The pure states of a quantum system are given by the eigenvectors of a Hamiltonian operator, which is Hermitian. The states are stationary, in the sense that only their phases change over time: |ψ_t> = exp(-iHt/ℏ) |ψ_0>. If you want something to *happen*, then you need to perturb the Hamiltonian for some period of time. If you want to measure the state, you typically start with two independent Hamiltonians (one for the quantum system and one for a pointer state) and then add a coupling perturbation for some period of time. On Fri, Dec 7, 2018 at 12:22 PM Henry Baker <hbaker1@pipeline.com> wrote:

As I understand it, a simple quantum system undergoes transitions via unitary transformations.

Unitary transformations preserve inner products, so they are "rigid" rotations of a "configuration" in N-space (assuming that a finite dimensional quantum system can exist).

Nevertheless, from a given "state" (whatever that means), not all unitary transformations seem possible.

For example, if a system is isolated from its environment, wouldn't it "evolve" in an "inertial-like frame" manner? If the rotation analogy is correct, wouldn't this be a rigid constant-speed spin about some axis/plane? Wouldn't this spin have some "angular momentum" (resistance to slowing down) ?

Aren't there other unitary evolutions which will require some external "effort" to change its course -- i.e., axis ? And in this case, wouldn't reversibility require the re-appearance of the conjugate of this "effort" to change the course back?

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