Light

You are reading this because light left a screen or a page, crossed the gap to your eyes, and was absorbed by cells in your retina. Light is how almost everything we know about the universe reaches us, from the words on this page to the glow of galaxies at the edge of the observable cosmos. It is also one of the strangest things in physics: a wave that is also a particle, with a speed that nothing can exceed and that never changes no matter how you move.

Waves

Light is an electromagnetic wave, a ripple in electric and magnetic fields travelling through space. It needs no medium; unlike sound, it crosses a vacuum without trouble. The distance between one crest and the next is its wavelength, and wavelength is colour. Red light has a wavelength of about 700 nanometres, billionths of a metre; violet about 400. White light is all the colours together, and a prism separates them because each wavelength bends by a different amount as it passes through glass. A rainbow is the same trick performed by millions of raindrops.

The colours we see are a tiny slice of a vastly wider range. Make the waves longer than red and you have infrared, which you feel as heat from a fire; longer still, microwaves and radio waves, metres or kilometres from crest to crest. Make them shorter than violet and you have ultraviolet, which tans skin; then X-rays, which pass through flesh but not bone; then gamma rays, the most energetic of all. All of them are light. Our eyes evolved to see the band the Sun emits most strongly and the atmosphere lets through, and nothing more.

Particles

In 1905 Einstein showed that light also comes in packets. Shine light on a metal and it knocks electrons out, but only if the light's wavelength is short enough, no matter how bright it is. That makes no sense for a wave and perfect sense if light arrives in individual lumps, photons, each carrying an amount of energy set by its colour. A blue photon carries more energy than a red one; an X-ray photon far more. A dim light is not a weaker wave but fewer photons.

Light is both. In the famous double-slit experiment, photons sent one at a time through two narrow openings build up, dot by dot, a striped pattern that only a wave passing through both slits could produce. Each photon, somehow, interferes with itself. This is not a trick of light alone: electrons and even whole atoms do the same. It is how the universe works at small scales, and nobody finds it intuitive.

The speed limit

Light in a vacuum travels at 299,792 kilometres per second, fast enough to go round the Earth more than seven times in a second, to the Moon in 1.3 seconds and to the Sun in eight minutes. Nothing with mass can reach that speed, and the faster something goes the harder it becomes to go faster, which is why no spaceship will ever catch a light beam.

Stranger still, the speed is the same for everyone. If you race toward a beam of light at half light speed, you measure it passing you at exactly the usual speed, not one and a half times it. Einstein took this fact seriously in 1905 and worked out the consequences: moving clocks run slow, moving objects shorten, and mass and energy are two forms of the same thing, related by E = mc². The whole of special relativity follows from the stubborn constancy of light.

Because light takes time to travel, looking far away means looking into the past. You see the Sun as it was eight minutes ago, the nearest star as it was four years ago, and the Andromeda galaxy as it was before humans existed. Telescopes are time machines, and the most distant light we have detected set out more than 13 billion years ago.

Making light

Light is born whenever charged particles lose energy. An electron in an atom dropping from a higher energy to a lower one emits a photon of a precise colour, which is why neon glows red and sodium streetlamps orange. Hot objects glow because their jiggling atoms radiate; a poker goes red, then yellow, then white as it heats, and a star's colour tells its temperature. Fireflies and deep-sea fish make light with chemistry. A laser makes it by persuading a crowd of atoms to emit identical photons in step, producing a beam so pure and straight it can be bounced off the Moon.

Why it matters

Light carries nearly all the information we have about the world and almost everything about the universe beyond it. It drives photosynthesis, and so feeds the planet. It runs the fibre-optic cables that carry the internet, the lasers in surgery and manufacturing, the X-rays and scans of medicine. And in its double nature and absolute speed, it revealed two of the deepest truths of physics, quantum mechanics and relativity, both of which began as attempts to understand what light is.