The Speed of Light

Light travels at 299,792,458 metres per second in a vacuum: nearly 300,000 kilometres per second, fast enough to travel around the Earths equator 7.5 times in a single second. This is the fastest speed possible in the universe. According to Einsteins theory of relativity, nothing with mass can ever reach the speed of light, let alone go faster. The speed of light is one of the most important numbers in physics and was the basis for the whole modern theory of space, time and gravity.

• Exact value299,792,458 m/sIn a vacuum
• Approximation300,000 km/sEasy to remember
• Around the Earth7.5 times per secondA blink
• To the Moon1.3 secondsEach way
• To the Sun8 minutes 20 secondsLight travel time
• Speed limitCannot be exceededFor anything with mass

How fast is it really?

Light is so fast that on human scales, it appears almost instantaneous. Turning on a torch lights a wall across the room essentially immediately. But on cosmic scales, light has a definite finite travel time:

• From the Sun to Earth: 8 minutes 20 seconds. If the Sun went out, we would not know for 8 minutes.
• From the Sun to Pluto: about 5 hours 30 minutes.
• From the Sun to the edge of the solar system: about a day.
• From the nearest star (Proxima Centauri): 4.2 years.
• From the centre of our galaxy: 26,000 years.
• From the next galaxy (Andromeda): 2.5 million years.
• From the most distant galaxies we can see: over 13 billion years.

When you look at the night sky, you are looking back in time. The light from the most distant galaxies left them when the universe was a tiny fraction of its current age. We see the universe as a kind of historical document, with closer objects showing more recent times.

How we measured it

Measuring the speed of light is hard. Light is so fast that early attempts (passing a flame between two distant hilltops with mirrors, for example) failed entirely.

The first reasonably accurate measurement came from a Danish astronomer called Ole Roemer in 1676. He noticed that the orbital period of Jupiters moon Io seemed slightly different depending on whether Earth was approaching or receding from Jupiter. He realised the difference was because light from Io had different distances to travel as Earth moved in its orbit. From this, he calculated a speed of light of about 220,000 km/s (close to the modern value of nearly 300,000 km/s).

Later experiments by Hippolyte Fizeau (1849, using a fast-spinning toothed wheel and a long distance) and Albert Michelson (1879, using a spinning mirror and ultra-precise timing) refined the value. By the 20th century, scientists could measure the speed of light to a few metres per second.

Today the speed of light is so well known that it is now used as a DEFINITION. Since 1983, the metre has been defined as the distance light travels in 1/299,792,458 of a second. The speed of light is, by definition, exactly 299,792,458 metres per second.

Fact Light is slower inside materials than in a vacuum. In water, it travels at 75 per cent of vacuum speed. In glass, around 67 per cent. In diamond, 41 per cent. In some experimental materials, light has been temporarily slowed to as little as 17 metres per second: about the speed of a cyclist. But in pure vacuum, no signal has ever been measured travelling faster than the universal speed limit.

The cosmic speed limit

According to Einsteins theory of special relativity, nothing with mass can travel faster than the speed of light. As an object approaches light speed, three strange things happen:

• Its mass increases.
• It contracts in length (in the direction of motion).
• Time runs slower for it (as seen from someone at rest).

To accelerate an object to the speed of light would take infinite energy. Reaching the limit is impossible for anything with even a tiny mass (even a single electron).

Light itself is made of photons, which have no mass. That is why they can travel at the speed of light, while everything else with mass cannot.

Light-years

For huge cosmic distances, astronomers use a unit called the light-year: the distance light travels in a year. One light-year is about 9.5 trillion kilometres (9,500,000,000,000 km).

Light-year is a measure of distance, NOT time, despite the name. The nearest star is 4.2 light-years away, meaning its light takes 4.2 years to reach us.

Did you know? Many things in your daily life rely on knowing the speed of light precisely. GPS satellites broadcast signals at the speed of light, and your phone calculates your position by timing how long they take to arrive (to within a billionth of a second). Without taking Einsteins corrections to account for the curvature of space and time, GPS would be wrong by 10 km after just a single day.

Light from the past

Because of the finite speed of light, looking out into the universe is the same as looking back in time:

• The Moon you see at night is 1.3 seconds old.
• The Sun is 8 minutes old.
• The North Star (Polaris) is over 400 years old.
• The centre of the Milky Way is 26,000 years old.
• The Andromeda Galaxy is 2.5 million years old.
• The most distant galaxies discovered by the James Webb Telescope are over 13 billion years old.

Astronomers use this to study the history of the universe. By looking at older and older light, they can essentially watch the universe develop, from its earliest moments to the current era.

Speeds of light in other media

• Vacuum: 299,792,458 m/s (exactly).
• Air: 299,704,000 m/s (just slightly slower than vacuum).
• Water: 225,000,000 m/s (75% of vacuum).
• Glass: 200,000,000 m/s (67% of vacuum).
• Diamond: 124,000,000 m/s (41% of vacuum).
• Special Bose-Einstein condensates in lab: as slow as 17 m/s (yes, walking pace).

Try this Stand outside on a clear night and look at a bright star. Hold up a finger and cover the star. The starlight you were just seeing took years (sometimes hundreds or thousands of years) to reach your eye, only to be blocked by your finger at the last moment. Every photon that hits your retina has been travelling unhindered through space for huge distances, only to land on a wet sphere of jelly in your skull.

Deeper dive: why nothing can go faster than light

The speed of light is not just a "fast number". It is a fundamental limit built into the structure of space and time. Albert Einsteins theory of special relativity (published in 1905) showed why no object with mass can ever reach this limit.

The key insight is that, contrary to our everyday experience, time and space are not absolute. Two observers moving differently disagree about how long things take and how far apart things are. The only thing they ALL agree on is the speed of light: it is exactly the same value for every observer, regardless of how fast they themselves are moving.

This single fact forces a redesign of physics. To keep the speed of light constant for everyone, time must run slower for a fast-moving observer (compared to a stationary one). Lengths must shrink in the direction of motion. Mass must seem to increase. The faster you go, the more these effects build up.

As you approach the speed of light, the effects become extreme. Your mass approaches infinity. The energy needed to accelerate you further also approaches infinity. The universe cannot supply that, so you can never actually reach light speed. You can get extraordinarily close (in particle accelerators, protons reach 99.999999 per cent of light speed), but never the full 100 per cent.

Light itself is the exception. Photons have no mass, and they always travel at exactly the speed limit. They do not gradually accelerate from rest; they exist at light speed from the moment they are emitted to the moment they are absorbed.

Could anything go faster? Some theoretical particles called tachyons have been proposed, but no evidence for them exists. Some shortcut routes through space (wormholes) might allow faster effective travel, but they are entirely speculative. As far as anyone has tested, the speed of light is the universal speed limit, and nothing in the real world has broken it.

For more, see what is light and the visible spectrum.