In physics, the HamiltonâÂÂJacobi equation, named after William Rowan Hamilton and Carl Gustav Jacob Jacobi, is an alternative formulation of classical mechanics, equivalent to other formulations such as Newton's laws of motion, Lagrangian mechanics and Hamiltonian mechanics.
The HamiltonâÂÂJacobi equation is a formulation of mechanics in which the motion of a particle can be represented as a wave. In this sense, it fulfilled a long-held goal of theoretical physics (dating at least to Johann Bernoulli in the eighteenth century) of finding an analogy between the propagation of light and the motion of a particle. The wave equation followed by mechanical systems is similar to, but not identical with, the Schrödinger equation, as described below; for this reason, the HamiltonâÂÂJacobi equation is considered the "closest approach" of classical mechanics to quantum mechanics. The qualitative form of this connection is called Hamilton's optico-mechanical analogy.
In mathematics, the HamiltonâÂÂJacobi equation is a necessary condition describing extremal geometry in generalizations of problems from the calculus of variations. It can be understood as a special case of the HamiltonâÂÂJacobiâÂÂBellman equation from dynamic programming.
The HamiltonâÂÂJacobi equation is a first-order, non-linear partial differential equation
for a system of particles at coordinates . The function is the system's Hamiltonian giving the system's energy. The solution of this equation is the action, , called Hamilton's principal function. The solution can be related to the system Lagrangian by an indefinite integral of the form used in the principle of least action:
Geometrical surfaces of constant action are perpendicular to system trajectories, creating a wavefront-like view of the system dynamics. This property of the HamiltonâÂÂJacobi equation connects classical mechanics to quantum mechanics.
Boldface variables such as represent a list of generalized coordinates,
A dot over a variable or list signifies the time derivative (see Newton's notation). For example,
The dot product notation between two lists of the same number of coordinates is a shorthand for the sum of the products of corresponding components, such as
Let the Hessian matrix be invertible. The relation
shows that the EulerâÂÂLagrange equations form a system of second-order ordinary differential equations. Inverting the matrix transforms this system into
Let a time instant and a point in the configuration space be fixed. The existence and uniqueness theorems guarantee that, for every the initial value problem with the conditions and has a locally unique solution Additionally, let there be a sufficiently small time interval such that extremals with different initial velocities would not intersect in The latter means that, for any and any there can be at most one extremal for which and Substituting into the action functional results in the Hamilton's principal function (HPF)
where
The momenta are defined as the quantities This section shows that the dependency of on disappears, once the HPF is known.
Indeed, let a time instant and a point in the configuration space be fixed. For every time instant and a point let be the (unique) extremal from the definition of the Hamilton's principal function . Call the velocity at . Then
Given the Hamiltonian of a mechanical system, the HamiltonâÂÂJacobi equation is a first-order, non-linear partial differential equation for the Hamilton's principal function ,
Alternatively, as described below, the HamiltonâÂÂJacobi equation may be derived from Hamiltonian mechanics by treating as the generating function for a canonical transformation of the classical Hamiltonian
The conjugate momenta correspond to the first derivatives of with respect to the generalized coordinates
As a solution to the HamiltonâÂÂJacobi equation, the principal function contains undetermined constants, the first of them denoted as , and the last one coming from the integration of .
The relationship between and then describes the orbit in phase space in terms of these constants of motion. Furthermore, the quantities
are also constants of motion, and these equations can be inverted to find as a function of all the and constants and time.
The HamiltonâÂÂJacobi equation is a single, first-order partial differential equation for the function of the generalized coordinates and the time . The generalized momenta do not appear, except as derivatives of , the classical action.
For comparison, in the equivalent EulerâÂÂLagrange equations of motion of Lagrangian mechanics, the conjugate momenta also do not appear; however, those equations are a system of , generally second-order equations for the time evolution of the generalized coordinates. Similarly, Hamilton's equations of motion are another system of 2N first-order equations for the time evolution of the generalized coordinates and their conjugate momenta .
Since the HJE is an equivalent expression of an integral minimization problem such as Hamilton's principle, the HJE can be useful in other problems of the calculus of variations and, more generally, in other branches of mathematics and physics, such as dynamical systems, symplectic geometry and quantum chaos. For example, the HamiltonâÂÂJacobi equations can be used to determine the geodesics on a Riemannian manifold, an important variational problem in Riemannian geometry. However as a computational tool, the partial differential equations are notoriously complicated to solve except when is it possible to separate the independent variables; in this case the HJE become computationally useful.
Any canonical transformation involving a type-2 generating function leads to the relations
and Hamilton's equations in terms of the new variables and new Hamiltonian have the same form:
To derive the HJE, a generating function is chosen in such a way that, it will make the new Hamiltonian . Hence, all its derivatives are also zero, and the transformed Hamilton's equations become trivial
so the new generalized coordinates and momenta are constants of motion. As they are constants, in this context the new generalized momenta are usually denoted , i.e. and the new generalized coordinates are typically denoted as , so .
Setting the generating function equal to Hamilton's principal function, plus an arbitrary constant :
the HJE automatically arises
When solved for , these also give us the useful equations
or written in components for clarity
Ideally, these equations can be inverted to find the original generalized coordinates as a function of the constants and , thus solving the original problem.
