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

HUB & accurate single measurement

  1. Feb 9, 2013 #1
    Hello,

    I recently read some texts which says that in a SINGLE measurement you can measure position and momentum to an arbitrary accuracy, assuring that that's not what HUB talks about.

    That confuses me, because I learnt that HUB is there NOT simply because some property of QM prevents us from knowing them simultaneously accurately, BUT BECAUSE the particle really don't have such things as position and momentum at the same time.... ......... I'm confused..... HELP me!
     
  2. jcsd
  3. Feb 9, 2013 #2

    ZapperZ

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor
    2016 Award

    It doesn't have definite position and momentum BEFORE a measurement. But it doesn't mean that when you make the measurement, you can't make it with arbitrary accuracy. There's nothing to prevent you from making as accurate of a single measurement of those values, other than the technological limit of your instrument.

    https://www.physicsforums.com/blog.php?b=4364 [Broken]

    Zz.
     
    Last edited by a moderator: May 6, 2017
  4. Feb 9, 2013 #3
    Let's take the double slit experiment, you want to measure position and momentum when it's at the slit, say.
    This is all before the measurement (that is, when the electron hits the screen).

    or you mean the particle determines to have a specific past when you measure it in the present, ?
     
  5. Feb 9, 2013 #4

    ZapperZ

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor
    2016 Award

    Do you know what those [itex]\Delta[/itex]'s mean in the HUP? They correspond to the variance and standard deviation of a value.

    So what are the variance/standard deviation of ONE single measurement?

    You are also now confusing superposition principle with the HUP by bringing up the double slit.

    Zz.
     
  6. Feb 9, 2013 #5
    Excuse me, I meant a single slit whose width determines the accuracy of position measurement..

    So again, can we measure position and momentum at the slit at a later time?
     
  7. Feb 9, 2013 #6

    ZapperZ

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor
    2016 Award

    "momentum at the slit"?

    The location of the slit is the "position". The width of the slit is the uncertainty in position.

    AFTER it passed through the slit, you THEN measure its transverse momentum (i.e. momentum in the same direction as the slit's width).

    I have no idea where this is going. However, this is all MOOT if you simply answer my question regarding the definition of each of the values in the HUP.

    Zz.
     
  8. Feb 9, 2013 #7

    vanhees71

    User Avatar
    Science Advisor
    2016 Award

    Perhaps this is what you refer to:

    https://www.physicsforums.com/showthread.php?t=664972&highlight=Uncertainty+relation

    It's about spin measurements, but the principle difference between the Heisenberg-Robertson uncertainty relation and the measurement-disturbance should become very clear. Ironically this misinterpretation of the Heisenberg-Robertson uncertainty relation as the disturbance of one observable by the measurement of another incompatible observable goes back to Heisenberg's very first publication of the subject and has been corrected already by Bohr shortly after that publication.

    It seems to be a common manifestation of Murphy's Law that the rare occasions where great minds publish a mistake the wrong implications stick, particularly in the textbook literature, often even enhanced by educationalists of sciene. :devil: A notorious example it the claim that the photoeffect would be evidence for the particle nature of the electromagnetic field and thus the existence of photons. This you even read in university texts and in very recent publications in journals on didactics of physics!
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook