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Achilles.Jr

With modest,but glorious temper

 
 
 

日志

 
 

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

  下载LOFTER 我的照片书  |

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

量子理论是一项科学的杰作,但物理学家至今仍不知道该如何来理解它。

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.

整整一百年前,第一届国际物理学会议在比利时布鲁塞尔举行。会议的议题是讨论如何认识新奇的量子理论并把它同我们的日常生活经验联系起来,以期给我们一个对世界清晰自洽的描述。

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.

然而,这个问题现在依然困扰着物理学家。微观粒子所具有的一些性质实在是出乎寻常,比如原子和分子就具有可以在不同地方同时出现的神奇能力,可以同时顺时针和逆时针旋转,或者即使相隔半个宇宙也可瞬间影响到对方。问题是,我们人也是分子和原子组成的,为什么我们就没有上述性质呢?“量子力学的应用立足于何处?”牛津大学的科学哲学家哈维?布朗这样问道。

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.

尽管最终答案还未出现,人类探寻的努力还是有回报的。比如,一个已经引起高科技产业和情报机构注意的全新领域已经诞生。这就是量子信息学。量子信息学可以让我们从一个崭新的角度来探索物理终极理论,它或许还可以告诉我们宇宙的起源。对于一个被量子理论的怀疑者——阿尔伯特?爱因斯坦——嗤之为让优秀物理学家沉睡不醒的“柔软枕头”的理论来说,这已经算是硕果颇丰了。

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?

出乎爱因斯坦所料,量子理论如今已经成为一项杰作。迄今尚无实验与量子理论所做的预言相抵触,并且人们相信它可以在微观尺度上很好地描述宇宙规律。这就导致了最后一个问题:量子理论意味着什么?

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.

物理学家是用“诠释”——一种和实验完全相符的对量子理论本质的哲学思考,来试着回答这个问题的。“现在我们有一大堆诠释。”在牛津大学和新加坡量子技术中心同时任职的弗拉托克?维德勒如是说。

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?

没有一种科学理论可以像量子力学这样可以从这么多角度来理解。为什么会有这样的情况?这么多的诠释中有没有一种可以胜过其他的?

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.

举个现在被称为哥本哈根诠释的量子论诠释作为例子,它是由丹麦物理学家尼尔斯?波尔提出的。该诠释的一个观点是说,任何不通过测量来谈论电子在原子中的位置的尝试都是无意义的。只有当我们用一个非量子的或“经典的”仪器去观察的时候,它才会显示出我们称之为物理性质的属性,进而才会成为现实的一部分。

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.

接着我们还有“多世界诠释”,在该体系中量子奇异性可以通过任何事物在无数平行宇宙的多重存在性得到解释。也许你更喜好“德布罗意-玻姆诠释”,在这里量子理论被认为是不完备的:我们还缺少一些隐藏属性,如果知道它们,我们就能理解所有东西。

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).

还有许多其他的诠释,比如吉亚尔迪-里米尼-韦伯诠释,交易诠释(这其中有逆时间而行的粒子),罗杰?彭罗斯的引力诱导坍缩诠释,模态诠释……在过去的一百年里,量子世界已经变得拥挤而热闹。

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

撇开这些熙攘热闹的景象,对大多数物理学家来说,只有少数解释至关重要。


Wonderful Copenhagen

美妙的哥本哈根

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.

最受欢迎的诠释莫过于波尔的哥本哈根诠释了。它之所以受欢迎,是得益于大多数物理学家不想费神去考虑哲学问题。类似于“到底什么构成了测量”或者“为什么它可能导致现实的改变”这样的问题是可以被忽略的——物理学家只想从量子理论中得到有用的结论。

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.

这就是为什么被不加怀疑而使用的哥本哈根诠释有时也被叫做“闭嘴,乖乖计算”诠释。“考虑到大多数物理学家只是想做计算并将所得结果应用于实际,他们中的绝大多数都是站在‘闭嘴,乖乖计算’这一边的。”维德勒说。

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.

没有其他的领域能比量子信息学更明显地表明这一点了。“如果人们没有担忧过量子物理的基础,量子信息学这个领域甚至不会存在。”奥地利维也纳大学的安东?蔡林格说。

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.

这个领域的核心是量子纠缠现象——一部分粒子的性质的信息被全体粒子所共有。这就导致了被爱因斯坦称为“幽灵般的超距作用”,即测量一个粒子的性质会瞬间影响到另一个和它纠缠的同伴的性质,不管它们之间距离有多远。

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.

当纠缠现象第一次在量子理论的方程中被发现时,它被当作过于奇怪的想法,以至于爱尔兰物理学家约翰?贝尔创造了一个思想实验来表明纠缠现象无法在真实世界中显现。而当真的可以做出这个实验真的之后,它证明了贝尔是错的,并且告诉物理学家有关量子测量的大量细节。它还为量子计算奠定了基础,通过量子计算,以前对粒子进行成千上万的并行测量才能得到的结果,现在单个的测量就可以告诉你答案。此外的应用还有量子密码学,通过利用量子测量的特殊性质来保护信息安全。

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.

不难理解,所有这些技术吸引了政府和渴望最高端技术的工业界的关注——同时防止它们落入敌手。

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.

然而物理学家更感兴趣的是这些现象可以告诉我们哪些自然界的本质规律。量子信息实验暗含的一个结论是说微观粒子包含的信息是实在的根源。

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.

