PHY411 Lecture notes Part 2

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1 PHY411 Lecture notes Part 2 Alice Quillen April 6, 2017 Contents 1 Canonical Transformations Poisson Brackets Canonical transformations Canonical Transformations are Symplectic Generating Functions for Canonical Transformations Example canonical transformation - action angle coordinates for the harmonic oscillator Some Geometry The Tangent Bundle Differential forms and Wedge product The Symplectic form Generating Functions Geometrically Vectors generate Flows and trajectories The Hamiltonian flow and the symplectic two-form Extended Phase space Hamiltonian flows preserve the symplectic two-form The symplectic two-form and the Poisson bracket On connection to the Lagrangian Discretized Systems and Symplectic integrators Surfaces of section The Hamiltonian following a canonical transformation Examples of Canonical transformations Orbits in the plane of a galaxy or around a massive body Epicyclic motion Epicyclic motion for a tidal tail The Jacobi integral The Shearing Sheet

2 4 Symmetries and Conserved Quantities Functions that commute with the Hamiltonian Noether s theorem Integrability Canonical Transformations It is straightforward to transfer coordinate systems using the Lagrangian formulation as minimization of the action can be done in any coordinate system. However, in the Hamiltonian formulation, only some coordinate transformations preserve Hamilton s equations. Canonical transformations, defined here as those that preserve the Poisson brackets or equivalently the symplectic 2-form, also preserve Hamilton s equations. A search for conserved quantities and symmetries is equivalent to a search for a nice coordinate system that preserves Hamilton s equations. 1.1 Poisson Brackets Consider a function f(q, p, t) and a Hamiltonian H(p, q) where p, q coordinates and momenta. The time dependence of f df dt = f f q + ṗ + f t Using Hamilton s equations we can write this as df dt = f H f H + f t We can write this short hand with a commutation relation known as the Poisson bracket df dt with the Poisson bracket for two functions f, g = {f, H} + f t {f, g} f g f g (1) For more than one dimension {f, g} = i ( f g f ) g i i i i and using summation notation we neglect the symbol. 2

3 What are the Poisson brackets of functions equal to the coordinates and momenta, f(q, p) = q i and g(q, p) = p i? We calculate where {p i, p j } = 0 {q i, q j } = 0 {q i, p j } = δ ij δ ij = { 1 for i = j 0 for i j Coordinates and momenta form a kind of basis. We can consider functions f(q, p) = q and g(q, p) = p. Inserting these functions into equation 1 we recover Hamilton s equations in terms of Poisson brackets q = H = {q, H} ṗ = H = {p, H} The Poisson bracket satisfies the conditions for a Lie algebra. For functions f, g, h, {{f, g}, h} + {{g, h}, f} + {{h, f}, g} = 0 {f, g} + {g, f} = 0 The first of these is called a Jacobi identity, the second is antisymmetry. In addition they satisfy another condition known as a Leibnitz type of product rule {f, gh} = g {f, h} + {f, g} h A Lie algebra with this extra rule is called a Poisson algebra. Remark In what contexts are these extra rules important? These relations give the Poisson bracket Lie bracket-like constraints. The Leibnitz rule is automatically satisfied by a Poisson bracket with a metric-like form. Infinite dimensional continuous systems with equations of motion corresponding to partial differential equations, a Hamiltonian and a Poisson bracket can be adopted (obeying the above relations) that give the equations of motion, but we might lack canonical coordinates. 1.2 Canonical transformations A canonical transformation is a transformation from one set of coordinates q, p to a new one Q(q, p), P (q, p) that satisfies the Poisson brackets {P i, P j } = 0 {Q i, Q j } = 0 {Q i, P j } = δ ij (2) The above Poisson brackets are computed using derivatives of p, q. 3

4 Using x = (q 1, q 2..., p 1, p 2,...p N ) with {x i, x j } = ω ij ω = ( 0 I I 0 and I the identity matrix. Given a Hamiltonian, H(p, q, t), we will show that we can find a new Hamiltonian K(Q, P, t) such that Hamilton s equations are obeyed in the new coordinate system. For a function f(q, P) using the chain rule and using summation notation The Poisson bracket f = f Q j i Q j i f = f Q j i Q j i ) + f P j P j i + f P j P j i {f, g} = f g f g i i i ( i f Q j = + f ) ( P j g Q k + g ) P k Q j i P j i Q k i P k i ( f Q j + f ) ( P j g Q k + g ) P k Q j i P j i Q k i P k i = f ( g Qj P k Q ) j P k f ( g Qk P j Q ) k P j Q j P k i i i i P j Q k i i i i + f ( g Qj Q k Q ) j Q k + f ( g Pj P k P ) j P k Q j Q k i i i i P j P k i i i i = f g {Q j, P k } f g {Q k, P j } Q j P k P j Q k + f Q j g {Q j, Q k } + f Q k P j g P k {P j, P k } If the new coordinates obey the Poisson brackets in equation 2 (so that the transformation is canonical) then we can insert these relations into the above equation. {f, g} = f g δ jk f Q j P k P j = f Q j g f P j P j g δ jk Q k g Q j 4

5 This is just the definition of the Poisson bracket but with respect to our new coordinates, P, Q rather than p, q. If the transformation is canonical then we can compute Poisson brackets using the new coordinates and momenta. If the coordinate transformation is canonical (the Poisson brackets of equation 2 are obeyed in the new coordinate system) then the Poisson bracket can be computed in the new coordinate system {f, g} pq = {f, g} P Q Above we defined canonical transformations without even specifying a Hamiltonian function. Given H(q, p), Hamilton s equations give q = {q, H} pq ṗ = {p, H} pq and using the q, p coordinates. But this is true for any time independent function including Q(q, p) and P(q, p) so Q = {Q, H} pq Ṗ = {P, H} pq and the Poisson bracket is computed using the p, q coordinate system. However if the transformation is canonical then the Poisson brackets can be computed using either coordinate system. So Q = {Q, H} P Q Ṗ = {P, H} P Q but now we compute the Poisson bracket with the new coordinates P, Q. Thus the new Hamiltonian is equivalent to the old Hamiltonian but using the new variables; K(Q, P) = H (q(q, P), p(q, P)) You may notice that there is a term missing from this expression. We will discuss time dependent transformations below. 1.3 Canonical Transformations are Symplectic A symplectic transformation S, obeys where J J = S t JS ( 0 I I 0 and S t is the transpose of S and I the identity matrix. Consider a canonical transformation P (p, q), Q(p, q) and the Jacobian matrix Q(q, p) Q(q, p) S = P (q, p) P (q, p) 5 )

