  Understanding physics in domains of critical (quantum unstable) fields
requires investigating the classical and quantum particle dynamics at the
critical acceleration, $\dot u \to 1$ [natural units]. This regime of physics
remains today experimentally practically untested. Particle and light collision
experiments reaching critical acceleration are becoming feasible, in particular
applying available high intensity laser technology. Ultra-relativistic heavy
ion collisions breach the critical domain but are complicated by the presence
of much other physics. The infamous problem of radiation reaction and the
challenging environment of quantum vacuum instability arising in the high field
domain signal the need for a thorough redress of the present theoretical
framework.
