Hamiltonian mechanics - Wikipedia, the free encyclopedia
Page 1 of 12
Hamiltonian mechanics
From Wikipedia, the free encyclopedia
Classical mechanics
Hamiltonian mechanics is a reformulation of classical
mechanics that was introduced in 1833 by Irish
mathematician William Rowan Hamilton.
It arose from Lagrangian mechanics, a previous
reformulation of classical mechanics introduced by Joseph
Louis Lagrange in 1788, but can be formulated without
recourse to Lagrangian mechanics using symplectic spaces
(see Mathematical formalism, below). The Hamiltonian
method differs from the Lagrangian method in that instead
of expressing second-order differential constraints on an ndimensional coordinate space (where n is the number of
degrees of freedom of the system), it expresses first-order
constraints on a 2n-dimensional phase space.[1]
Newton's Second Law
History of classical mechanics ·
Timeline of classical mechanics
Branches
Statics · Dynamics / Kinetics · Kinematics ·
Applied mechanics · Celestial mechanics ·
Continuum mechanics ·
Statistical mechanics
Formulations
As with Lagrangian mechanics, Hamilton's equations
provide a new and equivalent way of looking at classical
mechanics. Generally, these equations do not provide a
more convenient way of solving a particular problem.
Rather, they provide deeper insights into both the general
structure of classical mechanics and its connection to
quantum mechanics as understood through Hamiltonian
mechanics, as well as its connection to other areas of
science.
Contents
1 Simplified overview of uses
1.1 Basic physical interpretation
1.2 Using Hamilton's equations
1.3 Notes
2 Deriving Hamilton's equations
3 As a reformulation of Lagrangian mechanics
4 Geometry of Hamiltonian systems
5 Generalization to quantum mechanics through
Poisson bracket
6 Mathematical formalism
7 Riemannian manifolds
8 Sub-Riemannian manifolds
9 Poisson algebras
10 Charged particle in an electromagnetic field
11 Relativistic charged particle in an
electromagnetic field
12 See also
13 Notes and references
13.1 Notes
http://en.wikipedia.org/wiki/Hamiltonian_mechanics
Newtonian mechanics (Vectorial
mechanics)
Analytical mechanics:
Lagrangian mechanics
Hamiltonian mechanics
Fundamental concepts
Space · Time · Velocity · Speed · Mass ·
Acceleration · Gravity · Force · Impulse ·
Torque / Moment / Couple · Momentum ·
Angular momentum · Inertia ·
Moment of inertia · Reference frame ·
Energy · Kinetic energy · Potential energy ·
Mechanical work · Virtual work ·
D'Alembert's principle
Core topics
Rigid body · Rigid body dynamics ·
Euler's equations (rigid body dynamics) ·
Motion · Newton's laws of motion ·
Newton's law of universal gravitation ·
Equations of motion ·
Inertial frame of reference ·
Non-inertial reference frame ·
Rotating reference frame · Fictitious force ·
Linear motion ·
Mechanics of planar particle motion ·
Displacement (vector) · Relative velocity ·
Friction · Simple harmonic motion ·
5/23/2011
Hamiltonian mechanics - Wikipedia, the free encyclopedia
13.2 References
14 External links
Simplified overview of uses
The value of the Hamiltonian is the total energy of the
system being described. For a closed system, it is the sum
of the kinetic and potential energy in the system. There is a
set of differential equations known as the Hamilton
equations which give the time evolution of the system.
Hamiltonians can be used to describe such simple systems
as a bouncing ball, a pendulum or an oscillating spring in
which energy changes from kinetic to potential and back
again over time. Hamiltonians can also be employed to
model the energy of other more complex dynamic systems
such as planetary orbits in celestial mechanics and also in
quantum mechanics.[2]
The Hamilton equations are generally written as follows:
Page 2 of 12
Harmonic oscillator · Vibration · Damping ·
Damping ratio · Rotational motion ·
Circular motion · Uniform circular motion ·
Non-uniform circular motion ·
Centripetal force · Centrifugal force ·
Centrifugal force (rotating reference frame) ·
Reactive centrifugal force · Coriolis force ·
Pendulum · Rotational speed ·
Angular acceleration · Angular velocity ·
Angular frequency · Angular displacement
Scientists
Galileo Galilei · Isaac Newton ·
Jeremiah Horrocks · Leonhard Euler ·
Jean le Rond d'Alembert · Alexis Clairaut ·
Joseph Louis Lagrange ·
Pierre-Simon Laplace ·
William Rowan Hamilton ·
Siméon-Denis Poisson
In the above equations, the dot denotes the ordinary derivative with respect to time of the functions
p = p(t) (called generalized momenta) and q = q(t) (called generalized coordinates), taking values in
some vector space, and
=
is the so-called Hamiltonian, or (scalar valued) Hamiltonian
function. Thus, more explicitly, one can equivalently write
and specify the domain of values in which the parameter t (time) varies.
