In Hamiltonian mechanics, a canonical transformation is a change of canonical coordinates that preserves the form of Hamilton's equations. This is sometimes known as form invariance. Although Hamilton's equations are preserved, it need not preserve the explicit form of the Hamiltonian itself. Canonical transformations are useful in their own right, and also form the basis for the Hamilton–Jacobi equations (a useful method for calculating conserved quantities) and Liouville's theorem (itself the basis for classical statistical mechanics).
Since Lagrangian mechanics is based on generalized coordinates, transformations of the coordinates do not affect the form of Lagrange's equations and, hence, do not affect the form of Hamilton's equations if the momentum is simultaneously changed by a Legendre transformation intowhere
\left\{ (P1,Q1), (P2,Q2), (P3,Q3), \ldots \right\}
Pi
Qi,
i=1,2,\ldots N,
N
Therefore, coordinate transformations (also called point transformations) are a type of canonical transformation. However, the class of canonical transformations is much broader, since the old generalized coordinates, momenta and even time may be combined to form the new generalized coordinates and momenta. Canonical transformations that do not include the time explicitly are called restricted canonical transformations (many textbooks consider only this type).
Modern mathematical descriptions of canonical transformations are considered under the broader topic of symplectomorphism which covers the subject with advanced mathematical prerequisites such as cotangent bundles, exterior derivatives and symplectic manifolds.
Boldface variables such as represent a list of generalized coordinates that need not transform like a vector under rotation and similarly represents the corresponding generalized momentum, e.g.,
A dot over a variable or list signifies the time derivative, e.g.,and the equalities are read to be satisfied for all coordinates, 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, e.g.,
The dot product (also known as an "inner product") maps the two coordinate lists into one variable representing a single numerical value. The coordinates after transformation are similarly labelled with for transformed generalized coordinates and for transformed generalized momentum.
Restricted canonical transformations are coordinate transformations where transformed coordinates and do not have explicit time dependence, ie. and . The functional form of Hamilton's equations isIn general, a transformation does not preserve the form of Hamilton's equations but in the absence of time dependence in transformation, some simplifications are possible. Following the formal definition for a canonical transformation, it can be shown that for this type of transformation, the new Hamiltonian (sometimes called the Kamiltonian) can be expressed as:where it differs by a partial time derivative of a function known as generator, which reduces to being only a function of time for restricted canonical transformations.
In addition to leaving the form of the Hamiltonian unchanged, it is also permits the use of the unchanged Hamiltonian in the Hamilton's equations of motion due to the above form as:
Although canonical transformations refers to a more general set of transformations of phase space corresponding with less permissive transformations of the Hamiltonian, it provides simpler conditions to obtain results that can be further generalized. All of the following conditions, with the exception of bilinear invariance condition, can be generalized for canonical transformations, including time dependance.
Since restricted transformations have no explicit time dependence (by definition), the time derivative of a new generalized coordinate is
where is the Poisson bracket.
Similarly for the identity for the conjugate momentum, Pm using the form of the Kamiltonian it follows that:
Due to the form of the Hamiltonian equations of motion,
if the transformation is canonical, the two derived results must be equal, resulting in the equations:
The analogous argument for the generalized momenta Pm leads to two other sets of equations:
These are the indirect conditions to check whether a given transformation is canonical.
Sometimes the Hamiltonian relations are represented as:
Where
and . Similarly, let .
From the relation of partial derivatives, converting the
η |
=J\nablaηH
η |
=J(MT\nabla\varepsilonH)
Due to form of the Hamiltonian equations for ,
where can be used due to the form of Kamiltonian. Equating the two equations gives the symplectic condition as:
The left hand side of the above is called the Poisson matrix of
\varepsilon
η
The Poisson bracket which is defined as:can be represented in matrix form as:Hence using partial derivative relations and symplectic condition gives:
The symplectic condition can also be recovered by taking and which shows that . Thus these conditions are equivalent to symplectic conditions. Furthermore, it can be seen that , which is also the result of explicitly calculating the matrix element by expanding it.
The Lagrange bracket which is defined as:
can be represented in matrix form as:
Using similar derivation, gives:
The symplectic condition can also be recovered by taking and which shows that . Thus these conditions are equivalent to symplectic conditions. Furthermore, it can be seen that , which is also the result of explicitly calculating the matrix element by expanding it.
