The **Schwarzschild radius** (sometimes historically referred to as the **gravitational radius**) is the distance from the center of an object such that, if all the mass of the object were compressed within that sphere, the escape speed from the surface would equal the speed of light. An example of an object smaller than its Schwarzschild radius is a black hole. Once a stellar remnant collapses within this radius, light cannot escape and the object is no longer visible. It is a characteristic radius associated with every quantity of mass. The *Schwarzschild radius* was named after the German astronomer Karl Schwarzschild who calculated this exact solution for the theory of general relativity in 1915.

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