# Wavefunction expansion coefficients

1. May 18, 2005

### nd

I'm working in Liboff, 4e, QM, page 114, problem 4.35.
An electron in a 1-D box with walls at x= 0,a is in the state $$\psi(x) = A$$ for $$x\in (0,a/2)$$ and $$\psi(x) = -A$$ for $$x\in (a/2,a)$$. What is the lowest possible energy that can be measured?

From my understanding, the answer to this question will be the integer of first nonzero coefficient in the expansion $$\psi = \Sigma \limits_{n=1}^{\infty} a_n \phi_n$$, where $$\phi_n = \sqrt{\frac{2}{a}}\sin(\frac{n\pi x}{a})$$ are the basis functions given in eq (4.15) from the book (the eigenstates for the 1D box Hamiltonian). I do this and I get $$a_n = \frac{\sqrt{2}}{n\pi}(1+\cos(n\pi) - 2\cos(n\pi/2))$$. Now correct me if I'm wrong, but is it not true that $$\psi(x)=A$$ for $$x\in (0,a)$$ represents the same state since only the square of the wavefunction is given significance? In that case, however, I get $$a_n=\frac{\sqrt{2}}{n\pi}(1-\cos(n\pi))$$. It is my understanding that $$a_n^2$$ represents the probability of measuring the particle to be in the state $$\phi_n$$. But in these two cases, we will get different $$a_n^2$$ indicating that the two states are physically different.

Can anyone point out my mistake?

2. May 18, 2005

### dextercioby

Advice:read more on Fourier series.U could see post #5 from here https://www.physicsforums.com/showthread.php?p=571801#post571801 which deals exactly with a function of type $\psi$ in your problem.

Your problem should reduce to the question:does the fundamental state $\phi_{1}$-which has the lowest possible energy-have a nonzero probability ...?

U'll need to compute the Fourier coefficients of that wavefunction (whose sq.moduli give probabilities),so that's why i advised you to read.

Daniel.

3. May 19, 2005

### vanesch

Staff Emeritus
No, that is not true. It is not because |psi1|^2 = |psi2|^2 that psi1 and psi2 represent the same physical states. In order for two descriptions A and B to represent the same physical state, you have to have that ALL POSSIBLE measurements cannot make any difference. |psi1| = |psi2| only means that all possible POSITION measurements cannot make a difference. But a momentum measurement can, for instance.

cheers,
Patrick.