In the quantum world, electrons can behave like waves. If you create the right conditions for them, these waves start to feel something akin to gravity. In crystals known as Weyl semimetals, electrons move as if they were massless, and their energy depends on direction, like a tilted cone. When the tilt changes smoothly from point to point, an invisible boundary emerges for the electron wave. This is an analog of a black hole's event horizon. But instead of light and spacetime, here we have electronic states and the crystal's internal structure.

The authors compared two types of such semimetals. In the first, a wave packet, a compact group of electron waves, hits the horizon and is completely reflected, as if bouncing off a mirror wall. In the second, the packet passes through, but with great difficulty. The most interesting thing happens when the packet starts with zero momentum. It slows down sharply, almost freezing at the horizon, and spends an unusually long time there. This delay comes at a cost: part of the probability of finding the electron at that spot simply disappears. The longer the delay, the greater the loss. This effect resembles what happens to information near a real black hole. But here, it can be studied in the lab by tweaking the crystal's properties.