6 Let us compute S t JS S t JS = = = = Q P ( ) Q Q 0 I Q P I 0 P P Q P P P Q P Q Q Q P Q P Q P Q P Q P Q P Q P Q P ( ) 0 {Q, P } {Q, P } 0 If the transformation is canonical and the Poisson brackets are satisfied, then the transformation is symplectic. Remark Here we used the Jacobian matrix to see that the infinitesimal transformation is symplectic. Using an exponential, the coordinate transformation is likely to be symplectic even when we can t write it as a matrix. It can be useful in numerical integrations to use discrete transformations (approximating continuous sytems) that are symplectic. 1.4 Generating Functions for Canonical Transformations Not every coordinate transformation is canonical. The requirement that Poisson brackets are satisfied does not strongly restrict the transformation. Suppose we take a generating function F 2 (q, P ) of old coordinates and new momenta and define p = F 2(q, P ) Q = F 2(q, P ) P Really we describe the generating function in terms of p, q or P, Q so what we mean by F 2 (q, P ) is F 2 (q, P (q, p)) or F 2 (q(q, P ), P ). The function F 2 has two arguments so p = 1 F 2 (q, P ) Q = 2 F 2 (q, P ) (3) 6

7 where 1 is the derivative with respect to the first argument of the function F 2 (, ), and 2 is the derivative with respect to the second argument. Taking the second equation we can write Taking the equation for p we can write Q = 2 F 2 (q(q, P ), P ) Q Q = 1 = [ 1 2 F 2 ] Q p(q, P ) = 1 F 2 (q(q, P ), P ) Q = [ 1 1 F 2 ] Q P = [ 1 1 F 2 ] P F 2 We have computed relations for Q, P, Q because it will cancel out from our next computation. Take the Poisson bracket and insert the relations for Q {q, p} = Q P P = Q = Q 1 2 F 2 = 1 (4). We don t need to compute a relation for P Q ( 1 1 F 2 P F 2 and P ) P 1 1 F 2 Q where the last step uses equation 4. The coordinate transformation is canonical as long as we define the new coordinate and momenta using equations 3. Similar choices can be made for generating functions that depend on old and new coordinates, old and new momenta or old momenta and new coordinates. These are denoted F 1, F 2, F 3, F 4 traditionally Example canonical transformation - action angle coordinates for the harmonic oscillator Given coordinates φ, I we consider new coordinates q(i, φ) = 2I sin φ p(i, φ) = 2I cos φ (5) We check the Poisson bracket {q, p} = φ I I φ = cos 2 φ + sin 2 φ = 1 7

8 verifying that this is a canonical transformation. Note that we need a factor of two within the square root in equation 5 so that the Poisson bracket gives 1 instead of 1/2. This is a handy canonical transformation for the harmonic oscillator with Hamiltonian H(p, q) = 1 2 ( p 2 + q 2) In the coordinates I, φ the Hamiltonian is particularly simple K(I, φ) = I This system is said to be in action angle variables as I (the action) is conserved and φ is constant. Can we find a generating function that gives this canonical transformation from p, q to I, φ? Consider the generating function of old momenta, p, and new (φ) coordinates, with F 3 (p, φ) = p2 2 tan φ F 3 F 3 φ = p tan φ = q = p2 2 sec2 φ = I From this we find that or and consistent with equation 5. tan 2 φ = (q/p) 2 sec 2 φ = tan 2 φ + 1 = (q/p) q/p = atanφ 1 2 (q2 + p 2 ) = I Remark We defined canonical transformations as transformations that preserve the Poisson brackets and below we will show that an equivalent statement is that they preserve the symplectic two-form. We did not even mention a Hamiltonian in our definition. Often you see symplectic transformations defined as those that preserved Hamilton s equations. However, Arnold gives an example of a transformation that preserves Hamilton s equations and does not preserve the Poisson bracket. 8

9 2 Some Geometry Why introduce all this mathematical jargon? Dynamics can be reformulated in a coordinatefree manor. It s hard to read applied maths books on dynamics without this jargon. There might be other reasons. Consider coordinates p as positions or points on a manifold, M. A manifold is a topological space where each point has a neighborhood that resembles a Euclidean space of dimension n. A map to the Euclidean space is called a chart. A collection of charts is called an atlas. Here we assume that our manifold is differentiable which implies that nearby charts can be smoothly related to one another. Namely, there are transition maps from one chart to another that are differentiable. 2.1 The Tangent Bundle A curve is a map from R (parametrized by time t) to the manifold. On each chart (and near a point p) this gives at each time a vector. If we have a curve c in the manifold we can consider its image in the local Euclidean space at point x. The tangents to the curve at the point p are in the tangent space. The tangent space at position p we can call T p M. The tangent bundle TM consists of the manifold M with the collection of all its tangent spaces. It is convenient to use a coordinate basis to describe vectors in the tangent space, for example (e x, e y, e z ) that is sometimes written ( x, y, z Given a curve on the manifold (described by t), the tangent to the curve is q. In the Lagrangian formalism the manifold coordinates are specified by q. The Lagrangian is a function of q (coordinate on the manifold) and q (in the tangent space) and time t (along the curve). In the Hamiltonian formalism the manifold coordinates are specified by phase space q, p and curves given as a function of the phase space coordinates. Here q and ṗ are both vectors in the tangent space. A flow on the manifold is described by velocities which lie in the tangent space at each point p on the manifold. A vector field on the manifold generates a flow on the manifold. It is sometimes useful to keep track of the map from the manifold to the Euclidean space φ(p) which gives a point x in Euclidean space R 2n. A function on the manifold f(p) then can act effectively in the Euclidean space with f φ 1 (x) Differential forms and Wedge product We define vectors as lying in the tangent space T p M. We can write vectors as ). V = V i + V j + V k x i x j x k 9

