Today I Learned something really awesome and mind-boggling! So naturally I’m going to geek out about it.

Relativity, in extremely oversimplified terms, is the universal law that when things go fast everything gets weird. Time slows down, mass increases, all the things we think of as constants turn out not to be so constant, you and your twin have an awkward birthday reunion. However, when we say fast, we mean really really fast, measurable fractions of the speed of light: the kind of fast that, in general, does not apply to our day-to-day lives. Relativity is therefore associated most strongly with astrophysics and space technology (satellite-based GPS is the classic example of a real life application) and things that operate on a very, very large scale.

However! It turns out it’s also really important on a very, very small scale.

Atoms, as you may or may not recall, have a nucleus made of protons and neutrons (except for hydrogen, which don’t need no neutrons kthx), and then electrons orbit around this centre like tiny planets around a sun. The number of electrons in particular (and the way they are arranged into layers or shells around the nucleus) is important because they tend to have a big effect on the chemical behaviour of an atom.

But as you start getting into really heavy elements, with big bulky centres, the electrons have to move faster to keep up their orbits. So fast that they are actually moving at… measurable fractions of the speed of light.

And so relativity becomes a Thing.

Which brings us to the title: the reason that gold is yellow in colour is because its electrons are moving fast enough for relativistic effects to apply. Most other metals (apart from copper, which has its own shit going on) are silvery or grey in colour; gold would be too, except that the effects of relativity cause it to absorb more blue light than other colours, making it appear yellow to our eyes.

(It gets even weirder when you go much further along the table, into the semi-charted territory of superheavy elements that are so unstable they don’t exist in nature.)

So there you go. Next time you look at something made of gold, remember that its soft warm pretty colour is because its electrons are screaming around at unthinkable speeds that warp the fabric of spacetime and gobble up blue light!

I think that’s neat.

Further reading: Relativistic Quantum Chemistry (Wikipedia)