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Figure 1. Wave fronts C (in red) of light emitted at point r0 at Figure 3. The trajectory chosen by an object ( line) op- t0. Blue lines are the rays path. The Huygens principle timizes the compared to all other possible trajectories states that the wave front at time t can be seen as the wave (dashed lines) (1) front of light emitted at time t − tα by the wave front at this time (red dashed lines). jects linked together through an orthogonality. Even if the medium is not isotropic, we can still com- pute the wave front from the rays, and vice versa. All we need is a relation between the to the ray path (let’s call it q˙ ) at a point and the normal to the wave front (call it p) at the same point. We will come to this subject in more general detail in the next sections.

III. BASIC NOTIONS OF .

Figure 2. In geometrical optics in isotropic media, trajectories Very soon after the publication of Principia by Newton Pt of the light rays and wave fronts are orthogonal. Therefore, (1684), Bernoulli challenged (1696) the scientific commu- trajectories can be recovered from the wave front: from the nity to find the fastest path that, under , brings a C point Pt on the wave front t, draw the orthogonal to the from point A to point B. The analogy with optics wave front and recover the point Pt+dt at which it intercepts and the Fermat’s principle was not lost on the mathe- the wave front Ct+dt. Proceeds by recurrence. maticians who responded to the challenge[11]. This anal- ogy was then fully developed in subsequent years [12] and took its definitive form under the name of Euler-Lagrange called the Huygens principle. Finally, note that if r0 r, ≫ equation. r r0 r0 (r0/r0).r and we can approximate the The foundation of analytical mechanics is based on a kspherical− k ≈wave− by a plane one of the form S(r,t)= u.r − variational principles: Given a Lagrangian (q,q,t ˙ ), an (c/n)t where u = (r0/r0) is the direction of the plane object (be it a particle or a ray of light) Lchooses the wave propagation.− trajectory q(t) that makes the action If the medium is not homogeneous (n = n(r) ), the wave fronts are not spherical any more. The principle of t1,q1 Fermat states that the path taken by a ray to go from S = (q,q,t ˙ )dt (1) ˆ L a point A to a point B is the one that minimizes the t0,q0 traveling time : stationary (figure 3). The action depends on the end points (t0, q0) and (t1, q1) and the trajectory q(t) must 1 B T = nds obey the Euler-Lagrange equation c ˆ A d ∂ ∂ L L =0 (2) where ds is the of along a path. In dt ∂q˙ − ∂q order to compute a wave front now, one has to compute For a classical particle, the Lagrangian is the difference the ray paths and collect points along the path that have between the kinetic and the potential = T been reached at a given time t. If the medium is isotropic V , while for geometrical optics, the LagrangianL is the− (i.e. not like a crystal with particular directions of prop- traveling time. agation), it can be shown that ray paths and wave fronts We can reformulate equation (2) by making a Legendre are orthogonal (see below). In this case, deducing the transform. Defining the wave fronts from the ray paths is simple. On the other hand, if we knew the wave fronts, we could compute the ∂ p = L (3) ray paths (figure 2). Paths and wave fronts are dual ob- ∂q˙ 3

t1 d ∂ = Lδq dt ˆ dt ∂q˙ t0   ∂ t1 ∂ = Lδq = L dq ∂q˙ ∂q˙  t0 t1

As we have kept the final time fixed, δS = (∂S/∂q) dq and therefore ∂S ∂ = L = p(t1) (7) ∂q ∂q˙ t1

