  We present new Neutron Spin Echo (NSE) results and a revisited analysis of
historical data on spin glasses, which reveal a pure power-law time decay of
the spin autocorrelation function $s(Q,t) = S(Q,t)/S(Q)$ at the glass
temperature $T_g$, each power law exponent being in excellent agreement with
that calculated from dynamic and static critical exponents deduced from
macroscopic susceptibility measurements made on a quite different time scale.
It is the first time that this scaling relation involving exponents of
different physical quantities determined by completely independent experimental
methods is stringently verified experimentally in a spin glass. As spin glasses
are a subgroup of the vast family of glassy systems also comprising structural
glasses, other non-crystalline systems living matter the observed strict
critical scaling behaviour is important. Above the phase transition the
strikingly non-exponential relaxation, best fitted by the Ogielski (power-law
times stretched exponential) function, appears as an intrinsic, homogeneous
feature of spin glasses.
