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.It is baffling how someone could think that probability is some sort of physical thing. Probability is just a mathematical interpretation, and the claims of this paper do not make any sense. And even if probabilistic aspects of observable events are attributable to quantum mechanics, that still would not say anything about the multiverse.

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. ...

There is just one problem. The Born rule breaks down in some situations. The latest theories in cosmology say that our universe is just one part of a vast multiverse containing a large or even infinite number of other "pocket" universes. Some of those universes will be exact copies of our own, right down to a duplicate you. The mathematics behind the Born rule can't cope with this.

"In these situations, the quantum wave function can tell you nothing about which pocket you are in," says Albrecht. That's a problem if we want to predict the properties of our universe, which will look identical to many others at a given point in time, but which can eventually evolve differently due to quantum uncertainty.

Until now, physicists seeking to predict the properties and behaviour of the multiverse have added a sprinkling of conventional probability to reflect the chance of us being in a particular universe. For example, in a multiverse with just two universes, you might add a 50-50 chance of being in either one, just as we instinctively assign the same odds to a coin toss.

## Saturday, January 12, 2013

### Paper says all probability is quantum

NewScientist reports:

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yeah i am at a loss to fathom what their aim is... other than notoreity. if my doctor tells me my daughter has an operable tumor with an extremely high success rate if removed via a simple day procedure, i am not going to risk leaving it there because there are no available studies on comparative observed quantum fluctuations during that operation or because i cant find any published results of the same success rates from other universes. what possible use could their assertions ever have??

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