Constant of motion

From Wikipedia, the free encyclopedia
Jump to: navigation, search

In mechanics, a constant of motion is a quantity that is conserved throughout the motion, imposing in effect a constraint on the motion. However, it is a mathematical constraint, the natural consequence of the equations of motion, rather than a physical constraint (which would require extra constraint forces). Common examples include specific energy, specific linear momentum, specific angular momentum and the Laplace–Runge–Lenz vector (for inverse-square force laws).

Applications[edit]

Constants of motion are useful because they allow properties of the motion to be derived without solving the equations of motion. In fortunate cases, even the trajectory of the motion can be derived as the intersection of isosurfaces corresponding to the constants of motion. For example, Poinsot's construction shows that the torque-free rotation of a rigid body is the intersection of a sphere (conservation of total angular momentum) and an ellipsoid (conservation of energy), a trajectory that might be otherwise hard to derive and visualize. Therefore, the identification of constants of motion is an important objective in mechanics.

Methods for identifying constants of motion[edit]

There are several methods for identifying constants of motion.

Another useful result is Poisson's theorem, which states that if two quantities and are constants of motion, so is their Poisson bracket .

A system with n degrees of freedom, and n constants of motion, such that the Poisson bracket of any pair of constants of motion vanishes, is known as a completely integrable system. Such a collection of constants of motion are said to be in involution with each other.

In quantum mechanics[edit]

An observable quantity Q will be a constant of motion if it commutes with the hamiltonian, H, and it does not itself depend explicitly on time. This is because

where

is the commutator relation.

Derivation[edit]

Say there is some observable quantity Q which depends on position, momentum and time,

And also, that there is a wave function which obeys Schrödinger's equation

Taking the time derivative of the expectation value of Q requires use of the product rule, and results in

So finally,

Comment[edit]

For an arbitrary state of a Quantum Mechanical system, if H and Q commute, i.e. if

and Q is not explicitly dependent on time, then

But if is an eigenfunction of Hamiltonian, then even if

it is still the case that

provided Q is independent on time.

Derivation[edit]

Since

then

This is the reason why Eigenstates of the Hamiltonian are also called stationary states.

Relevance for quantum chaos[edit]

In general, an integrable system has constants of motion other than the energy. By contrast, energy is the only constant of motion in a non-integrable system; such systems are termed chaotic. In general, a classical mechanical system can be quantized only if it is integrable; as of 2006, there is no known consistent method for quantizing chaotic dynamical systems.

Integral of motion[edit]

A constant of motion may be defined in a given force field as any function of phase-space coordinates (position and velocity, or position and momentum) and time that is constant throughout a trajectory. A subset of the constants of motion are the integrals of motion, or first integrals, defined as any functions of only the phase-space coordinates that are constant along an orbit. Every integral of motion is a constant of motion, but the converse is not true because a constant of motion may depend on time.[1] Examples of integrals of motion are the angular momentum vector, , or a Hamiltonian without time dependence, such as . An example of a function that is a constant of motion but not an integral of motion would be the function for an object moving at a constant speed in one dimension.

Dirac observables[edit]

In order to extract physical information from gauge theories, one either constructs gauge invariant observables or fixes a gauge. In a canonical language, this usually means either constructing functions which Poisson-commute on the constraint surface with the gauge generating first class constraints or to fix the flow of the latter by singling out points within each gauge orbit. Such gauge invariant observables are thus the `constants of motion' of the gauge generators and referred to as Dirac observables.

References[edit]

  1. ^ "Binney, J. and Tremaine, S.: Galactic Dynamics.". Princeton University Press. Retrieved 2011-05-05.