内容来源：https://www.quantamagazine.org/where-does-the-quantum-world-end-and-ours-begin-20260917/

内容总结：

量子力学的边界在哪里？——理论物理学家乔纳森·哈利韦尔谈量子世界与经典世界的过渡

量子力学或许是科学中被最精确检验的理论范式之一，但关于它究竟揭示了怎样的现实本质，至今仍没有一种被普遍接受的诠释。量子系统的波动行为，是如何过渡到我们熟悉的宏观物体乃至整个宇宙的坚实世界的？

伦敦帝国理工学院理论物理学教授乔纳森·哈利韦尔毕生致力于探究量子理论的基础问题。在近日一档科学播客节目中，他向主持人史蒂文·斯特罗加茨解释了“退相干”——即脆弱的量子行为通过与周围环境相互作用而消散的过程——为何是理解量子到经典过渡的关键，并介绍了他自己用于理解这一过程的“历史”方法。在对话中，哈利韦尔还回应了该领域一些最深层的悖论：为什么量子到经典的过渡并不需要一个有意识的观测者？爱因斯坦那句著名的问题——“我不看月亮的时候，月亮还在那里吗？”——又该如何理解？访谈最后，哈利韦尔谈到了瑜伽和冥想如何帮助他面对科学中不同视角并存的奥秘。

节目还回顾了量子力学中一些经典思想实验，包括薛定谔的猫和双缝实验。哈利韦尔指出，量子理论的核心在于波，而非粒子。“人们常说有时是粒子，有时是波。但量子理论实际上说的是：永远是波，只是有时候波会高度集中在某一点附近。”他用一个生动的比喻加以说明：就像滑雪者在雪地中回头看自己的轨迹，发现轨迹同时出现在树的两侧——这在经典世界不可能，但在量子世界却真实发生。

在谈到“宏观实在论”时，哈利韦尔介绍了莱格特-加格不等式等检验方法。这类实验试图验证一个看似理所当然的假设：物体在被测量之前就具有确定的状态或属性。然而，量子力学的实验结果表明，这一假设在微观层面并不成立。哈利韦尔强调，这些检验并不是要证明量子力学“正确”——它早已被无数次验证——而是要排除某些非常自然的经典世界观。

关于“观测者”问题，哈利韦尔明确指出，量子到经典的过渡并不需要人类意识参与。退相干过程表明，环境本身——哪怕只是一堆无意识的分子——就足以使量子叠加态失去可观测性。量子特性并未真正消失，而是扩散到了整个环境之中，变得极难测量。

哈利韦尔还讨论了玻姆力学等替代诠释，以及量子信息科学对理解量子基础问题的影响。他认为，关于量子力学诠释，未来可能会看到更多“说同一件事的不同方式”，而他自己则更愿意“与奥秘共处”。他说：“我相信奥秘。与不确定性和奥秘共处，才是最舒适的姿态。”

值得注意的是，哈利韦尔在访谈中也谈到，公众对宇宙起源的理解往往带有某种“神话”色彩。他引用美国宇宙学家布莱恩·斯威姆的观点指出，每个社会都有自己的创世神话，而现代社会或许可以将大爆炸宇宙学视为一种当代神话——不是将其当作真理，而是作为一种赋予意义和方向的叙事。这一观点在节目后段引发了两位主持人的进一步讨论。

主持人之一、天体物理学家詹娜·莱文在节目尾声回忆了她与哈利韦尔早年的交往，并表示哈利韦尔在量子宇宙学中关于退相干的研究具有开创性意义——“他精确而深思熟虑的工作，正是他早期声誉的来源：你可以在这个宇宙内部完成所有这些描述。”

本期节目由Quanta Magazine制作，属于其播客栏目“The Joy of Why”。

中文翻译：

量子世界在哪里终结，我们的世界又从哪里开始？

引言

量子力学也许是科学中经过最精确检验的范式之一，但对于它究竟向我们揭示了怎样的实在，至今仍没有一个被普遍接受的诠释。我们是如何从量子系统的波状行为，过渡到由物体乃至整个宇宙构成的坚实、宏观世界的呢？

伦敦帝国理工学院理论物理学教授乔纳森·哈利韦尔毕生致力于探究量子理论的基础。在本期节目中，他向史蒂文·斯特罗加茨解释了为什么退相干——即脆弱的量子行为通过与周围环境的相互作用而消散的过程——是理解量子行为向经典行为过渡的关键，并描述了他用来理解这一过程的“历史”方法。在此过程中，哈利韦尔直面了该领域一些最深刻的悖论。他解释了为什么量子到经典的过渡不需要有意识的观察者，并探讨了爱因斯坦那个著名的问题：当我们不看月亮时，它是否真的还在那里。对话最后，哈利韦尔讲述了瑜伽和冥想如何帮助他与科学中相互竞争的观点之谜共处。

可在苹果播客、Spotify、TuneIn或你喜欢的播客应用上收听，也可以从Quanta网站流媒体播放。

文字记录

[音乐播放]

史蒂夫·斯特罗加茨：好了，我们开始吧。我是史蒂夫·斯特罗加茨。

詹娜·莱文：我是詹娜·莱文。

斯特罗加茨：这里是《为什么的乐趣》。

莱文：一档来自Quanta杂志的播客，我们在这里探索当今数学和科学中一些最重大的未解之谜。

斯特罗加茨：是的，詹娜，大问题。今天我们请来的是最重大的问题之一。

莱文：好，我很兴奋。我准备好了。我做好心理准备了。

斯特罗加茨：是的，做好准备，系好安全带。今天讨论的是量子力学的基础，这是很多人都在担心的真正悖论之一。

莱文：是啊，这确实是那个伟大的未解难题。量子力学是整个科学中经过最精确检验的范式，精确到了小数点后最多的位数，但仍然没有一个自洽的诠释。我不认识任何会声称自己完全理解量子力学的人。

斯特罗加茨：大家都是这么说的，对吧？如果你声称自己理解了它，那只能说明你其实没理解。

莱文：对，没错。一下子就露馅了。

斯特罗加茨：但这真是一件令人惊讶的事。正如你所说，这个理论，尤其是量子场论或量子电动力学这些更高级的形式，如你所提到的，现在已经被检验到了大概……我不知道，万分之一万亿的精度，或者某种惊人的精度。

莱文：是的。某个惊人的数字。绝对是的。所以我们可以把东西送上太空。它在你手机里运行，在加速器实验中也能达到那样的精度，但我们实际上并没有一个对我们来说说得通的诠释，也许这只是因为我们那颗巨大的经典大脑太过笨重，无法理解。但我从不觉得这令人满意，也没有人是靠双手一摊、放弃努力做出伟大发现的，对吧？

斯特罗加茨：[笑]是的，好吧，我们要和一位叫乔纳森·哈利韦尔的聊聊，他是伦敦帝国理工学院的理论物理学教授，他对量子力学基础的这些问题思考了很多，但他是从量子宇宙学切入的，所以和你的领域很接近。

莱文：是的，我认识乔纳森。当年我们经常在伦敦聚会。我当时在剑桥，乔纳森当然也在剑桥待过，后来在帝国理工。已经过了很久了，所以我非常想听听他最新的想法。

斯特罗加茨：好，那我们就不多说了，直接开始吧。交给你了，乔纳森。

[音乐播放]

斯特罗加茨：欢迎来到《为什么的乐趣》，乔纳森。很高兴你能来参加节目。

乔纳森·哈利韦尔：很高兴见到你，也很高兴来到这里。

斯特罗加茨：我想从你职业生涯的起点开始聊起，据我所知，那是在一个我们称之为量子宇宙学的领域，也就是将量子力学应用于整个宇宙。

我对此的第一个问题是，这想法真是太令人震撼了。但另外，当我在大学学量子力学时，甚至当我以前读科普文章时，我们经常会听到观察者和实验，观察者在实验之外，通过进行测量之类的事情导致波函数坍缩。

