Speed of Light: Constants, Measurements, and Physical Implications

Light is the fastest known entity in the universe, serving as a fundamental constant that governs the laws of physics. Because light travels at a finite speed, there is a measurable delay between when an event occurs and when we observe it. For instance, sunlight takes between 8 minutes and 10 seconds to 8 minutes and 27 seconds to reach Earth, depending on the time of year.

Key Facts

Three pairs of coordinate axes are depicted with the same origin A; in the green frame, the x-axis is horizontal and the ct-axis is vertical; in the red frame, the x′-axis is slightly skewed upwards, and the ct′-axis slightly skewed rightwards, relative to the green axes; in the blue frame, the x″-axis is somewhat skewed downwards, and the ct″-axis somewhat skewed leftwards, relative to the green axes. A point B on the green x-axis, to the left of A, has zero ct, positive ct′, and negative ct″. — Event A precedes B in the red frame, is simultaneous with B in the green frame, and follows B in the blue frame.

Exact Speed: In a vacuum, light travels at exactly 299,792,458 metres per second.
Universal Limit: The speed of light in a vacuum (denoted as c) acts as an upper limit for all speeds.
Defining the Metre: Since 1983, the metre has been defined by the distance light travels in a vacuum during 1/299,792,458 of a second.
Medium Influence: Light slows down when passing through transparent materials like glass or air.
Astronomical Scale: Light takes approximately 1.3 seconds to travel from the Moon to Earth and 4.2 years to reach us from the nearest star.

The Exact Value and Unit Definitions

The diameter of the moon is about one quarter of that of Earth, and their distance is about thirty times the diameter of Earth. A beam of light starts from the Earth and reaches the Moon in about a second and a quarter. — A beam of light is depicted travelling between the Earth and the Moon in the time it takes a light pulse to move between them: 1.255 seconds at their mean orbital (surface-to-surface) distance. The relative sizes and separation of the Earth–Moon system are shown to scale.

For decades, scientists measured the speed of light experimentally. However, in 1983, the 17th General Conference on Weights and Measures (CGPM) redefined the metre to ensure higher reproducibility. By fixing the speed of light as a defined constant, any future improvements in measurement accuracy will refine the length of the metre rather than change the value of c.

This precision extends to astronomy. In 2012, the astronomical unit (AU)—the mean distance from the Earth to the Sun—was redefined as exactly 149,597,870,700 metres, further aligning astronomical measurements with the fixed speed of light.

Approximate Speed of Light and Travel Times
Unit/Distance	Value/Time
Metres per second	299,792,458
Kilometres per hour	~1,080,000,000
Miles per second	~186,000
From Moon to Earth	1.3 seconds
From Sun to Earth (1 AU)	8.3 minutes
Across the Milky Way	87,400 years
From Andromeda Galaxy to Earth	2.5 million years

Propagation in Different Media

Measurement of the speed of light from the time it takes Io to orbit Jupiter, using eclipses of Io by Jupiter's shadow to precisely measure its orbit.

While c is the speed of light in a vacuum, light slows down when it travels through transparent materials. This phenomenon is described by the refractive index (n), which is the ratio between the speed of light in a vacuum and its speed in a specific material (n = c / v).

For example, visible light in glass has a refractive index of approximately 1.5, meaning it travels at roughly 200,000 km/s. In air, the refractive index is about 1.0003, making light travel approximately 90 km/s slower than it would in a vacuum.

A modulated wave moves from left to right. There are three points marked with a dot: A blue dot at a node of the carrier wave, a green dot at the maximum of the envelope, and a red dot at the front of the envelope. — The blue dot moves at the speed of the ripples, the phase velocity; the green dot moves with the speed of the envelope, the group velocity; and the red dot moves with the speed of the foremost part of the pulse, the front velocity.

Historical Determination of the Speed of Light

A light ray passes horizontally through a half-mirror and a rotating cog wheel, is reflected back by a mirror, passes through the cog wheel, and is reflected by the half-mirror into a monocular. — Diagram of the Fizeau apparatus:Light sourceBeam-splitting semi-transparent mirrorToothed wheel-breaker of the light beamRemote mirrorTelescopic tube

The realization that light does not travel instantaneously began in 1676 with Ole Rømer. By observing the eclipses of Jupiter's moon, Io, Rømer noticed that the orbital periods appeared shorter when Earth was moving toward Jupiter and longer when moving away. He concluded that light travels at a finite speed, estimating it took 22 minutes to cross the diameter of Earth's orbit.

A diagram of a planet's orbit around the Sun and of a moon's orbit around another planet. The shadow of the latter planet is shaded. — Rømer's observations of the occultations of Io from Earth

Following Rømer, other scientists refined these measurements. Christiaan Huygens estimated the speed at 220,000 km/s. Later, more sophisticated methods were developed, including the use of rotating mirrors and vacuum chambers. Between 1930 and 1935, Michelson, Pease, and Pearson used a one-mile vacuum chamber to achieve an accuracy of ±11 km/s.

One of the last and most accurate time of flight measurements, Michelson, Pease and Pearson's 1930–1935 experiment used a rotating mirror and a one-mile (1.6 km) long vacuum chamber which the light beam traversed 10 times. It achieved accuracy of ±11 km/s.

Physical Implications and Relativity

A box with three waves in it; there are one and a half wavelength of the top wave, one of the middle one, and a half of the bottom one. — Electromagnetic standing waves in a cavity

The finite and constant nature of the speed of light is a cornerstone of special relativity. As an object's velocity approaches c, the Lorentz factor (γ) increases, approaching infinity. This implies that light serves as a universal speed limit for the propagation of information and matter.

γ starts at 1 when v equals zero and stays nearly constant for small v, then it sharply curves upwards and has a vertical asymptote, diverging to positive infinity as v approaches c. — The Lorentz factor γ as a function of velocity. It starts at 1 and approaches infinity as v approaches c.

The finiteness of light speed also leads to aberration of light, where a moving telescope perceives light from a distant source as coming from a slightly different location due to the time it takes for the light to travel.

A star emits a light ray that hits the objective of a telescope. While the light travels down the telescope to its eyepiece, the telescope moves to the right. For the light to stay inside the telescope, the telescope must be tilted to the right, causing the distant source to appear at a different location to the right. — Aberration of light: light from a distant source appears to be from a different location for a moving telescope due to the finite speed of light.

Frequently Asked Questions

Schematic of the working of a Michelson interferometer. — An interferometric determination of length. Left: constructive interference; Right: destructive interference.

Hendrik Lorentz (right) with Albert Einstein (1921)

Why is the speed of light considered a constant?

In a vacuum, the speed of light is a fundamental constant of nature. To ensure global scientific consistency, it was formally fixed by definition in 1983, allowing the metre to be defined based on this constant.

Does light always travel at the same speed?

No. While it is constant in a vacuum, light slows down when passing through media such as air, water, or glass. This reduction in speed is measured by the material's refractive index.

How did Ole Rømer prove light has a finite speed?

Rømer observed that the timing of Io's eclipses by Jupiter's shadow varied depending on the distance between Earth and Jupiter, proving that light takes time to travel across space.

Can anything travel faster than the speed of light?

According to the laws of physics and special relativity, the speed of light in a vacuum (c) is the absolute upper limit for the speed of any matter or signal.

What is the difference between the speed of light and the speed of electricity?

The speed of electricity refers to the speed of electromagnetic waves traveling through wire cables, which is slower than the speed of light in a vacuum.