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American Journal of Physics

Development of canonical transformations from Hamilton’s principle

Charles Richard Quade

Published Jun 1, 1979• 3 citations
Publisher

Abstract

It is shown that the variations δqj and δQi cannot both be simultaneously zero at the limits of integration in application of Hamilton’s principle to the development of the theory of canonical transformations. This means that δF1 does not vanish. However, the derivation requires only that the δqj be zero at the end points. This lack of symmetry between the qj and Qi in the formulaion is noted with respect to the Lagrangian and the fundamental commutator relations of quantum mechanics. Further, what is meant by an independent variation of pj and qj (or Qi and Pi) in the formalism is discussed in the use of Hamilton’s principle in the derivation of Hamilton’s equations of motion.

DOI
10.1119/1.11780

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It is shown that the variations δqj and δQi cannot both be simultaneously zero at the limits of integration in application of Hamilton’s principle to the development of the theory of canonical transformations. This means that δF1 does not vanish. However, the derivation requires only that the δqj be zero at the end points. This lack of symmetry between the qj and Qi in the formulaion is noted with respect to the Lagrangian and the fundamental commutator relations of quantum mechanics. Further, what is meant by an independent variation of pj and qj (or Qi and Pi) in the formalism is discussed in the use of Hamilton’s principle in the derivation of Hamilton’s equations of motion.
Consistency between the Lagrangean and the Hamiltonian formalisms in the quantum mechanics is investigatedc for the type of Lagrangean L=½\dotqigij(q)\dotqj-v(q) as an extension of a previous paper. The variations δqi and δ\dotqi should be considered as q-numbers. When the Lagrangean can be transformed into the standard form L=½\dotQα2-V(Q), the commutation relations of δqi and δ\dotqi with qj and \dotqj are found with the help of the Q-coordinate system. It is shown that using the commutation relations, the variation principle leads to the same equation of motion as the canonical equation of motion which is obtained in the previous paper.
In a previous article in this series [E. M. Corson, J. Math. Phys. 4, 42(1963); herein referred to as I], we showed that a new variational principle, of Lagrangian structure, includes Hamilton's principle as a unique derived consequent and encompasses both the classical and quantal definitions of ``state'' and the corresponding Lagrangian and Schrödinger equations of motion, respectively. In this theory, the essential difference between classical and quantal domains was shown to arise solely from the superadded postulates of determinism vs indeterminism; other than this, the two basic postulates may be reduced to (a) Newton's equations and (b) the new variation principle. It was shown that in the quantal domain, Schrödinger's equations obtain for the transition amplitudes (propagators) between pure eigenstates of homologous variables in configuration space, at different times, thus clearly implying all the results of conventional Hamiltonian-operator quantum formalism. In the present article we show that our variational formulation, under the familiar classical canonical transformation to momentum space, also automatically leads to the corresponding Schrödinger equations for the transition amplitudes between pure eigenstates of homologous variables in momentum space, at different times, the unitarity of the formalism and the reciprocal Fourier integral relations between coordinate and momentum representations. Thus we bridge a long-standing gap in theory and derive, uniquely and apparently for the first time, the basic quantum commutators (P.B.'s) which are postulated in the more familiar Hamiltonian theory.
Quantum theory is developed from a q-number (operator) action principle with a representation-invariant technique for limiting the number of independent system variables. It is shown that in a q-number theory such a limitation on the number of variations is necessary, since a completely arbitrary q-number variation implies an infinite number of conditions to be satisfied. This implication arises because, in a q-number theory, the system variables are matrices which, in most cases, have an infinite number of elements, each of which is independently varied in a completely arbitrary q-number variation. A review of Schwinger's quantum field theory formalism shows that the limitation to c- number variations, while accomplishing the desired limitation on the number of independent variations, is not representation-invariant, i.e., under a change of representation, c-number variations applied to the original system variables transform to q-number variations of the new system variables. In the Hamiltonian formulation of classical mechanics, invariant infinitesimal canonical transformations are generated by constants of the motion. It is also possible to generate infinitesimal invariant transformations from constants of the motion in the classical Lagrangian formulation. This latter possibility is exploited to obtain a q-number theory. It is demanded that the variations of the system variablesmore » in the action principle be the invariant infinitesimal transformations generated by constants of the motion. This requirement accomplishes the desired limitation of the number of independent variations in a representation-invariant manner. The resulting theory is developed in detail for a certain class of Lagrangian functions. A set of commutation relations is derived and a dynamical principle for determining the temporal development of the system is postulated. A discussion of the results, including consideration of the self-consistency of the formalism developed, is presented. Finally, a specific problem is worked through and it is shown that the results obtained correspond with those of the canonical method of quantization.« less
Schwinger’s variational principle is formulated for the multi-dimensional quantum system which corresponds to the classical system described by the Lagrangian Lc(\dotx,x)=(M/2)gij(x)\dotxi\dotxj-v(x). The c-number variations of coordinates and time are sufficient to give the laws of quantum mechanics. The Euler-Lagrange equation, the canonical equations of motion and the canonical commutation relations are derived from this principle and there is no inconsistency. An appropriate choice of the Lagrangian operator is essential in our formulation. It is shown that an arbitrary point transformation is entitled to be called a canonical transformation. Ambiguities in the quantal Lagrangian are also discussed.