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An r with a subscript s, the size of a black hole's horizon.
Also known as: Schwarzschild radius, Gravitational radius, Event horizon radius
The Schwarzschild radius is where the escape velocity reaches the speed of light, and every mass has one — the Earth's is about nine millimetres. It marks the event horizon, not a physical surface, and nothing locally unusual happens to an object crossing it.
Denotes the Schwarzschild radius: the radius of the event horizon of a non-rotating black hole of a given mass.
Contexts: Laboratory
Is defined for any mass, not only black holes; the Earth's is around nine millimetres.
Contexts: Laboratory
Marks a coordinate horizon rather than a material surface: nothing locally detectable happens to a freely falling object at the crossing.
Contexts: Laboratory
An italic lowercase letter r with a small lowercase s set below and to the right.