  It is estimated that the Weibel instability is not generally an effective
mechanism for generating ultrarelativistic astrophysical shocks. Even if the
upstream magnetic field is as low as in the interstellar medium, the shock is
mediated not by the Weibel instability but by the Larmor rotation of protons in
the background magnetic field. Future simulations should be able to verify or
falsify our conclusion.
