I started a medical degree straight out of school, and stuck with it for one semester before transferring to journalism, then bailing on that a couple of years later to become an entry-level ski instructor. I’d finished school with the score to get into medicine, and there were expectations. Not explicit, but hovering. From that single unsuccessful semester, one physics problem, about the last breath of Julius Caesar, has always stuck with me.

Caesar, succumbing to the stab wounds of conspirators, breathed out. A final, emptying breath – lost thoughts of glory and betrayal, nitrogen mostly, some oxygen, a small amount of carbon dioxide. The oxygen and carbon dioxide went on to active chemical lives, decoupling and recoupling, making water, gas, organic molecules. But most of the nitrogen didn’t. It blew away, circulated. From ground level to the outer limits of the atmosphere. Calculate the number of nitrogen molecules in that last breath, allow two thousand years for dispersal – that’s why it’s Caesar – and you can work out how many of those molecules are likely to be in any random breath of air at sea level in the present. The answer, I think, is thirteen. Thirteen molecules of Caesar’s last breath in each breath we take. Not long ago, I googled it and found different approaches to the problem giving different numbers, but I recall it being thirteen the way we did it.

In the exam, I remembered that problem from lectures, but bombed most of the others. It was the story that got me, not the science or the number itself. All around the world, people breathing – a child asleep in Ghana, home of Walter Baker’s unnamed ancestor, a commuter on the Tube in London, every despot in modern times from his first breath in power to his last, all breathing some of Caesar’s final breath. And the final breath of someone killed in Pompeii, and one day, when time has sown his nitrogen far and wide, Kennedy’s. And mine, though that will be no-one’s physics problem two thousand years from now.

But, for me, the big issue isn’t that. It’s not the movement of nitrogen and some interesting maths to go with it. It’s what it means. It’s begging to be viewed metaphorically, as a sign that we’re all connected. But, at the same time, it’s just physics being physics, and physics doesn’t do metaphors, and doesn’t beg. It is what it is. People and things and their component parts in a confined space forever, however big that space might feel. We are all here, on this one small, festering, wet lump of rock. Collisions will occur, millions of them, most of them unwitnessed. It’s a closed system, this planet, plus or minus a few meteorites and space missions, and a closed system always makes for more encounters than you realise.