These set of conditions only apply to restricted canonical transformations or canonical transformations that are independent of time variable.
Consider arbitrary variations of two kinds, in a single pair of generalized coordinate and the corresponding momentum:
The area of the infinitesimal parallelogram is given by:
It follows from the symplectic condition that the infinitesimal area is conserved under canonical transformation:
Note that the new coordinates need not be completely oriented in one coordinate momentum plane.
Hence, the condition is more generally stated as an invariance of the form under canonical transformation, expanded as:If the above is obeyed for any arbitrary variations, it would be only possible if the indirect conditions are met. The form of the equation, is also known as a symplectic product of the vectors and and the bilinear invariance condition can be stated as a local conservation of the symplectic product.
The indirect conditions allow us to prove Liouville's theorem, which states that the volume in phase space is conserved under canonical transformations, i.e.,
By calculus, the latter integral must equal the former times the determinant of Jacobian Where
Exploiting the "division" property of Jacobians yields
Eliminating the repeated variables gives
Application of the indirect conditions above yields .
l{L}qp=p ⋅
q |
-H(q,p,t)
l{L}QP=P ⋅
Q |
-K(Q,P,t)
One way for both variational integral equalities to be satisfied is to have
Lagrangians are not unique: one can always multiply by a constant and add a total time derivative and yield the same equations of motion (as discussed on Wikibooks). In general, the scaling factor is set equal to one; canonical transformations for which are called extended canonical transformations. is kept, otherwise the problem would be rendered trivial and there would be not much freedom for the new canonical variables to differ from the old ones.
Here is a generating function of one old canonical coordinate (or), one new canonical coordinate (or) and (possibly) the time . Thus, there are four basic types of generating functions (although mixtures of these four types can exist), depending on the choice of variables. As will be shown below, the generating function will define a transformation from old to new canonical coordinates, and any such transformation is guaranteed to be canonical.
The various generating functions and its properties tabulated below is discussed in detail:
Trivial Cases | ||||||||||||
G=G1(q,Q,t) | p=
| P=-
| G1=qQ | Q=p | P=-q | |||||||
G=G2(q,P,t)-QP | p=
| Q=
| G2=qP | Q=q | P=p | |||||||
G=G3(p,Q,t)+qp | q=-
| P=-
| G3=pQ | Q=-q | P=-p | |||||||
G=G4(p,P,t)+qp-QP | q=-
| Q=
| G4=pP | Q=p | P=-q |
The type 1 generating function depends only on the old and new generalized coordinatesTo derive the implicit transformation, we expand the defining equation above
Since the new and old coordinates are each independent, the following equations must hold
These equations define the transformation as follows: The first set of equationsdefine relations between the new generalized coordinates and the old canonical coordinates . Ideally, one can invert these relations to obtain formulae for each as a function of the old canonical coordinates. Substitution of these formulae for the coordinates into the second set of equationsyields analogous formulae for the new generalized momenta in terms of the old canonical coordinates . We then invert both sets of formulae to obtain the old canonical coordinates as functions of the new canonical coordinates . Substitution of the inverted formulae into the final equation yields a formula for as a function of the new canonical coordinates .
In practice, this procedure is easier than it sounds, because the generating function is usually simple. For example, let This results in swapping the generalized coordinates for the momenta and vice versa and . This example illustrates how independent the coordinates and momenta are in the Hamiltonian formulation; they are equivalent variables.
The type 2 generating function
G2(q,P,t)
-Q ⋅ P
Since the old coordinates and new momenta are each independent, the following equations must hold
These equations define the transformation as follows: The first set of equationsdefine relations between the new generalized momenta and the old canonical coordinates . Ideally, one can invert these relations to obtain formulae for each as a function of the old canonical coordinates. Substitution of these formulae for the coordinates into the second set of equationsyields analogous formulae for the new generalized coordinates in terms of the old canonical coordinates . We then invert both sets of formulae to obtain the old canonical coordinates as functions of the new canonical coordinates . Substitution of the inverted formulae into the final equation yields a formula for as a function of the new canonical coordinates .
