The velocity of magnetohydrodynamic waves in general relativity

The energy-momentum tensor for a perfect fluid with infinite conductivity is modified slightly by attributing a proper energy content to a field of tension, as proposed by Cattaneo, for a perfect nonconducting fluid, and it is shown that with this modification the velocities of the hydrodynamic waves in the fluid do not exceed C. This contrasts with the standard theory in which a lower limit must be placed on the compressibility of the fluid to prevent the velocity of the ‘fast’ hydrodynamic wave fron exceeding c. The velocity of the Alfvén wave is changed only slightly and is always less than c as in the standard theory.