For a detailed derivation of these equations from Lagrangian mechanics, see below.
Basic physical interpretation
The simplest interpretation of the Hamilton equations is as follows, applying them to a one-dimensional
system consisting of one particle of mass m under time-independent boundary conditions: The
Hamiltonian
represents the energy of the system, which is the sum of kinetic and potential energy,
traditionally denoted T and V, respectively. Here q is the x coordinate and p is the momentum, mv. Then
http://en.wikipedia.org/wiki/Hamiltonian_mechanics
5/23/2011
Hamiltonian mechanics - Wikipedia, the free encyclopedia
Page 3 of 12
Note that T is a function of p alone, while V is a function of x (or q) alone.
Now the time-derivative of the momentum p equals the Newtonian force, and so here the first Hamilton
equation means that the force on the particle equals the rate at which it loses potential energy with
respect to changes in x, its location. (Force equals the negative gradient of potential energy.)
The time-derivative of q here means the velocity: the second Hamilton equation here means that the
particle’s velocity equals the derivative of its kinetic energy with respect to its momentum. (Because the
derivative with respect to p of p2/2m equals p/m = mv/m = v.)
Using Hamilton's equations
In terms of the generalized coordinates
and generalized velocities
1. First write out the Lagrangian
Express T and V as though you were going to use
Lagrange's equation.
2. Calculate the momenta by differentiating the Lagrangian with respect to velocity:
.
3. Express the velocities in terms of the momenta by inverting the expressions in step (2).
4. Calculate the Hamiltonian using the usual definition of H as the Legendre transformation of L:
. Substitute for the velocities using the results in step
(3).
5. Apply Hamilton's equations.
Notes
Hamilton's equations are appealing in view of their beautiful simplicity and (slightly broken) symmetry.
They have been analyzed under almost every imaginable angle of view, from basic physics up to
symplectic geometry. A lot is known about solutions of these equations, yet the exact general case
solution of the equations of motion cannot be given explicitly for a system of more than two massive
point particles. The finding of conserved quantities plays an important role in the search for solutions or
information about their nature. In models with an infinite number of degrees of freedom, this is of
course even more complicated. An interesting and promising area of research is the study of integrable
systems, where an infinite number of independent conserved quantities can be constructed.
Deriving Hamilton's equations
We can derive Hamilton's equations by looking at how the total differential of the Lagrangian depends
[3]
on time, generalized positions and generalized velocities
http://en.wikipedia.org/wiki/Hamiltonian_mechanics
5/23/2011
Hamiltonian mechanics - Wikipedia, the free encyclopedia
Now the generalized momenta were defined as
Page 4 of 12
and Lagrange's equations tell us that
We can rearrange this to get
and substitute the result into the total differential of the Lagrangian
We can rewrite this as
and rearrange again to get
The term on the left-hand side is just the Hamiltonian that we have defined before, so we find that
where the second equality holds because of the definition of the total differential of
in terms of its
partial derivatives. Associating terms from both sides of the equation above yields Hamilton's equations
As a reformulation of Lagrangian mechanics
http://en.wikipedia.org/wiki/Hamiltonian_mechanics
5/23/2011
Hamiltonian mechanics - Wikipedia, the free encyclopedia
Page 5 of 12
Starting with Lagrangian mechanics, the equations of motion are based on generalized coordinates
and matching generalized velocities
We write the Lagrangian as
with the subscripted variables understood to represent all N variables of that type. Hamiltonian
mechanics aims to replace the generalized velocity variables with generalized momentum variables, also
known as conjugate momenta. By doing so, it is possible to handle certain systems, such as aspects of
quantum mechanics, that would otherwise be even more complicated.
For each generalized velocity, there is one corresponding conjugate momentum, defined as:
In Cartesian coordinates, the generalized momenta are precisely the physical linear momenta. In circular
polar coordinates, the generalized momentum corresponding to the angular velocity is the physical
angular momentum. For an arbitrary choice of generalized coordinates, it may not be possible to obtain
an intuitive interpretation of the conjugate momenta.