In practice, this procedure is easier than it sounds, because the generating function is usually simple. For example, let where is a set of functions. This results in a point transformation of the generalized coordinates
The type 3 generating function
G3(p,Q,t)
q ⋅ p
Since the new and old coordinates are each independent, the following equations must hold
These equations define the transformation as follows: The first set of equationsdefine relations between the new generalized coordinates and the old canonical coordinates . Ideally, one can invert these relations to obtain formulae for each as a function of the old canonical coordinates. Substitution of these formulae for the coordinates into the second set of equationsyields analogous formulae for the new generalized momenta in terms of the old canonical coordinates . We then invert both sets of formulae to obtain the old canonical coordinates as functions of the new canonical coordinates . Substitution of the inverted formulae into the final equation yields a formula for as a function of the new canonical coordinates .
In practice, this procedure is easier than it sounds, because the generating function is usually simple.
The type 4 generating function
G4(p,P,t)
q ⋅ p-Q ⋅ P
Since the new and old coordinates are each independent, the following equations must hold
These equations define the transformation as follows: The first set of equationsdefine relations between the new generalized momenta and the old canonical coordinates . Ideally, one can invert these relations to obtain formulae for each as a function of the old canonical coordinates. Substitution of these formulae for the coordinates into the second set of equationsyields analogous formulae for the new generalized coordinates in terms of the old canonical coordinates . We then invert both sets of formulae to obtain the old canonical coordinates as functions of the new canonical coordinates . Substitution of the inverted formulae into the final equation yields a formula for as a function of the new canonical coordinates .
For example, using generating function of second kind: and , the first set of equations consisting of variables , and has to be inverted to get . This process is possible when the matrix defined by is non-singular.
Hence, restrictions are placed on generating functions to have the matrices: , , and , being non-singular.
Since is non-singular, it implies that is also non-singular. Since the matrix is inverse of , the transformations of type 2 generating functions always have a non-singular matrix. Similarly, it can be stated that type 1 and type 4 generating functions always have a non-singular matrix whereas type 2 and type 3 generating functions always have a non-singular matrix. Hence, the canonical transformations resulting from these generating functions are not completely general.
In other words, since and are each independent functions, it follows that to have generating function of the form and
G4(p,P,t)
G2(q,P,t)
G3(p,Q,t)
From:
K=H+
\partialG | |
\partialt |
Since the left hand side is which is independent of dynamics of the particles, equating coefficients of and to zero, canonical transformation rules are obtained. This step is equivalent to equating the left hand side as .
Similarly:
Similarly the canonical transformation rules are obtained by equating the left hand side as .
The above two relations can be combined in matrix form as: (which will also retain same form for extended canonical transformation) where the result , has been used. The canonical transformation relations are hence said to be equivalent to in this context.
The canonical transformation relations can now be restated to include time dependance:Since and , if and do not explicitly depend on time, can be taken. The analysis of restricted canonical transformations is hence consistent with this generalization.
Applying transformation of co-ordinates formula for
\nablaηH=MT\nabla\varepsilonH
Similarly for :or:Where the last terms of each equation cancel due to condition from canonical transformations. Hence leaving the symplectic relation: which is also equivalent with the condition . It follows from the above two equations that the symplectic condition implies the equation , from which the indirect conditions can be recovered. Thus, symplectic conditions and indirect conditions can be said to be equivalent in the context of using generating functions.
Since and where the symplectic condition is used in the last equalities. Using , the equalities and are obtained which imply the invariance of Poisson and Lagrange brackets.
By solving for:with various forms of generating function, the relation between K and H goes as instead, which also applies for case.
All results presented below can also be obtained by replacing , and from known solutions, since it retains the form of Hamilton's equations. The extended canonical transformations are hence said to be result of a canonical transformation () and a trivial canonical transformation () which has (for the given example, which satisfies the condition).
Using same steps previously used in previous generalization, with in the general case, and retaining the equation , extended canonical transformation partial differential relations are obtained as:
Following the same steps to derive the symplectic conditions, as: and
where using instead gives:The second part of each equation cancel. Hence the condition for extended canonical transformation instead becomes: .