10 in three dimensions in terms of coordinates in a chart at a particular position in the manifold. Tangents to a curve give vectors. Curves are parametrized by a distance along a curve or time. Another example of a vector is given by a trajectory ṗ, q with V = q + ṗ We can define a cotangent space T pm dual to the tangent space. An element in the cotangent space ω is a map T p M R. A differential of a function f is in the cotangent space df T p M and acts on a vector V in the tangent space giving a real number df, V = V i f x i R df is called a one form. In three dimensions we can write df = f x i dx i + f x j dx j + f x k dx k In a tangent space we have the tangent of a curve. Trajectories on the manifold give tangent vectors. In the cotangent space we have the gradient of a function. One forms are members of the cotangent space at a point p or T pm. As the tangent bundle TM is formed of M and its tangent spaces T q M, the cotangent bundle T M is formed of M and its cotangent spaces T pm. Differential forms can be integrated. A one-form can be integrated along a path with a sum of the values of the form along a bunch of tangent vectors to the path. A q form maps q vectors in T p M R and can be described as sums of products of one forms, for example a ijk dx i dx j dx k For example the two vectors V, W are operated on by the two-form ω = dx dy ω(v, W) = v x w y The wedge product,, of one forms is an antisymmetric sum dx i dx j = dx i dx j dx j dx i dx i dx j dx k = dx i dx j dx k + dx j dx k dx i + dx k dx i dx j dx i dx k dx j dx j dx i dx k dx k dx j dx i With ω = dx i dx j 10

11 we compute ω(v, W) = v i w j v j w i A differential form is an antisymmetric q form ω = 1 r! ω µ 1...µ r dx µ 1... dx µr Differential forms can be thought of as volume elements (think volume of a parallelpiped calculated from vectors). The exterior derivative dω = 1 ( ) ω µ1...µ r! x r dx ν dx µ 1... dx µr ν The exterior derivative gives the boundary of a volume element. As the exterior derivative is antisymmetric d 2 ω = 0 If ω = w x dx + w y dy then dω = ( wy x w ) x dx dy y This is reminiscent of the cross product. The geometric formulation is independent of the coordinate system used in the charts. The coefficients of a vector or a differential-form transform as a tensor. A form ω is exact if there is a form θ such that dθ = ω. A form is closed if dω = 0. Every exact form is closed, as d 2 θ = 0, but not every closed form is exact. The generalized version of Stokes theorem relates the integral of a form ω over the boundary of a region C to the exterior derivative dω and the region C. dω = ω Compare the above to Stokes theorem in three dimensions F da = F ds A C where A is a surface bounded by a loop S = A. Stokes theorem is also equivalent to Gaus law FdV = F da V where V is a volume with boundary A = V. In three dimensions the exterior derivative gives or depending on the dimension of the object that is being integrated. In three dimensions ds is a one form, da is related to a two form and dv is related to a three form. 11 C A S

12 2.2 The Symplectic form The one form has exterior derivative θ = q i dp i ω = dθ = dq i dp i which is a two form. A manifold with such a two-form (that is not degenerate) is known as a symplectic manifold. Since ω is a derivative of θ dω = 0 The symplectic form is non-degenerate and exact. (A form ω is exact if there is a form θ such that dθ = ω). What does it mean to be non-degenerate? A two-form maps two vectors, η, ξ to a real number. For every η 0 there exists a ξ such that ω(η, ξ) 0. The symplectic form is connected with areas. Consider two vector fields V, W at a point q, p. V = v qi + v pi i i W = w qi + w pi i i ( v qi ω(v, W) = + v pi i i ( v qi + v pi i i = v qi w pi v pi w qi ) dq i ( w qi ) dp i ( w qi + w pi i i + w pi i i ) dp i ) dq i For each (q i, p i ) pair we have the area of the parallelogram defined by (v qi, w pi ) and (w qi, w pi ). The total is the sum of the areas of the n parallelograms. Is this is related to Liouville s volume theorem? Yes, as we will show the symplectic form is preserved by Hamiltonian flows. Consider the two form ω = dq dp in a new coordinate system Q, P so that q(p, Q), p(p, Q). We compute dq = dq + Q P dp dp = dq + Q P dp 12

13 Inserting these into ω ( ) ω = dq + Q P dp ( = Q P P Q = {q, p} P,Q dq dp ( dq + Q P dp ) dq dp where I have written {q, p} P,Q as the Poisson brackets computed with P, Q. If the coordinate transformation is canonical then the two form can be written ω = dq dp Canonical transformations preserve the two form ω. 2.3 Generating Functions Geometrically Consider p, q and P, Q both canonical sets of coordinates. We can look at the one forms ) θ 1 = p i dq i θ 2 = P i dq i The two form Consider Because dp i dq i = dp i dq i we know that ω = dθ 1 = dθ 2 = dp i dq i = dp i dq i θ 1 θ 2 = p i dq i P i dq i d(θ 1 θ 2 ) = 0 If we can find a function F (the generating function) such that p i dq i P i dq i = df then we ensure that the coordinate transformation is canonical. By specifying that the difference θ 1 θ 2 is the differential of a function we guarantee that dp i dq i = dp i dq i and so that the transformation is canonical. F serves as our generating function. If we start with the generating function and then use it to give us the coordinate transformation then we ensure that the coordinate transformation is canonical. Let our difference in forms be pdq P dq = df 1 (q, Q) = F 1(q, Q) dq + F 1(q, Q) Q dq 13

14 We associate p = F 1(q, Q) and the transformation is canonical. Let We associate Let The plus sign here arises because qdp QdP = df 4 (p, P ) = F 1(p, P ) q = F 4(p, P ) P = F 1(q, Q) Q dp + F 4(p, P ) dp P Q = F 4(p, P ) P pdq + QdP = df 2 (q, P ) d(pdq + QdP ) = dp dq + dq dp = dp dq dp dq and this we would require is zero for the transformation to be canonical. df 2 (q, P ) = F 2(q, P ) dq + F 2(q, P ) dp P giving for a canonical transformation. p = F 2(q, P ) Q = F 2(q, P ) P Remark The sign of the generating functions can be flipped and the transformation is still canonical. Given a transformation Q(q, p) and P (q, p) the transformation is canonical (symplectic) if one of the following forms is exact σ 1 = pdq P dq σ 2 = pdq + QdP σ 3 = qdp + P dq σ 4 = qdp QdP 14