哥本哈根诠释的支持者诸如蔡林格,把量子系统看作信息的载体,而用经典仪器进行的测量不过是记录和显示系统所包含的信息的过程。“测量是在更新信息。”蔡林格说。这个把信息作为实在的基本组成的新观点导致了有人猜测宇宙本身或许就是一台巨大的量子计算机

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.

尽管哥本哈根诠释在大踏步前进,仍然有不少物理学家盯着它的弱点不放。这在很大程度上是由于哥本哈根诠释要求微观量子系统和对它的测量的经典仪器或观察者,二者必须人为区分开。

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.

以宇宙的尺度来考虑事物的本性也给哥本哈根诠释的批评者提供了弹药。如果经典观察者的测量过程对于创造我们观察到的实在是必不可少的,那么是谁的观测使得现有宇宙得以存在?“你确实需要一个在系统外的观察者才能让哥本哈根诠释是合理的——但根据定义,宇宙外没有任何东西。”布朗说。

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.

这就是为什么,布朗说,宇宙学家更倾向于赞同由普林斯顿的物理学家休?埃弗里特在上世纪50年代晚期创立的诠释。他的“多世界诠释”宣称实在并不受限于测量概念。

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.

作为替代的是,量子系统固有的无限可能性在它们自身的宇宙各自显现。大卫?多伊奇,牛津大学的物理学家并曾经为第一台量子计算机拟定蓝图,说他现在只能用平行宇宙的概念来考虑计算机的运行。对他来说,其他的诠释都是无意义的。

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.

并不是说多世界诠释就没有受到批评——事实恰恰相反。新泽西罗格斯大学的科学哲学家蒂姆?莫德林很赞同放弃把测量这一概念当作一个特殊过程。但同时,他也不相信多世界诠释可以提供一个很好的框架来解释为什么一些量子结果要比其他的更有可能出现。

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.

当量子理论预言一个测量的结果出现的可能性要高十倍于另一个,反复的实验可以证明这一点。依照莫德林所说,多世界诠释认为由于世界的多重性,所有的可能都会发生,但它并没有解释为什么观察者看到的总是(通过计算算出的)最可能出现的结果。“这里有个深层问题需要解决。”他说。

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.

多伊奇说这些问题在这一两年内已经被解决。“埃弗里特处理概率的方式是有缺陷的,但这几年里多世界诠释的理论家们已经清除掉了这些缺陷——问题已经解决了。”他说。

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.

然而多伊奇的论证太玄奥了以至于并不是每个人都承认他的说法。更难回答的问题还有被多世界诠释支持者称为“怀疑眼神的反对”。多世界诠释一个明显的推论是说宇宙中有很多你的复制品——比如,猫王现在仍然在另一个宇宙中的拉斯维加斯进行表演。很少有人能接受这种想法。

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."

这个问题只有靠时间来解决了,布朗认为。“人们普遍难以接受存在许多你和其他人的复制品这种想法,”他说,“但这只是人们能否逐渐习惯的问题。”

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).

他和布朗都宣称多世界的概念已经得到宇宙学家的支持。来自弦论、宇宙学和观测天文学的论证已经让宇宙学家猜测我们生活在多重的宇宙中。去年,加州大学圣克鲁兹分校的安东尼?阿奎尔,麻省理工的马克斯?蒂格马克以及哈佛大学的大卫?莱泽完成了把宇宙学和多世界的想法联系起来的大致方案。

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.

这个诠释背后的思想是通过把这些隐变量考虑进去就可以解释量子世界的奇异行为,而这些隐变量会在宇宙微波背景辐射的一些细节处留下踪迹。瓦伦蒂尼说隐变量理论可以比标准的量子力学更好地符合已观测到的宇宙微波背景辐射的结构。

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.

尽管这是个漂亮的想法,但仍然没有确凿的证据可以证明这个理论会大有作为。另外,如果真的观测到宇宙微波背景辐射出乎意料的结果,这也不能作为瓦伦蒂尼假说的证据。维德勒认为:任何诠释都可以声称早期宇宙的环境会导致出人意料的结果。

"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."

我们正陷入一种境地,或许我们永远也不能用实验判定埃弗里特和德布罗意-玻姆诠释的对错。”布朗承认说。但是,他补充说,也不应当因此悲观。“我想我们已经有很多显著的进展了。许多人说因为缺少能区分各种诠释的判决实验,我们什么结论也做不了。但事实上一些诠释确实比另一些要好。”

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.

目前,布朗、多伊奇和蔡林格都不愿放弃他们各自喜欢的量子力学诠释。尽管关于量子力学到底意味着什么的争论没有结束的意思,蔡林格仍然感到高兴。

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."

维德勒对此表示赞同。尽管他把自己放在“多世界俱乐部”里,他也承认,你选择去相信什么样的诠释在很大程度上取决于个人喜好。“在多数情况下你是不能用实验来区分这些诠释的,因此你要做的就是跟着自己的直觉走。”

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.

物理学家们漫游于量子世界,随心所欲地选择他们喜欢的诠释,这看起来有几分不科学,但至少目前看来也没什么坏处。

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."

量子理论已经通过它创造出的产品——比如晶体管和激光——改变了世界,并将继续改变。不同的诠释让物理学家们用不同的思想和不同的方式去做实验。如果历史可以靠得住的话,用开放的眼光来看待量子理论的意义或许可以开启物理学的另一个全新领域,维德勒说:“那就真的很令人兴奋了。”

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