Upper Bound on the Speed of Sound in Nuclear Matter from Transport

We point out that there is an upper bound on the speed of sound squared givenby c_s^2 ≤ 0.781 valid for all known systems described by relativistictransient hydrodynamics where calculations of certain ratios of hydrodynamictransport coefficients can be performed from first principles. Assuming thisbound is valid for ultradense matter implies that the maximum mass of isolated(non-rotating) neutron stars cannot be larger than 2.7 solar masses.