15 For the different related generating functions F 1 (q, Q) F 2 (q, P ) F 3 (p, Q) F 4 (p, P ) = p = p = q = q F 1 (q, Q) Q F 2 (q, P ) P F 3 (p, Q) Q F 4 (p, P ) P = P = Q = P = Q 2.4 Vectors generate Flows and trajectories Consider a vector field, X, and a curve on the manifold, σ(t, x), that is a map R M M such that the tangent vector at each point, x, on the curve is X. An integral curve σ(t, x 0 ) that goes through a point x 0 on the manifold at t = 0 has tangent d dt σµ (t, x 0 ) = X µ t=0 I have used local coordinates. The initial condition σ µ (t = 0, x 0 ) = x µ 0. Now instead of requiring σ to only contain a single curve going through x 0 we extend σ so that it includes curves that go through all the points on the manifold. The map σ is known as a flow generated by the vector field X. We can define an exponential function exp(xt) from M to M so that we can move along the flow from one point to another, σ µ (t, x) = exp(xt)x µ. xxxx 2.5 The Hamiltonian flow and the symplectic two-form The symplectic two-form ω = dq dp is a map from two vectors to a real number. Consider two vectors V, W, recall that V = v q + v p W = w q + w p ω(v, W ) = v q w p w q v p Now what if we consider the map only using vector V ω(v,?) = v q dp v p dq (6) 15

16 This gives a one-form. In this way the symplectic two-form gives us a way to take a vector (in the tangent space) and generate a one-form (in the cotangent space) from it. Conversely using the symplectic form ω, we can take a one-form and construct a vector from it. Remark By providing an invertible map between vectors and one-forms, ω serves like the metric tensor in general relativity or Riemannian geometry. However the map is antisymmetric rather than symmetric and so it is not a metric. Recall that by specifying a direction, a vector generates a flow, or given an initial condition, a trajectory. We can produces trajectories ( q, ṗ) from a function (H) using this map. Consider the one form associated with a Hamiltonian, H(q, p) and use ω to generate a vector from it. dh = H H dq + dp = ω(v,?) = v q dp v p dq On the second line we have used vector V (as written out in equation 6). The V vector has components v q = H v p = H These are associated with q and ṗ for the flow. Let us define (as Arnold) the vector generated by dh as IdH. The equations of motion (Hamilton s equations) are equivalent to ẋ = IdH In this way, a Hamiltonian flow is generated from the Hamiltonian function. 2.6 Extended Phase space The statements relating the symplectic two-form to Hamiltonian flows can be illustrated with the one-form (known as the Poincaré-Cartan integral invariant) σ 1 = pdq Hdt in what is known as extended phase space or a R 2n+1 space that consists of phase space (R 2n ) with addition of time or (p, q, t). The exterior derivative dσ 1 is a two form. The odd dimension implies that there is always some direction (or vector V ) such that dσ 1 (V, η) = 0 for all vectors η and this direction, V, we associate with the equations of motion. Flows along this direction are also called vortex lines. The exterior derivative dσ 1 = dp dq H H dq dt dp dt (7) 16

17 Consider the vector Let us insert this into dσ 1 V = H dσ 1 (V, ) = H H = 0 + H + t H H dq dp + H H H dt + dt dp + H dq So our vector V is in fact the direction defining the vortex lines. But look again at the vector V V = H ṗ + H + q + t + t The flow generated by vector V satisfies Hamilton s equations. Figure 1: Vortex lines 2.7 Hamiltonian flows preserve the symplectic two-form Here we are going to use the generalized Stokes theorem in extended phase space to prove that the symplectic two-form is conserved by a Hamiltonian flow. 17

18 Consider a surface region in (p, q) (call this c) and its Hamiltonian flow along t to a time τ causing a transformation g t c (see Figure 1). The boundary of the surface at t = 0 would be c. Boundary at t = τ would be g t c. Flow in time from t = 0 to τ is created with an operator J. Every point on c is given a trajectory from t = 0 τ using J. These trajectories are vortex lines and so have tangent equal to the vector V such that dσ(v ) = 0. The total volume of the cylinder is Jc. The sides of the cylinder are covered by J c. The boundary of Jc or (Jc) is the sum of the top and lower faces, c and g t c, and the sides, J c. The generalized version of Stokes theorem gives dω = ω c The integral of a differential form of a boundary of a region in an orientable manifold c is equivalent to the integral of the exterior derivative of the form dω in the region c. One consequence of the generalized version of Stokes theorem is that the integral of an exact form over the boundary of a region is zero. In other words we apply Stokes theorem dσ = d 2 σ = 0 c where the last step is zero because d 2 = 0. Let us integrate the exterior derivative of dσ 1 over the surface of the cylindrical volume (Jc) or dσ 1 Because of Stokes theorem (Jc) (Jc) c dσ 1 = Jc c d 2 σ 1 = 0 Now we divide (Jc) into three pieces, the bottom of the cylinder c, the top of the cylinder g t c and the cylindrical surface J c. The sum of these three integrals of dσ must be zero. Now we perform this sum for the exact two-form dσ 1 with σ 1 = pdq H. Integrating dσ 1 on the surface J c gives zero because the surface is comprised of vortex lines or null vectors. Consequently integrating dσ 1 on the top and both of the cylinder c and g t c must give the same result. On these surfaces the vectors are perpendicular to t so dσ1 = ω the symplectic two-form (see equation 7) and we find that g t preserves the integral of the symplectic two-form. Since the symplectic two-form gives the volume in phase space, this is equivalent to Liouville s theorem and also implies that a transformations generated by a Hamiltonian flow are canonical transformations. Previously we showed that a Hamiltonian flow preserved phase space volume (Liouville s theorem). Phase space volume conservation can also be written in terms of ω, hence 18

