A boundary, not a surface
The event horizon is a causal boundary in spacetime. It is not a solid shell, and no physical information travels outward across it.
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RELATIVITY & SPACE · INTERACTIVE LAB 55
Black-hole spacetime visualiser
Explore horizons, curved trajectories, gravitational clocks and light paths around a Schwarzschild or rotating black hole.Event horizonPhoton sphere · 1.5rₛISCO · 3rₛ
Schwarzschild radius29.5 km
Photon sphere44.3 km
ISCO88.6 km
Clock rate0.894
Redshift0.118
Far and near clocks
Radius–time diagram
Light and orbits
The photon sphere supports unstable circular light paths. For a Schwarzschild black hole, stable circular massive-particle orbits lie outside the ISCO.
Gravitational clocks
A stationary clock nearer the horizon accumulates less proper time than a clock far away. No stationary observer can remain at or below the horizon.
Model note: Horizon scales and stationary Schwarzschild clock rates are calculated. Particle paths, the Kerr view and lensing are educational approximations rather than a full numerical geodesic solver. The grid is not a literal physical surface.