candidate: It has been argued that to conserve energy and momentum , any black holes created in a collision between an UHECR and local matter would necessarily be produced moving at relativistic speed with respect to the Earth , and should escape into space , as their accretion and growth rate should be very slow , while black holes produced in colliders ( with components of equal mass ) would have some chance of having a velocity less than Earth escape velocity , 11.2 km per sec , and would be liable to capture and subsequent growth .