19 Liouville s theorem is equivalent to saying that Hamiltonian flows preserve the two-form ω. Because ω is preserved by a Hamiltonian flow, the flow also generates canonical transformations between coordinates at any two times. 2.8 The symplectic two-form and the Poisson bracket We ask, if canonical transformations preserve the Poisson bracket and they preserve the two-form ω what is the relation between the Poisson bracket and the symplectic two form? The Poisson bracket takes derivatives of functions. {g, h} = g h g h i i i i = x g t J x h = ω( x g, x h) where x = (q, p) and we consider x g, x h as vectors. Here ( ) 0 I J = I 0 and ω = dq dp is the symplectic two form. The above is true only in a canonical basis where the two-form provides a direct and trivial way to convert between one-forms and vectors. Really the gradient of a function should be considered a one-form, not a vector. What if we write the two-form in a non-canonical basis with ω = f ij dx i dx j and x variables are functions of p, q a canonical set. Here f ij an antisymmetric matrix that effectively gives the wedge product. Now we use the one form to convert between the differential forms of g, h and vectors ω(v,?) = dg = g x j dxj v i f ij dx j = g x j dxj v i f ij = g x j Likewise we do the same for h but in the second location in the two-form ω(?, W ) = dh = h x j dxj w j f ji dx i = h x i dxi w j f ji = h x i 19

20 Since the symplectic two-form is not degenerate we can invert the matrix f ij. We call the inverse of the matrix F with F ij f jk = δk i and both f and F are antisymmetric matrices. We invert the above relations g x j F jk = v i f ij F jk = v i δ k i = v k v k = g x j F jk w i = F ij h x j Now let us compute the two-form on these two vectors ω(v, W ) = g x j F jk il h f ki F x l = g x j δj il h i F x l = g il h F xi x l The expression on the right we recognize as similar to a Poisson bracket. So it makes sense to define {g, h} = g jl h F xj x l in the non-canonical basis. In this sense we can consider the two-form like an inverse of the Poisson bracket On connection to the Lagrangian Integrating the one form σ 1 = pdq Hdt is like integrating the Lagrangian Ldt = (p q H)dt So σ 1 gives us a way to generate the action for the associated Lagrangian Discretized Systems and Symplectic integrators Symplectic integrators provide maps from phase space to phase space separated by a time t. These are symplectic transformations, preserving phase space volume and the symplectic two-form. 20

21 2.8.3 Surfaces of section We can also consider surfaces at an angle in the flow of vortex lines (see Figure 2) and a two form computed at the p, q on this surface but at different times. We can consider maps generated from between the times it takes to cross a planar subspace (see Figure 3). There are a number of ways to generate area or volume preserving maps from Hamiltonian flows. The key point here is that the vortex lines are flow lines and these are null vectors with respect to the symplectic form. Figure 2: Vortex lines going through a tilted plane. A two-form can be constructed with degrees of freedom lying in the tilted plane. Because the vortex lines give no area, this two-form is equivalent to one at a single time. 2.9 The Hamiltonian following a canonical transformation We defined a canonical transformation as one that satisfied the Poisson brackets or equivalently preserved the symplectic 2-form. We did not require that Hamilton s equations were preserved, or even required a Hamiltonian to determine whether a coordinate transformation was canonical. However we showed that Hamilton s equations were satisfied using a new Hamiltonian in the old coordinates. This is true as long as coordinate transformation is time independent. What happens if the canonical transformation is time-dependent? For any function f recall that f = {f, H} + f t 21

22 Figure 3: Orbits covering a torus. A map can be constructed from the position of the orbits each time they pass through a plane. This is known as a surface of section and is another way to create an area preserving map from a Hamiltonian flow in 4-dimensional phase space (with a time independent Hamiltonian). The flow lines are vortex lines and so are null vectors with respect to the symplectic two-form in extended phase space. On a plane that slices the torus one piece of the two form can be set to zero. The remain degrees of freedom (in that plane) give a two form that is preserved by the map. and the map is symplectic. Following a time dependent canonical transformation Q(p, q, t), P (q, p, t), (satisfying Poisson brackets) we can insert Q or P finding Q = {Q, H} + Q t P = {P, H} + P t A new Hamiltonian is required so that the new coordinates satisfy Hamilton s equations. Suppose that we can find a function K such that Q = {Q, K} P = {P, K} Subtract these from the previous equation Consider an extended phase space defined by q, p, t. We are adding time as an extra dimension. We can construct a form θ = pdq Hdt + QdP + Kdt Remember we defined a one form σ 1 = pdq Hdt such that dσ(v, η) = 0 for V giving the Hamiltonian flow for all vectors η. Let θ = df 2 where F 2 (q, P, t) is a generating function that now depends on t. Remember we are now working in extended phase space so df 2 (q, P, t) = F 2 dq + F 2 P dp + F 2 t 22

23 We match p = F 2 Q = F 2 P K H = F 2 t Thus our new Hamiltonian (and one that by definition satisfies Hamilton s equations) is K = H + F 2 t Following the same procedure for the other classes of generating functions we find and similarly for F 3 and F 4. K = H + F 1 t 3 Examples of Canonical transformations 3.1 Orbits in the plane of a galaxy or around a massive body The Keplerian problem of a massless particle in orbit about a massive object of mass M can be written in polar coordinates and restricted to a plane L(r, θ; ṙ, θ) = 1 2 (ṙ2 + r 2 θ2 ) V (r) where V (r) = GM/r. The associated momentum are p r = L ṙ = ṙ p θ = L θ = r2 θ = L where L is the angular momentum. We find a Hamiltonian H(r, θ; p r, L) = p2 r 2 + L2 + V (r) (8) 2r2 Because the Hamiltonian is independent of θ, the angular momentum is conserved. 23