If we vary the end point q1, the relative variation in S is Figure 4. Varying the end points of a movement. the momentum p at the end point. To compute the variation of S as a function of the end point’s time, consider letting the original trajectory to expressing q˙ as a function of p and defining H(p,q,t) = continue along its optimal path. Then dS = dt. On the pq˙ , we obtain the Hamilton equations other hand L − L dq ∂H dp ∂H ∂S ∂S = ; = (4) dS = dt = dq + dt dt ∂p dt − ∂q L ∂q ∂t which allows us to move to the and have a Using our previous result (7), we have more geometrical view of the trajectories. One conse- quence of the above equation is the variation of H as a ∂S ∂S function of time along a trajectory: dt = pdq + dt = pq˙ + dt L ∂t ∂t ∂H ∂H ∂H ∂H   dH = dp + dq + dt = dt (5) ∂p ∂q ∂t ∂t and therefore ∂S Therefore, if the Hamiltonian does not depend explicitly = pq˙ = H (8) on time, the Hamiltonian is conserved along a trajectory: ∂t L− − H = E. In the above two formulation of analytical mechanics, Relation (7,8) are very general results of variational cal- the action S() itself plays little explicit role; what is im- culus with varying end points and are not restricted to portant is the differential equations (2) or (4) whose so- mechanics. The contact of a liquid droplet on a lution determines the trajectory. However, Let us have a solid surface is obtained for example by these compu- closer look at the action itself. By action S here we mean tations. Note also that even though we derived these the expression (1) when the particle moves along equations in one of space, they are trivially the optimal path. Even though the absolute value of S generalized to any dimension. can be hard to compute analytically, we can compute its variation if we vary the end points (figure 4). We will IV. GENERAL WAVE FRONTS. keep here the initial point fixed and vary the final end point either by dt or dq. We begin by keeping the final time fixed at t1 but move In geometrical optics, we had used the traveling time to the final position by dq (figure 4). The trajectory q(t) define the wave front. But the traveling time is just one will vary by δq(t) where δq(t0)=0 and δq(t1)= dq. The example of action and variational principles. In analogy variation in S is with optics, let us define the function Sq0,t0 (q,t) as the action of a particle that arrives at (q,t) after leaving t1 ∂ ∂ δS = Lδq˙ + Lδq dt (6) (q0,t0), following its optimal path. By this function, we ˆ ∂q˙ ∂q q t0   can associate to each point ( ,t) a value in space-time. Then, S(q,t)= C defines an n 1 dimensional surface , However, the trajectories obey the Euler-Lagrange equa- t i.e. the collection of points q that− have the same valueCC tion (2) and we must have of action at time t. Figure 1 that illustrated wave front ∂ d ∂ in optics illustrates similarly the general wavefronts of L = L ∂q dt ∂q˙ action. Consider for example a classical free particle, whose On the other hand, δq˙ = d(δq)/dt. Using these relations, trajectories are straight lines with constant speed v = we can rewrite equation (6) as q q0 /(t t0). The action is therefore k − k − t1 ∂ d (δq) d ∂ δS = L + L δq dt m 2 m 2 ˆ ∂q˙ dt dt ∂q˙ S(q,t)= v (t t0)= q q0 /(t t0) t0     2 − 2 k − k − 4 and the are spheres of radius proportional to Ct 2(t t0)/m. If the initial point is far away from the − region of interest ( t t0 , q q0), we can develop the pabove expression and| | ≪ write | | it,≪ to the first order in q,t : m 2 S(q,t) q0 2q0.q ( 1 t/t0) ≈ 2t0 − − − = S0 +p.q Et  (9) − where we have defined the constants p = mq0/t0 and 2 2 E = (1/2)mq0/t0. In this case, the action is a plane wave. We have defined the wave front as the collection of Figure 5. From known wave fronts Ct(in red) to trajectories points q at time t for which S(q,t)= const. To compute : at each point, the normal to the wave front p = ∂S/∂q the wavefronts however, we have relied on the knowl- (in blue) can be computed ; knowing p, we can compute the edge of trajectories. To go further, we need to derive an tangent to the trajectory q˙ ( in green ) and find a trajectory independent equation from which S() can be computed following a given line of . The procedure is trivially directly, without any a priori knowledge of trajectories. generalized to higher dimensional space where q collects the For this purpose, we just have to recall from the last sec- coordinates of many particles. tion (7,8) that we can compute the variation of S as a function of the variation of its end points:

∂S ∂S = p ; = H (10) ∂q ∂t − where ∂S/∂q = (∂q1 S, ∂q2 S, ...). Note that this a gener- alization of the free particle case where (according to 9), dS = pdq Edt. Now, we know that H = H(q, p,t), therefore combining− the above two expressions, we have

∂S ∂S Figure 6. An illustration of the wave front in a two dimen- + H q, ,t =0 (11) q ∂t ∂q sional space where the function W ( ) is represented as a sur-   face in three dimension. The wave front CS is the contour plot which is a first order PDE and called the Hamilton- of the function W (q). At any given point q,the momentum Jacobi equation (HJE). If we can solve this equation and is given by p = ∇W find the wave fronts, then we can deduce the trajectories from the wavefronts. The procedure is similar to what we did in geometrical optics : At each time t, we know the where the function W () (often called Hamilton principal wave front S, and therefore, we can compute the momen- function) obeys the relation tum at points q: p(q) = ∂S/∂q (figure 5). This vector ∂W is related to the tangent to a trajectory q˙ through the H q, = E ∂q relation   ∂ Once W () is solved for, we can find the wave fronts by p = L ∂q˙ slicing the function W () at different “heights” : at a given time t, we collects all points q such that W (q) = Et + By resolving the above relation, we can compute q˙ at const. into the wave front (figure 6). each point of space at each time : Ct q˙ = f(q,t) (12) V. EXAMPLES. If we knew the wave fronts, the second order differen- tial equations of Euler-Lagrange (equation 2) are trans- A. One particle. formed into ordinary first order differential equations (12) as above. For the simplest mechanical systems with one particle and a potential V (q,t), p and ˙q are co-linear and Consider one classical free particle with the Lagrangian q2 p q p2 the construction is really similar to optics. = (1/2)m ˙ , = m ˙ and H = /2m where we L u2 u u We can further simplify the HJE (eq. 11) if the func- use the square of a vector as a shorthand: = . . tion H does not contain t explicitly. In this case, we can Therefore, the HJE is simply separate the function S into 2 ∂S 1 ∂S + =0 (14) S(q,t)= W (q) Et (13) ∂t 2m ∂q −   5

