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什么是真实的?——对量子物理意义的未完成的追求

(2018-03-28 21:37:56)
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   什么是真实的?
——对量子物理意义的未完成的追求


大约90年前,物理学中的所有东西都被打破了。量子理论出现了 - 部分原因是阿尔伯特爱因斯坦和尼尔斯玻尔之间的激烈冲突。它对科学的本质提出了挑战,并且可以继续这样做,通过严重地束缚理论与现实性质之间的关系。科学作家和天体物理学家亚当贝克尔在“真实的东西”中探讨了这个纠结的故事。

贝克尔质疑哥本哈根对量子力学的解释霸权。玻尔和维尔纳海森堡在20世纪20年代提出了这一理论,该理论认为物理系统只有在测量之前才具有可能性,而不是具体属性。贝克认为,试图解析这种解释如何反映我们生活的世界是一种不透明的运动。显示科学的进化受到历史事件的影响 - 包括社会学,文化,政治和经济因素 - 他探索了替代解释。如果事情在二十世纪二十年代有不同的表现,他断言,我们对物理的看法可能会非常不同。

贝克尔在布鲁塞尔举行的1927年索尔维会议上徘徊,那里有29位杰出的科学家聚集在一起讨论刚刚起步的量子理论。在这里,玻尔,爱因斯坦和其他人,包括欧文薛定谔和路易斯德布罗意之间的分歧达到了顶点。玻尔提出,如果实体(如电子)没有被观察到,那么它们只有概率,但爱因斯坦认为它们具有独立的现实,这促使他着名的声称“上帝不玩骰子”。多年以后,他补充道:“我们称之为科学的唯一目的就是确定是什么。”突然间,科学实在论 - 确认科学理论大致反映了现实的观点 - 受到了威胁。

量子现象对许多人来说显然令人困惑。首先是波粒二象性,其中光可以像粒子一样起作用,而诸如电子之类的粒子像光波一样干扰。根据波尔的观点,一个系统根据上下文而表现为一个波或一个粒子,但是你无法预测它会做什么。

其次,海森堡表明,不确定性,例如关于粒子的位置和动量,是硬连接到物理学的。第三,玻尔争辩说,我们可能只有一个系统的概率知识:在薛定谔的思想实验中,一个盒子里的猫既是死又活的,直到它被看到。第四,粒子可能会纠缠在一起。例如,两个粒子可能有相反的自旋,无论它们有多远:如果你测量一个旋转起来,你立即知道另一个旋转下来。 (爱因斯坦称之为“远处的幽灵行动”)。
贝克尔解释了这些观察结果如何挑战局部性,因果性和决定论。在经典的台球,从树上落下的射弹和苹果的世界里,它们从来都不是问题。

通过对历史的回顾,贝克展示了玻尔作为一个反现实主义者如何将海森堡,沃尔夫冈波利和马克斯波恩等许多新兴物理学家带到他的身边。然而,爱因斯坦坚持认为哥本哈根的解释是不完整的。他猜测可能存在潜在的量子现象的变量或过程;或者也许是由德布罗意提出的“先导浪潮”控制着粒子的行为。 1932年,数学家约翰·冯·诺伊曼(John von Neumann)提出了一个证明,即量子力学中可能没有隐含的变量。尽管在数学上是正确的,但几十年后却发现它存在缺陷。但是损害已经完成:爱因斯坦和德布罗意构想的潜在可行的备选方案仍然相对未开发。哥本哈根的解释在20世纪30年代就已经实现,今天的教科书指出玻尔的观点“获胜”。

因此,索尔维会议可以被看作是两个数学等价但根本不同的范式之间的一个对立面:玻尔的量子物理的工具主义观点和爱因斯坦的现实主义观点。在科学领域,一个主导范式决定了哪些实验已经完成,如何解释它们以及研究计划遵循何种途径。

但是如果一个领域选择了错误的范式呢?贝克尔展示了在20世纪50年代和60年代,少数物理学家如何摒弃爱因斯坦和德布罗意的理论,并将其转化为能够改变现状的完全的解释。 David Bohm认为量子系统中存在的粒子无论是否被观测到,并且他们有可预测的位置和运动由导波决定。约翰·贝尔接着表明,爱因斯坦对哥本哈根解释中的地点和不完整性的担忧是有效的。正是他揭示了冯·诺依曼的证据,揭示它只排除了一小类隐变量理论。

科学界冷静地迎接了博姆的想法。前任导师J.罗伯特奥本海默说:“如果我们不能反驳博姆,那么我们必须同意忽视他。”而且,正如贝克尔所表明的,博姆的左派观点导致众议院非美活动委员会出现,并随后被排斥。

博姆的当代物理学家休·埃弗里特对哥本哈根的解释提出了另一个挑战。 1957年,埃弗里特开始着手解决量子理论中的“测量问题” - 量子级别上的粒子的概率性质与其“崩溃”之间的矛盾,在测量时,在宏观层面上成为一种状态。

埃弗雷特的许多世界的解释不会崩溃。相反,在测量时,概率分叉成平行的宇宙 - 例如薛定谔的猫活着,另一个死亡。尽管无数不可测试的宇宙似乎对某些人来说是不科学的,但今天许多物理学家认为这一理论很重要。

这本书有一些小缺点。贝克尔给予贝尔研究基础上最近的应用程序提供了太多的空间,对科学哲学的新发展太少了。然而,就像宇宙学家肖恩·卡罗尔(Sean Carroll)在其2016年的大图(R. P. Crease Nature 533,34; 2016)中所做的那样,确实为哲学的重要性提供了一个明确的例子。这是一个关键的呼吁,像尼尔德格拉斯泰森这样的有影响力的科学家将纪律视为浪费时间。

什么是真实的?是保持开放思想的一个论据。贝克尔提醒我们,当我们调查解释相同数据的无数解释和叙述时,我们需要谦逊。

Nature 555,582-584(2018)


What Is Real?: The Unfinished Quest for the Meaning of Quantum Physics Adam Becker Basic: 2018.