One thing which is not too obvious in this coordinate dependent formulation is that different generalized
coordinates are really nothing more than different coordinatizations of the same symplectic manifold.
The Hamiltonian is the Legendre transform of the Lagrangian:
If the transformation equations defining the generalized coordinates are independent of t, and the
Lagrangian is a sum of products of functions (in the generalised coordinates) which are homogeneous of
order 0, 1 or 2, then it can be shown that H is equal to the total energy E = T + V.
Each side in the definition of
produces a differential:
http://en.wikipedia.org/wiki/Hamiltonian_mechanics
5/23/2011
Hamiltonian mechanics - Wikipedia, the free encyclopedia
Page 6 of 12
Substituting the previous definition of the conjugate momenta into this equation and matching
coefficients, we obtain the equations of motion of Hamiltonian mechanics, known as the canonical
equations of Hamilton:
Hamilton's equations are first-order differential equations, and thus easier to solve than Lagrange's
equations, which are second-order. Hamilton's equations have another advantage over Lagrange's
equations: if a system has a symmetry, such that a coordinate does not occur in the Hamiltonian, the
corresponding momentum is conserved, and that coordinate can be ignored in the other equations of the
set. Effectively, this reduces the problem from n coordinates to (n-1) coordinates. In the Lagrangian
framework, of course the result that the corresponding momentum is conserved still follows
immediately, but all the generalized velocities still occur in the Lagrangian - we still have to solve a
system of equations in n coordinates.[4]
The Lagrangian and Hamiltonian approaches provide the groundwork for deeper results in the theory of
classical mechanics, and for formulations of quantum mechanics.
Geometry of Hamiltonian systems
A Hamiltonian system may be understood as a fiber bundle E over time R, with the fibers Et, t R
being the position space. The Lagrangian is thus a function on the jet bundle J over E; taking the
fiberwise Legendre transform of the Lagrangian produces a function on the dual bundle over time whose
fiber at t is the cotangent space T*Et, which comes equipped with a natural symplectic form, and this
latter function is the Hamiltonian.
Generalization to quantum mechanics through Poisson bracket
Hamilton's equations above work well for classical mechanics, but not for quantum mechanics, since the
differential equations discussed assume that one can specify the exact position and momentum of the
particle simultaneously at any point in time. However, the equations can be further generalized to then
be extended to apply to quantum mechanics as well as to classical mechanics, through the deformation
of the Poisson algebra over p and q to the algebra of Moyal brackets.
Specifically, the more general form of the Hamilton's equation reads
http://en.wikipedia.org/wiki/Hamiltonian_mechanics
5/23/2011
Hamiltonian mechanics - Wikipedia, the free encyclopedia
Page 7 of 12
where f is some function of p and q, and H is the Hamiltonian. To find out the rules for evaluating a
Poisson bracket without resorting to differential equations, see Lie algebra; a Poisson bracket is the
name for the Lie bracket in a Poisson algebra. These Poisson brackets can then be extended to Moyal
brackets comporting to an inequivalent Lie algebra, as proven by H Groenewold, and thereby describe
quantum mechanical diffusion in phase space (See the uncertainty principle and Weyl quantization).
This more algebraic approach not only permits ultimately extending probability distributions in phase
space to Wigner quasi-probability distributions, but, at the mere Poisson bracket classical setting, also
provides more power in helping analyze the relevant conserved quantities in a system.
Mathematical formalism
Any smooth real-valued function H on a symplectic manifold can be used to define a Hamiltonian
system. The function H is known as the Hamiltonian or the energy function. The symplectic manifold
is then called the phase space. The Hamiltonian induces a special vector field on the symplectic
manifold, known as the symplectic vector field.
The symplectic vector field, also called the Hamiltonian vector field, induces a Hamiltonian flow on the
manifold. The integral curves of the vector field are a one-parameter family of transformations of the
manifold; the parameter of the curves is commonly called the time. The time evolution is given by
symplectomorphisms. By Liouville's theorem, each symplectomorphism preserves the volume form on
the phase space. The collection of symplectomorphisms induced by the Hamiltonian flow is commonly
called the Hamiltonian mechanics of the Hamiltonian system.
The symplectic structure induces a Poisson bracket. The Poisson bracket gives the space of functions on
the manifold the structure of a Lie algebra.