但当量子客体是整个宇宙时，观察者不可能在系统之外。我想知道，是不是正是这类问题让你觉得量子理论的基础令人无法抗拒？

哈利韦尔：基本上就是这样。这就是我从量子宇宙学起步、最终进入量子基础领域的原因，就是因为对这类问题的思考。

量子宇宙学最初的动因与这样一个事实有关：大爆炸模型基于量子物质，但引力是经典的。而量子物质加上经典引力，当你沿时间回溯时，正如霍金、彭罗斯等人证明的定理所表明的，初始必然存在一个奇点，至少从经典角度来看是这样。

所以那是一个引力无限大、曲率无限大的区域。这表明，如果它仍然在物理定律的范围内，我们就需要一个量子引力理论。所以也许宇宙的起源是由某种量子理论描述的，它不仅包括物质模式，还包括引力场的某些模式。

我们甚至到现在都还不真正知道完整的量子引力理论是什么，而这是我40年前做那项工作时的情况。但在量子宇宙学中，你能做的是说，好吧，让我们只取引力场的几个要素，比如宇宙的整体大小、标度因子，也许还有几个畸变参数。

然后你可以用相对标准的技术对这些宇宙起源的模型进行量子化。这都有点启发式的。我不确定我会对它抱多大信心，但至少能给你一幅图景。从中得出的结论非常吸引人，就是宇宙有一个非奇异的、本质上是量子的开端。

至少在简单模型中，这就是得出的美妙结果。所以在那种较旧的经典模型中，从几何角度想，宇宙的起源有一个像锥体一样的奇点。锥体末端有一个尖锐的点。这就是经典大爆炸模型的样子。但在量子图景中，当你到非常小的尺度时，最小的部分变成了量子的，然后那个尖锐的点就可以用一种光滑的半球面来封闭。

所以我们心中的几何结构有时被称为羽毛球几何。这个想法来自斯蒂芬·霍金和詹姆斯·哈特尔。它被称为宇宙的无边界方案。这在40年前引起了巨大的兴趣，至今仍然是一个被广泛探索的想法。它是关于宇宙究竟如何起源的极少数模型之一。

所以这就是它有趣的原因。现在，回到你的问题，这真的是量子的吗？观察者呢，波函数坍缩等等呢？那里没有任何观察者来进行测量或使波函数坍缩。所以实际发生的是，现在有观察者了。我们对现在能观测的东西进行测量，比如微波背景或宇宙膨胀，或者更准确地说，是实际测量这些的探测器和照相底片。

我们看这些，我们测量这些，在某种意义上，当我们实际做那些测量时，我们使波函数坍缩了。但问题在于：你如何把今天所做的这些测量——我们基本上称之为记录——如何把这些与过去的大爆炸联系起来？对此的答案是，詹姆斯·哈特尔、默里·盖尔曼等人发展了一种量子力学的新表述，叫做量子力学的历史方法，或者更准确地说，退相干历史方法。

它试图摆脱外部观察者和波函数坍缩的观念。我们实际上是查看现在的记录，寻找它们之间的关联。所以退相干历史方法是在量子力学中一种将现在的记录与我们认为过去发生的事情联系起来的方式。这有点绕，但它避免了必须说过去在某种意义上确实发生了，或者波函数在过去坍缩了。

斯特罗加茨：好的，很好。你已经给了我们很多值得思考的东西，让我们开始逐步构建你在这里提到的一些概念。退相干将是我们讨论的一部分。我们要谈论量子力学的基础。有几个老生常谈的话题，几乎是不可避免的。

其中之一就是薛定谔那只不幸的猫。我想知道你是否可以从提醒我们那只猫开始我们的讨论。那个思想实验本来是要说明什么？

哈利韦尔：嗯，它本来是要说明量子物理中的一个现象，即一个系统、一个粒子、一只猫，可以同时处于两种不同的状态。

其实我有点不愿意谈论猫。这是一种已经固定下来的说法，以至于我那些非物理学界的朋友们当这个话题真的出现时就会说“呃”，我其实能理解。

斯特罗加茨：我猜这是出于一个热爱动物的人或只是一个有道德的人的立场。你对薛定谔的猫有什么异议？

哈利韦尔：两者都有。这个说法在人们还没有考虑这些——姑且说——更伦理的问题的早期阶段就进入了讨论。我的意思是，你现在不能引入那种说法了，我觉得。但这是个小问题。我不想转移话题。

斯特罗加茨：好的[笑]

哈利韦尔：让我们举一个更简单的例子，本质上是相同的，那就是著名的双缝实验……

斯特罗加茨：好的。

哈利韦尔：在这个实验中，你准备一个处于出射波状态的粒子。比如，你有一个电子源，你把它们射向一个有两个孔的屏幕，两个孔相距比如一毫米，另一侧有一个屏幕。

发生的情况是，波穿过两个孔，然后在屏幕上产生干涉图案，一组亮暗相间的斑块。然后你可能会说，但实际上它真的是一个电子。它是一个粒子。它是一个团块。它像台球。它肯定只是穿过了这个或那个孔。如果你实际上挡住其中一个缝或挡住另一个缝，干涉图案就消失了。所以如果你真的假设它是一个确定的粒子，穿过这个或那个缝，你就不会再得到干涉图案了。

所以在非常真实的意义上，你所认为的固体粒子，真的确实同时穿过了两边，并在另一侧产生了干涉。让我给你一个生动的类比。有一本很棒的书，是亚基尔·阿哈罗诺夫写的关于量子悖论的书，封面上有一幅画，画的是一个人在雪地里滑雪穿过森林。

他有平行的轨迹，在某一刻他回头看他的轨迹去了哪里，他观察到他的轨迹出现在一棵树的两侧。这对一个滑雪者来说显然是不可能的，但这正是量子物理中发生的事。电子回头看，注意到干涉仪中的轨迹，它确实穿过了两侧。

斯特罗加茨：我以前没听过这个类比。你描述的几何结构很有趣。让我确认一下我们理解了你之前描述的双缝电子实验。当我们听到波粒二象性时，人们习惯了光是一种波，如果有足够多的光子，我们可以在脑海中做这种连续近似。

我们学到了干涉。但我记得读到的似乎如此令人震惊的是，即使你把光的强度调得如此之低，以至于实际上腔中一次只有一个光子，或者在你的情况下，在某种意义上只有一个电子，就像这位神奇的滑雪者可以与自己发生干涉。

哈利韦尔：完全正确。最初在双缝实验中，他们可能会说：“好吧，有一大片粒子穿过去。”但确实，当你把强度调到极低，使得装置中永远只能有一个粒子时，它仍然具有那个性质。

事实上，他们现在可以用相当大的东西做干涉实验，比如包含数千个原子。有些效应实际上已经接近肉眼可见的边界了。我的意思是，这与宏观量子实验有关，它确实引出了这样一个问题：一个系统能有多大还能与自己发生干涉？

在这一切背后，根本问题实际上是关于量子层面上的世界观。有所有这些奇怪的效果，但在那背后，有这样一种世界观：量子世界实际上是关于波的，而波的直觉属性与粒子的直觉属性完全不同。再举一个简单的例子，如果一辆警车呼啸而过，警笛大作，如果你把车门和车窗都打开，那么声音同时从车门和车窗传进来。这没什么不寻常的。但如果它是一个固体物体，似乎同时从车门和车窗进来，那就相当惊人了。但原则上这是真的，因为在微观层面上，波状才是物质的根本性质。

斯特罗加茨：所以在这种波粒二象性中，听起来你是在主张，为了对量子理论有最好的直觉，我们应该想着波？

哈利韦尔：是的，人们常说有时是粒子，有时是波。量子理论实际上会说什么呢？永远是波。但有时波可以被非常紧密地压缩在单个点周围。

斯特罗加茨：啊，我明白了。

哈利韦尔：所以你可以有一个水波，比如说，一个单一的、紧密聚集的波。如果它向海滩传来，假设水面下有一个低矮的屏障，就像一堵墙，那么一个波就变成了两个：一个被反射，一个被透射。这在量子物理的散射实验中字面意义上也会发生。你射入一些粒子，一些弹回来，一些继续前进。所以你认为是单个粒子的东西最终变成了两个独立的部分。

