Publication Type
Journal
Journal Name
Physical Review Letters
Publication Date
Page Number
031103
Volume
114
Issue
3
Abstract
It has been conjectured that the velocity of sound in any medium is smaller than the velocity of light in
vacuum divided by sqrt(3). Simple arguments support this bound in nonrelativistic and/or weakly coupled
theories. The bound has been demonstrated in several classes of strongly coupled theories with gravity duals and is saturated only in conformal theories. We point out that the existence of neutron stars with masses around two solar masses combined with the knowledge of the equation of state of hadronic matter at “low” densities is in strong tension with this bound.