24 Let us expand this Hamiltonian using y = r R 0 and expand assuming that y R 0. H(y, θ, p r, L) = p2 r 2 + L 2 2(y + R 0 ) 2 + V (y + R 0) = p2 r 2 + y [V (R 0 ) L2 R 3 0 ] + y2 2 ] [V (R 0 ) + 3L2 Near a circular orbit the term proportional to y must be zero. L = R0 2Ω with Ω = θ and the angular rotation rate V Ω(R 0 ) = (R 0 ) R 0 We identify κ 2 (R 0 ) = V (R 0 ) + 3L2 R0 4 = V (R 0 ) + 3Ω(R 0 ) 2 as the frequency of radial oscillations or the epicyclic frequency. The above Hamiltonian (equation 8) is useful to study dynamics of stars in the midplane of a galaxy but with a different potential V (r). A circular orbit has velocity v c (r) = rv (r). Many galaxies have nearly flat rotation curves with v c (r) v c with v c a constant, corresponding to a logarithmic potential V (r) = vc 2 ln r. Above we used a Lagrangian in cylindrical coordinates to find the Hamiltonian system. However we could have started with a Hamiltonian in cartesian coordinates H(x, p x ; y, p y ) = 1 2 (p2 x + p 2 y) + V ( x 2 + y 2 ) The transformation is not obviously canonical p r = p xx r + p yy r L = xp y yp x r = x 2 + y 2 θ = atan(y/x) however we can check that it is using Poisson brackets. Computing derivatives r x = (p xy p y x) y r 3/2 r y = (p yx p x y) x r 3/2 R

25 We evaluate the Poisson bracket {p r, L} = r x L r L x x x + r y L r L y y y = (p x y p y x) y r 3/2 ( y) x r p y + (p y x p x y) x r 3/2 x + y r p x = L r L r = 0 and likewise for the other brackets. 3.2 Epicyclic motion Orbits in a disk galaxy are nearly circular. The epicylic approximation assumes that the orbit can be described by a radial oscillation around a circular orbit. In the Keplerian setting the radial oscillation period is the same as the orbital period. But in the galactic setting radial oscillations are faster that the rotation and the orbit does not close (see Figure 4). By setting p r = 0, we can define a function E(L) giving the energy of a circular orbit and this can be inverted L(E) to give the angular momentum of a circular orbit with energy E. We can also define a function r c (L) that gives the radius of a circular orbit with angular momentum L. These are related by E(L) = L2 2r c (L) 2 + V (r c(l)) L = rc 2 (L)Ω(L) V Ω(L) = (r c (L)) r c (L) where Ω(L) is the angular rotation rate θ for the circular orbit with angular momentum L. Example: For a flat rotation curve what is Ω(L) and r c (L)? For the flat rotation curve v c = r c (L)Ω(L) is constant. The angular momentum L = r c (L)v c, consequently r c (L) = L v c and The energy E(L) = Ω(L) = v c = v2 c r c L L2 2r c (L) 2 + V (r c) = v 2 c ln L + constant 25

26 Figure 4: A rosette orbit. The orbit can be described in terms of radial oscillations or epicycles about a circular orbit. 26

27 We can consider orbits with angular momentum and energy near those of the circular orbit. Consider the following generating function that is a function of old momenta (p r, L) and new coordinates (θ r, θ new ) F 3 (p r, L; θ r, θ new ) = The canonical transformation gives p2 r 2κ(L) cot θ r r c (L)p r Lθ new r = F 3 = r c (L) + p r r κ(l) cot θ r J r = F 3 θ r = p2 r 2κ(L) sin 2 θ r θ = F 3 L = θ new r c(l)p r + κ (L)p 2 r 2κ 2 (L) cot θ r L new = F 3 θ new = L We can rewrite this so that old coordinates are written in terms of new ones that can be directly inserted into the Hamiltonian 2J r r = r c (L) + κ(l) cos θ r p r = 2J r κ(l) sin θ r L = L new θ = θ new r c(l) 2J r κ(l) sin θ r + κ (L)J r 2κ cos(2θ r ) Inserting the new coordinates into the Hamiltonian and expanding to first order in J r the Hamiltonian is H(θ r, θ new, J r, L) = g 0 (L) + κ(l)j r where and g 0(L) = Ω(L) κ(l) 2 = V (r c (L)) + 3Ω 2 (L) This is known as the epicyclic approximation. The action variable J r sets the amplitude of radial oscillations and the frequency κ(l) is the epicyclic frequency and governs the frequency of radial oscillations. 27

28 3.2.1 Epicyclic motion for a tidal tail Using θ for the azimuthal angle and φ for the epicyclic angle, L for the angular momentum and J for the radial action variable giving H(φ, θ, J, L) = g 0 (L) + κ(l)j θ = Ω(L) + κ (L)J r φ = κ(l) If we have a nearby population of stars with a distribution around L 0, J 0 with width δl, δj and at initially the same angles θ 0, φ 0 After a time t the range of angles will be θ(δl, δj) = θ 0 + [ Ω(L 0 ) + κ (L 0 )J 0 + Ω (L 0 )δl + κ (L 0 )J 0 δl + κ (L 0 )δj ] t φ(δl, δj) = φ 0 + [ κ(l 0 ) + κ (L 0 )δl ] t For a flat rotation curve Ω(L) = v2 c L κ(l) = 2 v2 c L Ω (L) = 2 v2 c L 2 κ (L) = 2 2 v2 c L 2 κ (L) = 6 2 v2 c L 3 Plugging these into the angles and looking at the spread [ θ(δl, δj) = const + 2 v2 c L 2 t ( J 0 )δl ] 2δJ 0 L 0 φ(δl, δj) = const 2 2 v2 c L 2 δl t 0 This regime cannot capture the radial motion without angular momentum, but it does okay in the rotating sense. Perhaps we need θ to be extremely sensitive to L at small L because at small L we know that θ has to be very small. If we find the regime where θ is very sensitive to J, L we might find the regime for the hockey sticks. 28

