  Existing theories explain spin glass transition in terms of a phase
transition and order parameters, and assume the existence of a distinct spin
glass phase. In addition to problems related to clarifying the nature of this
phase, the common challenge is to explain profound dynamic effects. Here, we
propose that the main experimental results of spin glass transition can be
understood in an entirely dynamic picture, without a reference to a distinct
spin glass phase, phase transition and order parameters. In this theory, the
susceptibility cusp at the glass transition temperature is due to the dynamic
crossover between the high-temperature relaxational and low-temperature spin
wave, or elastic, regime. The crossover takes place when $t=\tau$, where $t$ is
observation time and $\tau$ is relaxation time. Time-dependent effects,
inconsistent with the phase transition approach, and the logarithmic increase
of $T_g$ with field frequency in particular, originate as the immediate
consequence of the proposed picture. We comment on the behavior of non-linear
susceptibility. In our discussion, we explore similarities between the spin and
structural glass transitions.
