杏吧原创

THE world used to be a much simpler place. A hundred years or so ago, we lived in a very normal, classical universe where everything made sense, and nothing behaved strangely. Then along came quantum theory.

Suddenly, stuff didn鈥檛 always behave as any rational person would expect. At the fundamental level of atoms and particles, things could be in two places at once. They could even move in two different directions at once. And it seemed they could also be entangled 鈥� engaged in a quantum version of telepathy in which they are somehow able to sense and affect each other instantaneously from a distance.

Adjusting to this new universe was a tall order. Some physicists constructed elaborate philosophies to deal with the implications. Albert Einstein, on the other hand, famously rejected entanglement as 鈥渟pooky鈥�. He was convinced that quantum entanglement couldn鈥檛 be real because the implications would run too deep: any effort to produce a unified theory, one that tied quantum mechanics together with relativity and other physical theories, would need to reconcile the weirdness of entanglement with relativity鈥檚 rather more practical grasp of time and space. That just seemed altogether too hard.

Not that he ever gave up on a theory of everything. Einstein spent the latter part of his life trying to construct a unified universe, without success. He also continued to wrestle with quantum spookiness on his own. To most physicists, quantum theory was useful if you wanted to design a laser or transistor, but it didn鈥檛 do to think about it too deeply.

That attitude prevailed even among those who wanted to understand the intimate workings of the universe. So the 鈥渇oundations鈥� of quantum theory 鈥� the assumptions behind how it describes particles, fields and reality itself 鈥� has taken a back seat in the quest for a unified theory. 鈥淓instein鈥檚 strong belief that the foundational issues in quantum mechanics are a necessary part of solving the problem of unification got suppressed and lost,鈥� says Lee Smolin of the Perimeter Institute in Waterloo, Canada.

But what was lost has now been found again. At the core of this renaissance is a growing tally of results showing that entanglement has profound implications for our view of reality. Recent experiments led by a group at the University of Vienna, Austria, provide the most compelling evidence yet that there is no objective reality beyond what we observe. This idea, that our measurements create reality, is controversial and scarcely new, but the mounting evidence for it could have major implications in the search for a theory of everything. Indeed, we are at the 鈥渃onceptual beginnings of a second quantum revolution鈥�, according to Alain Aspect of the Institute of Optics at Palaiseau in France.

The original quantum revolution came in the 1920s. Einstein鈥檚 big problem with quantum mechanics was that it contradicted the intuition supported by all other theories of physics. In our experience, objects have definite locations in space and a limited range of influence. According to quantum theory, however, a pair of particles would be able to share information about each other鈥檚 quantum states 鈥� and sometimes influence them 鈥� even where the distance and timing involved meant that no signal could have passed between them.

This suggested to Einstein that when it came to describing physical reality, quantum theory was lacking something. It is not that the information about the particles鈥� states is shared in a spooky link between them, he thought 鈥� it is simply that we don鈥檛 know where to find all the factors working on a particle to determine its momentum, say. Uncover these 鈥渉idden variables鈥�, Einstein said, and all the mystical spookiness would melt away. In its place would be a quantum theory that operates by the rules of common sense.

True to form, Einstein didn鈥檛 leave it at that: he formulated a mathematical argument to bolster his case. Working with Boris Podolsky and Nathan Rosen, he threw down the gauntlet to the quantum camp.

In 1935, the three theorists published the Einstein-Podolsky-Rosen thought experiment, known as . It said that if quantum theory was correct and complete, you should be able to perform an experiment in which a measurement on one entangled particle instantaneously affects the quantum state of its distant twin. At the time, this seemed to violate the known laws of physics and cast doubt on whether quantum theory could be considered to be a complete description of reality.

Much gnashing of teeth and triumphalism followed. Erwin Schr枚dinger, who had also wrestled with the implications of quantum mechanics, gleefully told Einstein the EPR paper had caught quantum theory 鈥渂y the throat鈥�. No one knew how to turn the thought experiment into a real one, however, so the two camps 鈥� Einstein鈥檚 opposition was spearheaded by the fearsome Danish physicist Niels Bohr 鈥� spent the next two decades shaking their fists at each other.

In 1964, nine years after Einstein鈥檚 death, physicist John Bell worked out a scheme to test EPR. He believed, as Einstein did, in the intuitive idea of 鈥渓ocal realism鈥�: that a particle cannot be instantly influenced by a distant event, and that its properties exist independently of any measurement.

Bell derived a mathematical formula that quantified what you would get if you made measurements on an entangled pair of particles. If local realism was correct, the correlation between measurements made on one of the pair and those made on its partner could not exceed a certain amount, because of each particle鈥檚 limited influence. The stage was set for a definitive experiment.

Many years passed before Aspect built the necessary equipment in his basement laboratory at the University of Paris, but by 1982 he had a result: Bell鈥檚 formula did not agree with quantum experiments (New 杏吧原创, 24 November 1990, p 43). The world, Aspect announced, could not be both local and real 鈥� Einstein was wrong. But which idea had to go, realism or locality? Do particles only acquire real properties when they are measured? Or are distant, instant influences possible between particles?

