Penrose diagram

These points and boundaries represent conformal infinity for spacetime, which was first introduced by Penrose in 1963.

So, a Penrose diagram can be used as a concise illustration of spacetime regions that are accessible to observation.

The diagonal boundary lines of a Penrose diagram correspond to the region called "null infinity", or to singularities where light rays must end.

Penrose diagrams are frequently used to illustrate the causal structure of spacetimes containing black holes.

Penrose diagrams are often used to illustrate the hypothetical Einstein–Rosen bridge connecting two separate universes in the maximally extended Schwarzschild black hole solution.

The Einstein–Rosen bridge closes off (forming "future" singularities) so rapidly that passage between the two asymptotically flat exterior regions would require faster-than-light velocity, and is therefore impossible.

In addition, highly blue-shifted light rays (called a blue sheet) would make it impossible for anyone to pass through.

The maximally extended solution does not describe a typical black hole created from the collapse of a star, as the surface of the collapsed star replaces the sector of the solution containing the past-oriented white hole geometry and other universe.

While the basic space-like passage of a static black hole cannot be traversed, the Penrose diagrams for solutions representing rotating and/or electrically charged black holes illustrate these solutions' inner event horizons (lying in the future) and vertically oriented singularities, which open up what is known as a time-like "wormhole" allowing passage into future universes.