurements, and in particular about the interpretation of quantum physics applied to the far past before experiments were possible.
    I think that this is a is a serious gap, but since the interpretation is silent here it is not wrong or broken (kaputt), just very incomplete (as it should be for a ''minimal'' interpretation).
    ** The Copenhagen interpretation that claims that nothing can be asserted in the absence of a measurement is also consistent, but it is part of the reason why quantum mechanics is considered to be weird - a tree fallen in the wood has fallen only after someone has seen it.
    ** In a many-world interpretation anything goes, and at not even specifiable times the world splits and splits, completely unnoticed by us. This is already weird by conception.
    Thus neither interpretation is satisfactory.

    A derivation of quantum mechanics must derive quantum mechanics from general assumptions, and hence must be applicable to all of quantum mechanics.
    If it cannot derive how QM treats a harmonic oscillator it is worthless.
    If it needs measurement devices as inputs it is worthless, too, since it cannot explain why QM worked before the first human measured something.
    Hardy claims in his abstract that ''it is shown that quantum theory can be derived from five very reasonable axioms''. But his derivation fails on both accounts. He derives quantum information theory, not quantum mechanics.
     
  6. Jan 5, 2016 #5

    bhobba

    User Avatar
    Science Advisor
    Gold Member

    Why cant a derivation that starts from the basis of the ensemble interpretation (ie the statistical theory of observation as per the diagrams in Hardy an Ballentine) be valid?

    That I cant follow. A green leaf was green regardless of it is observed to be green - that's more or less the objective view of the world. Its really only philosophers that argue about such. If a state is the equivalence class of preparation procedures it does not mean that preparation devices and intelligent beings have to exist for it to be in such a class. It simply means, conceptually, if it was then that's what you would get.

    Thanks
    Bill
     
    Last edited: Jan 5, 2016
  7. Jan 5, 2016 #6

    A. Neumaier

    User Avatar
    Science Advisor
    2016 Award

    Everywhere in logic, if an assumption is invalid, the derivation carries no weight.

    The ensemble interpretation derives its assertions using, among others, the assumption that there have been observations.
    But there were surely no observations when the Sun formed - which is analyzed in astrophysics as a quantum process.
    Moreover, the ensemble had size 1 only, which makes any statistical interpretation meaningless.

    Similarly, Hardy specifies as one of his assumption (still before the first axiom) that ''The number of degrees of freedom, K, is defined as the minimum number of probability measurements needed to determine the state''. There is no such minimum number for a harmonic oscillator, since its Hilbert space is infinite-dimensional. How can his derivation account for the building block of all QM (beyond a manipulation of qubits) if it doesn't satisfy his assumptions? It cannot. So it says very little about quantum mechanics. it is only a consistency check on toy problems.

    [That = ''it cannot explain why QM worked before the first human measured something'']
    The derivation of Hardy begins with ''The state associated with a particular preparation is defined to be (that thing represented by) any mathematical object that can be used to determine the probability associated with the outcomes of any measurement that may be performed on a system prepared by the given preparation.''.
    If the system was not prepared and no measurement was performed, there was no outcome, hence the state is undefined. You may argue that Hardy gives a counterfactual definition, but this makes a very poor derivation.
     
  8. Jan 5, 2016 #7

    bhobba

    User Avatar
    Science Advisor
    Gold Member

    There I disagree. Its a conceptualisation that if you did it then that is the equivalence class it belongs to. But on this we will likely not reach agreement.

    Thanks
    Bill
     
  9. Jan 5, 2016 #8

    A. Neumaier

    User Avatar
    Science Advisor
    2016 Award

    Oh, so there is no logic involved - where deducing something from a false statement never implies that the conclusion is correct.
    It is then just a plausibility argument that the reader has to fill with his own details to make it logically sound.
    Calling such an argument a ''derivation'' is inappropriate. At best it is a blueprint for a potential derivation.
     
  10. Jan 5, 2016 #9
    QM weird or not?
    Well, the more you think about QM, the more weird it is.
    It fills our world with half dead cats and partially pregnant women (until pregnancy test is made).

    I was also thinking about American/Russian drone killing Syrian peasant far away, in the desert, with noone there to observe it or finding his body.
    The question is: "Did this poor peasant even exist?"

    Now more seriously:
    How to describe results of famous double slit experiment with better word than "weird"?

    I suspect that many peoples work with QM and are getting good results but very few of those are understanding it even partially.
     
    Last edited: Jan 5, 2016
  11. Jan 5, 2016 #10

    phinds

    User Avatar
    Gold Member
    2016 Award

    Possibly not to STEM types who are comfortable with math and complicated topics. To the layman, of course it is weird. Both QM (the very small) and cosmology (the very large) are such a great many orders of magnitude outside of the realm of human experience during the millions of years of our evolution that it would be a bit surprising were it otherwise.
     
