Light cone

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In special relativity, a light cone (or null cone) is
the surface describing the temporal evolution of a flash of light in Minkowski spacetime.

This can be visualized in 3-space if the two horizontal axes are chosen to be
spatial dimensions, while the vertical axis is time.[3]



If using a system of units where the speed of light in vacuum is defined as exactly 1,
for example if space is measured in light-seconds and
time is measured in seconds, then, provided the time axis is drawn orthogonally to
the spatial axes, as the cone bisects the time and space axes,
it will show a slope of 45ยฐ, because light travels a distance of one light-second in
vacuum during one second.

Since special relativity requires the speed of light to be equal in every inertial frame,
all observers must arrive at the same angle of 45ยฐ for their light cones.

Commonly a Minkowski diagram is used to illustrate
this property of Lorentz transformations.

Elsewhere, an integral part of light cones is the region of spacetime outside
the light cone at a given event (a point in spacetime).
Events that are elsewhere from each other are mutually unobservable,
and cannot be causally connected.