I understand that this question might arise from multiple misconceptions and misunderstanding, I'm not from a physics background. If in a vacuum we had two particles A and B moving towards each other at sufficient speed. The way I understand things is that as the speed gets closer to the speed of light, the energy of the particle grows asymptotically.
Now here's what I don't understand: what happens if the speed is sufficient enough that from A's "perspective", particle B should have so much energy that the energy density is such that B should collapse into a black hole? I assume this doesn't happen, since from B's "perspective", the same should be happening to A.
But then what am I missing? Does kinetic energy not contribute to energy density? Or would the very high required speeds be so great that one particle could only perceive the other as a wave so spread out that the energy density isn't actually high enough?
EDIT: The linked question doesn't answer my question. I explicitely state in my question that I do understand that the accepted answer's description of what would happen would happen, but this answer doesn't tell me why, which is what my question is about. Except the last one (which I have no idea if I should trust), other answers are not satisfactory in my opinion, I know the particles wouldn't turn into a black hole, but my questions about kinetic energy's relation to relativity and energy density still remains.
