  Linear eigenmodes of a spherically symmetrical ultra-relativistic blast wave
(the Blandford-McKee, BMK, solution) are calculated. The BMK solution is shown
to be stable and strongly non-universal. It is stable because all the
eigenmodes decay. Non-universality of the BMK solution (BMK is not an
intermediate asymptotic) is a consequence of causality. In terms of eigenmodes
-- some eigenmodes decay too slowly. For each degree l, there exists an
eigenmode which decays at the smallest rate. The amplitude of this eigenmode,
defined as relative perturbation of energy behind the shock front, is constant
at early time. Later, when the blast wave slows down to Gamma = l/10, an
oscillatory decay commences, and the amplitude drops to less than 10% of the
initial value at Gamma = l/100. The non-universality is surprisingly strong.
Near the end of the ultra-relativistic stage, perturbations with l<200, that is
more than 10,000 different harmonics, are suppressed by less than a factor of
10. Spherical symmetry is only reached by the time the blast wave slows down to
non-relativistic velocities, when the Sedov/Taylor/von Neumann solution sets
in.
