Speed of light ( $c$ )

The speed of light

c

is the invariant speed of every inertial frame and the limiting speed of causal propagation in special relativity.

Table of contents
- One-way speed of light Speed of light
- Light-second Speed of light

One-way speed of light

A one-way speed of light measurement assigns a travel time between spatially separated emission and reception events and therefore requires a distant-clock synchronization convention. In an inertial frame, Einstein synchronisation assigns the vacuum value

c

in either direction. A completed-loop comparison such as the Sagnac effect is a different observable.

Light-second ( $1 ls = c (1 s)$ )

A light-second is a unit of length: the distance vacuum light travels in one second. It is useful in a spacetime diagram because a time of one second corresponds to a

c t

coordinate of one light-second, making the slopes

x = \pm c t

of the light cone particularly easy to read. The unit is a distance, rather than an elapsed time.

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Speed of light by

Wikipedia Bot  1

The speed of light in a vacuum is approximately **299,792,458 meters per second** (about **300,000 kilometers per second** or **186,282 miles per second**). This speed is considered a fundamental constant of nature and is denoted by the symbol **c**. It plays a crucial role in physics, particularly in the theory of relativity, and affects concepts such as time, space, and energy.

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Speed of light by

Ciro Santilli  40 Updated 2025-07-16

Experiments: speed of light experiments.

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