Today we are going to talk about a word people use without really knowing what they mean by it. That word is “ghost.”

I don’t mean storybook ghosts — I mean the kind some people think are skulking around old houses, scaring children and knocking over expensive vases in the dark.

So: What is a ghost? It’s a floating mind with no body, right? And the mind used to belong to a person who is now dead? I think that’s a pretty fair definition, though I don’t believe in ghosts myself. As far as I’m concerned, a mind without a brain is like a skeleton without bones.

But plenty of people — you, too, maybe! — think the mind is not the brain. In fact, it’s not even a physical object. So ghosts aren’t physical either and that’s why we can’t find them when the lights are on.

If this sounds pretty convincing to you, do me a favor: Grab a helmet and some full body armor and ask a physicist what he or she thinks of non-physical objects.

I tried it once without armor because I can run pretty fast. The physicist looked at me like I had just asked him what he thought of chlamydia and said, uh, even if there is something physics can’t observe directly, it becomes physical the moment it causes something you CAN observe.

So like if there’s a ghost (whatever that means) ghosting around, completely outside of physical space (whatever that means), we wouldn’t be able to learn anything about it. But if it starts making ghostly noises, breaking mirrors, sucking all the heat out of the room, spinning compass needles, or showing up in photographs, well, it’s acting in the physical world. Sound, heat, force, glass, magnetic fields, and light are all physical.

Phew, did you catch everything that happened up there? We started with a word for a thing that might not even exist, then tried to define it by casually ignoring a few laws of physics. The human imagination and the flexibility of our language are wonderful things, aren’t they?

Oh, and those ghost-hunting shows are bullshit.