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Penrose Diagrams Explained (Interactive)
What is a Penrose diagram? An interactive, visual explainer of the spacetime diagrams Hawking and Penrose used to map black holes, from flat space to a collapsing star.
Last reviewed 23 May 2026 · How we research
In empty space, light rays (at 45 degrees) travel freely. The diamond represents all of space and time, squashed so that infinity sits at the edges. Everything you could ever see or reach lies within your future and past light cones.
A Penrose diagram squashes all of infinite space and time onto a finite picture, while keeping light rays at 45 degrees. It is the standard tool physicists, including Hawking, used to reason about the causal structure of black holes. This is a simplified, schematic illustration.
Penrose diagrams are one of the most powerful tools in a physicist's kit: a way to draw the entire infinite history of a universe, or a black hole, on a single finite picture. They were central to the work Roger Penrose and Stephen Hawking did together on singularities. Use the buttons below to see how the picture changes from empty space to a fully formed black hole.
How to read it
The trick of a Penrose diagram is that it keeps light rays travelling at 45 degrees no matter how much space and time are squashed. That means the causal structure, what can affect what, stays clear even as distances are distorted. Time runs roughly upward, space across. The edges represent infinity, brought in to a finite boundary.
The most striking lesson, visible when you select the black hole, is that the singularity is drawn as a horizontal line at the top: it is a moment in time, not a place in space. Once you cross the event horizon, hitting the singularity becomes as unavoidable as next week. This is the insight that the singularity theorems made rigorous.