# Temperature as the independent variable of Lagrangian

I was thinking about applications of the Lagrangian and I started to toy with some ideas and tried to come up with interesting twists. Immediately I thought it would be interesting to use temperature as oppose to time as the independent variable.

$\int f[x, \dot{x}; t]dt$

Something like

$\int f[x, x'; T]dT$

There are a few issues with this:

1. Is this even a legitimate method? Given that we use time and take the derivative with respect to it I'm a tad bit skeptical as to whether this can be legitimately done. What would the derivative mean in this context.

2. The other issue with this is that temperature really is just giving us stochastic information, so how would one be able to create kinetic energy equation that is inherently random? Even if the idea of using temperature is invalid the question about randomness could still be useful for time based Lagrangians.

• Lagrangian mechanics is formulated in terms of time for a reason. It wouldn't reproduce Newtonian physics otherwise. – lemon Jan 26 '15 at 8:58
• There is indeed a significant amount of work in presenting a Lagrangian formulation of Thermodynamics due to the fact that both temperature and an entropy may be associated with horizons in semi-classical general relativity. – Autolatry Jan 26 '15 at 10:03
• Some thermodynamic variables are derivatives with respect to $T$, so $dx/dT$ isn't terribly wrong in itself. But we get the Euler-Lagrange equations from minimizing the action, I don't see what can come about with your "interesting twist" in that manner. – Kyle Kanos Jan 26 '15 at 13:40
• @Autolatry Any references you could point me to for that – Skyler Jan 27 '15 at 11:41
• Sure, I recently read arxiv.org/pdf/1110.6152v1.pdf as an introduction. – Autolatry Jan 27 '15 at 12:19