Imagine an ultra relativistic object with a very small rest mass, but an extremely high kinetic energy in our reference frame. So high that a stationary object with the same amount of energy would collapse into a black hole. However, our moving object obviously doesn't collapse according to several earlier answers. For example:
Now imagine a second object that is exactly the same, except it is moving in the opposite direction. The total combined momentum of both is zero in our reference frame. The objects collide head on with each other. The entire kinetic energy of their movement is transferred into the energy of something else, such as heat, explosion, etc.
Would this collision create a black hole?
My intuition is that perhaps it would, as I can't see why not, but I would defer the answer to the experts here. My thinking is that, no matter how spectacular the explosion may be, it would expand from the point of collision slower than light, while gravity (spacetime curvature) propagates at the speed of light and outruns the explosion. So, on one hand, the event horizon should form before the explosion can get out. On the other hand, however, this is not a static case, so the Schwarzschild solution does not describe it precisely.
It would be great if someone could shed some light on this case and how its different aspects may change the outcome (e.g. an elastic vs. inelastic collision, if such a collision can even be "elastic").