29 3.3 The Jacobi integral Consider the Hamiltonian H(r, θ, p r, L) = p2 r 2 + L2 2r 2 + V (r) + ɛg(θ Ω bt) where ɛ is small and g(θ Ω b t) is a perturbation to the potential that is fixed in a frame rotating with angular rotation rate Ω b. The perturbation is what one would expect for an oval or bar perturbation, as is found in barred galaxies, which is moving through the galaxy with a pattern speed or angular rotation rate Ω b. Similarly a planet in a circular orbit about the Sun can cause a periodic perturbation (with a constant angular rotation rate) on an asteroid, also in orbit about the Sun. We take a time dependent generating function of old coordinate θ and new momenta L giving new coordinates F 2 (θ, L, t) = (θ Ω b t)l L = F 2 θ = L θ new = F 2 L = θ Ω b t The transformation only involves θ, L so we neglect p r, r in the transformation. Because the generating function is time dependent we must add F 2 t = Ω b L to the new Hamiltonian. The new Hamiltonian in the new coordinates K(r.θ new, p r, L ) = H F 2 t = p2 r 2 + L2 2r 2 LΩ b + V (r) + ɛg(θ new ) We note that the new Hamiltonian is time independent and so is conserved. This conserved energy, computed in the rotating frame is called the Jacobi integral. It is equivalent to the Tisserand relation when written in terms of orbital elements in the context of celestial mechanics (orbital dynamics in a frame rotating with a planet that is in a circular orbit around the Sun). The Tisserand relation is used to classify comets and estimate the range of orbital changes that can be caused by gravitational assists. 29

30 Figure 5: The Shearing Sheet with Hamiltonian in equation 9 and equations of motion (equations 11). A central particle remains fixed. Particles with no epicyclic oscillations (in circular orbits) exhibit shear in their horizontal (x) velocities as a function of y. Often the shearing sheet is simulated with periodic boundary conditions in both x and y. 3.4 The Shearing Sheet A two-dimensional system in the plane H(r, θ; p r, L) = p2 r 2 + L2 2r 2 + V (r) For a Keplerian system the potential V (r) = GM/r. In a disk galaxy with a flat rotation curve V (r) = vc 2 ln r. We want to consider motion near a particle that is in a circular orbit with radius R 0 and has angular rotation rate Ω 0 with ( ) 1 Ω 2 dv 0 = r dr R 0 First let us rescale units so that units of time are in Ω 1 0 and units of distance are R 0. We want to go into a rotating frame moving with the particle at r = R 0 = 1 and has θ = Ω 0 t = t. In our rescaled units we can do this using a generating function using old coordinates and new momenta F 2 (θ, r; p x, p y ; t) = (θ t)(p x + 1) + (r 1)p y 30

31 giving x = F 2 x = θ t y = F 2 y = r 1 L = F 2 θ = p x + 1 p r = F 2 r = p y F 2 = p x 1 t Our new Hamiltonian (neglecting constants) H(x, y; p x, p y ) = p2 y 2 + (p x + 1) 2 2(1 + y) 2 + V (1 + y) p x The Hamiltonian is independent of x so p x is a conserved quantity. The Hamiltonian is time independent so H is constant. There is a fixed point at x = 0, y = 0, p x = 0, p y = 0. For small y we can expand V (1 + y) = V (1) + V (1)y + V (1)y 2 /2 Recall that Ω 2 0 = R 1 0 V (R 0 ) so with our choice of units V (1) = 1. Below it will be useful to write κ 2 = V (1) + 3 (usually one writes κ 2 0 = V (R 0 ) + 3Ω 2 0 but with Ω 0 = R 0 = 1 this simplifies). Expanding the Hamiltonian for small y, H(x, y; p x, p y ) p2 y 2 + (p x + 1) 2 (1 2y + 3y 2 ) + y + V (1) y2 2 2 p x I have have dropped V (1) as it is a constant. We can also drop terms that are higher than second order in all coordinates and momenta, (dropping terms p 2 xy and p x y 2 ) giving us H(x, y; p x, p y ) p2 y 2 + p2 x 2 2p xy + κ2 y 2 (9) 2 and I have used κ 2 V (1) + 3 for the epicyclic frequency. Hamilton s equations gives H x = 2y + p x = ẋ H y = p y = ẏ H x = 0 = ṗ x H y = 2p x + κ 2 y = ṗ y 31

32 We can solve for and because p x is conserved p x = ẋ + 2y ẍ = 2ẏ Inserting p x = ẋ + 2y into the expression for ṗ y For the Keplerian system κ 2 = Ω 2 = 1 ÿ = 2ẋ + (4 κ 2 )y ẍ = 2ẏ ÿ = 2ẋ + 3y (10) With the addition of an additional local potential these are known as Hill s equations (and originally derived for orbits near the Moon and usually x and y are interchanged). But in a system with a different rotation curve (like a disk galaxy) ẍ = 2ẏ ÿ = 2ẋ + (4 κ 2 )y (11) If we added in a local potential (that is a function of distance from the origin) that could be due to a local mass at the origin then the Hamiltonian would look like this H(x, y; p x, p y ) p2 y 2 + p2 x 2 2p xy + κ2 y 2 + W ( x y 2 ) (12) Going back to the shearing sheet without any extra perturbations H(x, y; p x, p y ) p2 y 2 + p2 x 2 2p xy + κ2 y 2 2 (13) The momentum p x sets the mean y value about which y oscillates. The oscillation frequency for y (radial) oscillations is the epicylic frequency κ and it s independent of the mean value of y. The angular rotation rate (here ẋ) is set by p x and this sets the mean y value. As there is a fixed point at x = 0, y = 0, p x = 0, p y = 0 this Hamiltonian can be written in the form H = 1 2 xmx with M the Hessian matrix and ẋ = ωmx and ẍ = (ωm) 2 x. As H is second order in all coordinates, M contains no variables (is just constants), conserved quantities can be identified from zero value eigenvalues of ωm and frequencies of oscillation from eigenvalues of (ωm) 2. 32

