Abstract We study the probability that a horizon appears when concentric shells of matter collide. by computing the horizon wave-function of the system. We mostly consider the collision of two ultra-relativistic shells. both shrinking and expanding. https://www.roneverhart.com/HP-EliteDisplay-S14-14-FHD-LED-Monitor-1080p-Portable-Display-60Hz-5ms-16-9-200-cd-m-700-1-USB-C-3HX46A8/