Gold is yellow because it swallows some of the blue in white light, and white light with the blue taken out looks yellow. Most metals absorb only in the ultraviolet, which our eyes cannot see, so they send back every visible colour and look silver. Gold’s absorption has been dragged down into the blue by Einstein’s special relativity. Its heavy nucleus whips the innermost electrons round at more than half the speed of light, which pulls gold’s outer 6s orbital inwards and shortens the step a 5d electron must climb. Switch relativity off in the calculations, and gold comes out looking like silver.
Element 79. Gold. Here is the fact nobody believes: line up the metals, and nearly all of them wear the same dull silver. It is the sort of thing you have seen all your life and never once noticed. Only a few have a colour of their own. Copper blushes a reddish colour. And gold, of course, is gold.
Gold has a second trick, too. It refuses to tarnish. Gold buried for thousands of years comes out of the ground still shining, as if it went in yesterday. Two odd facts, one cause, and it is not the chemistry most of us were taught at school. It is relativity, the physics of things moving close to the speed of light. Let us open the file.
Why metals look silver
To understand gold, we first need to understand why everything else is so boringly grey. Picture a metal up close: a neat lattice of atoms sitting in a sea of free electrons. Those electrons belong to no single atom; they roam the whole metal, like a crowd in a square. Shine light on it and the free electrons shake in time with the light, and shaking electrons send the light straight back out again. That, quite simply, is a mirror.
Most metals do this for every colour of visible light, more or less equally. Red, green and blue are all returned in the same measure, and mixed back together they make silver, or grey.
But there is a catch, and it is where the story really begins. Every metal also has a colour of light it can swallow rather than reflect. That light is absorbed by lifting an electron to a higher energy level, like kicking a ball up a staircase. For silver, the absorption sits in the ultraviolet, at about 3.7 electronvolts. We cannot see ultraviolet, so whatever silver swallows there we never miss, and silver looks white and gleaming.
Gold is different. Its absorption starts lower, at about 2.4 electronvolts, squarely in the blue part of the visible spectrum. So gold quietly eats some of the blue out of white light, and what remains looks yellow. That warm glow is not something gold adds. It is what is left once the blue has gone. The real question in this case is why gold’s absorption slipped out of the ultraviolet and into the blue.
Why is gold yellow? Follow the electrons
Start at the heart of the gold atom. Its nucleus holds 79 protons, 79 positive charges packed into a tiny space, and its pull on the innermost electrons is enormous. The harder the pull, the faster they whirl.
There is a lovely simple estimate of how fast. Take the number of protons, 79, and divide it by 137 — the same 137 that sits inside the fine-structure constant. The answer is a fraction of the speed of light: for gold, about 58 per cent. More than half the speed of light, inside something you could wear on your finger.
At those speeds, special relativity stops being a footnote and starts running the show. The electrons behave as if they were heavier, and heavier electrons settle into orbits pulled in closer to the nucleus. Picture the cloud of the innermost electrons tightening, like a drawstring bag being pulled shut. The shrinking is not confined to the inner shells: it affects all of gold’s s-orbitals, including the outermost one, which chemists call 6s.
The 5d orbitals do something rather different. Shielded from the nucleus by those shrunken inner orbitals, they feel less of its pull and spread out a little. One level is drawn inward; the other drifts outward. The energy gap between 5d and 6s narrows, and the step on the staircase gets shorter. Lifting a 5d electron now takes less energy, enough to move the absorption out of the ultraviolet and into the blue. Gold absorbs blue, and so gold looks yellow.
Then comes the clincher. Researchers have run computer calculations of gold with relativity simply switched off, and the gold that comes out would look like silver. The chemist Pekka Pyykkö set out the case in the journal Chemical Reviews in 1988. The video animates the shells tightening, which is worth seeing once. The colour of every gold ring is, in a very real sense, a relativistic effect.
Why gold never tarnishes
Relativity has one more gift for gold, and it comes from the very same shrinking. The outermost 6s electron is held tightly, close in to the nucleus, and an electron held that tightly is very hard to prise away. Gold is deeply reluctant to give it up, or even to share it, so the air around it finds almost nothing to grab. Gold does not rust. It does not tarnish. Chemists have a rather grand name for metals like this: noble. Relativity, in other words, made gold aloof.
For proof, consult a boy king. Tutankhamun’s golden funeral mask was made around 1323 BC. When Howard Carter’s team opened the innermost of the king’s three coffins in 1925, the mask was waiting inside, and more than 3,000 years after it was made, it still shines. It is on display today in the Grand Egyptian Museum in Cairo.
That stubborn shine is exactly why people have prized gold for so long. It became the metal of kings, of temples and of coins. The Inca, it is said, called it the sweat of the sun. Alchemists spent centuries hunched over their furnaces, trying to make it.
One last strange detail for the file. All the gold ever mined, in all of history, comes to about 216,000 tonnes, on the World Gold Council’s 2024 estimate. Gather it together and it would fit in a cube roughly 22 metres on each side, because gold is so dense: 19.3 grams in every cubic centimetre. The arithmetic is in all the gold ever mined in one cube.
Relativity in your pocket
Gold is not the only element on this street where relativity is quietly at work. Move one step along the periodic table to element 80, mercury, with 80 protons to gold’s 79. Mercury is famously strange: a metal that is liquid at room temperature. In 2013 Florent Calvo and colleagues calculated what relativity does to it, and found that relativity lowers mercury’s melting point by about 100 degrees (105 kelvin, in their figure). Without it, mercury would be a solid sitting on the bench, not a silver puddle. That shimmering liquid is, in a sense, relativity you can pour.
Now to lead, and the box under your car’s bonnet. A lead-acid battery is built from cells, and each cell gives about 2.1 volts. In 2011 Rajeev Ahuja and colleagues, Pyykkö among them, asked how much of that voltage relativity was responsible for. Their answer was about 1.7 volts. Most of the voltage, in other words, comes from relativistic effects on lead’s electrons. Without relativity the battery would be a feeble shadow of itself, and cars would not start. Think about that on a cold morning, when the engine turns over.
We tend to file relativity away with rockets and black holes: far away, enormously fast, nothing to do with daily life. Or so we tell ourselves. But it is right here, in the colour of a wedding ring, in the silver drop of mercury, in the battery under the bonnet. Einstein’s physics, folded quietly into the ordinary things around you, and it all began with a simple question: why is gold gold?
Every element has a story. That was gold’s.
