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Does the uncertainty relation apply to the past?

  1. Apr 13, 2012 #1
    Can anybody clear this up for me?

    In his Chicago lectures in 1930, Heisenberg is quoted as saying

    “The uncertainty relation does not hold for the past…If the velocity of the electron is at first known, and the position then exactly measured, the position of the electron for times previous to the position measurement may be calculated. For these past times, δpδq is smaller than the usual bound”.


    Is this the current view, and, if so, does this apply to the famous Bohr-Einstein debate over measuring to arbitrary accuracy both the time duration of the emission of a photon and its energy (using ‘Einstein’s box’)?


    After all, the results would tell you how long the hole was open for to let the photon escape, and what energy the photon had - both measurements applying to the photon in the past.
  2. jcsd
  3. Apr 13, 2012 #2


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    In 1930, it was not clear that measurement itself can change the properties of the system, sometimes even in a nonlocal way. In particular, the case of Einstein box and energy-time uncertainty is discussed in detail from a modern point of view here:

    So in modern view of QM, uncertainty relations refer to the past as well.
  4. Apr 15, 2012 #3
    Thanks, Demystifier. Interesting paper.

    The idea of nonlocality ias applied to this example - that the very act of measuring the box's mass to determine the energy of the emitted photon can actually influence the uncertainty of the photon's energy - makes sense.
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