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With modest,but glorious temper




THERE is MORE than one way to think about the UNIVERSE.  

2011-05-07 16:18:04|  分类: 默认分类 |  标签: |举报 |字号 订阅

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Quantum theory is a scientific masterpiece – but physicists still aren't sure what to make of it.


A CENTURY, it seems, is not enough. One hundred years ago this year, the first world physics conference took place in Brussels, Belgium. The topic under discussion was how to deal with the strange new quantum theory and whether it would ever be possible to marry it to our everyday experience, leaving us with one coherent description of the world.


It is a question physicists are still wrestling with today. Quantum particles such as atoms and molecules have an uncanny ability to appear in two places at once, spin clockwise and anticlockwise at the same time, or instantaneously influence each other when they are half a universe apart. The thing is, we are made of atoms and molecules, and we can't do any of that. Why? "At what point does quantum mechanics cease to apply?" asks Harvey Brown, a philosopher of science at the University of Oxford.


Although an answer has yet to emerge, the struggle to come up with one is proving to be its own reward. It has, for instance, given birth to the new field of quantum information that has gained the attention of high-tech industries and government spies. It is giving us a new angle of attack on the problem of finding the ultimate theory of physics, and it might even tell us where the universe came from. Not bad for a pursuit that a quantum cynic - one Albert Einstein - dismissed as a "gentle pillow" that lulls good physicists to sleep.


Unfortunately for Einstein quantum theory has turned out to be a masterpiece. No experiment has ever disagreed with its predictions, and we can be confident that it is a good way to describe how the universe works on the smallest scales. Which leaves us with only one problem: what does it mean?


Physicists try to answer this with "interpretations" - philosophical speculations, fully compliant with experiments, of what lies beneath quantum theory. "There is a zoo of interpretations," says Vlatko Vedral, who divides his time between the University of Oxford and the Centre for Quantum Technologies in Singapore.


No other theory in science has so many different ways of looking at it. How so? And will any one win out over the others?


Take what is now known as the Copenhagen interpretation, for example, introduced by the Danish physicist Niels Bohr. It says that any attempt to talk about an electron's location within an atom, for instance, is meaningless without making a measurement of it. Only when we interact with an electron by trying to observe it with a non-quantum, or "classical", device does it take on any attribute that we would call a physical property and therefore become part of reality.


Then there is the "many worlds" interpretation, where quantum strangeness is explained by everything having multiple existences in myriad parallel universes. Or you might prefer the de Broglie-Bohm interpretation, where quantum theory is considered incomplete: we are lacking some hidden properties that, if we knew them, would make sense of everything.


There are plenty more, such as the Ghirardi-Rimini-Weber interpretation, the transactional interpretation (which has particles travelling backwards in time), Roger Penrose's gravity-induced collapse interpretation, the modal interpretation... in the last 100 years, the quantum zoo has become a crowded and noisy place (see diagram).


For all the hustle and bustle, though, there are only a few interpretations that seem to matter to most physicists.


Wonderful Copenhagen


The most popular of all is Bohr's Copenhagen interpretation. Its popularity is largely due to the fact that physicists don't, by and large, want to trouble themselves with philosophy. Questions over what, exactly, constitutes a measurement, or why it might induce a change in the fabric of reality, can be ignored in favour of simply getting a useful answer from quantum theory.


That is why unquestioning use of the Copenhagen interpretation is sometimes known as the "shut up and calculate" interpretation. "Given that most physicists just want to do calculations and apply their results, the majority of them are in the shut up and calculate group," Vedral says.


This approach has a couple of downsides, though. First, it is never going to teach us anything about the fundamental nature of reality. That requires a willingness to look for places where quantum theory might fail, rather than where it succeeds (New Scientist, 26 June 2010, p 34). "If there is going to be some new theory, I don't think it's going to come from solid state physics, where the majority of physicists work," says Vedral.

然而这种方式也有不足之处。首先它不会告诉我们任何关于实在的根本性质。那需要通过去寻找量子理论可能失效的地方来获得,而不是成功的地方。(New Scientist, 26 June 2010, p 34)“如果真要有什么新的理论出现的话,我不认为它会来自大多数物理学家工作的固体物理学领域。” 维德勒说。

Second, working in a self-imposed box also means that new applications of quantum theory are unlikely to emerge. The many perspectives we can take on quantum mechanics can be the catalyst for new ideas. "If you're solving different problems, it's useful to be able to think in terms of different interpretations," Vedral says.

其次,作茧自缚式的研究也意味着不大可能出现量子理论的新的应用。我们对量子理论可以采取的多方面的视角正是新想法产生的催化剂。“如果你正在解决不同的问题,那么用不同的诠释来思考会有好处。” 维德勒说。

Nowhere is this more evident than in the field of quantum information. "This field wouldn't even exist if people hadn't worried about the foundations of quantum physics," says Anton Zeilinger of the University of Vienna in Austria.


At the heart of this field is the phenomenon of entanglement, where the information about the properties of a set of quantum particles becomes shared between all of them. The result is what Einstein famously termed "spooky action at a distance": measuring a property of one particle will instantaneously affect the properties of its entangled partners, no matter how far apart they are.


When first spotted in the equations of quantum theory, entanglement seemed such a weird idea that the Irish physicist John Bell created a thought experiment to show that it couldn't possibly manifest itself in the real world. When it became possible to do the experiment, it proved Bell wrong and told physicists a great deal about the subtleties of quantum measurement. It also created the foundations of quantum computing, where a single measurement could give you the answer to thousands, perhaps millions, of calculations done in parallel by quantum particles, and quantum cryptography, which protects information by exploiting the very nature of quantum measurement.


Both of these technologies have, understandably, attracted the attention of governments and industry keen to possess the best technologies - and to prevent them falling into the wrong hands.


Physicists, however, are actually more interested in what these phenomena tell us about the nature of reality. One implication of quantum information experiments seems to be that information held in quantum particles lies at the root of reality.


Adherents of the Copenhagen interpretation, such as Zeilinger, see quantum systems as carriers of information, and measurement using classical apparatus as nothing special: it's just a way of registering change in the information content of the system. "Measurement updates the information," Zeilinger says. This new focus on information as a fundamental component of reality has also led some to suggest that the universe itself is a vast quantum computer.


However, for all the strides taken as a result of the Copenhagen interpretation, there are plenty of physicists who would like to see the back of it. That is largely because it requires what seems like an artificial distinction between tiny quantum systems and the classical apparatus or observers that perform the measurement on them.


Vedral, for instance, has been probing the role of quantum mechanics in biology: various processes and mechanisms in the cell are quantum at heart, as are photosynthesis and radiation-sensing systems (New Scientist, 27 November, p 42). "We are discovering that more and more of the world can be described quantum mechanically - I don't think there is a hard boundary between quantum and classical," he says.

例如,维德勒曾经探寻过量子力学在生物中所扮演的角色:细胞中各种各样的过程和机制本质上都是量子的,比如光和作用和光线感知系统(New Scientist, 27 November, p 42)。“我们发现世界上越来越多的东西可以用量子力学来描述——我并不认为在‘量子’和‘经典’之间有明确的界限。”他说。

Considering the nature of things on the scale of the universe has also provided Copenhagen's critics with ammunition. If the process of measurement by a classical observer is fundamental to creating the reality we observe, what performed the observations that brought the contents of the universe into existence? "You really need to have an observer outside the system to make sense - but there's nothing outside the universe by definition," says Brown.


That's why, Brown says, cosmologists now tend to be more sympathetic to an interpretation created in the late 1950s by Princeton University physicist Hugh Everett. His "many worlds" interpretation of quantum mechanics says that reality is not bound to a concept of measurement.


Instead, the myriad different possibilities inherent in a quantum system each manifest in their own universe. David Deutsch, a physicist at the University of Oxford and the person who drew up the blueprint for the first quantum computer, says he can now only think of the computer's operation in terms of these multiple universes. To him, no other interpretation makes sense.


Not that many worlds is without its critics - far from it. Tim Maudlin, a philosopher of science based at Rutgers University in New Jersey, applauds its attempt to demote measurement from the status of a special process. At the same time, though, he is not convinced that many worlds provides a good framework for explaining why some quantum outcomes are more probable than others.


When quantum theory predicts that one outcome of a measurement is 10 times more probable than another, repeated experiments have always borne that out. According to Maudlin, many worlds says all possible outcomes will occur, given the multiplicity of worlds, but doesn't explain why observers still see the most probable outcome. "There's a very deep problem here," he says.


