Operator (physics)


In physics, an operator is a function over a space of physical states to another space of physical states. The simplest example of the utility of operators is the study of symmetry. Because of this, they are very useful tools in classical mechanics. Operators are even more important in quantum mechanics, where they form an intrinsic part of the formulation of the theory.

Operators in classical mechanics

In classical mechanics, the movement of a particle is completely determined by the Lagrangian or equivalently the Hamiltonian, a function of the generalized coordinates q, generalized velocities and its conjugate momenta:
If either L or H is independent of a generalized coordinate q, meaning the L and H do not change when q is changed, which in turn means the dynamics of the particle are still the same even when q changes, the corresponding momenta conjugate to those coordinates will be conserved. Operators in classical mechanics are related to these symmetries.
More technically, when H is invariant under the action of a certain group of transformations G:
the elements of G are physical operators, which map physical states among themselves.

Table of classical mechanics operators

where is the rotation matrix about an axis defined by the unit vector and angle θ.

Generators

If the transformation is infinitesimal, the operator action should be of the form
where is the identity operator, is a parameter with a small value, and will depend on the transformation at hand, and is called a generator of the group. Again, as a simple example, we will derive the generator of the space translations on 1D functions.
As it was stated,. If is infinitesimal, then we may write
This formula may be rewritten as
where is the generator of the translation group, which in this case happens to be the derivative operator. Thus, it is said that the generator of translations is the derivative.

The exponential map

The whole group may be recovered, under normal circumstances, from the generators, via the exponential map. In the case of the translations the idea works like this.
The translation for a finite value of may be obtained by repeated application of the infinitesimal translation:
with the standing for the application times. If is large, each of the factors may be considered to be infinitesimal:
But this limit may be rewritten as an exponential:
To be convinced of the validity of this formal expression, we may expand the exponential in a power series:
The right-hand side may be rewritten as
which is just the Taylor expansion of, which was our original value for.
The mathematical properties of physical operators are a topic of great importance in itself. For further information, see C*-algebra and Gelfand-Naimark theorem.

Operators in quantum mechanics

The mathematical formulation of quantum mechanics is built upon the concept of an operator.
The wavefunction represents the probability amplitude of finding the system in that state.
Physical pure states in quantum mechanics are represented as unit-norm vectors in a special complex Hilbert space. Time evolution in this vector space is given by the application of the evolution operator.
Any observable, i.e., any quantity which can be measured in a physical experiment, should be associated with a self-adjoint linear operator. The operators must yield real eigenvalues, since they are values which may come up as the result of the experiment. Although traditionally physicists associated real eigenvalues with Hermiticity, in 1998 physicists realized that there also exist non-Hermitian operators with all-real spectra; namely, parity-time symmetric operators. For Hermitian operators, the probability of each eigenvalue is related to the projection of the physical state on the subspace related to that eigenvalue. See below for mathematical details about Hermitian operators.
In the wave mechanics formulation of QM, the wavefunction varies with space and time, or equivalently momentum and time, so observables are differential operators.
In the matrix mechanics formulation, the norm of the physical state should stay fixed, so the evolution operator should be unitary, and the operators can be represented as matrices. Any other symmetry, mapping a physical state into another, should keep this restriction.

Wavefunction

The wavefunction must be square-integrable, meaning:
and normalizable, so that:
Two cases of eigenstates are:
Let ψ be the wavefunction for a quantum system, and be any linear operator for some observable A, then
where:
If ψ is an eigenfunction of a given operator A, then a definite quantity will be observed if a measurement of the observable A is made on the state ψ. Conversely, if ψ is not an eigenfunction of A, then it has no eigenvalue for A, and the observable does not have a single definite value in that case. Instead, measurements of the observable A will yield each eigenvalue with a certain probability.
In bra–ket notation the above can be written;
in which case is an eigenvector, or eigenket.
Due to linearity, vectors can be defined in any number of dimensions, as each component of the vector acts on the function separately. One mathematical example is the del operator, which is itself a vector.
An operator in n-dimensional space can be written:
where ej are basis vectors corresponding to each component operator Aj. Each component will yield a corresponding eigenvalue. Acting this on the wave function ψ:
in which
In bra–ket notation:

