What is a light cone?

In physics — especially in relativity — a light cone is a way of visualizing how light (and information) travels through spacetime.

Imagine you are at a specific event in spacetime — a single point in space and time (for example, “you snapping your fingers right now”). From that event, light travels outward in all directions at the speed of light (c).

If we graph space on the horizontal axis and time on the vertical axis, the paths that light can take from that event form a cone shape — the light cone.

 The Two Halves of the Light Cone

 Future and Past Light Cones

Future Light Cone — All events that can be reached by light (or anything moving slower than light) emitted from the original event.
 ➤ Represents all possible effects of the event.

Past Light Cone — All events that could have sent light (or information) to the original event.
 ➤ Represents all possible causes of the event.

Events outside the cone are called spacelike separated — they cannot affect or be affected by the event, because doing so would require something to move faster than light, which relativity forbids.

Why It Matters

Causality: The light cone defines the boundary between what can and cannot influence an event.

Relativity: Every observer, no matter how they move, will agree on which events are inside or outside a given light cone — it’s an invariant structure.

Cosmology: The concept helps define the observable universe — the region of space from which light has had time to reach us since the Big Bang.

Can People Be Considered Light Cones?

The Human Worldline

  1. Every person occupies a “worldline” in spacetime — the path traced through space and time by their existence.
  2. Each moment of your life has its own light cone — your past and future possibilities.
  3. Stacking all these cones along your worldline forms a causal envelope — the region of spacetime you can affect or be affected by.
  4. You are not just a body in space, but a process in spacetime — a continuous chain of light cones bounded by causality.

Letters to Leading Physicists

Open Correspondence: Consciousness and Light

Dear Edward Witten, Lee Smolin, Juan Maldacena, Gail Hanson, Lisa Randall,

Why would consciousness, which illuminates the universe around it, choose to go dark?

Responses from the Great Minds of Physics

The author will be corresponding with some of the world’s leading physicists to explore this question.
 Check back soon to read their responses and reflections.