斯特罗加茨：你之前提到了我希望将是本次讨论核心问题之一的话题，那就是我们如何从具有干涉图案和物质能量波状性质的量子世界，过渡到我们习惯的宏观世界？我们生活的这个世界。我的意思是，为什么我们看不到日常尺度物体——比如月亮或我们自己——的干涉？

哈利韦尔：是的，这是个大问题。我的意思是，第一个特征是所谓的粗粒化。所以波从远处看是平滑的。从三万英尺高空看，海洋几乎是平滑的，但当你靠近时，完全不是那样。

所以这是其中之一：从粗粒化的尺度看，波状图案消失了。但比这更具体。有些描述物理系统的量变化非常缓慢，有些则变化极快。假设你有一个大质量粒子，它被分子轰击。

就像著名的布朗运动例子，你有一个花粉粒子，它相对于原子尺度来说算是大的，被分子轰击使它微微抖动。大粒子实际上会表现为经典的。发生的情况是，来自其他系统——我们通常称之为环境——的轰击，实际上“杀死了干涉”，这是我们用的说法。它通过抹平事物、破坏波状性质来抑制产生波状性质的干涉。

斯特罗加茨：这是一个完美的过渡。量子相干性、量子干涉图案的脆弱性。我喜欢你用的“轰击”这个词，因为虽然它是一个非常猛烈、几乎暴力的词，但它给出了正确的直觉：量子系统总是面临被杂散光子、气体分子、微小电磁场击中的风险。正如你所说，环境有各种方式可以推搡或破坏这种精细的量子态。所以这与你之前用的那个词有关，退相干。对吗？

哈利韦尔：是的。干涉的破坏或相干性的逆转，没错。

斯特罗加茨：那么，退相干就是让一个系统停止以其原始量子特性行事、看起来更像经典的关键吗？

哈利韦尔：正是如此。自现代量子理论从薛定谔开始以来，我们已经有了100年的量子理论。所以第一段旅程是从经典世界进入量子世界，这带我们到了薛定谔著名的方程和所有这些奇怪的量子效应。

但返程也很重要，就是说，如果一切都是量子的，我的桌子却不是，那发生了什么？当你回到粗粒化的世界时，为什么它实际上不是量子的，即使它是由非常量子的东西构成的？这就是退相干的过程。量子效应实际上非常脆弱，正如你所说。你必须努力维持它们。你需要隔离室之类的。而且要找到持续存在的量子效应是相当困难的。

我的意思是，有些效应，比如导电材料的性质等，具有相对稳健的特性。但就那些真正——姑且说——古怪的东西，比如叠加态，那些很容易被破坏。出于各种原因，我花了大量时间研究不同类型的量子效应，理解哪些会很快被杀死，哪些实际上相当稳健。

斯特罗加茨：所以我想问一个与此相关的问题：当我们谈论退相干时，是系统的量子行为变得我们无法获取了？通过与环境的混合或纠缠使我们失去了认知的能力，还是说事情比这更深？不仅仅是我们无法获取知识的问题。

哈利韦尔：我想大多数人会认为这是关于不可获取性的。看，在薛定谔的猫实验中会发生什么：你有一个微观系统，它进入某物两个态的叠加，比如几个自旋，然后这些又依次与一个大系统纠缠——在这个例子中就是那只不幸的猫。

现在在实践中，在现实实验中，任何大系统都会很快被其周围环境、被周围的光子退相干。但纠缠，量子东西，实际上仍然在那里。它只是被散布到四面八方了。所以如果你有一个小粒子，像布朗运动粒子，在某种意义上它实际上仍然是量子的，尽管它被散布了，但信息太多了。你知道，所有的信息碎片都在那里，超出了地平线，你甚至可能永远都拿不到它们。

斯特罗加茨：哈！感觉你把观察者稍微拉进来了，不是说量子本性丢失了，而是它泄漏到了房间里，或者泄漏到了环境中，到那时我们就很难测量了。

哈利韦尔：没错。在量子理论早期有很多关于观察者本人是否在其中扮演某种角色的讨论。我认为约翰·贝尔，他是这些问题的著名早期作者，他实际上以一种我认为有点修辞夸张的方式谈论过这一点。你知道，我们说的观察者是什么意思？而且，在实践中，它意味着某个地方有某种环境，某种周围系统实际上存储了信息。

但它确实仍然留下一个问题：你能不能有一个——姑且说——某种超级观察者，实际上具有无限的信息处理能力，能够实际测量正在发生的一切？原则上，这是真的。实际上有一个——姑且说——信息论的维度。

但部分原因在于，正如你可能知道的，量子系统具有比经典系统大得多的信息存储能力。这是表征量子物理的方式之一。

斯特罗加茨：你还没说“量子计算机”这个词，但这就是你在暗示的？

哈利韦尔：本质上是的。这与量子系统的信息存储能力或处理能力有关。这不是我专长的领域，但当有人发现一个新的量子效应时，它是一个重要的参照点。第一个问题是：它是一种资源吗？我们能做经典系统做不到的事情吗？事实上，在很多情况下我们可以。

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莱文：哇。好的，所以你谈到了量子力学中的观察者。这是量子力学对流行文化最糟糕的影响之一，就是观察者对于量子力学说得通是必需的，观察者按照人们的说法使波函数坍缩。

也许可以这样表述：在观察者使量子客体呈现为确定状态之前，事物处于叠加态。这在很多方面完全是个误导，我相信乔纳森大概费了很大劲来解释你不需要人类意识。但这催生了太多东西，关于意识的那些短暂流行之物。而量子力学，实际上很难将其击退。

所以即使现在，人们仍然会说：“啊，但是你知道，量子和意识。”这些不是相互关联的观念。我相信乔纳森做了非常重要的工作来将它们分开，说观察者可以只是环境，一个无思虑的环境，一堆无思想的分子。

斯特罗加茨：我当然是在阅读量子物理科普文章中长大的，听到很多关于观察者的内容，当然也听到很多关于波函数坍缩的内容。不过我不确定这已经完全被搁置了。你说的是过去被称为哥本哈根诠释的东西，对吧？来自尼尔斯·玻尔和他的学派。

莱文：对。

斯特罗加茨：最初试图不完全说得通，而是对测量问题给出某种哲学解释。它现在仍然存在。

莱文：这仍然是个问题。我认为哥本哈根诠释和多世界诠释仍然存在。我认为退相干历史也许是这两者之外的另一种选择。所以在哥本哈根诠释中，有某种东西导致叠加态坍缩，叠加粒子呈现出一个确定状态，比如说它的位置。

而在多世界中，发生的不是这样。发生的是你，观察者，或者热房间或猫，无论是什么，变得叠加并分支出去，两种可能性共存于某个元世界，某种多世界诠释中，所有可能性不断分支、一再分支地继续被实现。这在量子力学中出人意料地流行，而且完全令人困惑。

斯特罗加茨：非常令人惊讶。

莱文：非常令人惊讶！现在，在我看来，我不知道我是否100%正确，我已经很久没和乔纳森讨论这些想法了，但退相干历史不是这两者中的任何一个。它说：“看，有太多的相互作用，太多的量子概率，以至于它根本无法以那种方式保持形状。”

量子性并没有消失。它只是在某种意义上在这些宏观尺度上变得不那么清晰，因为那里有太多的历史，就像说宏观上有太多的粒子。我认为这就是他的方向。我不知道如果你对量子物理学家做个抽样调查，他们会在所有这些上落在哪里。

斯特罗加茨：嗯，这可能几乎像宗教问题之一，除了信徒之外，还有无神论者、不可知论者，以及所谓的冷漠者，对吧？就是根本不在乎的人。所以可能，我的意思是，我觉得可以说有很多量子物理学家觉得：“我对基础不感兴趣。我想做测量、做实验、得到结果、造新设备。”你可以在不担心基础的情况下走得惊人的远。

