Quantum Mechanics wavefuction collapse

[sty2004]

Homework Statement

Consider a harmonic oscillator. It is in the ground state. Momentum is
measured and is found to be between 0.2 \sqrt{}m\omega hand 1.5 \sqrt{}m\omega h. (h here is h bar)
Energy is now immediately measured. What is the probability that the energy is
unchanged? You may want to do it numerically.

Homework Equations

The Attempt at a Solution

I don't know what to start with. Maybe P(E unchange)=|<\varphi₀|wave after measurement>|²

 

[vela]

That's right. If the energy is unchanged, it's in the ground state, so you want to find the probability the oscillator is in the ground state after the momentum is measured.

 

[sty2004]

how to find wave after measurement then?

 

[vela]

Making a measurement causes the wave function to collapse. How does the collapse work in general?