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2 years ago in Astronomy By Trisha

What is the event horizon, and can light escape it?

The popular science description of the event horizon as a "point of no return" is clear, but in my GR studies, it's a coordinate-dependent boundary. With the EHT images now showing black hole shadows, I'm trying to reconcile the mathematical definition with what we're actually observing. Is the shadow edge precisely the event horizon, and how does the infalling matter's behavior inform us about its properties?

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By Vinod D Answered 2 years ago

This is an excellent point of confusion. Theoretically, the event horizon is a null surface in spacetime, a one-way membrane. Observationally, we don't see the horizon itself. What the Event Horizon Telescope images is the "shadow" a dark region encircled by a bright ring of lensed light from the accretion disk. This shadow is about 2.5 times larger than the actual event horizon, defined by the photon capture orbit. The infalling matter we see is just outside the horizon, and its extreme redshift and orbital dynamics are our primary probes of the horizon's gravitational influence.

 

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