How long is a piece of string?

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Alan Davies leaves behind his role in the TV quiz show QI to explore the world of quantum mechanics for the BBC science programme Horizon.

The stand-up comic admits to deliberately failing at physics so he wouldn't have to take the O-level. But after making a TV documentary, he says he now understands aspects of quantum mechanics.

"There is no point in my making the programme if I don't," he says. "They have to explain it again and again until I do understand."

"They" are the physicists he meets in the programme - "I was always getting into trouble for calling them physicians instead of physicists," says Alan. "They showed me the aspects of quantum mechanics that are explicable."

Horizon set Alan what appeared to be a simple challenge. All he had to do was measure a piece of string.

But the catch was he had to do it with absolute precision.

And that turned into a mind-bending journey though the worlds of maths and physics, using quantum mechanics to try to work out where the individual atoms and particles that make up the string actually are.

Laser experiment

Quantum mechanics allows matter to behave as both particles and waves. Quantum objects can be in different states at the same time, and can sometimes seem to be in two places at once.

Alan describes how this was demonstrated in a simple experiment using a laser and a special camera. A single wave of light, a photon, passes through two slits at the same time and creates an interference pattern.

A simple experiment in quantum mechanics

"A photon... can't be split," says Alan "but it's clearly in two places at the same time which is very exciting".

"I didn't understand that whole side of physics," he says. "The principle that light can be in two places at the same time is absolutely extraordinary".

But even if he accepts the idea now, he still finds it uncomfortable, which puts him "firmly in the Einstein Camp". That's what he was told by MIT professor of quantum physics Seth Lloyd.

"It's the only time in my life that I'll be compared to Einstein," says the comedian, "unless my hair goes white and I comb it backwards."

Alan's exploration of the complexities of sub-atomic physics in the programme involves playing the role of layperson, repeatedly posing the question, "How Long is a Piece of String?" of some of the world's top scientists?

"Scientists love that sort of question," says Alan, who claims to "thoroughly enjoy being told stuff.

"I'm quite a curious person. I don't mind being the one who doesn't know things," he says, "a role I often play in QI."

But he admits his understanding of quantum mechanics is somewhat superficial. "It's like showing someone a microphone and expecting them to be a stand-up comic."

"In the intervals waiting for a camera or the director, when you overhear them (the scientists) talking, their conversation may as well be in a different language."

But he is capable of dropping a reference to quantum computers into conversation.

"If you ask what will the world look like in the next century? These are the computers that are going to be changing the world. Seth builds them. I don't understand them."

As for his understanding of theoretical physics, "I liked the idea of all of humanity fitting inside a sugar cube because more than 99.9% of matter is space," he says.

"It's not QI. It's VI, very interesting."

Have his attitudes to science changed? "I'm more inclined to linger in the science pages of The Week magazine," he says, "But my principle obsessions are still watching sitcoms and football."

Alan Davies presents Horizon: How Long is a Piece of String at 9pm on Tue 17 Nov on BBC Two