Watching Free Birds, a cartoon about turkeys with a time machine, with my boy, he asked me what the invisible spaceship/ time machine was made with. He’d never seen an invisible spaceship before!

What could I say? What would you say? I said it was a force field, but in my head, I wondered, is it? Or is it an invisible wall? And for the first time, I questioned, is there a difference? Or has it always been just a word game?

(Now, you may well say a wall is composed of matter, and a force field is energy. But then you should talk to Einstein. Since he’s dead, I’ll say this: what is a wall, but an array of pinpoint masses floating in mostly empty space, holding each other in place, and keeping us out, with an electromagnetic field?)