"God does not play dice" Albert Einstein once said, expressing his contempt for the notion that the universe is governed by probability - an idea fundamental to quantum theory (see "Quantum uncertainty" in Issue No 5). Since then the undisputable successes of the quantum theory have convinced all but a handful of contemporary physicists that God does indeed play dice. The question some physicists are now asking is why does God play dice? What's the point? Popescu, a Cambridge physicist, believes he has the beginnings of an answer to this seemingly monumental question.

There's now no doubt that God plays dice, but why should our universe be probabilistic at all?

"Quantum entanglement" is the term used to describe the bizarre link that may develop between two or more particles, even when they are separated by cosmological distances. A photon may decay into an electron and a positron (the positron being the antiparticle of the electron). Electrons and positrons possess an observable quantity called spin, a kind of internal angular momentum, which when measured in some direction will either be "up" or "down". According to quantum mechanics, if one measures say the electron spin to be "up" in some direction, at that very instant the spin of the positron will be "down" in the same direction, no matter how far apart the particles. This poses a serious problem: according to relativity, no signal can travel faster than light, the "speed limit of the universe". Imagine it; we could measure the electron spin here on earth, and instantaneously the positron, which could be in another galaxy, would "know" which direction its spin should point. Weird.

Popescu believes he can resolve this apparent paradox between the two theories. He believes that God "plays dice" precisely to allow quantum theory and relativity theory to coexist.