Hamilton s principle and Symmetries

Transcription

1 Hamilton s principle and Symmetries Sourendu Gupta TIFR, Mumbai, India Classical Mechanics 2011 August 18, 2011

2 The Hamiltonian The change in the Lagrangian due to a virtual change of coordinates is dl = k ṗ k dq k +p k d q k. Using this, one can define a function, called the Hamiltonian, H(p k,q k ), by eliminating q k to write [ ] dh = d p k q k L = q k dp k ṗ k dq k. k k Since p k and q k are independent variables, one has Hamilton s equations q k = H and ṗ k = H. p k q k

3 An example For a single particle moving in a potential, one can write L = 1 2 m ẋ 2 V(x), and p = L ẋ = mẋ. Eliminating ẋ, the Hamiltonian is Hamilton s equations are H = p ẋ L = 1 2m p 2 +V(x). ẋ = 1 p, and ṗ = V(x). m These are exactly the usual equations of motion.

4 Poisson brackets Take any two functions on phase space, f(q k,p k ) and g(q k,p k ). Then we define the Poisson bracket of these two functions as [f,g] = [ f g g ] f. q k p k q k p k k Clearly, interchanging f and g in the expression on the right changes the sign. So [f,g] = [g,f]. Also, because of this antisymmetry, [f,f] = 0. Clearly, the time derivative of f is given by the expression df dt = k = k [ f q k + f ] ṗ k q k p k [ f H f H q k p k p k q k ] = [f,h]. We have used the EoM to get the second line from the first.

5 Time independent Hamiltonians are conserved More generally, for any time-dependent function on phase space f(q k,p k,t), one has df dt = f t +[f,h]. It follows trivially that, for the Hamiltonian itself we have the identity dh dt = H t, so that if the Hamiltonian is time independent, then it is conserved. Other identities for the Poisson bracket include the Jacobi identity [[f,g],h]+[[g,h],f]+[[h,f],g] = 0. Problem 11 Prove the Jacobi identity.

6 Elementary Poisson brackets Since all the q k and p k are independent variables, their derivatives with respect to each other vanish. So we have [q j,q k ] = 0 and [p j,p k ] = 0. Also, [q i,p j ] = [ qi p j p ] j q i = δ ik δ jk = δ ij. q k p k q k p k k k The E0M can be written in the form q i = [q i,h] and ṗ i = [p i,h]. Using the definition of the Poisson bracket, one sees that these reproduce the canonical equations. If some momenta are conserved, then one clearly has H/ q i = 0. The Hamiltonian does not depend on the corresponding coordinates, i.e., we have a symmetry.

7 Angular momenta rotate vectors We write the components of the angular momentum L = x p using the Levi-Civita tensor as L i = ǫ ijk x j p k. In order to evaluate the Poisson bracket [L i,x j ] we note that for any three functions on phase space, f, g, h, Using this we find [fg,h] = f[g,h]+g[f,h]. [L i,x j ] = ǫ iαβ [x α p β,x j ] = ǫ iαβ {x α [p β,x j ]+p β [x α,x j ]} = ǫ iαβ x α δ βj = ǫ ijk x k. This is the definition of a vector function on phase space. Problem 12 Prove that the momentum p is a vector function on phase space.

8 Poisson brackets of angular momenta The Poisson bracket of two components can be easily worked out [L i,l j ] = ǫ iαβ ǫ jγδ [x α p β,x γ p δ ]. Using the identity for Poisson brackets of products, we can write [L i,l j ] = ǫ iαβ ǫ jγδ {x α [p β,x γ p δ ]+p β [x α,x γ p δ ]}. Using the identity a second time we can write [L i,l j ] = ǫ iαβ ǫ jγδ {x α x γ [p β,p δ ]+x α p δ [p β,x γ ] +p β x γ [x α,p δ ]+p β p δ [x α,x γ ]} = ǫ iαβ ǫ jγδ { x α p δ δ βγ +p β x γ δ αδ }

9 The angular momentum is a vector We have the tensor identity ǫ iαβ ǫ jγβ = δ ij δ αγ δ iγ δ jα, where we have used the summation convention repeated indices are summed. Using this we reduce the Poisson bracket [L i,l j ] = ǫ iαβ ǫ jγδ { x α p δ δ βγ +p β x γ δ αδ } = ǫ iαβ ǫ jγβ {x α p γ x γ p α } renaming dummy indices = {δ ij δ αγ δ iγ δ jα } {x α p γ x γ p α } = x i p j x j p i = ǫ ijk L k. This completes the demonstration that the angular momentum itself is a vector.

10 Keywords and References Keywords Hamiltonian, Hamilton s equations, Poisson bracket, antisymmetry, Jacobi identity, conserved, symmetry, Levi-Civita tensor, vector function References Goldstein, Chapters 8, 9 Landau, Sections 40, 42