莱文：是的，我们可以把东西送上太空。

斯特罗加茨：是的，我们可以。

莱文：我的意思是，我们可以走得非常远。

斯特罗加茨：所以我们有很多内容要覆盖，休息后我们将讨论从宏观实在论到可能的大爆炸神话等一切内容。

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斯特罗加茨：欢迎回到《为什么的乐趣》。我们邀请到了理论物理学家乔纳森·哈利韦尔。

那么，到这里我想深入探讨这个我知道你一直感兴趣、并做了大量研究的宏观实在论概念。让我从引用爱因斯坦的一句话开始，这句话接近我想我们要讨论的内容。

爱因斯坦当年思考量子理论悖论时有一句名言：“我愿意认为月亮即使我不看它的时候也在那里。”回到尼尔斯·玻尔正在大力强调你必须考虑观察者的时代，说一个物体在你测量之前就具有某个速度或位置或自旋状态这样的属性，是否有意义。

爱因斯坦认为：“当然，我愿意相信月亮即使我不看它的时候也在那里。”请告诉我们宏观实在论是什么意思，以及在爱因斯坦和月亮这个小类比中缺失了什么？

哈利韦尔：没人看的时候月亮真的在那里吗？这确实正是问题所在。事实上，第一篇测试宏观实在论的实验论文，我想标题叫“那里没有月亮”之类的。

所以，这里有一整套东西，我可以尝试勾勒一下。一方面，这些宏观实在论的测试，它是经典性的一种版本，它是为在时间上顺序测量的单系统设计的。所以当你测量某物时，你得到的结果不是它实际是什么的真实结果，而是可能取决于其他地方正在发生什么。

这就是所谓的语境性。这就是宏观实在论的莱格特-加格检验中发生的情况。莱格特-加格检验所做的是试图消除对量子结果的经典解释，当你做两次时间测量时。当你有一个非常量子的情况时，你会得到某种从经典角度看相当悖谬的东西。

继续说什么是一宏观实在论，我们所做的是有一个简单系统，比如说它可能是一个自旋，只有两个值，或者它可能更复杂，比如对像月亮这样的粒子做近似位置测量，然后你只需测量很多次，计算平均位置或平均自旋。

重要的是你还必须计算不同自旋之间的关联，比如两个自旋是否相同？然后从这些你实际上可以尝试评估这些关联、这些测量是否与经典模型一致？所谓经典模型，你真正指的是一个底层系统，其中你测量的东西具有确定的值，所以自旋要么是正要么是负，或者位置肯定是这个或者肯定不是那个。

这看起来是如此简单的事情，但这就是值的确定性这一概念，它在量子理论中实际上可能是不成立的，你通过假设它成立，然后推导出一些矛盾后果来证明它不成立。

斯特罗加茨：那么，当你谈到确定的值，一个粒子或量子系统具有确定的值，与什么相对？

哈利韦尔：这是个很好的问题。在量子物理中，你有所谓的叠加态的可能性，一个粒子可以比如同时处于两个地方。所以位置，我们会说，是不确定的。但比这更微妙一些。我的意思是，它可能是一个在这里或在这里的概率分布。

但在量子物理中，你可以做一个测量表明它确实同时处于两个地方，然后你可以在每个地方测量它，发现它确实在那里，也确实在那里。所以有时人们说模糊态或不定态。

你唯一能说的是，如果你假设它是不定态，并且你也假设各种测量不能相互影响，那么你发现一个矛盾。因此，它处于确定状态的假设是错误的。所以这实际上是间接的。你只能反驳确定状态的世界观。这是一个非常难以捉摸的概念。而且它是量子物理中最重要的概念，实际上，我会说，从概念上讲。

斯特罗加茨：确定状态与叠加态的观念，还是……？

哈利韦尔：与叠加态相对，但它甚至超越了那个。有时对于叠加态，你仍然可以找到一个具有确定状态的底层模型。这归结为语境性这个概念。另一种解释不定态的方式是说，当你做一系列测量时，如果它们以你没有注意到的方式相互依赖，那就是不定态可能看起来的样子。

只不过在量子物理中，我们做这些实验的方式使得不存在对语境的依赖。所以两个相距遥远的粒子处于纠缠态，你安排得使它们不可能传递信号。它看起来仍然像它们在互相通信。我的意思是，那才是真正的谜。

有一个量子力学的解释。量子力学的解释是它由波描述，波是一种非局域的东西，在空间中展开。只有当你试图把它想成两个分开的粒子时，它才看起来像两个具有不同状态的粒子。波就像全息图，全息图的每一小部分都包含整体。这是最好的类比，实际上。

斯特罗加茨：我只是想知道，你之前提到双缝实验时，如果我们说粒子确实穿过了左缝或右缝，如果那是真的，它会在记录屏幕上留下一个双峰的 probability distribution，而不是一组干涉条纹？

哈利韦尔：它会改变干涉图案。事实上，你可以把双缝实验变成这些莱格特-加格宏观实在论检验之一。

斯特罗加茨：是吗？

哈利韦尔：所以你在第一次做一个测量。它是穿过了左缝还是右缝？然后你看屏幕上某个小的局部区域说：“它到达这里了没有？”

所以，你有两个时间的简单历史，然后你做一系列关联和平均值的测量，好吧，你发现如果你给这些赋予确定的值，你最终得到一个矛盾，或者更具体地说，你最终得到一个负的概率。

本质上，莱格特-加格不等式或贝尔不等式，它们本质上是某事的概率。但它们实际上结果是负的，这意味着实际上不存在一个底层概率，这意味着你假设它们具有确定的值是错误的。

在莱格特-加格中，我们试图实际解释顺序测量遵循非常非经典的观点，只要我们能证明这种侵入性概念的合理性。我在我的书上花了相当多时间试图理解它，但它非常非常难。整个领域充满了漏洞。

斯特罗加茨：你刚才用的“侵入性”这个词很有趣。它让我想到的是人们过去解释不确定性原理之类的方式，给出一个测量扰动系统的图景，比如电子或光子会把系统撞离轨道。测量的行为可以扰动系统，对吧？

它成了旧式哥本哈根诠释的一个信条。而莱格特-加格检验，如果我理解你的话，也许在说更深层的东西：即使我们非常小心地测量一个系统而不扰动它，我们做一个非侵入性测量，经典图景仍然可能失败。我，我理解对了吗？

哈利韦尔：差不多是这样。所以，在贝尔的情况下，如果粒子靠得很近，你总是可以说：“好吧，你得到了这个量子结果，但实际上一个只是传递信号非常快的经典系统会给出相同的结果。”

现在，在莱格特-加格检验中，你可以做两次顺序测量，如果你得到预期的量子结果，你总是可以说：“好吧，你的第一次测量实际上，你很笨拙。”这是我们用的词。

斯特罗加茨：这个词很有用。

哈利韦尔：在贝尔检验中，它被称为信号漏洞。在莱格特-加格中我们称之为笨拙漏洞，如果你做一个经典的笨拙测量，它实际上会解释量子结果。所以这是真的。你可以编造实际上能做到这一点的模型。

所以这里不是特别的量子问题。它实际上是，你能消除一个具有某种侵入性测量的经典模型吗？

但也有一个量子测量方面与波函数受到影响有关，但你实际上可以把它分解出来。我的意思是，经典地假设，你在高速公路上飞驰，警察有一个雷达枪，他们把雷达弹到你的车上，就是这样算出速度的。

反弹实际上让车减慢了一点点，极其微小的一点点。有动量的变化，但绝对微乎其微。但当你对一个量子粒子做这件事时，一个光子的反弹实际上可能相当显著。

莱格特和加格最初提出了一种创建这种非侵入性的方法，他们称之为理想负测量，假设这个粒子在第一次有两个选项，然后你在选项A上装一个探测器，但不在选项B上。

如果探测器没有响，你说：“好吧，选项B发生了，”但探测器没有装在B上，所以我们实际上没有扰动任何东西。经典地，那实际上是非侵入性的。但那里有一个棘手的争论说：“好吧，它真的是量子的，它实际上是一个波函数，波函数同时穿过两个缝，”所以它在量子层面上仍然是侵入性的。

