A particle of mass m is initially in the ground state of the (one-dimensional) infinite square well. At time t=0 a "brick" is dropped into the well, so that the potential becomes
V(x)=⎩⎨⎧V0,0,∞,0≤x≤a/2,a/2<x≤a,otherwise,
where V0≪E1. After a time T, the brick is removed, and the energy of the particle is measured. Find the probability (in first-order perturbation theory) that the result is now E2.