Event Horizon
The boundary surrounding a black hole beyond which escape velocity exceeds the speed of light, preventing any matter or signal from escaping.
Governing formula
R_s = 2 G M / c²
SI unit
km or m (Boundary Radius)
In depth
Acts as a one-way causal membrane in general relativity. To outside observers, time dilation approaches infinity at the event horizon, causing infalling matter to appear frozen at the boundary.
Examples in the real world
The Event Horizon Telescope (EHT) collaboration imaging the shadow boundary of the supermassive black hole in galaxy M87.