Dirac Brackets and Bihamiltonian Structures
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Dirac brackets and bihamiltonian structures Charles-Michel Marle [email protected] Universite´ Pierre et Marie Curie Paris, France Thirty years of bihamiltonian systems, Be¸dlewo, August 3–9, 2008 Dirac brackets and bihamiltonian systems – p. 1/46 Summary (1) Introduction Lagrangian formalism Lagrangian formalism and Legendre map Generalized Hamiltonian formalism Primary constraints Generalized Hamiltonian dynamics Secondary constraints Dirac’s algorithm Final constraint submanifold First and second class constraints Poisson brackets of second class constraints Symplectic explanation Thirty years of bihamiltonian systems, Be¸dlewo, August 3–9, 2008 Dirac brackets and bihamiltonian systems – p. 2/46 Summary (2) The Dirac bracket The Poisson-Dirac bivector Compatibility of Λ anΛD Example Thanks Thirty years of bihamiltonian systems, Be¸dlewo, August 3–9, 2008 Dirac brackets and bihamiltonian systems – p. 3/46 Introduction Around 1950, P. Dirac developed a Generalized Hamiltonian dynamics for Lagrangian systems with degenerate Lagrangians. In this theory, the phase space of the system (i.e. the cotangent bundle to the configuration manifold) is endowed with two Poisson brackets: Thirty years of bihamiltonian systems, Be¸dlewo, August 3–9, 2008 Dirac brackets and bihamiltonian systems – p. 4/46 Introduction Around 1950, P. Dirac developed a Generalized Hamiltonian dynamics for Lagrangian systems with degenerate Lagrangians. In this theory, the phase space of the system (i.e. the cotangent bundle to the configuration manifold) is endowed with two Poisson brackets: the usual Poisson bracket associated to its symplectic structure, Thirty years of bihamiltonian systems, Be¸dlewo, August 3–9, 2008 Dirac brackets and bihamiltonian systems – p. 4/46 Introduction Around 1950, P. Dirac developed a Generalized Hamiltonian dynamics for Lagrangian systems with degenerate Lagrangians. In this theory, the phase space of the system (i.e. the cotangent bundle to the configuration manifold) is endowed with two Poisson brackets: the usual Poisson bracket associated to its symplectic structure, a modified Poisson bracket (today known as the Dirac bracket), used by Dirac for the canonical quantization of the system. Thirty years of bihamiltonian systems, Be¸dlewo, August 3–9, 2008 Dirac brackets and bihamiltonian systems – p. 4/46 Introduction Around 1950, P. Dirac developed a Generalized Hamiltonian dynamics for Lagrangian systems with degenerate Lagrangians. In this theory, the phase space of the system (i.e. the cotangent bundle to the configuration manifold) is endowed with two Poisson brackets: the usual Poisson bracket associated to its symplectic structure, a modified Poisson bracket (today known as the Dirac bracket), used by Dirac for the canonical quantization of the system. I what follows I will describe Dirac’s theory of generalized Hamiltonian dynamics, and I will consider its links with the theory of bihamiltonian systems. Thirty years of bihamiltonian systems, Be¸dlewo, August 3–9, 2008 Dirac brackets and bihamiltonian systems – p. 4/46 Lagrangian formalism We consider a mechanical system with a smooth manifold Q as configuration space. The dynamical properties of the system are described by a smooth Lagrangian L : T Q → R. Possible motions of the system are curves t 7→ q(t), parametrized by the time t, defined on intervals [t0, t1] ⊂ R, which are extremals of the action integral t1 dq(t) I = L dt , Zt0 dt with fixed endpoints. Thirty years of bihamiltonian systems, Be¸dlewo, August 3–9, 2008 Dirac brackets and bihamiltonian systems – p. 5/46 Lagrangian formalism and Legendre map (1) The curve t 7→ q(t) is an extremal of the action integral if and only if it satisfies Lagrange equations, which, in a chart of Q with local coordinates (q1,...,qn), and the associated chart of T Q with local coordinates (q1,...,qn, q˙1,..., q˙n), are d ∂L(q, q˙) ∂L(q, q˙) − = 0 , 1 ≤ i ≤ n . dt ∂q˙i ∂qi Thirty years of bihamiltonian systems, Be¸dlewo, August 3–9, 2008 Dirac brackets and bihamiltonian systems – p. 6/46 Lagrangian formalism and Legendre map (1) The curve t 7→ q(t) is an extremal of the action integral if and only if it satisfies Lagrange equations, which, in a chart of Q with local coordinates (q1,...,qn), and the associated chart of T Q with local coordinates (q1,...,qn, q˙1,..., q˙n), are d ∂L(q, q˙) ∂L(q, q˙) − = 0 , 1 ≤ i ≤ n . dt ∂q˙i ∂qi When the Legendre map ∗ ∂L(q, q˙) L : T Q → T Q , (q, q˙) 7→ (q,p) with p = i ∂q˙i is a (local) diffeomorphism, one may (locally) define a Hamiltonian H on T ∗Q by setting n i H(q,p) = q˙ pi − L(q, q˙) , Xi=1 Thirty years of bihamiltonian systems, Be¸dlewo, August 3–9, 2008 Dirac brackets and bihamiltonian systems – p. 6/46 Lagrangian formalism and Legendre map (2) where q˙i and (q, q˙) = L−1(q,p) are expressed in terms of (q,p) by means of the (local) inverse L−1 of the Legendre map. Under these assumptions, Lagrange equations are (locally) equivalent to Hamilton equations, i dq ∂H(q,p) dpi ∂H(q,p) = , = − i . dt ∂pi dt ∂q Thirty years of bihamiltonian systems, Be¸dlewo, August 3–9, 2008 Dirac brackets and bihamiltonian systems – p. 7/46 Lagrangian formalism and Legendre map (2) where q˙i and (q, q˙) = L−1(q,p) are expressed in terms of (q,p) by means of the (local) inverse L−1 of the Legendre map. Under these assumptions, Lagrange equations are (locally) equivalent to Hamilton equations, i dq ∂H(q,p) dpi ∂H(q,p) = , = − i . dt ∂pi dt ∂q When the Legendre map is not a (local) diffeomorphism, we still can define a Hamiltonian on T Q ⊕ T ∗Q by n i H(q, q,p˙ ) = q˙ pi − L(q, q˙) . Xi=1 That Hamiltonian is not a function defined on T ∗Q. Thirty years of bihamiltonian systems, Be¸dlewo, August 3–9, 2008 Dirac brackets and bihamiltonian systems – p. 7/46 Primary constraints (1) Dirac does not assume that the Legendre map is a local diffeomorphism. Instead, he (tacitly) assumes that it is a map of constant rank 2n − r, with 1 ≤ r ≤ n. Let D0 = L(T Q) be its image. Dirac assumes that there exist r ∗ smooth functions Φα : T Q → R, 1 ≤ α ≤ r, such that D0 is defined by the equations Φα = 0 , 1 ≤ α ≤ r , the dΦα being linearly independent on D0. These equations are called primary constraints. The functions Φα are called the primary constraint functions. Thirty years of bihamiltonian systems, Be¸dlewo, August 3–9, 2008 Dirac brackets and bihamiltonian systems – p. 8/46 Primary constraints (2) These assumptions are valid locally: since L is of constant rank 2n − r, each point in T Q has an open neighbourhood U in T Q such that L(U) is a smooth (2n − r)-dimensional submanifold of T ∗Q defined by equations of that form, the Φi being smooth functions defined on some open subset V of T ∗Q containing L(U). Thirty years of bihamiltonian systems, Be¸dlewo, August 3–9, 2008 Dirac brackets and bihamiltonian systems – p. 9/46 Primary constraints (2) These assumptions are valid locally: since L is of constant rank 2n − r, each point in T Q has an open neighbourhood U in T Q such that L(U) is a smooth (2n − r)-dimensional submanifold of T ∗Q defined by equations of that form, the Φi being smooth functions defined on some open subset V of T ∗Q containing L(U). ∗ Globally, D0 may not be a “true” submanifold of T Q: it may be self-intersecting, with multiple points. Thirty years of bihamiltonian systems, Be¸dlewo, August 3–9, 2008 Dirac brackets and bihamiltonian systems – p. 9/46 Primary constraints (2) These assumptions are valid locally: since L is of constant rank 2n − r, each point in T Q has an open neighbourhood U in T Q such that L(U) is a smooth (2n − r)-dimensional submanifold of T ∗Q defined by equations of that form, the Φi being smooth functions defined on some open subset V of T ∗Q containing L(U). ∗ Globally, D0 may not be a “true” submanifold of T Q: it may be self-intersecting, with multiple points. The partial derivatives of H : T Q ⊕ T ∗Q → R are ∂H(q, q,p˙ ) ∂L(q, q˙) ∂H(q, q,p˙ ) ∂L(q, q˙) = − , = pi − , ∂qi ∂qi ∂q˙i ∂q˙i ∂H(q, q,p˙ ) =q ˙i . ∂pi Thirty years of bihamiltonian systems, Be¸dlewo, August 3–9, 2008 Dirac brackets and bihamiltonian systems – p. 9/46 Generalized Hamiltonian dynamics (1) Let ∗ ΓL = (q, q,p˙ ) ∈ T Q ⊕ T Q;(q,p) = L(q, q˙) be the graph of the Legendre map. We see that ∂H(q, q,p˙ ) = 0 when (q, q,p˙ ) ∈ ΓL . ∂q˙i Thirty years of bihamiltonian systems, Be¸dlewo, August 3–9, 2008 Dirac brackets and bihamiltonian systems – p. 10/46 Generalized Hamiltonian dynamics (1) Let ∗ ΓL = (q, q,p˙ ) ∈ T Q ⊕ T Q;(q,p) = L(q, q˙) be the graph of the Legendre map. We see that ∂H(q, q,p˙ ) = 0 when (q, q,p˙ ) ∈ ΓL . ∂q˙i In other words, on the graph ΓL (which is a 2n-dimensional submanifold of the 3n-dimensional manifold T Q ⊕ T ∗Q), the Hamiltonian H does not depend on the variables q˙i. Thirty years of bihamiltonian systems, Be¸dlewo, August 3–9, 2008 Dirac brackets and bihamiltonian systems – p. 10/46 Generalized Hamiltonian dynamics (1) Let ∗ ΓL = (q, q,p˙ ) ∈ T Q ⊕ T Q;(q,p) = L(q, q˙) be the graph of the Legendre map. We see that ∂H(q, q,p˙ ) = 0 when (q, q,p˙ ) ∈ ΓL .