For three hundred years, some of the best minds in science argued about what light is. They were all partly right. Every chapter of this exhibition holds one of the observations that settled the argument, and the answer is stranger than either side guessed.
Epilogue
What Light Is
How the argument ran and how each step was tested; what you can now explain; what nobody knows yet; and what light is doing to the world we live in.
The long argument
Science rarely settles a question by thinking harder. It settles it by finding an observation that one idea predicts and the other forbids, and then making it. Again and again in the story of light, the decisive experiment was one that somebody expected to come out the other way.
How a hard question was cracked
Light is a hard thing to study. You cannot hold it, it travels a billion kilometres an hour, and every instrument you use to look at it is made of it. Looking back across these chapters, the same four moves did most of the work.
Make it predict something risky
Fresnel’s wave theory predicted a bright spot at the centre of a round shadow, which a sceptic thought absurd. It was there. A theory earns trust by making predictions that could have failed.
Find where two ideas disagree
Newton’s particles had to speed up in water; waves had to slow down. Foucault measured it in 1850, and light slowed. Find the case the ideas disagree on, and look.
Turn it down until it breaks
Dim the lamp until it gives single clicks (chapter 5); turn up the energy until each grain kicks an electron (chapter 12). Extremes show what averages hide.
Follow the anomaly
A thermometer that warmed in the dark (chapter 7), a screen that glowed through black card (chapter 11), a hiss that kept the stars’ time (chapter 10). The great finds were things that should not have happened, taken seriously.
Explain it back
The best test of understanding is explaining. Try each question aloud, or in your head, before you open the answer.
If someone asks you what light is
Light is a ripple in electric and magnetic fields that travels at about 300,000 kilometres a second. It spreads, bends and interferes like a wave, and it is emitted and absorbed in whole grains, photons, each carrying energy and a push in proportion to its frequency.
Radio waves, the warmth of your skin, the colours you see, sunburn, X-rays and gamma rays are all the same thing at different wavelengths, from longer than a bus to smaller than an atom. Your eyes use one narrow band, the one water lets through. And everything you see has been shaped on the way: by the source, by what the light passed through, by the opening that gathered it, by the detector, and finally by a brain deciding what was there.
What nobody knows yet
The theory of light and matter, quantum electrodynamics, is the most precisely tested theory in science. That makes the questions it leaves open all the more interesting.
What happens when light is detected
The theory predicts the chances of every outcome perfectly: where in chapter 5’s pattern the next grain will land, how often a filter passes it. It does not say why one outcome happens rather than another, or what a photon is between being sent and being caught. Physicists still disagree about how to read it.
How light becomes seeing
Chapter 6 followed the light as far as a decision. How a pattern of firing cells becomes the experience of seeing a diatom, the thing this exhibition was built to reach, is not explained by any equation here, or anywhere yet.
The dark matter
About five-sixths of the matter in the universe neither gives off light nor absorbs it, at any wavelength. We know it is there only by its pull on the matter we can see. Every window in this exhibition is blind to it.
The first light
The oldest light we can see, the microwave glow of the Big Bang, left 380,000 years after it began. Before the first stars, hydrogen should have left a faint mark at 21 centimetres, the line from chapter 10. A detection was announced in 2018; other experiments have not confirmed it, and the search goes on.
Flashes with no explanation
Since 2007, radio telescopes have caught bursts lasting a few thousandths of a second from distant galaxies, each giving off as much energy in that moment as the Sun does in days. At least some come from intensely magnetic dead stars. Most are still unexplained.
How it is changing the world
None of this is only history. Each chapter’s idea is at work right now, and some are about to change what is possible.
Seeing past Abbe’s limit · chapter 2
A lens cannot resolve much finer than half a wavelength. In the 2000s microscopists sidestepped the limit by switching glowing molecules on and off a few at a time, and now map the inside of living cells at a few tens of nanometres. It won the 2014 Nobel Prize in Chemistry.
Clocks that feel gravity · chapter 3
Lasers locked to atoms keep time so precisely that in 2022 one clock measured time passing at different rates across a millimetre of height, as Einstein’s gravity predicts. The world’s definition of the second is likely to move to clocks like these.
Secrets in single photons · chapters 4 and 5
A message key carried by single polarised photons cannot be read in transit without disturbing it, so eavesdropping shows. In 2017 a Chinese satellite shared entangled photons between stations 1,200 kilometres apart.
A ruler made of light · chapter 3
In 2015 two interferometers four kilometres long detected a gravitational wave from two black holes merging, by measuring a change in their length of a few thousandths of a proton’s width. It opened a new way to sense the universe: not light at all, but the stretching of space itself, measured with light.
Looking back to the first galaxies · chapter 7
The James Webb Space Telescope works in Herschel’s infrared, because light from the first galaxies has been stretched past the red by the expansion of the universe, and because infrared slips through dust. It is now seeing galaxies that formed in the universe’s first few hundred million years.
Back to the front door
You began with a diatom and a question: when you look at something, what are you actually seeing? You have now followed light from a lamp through a hole, a shell of glass, a lens, a pair of filters, a scatter of single grains and into an eye; and beyond, through water and air, through flesh and bone, across the whole width of the Earth and into a collision with a single electron.
So: what is the shape of light?