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).
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 Q for transformed generalized coordinates and P for transformed generalized momentum.
Conditions for restricted canonical transformation
Restricted canonical transformations are coordinate transformations where transformed coordinates Q and P do not have explicit time dependence, i.e., and . The functional form of Hamilton's equations is
In general, a transformation (q, p) → (Q, P) 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[1]) can be expressed as:where it differs by a partial time derivative of a function known as a 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.
Sometimes the Hamiltonian relations are represented as:
Where
and . Similarly, let .
From the relation of partial derivatives, converting the relation in terms of partial derivatives with new variables gives where . Similarly for ,
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:[2]
The left hand side of the above is called the Poisson matrix of , denoted as . Similarly, a Lagrange matrix of can be constructed as .[3] It can be shown that the symplectic condition is also equivalent to by using the property. The set of all matrices which satisfy symplectic conditions form a symplectic group. The symplectic conditions are equivalent with indirect conditions as they both lead to the equation , which is used in both of the derivations.
The Poisson bracket which is defined as:can be represented in matrix form as:Hence using partial derivative relations and symplectic condition gives:[4]
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.[3]
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.[3]
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:[5]
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.[6][7]
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.[8]
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 JacobianMWhere
Exploiting the "division" property of Jacobians yields
Eliminating the repeated variables gives
Application of the indirect conditions above yields .[9]
To guarantee a valid transformation between (q, p, H) and (Q, P, K), we may resort to a direct generating function approach. Both sets of variables must obey Hamilton's principle. That is the action integral over the Lagrangians and , obtained from the respective Hamiltonian via an "inverse" Legendre transformation, must be stationary in both cases (so that one can use the Euler–Lagrange equations to arrive at Hamiltonian equations of motion of the designated form; as it is shown for example here):
Lagrangians are not unique: one can always multiply by a constant λ and add a total time derivative dG/dt and yield the same equations of motion (as discussed on Wikibooks). In general, the scaling factor λ is set equal to one; canonical transformations for which λ ≠ 1 are called extended canonical transformations. dG/dt 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 G is a generating function of one old canonical coordinate (q or p), one new canonical coordinate (Q or P) and (possibly) the time t. 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 (q, p) → (Q, P) is guaranteed to be canonical.
The various generating functions and its properties tabulated below is discussed in detail:
Properties of four basic Canonical Transformations[10]
The type 1 generating function G1 depends only on the old and new generalized coordinates
To derive the implicit transformation, we expand the defining equation above
Since the new and old coordinates are each independent, the following 2N + 1 equations must hold
These equations define the transformation (q, p) → (Q, P) as follows: The first set of N equations
define relations between the new generalized coordinatesQ and the old canonical coordinates(q, p). Ideally, one can invert these relations to obtain formulae for each Qk as a function of the old canonical coordinates. Substitution of these formulae for the Q coordinates into the second set of N equations
yields analogous formulae for the new generalized momenta P in terms of the old canonical coordinates(q, p). We then invert both sets of formulae to obtain the oldcanonical coordinates(q, p) as functions of the newcanonical coordinates(Q, P). Substitution of the inverted formulae into the final equation
yields a formula for K as a function of the new canonical coordinates(Q, P).
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 K = H. This example illustrates how independent the coordinates and momenta are in the Hamiltonian formulation; they are equivalent variables.
The type 2 generating function depends only on the old generalized coordinates and the new generalized momenta
where the terms represent a Legendre transformation to change the right-hand side of the equation below. To derive the implicit transformation, we expand the defining equation above
Since the old coordinates and new momenta are each independent, the following 2N + 1 equations must hold
These equations define the transformation (q, p) → (Q, P) as follows: The first set of N equations
define relations between the new generalized momenta P and the old canonical coordinates(q, p). Ideally, one can invert these relations to obtain formulae for each Pk as a function of the old canonical coordinates. Substitution of these formulae for the P coordinates into the second set of N equations
yields analogous formulae for the new generalized coordinates Q in terms of the old canonical coordinates(q, p). We then invert both sets of formulae to obtain the oldcanonical coordinates(q, p) as functions of the newcanonical coordinates(Q, P). Substitution of the inverted formulae into the final equation
yields a formula for K as a function of the new canonical coordinates(Q, P).
In practice, this procedure is easier than it sounds, because the generating function is usually simple. For example, let
where g is a set of N functions. This results in a point transformation of the generalized coordinates
The type 3 generating function depends only on the old generalized momenta and the new generalized coordinates
where the terms represent a Legendre transformation to change the left-hand side of the equation below. To derive the implicit transformation, we expand the defining equation above
Since the new and old coordinates are each independent, the following 2N + 1 equations must hold
These equations define the transformation (q, p) → (Q, P) as follows: The first set of N equations
define relations between the new generalized coordinatesQ and the old canonical coordinates(q, p). Ideally, one can invert these relations to obtain formulae for each Qk as a function of the old canonical coordinates. Substitution of these formulae for the Q coordinates into the second set of N equations
yields analogous formulae for the new generalized momenta P in terms of the old canonical coordinates(q, p). We then invert both sets of formulae to obtain the oldcanonical coordinates(q, p) as functions of the newcanonical coordinates(Q, P). Substitution of the inverted formulae into the final equation yields a formula for K as a function of the new canonical coordinates(Q, P).
