Measuring Spin in the Stern Gerlach Experiment

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Discussion Overview

The discussion revolves around the measurement of electron spin in the Stern-Gerlach experiment, specifically addressing the nature of spin eigenstates and their time dependence in relation to the Schrödinger Equation. Participants explore theoretical aspects of quantum mechanics, including the relationship between time-dependent state vectors and time-independent eigenstates.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant suggests that spin eigenstates could be time dependent and questions whether the problem can be approached using the time-independent Schrödinger Equation, similar to energy measurements.
  • Another participant clarifies that the state vector is time dependent, while the eigenstates themselves are solutions to the time-independent Schrödinger Equation.
  • A participant asserts that only energy eigenstates are stationary and do not depend on time, implying that spin eigenstates might exhibit time dependence.
  • It is noted that if two operators commute, they can share eigenstates, and since the spin operator ##S_z## commutes with the Hamiltonian, there exist states that are eigenstates of both energy and spin, remaining constant over time.

Areas of Agreement / Disagreement

Participants express differing views on the time dependence of spin eigenstates, with some arguing that they could be time dependent while others maintain that eigenstates of commuting operators can be stationary. The discussion remains unresolved regarding the nature of spin eigenstates in this context.

Contextual Notes

Participants highlight the dependence of their arguments on the definitions of eigenstates and the conditions under which operators commute. There is an acknowledgment of the complexity involved in the relationship between time dependence and the Schrödinger Equation.

klen
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When we are measuring the spin of the electron in the experiment, we choose the spin property as its eigen state for the measurement. The eigen vectors corresponding to these states could be time dependent. Can we still break the problem into solving time independent Schrödinger Equation and then multiplying by a time dependent function, like we do for the case of measurement of energy? How do we calculate the spin eigen vectors using Schrödinger Equation?
 
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In the representation you are talking about, it is the state vector that is time dependent... not the eigenstates.
##\psi = c_1(t)\psi_1+c_2(t)\psi_2: c_1^\star c_1+c_2^\star c_2 = 1## ... so ##\psi_1## and ##\psi_2##, the eigenstates, are solutions to the time independent Schrödinger equation.
... so what was your question there?
 
Simon Bridge said:
In the representation you are talking about, it is the state vector that is time dependent... not the eigenstates.
##\psi = c_1(t)\psi_1+c_2(t)\psi_2: c_1^\star c_1+c_2^\star c_2 = 1## ... so ##\psi_1## and ##\psi_2##, the eigenstates, are solutions to the time independent Schrödinger equation.
... so what was your question there?

I believe only the eigen states of energy are the stationary states and do not depend on time, so if we are measuring spin eigen states, they could be time dependent. Isn't the time independent Schrödinger equation a energy eigen value equation?
 
Last edited:
klen said:
I believe only the eigen states of energy are the stationary states and do not depend on time, so if we are measuring spin eigenstates, they could be time dependent. Isn't the time independent Schrödinger equation a energy eigenvalue equation?

It is. However, if two operators commute they will have shared eigenstates, and ##S_z## commutes with the Hamiltonian. Thus there are states that are eigenstates of both energy and spin, and both remain constant over time.
 

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