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# quantum mechanics/solving a DE. watch

1. question is attached.

i'm stuck on the part of the question in the second paragraph. we have that at time 0, f = (f_1+f_2)/rt2 = f_+ (where i'm writing f for phi), and it evolves from that point by the Schrödinger equation. And i want to find f.

err. so the answer looks like sin and cos are going to appear. but that would require, i imagine, second time derivatives. and i can't see how they'd come about.
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2. (Original post by Chewwy)
stuff
Write down the time evolution of the system, and then form the product |<S(t=0)|S(t)>|^2
(i.e. the probability of remeasuring the initial state S(t=0) at some later time t)

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You know how stationary states evolve right? Just solve E f_ = -ih f_/2pi to get the form of S(t)

Spoiler:
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This will leave some sum of complex exponentials, which you can form a cos term from

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