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# Time evolution of spin

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See attached file for equations and answer the following:

(a) Calculate the matrix representing in the { | + >, | - > } basis, the operator H, the Hamiltonian of the system.
(b) Calculate the eigenvalues and the eigenvalues of H
(c) The system at the time t = 0 is in the state | _ >. What values can be found if the energy is measured, and with what probabilities?
(d) Calculate the state vector at time t. At this instant, S, is measured; what is the mean value of the reuslts that can be obtained? Give a geometrical interpretation.

https://brainmass.com/physics/schrodinger/time-evolution-spin-571766

#### Solution Preview

The magnetic field is:
(1.1)
The spin operator is a vector sum as well:
(1.2)
The magnetic moment of the particle is:
(1.3)
Where is the mass of the electron, e is the charge and c is the speed of light. In other words we can write the magnetic moment as
(1.4)
Where is a constant.
The energy (Hamiltonian) of the interaction between the field and the spin is
(1.5)
The matrix representation of the spin operators in the basis of are:
(1.6)
Thus the matrix representation of the Hamiltonian is:
(1.7)

The eigenvalues of the matrix are:

(1.8)
Thus, the eigenvalues (energy levels) of the Hamiltonian are
(1.9)
Where the ...

#### Solution Summary

This step-by-step solution demonstrates how to use vector mechanics to calculate the time evolution and expectation value of the spin.

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