  12. Jan 5, 2016 #11

    fresh_42

    Staff: Mentor

    This must not be the benchmark. Have you ever seen a quiz show in which the pure remembering of Pythagoras has been called math?
    People like Hawking or Kaku do their best to explain physics in a common manner. And in contrast to many they don't insist on their ivory tower. It must be the goal to explain complicated issues such that most people can follow, which IMO requires an education to people so they can follow the explanations without being an expert. The current gap is by far to wide. On the other hand it requires a lot more honesty on the experts' side. Many things are simply unknown. Admit it and don't hide behind mathematical constructions or their failure. I've followed the discussion here about virtual particles and their non-existence. Pair production was one of the first things at all I've read about elementary particles long, long ago. I find they are still a good vehicle for explanations. In mathematics theorems are widely regarded as beautiful if they are simple (to state and to prove). I like to think of physics in a similar way. Let us assume for a second a SUSY will be a feasible way to model a GUT. That wouldn't mean there is an even better way to do so. We just might not haven't found the right tools.
     
  13. Jan 5, 2016 #12

    phinds

    User Avatar
    Gold Member
    2016 Award

    I think you totally missed the point of my comment. I am not commenting on helpful teaching styles, just on the fact that QM and cosmology are so WAY far outside of normal human experience that many of the concepts involved will almost of necessity seem weird at first.
     
  14. Jan 5, 2016 #13

    fresh_42

    Staff: Mentor

    Agreed. Yesterday I've read about our home address in Lanikea. It's so huge and yet a small part. Lifetime of a pion is so short. I bet although we all can handle the number it's not really imaginable. Not to speak about the Planck scale. But these are true for all of us.
     
  15. Jan 5, 2016 #14

    phinds

    User Avatar
    Gold Member
    2016 Award

    Well, true for all of us, yes, but I think the normal STEM type person, rather that being weirded out by the unfamiliar simply says to him/herself, well this is stuff that other people understand and yeah there's going to be some math involved but that's fine. I can learn this stuff.
     
  16. Jan 5, 2016 #15

    fresh_42

    Staff: Mentor

    I still try to figure out what STEM means. I know what it's about by reading your comments but what exactly?
     
  17. Jan 5, 2016 #16
    STEM is an acronym for the fields of Science, Technology, Engineering and Mathematics
     
  18. Jan 5, 2016 #17

    bhobba

    User Avatar
    Science Advisor
    Gold Member

    The issue of state determination for states from infinite dimensional spaces is a problem. Personally I preclude them from discussions of QM foundations and only have finite dimensional states - infinite dimensions are introduced for mathematical convenience. This is the Rigged Hilbert Space approach where the physically realisable states are the space of all vectors of finite dimension but its dual is introduced for convenience.

    That's the other insights paper I am thinking of writing - but its cricket and tennis season here in Australia and I am too bleary eyed from staying up late recording and watching it.

    Thanks
    Bill
     
  19. Jan 5, 2016 #18

    fresh_42

    Staff: Mentor

    Tennis, ok, but cricket? Are you that disappointed from losing the final last year that you decided to watch cricket?

    However, the roles of finite and infinite vector spaces and their meaning in physics would be interesting to read. Especially if it's about the difference between necessity and convenience and the problem of convergence.
     
  20. Jan 5, 2016 #19

    bhobba

    User Avatar
    Science Advisor
    Gold Member

    I am a cricket tragic from way back ever since I saw Jeff Thomson bowl so fast it made the colour drain from batsman's faces:
    http://www.dailytelegraph.com.au/sport/cricket/ian-chappell-compares-pace-bowling-enforcers-jeff-thomson-and-mitchell-johnson/story-fni2fnmo-1226834729518 [Broken]
    'He unleashed a delivery that didn’t hit a batsman, nor slam into the wicketkeeper’s gloves but it did more psychological damage than any other in a series where many English batsmen were traumatised. The delivery landed mid-pitch and it’s next bounce half-volleyed the sightboard. The batsman saw where the ball landed and the colour immediately drained from his face; that delivery, from takeoff to landing must have travelled at least sixty metres.'

    There is nothing like watching the battle between fast bowler and the courage of batsman willing to face up to a cricket ball hurling at you at 100mph.

    Its nothing earth shattering. All you do is take the space of all vectors of finite dimension. Then you consider its dual as approximations to the vectors of large dimension that are easier to handle mathematically. Its like in solving problems of hammer strikes and such you model it as a Dirac Delta function. It isn't really - but to get a mathematical grip on the problem you model it that way. Convergence is also interesting - you use so called weak convergence that is a whole lot easier - but you need to wait for the paper if you haven't come across it before.

    Thanks
    Bill
     
    Last edited by a moderator: May 7, 2017
  21. Jan 5, 2016 #20

    A. Neumaier

    User Avatar
    Science Advisor
    2016 Award

    No, the more I think about quantum mechanics, the less weird it is. I have written a whole book about it, without any weirdness; see post #2.

    Quantum mechanics is weird only in the eyes of those who take the talk about it too serious and neglect the formal grounding which contains the real meaning.
     
    Last edited: Jan 11, 2016
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Quantum mechanics is not weird, unless presented as such
  1. Quantum Weirdness (Replies: 8)

Loading...