2.4 Characteristic speeds

The system of field equations (1), (2) may be written as a quasilinear system of n equations in the form
Such a system allows the propagation of weak waves, so-called acceleration waves. There are n such waves and their speeds are called characteristic speeds, which are not necessarily all different. The fastest characteristic speed is the pulse speed. This is the largest speed by which information can propagate.

Let define the wave front; thus

define its unit normal and the speed . An easy manipulation provides

Since in a weak wave the fields have no jump across the front, the jumps in the gradients must have the direction of and we may write

is the magnitude of the jump of the gradient of u. The square brackets denote differences between the front side and the back side of the wave.

In the field equations (10) the matrix and the productions are equal on both sides of the wave, since both only depend on and since is continuous. Thus, if we take the difference of the equations on the two sides and use (13) and (11), we obtain

Non-trivial solutions for require that this linear homogeneous system have a vanishing determinant
Insertion of (11) into (15) provides an algebraic equation for whose solutions – for a prescribed direction n – determine n wave speeds , of which the largest one is the pulse speed. Equation (15) is called the characteristic equation of the system (10) of field equations. By (11) it may be written in the form