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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:

If a mass moves close to the speed of light, does it turn into a black hole?

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").

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  • $\begingroup$ According to arxiv.org/abs/0709.1107, “Once we reach collision energies exceeding the Planck mass ... collisions of particles can form black holes.” $\endgroup$ – G. Smith Jun 16 at 5:23
  • $\begingroup$ Closely related question: physics.stackexchange.com/q/3584 $\endgroup$ – G. Smith Jun 16 at 5:32
  • $\begingroup$ @G.Smith Thanks for the great links! $\endgroup$ – safesphere Jun 16 at 5:34
  • $\begingroup$ It is worth taking at look at the mathematics that underpin the justification of colliders over fixed target accelerators in the high energy world. If you were trying to create the situation artificially you would really, really prefer to do a nearly or exactly symmetric case rather that a fixed target case. $\endgroup$ – dmckee Jun 16 at 23:44

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