In this series we are exploring the weird and wonderful world of astronomy jargon! You won’t ever stop reading about today’s topic: event horizons!
An event horizon is the ultimate wall. It’s a boundary that separates one region of the universe from another. This separation is so complete that with event horizons it becomes utterly impossible for events on one side of the boundary to ever interact with or influence anything on the other side.
Event horizons appear in two situations in astronomy. The first is in cosmology and the expansion of the universe. Since the universe expands at a constant rate, that means that more distant objects appear to recede faster and faster away from us. At some distance, that recession exceeds the speed of light. This is not big deal, by the way, since the speed of light limitations in special relativity only apply to measurements of nearby objects.
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This creates an event horizon. If an extremely distant galaxy emits light right now, that light will never reach us, because the light can’t overcome the expansion of the universe. We can still see the galaxy, at least for awhile, because we are looking at light emitted when the galaxy was closer to us. So we are completely and totally separated from that galaxy. Whatever’s going on there, we’ll never know about it, and any aliens living there will never know about us.
The other situation that creates event horizons is black holes. When matter collapses into too small a volume, nothing can prevent a catastrophic gravitational collapse that pulls that matter into an infinitely tiny point, called a singularity. The gravity of that singularity is exceptionally strong, so much so that if you get too close to it the singularity will pull you in faster than the speed of light.
Once again, that creates an event horizon. Every black hole has one, and it’s the boundary where gravity becomes too strong to overcome. If you were to fall beyond the event horizon, you can still be influenced by events outside the black hole, because the light can follow you in afterwards, but you cannot influence anything in the outside world. You’re cut off, forever trapped inside the black hole.