Oscillation Theorem

Quantum Mechanics A

Schrödinger Equation The most fundamental equation of quantum mechanics; given a Hamiltonian ${\mathcal {H}}$, it describes how a state $|\Psi \rangle$ evolves in time.

Physical Basis of Quantum Mechanics

Basic Concepts and Theory of Motion UV Catastrophe (Black-Body Radiation) Photoelectric Effect Stability of Matter Double Slit Experiment Stern-Gerlach Experiment The Principle of Complementarity The Correspondence Principle The Philosophy of Quantum Theory

Schrödinger Equation

Brief Derivation of Schrödinger Equation Relation Between the Wave Function and Probability Density Stationary States Heisenberg Uncertainty Principle Some Consequences of the Uncertainty Principle

Operators, Eigenfunctions, and Symmetry

Linear Vector Spaces and Operators Commutation Relations and Simultaneous Eigenvalues The Schrödinger Equation in Dirac Notation Transformations of Operators and Symmetry Time Evolution of Expectation Values and Ehrenfest's Theorem

Motion in One Dimension

One-Dimensional Bound States Oscillation Theorem The Dirac Delta Function Potential Scattering States, Transmission and Reflection Motion in a Periodic Potential Summary of One-Dimensional Systems

Discrete Eigenvalues and Bound States

Harmonic Oscillator Spectrum and Eigenstates Analytical Method for Solving the Simple Harmonic Oscillator Coherent States Charged Particles in an Electromagnetic Field WKB Approximation

Time Evolution and the Pictures of Quantum Mechanics

The Heisenberg Picture: Equations of Motion for Operators The Interaction Picture The Virial Theorem

Angular Momentum

Commutation Relations Angular Momentum as a Generator of Rotations in 3D Spherical Coordinates Eigenvalue Quantization Orbital Angular Momentum Eigenfunctions

Central Forces

General Formalism Free Particle in Spherical Coordinates Spherical Well Isotropic Harmonic Oscillator Hydrogen Atom WKB in Spherical Coordinates

The Path Integral Formulation of Quantum Mechanics

Feynman Path Integrals The Free-Particle Propagator Propagator for the Harmonic Oscillator

Continuous Eigenvalues and Collision Theory

Differential Cross Section and the Green's Function Formulation of Scattering Central Potential Scattering and Phase Shifts Coulomb Potential Scattering

Let $\psi _{1}$ be a bound state of a given potential with energy $E_{1}$ and $\psi _{2}$ be another bound state with energy $E_{2}$ such that $E_{2}>E_{1}$. Let $x_{0}$ be a point at which both $\psi _{1}$ and $\psi _{2}$ vanish; this is guaranteed to happen at least for $x_{0}\to \pm \infty .$ We will now prove that, between any two points $x_{0}$ and $x_{1}$ at which $\psi _{1}$ vanishes, there must be at least one point at which $\psi _{2}$ vanishes. Let us begin by writing down the Schrödinger equation for each wave function:

$-{\frac {\hbar ^{2}}{2m}}{\frac {d^{2}\psi _{1}}{dx^{2}}}+V(x)\psi _{1}=E_{1}\psi _{1}\!$

$-{\frac {\hbar ^{2}}{2m}}{\frac {d^{2}\psi _{2}}{dx^{2}}}+V(x)\psi _{2}=E_{2}\psi _{2}\!$

Multiplying the first equation by $\psi _{2}$ and the second by $\psi _{1},$ subtracting the second equation from the first, and simplifying, we see that

${\frac {d}{dx}}\left(-\psi _{2}{\frac {d\psi _{1}}{dx}}+\psi _{1}{\frac {d\psi _{2}}{dx}}\right)={\frac {2m}{\hbar ^{2}}}\left(E_{1}-E_{2}\right)\psi _{1}\psi _{2}.\!$

If we now integrate both sides of this equation from $x_{0}$ to any position $x'$ and simplify, we see that

$-\psi _{2}(x'){\frac {d\psi _{1}(x')}{dx}}+\psi _{1}(x'){\frac {d\psi _{2}(x')}{dx}}={\frac {2m}{\hbar ^{2}}}(E_{1}-E_{2})\int _{x_{0}}^{x'}\psi _{1}\psi _{2}\,dx\!$

The key is to now let $x'=x_{1}\!$ be the first position to the right of $x_{0}\!$ where $\psi _{1}\!$ vanishes.

$-\psi _{2}(x_{1}){\frac {\partial \psi _{1}(x_{1})}{\partial x}}={\frac {2m}{\hbar ^{2}}}(E_{1}-E_{2})\int _{x_{0}}^{x_{1}}\psi _{1}\psi _{2}\,dx\!$

Now, if we assume that $\psi _{2}$ does not vanish at or between $x_{0}$ and $x_{1}$, then it is easy to see that the left hand side of the previous equation has a different sign from that of the right hand side, and thus it must be true that $\psi _{2}$ must vanish at least once between $x_{0}$ and $x_{1}$ if $E_{2}>E_{1}.$