Deutsch says these issues have been resolved in the last year or so. "The way that Everett dealt with probabilities was deficient, but over the years many-worlds theorists have been picking away at this - and we have solved it," he says.


However Deutsch's argument is abstruse and his claim has yet to convince everyone. Even more difficult to answer is what proponents of many worlds call the "incredulous stare objection". The obvious implication of many worlds is that there are multiple copies of you, for instance - and that Elvis is still performing in Vegas in another universe. Few people can stomach this idea.


Persistence will be the only solution here, Brown reckons. "There is a widespread reluctance to accept the multiplicity of yourself and others," he says. "But it's just a question of getting used to it."


Deutsch thinks this will happen when technology starts to use the quantum world's stranger sides. Once we have quantum computers that perform tasks by being in many states at the same time, we will not be able to think of these worlds as anything other than physically real. "It will be very difficult to maintain the idea that this is just a manner of speaking," Deutsch says.

多伊奇认为当量子世界奇怪方面可以用到现实技术中时,人们将能接受多世界的概念。一旦量子计算机可以实现在同一时间在不同的状态来处理任务,我们将不会认为这些多重的世界不是物理层面的事实。“到时候人们将会很难坚持说多世界的想法只是嘴上说说而已。” 多伊奇说。

He and Brown both claim that many worlds is already gaining traction among cosmologists. Arguments from string theory, cosmology and observational astronomy have led some cosmologists to suggest we live in one of many universes. Last year, Anthony Aguirre of the University of California, Santa Cruz, Max Tegmark of the Massachusetts Institute of Technology, and David Layzer of Harvard University laid out a scheme that ties together ideas from cosmology and many worlds (New Scientist, 28 August 2010, p 6).


But many worlds is not the only interpretation laying claim to cosmologists' attention. In 2008, Anthony Valentini of Imperial College London suggested that the cosmic microwave background radiation (CMB) that has filled space since just after the big bang might support the de Broglie-Bohm interpretation. In this scheme, quantum particles possess as yet undiscovered properties dubbed hidden variables.

多世界诠释并不是引起宇宙学家注意的唯一的诠释。在2008年,伦敦帝国理工的安东尼?瓦伦蒂尼指出在大爆炸之后就充满宇宙的宇宙微波背景辐射或许能支持德布罗意-玻姆诠释。在这个方案下,微观粒子具有未被发现的被称为“隐变量”的性质。(New Scientist, 28 August 2010, p 6)

The idea behind this interpretation is that taking these hidden variables into account would explain the strange behaviours of the quantum world, which would leave an imprint on detailed maps of the CMB. Valentini says that hidden variables could provide a closer match with the observed CMB structure than standard quantum mechanics does.


Though it is a nice idea, as yet there is no conclusive evidence that he might be onto something. What's more, if something unexpected does turn up in the CMB, it won't be proof of Valentini's hypothesis, Vedral reckons: any of the interpretations could claim that the conditions of the early universe would lead to unexpected results.


"We're stuck in a situation where we probably won't ever be able to decide experimentally between Everett and de Broglie-Bohm," Brown admits. But, he adds, that is no reason for pessimism. "I think there has been significant progress. A lot of people say we can't do anything because of a lack of a crucial differentiating experiment but it is definitely the case that some interpretations are better than others."


For now, Brown, Deutsch and Zeilinger are refusing to relinquish their favourite views of quantum mechanics. Zeilinger is happy, though, that the debate about what quantum theory means shows no sign of going away.


Vedral agrees. Although he puts himself "in the many worlds club", which interpretation you choose to follow is largely a matter of taste, he reckons. "In most of these cases you can't discriminate experimentally, so you really just have to follow your instincts."


The idea that physicists wander round the quantum zoo, choosing a favourite creature on a whim might seem rather unscientific, but it hasn't done us any harm so far.


Quantum theory has transformed the world through its spin-offs - the transistor and the laser, for example - and there may be more to come. Having different interpretations to follow gives physicists ideas for doing experiments in different ways. If history is anything to go by, keeping an open mind about what quantum theory means might yet open up another new field of physics, Vedral says. "Now that really would be exciting."


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