In practice, this procedure is easier than it sounds, because the generating function is usually simple.
The type 4 generating function depends only on the old and new generalized momenta
where the terms represent a Legendre transformation to change both sides of the equation below. To derive the implicit transformation, we expand the defining equation above
Since the new and old coordinates are each independent, the following 2N + 1 equations must hold
These equations define the transformation (q, p) → (Q, P) as follows: The first set of N equations
define relations between the new generalized momenta P and the old canonical coordinates(q, p). Ideally, one can invert these relations to obtain formulae for each Pk as a function of the old canonical coordinates. Substitution of these formulae for the P coordinates into the second set of N equations
yields analogous formulae for the new generalized coordinates Q in terms of the old canonical coordinates(q, p). We then invert both sets of formulae to obtain the oldcanonical coordinates(q, p) as functions of the newcanonical coordinates(Q, P). Substitution of the inverted formulae into the final equation
yields a formula for K as a function of the new canonical coordinates(Q, P).
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.[11]
Hence, restrictions are placed on generating functions to have the matrices: , , and , being non-singular.[12][13]
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.[14]
In other words, since (Q, P) and (q, p) are each 2N independent functions, it follows that to have generating function of the form and or and , the corresponding Jacobian matrices and are restricted to be non singular, ensuring that the generating function is a function of 2N + 1 independent variables. However, as a feature of canonical transformations, it is always possible to choose 2N such independent functions from sets (q, p) or (Q, P), to form a generating function representation of canonical transformations, including the time variable. Hence, it can be proved that every finite canonical transformation can be given as a closed but implicit form that is a variant of the given four simple forms.[15]
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 Q and P 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 , in Hamiltonian's equations gives:
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).[16]
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:
Consider the canonical transformation that depends on a continuous parameter , as follows:
For infinitesimal values of , the corresponding transformations are called as infinitesimal canonical transformations which are also known as differential canonical transformations.
Consider the following generating function:
Since for , has the resulting canonical transformation, and , this type of generating function can be used for infinitesimal canonical transformation by restricting to an infinitesimal value. From the conditions of generators of second type:Since , changing the variables of the function to and neglecting terms of higher order of , gives:[19]Infinitesimal canonical transformations can also be derived using the matrix form of the symplectic condition.[20]
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:
or as in matrix form.
For any function , it changes under active view of the transformation according to:
Considering the change of Hamiltonians in the active view, i.e., for a fixed point,where are mapped to the point, by the infinitesimal canonical transformation, and similar change of variables for to is considered up-to first order of . Hence, if the Hamiltonian is invariant for infinitesimal canonical transformations, its generator is a constant of motion.
Taking and , then . Thus the continuous application of such a transformation maps the coordinates to . Hence if the Hamiltonian is time translation invariant, i.e. does not have explicit time dependence, its value is conserved for the motion.
Taking , and . Hence, the canonical momentum generates a shift in the corresponding generalized coordinate and if the Hamiltonian is invariant of translation, the momentum is a constant of motion.
Consider an orthogonal system for an N-particle system:
Choosing the generator to be: and the infinitesimal value of , then the change in the coordinates is given for x by:
and similarly for y:
whereas the z component of all particles is unchanged: .
These transformations correspond to rotation about the z axis by angle in its first order approximation. Hence, repeated application of the infinitesimal canonical transformation generates a rotation of system of particles about the z axis. If the Hamiltonian is invariant under rotation about the z axis, the generator, the component of angular momentum along the axis of rotation, is an invariant of motion.[20]
Motion itself (or, equivalently, a shift in the time origin) is a canonical transformation. If and , then Hamilton's principle is automatically satisfiedsince a valid trajectory should always satisfy Hamilton's principle, regardless of the endpoints.
The translation where are two constant vectors is a canonical transformation. Indeed, the Jacobian matrix is the identity, which is symplectic: .
Set and , the transformation where is a rotation matrix of order 2 is canonical. Keeping in mind that special orthogonal matrices obey it's easy to see that the Jacobian is symplectic. However, this example only works in dimension 2: is the only special orthogonal group in which every matrix is symplectic. Note that the rotation here acts on and not on and independently, so these are not the same as a physical rotation of an orthogonal spatial coordinate system.
The transformation , where is an arbitrary function of , is canonical. Jacobian matrix is indeed given by which is symplectic.
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 as
up 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 coordinatesq is written here as a superscript (), not as a subscript as done above (). The superscript conveys the contravariant transformation properties of the generalized coordinates, and does not mean that the coordinate is being raised to a power. Further details may be found at the symplectomorphism article.
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.