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

B Why cant we choose?

  1. Aug 4, 2016 #1
    This I'd guess is a simple question for this forum. I'm just a middle age dude who never went to school and have taken an interest in physics, just trying learn how it works.

    The question is, suppose you have an electron with a spin in a superposition of both up spin and down spin. It is my understanding that you are 'not allowed' to choose which spin the electron ends up with. You can only have the electron interact with something at which time 'it' chooses which spin it gets somehow. Why can't we choose?

    Maybe just direct me to some material on the subject.

  2. jcsd
  3. Aug 4, 2016 #2
    You can choose to find the spin in the horizontal direction with the Stern/Gerlach apparatus arranged properly. But you are correct that you can't choose to make it up or down with a measurement.
    Fish around on line about quantum measurement.
  4. Aug 4, 2016 #3
    Is there any reason why we should expect to have a choice?
  5. Aug 4, 2016 #4
    There is if one selects a measurement operator with an eigenstate which equals the state to be measured.
  6. Aug 4, 2016 #5
    Behaviour of quantum objects is essentially statistical though.
    If the observer could choose the desired observation then the whole of QM theory crashes and burns.
    We would need to replace it with a theory of how minds affect matter.
    That would be very unpopular with a lot of people.
  7. Aug 5, 2016 #6

    Vanadium 50

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor
    2017 Award

    It's going to be 90 degrees out today. Why can't I choose to make it 70 instead?
  8. Aug 6, 2016 #7


    User Avatar
    Science Advisor
    Gold Member

    Actually, I did. A short trip to San Diego did the trick. Although it is a bit overcast.

  9. Aug 10, 2016 #8
    To be quite honest, this answer is also possible: we can't choose because we are not the main players of the quantum game. Some Main Players might well be choosing the outcome, somehow maintaining the Born rule as a rule of the Game, in order to hide themselves.
  10. Aug 10, 2016 #9


    User Avatar
    Science Advisor
    Gold Member

    Perhaps they are magical leprechauns. :smile:

    A similar thing is often said about Bell's Theorem - that there are pre-existing conditions that make Bell's Theorem appear to be violated - when it really isn't. The same, of course, could be said about ANY physical evidence we obtain. I.e. that it is somehow tainted and not representative of what really occurs.
  11. Aug 10, 2016 #10
    There you go . http://www.ams.org/notices/200902/r...AQFjAB&usg=AFQjCNE7L-k87yWE32ru0rDjkLOdg12LRQ

Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted