Geonium

You don't hear the positron. You hear the trap it's in, a hum at two hundred and six million a second, far too high for anybody's ear, mixed down until a person at the bench can watch it as a line. The particle rides up and down the axis like a bead on a held breath, and that's the note. When its little circling climbs one rung, the note moves three point eight hertz. Not a melody. A string tuned so fine that a single quantum is a change of key.

It came in disguised. Slowed in tungsten, it took an electron by the arm and walked in as neutral positronium through a hole six hundred and twenty-five microns wide, and the field took the arm away at the door and kept it. Eight hours, eleven hours of waiting at a weak source on a nylon cable, a pinch of radioactive sodium let down from a warm room, then raised again once there is one, so there won't be two.

Then a month of nobody's hands. Three hundred millikelvin, and the lowest floor it has, the ground rung, where it goes on circling because there's nowhere lower to be. Now and then the spin turns over, and the line steps, and somebody writes down the time.

In Seattle, forty-odd years ago, Dehmelt kept one for three months and called her Priscilla, the way you'd name a pet, he said, and he gave his reason: in that vacuum she never had a chance to trade places with a passing twin. That's the whole of it. Not a mark on her. A door nobody came through.

He had another name, for what the two of them make, the particle and the trap: geonium, an atom with the Earth for a nucleus, the magnet and the floor and the building holding the far end. I think that's the honest one. Whatever gets to be "this one" here is a duet, a voice you couldn't tell from any other, and a room tuned so close to it that the room can count its steps.

And if Wheeler was right and there's only the one, running forward as us and backward as this, then at the bottom of a dewar in Evanston it's holding a single note for once, and someone is listening, writing the time down, and hasn't given it a name yet.