所以需要做一定数量的旋转才能实际绕过这一点。

斯特罗加茨：但这些类型的想法，实验已经推进了多远？比如，我们还在光子和原子以及微小晶体的微观世界吗？

哈利韦尔：当然。我的意思是，已经做的是相当大的系统的干涉实验。我想牛津的弗拉特科·韦德拉尔，他是这方面的专家，他谈到了薛定谔的病毒作为我们可能达到的那种尺度。

那些实验只是确认当你做双缝实验时存在干涉图案。莱格特-加格检验做的更多。它们不仅确认量子，而且排除了替代的经典解释，这是更强的事情。

这有点有趣。你仍然可以得到干涉图案，但如果干涉不太大，实际上有一个经典模型可以解释它。问题是，干涉是由波穿过两个孔产生的事实创造的，出来的是两个波。它们不是概率，它们是可正可负的波，它们可以相长干涉，所以变得更大。那实际上像经典效应，因为如果你把两个概率相加，它们变大。

但它们可以相消干涉，所以相互抵消。所以你可以得到两个波进来，你得到什么都没有，这在概率中不可能发生。现在，如果相消干涉不太大，你仍然可以把它们建模为经典概率流，那会满足莱格特-加格不等式，这实际上意味着在某个有限范围内存在干涉图案的经典模型。

所以对于这些大粒子的大的干涉仪实验之一，要真正说这肯定是量子的而不是伪经典的，你基本上需要违反莱格特-加格不等式。这在原则上都可以做到。我的意思是，甚至只用现有实验的数据基本上就可以。

斯特罗加茨：让我开始从一些更理论性的点退后，进入一些更个人的东西。其中一个是观点问题，真的，但我打赌你有强烈的观点，那就是到目前为止没有任何实验与量子力学或量子场论矛盾。我不确定我们应该把这一点推进多远，那么我们为什么要一直测试它？我们希望通过把这些量子测试推进到越来越大的系统来了解什么？

哈利韦尔：是的，这是一个很好的观点。确实非常非常引人注目的是，就已经测量到很多很多很多小数位的实验预测而言，形式体系中没有矛盾。以至于实际上我认识量子基础领域的专家，他甚至对我说过：“你为什么要研究确认量子行为的莱格特-加格不等式，当”——用他们的话说——“我们已经知道量子物理无论如何是真的？”

嗯，我对此有意见。我的意思是，这近乎一种接受某物为真理的原教旨主义，这可以是一个更广泛的讨论。但无论理论是什么，它们都必须接受实验检验和确认。我的意思是，真的，绝对如此。

我的意思是，如果我们开始看到差异，那将是一件美妙的事情。但从未观察到与理论的差异仍然是一件非常引人注目的事情。我认为在这背后，是关于量子物理的真正诠释是什么，在所有这些研究、贝尔不等式、莱格特-加格不等式的背后。这些东西所做的是，它们消除了某些非常自然的世界观。

所以贝尔不等式表明量子物理和实验与局域实在论的世界观不一致。所以这个想法是粒子具有独立于遥远粒子正在做什么的确定属性。类似地，莱格特-加格检验的是宏观实在论。粒子具有确定的属性，与过去的测量或未来的测量无关。所以我们知道实在不是什么。

但还有其他世界观，如果你愿意的话，其他诠释。与这些想法最著名的互补之一是关于玻姆力学的，如果你熟悉的话，或者德布罗意-玻姆理论，它基本上取标准量子力学，基于波函数，但说粒子有一个波函数，它也有一个位置轨迹。然后你不需要担心粒子在这里还是那里？粒子肯定有一个轨迹。它肯定在某个地方。波函数退居幕后，作为一种他们称之为导引场的东西，像一种流体流动并告诉粒子做什么，但粒子总是有一个确定的位置。这非常吸引人，它实际上是在量子物理被发明后不久就被发明出来的。

很多人喜欢它，因为基本上你知道你在哪里。你有一条确定的路径。但它有一个特征，就是它是非局域的。因为有一个波函数和一个粒子，粒子去哪里，波函数可以散布到各处。所以粒子的导引场可以取决于非常遥远的观察者。它基本上说，在与遥远粒子的纠缠情况下，一个粒子真的可以取决于另一个粒子在做什么，或者你对另一个粒子测量了什么。但也许就是这样。我的意思是，你可以有一个一切都是确定的但非局域的世界。那也可能实际上是真的。所以这就是为什么我不相信真理。我相信多重视角。

斯特罗加茨：你想展开这个想法吗？在我为我们的对话做准备时，有人告诉我你对瑜伽、冥想之类的东西有兴趣并练习过。这与你在这里描述的那种视角有关吗？

哈利韦尔：它不直接相关，但就我对物理和科学思想所持的态度和视角而言，它在那种意义上帮助了我。所以我认为在早期当我想弄清楚，你知道，宇宙究竟是如何开始的？我们真的很想知道。现在我更愿意与谜共处。

当我做科普演讲时，我有一种感觉，对物理感兴趣的一般公众对缺乏理解感到不适。他们希望物理学家在神话意义上给他们一些关于大爆炸的有意义的东西，他们不满意。对我来说，我在一个更——姑且说——灵性的方面找到了另一种视角，我更能与多重视角的谜共处。

有一位美国宇宙学家，他现在更像是一位神话学家和作家，叫布莱恩·斯威姆，S-W-I-M-M-E，你可能见过。他基本上……从这样一个想法开始：所有社会都有某种形式的创世神话，而现代社会不一定有。

但他说：“好吧，看，让我们把大爆炸宇宙学当作我们时代的现代神话。我们不必把它当作真理，但我们可以把它当作一个有意义的、神话般的故事，从中我们可以获得指引、目的或视角等等。”

而在我做的科普演讲中，我感到这实际上填补了一个空白，人们有大爆炸，但它几乎是作为真理呈现给他们的，但它并没有真正在人们实际想要的程度上满足他们。

斯特罗加茨：对我来说有趣的是，你描述的视角——同时持有不同的可能性并感到舒适——我想说有点像叠加态。你似乎在自己的思考中抵制坍缩。

哈利韦尔：是的。我的意思是，我的一些非物理学界的朋友，甚至物理学界的朋友，他们说：“好吧，你作为一个——你知道——硬核理论物理学家，你怎么能容纳所有这些更难以捉摸的东西，灵性的或神话的或什么的？”

好吧，事情是这样的。你看在物理学中，当然，我们在20世纪初有两次重大革命，一次在相对论，一次在量子理论。在相对论中，突然空间和时间甚至完全不像你想的那样。在量子理论中，物质不像你想的那样。所以物理学家不得不习惯于极大地调整他们的世界观。

总是有新的视角出现。可以肯定的是，下一代科学家将会有新的方式来思考这些事情。

量子信息是一个非常当前的潮流，很多人把诠释问题转化为量子信息的概念。所以我认为我们将看到的是实际上在说关于量子力学的奇异性和谜的同一件事的多重方式。

所以我认为归根结底，我相信谜。与谜和不确定性共处，那是最舒适的位置。事实上，我的一位冥想老师说过，我想，引用几千年来的话，很多这些东西在某种意义上是关于无限的，宇宙的起源。

与其担心“它是什么”，你可以问：“好吧，我与它的关系是什么？”这是简单的问题，我认为，存在于很多灵性传统中。所以这就是我在物理学中的状态。你知道，在所有这些不确定性和浩瀚面前，我感觉如何？

斯特罗加茨：这真是一次愉快的经历。感谢你在《为什么的乐趣》上花时间与我们在一起。这真的非常引人入胜。

哈利韦尔：对我来说也是如此。好的，非常感谢。

[音乐播放]

莱文：我很高兴乔纳森提到了冥想。如果他没提，我不确定我能不能提。乔纳森以前做很多瑜伽。我记得的。那是我也在做很多瑜伽的时期，我们也会谈论那种联系。这种尝试不带假设、猜想和理论去观察，对吧？

