Skip to main content
edited tags; edited title
Link
Qmechanic
  • 213.1k
  • 48
  • 590
  • 2.3k

Delta function eigenstate for nonzeronon-zero potential

added 39 characters in body; edited tags
Source Link
Qmechanic
  • 213.1k
  • 48
  • 590
  • 2.3k

Consider the potential $V(x)=\frac{2}{x^2}$ and let $\frac{\hbar^2}{2m}=1$ for convenience. Now consider the function $\psi(x)=\delta(x)$. According to Griffiths (electrodynamics book) problem 1.45(a), $x\delta'(x)=-\delta(x)$. I'm$$x\delta'(x)=-\delta(x)\tag{1}.$$ I'm not sure if I can do this but if I write

$$\delta'(x)=-\frac{\delta(x)}{x},$$$$\delta'(x)=-\frac{\delta(x)}{x},\tag{2}$$

$$\frac{d^2}{dx^2}\psi(x)=-\frac{d}{dx}\left[\frac{\delta(x)}{x}\right]=\frac{2\delta(x)}{x^2}.$$$$\frac{d^2}{dx^2}\psi(x)=-\frac{d}{dx}\left[\frac{\delta(x)}{x}\right]=\frac{2\delta(x)}{x^2}.\tag{3}$$

The Schrodinger equation now looks like

\begin{align} &-\frac{d^2\psi(x)}{dx^2}+V(x)\psi(x)=E \psi(x)\\ &-\frac{2\delta(x)}{x^2}+\frac{2}{x^2}\delta(x)=0 \delta(x). \end{align}\begin{align} &-\frac{d^2\psi(x)}{dx^2}+V(x)\psi(x)=E \psi(x)\tag{4}\\ &-\frac{2\delta(x)}{x^2}+\frac{2}{x^2}\delta(x)=0 \delta(x).\tag{5} \end{align}

So it looks like $\delta(x)$ is an eigenstate with eigenvalue zero. But this goes against my intuition and is probably wrong but I'm not sure where the fault lies. Is it the derivative of the delta function? Can energy (eigenvalue of Hamiltonian) be zero? Potential is maximum at $0$, so how can the probability be maximum at $0$?

Consider the potential $V(x)=\frac{2}{x^2}$ and let $\frac{\hbar^2}{2m}=1$ for convenience. Now consider the function $\psi(x)=\delta(x)$. According to Griffiths (electrodynamics book) problem 1.45(a), $x\delta'(x)=-\delta(x)$. I'm not sure if I can do this but if I write

$$\delta'(x)=-\frac{\delta(x)}{x},$$

$$\frac{d^2}{dx^2}\psi(x)=-\frac{d}{dx}\left[\frac{\delta(x)}{x}\right]=\frac{2\delta(x)}{x^2}.$$

The Schrodinger equation now looks like

\begin{align} &-\frac{d^2\psi(x)}{dx^2}+V(x)\psi(x)=E \psi(x)\\ &-\frac{2\delta(x)}{x^2}+\frac{2}{x^2}\delta(x)=0 \delta(x). \end{align}

So it looks like $\delta(x)$ is an eigenstate with eigenvalue zero. But this goes against my intuition and is probably wrong but I'm not sure where the fault lies. Is it the derivative of the delta function? Can energy (eigenvalue of Hamiltonian) be zero? Potential is maximum at $0$, so how can the probability be maximum at $0$?

Consider the potential $V(x)=\frac{2}{x^2}$ and let $\frac{\hbar^2}{2m}=1$ for convenience. Now consider the function $\psi(x)=\delta(x)$. According to Griffiths (electrodynamics book) problem 1.45(a), $$x\delta'(x)=-\delta(x)\tag{1}.$$ I'm not sure if I can do this but if I write

$$\delta'(x)=-\frac{\delta(x)}{x},\tag{2}$$

$$\frac{d^2}{dx^2}\psi(x)=-\frac{d}{dx}\left[\frac{\delta(x)}{x}\right]=\frac{2\delta(x)}{x^2}.\tag{3}$$

The Schrodinger equation now looks like

\begin{align} &-\frac{d^2\psi(x)}{dx^2}+V(x)\psi(x)=E \psi(x)\tag{4}\\ &-\frac{2\delta(x)}{x^2}+\frac{2}{x^2}\delta(x)=0 \delta(x).\tag{5} \end{align}

So it looks like $\delta(x)$ is an eigenstate with eigenvalue zero. But this goes against my intuition and is probably wrong but I'm not sure where the fault lies. Is it the derivative of the delta function? Can energy (eigenvalue of Hamiltonian) be zero? Potential is maximum at $0$, so how can the probability be maximum at $0$?

Source Link
Razor
  • 459
  • 1
  • 7
  • 22

Delta function eigenstate for nonzero potential

Consider the potential $V(x)=\frac{2}{x^2}$ and let $\frac{\hbar^2}{2m}=1$ for convenience. Now consider the function $\psi(x)=\delta(x)$. According to Griffiths (electrodynamics book) problem 1.45(a), $x\delta'(x)=-\delta(x)$. I'm not sure if I can do this but if I write

$$\delta'(x)=-\frac{\delta(x)}{x},$$

$$\frac{d^2}{dx^2}\psi(x)=-\frac{d}{dx}\left[\frac{\delta(x)}{x}\right]=\frac{2\delta(x)}{x^2}.$$

The Schrodinger equation now looks like

\begin{align} &-\frac{d^2\psi(x)}{dx^2}+V(x)\psi(x)=E \psi(x)\\ &-\frac{2\delta(x)}{x^2}+\frac{2}{x^2}\delta(x)=0 \delta(x). \end{align}

So it looks like $\delta(x)$ is an eigenstate with eigenvalue zero. But this goes against my intuition and is probably wrong but I'm not sure where the fault lies. Is it the derivative of the delta function? Can energy (eigenvalue of Hamiltonian) be zero? Potential is maximum at $0$, so how can the probability be maximum at $0$?