Using a new method of data processing in the investigation of interference correlations of identical particles, a direct proof of nonstationarity of the π^- production volume in central Mg-Mg collisions at a beam momentum of p_lab = 4.4 GeV/c per nucleon was obtained. The velocity of a source of correlated π^- meson pairs is introduced as a free parameter in the Lorentz transformed approximation formula of the interference peak. This velocity increases with increasing the velocity of a subset of π^- mesons which are chosen for the interference analysis. The source velocity is the new experimentally measurable physical variable of interference analysis, apart from space-time sizes of the source.