The Poisson brackets are changed as follows:whereas, the Lagrange brackets are changed as:
Hence, the Poisson bracket scales by the inverse of whereas the Lagrange bracket scales by a factor of .[1]
Consider the canonical transformation that depends on a continuous parameter
\alpha
\begin{align} &Q(q,p,t;\alpha) &Q(q,p,t;0)=q\ &P(q,p,t;\alpha) with &P(q,p,t;0)=p\ \end{align}
For infinitesimal values of
\alpha
Consider the following generating function:
G2(q,P,t)=qP+\alphaG(q,P,t)
Since for
\alpha=0
G2=qP
Q=q
P=p
\alpha
P=P(q,p,t;\alpha)
G
G(q,p,t)
\alpha
See also: Active and passive transformation. In the passive view of transformations, the coordinate system is changed without the physical system changing, whereas in the active view of transformation, the coordinate system is retained and the physical system is said to undergo transformations. Thus, using the relations from infinitesimal canonical transformations, the change in the system states under active view of the canonical transformation is said to be:
\begin{align} &\deltaq=\alpha
\partialG | |
\partialp |
(q,p,t) and \deltap=-\alpha
\partialG | |
\partialq |
(q,p,t),\ \end{align}
or as
\deltaη=\alphaJ\nablaηG
For any function
u(η)
\deltau=u(η+\deltaη)-u(η)=(\nablaηu)T\deltaη=\alpha(\nablaηu)TJ(\nablaηG)=\alpha\{u,G\}.
Considering the change of Hamiltonians in the active view, ie. for a fixed point,where are mapped to the point, by the infinitesimal canonical transformation, and similar change of variables for
G(q,P,t)
G(q,p,t)
\alpha
Taking
G(q,p,t)=H(q,p,t)
\alpha=dt
\deltaη=(J\nablaηH)dt=
η |
dt=dη
η(\tau)
η(\tau+t)
Taking
G(q,p,t)=pk
\deltapi=0
\deltaqi=\alpha\deltaik
Consider an orthogonal system for an N-particle system:
\begin{array}{l}{{q=\left(x1,y1,z1,\ldots,xn,yn,zn\right),}}\ {{p=\left(p1x,p1y,p1z,\ldots,pn,pn,pn\right).}}\end{array}
Choosing the generator to be:
G=Lz
n | |
=\sum | |
i=1 |
\left(xipi-yipi\right)
\alpha=\delta\phi
\begin{array}{c} {\deltaxi=\{xi,G\}\delta\phi=\displaystyle\sumj\{xi,xjpj-yjpj\}\delta\phi=\displaystyle\sumj(\underbrace{\{xi,xjpj\}}=0-{\{xi,yjpj\}}})\delta\phi\\ {{=\displaystyle-\sumjyj\underbrace{\{xi,pjx
\}} | |
=\deltaij |
\delta\phi=-yi\delta\phi}}\end{array}
and similarly for y:
\begin{array}{c} \deltayi=\{yi,G\}\delta\phi=\displaystyle\sumj\{yi,xjpj-yjpj\}\delta\phi=\displaystyle\sumj(\{yi,xjpj\}-\underbrace{\{yi,yjpj\}}=0)\delta\phi\ {=\displaystyle\sumjxj\underbrace{\{yi,pjy
\}} | |
=\deltaij |
\delta\phi=xi\delta\phi,}\end{array}
whereas the z component of all particles is unchanged:
\deltaz=\left\{zi,G\right\}\delta\phi=\sumj\left\{zi,xjpj-yjpj\right\}\delta\phi=0
These transformations correspond to rotation about the z axis by angle
\delta\phi
Motion itself (or, equivalently, a shift in the time origin) is a canonical transformation. If
Q(t)\equivq(t+\tau)
P(t)\equivp(t+\tau)
(q(t),p(t))
Q(q,p)=q+a,P(q,p)=p+b
a,b
ITJI=J
x=(q,p)
X=(Q,P)
X(x)=Rx
R\inSO(2)
RTR=I
SO(2)
(q,p)
q
p
(Q(q,p),P(q,p))=(q+f(p),p)
f(p)
p
In mathematical terms, canonical coordinates are any coordinates on the phase space (cotangent bundle) of the system that allow the canonical one-form to be written asup to a total differential (exact form). The change of variable between one set of canonical coordinates and another is a canonical transformation. The index of the generalized coordinates is written here as a superscript (
qi
qi
The first major application of the canonical transformation was in 1846, by Charles Delaunay, in the study of the Earth-Moon-Sun system. This work resulted in the publication of a pair of large volumes as Mémoires by the French Academy of Sciences, in 1860 and 1867.