Hamiltonian field theory
In theoretical physics, Hamiltonian field theory is the field-theoretic analogue to classical Hamiltonian mechanics. It is a formalism in classical field theory alongside Lagrangian field theory, and has applications in quantum field theory also.
The Hamiltonian for a system of discrete particles is a function of their generalized coordinates and conjugate momenta, and possibly, time. For continua and fields, Hamiltonian mechanics is unsuitable but can be extended by considering a large number of point masses, and taking the continuous limit, that is, infinitely many particles forming a continuum or field. Since each point mass has one or more degrees of freedom, the field formulation has infinitely many degrees of freedom.
One scalar field
The Hamiltonian density is the continuous analogue for fields; it is a function of the fields, the conjugate "momentum" fields, and possibly the space and time coordinates themselves. For one scalar field φ(x, t), the Hamiltonian density is defined from the Lagrangian density by[nb 1]
with ∇ the "del" or "nabla" operator, x is the position vector of some point in space, and t is time. The Lagrangian density is a function of the fields in the system, their space and time derivatives, and possibly the space and time coordinates themselves. It is the field analogue to the Lagrangian function for a system of discrete particles described by generalized coordinates.
As in Hamiltonian mechanics where every generalized coordinate has a corresponding generalized momentum, the field φ(x, t) has a conjugate momentum field π(x, t), defined as the partial derivative of the Lagrangian density with respect to the time derivative of the field,
Many scalar fields
For many fields φi(x, t) and their conjugates πi(x, t) the Hamiltonian density is a function of them all:
where each conjugate field is defined with respect to its field,
In general, for any number of fields, the volume integral of the Hamiltonian density gives the Hamiltonian, in three spatial dimensions:
Tensor and spinor fields
Equations of motion
The equations of motion for the fields are similar to the Hamiltonian equations for discrete particles. For any number of fields:
where again the overdots are partial time derivatives, the variational derivative with respect to the fields
where ∂μ is the four gradient.
The fields φi and conjugates πi form an infinite dimensional phase space, because fields have an infinite number of degrees of freedom.
For two functions which depend on the fields φi and πi, their spatial derivatives, and the space and time coordinates,
and the fields are zero on the boundary of the volume the integrals are taken over, the field theoretic Poisson bracket is defined as (not to be confused with the commutator from quantum mechanics).
where is the variational derivative
Under the same conditions of vanishing fields on the surface, the following result holds for the time evolution of A (similarly for B):
which can be found from the total time derivative of A, integration by parts, and using the above Poisson bracket.
The following results are true if the Lagrangian and Hamiltonian densities are explicitly time-independent (they can still have implicit time-dependence via the fields and their derivatives),
Kinetic and potential energy densities
The Hamiltonian density is the total energy density, the sum of the kinetic energy density (calligraphic T) and the potential energy density (calligraphic V),
Taking the partial time derivative of the definition of the Hamiltonian density above, and using the chain rule for implicit differentiation and the definition of the conjugate momentum field, gives the continuity equation:
in which the Hamiltonian density can be interpreted as the energy density, and
the energy flux, or flow of energy per unit time per unit surface area.
Relativistic field theory
Covariant Hamiltonian field theory is the relativistic formulation of Hamiltonian field theory.
Hamiltonian field theory usually means the symplectic Hamiltonian formalism when applied to classical field theory, that takes the form of the instantaneous Hamiltonian formalism on an infinite-dimensional phase space, and where canonical coordinates are field functions at some instant of time. This Hamiltonian formalism is applied to quantization of fields, e.g., in quantum gauge theory. In Covariant Hamiltonian field theory, canonical momenta pμi corresponds to derivatives of fields with respect to all world coordinates xμ. Covariant Hamilton equations are equivalent to the Euler-Lagrange equations in the case of hyperregular Lagrangians. Covariant Hamiltonian field theory is developed in the Hamilton–De Donder, polysymplectic, multisymplectic and k-symplectic variants. A phase space of covariant Hamiltonian field theory is a finite-dimensional polysymplectic or multisymplectic manifold.
- Analytical mechanics
- De Donder–Weyl theory
- Four vector
- Canonical quantization
- Hamiltonian fluid mechanics
- Covariant classical field theory
- Polysymplectic manifold
- Non-autonomous mechanics
- It is a standard abuse of notation to abbreviate all the derivatives and coordinates in the Lagrangian density as follows:
- This is standard notation in this context, most of the literature does not explicitly mention it is a partial derivative. In general total and partial time derivatives of a function are not the same.
- Greiner & Reinhardt 1996, Chapter 2
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