All hell broke loose in physics some 90 years ago. Quantum theory emerged — partly in heated clashes between Albert Einstein and Niels Bohr. It posed a challenge to the very nature of science, and arguably continues to do so, by severely straining the relationship between theory and the nature of reality. Adam Becker, a science writer and astrophysicist, explores this tangled tale in What Is Real?.

Becker questions the hegemony of the Copenhagen interpretation of quantum mechanics. Propounded by Bohr and Werner Heisenberg in the 1920s, this theory holds that physical systems have only probabilities, rather than specific properties, until they’re measured. Becker argues that trying to parse how this interpretation reflects the world we live in is an exercise in opacity. Showing that the evolution of science is affected by historical events — including sociological, cultural, political and economic factors — he explores alternative explanations. Had events played out differently in the 1920s, he asserts, our view of physics might be very different.

Becker lingers on the 1927 Solvay Conference in Brussels, where 29 brilliant scientists gathered to discuss the fledgling quantum theory. Here, the disagreements between Bohr, Einstein and others, including Erwin Schrödinger and Louis de Broglie, came to a head. Whereas Bohr proposed that entities (such as electrons) had only probabilities if they weren’t observed, Einstein argued that they had independent reality, prompting his famous claim that “God does not play dice”. Years later, he added a gloss: “What we call science has the sole purpose of determining what is.” Suddenly, scientific realism — the idea that confirmed scientific theories roughly reflect reality — was at stake.

Quantum phenomena were phenomenally baffling to many. First was wave–particle duality, in which light can act as particles and particles such as electrons interfere like light waves. According to Bohr, a system behaves as a wave or a particle depending on context, but you cannot predict which it will do.

Second, Heisenberg showed that uncertainty, for instance about a particle’s position and momentum, is hard-wired into physics. Third, Bohr argued that we could have only probabilistic knowledge of a system: in Schrödinger’s thought experiment, a cat in a box is both dead and alive until it is seen. Fourth, particles can become entangled. For example, two particles might have opposite spins, no matter how far apart they are: if you measure one to be spin up, you instantly know that the other is spin down. (Einstein called this “spooky action at a distance”.)

Becker explains how these observations challenge locality, causality and determinism. In the classical world of billiard balls, projectiles and apples falling from trees, they were never problems.

Sifting through the history, Becker shows how Bohr, as an anti-realist, brought to his side many rising physicists, including Heisenberg, Wolfgang Pauli and Max Born. Einstein, however, persistently argued that the Copenhagen interpretation was incomplete. He conjectured that there might be hidden variables or processes underlying quantum phenomena; or perhaps ‘pilot waves’, proposed by de Broglie, govern the behaviour of particles. In 1932, mathematician John von Neumann produced a proof that there could be no hidden variables in quantum mechanics. Although mathematically correct, it was revealed to be flawed decades later. But the damage had been done: the potentially viable alternatives conceived by Einstein and de Broglie remained relatively unexplored. The Copenhagen interpretation had taken hold by the 1930s, and textbooks today state that Bohr’s view ‘won’.

Thus, the Solvay Conference can be seen as a stand-off between two mathematically equivalent but fundamentally different paradigms: Bohr’s instrumentalist view of quantum physics and Einstein’s realist one. In science, a dominant paradigm determines which experiments are done, how they’re interpreted and what kind of path a research programme follows.

But what if a field picks the wrong paradigm? Becker shows how, in the 1950s and 1960s, a handful of physicists dusted off the theories of Einstein and de Broglie and turned them into a fully fledged interpretation capable of shaking up the status quo. David Bohm argued that particles in quantum systems existed whether observed or not, and that they have predictable positions and motions determined by pilot waves. John Bell then showed that Einstein’s concerns about locality and incompleteness in the Copenhagen interpretation were valid. It was he who refuted von Neumann’s proof by revealing that it ruled out only a narrow class of hidden-variables theories.

The scientific community greeted Bohm’s ideas coolly. A former mentor, J. Robert Oppenheimer, said: “if we cannot disprove Bohm, then we must agree to ignore him”. And, as Becker shows, Bohm’s leftist views led to an appearance before the House Un-American Activities Committee, and subsequent ostracization.

Bohm’s contemporary, physicist Hugh Everett, delivered another challenge to the Copenhagen interpretation. In 1957, Everett set out to resolve the ‘measurement problem’ in quantum theory — the contradiction between the probabilistic nature of particles at the quantum level and their ‘collapse’, when measured, into one state at the macroscopic level.

Everett’s many-worlds interpretation posited no collapse. Instead, probabilities bifurcate at the moment of measurement into parallel universes — such as one in which Schrödinger’s cat is alive and another in which it’s dead. Although an infinite number of untestable universesseems unscientific to some, many physicists today view the theory as important.

The book has a few minor shortcomings. Becker gives too much space to recent applications building on Bell’s research, and too little to new developments in the philosophy of science. Yet he, like cosmologist Sean Carroll in his 2016 The Big Picture (R. P. Crease Nature 533, 34; 2016), does make an explicit case for the importance of philosophy. That’s a key call, with influential scientists such as Neil deGrasse Tyson dismissing the discipline as a waste of time.

What Is Real? is an argument for keeping an open mind. Becker reminds us that we need humility as we investigate the myriad interpretations and narratives that explain the same data.

https://www.nature.com/articles/d41586-018-03793-2

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