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

Wave collapse

  1. Mar 10, 2004 #1

    hypnagogue

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Hate to ask another one of these questions, but I've just read something about the collapse of the wave function that does not seem consistent with other accounts I've read about it. From what I understand, the wave function of a system is collapsed automatically by interaction with another, macroscopic system. Would anyone care to comment on the following?

    from The Physics of Consciousness by Evan Harris Walker
     
    Last edited: Mar 10, 2004
  2. jcsd
  3. Mar 10, 2004 #2

    ZapperZ

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor

    What you quoted is only valid up to a certain extent. Let's say we have an isolated atom, and we solved for the wavefunction of this atom. If we start having many more of these atoms and they start to WEAKLY interact with each other, then the most general wavefunction for the entire system can be accurately approximated as a sum of each individual atomic wavefunction - the Psi_i's that you quoted. [Note that when you do this, the general wavefunction is no longer orthorgonal. In solid state physics, we now have to construct what is known as the Wannier function to preserve some degree of orthorgonality]. This is because we make very little perturbation to the original individual hamiltonian.

    However, once we make them interact even stronger that the modification to the potential part in the hamiltonian can no longer be ignored, then the ORIGINAL, individual wavefunctions are no longer the eigenfunction of the system. You cannot simply assume that the general wavefunction is the sum of all the atomic wavefunctions. One has to re-solve the new hamiltonian and find the new eigenfunctions, and typically, this can't be done analytically. In such cases, this is where many-body physics comes in and provide various models to formulate a new hamiltonian for the many-body system.

    As a side note, I am always skeptical of anyone trying to make any connection between a science and a pseudoscience, especially when there are concepts such as "consciousness" that are ambiguously defined. More often than not, those who tried to do such things tend to not be physicists, but think they know enough to interpret and use various physics ideas.

    Zz.
     
  4. Mar 10, 2004 #3

    hypnagogue

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Re: Re: Wave collapse

    Thanks for your response. Some of it is beyond my knowledge of physics but I think I have the general idea. However, I don't think Walker's claim here is that we simply do a linear combination of wave functions as much as it is that what we get, ultimately, is still a wave function with a superposition of states rather than just one state-- there is nothing in QM that states that the function has collapsed yet. Do you agree with this general statement?

    I'd like to stay away from this issue, as it is only my intent right now to assess the quoted passage. However, since you brought it up, I might as well list the information about the author given in the book:

    Evan Harris Walker is the founder of the Walker Cancer Institute. Since he received his Ph.D. in physics from the University of Maryland in 1964, he has published more than a hundred paper in scientific journals and holds a dozen patents. He lives in Aberdeen, Maryland.
     
  5. Mar 10, 2004 #4

    selfAdjoint

    User Avatar
    Staff Emeritus
    Gold Member
    Dearly Missed

    It seems to me that the Walker quote is saying that the measurement device is itself a quantum system, and since it is (stongly?) interacting with the given quantum system its state function becomes part of just such a larger nonlinear wavefunction as ZapperZ described. I believe Wigner was the first to state this clearly, and John Bell has also written on it.
     
  6. Mar 10, 2004 #5

    ZapperZ

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor

    Re: Re: Re: Wave collapse

    I don't agree with it in general. I think what he was trying to convey is the concept of decoherence whereby the act of measurement simply induces a larger degree of freedom to the whole overall system (which now includes the orginal system and the instrument). This is totally different than the mathematical operation A|psi> or <psi|A|psi> where A is an observable and |psi> is the complete set of basis function. It is where there is a selection of which states "survive" after a measurement (operation with an observable). If all we get after a measurement is just MORE superposition, there wouldn't be any reason for the brouhaha surrounding the Schrodinger Cat in the first place.

    You're right. I was making a general observation and not specifically directed to him. I may have actually heard of him, though I have not read the book you quoted. If it's any consolation, I'm also skepticaly of Stephen Wolfram and his latest book on complexities - or maybe I've made it worse by saying that! :)

    Zz.
     
  7. Mar 11, 2004 #6

    hypnagogue

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    So the measurement problem is such that we cannot be sure exactly when the wave function collapses, just that by the time we observe the results of the experiment the relevant functions will have been collapsed-- is that correct?
     
  8. Mar 11, 2004 #7

    hypnagogue

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Re: Re: Re: Re: Wave collapse

    OK, but how exactly are you defining 'measurement'?
     
  9. Mar 11, 2004 #8

    ZapperZ

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor

  10. Mar 13, 2004 #9
    hey ZapperZ,
    i visited the webpage you mentioned in the link...but where in that page does the author talk about what a measurement IS ?!
    i did not find a defenition of measurement there!
     
  11. Mar 13, 2004 #10

    ZapperZ

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor

    Postulates 3,4, and 5. They define what is meant as an act of measurement on a quantum system.

    Zz.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: Wave collapse
  1. Wave vector collapse (Replies: 6)

Loading...