The light cone defines the path of massless photons. It is represented here as a cone, but in fact it is a sphere expanding out into space.

Suppose an event at the origin; for example, a flash of light.

The photons, with zero mass, carrying the event information with the speed of light, move along the cone's outer surface, 45 degrees from the xy plane. With increasing time, a component of the photon moves along the x-y surface.

Particles with a non-zero mass move at a speed less than that for light, and are represented by a line through the interior of the cone, less than 45 degrees from the timeline. It will move less on the x-y surface in a similar time interval.

The past cone is for massless photons carrying information about some events in the past, to the observer at the central point. This would be the shortest possible time for the observer to get this information.

Heavy particles would take longer and their line would be through the interior of the cone.
A very heavy particle would be represented by a line almost along the time axis of the cone.

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