  Laminar-turbulent intermittency is intrinsic to the transitional regime of a
wide range of fluid flows including pipe, channel, boundary layer and Couette
flow. In the latter turbulent spots can grow and form continuous stripes, yet
in the stripe-normal direction they remain interspersed by laminar fluid. We
carry out direct numerical simulations in a long narrow domain and observe that
individual turbulent stripes are transient. In agreement with recent
observations in pipe flow we find that turbulence becomes sustained at a
distinct critical point once the spatial proliferation outweighs the inherent
decaying process. By resolving the asymptotic size distributions close to
criticality we can for the first time demonstrate scale invariance at the onset
of turbulence.
