杏吧原创

Read an interview with Eels singer Mark Everett, son of Hugh Everett

If you think of yourself as unique, think again. The days when physicists could ignore the concept of parallel universes may have come to an end. If that doesn鈥檛 send a shudder down your spine, think of it this way: our world is just one of many. You are just one version of many.

David Deutsch at the University of Oxford and colleagues have shown that key equations of quantum mechanics arise from the mathematics of parallel universes. 鈥淭his work will go down as one of the most important developments in the history of science,鈥� says Andy Albrecht, a physicist at the University of California at Davis. In one parallel universe, at least, it will 鈥� whether it does in our one remains to be seen.

鈥淭his work will go down as one of the most important developments in the history of science鈥�

The 鈥渕any worlds鈥� interpretation of quantum mechanics was proposed 50 years ago by Hugh Everett, a graduate student at Princeton University. Rather than apply one set of rules to the subatomic quantum world and another to the larger-scale everyday world, as physicists tend to do, Everett wanted to apply quantum mechanical equations to everything. This had some startling consequences.

According to quantum mechanics, particles do not have set properties before they are observed. Instead, particles are described by 鈥渨ave functions鈥� representing many mutually contradictory properties. It is only when an observer measures a property that the particle somehow settles into one of these multiple options. The paradox is exemplified by Schr枚dinger鈥檚 cat 鈥� the famous thought experiment in which a cat in a box can be said to be both alive and dead. It is traditionally thought that the act of observation, opening the box to check the cat, is what forces it to settle into a state, living or dead.

If, as Everett argued, quantum mechanics is applied to the whole universe, then it too should exist in a multitude of separate states. There would be a 鈥渕ultiverse鈥� of parallel universes 鈥� one for every physical possibility. So when you open the box holding Schr枚dinger鈥檚 cat, the universe splits, forming two new 鈥測ous鈥� 鈥� one whose future involves viewing the live cat and the other who sees the dead cat.

Dismissed by the scientific establishment as ridiculous for decades, the many-worlds scenario may at last come in from the cold thanks to Deutsch鈥檚 work.

The biggest criticism levelled at many worlds was that it seemed to make a puzzle about the outcomes of quantum experiments even worse. Physicists can predict the probability of getting a certain outcome from a quantum experiment from the square of its wave function, according to the Born rule. Nobody can explain why this rule works, it simply fits with experimental observations. The problem was there seemed to be no place for the Born rule in the multiverse. In fact, there didn鈥檛 seem to be any space for any probabilities at all, says Deutsch.

鈥淵ou toss a coin, but what does it mean to say that the probability of it coming up heads is 50 per cent?鈥� Deutsch asks. 鈥淎ccording to Everett, both outcomes must happen.鈥�

In the mid-1990s, Deutsch set out to put the uncertainty we see in quantum mechanical experiments back into the many-worlds scenario. Now, with additional work by Simon Saunders and David Wallace, also at Oxford, he believes they have succeeded. The trick is to examine a quantum experiment while excluding probability theory and accepting the many-worlds interpretation.

The multiverse has a branching structure, created as the universe splits into parallel versions of itself. The thickness of the branches can be calculated solely using deterministic equations, getting around the uncertainties usually associated with quantum physics. What the Oxford gang found is that the branching structure exactly reproduces the peculiar probabilities predicted by the Born rule. The branching also gives the illusion of probabilistic outcomes to measurements.

Deutsch believes this solves the problem of the origin of quantum probability once and for all. 鈥淧robabilities used to be regarded as the biggest problem for Everett, but ironically, they are now its most powerful success,鈥� he says.

鈥淲e鈥檝e cleared up the obscurities and come up with a pretty clear verdict that Everett works,鈥� says Saunders, who is presenting the work with Wallace at the Many Worlds at 50 conference at the Perimeter Institute for Theoretical Physics in Waterloo, Canada, this week. 鈥淚t鈥檚 a dramatic turnaround and it means that people now have to discuss Everett seriously.鈥�

Albrecht agrees that the work will shake up physicists鈥� worlds. 鈥淢any people are uncomfortable about the probabilities at the heart of quantum mechanics and attempt to get rid of quantum mechanics because of it,鈥� he says. 鈥淏ut this greatly amplifies the fundamental place of quantum mechanics in our understanding of the physical world.鈥�

David Papineau, a philosopher of physics at King鈥檚 College London, says that he has been converted from scepticism about many worlds to belief, based on its potential to one day solve this puzzle of quantum probabilities. He adds, though, that the work by Deutsch, Wallace and Saunders must now be scrutinised. 鈥淚t鈥檚 an ambitious claim and so we have to be careful,鈥� he says. For Papineau, the problem is whether a belief in parallel universes should affect the way we live our everyday lives (see 鈥淛ust another universe鈥�).

Max Tegmark at the Massachusetts Institute of Technology has long been a fan of the many-worlds scenario. But while he believes the new work on probability should help convince physicists of its reality, it will never be enough to win over die-hard sceptics. 鈥淭he critique of many worlds is shifting from 鈥榠t makes no sense and I hate it鈥� to simply 鈥業 hate it鈥�,鈥� he says.

David Albert, a philosopher of physics at Columbia University, New York, is sceptical. He argues there is good reason to be wary because the Oxford group may be guilty of sleight of hand. 鈥淲hen you first hear about this you feel euphoric,鈥� he says. 鈥淏ut then you think, maybe this is too good to be true.鈥� He believes that it is irrelevant that Deutsch and his colleagues can show that branching universes give the illusion of probabilistic outcomes to measurements. What we really want to know, says Albert, is why this branching happens in the first place. 鈥淭hey have answered a question, but I think it鈥檚 the wrong question,鈥� he says.

Wojciech Zurek at the Los Alamos National Laboratory in New Mexico believes that the Born rule is exactly the right question to tackle. However, he believes that it can be answered without resorting to parallel universes. Zurek points out that Everett never used the term 鈥渕any worlds鈥� in his papers, and says that his work can be interpreted in less controversial ways.

Zurek is also inspired by Everett鈥檚 ideas, particularly his insight that quantum mechanics must be applied to the entire universe rather than a limited quantum realm. He interprets this to mean that quantum entanglement 鈥� the process in which quantum particles can become inextricably linked and act in unison no matter how far apart they are 鈥� is a fundamental ingredient of quantum physics. Zurek has already used this property to explain why we see a single objective reality when we make a measurement of a quantum state (New 杏吧原创, 30 June, p 18). Zurek says that entanglement can also be used to derive the Born rule ().

鈥淚 could not have derived probability without using Everett,鈥� says Zurek, who is also presenting his work at the conference. 鈥淏ut at no point am I forced to assign equal reality to all other versions of the universe in the many-worlds scenario.鈥�

For Tegmark, the fact that many worlds is sparking such debate, 50 years after its conception, is a triumph in itself. He believes that physicists interested in quantum computing and cosmology are now warming to it. Will the majority be won over? 鈥淭hat depends on what parallel universe you live in,鈥� he says.