The wonder-ful NASA Earth Observatory this week sent me an image of the Grand Canyon created by overlaying satellite imagery with a detailed topographic map. And as I stared in wonder I could see clearly that the Grand Canyon did not in fact exist!
What I saw was that the Grand Canyon was a phenomenon defined by absence – the absence of the rock which has been removed. The rock which has been removed was – if you like – the ‘positive’ – the actual ‘stuff’ which exists. And what remains is the ‘negative’ a phenomenon made entirely out of… absence! And this made me think about electrons…
As you may know, Dirac’s conception of particles and anti-particles imagines a vacuum state, geographically analogous to the elevated high plains. He imagined that things could be added to this vacuum state to create a phenomenon we call a ‘particle’ , for which the geographic analogy would be a mountain. Nobody could doubt that a mountain exists!
But in Dirac’s conception, something can also be subtracted from this vacuum state to make an anti-particle, for which the geographic equivalent would be a valley. Or a canyon. Anti-particles would be defined by absence.
Staring at the undeniable existence of the Grand Canyon and realising that it didn’t really exist, I felt something click. I felt that the wonders and mysteries of the microscopic world were somehow being played out at another scale – a scale so large that this time I almost failed to recognise them for a different reason!
It’s a funny old world.