The answer would come from another source. In 1976, well before Aspect had carried out his experiment, physicist Anthony Leggett had what he calls 鈥渢he kernel鈥� of an idea to rework Bell鈥檚 formula with a twist: he quantified what you would get if you made measurements on entangled particles, assuming that distant, instant influences were in fact possible. Leggett eventually published this formula in 2003, the year he won the for his work on the quantum properties of helium-3.

Enter a team of Austrian and Polish physicists, who have now done experiments on pairs of entangled photons to test Leggett鈥檚 formula (see 鈥淭he end of reality鈥�). The team, led by Markus Aspelmeyer of the Austrian Academy of Sciences and Anton Zeilinger of the University of Vienna, managed to reduce the noise in their set-up by a necessary factor of 10, compared with Aspect鈥檚 work. They published their results in April .

What they found is that Leggett鈥檚 formula is violated as well: even if you allow for instantaneous influences, quantum measurements do not fit with the idea of an objective reality. This is surprising because you might expect that, once any spooky 鈥渘on-local鈥� action is allowed, you could account for almost any relationship between two particles, and there would be no reason to ditch our concepts of reality. 鈥淭his is not the case,鈥� says Aspelmeyer.

Although some loopholes remain 鈥� not all non-local models have been ruled out 鈥� we now have to face the possibility that there is nothing inherently real about the properties of an object that we measure. In other words, measuring those properties is what brings them into existence. 鈥淩ather than passively observing it, we in fact create reality,鈥� says quantum researcher Vlatko Vedral of the University of Leeds, UK.

鈥淩ather than passively observing it, we in fact create reality鈥�

This idea may not be new, but the evidence for it is, and it could have serious implications for a theory of everything: it tells us that what we think of as real is not necessarily so. 鈥淲e know from our experience that there is a 鈥榬eal鈥� world with 鈥榬eal鈥� physical events, starting from clicks in a detector in the laboratory to experiencing a headache after too many beers,鈥� Aspelmeyer says. But that doesn鈥檛 mean our physical theories ought to slavishly follow that experience, he points out 鈥� perhaps they need to dig deeper.

While quantum researchers may find this satisfying, it raises deep concerns for anyone attempting to unify the universe. General relativity, Einstein鈥檚 theory of gravity, is fully realistic 鈥� it relies on things existing independent of measurements. So the search for a theory of everything, which involves uniting quantum physics with general relativity, may be even more difficult than we thought. 鈥淚t is not at all clear how to construct a theory of gravity that is not real, which is what we need to do if we want to quantise gravity,鈥� Vedral says.

Spooky space-time

As Einstein suggested decades ago, entanglement could be the key. 鈥淯nderstanding entanglement means understanding a great deal about the principles upon which physical theories are based,鈥� Aspelmeyer says. His Vienna colleague 脟Caslav Brukner goes further. For more than two decades, Brukner points out, people have been saying that physics is almost wrapped up, yet if anything we seem to have reached a stalemate. 鈥淲e need to rethink and radically revise our basic physical concepts before we can make the next big breakthrough in physics,鈥� he says.

Some physicists working on unified theories are well aware of this. In terms of 鈥渘ew ideas about quantum gravity鈥�, says Smolin, 鈥渘on-locality is certainly at the core鈥�. His particular area, loop quantum gravity, does not presume that space and time behave as Einstein鈥檚 relativity dictates (New 杏吧原创, 12 August 2006, p 28). As well as allowing for spooky instantaneous signalling across space-time, those working on similar models are revisiting the fundamental aspects of quantum mechanics. A recent paper Smolin wrote with Perimeter colleague Fotini Markopoulou points out, for instance, that loop quantum gravity may conflict with common notions of entanglement ().

Translating these studies into a deeper theory won鈥檛 be easy. Physicist David Deutsch at the University of Oxford warns that even re-examining entanglement might not help us find the path to a theory of everything. According to Deutsch, we are blocked by something even more fundamental than that.

Entanglement is real, he says, but it tells us more about how information can be extracted from quantum systems than the nature of the physical universe. All the philosophical hand-wringing over entanglement is based on the 鈥渄elusion鈥� that we have a basic grasp on quantum theory, he adds. Just because we have made one leap away from the classical world doesn鈥檛 mean we鈥檝e reached the heart of quantum truth. 鈥淭his local realism stuff is all to do with whether it is possible to have a classical world view,鈥� Deutsch says. 鈥淚t鈥檚 a completely pointless controversy that should have ended in the 1950s.鈥�

While his world view is clearly quantum, all Deutsch will say about a theory of everything is that it is likely to come from uniting quantum theory and relativity at a more fundamental level than current entanglement experiments allow.

This is, of course, where we are still scrabbling for clues. 鈥淭he whole underlying problem, ultimately, is that we lack experimental observations in the region where quantum and gravitational effects both matter,鈥� Vedral says. 鈥淕ravity works well in its own domain and so does quantum physics.鈥� What we have to decide, he says, is whether gravity or quantum theory is more fundamental.

So does the universe exist independently of measurements? That is a question we will have to face. Maybe it is time to revisit Einstein鈥檚 lost quest, if we are serious about uncovering the basic laws of the universe; the money spent on particle smashers such as the Large Hadron Collider certainly suggests we are. Perhaps we need to move quantum entanglement and the nature of reality to the centre of the quest to find a theory of everything. What was once a quirky sideshow may yet prove to be the main event.