33 The Hamiltonian is independent of x (giving us a conserved quantity p x ). An associated Lagrangian would be independent of x. That means in a simulation we can change x of a particle and the equations of motion will not change. That means in a simulation, periodic boundary conditions in x would not change the equation of motion (equations 11). There is another symmetry in the equations of motion (is this because the Jacobi integral is conserved in the rotating frame?) ẋ ẋ + ɛ (κ2 4) 2 y y + ɛ This symmetry is exploited in simulations so that the y boundary condition can be periodic also. If we are simulating with the Hamiltonian system then we are change momenta and coordinates rather than velocities and coordinates. Equations of motion are ẋ = p x 2y ẏ = p y ṗ x = 0 ṗ y = 2p x κ 2 y Consider changing y by δy but not affecting any accelerations. To maintain ṗ y we require that δp x = κ2 2 δy The acceleration in x is ẍ = ṗ x 2p y is zero as long as we don t change p y. How does this transformation affect ẋ? δẋ = δp x 2δy = 1 2 (κ2 4)δy and this is equivalent to the transformation given above. In terms of momenta and coordinates the transformation is y y + ɛ p x p x + ɛ κ2 2 As the system has two independent conserved quantities (H, p x ), without any additional perturbations the system is integrable, and this is expected as this system is equivalent to the epicyclic approximation. The shearing sheet, because it is non-trivial, integrable and can be simulated with periodic boundary conditions is a nice place for particle integrations (see recent work by Hanno Rein and collaborators, and including short range interactions between particles). 33

34 4 Symmetries and Conserved Quantities 4.1 Functions that commute with the Hamiltonian Recall that for a function f(p, q, t) If the Poisson bracket and f t f = {f, H} + f t {f, H} = 0 (14) = 0 then f is a conserved quantity. Consider the gradient of a function on p, q or with one-form df = f f dq + dq. We can use the symplectic form to generate a flow or vectors V such that ω(v,?) = df. In this way f generates a direction in phase space or a tangent vector. The Hamiltonian generates a flow. The function f also generates a flow. The function f corresponding to a conserved quantity is also one that commutes with H and generates a flow that commutes with the flow generated by H. Using x = (q, p) and = ( q, p ) recall that we could write the equations of motion as ẋ = ω H where ω is the matrix made up of a positive and negative identity matrix ( ) 0 I ω = I 0 Using gradients it is possible to rewrite equation 14 as Using the equations of motion {f, H} = f t ω H = 0 f ẋ = 0 A conserved quantity is one that has a gradient (in phase space) perpendicular to the direction of motion. If there are many possible such directions (there are many conserved quantities) then the orbits must be of low dimension. The Hamiltonian formalism relates flows that commute with the Hamiltonian flow with conserved quantities. 4.2 Noether s theorem Noether s theorem relates coordinate symmetries of the Lagrangian with a conserved quantity. Consider a Lagrangian L(q, q, t) and a transformation of the coordinates q s = h(s, q) 34

35 with q = h(0, q) and s a continuous parameter. derivative We require that the transformed time q s = d dt q s so that h transforms both q and q consistently. The map h is a symmetry of the Lagrangian when The Lagrangian cannot depend on s so Using the chain rule We can write L(q s, q s, t) = L(q, q, t) L(q s, q s, t) s = 0 s=0 L s = L s s + L q q s s = d s dt s and using Lagrange s equation we can replace L 15 give L s = d [ ] L s s=0 dt q s Hence we have a conserved quantity or I = L q q s s with d dt s s I = L i,s q i s s=0 s=0 (15) (16) L q. These inserted into equation using summation notation and in more than one dimension. Note: Rather than conserving L we should really be thinking about conserving the action. In this case we should be considering infinitesimals in time and end points. The Lagrangian formalism relates a symmetry of the Lagrangian to a conserved quantity. In the Hamiltonian context, we use the symplectic two form and a conserved quantity (here a function) to generate a vector and so a flow in phase space. This flow commutes with the Hamiltonian flow. s=0 = 0 35

36 4.3 Integrability A time independent Hamiltonian with N degrees of freedom is said to be integrable if N smooth independent functions I i can be found that that are conserved quantities and {I i, H} = 0 {I i, I j } = 0 are in involution. The reason that the constants should be smooth and independent is that the equations I i (q, p) = c i, where the c i s are constants, must define N different surfaces of dimension 2N 1 in the 2N-dimensional phase space. If the conserved quantities are in involution then they can be used as canonical momenta. For example, consider a 4 dimensional phase space with a time independent Hamiltonian. An orbit is a trajectory in 4 dimensional phase space. Once we specify the energy (Hamiltonian) then there is a constraint on the orbit and the orbit must wander in a 3- dimensional subspace. If we specify an additional conserved quantity then the orbit must wander in a 2 dimensional subspace. At this point we say the system is integrable. The orbit could be a lower dimensional object if there are additional conserved quantities. A circular orbit in the plane of an axisymmetric galaxy is a one dimensional object (points only a function of angle). Epicyclic motion near the circular orbit covers a 2-dimensional surface (is a torus). I can describe it in terms of two angles given angular momentum and epicyclic action variable. This system has two conserved quantities (energy and angular momentum) so is integrable. If the potential is proportional to 1/r (Keplerian setting) then there is an additional conserved quantity (Runge-Lunz vector) and every orbit is one dimensional (an ellipse) instead of a 2-dimensional (torus). Systems with extra conserved quantities (past what is needed to make them integrable) are known as superintegrable. The Keplerian system is maximally superintegrable as there are 5 conserved quantities for every orbit in 6 dimensional phase space. Liouville integrability means there exists a maximal set of Poisson commuting invariants (i.e., functions on the phase space whose Poisson brackets with the Hamiltonian of the system, and with each other, vanish). Locally there could be a complete set of conserved quantities and it might be possible to construct canonical transformations that give you a Hamiltonian purely in terms of conserved quantities. But it might not be possible to find a complete set the covers the entire manifold (in the Liouville sense of integrability). Often we call a system integrable if the dimension covered by orbits is low. For example in a 4 dimensional phase space if there are 2 (non-degenerate) conserved quantities (one of them could be the Hamiltonian itself). Orbits are in a 4 dimensional space, and two conserved quantities drops the dimension by 2 so orbits cover a surface. They are like tori. If the system had only a single conserved quantity (like energy) we could make a surface 36

37 of section and find area filling orbits that corresponded to orbits filling a 3 dimensional volume. Periodic orbits would be fixed points in a surface of section and these would be 1 dimensional. 37