By the definition of symmetric hyperbolicity due to Friedrichs  the system is symmetric hyperbolic, if there exists at least one co-vector for which
It is true that in the relativistic case we have to rely on the privileged co-vector in this context and therefore on a privileged Lorentz frame whose entropy density is concave in . The significance of this choice is not really understood. Indeed, we might have preferred the privileged frame to be the local rest frame of the body. In that respect it is reassuring that is often co-linear to the four-velocity as we shall see in Section 4.3 below; but not always! A better understanding is needed.
Note that in the non-relativistic case the only time-like co-vector is , a constant vector. In that case all the above-mentioned complications are absent: Concavity of the one and only entropy density is equivalent to symmetric hyperbolicity, see Section 3 above.
Also note that the requirement (65) of symmetric hyperbolicity ensures finite characteristic speeds, not necessarily speeds smaller than c as we might have wished. [In this respect we may be tempted to replace Friedrich’s definition of symmetric hyperbolicity by one of our own making, which might require (65) to be true for all time-like co-vectors – instead of at least one. If we did that, we should anticipate the whole problem of speeds greater than c. Indeed, we recall the characteristic equation (15) which – for our system (64) – reads
If (65) were to hold for all time-like co-vectors , we could now conclude that is space-like, or light-like, so that holds. Thus (12) would imply . This is a clear case of assuming the desired result in a disguise and we do not follow this path.]
© Max Planck Society and the author(s)