Commutation Relations

The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.

Commutation Relations


fundamental relations in quantum mechanics that establish the connection between successive operations on the wave function, or state vector, of two operators (1 and L̂2) in opposite orders, that is, between 12 and 21. The commutation relations define the algebra of the operators. If the two operators commute, that is, 12 = L̂21, then the corresponding physical quantities L1 and L2 can have simultaneously defined values. But if their commutator is nonzero, that is, L̂L̂12 – L̂21 = c, then the uncertainty principle holds between the corresponding physical quantities: ΔL̂1ΔL̂2 ≤ ǀcǀ/2, where ΔL1 and ΔL2 are the uncertainties, or dispersions, of the measured values of the physical quantities L1 and L2. The commutation relation between the operator of the generalized coordinate and its conjugate generalized momentum (q̂p̂p̂ĝ = , where ħ is Planck’s constant) is very important in quantum mechanics. If the operator commutes with the operator of the total energy of the system (the Hamiltonian) , that is, L̂Ĥ = ĤL̂, then the physical quantity L (its average value, dispersion, and so on) preserves its value in time.

The commutation relations for the operators of the creation a+ and annihilation a of particles are of fundamental importance in the quantum mechanics of systems of identical particles and in quantum field theory. For a system of free (noninteracting) bosons, the particle creation operator in the state n, Commutation Relations and the annihilation operator for the same particle, Commutation Relations, satisfy the commutation relation Commutation Relations for fermions, the relation Commutation Relations holds. The latter commutation relation is a formal expression of the Pauli principle.


The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.
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