A Penrose Diagram shows the global causal structure of a spacetime.
It is a spacetime diagram in which all light rays travel in 45 degree
angles. Therefore massive particles must travel on trajectories that
lie within the light cone which sits at every point in the diagram.
In the following diagram the causal structure of a spherically symmetric
"Schwarzchild" type black hole is shown. Only radial and time directions
are represented while angular directions are suppressed. Once inside
the event horizon, the only way for a trajectory to escape to future infinity
is if it travelled faster than light which is not possible according to
the laws of special relativity. Therefore all physical trajectories
inevitably lead to the singularity.
[A penrose diagram is not meant to accurately portray distances,
only causal structure.]
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