Let $(Q,g)$ be a Riemannian manifold and let $q_0,q_1\in Q$ be two points that are joined by a unique geodesic $\gamma$ (this holds in particular if $q_1$ belongs to a normal neighborhood of $q_0$). Suppose that $q_0=\gamma(t_0)$ and $q_1=\gamma(t_1)$, then we define Synge's World function as \begin{equation} \label{Synge world function formula} \sigma(q_0,q_1):=\dfrac{1}{2}(t_1-t_0)\int_{t_0}^{t_1}g_{ij}\dot{\gamma}^i\dot{\gamma}^jdt,\tag{1} \end{equation} where $\gamma^i$ are the components of $\gamma$ in a chart that contains $q_0$ and $q_T$.
Consider the Hamiltonian \begin{equation} \label{geodesic Hamiltonian formula} \begin{array}{rcl} H:T^*Q & \rightarrow&\mathbb{R} \\ (q,p) & \mapsto&H(q,p):=\frac{1}{2}g^{ij}(q)p_ip_j. \end{array}\tag{2} \end{equation} It can be shown that its flow $\phi^t_H$ is the geodesic flow, which means that the integral curves of the Hamiltonian vector field $X_H$ are the geodesic.
I've read that Synge's World function can be thought as a generating function for the geodesic flow, which means that \begin{equation} \label{eq 1} p_0=-\dfrac{\partial\sigma}{\partial q_0}\qquad p_1=\dfrac{\partial\sigma}{\partial q_1},\tag{3} \end{equation} where \begin{equation} (q_1,p_1)=\phi^{t_1-t_0}_{H}(q_0,p_0).\tag{4} \end{equation} How to prove the relations (3)?


Hint: OP's eq. (3) follows from a Lemma eq. (11) in my Phys.SE answer here.

  • $\begingroup$ I know that kind of proofs. But I don't know how to apply it to Synge's World function. Where is the link between the general case of Hamilton's principal function and the particular case of Synge's World function? $\endgroup$ – Giulio Binosi Sep 22 '20 at 9:09
  • $\begingroup$ The trick is to keep everything in the Lagrangian formulation. $\endgroup$ – Qmechanic Sep 22 '20 at 9:38
  • $\begingroup$ The Lagrangian associated to the geodesic Hamiltonian is $L(q,\dot{q})=\frac{1}{2}g_{ij}\dot{q}^i\dot{q}^j$? $\endgroup$ – Giulio Binosi Sep 22 '20 at 13:32
  • $\begingroup$ $\uparrow$ Yes. $\endgroup$ – Qmechanic Sep 22 '20 at 14:10
  • $\begingroup$ So that's where Synge's World function comes from, right? $\endgroup$ – Giulio Binosi Sep 22 '20 at 15:09

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