If I can’t find my keys, they could be on the counter, or in the kitchen table hiding under some junk mail. Or maybe I left them hanging on the door. Until I find them, I obviously can’t say which. It’s a bit like sealing a radioactive atom in a box and leaving it isolated: until I open the box I can’t say whether it decayed. Sounds familiar?

The quantum superposition principle says that, until we observe the atom, it behaves as if it were both decayed and not. So are my keys also on the counter and under the mail and hanging on the door at the same time? Nope.

Not everything uncertain is a superposition. Whether I look for them or not, whether I find them or not, my keys are in a definite–albeit unknown–place. When I finally find them in my jacket, it’s not because me searching them made them materialize there: they were clearly there to begin with.

Atoms, electrons, photons, and other quantum stuff, instead, actually does behave as if it were in more places at once, as if our observation made them materialize here rather than there.

We know for a fact that a single atom has this ability. And so do two atoms. But a keyring (though composed of many atoms) does not. So somewhere between two and “keychain” a bunch of atoms stops obeying the weird rules of the quantum world. Where that border lies is still a mystery.

As is a mystery where the heck my phone is.

If you want more

• The superposition principle is really cool. We’ve talked about it a few times already.
• Superpositions are so weird that for decades physicists tried to get rid of them with alternative interpretations of quantum mechanics. Like “Bohmian mechanics”:

Cover photo: CC0 Uwe Baumann/pixabay