Commutation of operators on ''Ψ''

If two observables A and B have linear operators and, the commutator is defined by,
The commutator is itself a operator. Acting the commutator on ψ gives:
If ψ is an eigenfunction with eigenvalues a and b for observables A and B respectively, and if the operators commute:
then the observables A and B can be measured simultaneously with infinite precision i.e. uncertainties, simultaneously. ψ is then said to be the simultaneous eigenfunction of A and B. To illustrate this:
It shows that measurement of A and B does not cause any shift of state i.e. initial and final states are same. Suppose we measure A to get value a. We then measure B to get the value b. We measure A again. We still get the same value a. Clearly the state of the system is not destroyed and so we are able to measure A and B simultaneously with infinite precision.
If the operators do not commute:
they can't be prepared simultaneously to arbitrary precision, and there is an uncertainty relation between the observables,


even if ψ is an eigenfunction the above relation holds.. Notable pairs are position-and-momentum and energy-and-time uncertainty relations, and the angular momenta about any two orthogonal axes.

Expectation values of operators on ''Ψ''

The expectation value is the average measurement of an observable, for particle in region R. The expectation value of the operator is calculated from:
This can be generalized to any function F of an operator:
An example of F is the 2-fold action of A on ψ, i.e. squaring an operator or doing it twice:

Hermitian operators

The definition of a Hermitian operator is:
Following from this, in bra–ket notation:
Important properties of Hermitian operators include:
An operator can be written in matrix form to map one basis vector to another. Since the operators are linear, the matrix is a linear transformation between bases. Each basis element can be connected to another, by the expression:
which is a matrix element:
A further property of a Hermitian operator is that eigenfunctions corresponding to different eigenvalues are orthogonal. In matrix form, operators allow real eigenvalues to be found, corresponding to measurements. Orthogonality allows a suitable basis set of vectors to represent the state of the quantum system. The eigenvalues of the operator are also evaluated in the same way as for the square matrix, by solving the characteristic polynomial:
where I is the n × n identity matrix, as an operator it corresponds to the identity operator. For a discrete basis:
while for a continuous basis:

Inverse of an operator

A non-singular operator has an inverse defined by:
If an operator has no inverse, it is a singular operator. In a finite-dimensional space, an operator is non-singular if and only if its determinant is nonzero:
and hence the determinant is zero for a singular operator.

Table of QM operators

The operators used in quantum mechanics are collected in the table below. The bold-face vectors with circumflexes are not unit vectors, they are 3-vector operators; all three spatial components taken together.
J 2 −2Potential energyN/AJ 2 −2Total energyN/ATime-dependent potential:

Time-independent:
J 2 −2HamiltonianJ 2 −2Angular momentum operatorJ s = N s m 2 −1Spin angular momentum
where
are the pauli matrices for spin-½ particles.
where σ is the vector whose components are the pauli matrices.J s = N s m 2 −1Total angular momentumJ s = N s m 2 −1Transition dipole moment C m

Examples of applying quantum operators

The procedure for extracting information from a wave function is as follows. Consider the momentum p of a particle as an example. The momentum operator in position basis in one dimension is:
Letting this act on ψ we obtain:
if ψ is an eigenfunction of, then the momentum eigenvalue p is the value of the particle's momentum, found by:
For three dimensions the momentum operator uses the nabla operator to become:
In Cartesian coordinates this can be written;
that is:
The process of finding eigenvalues is the same. Since this is a vector and operator equation, if ψ is an eigenfunction, then each component of the momentum operator will have an eigenvalue corresponding to that component of momentum. Acting on ψ obtains: