# Variation of Action with time coordinate variations

I was trying to derive equation (65) in the following review:

http://relativity.livingreviews.org/open?pubNo=lrr-2004-4&page=articlesu23.html

This slightly unusual then usual classical mechanics because it includes a variation of time also, $\delta t$.

Usually one would define, $$\delta S~=~ \int \left[ L \left(\tilde{q}(t),\dot{\tilde{q}}(t),t\right) - L \left(q(t),\dot{q}(t),t\right)\right] dt ,$$ where,

$$\tilde{q}~=~q+\delta q.$$

We have then (before applying int by parts),

$$\delta S ~=~ \int \left(\frac{\partial L}{\partial q}\delta q +\frac{\partial L}{\partial \dot q}\delta \dot q\right)dt.$$

How does one proceed if both $q$ and $t$ vary and further that $q$ depends on a varying $t$?

Is the definition now

$$\delta S~=~ \int \left[ L \left(\tilde{q}(t+\delta t),\dot{\tilde{q}}( t+\delta t), t+\delta t\right) - L \left(q(t),\dot{q}(t),t\right)\right] dt~?$$

If so how does one proceed?

• this is the variational principle with varying boundary as well (begin/end times) which gives a generalised Euler-Lagrange problem and can determine end times also – Nikos M. Jun 14 '14 at 23:50

I) Hint: Decompose the full infinitesimal variation

$$\tag{A} \delta q~=~\delta_0 q + \dot{q} \delta t$$

in a vertical infinitesimal variation $\delta_0 q$ and a horizontal infinitesimal variation $\delta t$. Similarly the full infinitesimal variation becomes

$$\tag{B} \delta I~=~\delta_0 I + \left[ L ~\delta t \right]_{t_1}^{t_2},$$

where the vertical piece follows the standard Euler-Lagrange argument

$$\tag{C} \delta_0 I~=~ \int_{t_1}^{t_2}\! dt~\left[\frac{\partial L}{\partial q}-\dot{p} \right] \delta_0 q + \left[ p ~\delta_0q \right]_{t_1}^{t_2},$$

and we have for convenience defined the Lagrangian momenta

$$\tag{D} p~:=~\frac{\partial L}{\partial \dot{q}}.$$

Now combine eqs. (A-D) to derive eq. (65) in Ref. 1:

$$\tag{65} \delta I~=~ \int_{t_1}^{t_2}\! dt~\left[\frac{\partial L}{\partial q}-\dot{p} \right] \delta_0 q + \left[ p ~\delta q - (p\dot{q}-L)\delta t\right]_{t_1}^{t_2},$$

II) Ideologically, we should stress that Ref. 1 is not interested in proposing a variational principle for non-vertical variations (such as, e.g., Maupertuis' principle, or a variant of Pontryagin's maximum principle, etc). Ref. 1 is merely calculating non-vertical variations within a theory that is still governed by the principle of stationary action (for vertical variations).

III) Ref. 1 mainly uses eq. (65) to deduce properties of the on-shell Dirichlet action $^1$

$$\tag{E} S(q_2,t_2;q_1,t_1)~:=~I[q_{\rm cl};t_1,t_2],$$

cf. e.g. this Phys.SE post.

References:

1. L.B. Szabados, Quasi-Local Energy-Momentum and Angular Momentum in GR: A Review Article, Living Rev. Relativity 7 (2004) 4.

--

$^1$ Ref. 1 calls $S(q_2,t_2;q_1,t_1)$ the Hamilton-Jacobi principal function. Although related, the Hamilton-Jacobi principal function $S(q,P,t)$ is strictly speaking another function, cf. e.g. this Phys.SE post.

This the Variational Problem with unknown end-time and one proceeds like this:

$$\delta S= \int_{t_i}^{t_f+\delta t_f} L \left(q+\delta q,\dot{q}+\delta \dot{q},t\right) dt - \int_{t_i}^{t_f} L \left(q, \dot{q}, t\right) dt$$

After several transformations and integration by parts one finally gets the usual Euler-Lagrange diff eq plus a boundary condition involving $\delta t_f$:

$$0 = L(q, \dot{q}, t) \delta t_f + \frac{\partial L}{\partial \dot{q}}(\delta q_f - \dot{q} \delta t_f)$$

Derivation steps:

a. Expand the first integral in a Taylor series and keep terms of 1st order and splitting the limits of integration (and doing any cancelations):

$$\delta S= \int_{t_f}^{t_f+\delta{t_f}} \left[ L + \frac{\partial{L}}{\partial{q}}\delta{q} + \frac{\partial{L}}{\partial{\dot{q}}}\delta{\dot{q}} \right] dt + \int_{t_i}^{t_f} \left[ \frac{\partial{L}}{\partial{q}}\delta{q} + \frac{\partial{L}}{\partial{\dot{q}}}\delta{\dot{q}} \right] dt$$

b. The total variation consists of 2 variations; $\delta{q}$ and $\delta{t_f}$. Integrating over a small interval i.e $[t_f, t_f + \delta{t_f}]$ is effectively equivalent to multiplication by $\delta{t_f}$:

$$\delta S= \delta{t_f} \left[ L + \frac{\partial{L}}{\partial{q}}\delta{q} + \frac{\partial{L}}{\partial{\dot{q}}}\delta{\dot{q}} \right] + \int_{t_i}^{t_f} \left[ \frac{\partial{L}}{\partial{q}}\delta{q} + \frac{\partial{L}}{\partial{\dot{q}}}\delta{\dot{q}} \right] dt$$

c. Terms like $\delta{t_f}\delta{q}$ or $\delta{t_f}\delta{\dot{q}}$ are 2nd order variations and can be dropped:

$$\delta S= \delta{t_f} L + \int_{t_i}^{t_f} \left[ \frac{\partial{L}}{\partial{q}}\delta{q} + \frac{\partial{L}}{\partial{\dot{q}}}\delta{\dot{q}} \right] dt$$

d. Integration by parts yields a usual Euler-Lagrange diff eq. plus the boundary condition.

e. On the boundary condition at time $t_f$ one has:

Total variation of $q$ at $t_f$ is

$$\delta{q_f} = \delta{q(t_f)} + (\dot{q} + \delta{\dot{q}})\delta{t_f} = \delta{q(t_f)} + \dot{q}\delta{t_f}$$

or

$$\delta{q(t_f)} = \delta{q_f} - \dot{q}\delta{t_f}$$

• Comment to the answer (v2): Note that to have a well-posed variational problem with unknown end-time, one in general needs to specify conditions on the allowed variations. – Qmechanic Jun 16 '14 at 3:52
• @Qmechanic, correct, the answer follows the question regarding the extended formalism, the conditions are part of the problem and the final time is related to the boundary condition – Nikos M. Jun 16 '14 at 7:40
• So is there a way to decompose that first integral? – user50482 Jun 16 '14 at 23:34
• @user50482, yes there is, these are taken mostly from my notes on optimal control theory (a-la Pontryagin), so did not put every derivation step, it is possible one can find online, else will have to look at the notes and add more steps of the derivation – Nikos M. Jun 17 '14 at 0:30
• @user50482, added derivation steps – Nikos M. Jun 17 '14 at 0:55