When the problem allows additive separation of variables, the HJE leads directly to constants of motion. For example, the time t can be separated if the Hamiltonian does not depend on time explicitly. In that case, the time derivative in the HJE must be a constant, usually denoted (), giving the separated solution
where the time-independent function is sometimes called the abbreviated action or Hamilton's characteristic function and sometimes written (see action principle names). The reduced HamiltonâÂÂJacobi equation can then be written
To illustrate separability for other variables, a certain generalized coordinate and its derivative are assumed to appear together as a single function
in the Hamiltonian
In that case, the function S can be partitioned into two functions, one that depends only on q<sub>k</sub> and another that depends only on the remaining generalized coordinates
Substitution of these formulae into the HamiltonâÂÂJacobi equation shows that the function àmust be a constant (denoted here as ), yielding a first-order ordinary differential equation for
In fortunate cases, the function can be separated completely into functions
In such a case, the problem devolves to ordinary differential equations.
The separability of S depends both on the Hamiltonian and on the choice of generalized coordinates. For orthogonal coordinates and Hamiltonians that have no time dependence and are quadratic in the generalized momenta, will be completely separable if the potential energy is additively separable in each coordinate, where the potential energy term for each coordinate is multiplied by the coordinate-dependent factor in the corresponding momentum term of the Hamiltonian (the Staeckel conditions). For illustration, several examples in orthogonal coordinates are worked in the next sections.
In spherical coordinates the Hamiltonian of a free particle moving in a conservative potential U can be written
The HamiltonâÂÂJacobi equation is separable in these coordinates provided that there exist functions such that can be written in the analogous form
The last term has few physical applications. Dropping that term, the HJE becomes
The coordinate is cyclic and the solution can be written in the form
resulting in two ordinary differential equations for the remaining coordinates:
where , , and are constants of the motion. This reduces the HJE to the ordinary differential equations whose integration completes the solution for .
The HJE establishes a duality between trajectories and wavefronts. For example, in geometrical optics, light can be considered either as "rays" or waves. The wave front can be defined as the surface that the light emitted at time has reached at time . Light rays and wave fronts are dual: if one is known, the other can be deduced.
More precisely, geometrical optics is a variational problem where the "action" is the travel time along a path, where is the medium's index of refraction and is an infinitesimal arc length. From the above formulation, one can compute the ray paths using the EulerâÂÂLagrange formulation; alternatively, one can compute the wave fronts by solving the HamiltonâÂÂJacobi equation. Knowing one leads to knowing the other.
The above duality is very general and applies to all systems that derive from a variational principle: either compute the trajectories using EulerâÂÂLagrange equations or the wave fronts by using HamiltonâÂÂJacobi equation.
The wave front at time , for a system initially at at time , is defined as the collection of points such that . If is known, the momentum is immediately deduced.
Once is known, tangents to the trajectories are computed by solving the equationfor , where is the Lagrangian. The trajectories are then recovered from the knowledge of .
The isosurfaces of the function can be determined at any time t. The motion of an -isosurface as a function of time is defined by the motions of the particles beginning at the points on the isosurface. The motion of such an isosurface can be thought of as a wave moving through -space, although it does not obey the wave equation exactly. To show this, let S represent the phase of a wave
where is a constant (the Planck constant) introduced to make the exponential argument dimensionless, and is a scalar normalization constant; changes in the amplitude of the wave can be represented by having be a complex number. The HamiltonâÂÂJacobi equation is then rewritten as
which is the Schrödinger equation.
Conversely, starting with the Schrödinger equation and our ansatz for , it can be deduced that This is the Schrödinger equation in nonlinear Riccati form.
The classical limit () of the Schrödinger equation above becomes identical to the following variant of the HamiltonâÂÂJacobi equation,
The following table summarizes the simmilarities between optics and quantum mechancis. The Hamilton-Jacobi equation is to quantum mechanics what the eikonal equation is to the wave equation of optics. Both the eikonal equation and the Hamilton-Jacobi equation are slowly varying approximations (WKB approximations) of the corresponding more fundamental wave equations.
<div style="display:flex; justify-content:space-around; text-align:center;">
<div style="flex:1;"> <b>Optics</b><br/><br/> <br/> <small>Helmholtz equation</small><br/> ↓<br/> <br/> <small>Eikonal equation</small><br/> </div>
<div style="flex:1;"> <b>Quantum mechanics</b><br/><br/> <br/> <small>Schrödinger equation</small><br/> ↓<br/> <br/> <small>HamiltonâÂÂJacobi equation</small><br/> </div>
</div>
Using the energyâÂÂmomentum relation in the form
for a particle of rest mass travelling in curved space, where are the contravariant coordinates of the metric tensor (i.e., the inverse metric) solved from the Einstein field equations, and is the speed of light. Setting the four-momentum equal to the four-gradient of the action ,
gives the HamiltonâÂÂJacobi equation in the geometry determined by the metric :
in other words, in a gravitational field.
For a particle of rest mass and electric charge moving in electromagnetic field with four-potential in vacuum, the HamiltonâÂÂJacobi equation in geometry determined by the metric tensor has a form
and can be solved for the Hamilton principal action function to obtain further solution for the particle trajectory and momentum:
where and with the cycle average of the vector potential.
In the case of circular polarization,
Hence
where , implying the particle moving along a circular trajectory with a permanent radius and an invariable value of momentum directed along a magnetic field vector.
For the flat, monochromatic, linearly polarized wave with a field directed along the axis
hence
implying the particle figure-8 trajectory with its axis oriented along the electric field vector.
For the electromagnetic wave with axial (solenoidal) magnetic field:
hence
where is the magnetic field magnitude in a solenoid with the effective radius , inductivity , number of windings , and an electric current magnitude through the solenoid windings. The particle motion occurs along the figure-8 trajectory in plane set perpendicular to the solenoid axis with arbitrary azimuth angle due to axial symmetry of the solenoidal magnetic field.