# How do I derive geodesic equation using variational principle? [duplicate]

I am trying to derive the geodesic equation using variational principle. My Lagrangian is $$L = \sqrt{g_{jk}(x(t)) \frac{dx^j}{dt} \frac{dx^k}{dt}}$$ Using the Euler-Lagrange equation, I have got this. $$\frac{d^2 x^u}{dt^2} + \Gamma^u_{mk} \frac{dx^m}{dt} \frac{dx^k}{dt} = \frac{\frac{dx^u}{dt}}{g_{pq}\frac{dx^p}{dt}\frac{dx^q}{dt}} \frac{d}{dt}\left[g_{rs}\frac{dx^r}{dt}\frac{dx^s}{dt}\right]$$

How do I prove the right hand side to be zero to get the geodesic equation?

Basically, I can rewrite the above mentioned equation as $$\frac{d^2 x^\mu}{dt^2} + \Gamma^\mu_{\nu \lambda} \frac{dx^\nu}{dt} \frac{dx^\lambda}{dt} = \frac{dx^\mu}{dt} \frac{d\ \log(L)}{dt}$$

I need to know why is the following true? $$\frac{d\ \log(L)}{dt} =0$$

I know that the derivation might be simpler using a different Lagrangian but I want to do it using this one.

## marked as duplicate by Danu, Ryan Unger, John Rennie general-relativity StackExchange.ready(function() { if (StackExchange.options.isMobile) return; $('.dupe-hammer-message-hover:not(.hover-bound)').each(function() { var$hover = $(this).addClass('hover-bound'),$msg = $hover.siblings('.dupe-hammer-message');$hover.hover( function() { $hover.showInfoMessage('', { messageElement:$msg.clone().show(), transient: false, position: { my: 'bottom left', at: 'top center', offsetTop: -7 }, dismissable: false, relativeToBody: true }); }, function() { StackExchange.helpers.removeMessages(); } ); }); }); Feb 11 '16 at 7:18

• $g_{rs}\dot x^r\dot x^s=\text{const.}$ if you pick a nice parameterization... – Ryan Unger Feb 10 '16 at 16:11
The geodesic equation for a general parameterization takes the form $$\frac{d^2 x^\mu}{d\tau^2} + \Gamma^\mu_{\rho\sigma} \frac{dx^\rho}{d\tau} \frac{dx^\sigma}{d\tau} = K(x(\tau))\frac{dx^\mu}{d\tau}$$ The RHS of the above is only zero when $\tau$ is chosen to be an affine parameter along the geodesic.
If you choose the affine parameter then $$g_{\mu\nu}(x(\tau)) \frac{dx^\mu}{d\tau} \frac{ d x^\nu}{d\tau} = \varepsilon$$ where $\varepsilon$ is a constant. In this case, your equation takes the form required.