Given a function f
If we have a probability distribution, ρ, then (since the phase space velocity (
) has zero
divergence, and probability is conserved) its convective derivative can be shown to be zero and so
This is called Liouville's theorem. Every smooth function G over the symplectic manifold generates a
one-parameter family of symplectomorphisms and if { G, H } = 0, then G is conserved and the
symplectomorphisms are symmetry transformations.
A Hamiltonian may have multiple conserved quantities Gi. If the symplectic manifold has dimension 2n
and there are n functionally independent conserved quantities Gi which are in involution (i.e., { Gi, Gj }
= 0), then the Hamiltonian is Liouville integrable. The Liouville–Arnol'd theorem says that locally, any
Liouville integrable Hamiltonian can be transformed via a symplectomorphism in a new Hamiltonian
http://en.wikipedia.org/wiki/Hamiltonian_mechanics
5/23/2011
Hamiltonian mechanics - Wikipedia, the free encyclopedia
Page 8 of 12
with the conserved quantities Gi as coordinates; the new coordinates are called action-angle
coordinates. The transformed Hamiltonian depends only on the Gi, and hence the equations of motion
have the simple form
for some function F (Arnol'd et al., 1988). There is an entire field focusing on small deviations from
integrable systems governed by the KAM theorem.
The integrability of Hamiltonian vector fields is an open question. In general, Hamiltonian systems are
chaotic; concepts of measure, completeness, integrability and stability are poorly defined. At this time,
the study of dynamical systems is primarily qualitative, and not a quantitative science.
Riemannian manifolds
An important special case consists of those Hamiltonians that are quadratic forms, that is, Hamiltonians
that can be written as
where
is a smoothly varying inner product on the fibers
, the cotangent space to the point q
in the configuration space, sometimes called a cometric. This Hamiltonian consists entirely of the kinetic
term.
If one considers a Riemannian manifold or a pseudo-Riemannian manifold, the Riemannian metric
induces a linear isomorphism between the tangent and cotangent bundles. (See Musical isomorphism).
Using this isomorphism, one can define a cometric. (In coordinates, the matrix defining the cometric is
the inverse of the matrix defining the metric.) The solutions to the Hamilton–Jacobi equations for this
Hamiltonian are then the same as the geodesics on the manifold. In particular, the Hamiltonian flow in
this case is the same thing as the geodesic flow. The existence of such solutions, and the completeness
of the set of solutions, are discussed in detail in the article on geodesics. See also Geodesics as
Hamiltonian flows.
Sub-Riemannian manifolds
When the cometric is degenerate, then it is not invertible. In this case, one does not have a Riemannian
manifold, as one does not have a metric. However, the Hamiltonian still exists. In the case where the
cometric is degenerate at every point q of the configuration space manifold Q, so that the rank of the
cometric is less than the dimension of the manifold Q, one has a sub-Riemannian manifold.
The Hamiltonian in this case is known as a sub-Riemannian Hamiltonian. Every such Hamiltonian
uniquely determines the cometric, and vice-versa. This implies that every sub-Riemannian manifold is
uniquely determined by its sub-Riemannian Hamiltonian, and that the converse is true: every subRiemannian manifold has a unique sub-Riemannian Hamiltonian. The existence of sub-Riemannian
geodesics is given by the Chow-Rashevskii theorem.
The continuous, real-valued Heisenberg group provides a simple example of a sub-Riemannian
http://en.wikipedia.org/wiki/Hamiltonian_mechanics
5/23/2011
Hamiltonian mechanics - Wikipedia, the free encyclopedia
Page 9 of 12
manifold. For the Heisenberg group, the Hamiltonian is given by
pz is not involved in the Hamiltonian.
Poisson algebras
Hamiltonian systems can be generalized in various ways. Instead of simply looking at the algebra of
smooth functions over a symplectic manifold, Hamiltonian mechanics can be formulated on general
commutative unital real Poisson algebras. A state is a continuous linear functional on the Poisson
algebra (equipped with some suitable topology) such that for any element A of the algebra, A² maps to a
nonnegative real number.
A further generalization is given by Nambu dynamics.
Charged particle in an electromagnetic field
A good illustration of Hamiltonian mechanics is given by the Hamiltonian of a charged particle in an
electromagnetic field. In Cartesian coordinates (i.e. qi = xi), the Lagrangian of a non-relativistic
classical particle in an electromagnetic field is (in SI Units):
where e is the electric charge of the particle (not necessarily the electron charge), φ is the electric scalar
potential, and the Ai are the components of the magnetic vector potential (these may be modified
through a gauge transformation).
