Measuring the thermalization time

A new method of measuring the pressure anisotropy in the early stage of heavy-ion collisions is discussed. It is shown that the transverse momentum spectra, elliptic flow and interferometry radii are not sensitive to the early anisotropy. We propose the directed flow as a measure of the early transverse and longitudinal pressures. Calculations indicate that the transverse and longitudinal pressures equilibrate in less than 0.25 fm/c.