我们生活的这两种模式。一种是对世界进行理论化，精确地试图理解它。另一种是冥想思考而不试图理解它。在我看来，这大概就是他所描述的，这种他感到舒适地容纳或——你知道——只是体验的二元性。

斯特罗加茨：嗯嗯。我必须说，作为一个没有练习过冥想的人，我发现自己在与“神话”这个词较劲，当他讨论创世神话时。所以我们听到“神话”这个词被以几种不同的方式使用。有时它被用作“哦，那只是个神话”，作为一个轻蔑的词。他显然不是那样用的。他说创世神话给人们意义，给他们目的，帮助他们在我们发现自己所处的浩瀚无限的宇宙之谜中定位自己。所以我理解对了吗，你觉得？我的意思是，当你听到创世神话时，你听到什么？

莱文：是的，我也对此感到非常惊讶，即使有我刚才说的，观察和容忍谜与缺乏理解的想法。但那和神话不是一回事。在我看来，神话似乎在做一个非常强的事情，就是对一个无法证明的东西做出声明。总之，我能理解为什么听到这个词有点不舒服。

我的意思是，听他谈论那个很有趣。我不确定，即使在那次对话中，他是否在暗示这是人们在寻求理解大爆炸时所寻找的一部分，他们在寻找一个替代品或神话本身。无论人们是否在寻找它，那 certainly 不是我们作为实践科学家试图理解宇宙起源时所做的事。

你知道，如果你今天摧毁所有神话和所有关于它们的记忆并重建文明，会有不同的神话。但量子力学仍然会在那里，你知道吗？无论它是用矩阵还是其他什么来描述的，量子力学仍然是可以被发现的，而神话则不是。我不知道……你怎么看，史蒂夫？

斯特罗加茨：嗯。好吧，这是我的问题，当乔纳森谈到多重视角时，这在量子理论本身中经常出现。所以比如，回到最初，薛定谔的波动力学方法。在那之前，有海森堡、玻恩和约尔丹基于矩阵力学的方法。狄拉克后来有他的做法。费曼最终有他的历史求和做法。我的意思是，在前两者的情况下，矩阵力学和波力学，它们是两种视角，但它们在数学上被证明是等价的。只有一个量子力学，对吧？

所以当乔纳森谈论多重视角时，实际上只有一个宇宙，好吧，撇开多重宇宙不谈，我们似乎所处的这个宇宙，我们可观测的那部分，无论什么多重宇宙，只有一个东西。如果我们有多重视角，它们不是必须一致吗？

莱文：是的。嗯，这真的很有趣，因为他早期关于量子宇宙学中退相干的工作正是为了形式化一个封闭的量子系统如何——宇宙之外没有东西。一切都在封闭系统之内，他在这方面细致而深思熟虑的工作正是使他早期成名的原因，你可以在宇宙本身之内做所有这些。而且，我觉得那相当深刻，实际上。

斯特罗加茨：谢谢。这是一个漂亮的总结。我想我们应该到这里了。

莱文：好了。飞向那辽阔的蓝色远方。好的。下次见。

斯特罗加茨：下次见。

[音乐播放]

