* t0 is the proper time between events A and B for a slow-ticking observer within the gravitational field,

* tf is the coordinate time between events A and B for a fast-ticking observer at an arbitrarily large distance from the massive object (this assumes the fast-ticking observer is using Schwarzschild coordinates, a coordinate system where a clock at infinite distance from the massive sphere would tick at one second per second of coordinate time, while closer clocks would tick at less than that rate),

* G is the gravitational constant,

* M is the mass of the object creating the gravitational field,

* r is the radial coordinate of the observer (which is analogous to the classical distance from the center of the object, but is actually a Schwarzschild coordinate),

* c is the speed of light, and

* r0 = 2GM / c2 is the Schwarzschild radius of M. If a mass collapses so that its surface lies at less than this radial coordinate (or in other words covers an area of less than 4πG2M2 / c4), then the object exists within a black hole.