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

Is electron spin uniquely determined?

  1. Apr 4, 2012 #1
    If I give you a box, completely empty except for one hydrogen atom, could you theoretically measure the spin of its electron to tell me if it is +1/2 or -1/2? If this can be done, how can it be accomplished (at least in theory). Or if the question is complete nonsense, why is that?
    (Sorry for the newb question ... I don't yet know much about quantum mechanics)

  2. jcsd
  3. Apr 4, 2012 #2
    You can measure the spin, you can predict a probability for the values of the spin but you cannot say with certainty what the spin of the electron is before you measure the system.
    If there's nothing else in the system you can however tell with certainty what the spin is after it has been measured, infact it will remain the same as what it was measured as.
    Without measuring, however, you can only say that it has 50% chance of being up and 50% of being down
  4. Apr 4, 2012 #3


    User Avatar
    Science Advisor

    If you take a spin with unknown origin, there is absolutely nothing you can tell about it. If you measure it, you can only get +1/2 or -1/2, but you can do the measurement only once, so you can't say anything about the original state. In fact, if you measured the spin to be in +1/2 state, the only thing you can say with absolute certainty is that it wasn't in a pure -1/2 state, which you pretty much can guess from the start.

    However, if you need a spin in precise state, it can be prepared without too much trouble.
  5. Apr 4, 2012 #4
    > If you measure it, you can only get +1/2 or -1/2, but you can do the
    > measurement only once, so you can't say anything about the original state.

    So that's because the measurement itself disturbs the spin?

    I also have some more questions about the spin property:

    1.) Are up and down as different as say an apple and an orange or does it depend on how you are looking at it (i.e. comparing it with something, reference frame, etc.)

    2.) Once you measure the spin to be say "up", what could cause it to change? (i.e. bouncing off a wall of the container, shooting it with gamma rays, etc).

    3.) What does the experiment look like that we would need to make this measurement (in simple terms) ... unless it is not so simple, in which case just skip it.

    4.) If the electron is freed from the hydrogen atom, does it still posses a spin property?

  6. Apr 5, 2012 #5
    I wouldn't say the measurment 'disturbs' the spin, once you have measured it, any measurments after will give you the same result
    1. Up and down are 'orthogonal' states, this means that they are distinguishable, in that respect they are as different as an apple and an orange.
    2. Up and down do depend on how you are looking at it but not in any deep way, up and down usually refer to +1/2 and -1/2 along the z axis, so if you look at it in a rotated frame, someone elses z could be your x, but that's nothing odd.
    3. Not a clue
    4. Electrons are spin 1/2 particles, it'll always have a spin of 1/2 in some direction when measured.
  7. Apr 5, 2012 #6
    I think K2 means that you can only observe it once from whatever conditions it existed. Once you measure it, it begins a 'new journey'....however, just letting the electron and atom sit in the box, does not mean it is undisturbed....The very presence of any conatiner, in fact the atom structure itself, affects the 'state' of the particles. The electon cloud,the orbitals, for example, is different in different atoms....you can think of it as being different energy levels for example.

    Check out the illustrations here:

    In the real world, stuff is 'disturbed' all the time.....it's unavoidable...
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook