  Fundamental understanding of strongly-interacting systems necessarily
involves collective modes, but their nature and evolution is not generally
understood in dynamically disordered and strongly-interacting systems such as
liquids and supercritical fluids. We report the results of extensive molecular
dynamics simulations and provide direct evidence that liquids develop a gap in
solid-like transverse spectrum in the reciprocal space, with no propagating
modes between zero and a threshold value. In addition to the liquid state, this
result importantly applies to the supercritical state of matter. We show that
the emerging gap increases with the inverse of liquid relaxation time and
discuss how the gap affects properties of liquid and supercritical states.
