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## Main Question or Discussion Point

How reasonable would it be to model near-colliding of ultra-relativistic black holes by the collision of pp waves? Wkiki, for instance, says that the Aichelburg-Sexl ultraboost space-time models the space-time near an ultra-relativistic black hole, and that it's a type of pp wave spacetime, though it's not plane symmetric. This suggests that a pair of colliding pp waves (modelled on the Aichelburg-Sexl solution) might be a good model for a "near-miss" collision of ultra-relativistic black holes.

A related question is if we know anything definite about the results of such a collision. I gather there's quite a lot written about colliding pp waves, but it's unclear if anything that's been written would apply to this hypothetical scneario.

A related question is if we know anything definite about the results of such a collision. I gather there's quite a lot written about colliding pp waves, but it's unclear if anything that's been written would apply to this hypothetical scneario.