4

2

ℓ √ 0 q

-2

-4

− − -4 -2 0 2 4 q

Figure 7. Contour plot of W (q1, q2) of free particle in 2 di- mensions (relation 16) for u1 = cos θ = 1/2. Figure 8. The Hamilton principal function W (q) for the uni- dimensional .

It is straightforward to check that the spherical wave S = 2 There exist a systematic method to search for the so- m(q q0) /2(t t0) is a solution of the above equation, − − lution of this equation, called canonical transformations where q0 and t0 are some constants. We can also look for a separable solution of the form S = W Et, in which (see for example [4, section 10.4]). If however, the po- q case − tential is itself separable V ( ) = i Vi(qi), we can look for a separable solution of the HJE as before. As an il- 2 1 ∂W lustration, consider the simple oneP dimensional harmonic = E 2 2m ∂q oscillator with V (q) =(1/2)kq . Extension to higher di-   mensional case is trivial but harder to present graphically. To solve this PDE, we can search for further separability Setting S = W Et we have in the form of − dW x2 W (q)= √2m wi(qi) (15) = √2mE 1 dq − ℓ2 i r X 2 and solve the equations dwi/dqi = √ei where ei are in- where ℓ = 2E/k. Setting q = ℓ sin θ transforms the tegration constants. The solution of these equations are above equation into wi(qi)= √eiqi + Ci with the constraints ei = E and i dW 2 Ci another of integration constants. The complete = ℓ√2mE cos θ solution is then a plane wave with (figureP 7) dθ that integrates directly ′ W (q)= √2mE uiqi + C (16) i 1 1 i W (θ)= ℓ√2mE(θ + sin 2θ)+ C X 2 2 where u = e /E are the integration constants. We i i Figure 8 displays a plot of W (q) as a function of q. It collect the constants ui into a constant vector v such that v = vu p, E = (1/2)mv2 and write (figure 7) can be observed that the function W () is multivalued i i and at its “turning points”, p = ∂W/∂q = 0, a fact that W (q)= mv.q + C is common to all bounded mechanical systems. Now that we know the wave front, if we wish so, we can deduce the trajectories : the is given by B. Relativistic particle. p = ∂W/∂q = mv. From the Lagrangian, we know that q˙ = p/m, and therefore q˙ = v and q = vt + q0 where We distinguish here explicitly between time and space q0 is another integration constant. coordinate for more clarity at the expense of elegance. For a classical free particle, the HJE is obviously an Consider a free relativistic particle whose action is given overkill. The purpose of this example is to illustrate how by its Minkowski arc length the solution of the HJE with the integration constant v leads to the trajectories. It is straightforward to check B S = m ds that the spherical wave solution leads to the same result − ˆ for trajectories. A Adding a potential V (q) to the problem give rise to where (in natural units c = 1 ) dt = mds = L − the HJE m√dt2 dxdx = m√1 x˙ 2dt. We have 2 − − − − ∂S 1 ∂S ∂ mx˙ + = V (q) (17) p = L = ∂t 2m ∂q − ∂x˙ √1 x˙ 2   − 6 and therefore The HJE is then

