**Why is there a 1 in 2 chance of getting a tail when you flip a coin? It may seem like a simple question, but the humble coin toss is now at the heart of a lively row about the multiverse. At stake is the ability to calculate which, of an infinite number of parallel universes, is the one that we inhabit.**

**The debate comes in the wake of a paper posted online a couple of weeks ago by cosmologists Andreas Albrecht and Daniel Phillips, both at the University of California, Davis. They argue that conventional probability theory, the tool we all use to quantify uncertainty in the real world, has no basis in reality (arxiv.org/abs/1212.0953). Instead, all problems in probability are ultimately about quantum mechanics. "Every single time we use probability successfully, that use actually comes from quantum mechanics," says Albrecht.**

**This controversial claim traces back to the uncertainty principle, which says that it is impossible to know both a quantum particle's exact position and its momentum.**

**Albrecht and Phillips think particle collisions within gases and liquids amplify this uncertainty to the scale of everyday objects. This, they say, is what drives all events, including the outcome of a coin toss. Conventional probability - which says the outcome simply arises from two equally likely possibilities - is just a useful proxy for measuring the underlying quantum uncertainties.**

**click here.**