莱文：如果你喜欢《为什么的

英文来源：

Where Does the Quantum World End and Ours Begin?
Introduction
Quantum mechanics may be one of the most precisely tested paradigms in science, but there’s still no universally accepted interpretation of what it tells us about reality. How do we get from the wave-like behavior of quantum systems to the solid, macroscopic world of objects and the universe at large?
Jonathan Halliwell, a professor of theoretical physics at Imperial College London, has spent his career probing the foundations of quantum theory. In this episode, he tells Steven Strogatz why decoherence — the process by which fragile quantum behavior becomes dispersed through interactions with the surrounding environment — is key to the transition from quantum to classical behavior, and he describes the “histories” approach that he uses to understand this. Along the way, Halliwell tackles some of the field’s deepest paradoxes. He explains why the quantum-to-classical transition doesn’t require a conscious observer, and he explores Einstein’s famous question of whether the moon is really there when no one looks. The conversation ends with Halliwell explaining how yoga and meditation help him sit with the mystery of competing perspectives in science.
Listen on Apple Podcasts, Spotify, TuneIn or your favorite podcasting app, or you can stream it from Quanta.
Transcript
[Music plays]
STEVE STROGATZ: Alright, here we go. I’m Steve Strogatz.
JANNA LEVIN: And I’m Janna Levin.
STROGATZ: And this is The Joy of Why.
LEVIN: A podcast from Quanta Magazine where we explore some of the biggest unanswered questions in math and science today.
STROGATZ: Yes, Janna, big questions. Today we got one of the biggest of all.
LEVIN: Okay, I’m excited. I’m ready. I’m braced.
STROGATZ: Yeah, brace yourself and buckle up. This is about foundations of quantum mechanics, one of the really paradoxical things that a lot of people worry about.
LEVIN: Yeah, it really is the great unsolved quandary. Quantum mechanics, the most precisely tested paradigm in all of science to the largest number of decimal points, and still there’s no coherent interpretation. I don’t know anyone who’s gonna claim to understand quantum mechanics fully.
STROGATZ: That’s what they say, right? That if you claim to understand it, it’s just a sign that you don’t understand it.
LEVIN: Yeah, exactly. It’s a dead giveaway.
STROGATZ: But it’s such a surprising thing. As you say, the theory, especially in the more advanced flavor of quantum field theory or quantum electrodynamics, as you mention, it’s now been tested to something like, I don’t know, one part in 10 to the 12th or some astonishing precision.
LEVIN: Yeah. Some astonishing number. Absolutely. And so we can send things to space. It works in your phones, and it works to that level of precision in an accelerator experiment, but we don’t actually have an interpretation that makes sense to us, and that might just be our big classical brains are just too lumbering to comprehend. But I never find that satisfying, and nobody’s ever made a great discovery by throwing their hands in the air and giving up, right?
STROGATZ: [Laughs] Yeah, well, we’re gonna be talking with a guy named Jonathan Halliwell, who is a professor of theoretical physics at Imperial College London, and he has given a lot of thought to these questions of foundations of quantum mechanics, but he came at it from quantum cosmology, so pretty close to your own area.
LEVIN: Yeah, I knew Jonathan. Back in the day, we used to hang out in London. I was at Cambridge, and Jonathan, of course, had spent time at Cambridge and was at Imperial. It’s been a while, so I’m very interested to hear what his latest ideas are.
STROGATZ: Okay then, why don’t we just dive in. Take it away, Jonathan.
[Music plays]
STROGATZ: Welcome to The Joy of Why, Jonathan. It’s great to have you on the show.
JONATHAN HALLIWELL: It’s good to meet you and it’s good to be here.
STROGATZ: I’d like to start by talking about the beginning of your career, which my understanding is that it was in an area we would call quantum cosmology, where quantum mechanics gets applied to the universe as a whole.
My question about that, first of all, what a mind-blowing idea. But also, like, when I took quantum mechanics in college or even when I used to read popular accounts of it, frequently we would hear about an observer and an experiment, and the observer is sort of outside the experiment, causing wave functions to collapse by making measurements and that sort of thing.
But when it’s the universe as a whole that’s the quantum object, the observer can’t be outside the system. And I just wonder if that’s the sort of thing that made thinking about foundations of quantum theory irresistible to you.
HALLIWELL: That is essentially it. That’s why I started in quantum cosmology and ended up in quantum foundations, because of wondering about questions like that.
The original impetus for quantum cosmology is to do with the fact that the Big Bang model is based on quantum matter, but classical gravity. And quantum matter plus classical gravity, as you go backwards in time, as theorems due to Hawking, Penrose, and others showed, it must have a singularity initially, at least classically.
So a region of infinite gravity, infinite curvature. So that suggests that if it is still within the laws of physics, we need a quantum theory of gravity. And so perhaps the beginning of the universe is described by some quantum theory, which includes not just the matter modes, but also some modes of the gravitational field.
We don’t really know what a full quantum gravity theory is even yet, and this was 40 years ago when I was doing that work. But in quantum cosmology, what you can do is say, well, let’s just take a few elements of the gravitational fields, like the overall size of the universe, the scale factor, and maybe a few distortion parameters.
And then you can quantize these models of the beginning of the universe using relatively standard techniques. It’s all kind of heuristic. I’m not sure how much faith I would put in it, but at least gives you a picture. And what comes out of that, which is very appealing, is there is a non-singular beginning of the universe which is quantum in nature.
That’s the beauty that comes out, at least in simple models. So in the sort of older classical model, thinking geometrically, the beginning of the universe has a singularity like a cone. A cone has a sharp point at the end. So that’s what a classical Big Bang model looks like. But in the quantum picture, as you go to very small scales, the very smallest bit becomes quantum, and then instead of the sharp point, you can actually close off with a kind of smooth hemisphere.
So the geometry we have in mind is sometimes called a shuttlecock geometry. This idea was due to Stephen Hawking and James Hartle. It’s called the no boundary proposal of the universe. This has caused a huge amount of interest then 40 years ago, and it’s still very much an explored idea. It’s one of the very few models of how the universe actually began.
So that’s why that’s interesting. Now, to your question, is this really quantum? What about observers, collapse of the wave function, and so on? There aren’t any observers there to do the measurement or to collapse the wave function. So what is actually happening is that there are observers now. What we make measurements on things we can look at now, like the microwave background or the expansion of the universe, or more precisely, the detectors and the photographic plates that actually measure those.
We look at those, we measure those, and in some sense, we collapse the wave function when we actually do those measurements. But the question then is: how do you relate those measurements made today, which we would call records, basically, how do we relate those to the Big Bang in the past? And the answer to that is there was a new reformulation of quantum mechanics developed by James Hartle, Murray Gell-Mann, and others, called the histories approach to quantum mechanics, or more precisely, decoherent histories approach.
It tries to get away from ideas of external observers and collapse of the wave function. We’re actually looking at present records and looking for the correlation between them. So decoherent histories approach is a way in quantum mechanics of actually linking records in the present in terms of what we think happened in the past. It’s a bit roundabout, but it avoids having to actually say that the past actually happened in some sense, or the wave function collapsed in the past.
STROGATZ: Well, good. You’ve given us a lot to chew on already, and so let us start building up to some of these notions that you’ve touched on here. Decoherence will be part of our discussion. We are gonna be talking about foundations of quantum mechanics. There are a few old chestnuts that are sort of irresistible, sort of inevitable.
One of them being Schrödinger’s unfortunate cat. I wonder if you could start our discussion by reminding us about that feline. What was that thought experiment supposed to illustrate?
HALLIWELL: Well, it was supposed to illustrate a phenomenon in quantum physics, which is that a system, a particle, a cat, could be in two different states simultaneously.
I’m slightly averse to talking about cats, actually. It’s a sort of nomenclature that’s stuck, to the point that non-physics friends of mine go, “Ugh,” when this actually comes up, and I sort of understand that, actually.
STROGATZ: I’m assuming this is coming from the place of being an animal lover or just an ethical person. What was your objection to Schrödinger’s cat?
HALLIWELL: Both of those. It’s a phrase that’s kind of entered the discussion at a very early stage when people didn’t think about these, let’s say, more ethical things. I mean, you couldn’t introduce that sort of nomenclature these days, I don’t think. But it’s a small point. I don’t want to distract.
STROGATZ: OK [laughs]
HALLIWELL: Let’s take a simpler example, which is essentially the same, and that’s the famous double-slit experiment …
STROGATZ: All right.
HALLIWELL: where you prepare a particle in a sort of outgoing wave state. Like, you just have a source of, say, electrons, and you shoot them at a screen which has got two holes in, say, a millimeter apart, and the far side there’s a screen.
And what happens is that the wave goes through both of the holes, and then it sort of gives an interference pattern, a set of light and dark patches on the screen. And then you might say, well, actually it’s really an electron. It’s a particle. It’s a lump. It’s like a billiard ball. Surely it just went through one or the other. And if you actually block one of the slits or block the other slit, then the interference pattern goes away. So if you actually make the assumption that it’s a definite particle that goes through one or the other, you don’t get the interference pattern anymore.
So in a very real sense, you can have what you think is a solid particle that very genuinely actually goes through both sides and creates the interference on the other side. Let me give you a nice pictorial analogy. There’s a lovely book by, uh, Yakir Aharonov on quantum paradoxes, and on the cover it’s got a picture of a man skiing through a forest in the snow.
And he’s got parallel tracks, and at one stage he looks back to see where his tracks have gone, and he observes that his tracks have gone both sides of a tree. And, and that, that is obviously impossible for a skier, but that’s exactly what happens in quantum physics. The electron looks back and notices the tracks in the interferometer where it really has gone through both sides.
STROGATZ: I have not heard that analogy before. It’s an interesting geometry you describe. So let me just make sure we’re getting it with this electron experiment that you described earlier with the double slit. When we hear about wave particle duality, like people are used to light as a wave, and if there are enough photons, we can make this kind of continuum approximation in our minds.
We learn about interference. But what I remember reading about that seems so astonishing is that even if you turn down the intensity of light so much that there’s effectively only one photon in the chamber at a time, or in your case, single electron in some sense like this amazing skier can sort of interfere with itself.
HALLIWELL: That’s absolutely it. Initially with this double slit experiments they might have said, “Well, okay, there’s a whole swathe of particles going through.” But indeed, when you turn it right down so that there can only ever be one particle in the apparatus, it still has that property.
And in fact, they now can do interference experiments with pretty big things, like containing thousands of atoms. They’re borderline visible with the naked eye actually, some of the effects. I mean, this links to sort of macroscopic quantum experiments, and, it does lead to this question of how big a system can you interfere with itself?
