  Collisionless shocks in plasmas play an important role in space physics
(Earth's bow shock) and astrophysics (supernova remnants, relativistic jets,
gamma-ray bursts, high energy cosmic rays). While the formation of a fluid
shock through the steepening of a large amplitude sound wave has been
understood for long, there is currently no detailed picture of the mechanism
responsible for the formation of a collisionless shock. We unravel the physical
mechanism at work and show that an electromagnetic Weibel shock always forms
when two relativistic collisionless, initially unmagnetized, plasma shells
encounter. The predicted shock formation time is in good agreement with 2D and
3D particle-in-cell simulations of counterstreaming pair plasmas. By predicting
the shock formation time, experimental setups aiming at producing such shocks
can be optimised to favourable conditions.
