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

No experiment can be repeated? (2 questions)

  1. Apr 26, 2007 #1
    1) Hey, so I just started reading the Feynman lectures. In the second chapter he has a small intro to quantum and he says:

    ok, so far so good, I was aware of this part. But then he goes on to say:

    this is just the intro of the book so he might go into more detail later, but it caught my attention— I always figured that there were "internal wheels" and we just haven't discovered them yet, and that this is why we can't predict (or that the wheels were simply unobservable)...

    but his statement implies there are no wheels at all?? is this actually the case? it seems weird that reactions would happen without rhyme or reason yet still follow a semi-predictable pattern. How do we know that there aren't any internal wheels that we are yet to discover?
    Maybe I misunderstood what he meant (most probably :-) ).

    2) (EDIT) I realize now my second question reads kind of stupid because I didn't really explain that well what I meant... I meant, since the lectures are pretty much the outline of a physics class, if in the same physics class now a days there would be any subject that is not mentioned in the books due to recent advancements.

    I've read next to nothing on it, but I that know string theory is pretty new, even though some physicists find it to be loopy (haha end with a pun! everyone loves a pun, right? ... r-right?)
    Last edited: Apr 26, 2007
  2. jcsd
  3. Apr 26, 2007 #2


    User Avatar
    Science Advisor

    Moe, your point 1) is a good one.
    The claim that "there are no internal wheels" is nothing but one of quantum myths discussed in
    (See in particular Sec. 4)
    The fact is that we really do not know if internal wheels exist or not. A more common name for such internal wheels is "hidden variables". What we do know, however, is that if hidden variables exist, then they must have some strange properties, such as nonlocality and contextuality. (In the paper above you will also find more detailed explanations of these properties.) These strange properties make some physicists believe that they do not exist, but nobody knows with certainty if they exist or not.
  4. Apr 26, 2007 #3
    R.F:” For example, it is possible to arrange an atom which is ready to emit light, and we can measure when it has emitted light by picking up a photon particle, which we shall describe shortly. We cannot, however, predict when it is going to emit the light or, with several atoms, which one is going to… You may say that this is because there are some internal ‘wheels’ which we have not looked at closely enough. No, there are no internal wheels.”

    Sure. R.F. has in mind two fundamental properties of quantum systems: 1) all atoms, electrons and photons considered are the same (indistinguishable); 2) time in the quantum world has size (HUR). When you perform the measurement, you are looking at quantum world using classical eyes. In the classical world time have no size, the quantum size tie up to the same single point. All you may say is that all photons were emitted within that delta t and this is consistent with the fact that the identical particles are indistinguishable (otherwise you may distinguish them using time tag). The emitted photons are the wave packets. If you will measure delta E, you may calculate delta t (for example). Sure that in this measurement you must use repeatability. Notice that I did not use any internal “wheels” and I do not understand what is weird here. The R.F. example describes the transition from the upper energy state to the lower energy state exactly as in classical physics.

    The answer to your second question is yes.

    Regards, Dany.
    Last edited: Apr 26, 2007
  5. Apr 26, 2007 #4
    Sweet, so QM somehow proves that everyone loves a pun! :biggrin:

    So from the pdf (I read the sections that talked about the randomness) in a nutshell I get that there could be hidden variables, some physicists believe that there probably aren't, so I'm guessing, from his comment, that Feynman was in that group.

    I guess what confused me was that his wording seems to imply that we know for a fact that there aren't any hidden variables, which seems counter intuitive to me (and physics are often counter intuitive so I wanted to make sure).

    Like I said I just started reading the book so I have a long way to go until I can understand this in more detail, but that statement kind of struck me so I wanted to get it out of the way (it was bugging me all day actually lol).

    I realize now my second question reads kind of stupid because I didn't really explain that well what I meant... I meant, since the lectures are pretty much the outline of a physics class, if in the same physics class now a days there would be any subject that is not mentioned in the books.
    Last edited: Apr 26, 2007
  6. Apr 26, 2007 #5


    User Avatar
    Science Advisor

    For fermions we can say that there are no internal variables if we assume they obey the same quantum rules as all observable variables, because the Pauli exclusion principle, which implies that even if we hadn't discovered some variables yet we could tell if the set of variables we had so far was incomplete. But this doesn't rule out hidden variables which fail to obey the same rules as measurable variables.
  7. Apr 27, 2007 #6
    The assumption about existence of the “hidden” variables that describe the quantum world as the classical world is so counter intuitive to me that I never read even one paper in that direction (at least did not finished reading to the end). Similarly, for example, the assumption that the universe is described by SE seems to me outrageously stupid.

    Now I see that I understood your second question correctly and I did not consider it stupid.

    Regards, Dany.
  8. Apr 27, 2007 #7
    Perhaps it is worth to add something.

    In addition, it is pretty good group: A.Einstein, E. Schrödinger, W. Heisenberg, P.A.M. Dirac, J. von Neumann, etc. For me the case was closed after J. von Neumann discussion (I consider the statement made by youngster J.S.Bell as simply unmannerly kid).

