Commutation Relations

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

In many multidimensional problems, we often deal with rotational motion of particles, and thus we are interested in treating angular momentum in the framework of quantum mechanics. The (orbital) angular momentum operator in quantum mechanics is given by the cross product of the position of the particle with its momentum:

${\hat {\mathbf {L} }}={\hat {\mathbf {r} }}\times {\hat {\mathbf {p} }}$

Working in the position representation, this becomes

${\hat {\mathbf {L} }}=\mathbf {r} \times {\frac {\hbar }{i}}\nabla .$

Evaluating the cross product in the Cartesian coordinate system, we get a component of $\mathbf {L} \!$ in each direction; for example,

${\hat {L}}_{x}={\hat {y}}{\hat {p}}_{z}-{\hat {z}}{\hat {p}}_{y}={\frac {\hbar }{i}}\left(y{\frac {\partial }{\partial z}}-z{\frac {\partial }{\partial y}}\right),$

and similarly the other two components of the angular momentum operator. All of these can be written in a more compact form using the Levi-Civita symbol as

${\hat {L}}_{\mu }=\epsilon _{\mu \nu \lambda }{\hat {r}}_{\nu }{\hat {p}}_{\lambda },$

where

$\epsilon _{\mu \nu \lambda }={\begin{cases}+1,&{\mbox{if }}(\mu ,\nu ,\lambda ){\mbox{ is }}(1,2,3),(3,1,2){\mbox{ or }}(2,3,1),\\-1,&{\mbox{if }}(\mu ,\nu ,\lambda ){\mbox{ is }}(3,2,1),(1,3,2){\mbox{ or }}(2,1,3),\\0,&{\mbox{otherwise: }}\mu =\nu {\mbox{ or }}\nu =\lambda {\mbox{ or }}\lambda =\mu \end{cases}}$

and we use the Einstein summation convention, in which sums over repeated indices are omitted. The above definition of the Levi-Civita symbol gives the "sign" of a permutation of 123 (it is 1 for even permutations, or -1 for odd permutations).

We can immediately verify the following commutation relations:

$[{\hat {L}}_{\mu },{\hat {x}}_{\nu }]=i\hbar \epsilon _{\mu \nu \lambda }{\hat {x}}_{\lambda }$

$[{\hat {L}}_{\mu },{\hat {p}}_{\nu }]=i\hbar \epsilon _{\mu \nu \lambda }{\hat {p}}_{\lambda }$

$[{\hat {L}}_{\mu },{\hat {L}}_{\nu }]=i\hbar \epsilon _{\mu \nu \lambda }{\hat {L}}_{\lambda }$

The last relation may also be written as

$\mathbf {L} \times \mathbf {L} =i\hbar \mathbf {L} .$

Furthermore,

$[{\hat {\mathbf {n} }}\cdot {\hat {\mathbf {L} }},{\hat {\mathbf {r} }}]=i\hbar ({\hat {\mathbf {r} }}\times {\hat {\mathbf {n} }})$

$[{\hat {\mathbf {n} }}\cdot {\hat {\mathbf {L} }},{\hat {\mathbf {p} }}]=i\hbar ({\hat {\mathbf {p} }}\times {\hat {\mathbf {n} }})$

$[{\hat {\mathbf {n} }}\cdot {\hat {\mathbf {L} }},{\hat {\mathbf {L} }}]=i\hbar ({\hat {\mathbf {L} }}\times {\hat {\mathbf {n} }})$

For example,

${\begin{aligned}\left[{\hat {L}}_{\mu },{\hat {x}}_{\nu }\right]&=[\epsilon _{\mu \lambda \rho }{\hat {x}}_{\lambda }{\hat {p}}_{\rho },{\hat {x}}_{\nu }]=\epsilon _{\mu \lambda \rho }[{\hat {x}}_{\lambda }{\hat {p}}_{\rho },{\hat {x}}_{\nu }]=\epsilon _{\mu \lambda \rho }{\hat {x}}_{\lambda }[{\hat {p}}_{\rho },{\hat {x}}_{\nu }]\\&=\epsilon _{\mu \lambda \rho }{\hat {x}}_{\lambda }{\frac {\hbar }{i}}\delta _{\rho \nu }=\epsilon _{\mu \lambda \nu }{\hat {x}}_{\lambda }{\frac {\hbar }{i}}\\&=i\hbar \epsilon _{\mu \nu \lambda }{\hat {x}}_{\lambda }.\end{aligned}}$

Also, note that for ${\hat {mathbf{L}}}^{2}={\hat {L}}_{x}^{2}+{\hat {L}}_{y}^{2}+{\hat {L}}_{z}^{2}={\hat {L}}_{\mu }{\hat {L}}_{\mu },$

${\begin{aligned}\left[{\hat {L}}_{\mu },{\hat {L}}^{2}\right]&=\left[{\hat {L}}_{\mu },{\hat {L}}_{\nu }{\hat {L}}_{\nu }\right]\\&={\hat {L}}_{\nu }\left[{\hat {L}}_{\mu },{\hat {L}}_{\nu }\right]+\left[{\hat {L}}_{\mu },{\hat {L}}_{\nu }\right]{\hat {L}}_{\nu }\\&={\hat {L}}_{\nu }i\hbar \epsilon _{\mu \nu \lambda }{\hat {L}}_{\lambda }+i\hbar \epsilon _{\mu \nu \lambda }{\hat {L}}_{\lambda }{\hat {L}}_{\nu }\\&=i\hbar \epsilon _{\mu \nu \lambda }{\hat {L}}_{\nu }{\hat {L}}_{\lambda }-i\hbar \epsilon _{\mu \lambda \nu }{\hat {L}}_{\lambda }{\hat {L}}_{\nu }\\&=i\hbar \epsilon _{\mu \nu \lambda }{\hat {L}}_{\nu }{\hat {L}}_{\lambda }-i\hbar \epsilon _{\mu \nu \lambda }{\hat {L}}_{\nu }{\hat {L}}_{\lambda }\\&=0.\end{aligned}}$

Therefore, the magnitude of the angular momentum squared commutes with any one component of the angular momentum, and thus both may be specified exactly in a given measurement.