Indeed, suppose we have a single scalar field whose dynamics is governed by some generic Lagrangian , which is some arbitrary function of the field and its first derivatives (here we will follow the notation and ideas of ). In the general analysis that follows the previous irrotational and inviscid fluid system is included as a particular case; the dynamics of the scalar field is now much more general. We want to consider linearised fluctuations around some background solution of the equations of motion, and to this end we write
This can be given a nice clean geometrical interpretation in terms of a d’Alembertian wave equation – provided we define the effective spacetime metric by
Suppressing the except when necessary for clarity, this implies [in (d+1) dimensions, d space dimensions plus 1 time dimension]
It is important to realise just how general the result is (and where the limitations are): It works for any Lagrangian depending only on a single scalar field and its first derivatives. The linearised PDE will be hyperbolic (and so the linearised equations will have wave-like solutions) if and only if the effective metric has Lorentzian signature . Observe that if the Lagrangian contains nontrivial second derivatives you should not be too surprised to see terms beyond the d’Alembertian showing up in the linearised equations of motion.
As a specific example of the appearance of effective metrics due to Lagrangian dynamics we reiterate the fact that inviscid irrotational barotropic hydrodynamics naturally falls into this scheme (which is why, with hindsight, the derivation of the acoustic metric presented earlier in this review was so relatively straightforward). In inviscid irrotational barotropic hydrodynamics the lack of viscosity (dissipation) guarantees the existence of a Lagrangian; which a priori could depend on several fields. Since the flow is irrotational is a function only of the velocity potential, and the Lagrangian is a function only of this potential and the density. Finally, the equation of state can be used to eliminate the density leading to a Lagrangian that is a function only of the single field and its derivatives .
Living Rev. Relativity 14, (2011), 3
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