    If you intend to prove that somebody is wrong you will not have time to do something positive in your life.

    Regards, Dany.
  9. Apr 27, 2007 #8
    Well I'm obviously not anywhere near the level of being able to understand arguments on either side. My understanding of QM is superficial at best, so my comment also came form a purely superficial point of view.— The idea of something being random is very hard for me to understand, harder than other "weird" concepts like infinity and extra dimensions, I just can't fit it in my head.

    By the way, you listed Einstein, I thought he did have a problem with the idea (the whole "god does not play dice" thing).

    Well, I hope I'll get a bit clearer picture once I get to the 3rd book that has all the QM stuff.
  10. Apr 27, 2007 #9


    User Avatar
    Science Advisor

    Einstein was definitely NOT in that group. He never accepted that nature is probabilistic and that QM is a complete description of nature.
    Schrodinger was also not in that group, he never accepted the concept of "quantum jumps".
    The case was definitely NOT closed by Neumann, while your statement on Bell is simply meaningless.
    Nevertheless, it would be interesting to see what words would you choose to characterize the contribution of D. Bohm. :smile:
  11. Apr 27, 2007 #10


    User Avatar
    Science Advisor

    1. Who said that hidden variables are classical? We know for sure that they must have at least one nonclassical property - nonlocality.

    2. So, the universe is described neither by hidden variables, nor by SE. Then, how it is described?
  12. Apr 27, 2007 #11
    I don’t know what your current background is. The Feynman Lectures on Physics are extremely difficult since they are extremely deep (in addition they are the experiment of the one-to-one translation of R.P. Feynman volume of knowledge of the mathematics into English). Certainly, it is not good introduction (the anecdote say that the students that were able to pass his exams successfully were accepted to be his Ph.D. students). When you will complete your degree and in order to check whether you also understood something you may return to the Lectures. There are number of excellent modern introductions in QM, for example, S. Gasiorowicz “Quantum Physics”.

    So, you are in pretty good group (see above). Notice, that in my post (#3) I did not use the statistical interpretation either.

    I agree with A.Einstein:” It, in my view, cheap consideration”.

    Regards, Dany.
  13. Apr 27, 2007 #12


    User Avatar
    Science Advisor

    Although I am an adherent of the Bohmian interpretation, I agree with Einstein that this interpretation is very cheap. But, unlike Einstein, I take it as a virtue, not as a drawback. For me, cheap means simple, and I prefer simple interpretations, not "deep" and "mysterious" ones. :approve:
  14. Apr 27, 2007 #13
    To the best of my knowledge the state-of-art description of the universe (large scale phenomena) is given in terms of A.Einstein general theory of gravitation. Indeed, a more complete description will be achieved in future by the kids like moe darklight.

    I think that the point here is that you ignore the fact that laws of nature are defined by God and not by you. And A.Einstein had better experience with that compare with you.

    Regards, Dany.
  15. Apr 27, 2007 #14


    User Avatar
    Science Advisor
    Gold Member

    Kind of an odd comment. I am not sure how you would know this about Demystifier.
  16. Apr 27, 2007 #15


    User Avatar
    Science Advisor

    That's easy: Everybody is less clever than Einstein, so Demystifier is less clever than Einstein too. :biggrin:
  17. Apr 27, 2007 #16


    User Avatar
    Science Advisor

    And what about the small scale phenomena of the universe?
  18. Apr 27, 2007 #17
    What do you mean? That Demystifier is not a God or that laws of nature are not defined by him? I do not know. It is interpretation and as you know I am very weak in interpretations. Therefore, I asked your help in Why don't the Slits collapse the wave function? The paper written not clears enough for me. I am still waiting your even comment.

    Regards, Dany.
  19. Apr 27, 2007 #18
    I doubt this since the degree I'm going for is digital film production :smile:, I just really enjoy science (I was torn between biology and film, but I chose film as a career). So the best I could offer is a very accurate science fiction movie some day :rofl:

    I have heard about the third book being hard, so I'm expecting a challenge.. I'm not in a school setting and I don't have the pressure of deadlines and exams, so I can take as long as it takes for me to understand it.

    Thanks for the explanations :biggrin:
  20. Apr 27, 2007 #19


    User Avatar

    I'd be satisfied with that. We could use accurate sci-fi's in place of the garbage we generally get from Hollywood.:biggrin:
  21. Apr 27, 2007 #20
    Who knows for sure about the garbage?

    Without the TV series "Quantum Leap" there MAY not have been string theory!

    (everything's connected o:) )

    and before TV, Well's "Time Machine" was written just a couple years before relativity came out.
    Last edited: Apr 27, 2007
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?

Similar Discussions: No experiment can be repeated? (2 questions)
  1. Question on experiment (Replies: 28)

  2. 2 slit experiment again (Replies: 49)