2 2 2 H = px˙ = m + p ∂S ∂S − L n2 =0 ∂x − s − ∂y The HJ equation is therefore p     or in other words, 2 ∂S ∂S = m2 + 2 2 ∂S ∂S 2 ∂t −s ∂x + = n (20)     ∂x ∂y or     2 2 The above expression, called the eikonal equation, is ∂S ∂S = m2 (18) the fundamental equation of geometrical optics. In the ∂t − ∂x Hamilton-Jacobi approach, its resemblance to relativis-     tic particle is obvious. We will see below that the eikonal Note that the parabolic PDE of a classical be- equation can be obtained through approximation of the comes a when we consider the relativistic wave equation. dynamics. This is exactly how the Schrodinger equation transforms into the Klein-Gordon one, i.e. the relativis- tic wave equation for spineless particles. This can be VI. WAVES AND PARTICLES. extended to the case of a particle with in an electromag- netic field by considering For about 50 years after its introduction, the Hamilton- Jacobi equation was considered a beautiful but use- dt = mds qds.A L − − less tool. With the advent of , where the four vector A = ( φ, A~), φ is the electromag- Schrodinger realized that this equation is the natural − road to formulating a “wave” equation for particles. The netic potential and A~ the (three) vector potential. approach was as follow : geometrical optic is an ap- proximation of the Maxwell equations that neglects in- terference effect. We know the Maxwell equation and C. Geometrical optics. the approximation procedure to get to geometrical op- tics. Schrodinger realized that can Consider light propagating in an isotropic medium. be such an approximation of a more complicated theory The action is the total traveling time and reverse engineered the geometrical optics approxi- mation to get to his famous equation in 1926. The detail B of this procedure and its connection to Hamilton-Jacobi S = nds ˆA equation is beautifully written by Massoliver and Ros[13] and we don’t develop it here. However, it is very simple where n(q) is the index of the medium at position q, to show that classical mechanics is an approximation of ds is the arc length along a trajectory and we have set the quantum mechanics. the speed of light in vacuum c = 1. This is called the Consider the Schrodinger equation principle of Fermat. For simplicity, we will consider a 2 2 two-dimensional medium where x is used as the integra- ∂ψ ~ ∂ ψ i~ = + V (x)ψ tion and ds = dx2 + dy2 = 1+ y′2dx ; the ∂t −2m ∂x2 Lagrangian is p p using a standard change of function ′2 = n(x, y) 1+ y iS/~ L ψ = e (21) p and by definition, the Schrodinger equation transforms into ′ ∂ y 2 2 p = L = n (19) ∂S i~ ∂ S 1 ∂S ∂y′ ′2 = + + V (x) (22) 1+ y − ∂t −2m ∂x2 2m ∂x   p if we set θ as the angle between the tangent to the we see that the above equation, when we neglect the term trajectory and the x axis, the above relation is simply in ~, reduces exactly to the classical HJE (17): the clas- p = n sin θ, which is the conserved quantity if n = n(x) ′ ′ sical mechanics is indeed the limit of quantum mechanics (Snell’s law). Solving relation 19 in y , we have y = when ~ 0. 2 2 p/ n p and therefore the Hamiltonian is The transformation→ (21), called the ansatz of Sommer- − field and Runge[14], was nothing unusual at the time of p ′ H = py = n2 p2 Schrodinger and is used to recover the geometrical optics − L − − p 7 from the wave equation ( see[15] for a review). Con- scale of variation in the index is large compared to the 2 2 sider the equation of an electromagnetic wave propagat- wave length, or equivalently, when A/A k0. Ne- 2 ∇ ≪ ing through space, where the index of refraction is not glecting the A term is relation (25), we obtain an equa- supposed to be constant : tion for the∇ phase φ alone:

2 2 2 ∂ ψ 2 2 ( φ) = n (27) = v ψ (23) ∇ ∂t2 ∇ which is the eikonal equation we had already obtained where ψ is any component of the electromagnetic from the principle of Fermat (eq. 20). or the vector potential and v = c/n where c is the speed of light and n the index of the medium. We look for a solution of the form VII. CONCLUSION.

ψ(t)= A(r)exp(ik0 (φ(r) ct)) (24) The Hamilton-Jacobi equation is one of the most ele- − gant and beautiful approach to mechanics with far reach- in analogy with plane waves when n = const. k0 =2π/λ0 ing consequences in many adjacent fields such as quan- is the wave and λ0 is the wave length in vacuum tum mechanics and probability theory. Unfortunately, ; A (the amplitude ) and φ (the phase) are real functions. its beauty is lost to many students learning the basics Note that the total phase of analytical mechanics. An informal and statistically non-significant inquiry of practicing suggests Φ(r)= φ(r) ct that even among , Hamilton-Jacobi brings up − mostly (if any) memories of arcane transformations with has the same structure as the function S in relation (13) no observable use. and φ() plays the same role as the function W (). The materials developed in this short article, which Plugging expression (24) into (23), separating the real does not contain the usual mathematical complexity and the imaginary part, we have: found in most textbooks, can be covered in one or two

2 2 2 2 2 lectures and I hope help students to get a basic under- A Ak0 ( φ) = k0n A (25) standing of the Hamilton-Jacobi approach to variational ∇ − ∇ − 2 ( φ) ( A)+ A 2φ =0 (26) systems. ∇ ∇ ∇ Acknowledgment. I’m grateful to Marcel Vallade for The geometrical optics is obtained from the wave equa- detailed reading of the manuscript and fruitful discus- tion by letting λ0 0, i.e. when we assume that the sions. →

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