What is the speed of light? And why nothing goes faster

The speed of light in a vacuum is exactly 299,792,458 meters per second, a little under 300,000 kilometers a second, or about 671 million miles an hour. Physicists call it c. It is exact because, since 1983, the meter itself has been defined by it. It is also the universe's speed limit: no object, signal or piece of information can travel through space faster.

In short

• Light crosses from the Moon in about 1.3 seconds and from the Sun in about 8 minutes 19 seconds.
• The number is fixed by definition; what scientists measure now is the length of the meter.
• Relativity makes c a limit because pushing anything with mass toward it takes ever more energy, without end.

How fast is light, in everyday terms?

Fast enough to circle the Earth about 7.5 times in one second, but not so fast that the delay disappears over large distances. Because light takes time to arrive, everything anyone sees is a little in the past, and the farther away it is, the older the view. Dividing a distance by c gives the delay:

Trip	Distance	Time for light
New York to London, in a straight line	5,570 km	0.019 seconds
Earth to the Moon	384,400 km	1.28 seconds
Earth to the Sun	149.6 million km	8 minutes 19 seconds
Earth to Mars, at its closest and farthest	about 55 to 400 million km	about 3 to 22 minutes
To the nearest star beyond the Sun, Proxima Centauri	4.24 light-years	4.24 years

Take the Sun: 149,600,000 km ÷ 299,792 km per second = 499 seconds, which is 8 minutes 19 seconds. If the Sun vanished, Earth would keep seeing it for that long. The Mars delay is why rovers there are not driven with a joystick, and why NASA plans for crews on a Mars mission to face a communication delay of up to 20 minutes one way. A light-year, the distance light travels in a year, is about 9.46 trillion kilometers, and it is the natural ruler for the universe: light from distant galaxies set out billions of years ago.

How was the speed of light first measured?

For most of history, people assumed light arrived instantly. In 1676 the Danish astronomer Ole Rømer noticed that the eclipses of Io, one of Jupiter's moons, came several minutes later than predicted when Earth was farther from Jupiter, and earlier when it was closer. He reasoned that light takes time to cross the extra distance, and estimated about 22 minutes to cross the full width of Earth's orbit. The true figure is closer to 16 minutes 40 seconds, but he had shown light has a finite speed, and his timing implied a speed of a little over 200,000 kilometers a second.

The first measurement on the ground came in 1849, when the French physicist Hippolyte Fizeau sent light through the gaps of a spinning toothed wheel to a mirror 8.6 kilometers away and back. At the right speed, the returning light was blocked by the next tooth, and from the wheel's speed he worked out a figure only about 5% too high. Later experiments with rotating mirrors, then microwaves and lasers, narrowed the figure down until the uncertainty was smaller than the uncertainty in the meter itself.

Why is the number exact?

By 1983, the speed of light was known better than the length of the meter, which was then defined using the wavelength of light from krypton atoms. So the General Conference on Weights and Measures turned it around. Its 1983 resolution defined the meter as the distance light travels in a vacuum in 1/299,792,458 of a second. Since then, c is exactly 299,792,458 meters per second, the value listed by NIST with zero uncertainty, and a better experiment would refine the meter, not c. The 2019 revision of the metric system kept that arrangement.

Why can't anything go faster than light?

In 1905, Albert Einstein built his special theory of relativity on two ideas: the laws of physics are the same for everyone moving at a steady speed, and light in a vacuum moves at the same speed for every observer, however fast they are moving. His paper, On the Electrodynamics of Moving Bodies, shows that these two ideas force time and space to stretch and shrink so that c always comes out the same. The fuller story is in the theory of relativity.

One consequence is that speed doesn't add up simply. The amount by which time slows and an object's energy grows is set by the Lorentz factor, γ (gamma), which equals 1 divided by the square root of 1 − v²/c², where v is the speed:

Speed	Lorentz factor γ	What it means for a clock on board
10% of c	1.005	Runs 0.5% slow
50% of c	1.155	Runs about 13% slow
90% of c	2.29	Ticks once for every 2.29 ticks on Earth
99% of c	7.09	Ticks once for every 7
99.9% of c	22.4	Ticks once for every 22
100% of c	infinite	Impossible for anything with mass

The energy of motion is (γ − 1) times the object's mass times c². Going from 90% to 99% of c multiplies it almost five times, from 1.29 to 6.09 units, and each further step costs more than the last. Reaching c itself would take infinite energy, so anything with mass can approach the speed of light but never reach it. Light gets there only because photons, its particles, have no mass and can't travel at any other speed. The limit also protects cause and effect: a signal faster than light would, for some observers, arrive before it was sent.

What about things that seem faster than light?

Several effects look like exceptions and aren't.

Light slowing in glass and water. Light in glass travels at about 200,000 kilometers a second, and in water about 225,000. Light's electric field sets the electrons in the material moving, and the waves they give off combine with the original into a wave that travels more slowly; between atoms, nothing is slower than c. That slowing is why light bends at a surface, and in a fiber-optic cable it adds up: a signal along the straight line from New York to London would take about 27 milliseconds through glass, against 19 in a vacuum. In air the slowing is only about 0.03%, though the way air scatters light is what explains why the sky is blue.

Particles outrunning light in a material. Because light slows in water, a fast charged particle can pass through water faster than light does there. It gives off a blue glow called Cherenkov radiation, seen in nuclear reactor pools. Nothing beats c.

The expansion of space. Very distant galaxies are moving away from Earth faster than light, as measured by how quickly the distance grows. That isn't motion through space but space itself stretching, which relativity allows, and no signal outruns light in the process.

Quantum entanglement. Measurements on linked particles far apart show correlations that appear instantly, but they can't be used to send a message, so no information travels faster than light.

Spots and shadows. Sweep a laser across the Moon and the dot can cross its face faster than c. Nothing travels from one side to the other; each part of the dot is new light from the laser.

In 2011, an experiment called OPERA reported neutrinos arriving slightly faster than light between Geneva and Italy. The team traced the result to a loose fiber-optic cable in its timing system, and repeat measurements found the neutrinos within the limit. Every test since has upheld c. That is how science treats a surprising result: check it, and keep the theory only if it survives.

Check yourself

• Without looking back, explain why the speed of light now has no measurement uncertainty.
• Why does it take infinite energy for a spaceship to reach c, but not for light?
• Why don't distant galaxies receding faster than light break relativity?

Sources

• Speed of light in vacuum, CODATA value — National Institute of Standards and Technology
• Resolution 1 of the 17th CGPM, definition of the metre — International Bureau of Weights and Measures, 1983
• On the Electrodynamics of Moving Bodies — Albert Einstein, 1905, English translation at Fourmilab
• Hazard: distance from Earth — NASA Human Research Program
• Speed of light — Wikipedia