Mercury's Relativity

Mercury has an interesting relationship with Einstein’s theory of relativity.

You’re probably thinking that there are two things I could mean by this. There’s mercury, the metal that’s liquid at room temperature. According to legend, the first emperor of unified China died of mercury poisoning, after taking pills that were intended to give him eternal life.

Then, there’s the planet Mercury, closest to the sun. This Mercury flies around the sun at a blistering speed, making its year 88 earth days. Like its next door neighbor Venus, Mercury’s day is extended so much that a year is close to a day, about 59 days. With almost no atmosphere, it experiences the most extreme temperature swings in our solar system, from as cold as -180°C, and as hot as 430°C.

So, which Mercury (or mercury) do I mean?

Both.

Let’s start with the planet. Isaac Newton laid out an incredible system to describe how celestial bodies like planets worked, and this framework was phenomenal. It worked virtually flawlessly!

Virtually, but not entirely. One of the spots astronomers couldn’t explain was Mercury’s orbit around the Sun. There was a tiny wobble that observation insisted was real, but which Newton’s theory insisted shouldn’t be there.

This wobble meant that Mercury didn’t go in a perfect ellipse around the Sun, but instead changed its course ever so slightly every time. This little shift is called the precession, and it really bothered astronomers for a few hundred years.

This conflict was finally resolved when Einstein worked out his general theory of relativity, showing that gravity wasn’t just a force, as Newton assumed, but instead the warping of spacetime. The Sun’s enormous gravity creates an intense curve that Mercury is pulled into, and as Mercury gets close to the Sun, the warped spacetime pulls the planet into an ellipse that’s angled off just a hair from the previous one.

This was a huge validation for Einstein’s theory: his calculations for Mercury agreed with observation completely.

What about the liquid, though?

We have to turn from the astronomically large to the world of the very tiny now. Mercury’s outer layer of electrons is almost full, meaning it doesn’t really want to bond with other mercury atoms. This provides some of the reason for mercury being a liquid at room temperature, but not all of it.

Mercury has 80 protons in its nucleus, making its pull significant on its orbiting electrons. And—wouldn’t you know it, just like with the planet, these electrons reach a substantial portion of the speed of light. This means we need to invoke relativity once again in order to get the right answer.

It also means that those ultra-fast inner electrons keep that outer layer uninterested in partnering up with other mercury atoms, so the element remains liquid all the way down to -39°C.

Mercury the planet and mercury the liquid are both named after Mercury, the god.

Mercury was the Roman god of lots of things, including financial gain and commerce, communication, and luck. Most importantly, his role as “messenger of the gods” meant he was really, really fast.

I love that the defining trait for a god devised 2400 years ago is speed, and speed is the defining reason we need to invoke relativity for both of the physical Mercuries.

Have you ever had the opportunity to play with mercury, the substance? Did you ever wonder why it was a liquid? Are you a huge fan of Roman mythology or relativity? Let’s talk!