The generalized momenta may be derived by:
Rearranging, we may express the velocities in terms of the momenta, as:
If we substitute the definition of the momenta, and the definitions of the velocities in terms of the
momenta, into the definition of the Hamiltonian given above, and then simplify and rearrange, we get:
http://en.wikipedia.org/wiki/Hamiltonian_mechanics
5/23/2011
Hamiltonian mechanics - Wikipedia, the free encyclopedia
Page 10 of 12
This equation is used frequently in quantum mechanics.
Relativistic charged particle in an electromagnetic field
The Lagrangian for a relativistic charged particle is given by:
Thus the particle's canonical (total) momentum is
that is, the sum of the kinetic momentum and the potential momentum.
Solving for the velocity, we get
So the Hamiltonian is
From this we get the force equation (equivalent to the Euler–Lagrange equation)
from which one can derive
An equivalent expression for the Hamiltonian as function of the relativistic (kinetic) momentum,
is
http://en.wikipedia.org/wiki/Hamiltonian_mechanics
5/23/2011
Hamiltonian mechanics - Wikipedia, the free encyclopedia
Page 11 of 12
This has the advantage that can be measured experimentally whereas
cannot. Notice that the
Hamiltonian (total energy) can be viewed as the sum of the relativistic energy (kinetic+rest) ,
plus the potential energy,
See also
Canonical transformation
Classical field theory
Classical mechanics
Dynamical systems theory
Hamilton–Jacobi equation
Lagrangian mechanics
Maxwell's equations
Hamiltonian (quantum mechanics)
Quantum Hamilton's equations
Quantum field theory
Hamiltonian optics
Notes and references
Notes
1. ^ LaValle, Steven M. (2006), "§13.4.4 Hamiltonian mechanics", Planning Algorithms, Cambridge
University Press, ISBN 978-0-521-86205-9, http://planning.cs.uiuc.edu/node707.html.
2. ^ "16.3 The Hamiltonian", MIT OpenCourseWare website 18.013A,
http://ocw.mit.edu/ans7870/18/18.013a/textbook/chapter16/section03.html, retrieved February 2007
3. ^ This derivation is along the lines as given in Arnol'd 1989, pp. 65–66
4. ^ Goldstein, H. (2001), Classical Mechanics (3rd ed.), Addison-Wesley, ISBN 0201657023
References
Arnol'd, V. I. (1989), Mathematical Methods of Classical Mechanics, Springer-Verlag, ISBN 0387-96890-3
Abraham, R.; Marsden, J.E. (1978), Foundations of Mechanics, London: Benjamin-Cummings,
ISBN 0-8053-0102-X
Arnol'd, V. I.; Kozlov, V. V.; Neĩshtadt, A. I. (1988), Mathematical aspects of classical and
celestial mechanics, 3, Springer-Verlag
Vinogradov, A. M.; Kupershmidt, B. A. (1981) (DjVu), The structure of Hamiltonian mechanics,
London Math. Soc. Lect. Notes Ser., 60, London: Cambridge Univ. Press,
http://diffiety.ac.ru/djvu/structures.djvu
External links
Binney, James J., Classical Mechanics (lecture notes), University of Oxford, http://wwwthphys.physics.ox.ac.uk/users/JamesBinney/cmech.pdf, retrieved 27 October 2010
http://en.wikipedia.org/wiki/Hamiltonian_mechanics
5/23/2011
Hamiltonian mechanics - Wikipedia, the free encyclopedia
Page 12 of 12
Tong, David, Classical Dynamics (Cambridge lecture notes), University of Cambridge,
http://www.damtp.cam.ac.uk/user/tong/dynamics.html, retrieved 27 October 2010
Retrieved from "http://en.wikipedia.org/wiki/Hamiltonian_mechanics"
Categories: Fundamental physics concepts | Classical mechanics | Hamiltonian mechanics | Dynamical
systems | Symplectic geometry
This page was last modified on 22 May 2011 at 04:19.
Text is available under the Creative Commons Attribution-ShareAlike License; additional terms
may apply. See Terms of Use for details.
Wikipedia® is a registered trademark of the Wikimedia Foundation, Inc., a non-profit
organization.
http://en.wikipedia.org/wiki/Hamiltonian_mechanics
5/23/2011