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Question about a QM postulate

  1. Oct 20, 2009 #1
    If a system is in state [itex]|\psi>[/itex] before a meassurement, then after we find a value a_n the system is in state

    so for instance if [itex]|\psi>=c_1|a_1>+c_2|a_2>+c_3|a_3>[/itex] after we find a_1 the system is in the state [itex]|\psi>'=e^{i\theta}|a_1>[/itex] where [itex]\theta=[/itex] is the argument of complex a_1.
    I know that [itex]|\psi>'[/itex] and [itex]|a_1>[/itex] represent two different vectors of the Hilbert space of states but the same physical state.
    My question is, is there any way that the phase factor [itex]e^{\theta}[/itex] manifest itself in a physical experiment? or put in another way, does this factor have any physical significance?
    I ask this because some systems when rotated 360 degrees transforms like [itex]|a> \rightarrow -|a>[/itex] and this sign can be detected exparimentally
  2. jcsd
  3. Oct 20, 2009 #2


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    you mean c_1.

    No, I don't think so.

  4. Oct 21, 2009 #3

    Ben Niehoff

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    Phase factors can be detected in interference experiments. However, the absolute phase cannot be detected; only phase differences cause interference.
  5. Oct 21, 2009 #4
    Thank you for your answers I think that know I've Got it
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