Roped Santas and Schrödinger's Cat

On the wall of my sister-in-law's very hospitable house in Aberdeen were several Santas on a string, reminiscent of a roped-up party of mountaineers. "What do you do if you and your climbing partner are walking along a ridge, roped together, and your partner falls off?" A discussion of the correct course of action (leaping off the other side of the ridge) led fairly quickly to Touching the Void and the awful decision faced by Simon Yates. On the end of the rope, was his climbing partner Joe Simpson alive or dead? As the world knows, Yates eventually had no option but to cut the rope and Simpson's subsequent survival forms the incredible heart of the book.

Already leaning to the tortuous, the conversation then got bendier: "like the cat in the box," said my daughter. She was drawing a parallel between the mental state of Simon Yates, not knowing if Joe Simpson was alive or dead on the end of the rope, and the hypothetical scientist of Erwin Schrödinger's famous thought experiment.

I guess most people reading this will have heard of Schrödinger's Cat. Here's a quick recap, just in case. Schrödinger, one of the founders of quantum mechanics, was seriously unhappy with its implications. To highlight what he felt to be their absurdity he devised the thought experiment involving the cat in the box. The box is sealed and we can't see inside it. There is a jar containing poison gas and a mechanical device which will break the jar and kill the cat, if a radioactive atomic nucleus decays and makes a Geiger counter type device go, "click". Before the box is opened the cat is in a superposition of dead and alive states. Only the opening of the box, and the act of observation, forces one of these outcomes or the other. The decay of the atomic nucleus is a quantum event which has to be understood in this way. The ingenious, if brutal experiment allows the strangeness of the quantum world to have consequences in the macroscopic world of living things. The idea of a superposition of living and dead cats appears so contrary to sense that Schrödinger hoped to persuade people it couldn't possibly be correct.

Joe Simpson, on the end of the rope, was alive. Simon Yates didn't know this but any "superposition" of a dead Simpson and a living one existed only in Yates' mind. The whole point of Schrödinger's Cat is that the cat is genuinely in a mix of alive and dead states until the intervention of an observer "collapses the wavefunction" and forces one or other of the two outcomes. It's about much more than just the state of knowledge of the experimenter. But we had quite a conversation about this point, subtle or possibly even daft to somebody who hasn't spent some time getting familiar with the background to quantum mechanics.

I was impressed that my daughter had heard of Schrödinger's Cat, an amusing diversion for students of physics. I guess this superposed pussy has earned Schrödinger much more fame than his formulation of wave mechanics, the starting point in the serious study of the quantum world, and the computational tool that underlies so many everyday devices in this era of digital, semiconductor electronics.

But don't worry, festive bloggees, it wasn't all quantum mechanics and gazing into the abyss until it gazes back. Under the watchful eyes of the roped-up Santas we also played Cluedo and the board game version of Pointless, ate and drank too much, inflicted Xmas pressie CD's on each other and went outside very little. Soon be Hogmanay.