Science › Space, time & gravity
The speed of light is constant
1905· Idea
ProvenTested many times; part of working physics.
Light in a vacuum always travels at about 186,000 miles per second, no matter how fast you or the light source is moving.
See it on the Science map →How it works
Normally speeds add: throw a ball forward from a moving truck and it goes faster. Light does not work that way. Whether you chase a light beam or flee from it, you measure the same 299,792,458 meters per second. Einstein took this as a rule of nature in 1905. Since speed is distance divided by time, if speed cannot change, then distances and time intervals themselves must change between observers.
Why it matters
This one fact forces every strange result in relativity: slowed clocks, shrunken lengths, relative "now," and a cosmic speed limit nothing with mass can reach.
Think of it like
Like a fixed cap rate that every appraiser must use: if the cap rate cannot move, the other numbers in the valuation have to adjust instead.
How we know
Michelson and Morley (1887) found no change with direction. Later tests with lasers, fast-moving sources and astronomy confirm it to tiny precision. Since 1983 the meter has been defined using this speed.
Still a mystery
Why the speed has this particular value, and whether it was always the same in the early universe, are open questions; no change has ever been detected.
Connections (5)
▸Light is an electromagnetic wave first hinted at The speed of light is constantProven
The relationship
Maxwell's equations predicted electromagnetic waves with one speed, calculated from two electrical constants measured in labs. That speed matched light, so light was an electromagnetic wave. The equations gave a single fixed speed without saying relative to what, which raised the question of what the speed was measured against. Einstein answered: the same against everything.
How the link was found
Maxwell calculated the speed in 1862-1865. Hertz produced radio waves in 1887-1888, confirming electromagnetic waves travel at light's speed.
Why it's proven
Measured light speed equals the value from electrical constants. Radio, microwaves and X-rays all travel at the same speed.
Open questions and mysteries
Why the electrical constants have the values they do, and therefore why light has this speed, is unexplained.
▸Michelson–Morley: no "ether wind" showed the same speed in every direction for The speed of light is constantProven
The relationship
Physicists expected light to move through an invisible "ether," so Earth's motion should create an ether wind changing light's speed by direction. Michelson and Morley split a light beam into two perpendicular paths and compared them. No difference appeared. That null result is exactly what follows if light's speed is the same for every observer.
How the link was found
Albert Michelson and Edward Morley, Cleveland, Ohio, 1887, using a rotating interferometer floating on mercury.
Why it's proven
Repeated many times with lasers and, since the 2000s, rotating optical cavities that rule out any direction effect to about one part in a hundred million billion.
Open questions and mysteries
Historians debate how much the experiment directly influenced Einstein, who said it played a smaller role in his thinking than many assume.
▸Albert Einstein made a founding rule of The speed of light is constantProven
The relationship
Others saw the constant speed of light as a puzzle needing explanation by ether effects. Einstein simply declared it a law of nature: every observer measures the same speed. Making it a starting rule, rather than something to explain away, forced space and time to adjust instead.
How the link was found
Einstein's second postulate, stated in his June 1905 paper. He said he had puzzled over riding a light beam since age 16.
Why it's proven
The constancy has been tested to extreme precision, and everything Einstein deduced from it has been confirmed.
Open questions and mysteries
Some theories of quantum gravity suggest tiny variations at extreme energies; none have been observed.
▸The speed of light is constant is the foundation of Special relativityProven
The relationship
If light's speed is fixed for everyone, observers moving differently must disagree about time and distance so that distance divided by time still gives the same number. Working this through gives the Lorentz equations, from which time dilation, length contraction and E = mc² all follow.
How the link was found
Einstein, 1905. Lorentz had found the same transformation formulas in 1904, within an ether theory.
Why it's proven
The derived predictions match experiments with particles, clocks and light to high precision.
Open questions and mysteries
Settled. Whether light's speed is truly exact at the very highest energies is still being checked.
▸The speed of light is constant sets the speed limit of Causality and light conesProven
The relationship
Since nothing can outrun light, light's speed marks the edge of what can influence what. Any event can only affect places that light could reach in time. The fixed speed therefore defines the light cones, the boundaries of cause and effect, for every observer.
How the link was found
Implicit in Einstein's 1905 work; made explicit geometrically by Minkowski in 1908.
Why it's proven
No signal or influence has ever been measured faster than light.
Open questions and mysteries
Quantum entanglement shows correlations that seem instant, though they cannot carry messages; why nature works this way is debated.