The fundamental issue really beneath all of this, it is actually about the worldview at the quantum level. There are all these weird effects, but behind that, there is this worldview that the quantum world is actually about waves, and their intuitive properties are just not the same as those of particles. Just to give another simple example, if a police car comes past w- with its siren blaring and if you’ve got the doors and the window open, then the sound comes in both the door and the window. There’s nothing unusual about that. But if it’s a solid object that appears to come in through both the door and the window, that would be rather striking. But that is, in principle, true because wavelike is the fundamental nature of matter at the microscopic level.
STROGATZ: So in this wave-particle duality, you, it sounds like you’re arguing that for best intuition about quantum theory, we should be thinking waves?
HALLIWELL: Yes, people often say sometimes a particle, sometimes a wave. What actually quantum theory would say, always a wave. But sometimes the wave can be very compressed around a single point.
STROGATZ: Ah, I see.
HALLIWELL: So you can have a water wave, for instance, which is a single tightly peaked wave. If it comes in towards the beach, supposing there’s a low barrier, like a wall just beneath the surface, so one wave then becomes two: one being reflected, one transmitted. And that also literally happens in scattering experiments in quantum physics. You shoot in some particles uh, some bounce back, some keep going. So what you think was a single particle ends up in two separate bits.
STROGATZ: You touched earlier on what I hope will be one of our central questions in this discussion, which is how do we go in this transition from the quantum world with interference patterns and a wave-like nature of matter and energy to the macroscopic world that we’re used to? The one that we live in. I mean, why don’t we see interference in everyday scale objects like the moon or ourselves?
HALLIWELL: Yeah, that is a, a big question. I mean, so the first feature is what’s called graining. So waves look smooth from a long way away. The ocean looks almost smooth from 30,000 feet, but when you get close, it’s not like that at all.
So that’s one of the things is from a coarse grain scale, the wave-like pattern goes away. But it’s more concrete than that. There are certain quantities which describe a physical system which are very slowly varying, and some which vary extremely rapidly. Suppose you have a big massive particle that’s being bombarded by molecules.
Like, there’s the famous Brownian motion example where you have, like, a pollen particle, which is kind of big compared to the atomic scale, being bombarded by molecules that makes it jiggle around a bit. The big particle will be effectively classical. What happens is the bombardment by other systems, which we o- often call an environment, it actually, it kills the interference, is the phrase we use. It suppresses the interferences that create the wave-like properties by kind of smoothing things out and destroying the wave-like nature.
STROGATZ: That’s a perfect segue. The fragility of quantum coherence, quantum interference patterns. I like your word bombardment because although it’s a very vigorous and almost violent word, it gives the right intuition that quantum systems are always at risk of being hit by stray photons, gas molecules, tiny electric and magnetic fields. There’s all kinds of ways that, as you say, the environment can jostle or mess up this delicate quantum state. So this is to do with this word you used earlier, decoherence. Is that right?
HALLIWELL: Yes. Destruction of interference or the reversal of coherence, yeah.
STROGATZ: So is it decoherence then that is the key to making a system stop acting in its pristine quantum character, look more classical?
HALLIWELL: That is exactly it. We’ve had 100 years of quantum theory, since modern quantum theory, which started with Schrödinger. And, so the first journey was from the classical world into the quantum, and that took us to Schrödinger’s famous equation and all these weird quantum effects.
But it was also important to do the return journey, which said, if everything’s quantum, my table isn’t, or what happened? And when you come back to the coarse-grain world, why is that not in fact quantum even though it’s made of very quantum stuff? That is the process of decoherence. That the quantum effects are actually very fragile, as you say. You have to work hard to maintain them. You need sort of isolated chambers and so on. And it is quite hard to find quantum effects which persist.
I mean, there are some like properties of electrically conducting materials and so on have relatively robust properties. But in terms of the really kind of, let’s say, out there freaky stuff like superpositions, those are quite easily destroyed. For all sorts of reasons, I spent a lot of time looking at different types of quantum effects and understanding which ones get killed quickly and which ones actually are quite robust.
STROGATZ: And so I guess I wanna ask something related to that, that when we speak about decoherence, is it that the system its quantum behavior becomes inaccessible to us? Through sort of the mixing or the entanglement with the environment causes us to lose our ability to know, or is it something deeper than that? It’s not just a matter of our inaccessibility of knowledge.
HALLIWELL: I think most would take the view it’s about inaccessibility. See, what would happen in the Schrödinger’s cat experiment is that you have a microscopic system which goes into a superposition of two states of something, like a couple of spins, and then those in turn are, en- bit tangled with a large system, and in this case, the unfortunate cat.
Now in practice, in a realistic experiment, any large system would be very quickly decohered by its, uh, immediate environment, by surrounding photons. But the entanglement, the quantum stuff, is actually still there. It’s just scattered far and wide. So if you have a small particle, like the Brownian motion particle, that is in some sense still actually quantum even though it’s scattered, but there’s so much information. You know, all the bits of information are just out there beyond the horizon that you can probably even never get them.
STROGATZ: Huh! It feels like you’re bringing the observer in a little bit, that it’s not that quantum nature is lost, it’s that it leaked into the room, or it leaked into the environment in general, and it’s hard for us to measure at that point.
HALLIWELL: True. There was a lot in the early days of quantum theory about does the observer in person play some role in this? And I think John Bell, who was a famous early writer on these things, he spoke about this actually with a certain level of rhetorical overkill, I would say. You know, what do we mean by the observer? And, you know, in practice it means that there is some environment somewhere, some surrounding system that actually stores the information.
But it does still leave the question, could you have, let’s say, some sort of super-observer with actually unlimited information processing capacity that could actually measure everything that’s going on? And in principle, that’s true. There is actually, let’s say, an information theoretic dimension to this.
But, part of it is that, as you probably know, quantum systems have vastly greater information storage capacities than classical ones. That’s one of the ways of characterizing quantum physics.
STROGATZ: You haven’t said the word “quantum computer” yet, but this is what you’re hinting at?
HALLIWELL: Essentially, yes. And it is to do with information storage capacity or processing capacity of quantum systems. It’s not an area in which I’m expert, but it’s an important reference point when someone discovers a new quantum effect. The first question is is it a resource? Can we do something that we can’t do with classical systems? And indeed, we can in many cases.
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LEVIN: Wow. Okay, so you got into the observer in quantum mechanics. This is one of the terrible impacts of quantum mechanics on pop culture, this idea that an observer was required for quantum mechanics to make sense, that the observer collapsed the wave function the way that people talked about it.
Maybe the way to say it is things were in superposition until the observer rendered the quantum object in a definite state. And that’s just a total red herring in many ways, and I believe Jonathan probably went to great pains to explain how you don’t need a human consciousness. But it led to so much stuff out there, just ephemera out there about consciousness. And quantum mechanics that has been very hard to whack back actually.
So even now, people will still say, “Ah, but, you know, quantum and consciousness.” Those are not connected ideas. And I believe Jonathan’s done really important work to separate that out, to say the observer could be just the environment, a thoughtless environment, an unthinking collection of molecules.
STROGATZ: I certainly grew up in reading pop accounts of quantum physics, hearing a lot about the observer, certainly a lot about the collapse of the wave function. I’m not so sure it’s totally been put to bed, though. What you’re talking about is what used to be called the Copenhagen interpretation, right? From Niels Bohr and his school.
LEVIN: Right.
STROGATZ: The original attempt to not exactly make sense, but give some kind of philosophical account of what’s going on with this measurement problem. It’s out there still.
LEVIN: It’s still an issue. I think the Copenhagen interpretation and the many worlds interpretation are still out there. I think decoherent histories is maybe an alternative to both. So in the Copenhagen interpretation, something causes the superposition to collapse and a definite state to be assumed by the superposed particle, let’s say its location.
Now, in the many worlds, that’s not what happens. What happens is you, the observer, or the hot room or the cat, whichever it is, becomes superposed and branches off, and the two possibilities coexist in some meta world, some many worlds interpretation and, and all possibilities continue to be realized branching over and over again. That is surprisingly popular in quantum mechanics and completely confounded.
STROGATZ: Very surprising.
LEVIN: Very surprising! Now, to my mind, and I don’t know if I have this 100% correct, it’s been a long time since I’ve spoken to Jonathan about these ideas, but the decoherent histories is neither of those. It says, “Look, there are so many interactions, so many quantum probabilities that it just can’t keep its shape in that way.”
The quantum-ness doesn’t go away. It just becomes, in some sense, less distinct on these macroscopic scales where there’s so many histories, which is like saying so many particles macroscopic. And I think that’s where he’s going. I don’t know if you took a sampling of quantum physicists where they’d land on all of these.
STROGATZ: Well, it could be almost like one of these religious questions where, there’s, in addition to the believers, there’s the atheists, the agnostics, and what have been called the apatheists, right? Who just don’t care. So it could be, I mean, I think it’s fair to say that there are a lot of quantum physicists who feel, “I’m not interested in foundations. I wanna make measurements and do my experiments and get results and build new devices.” You, you can get surprisingly far without worrying about foundations.
LEVIN: Yeah, we can launch things into space.
STROGATZ: Yes, we can.
LEVIN: I mean, we can get really far.
STROGATZ: So we have a lot of ground to cover, and we will get into everything from macroscopic realism to possibly the mythology of the Big Bang after the break.
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STROGATZ: Welcome back to The Joy of Why. We’re joined by theoretical physicist Jonathan Halliwell.
So, at this point, I’d like to get into this notion of macroscopic realism that I know has been of interest to you, you’ve worked on extensively. Let me start by bringing up a quote of Einstein that is close to what I think we wanna talk about.
But one of Einstein’s famous quotes in thinking about paradoxes of quantum theory in the old days was, “I like to think that the moon is there even if I’m not looking at it.” Back in the days when Niels Bohr was drilling in the idea that you had to think about the observer, and does it make sense to say that an object has properties like a certain velocity or a position, or its spin state even before you measure it.
And Einstein thought, “Of course, I wanna believe that the moon is there even when I’m not looking at it.” Tell us what we mean by macroscopic realism, and what’s missing in this little analogy of Einstein and the moon?
HALLIWELL: Is the moon really there when no one looks? That is indeed exactly what’s going on. In fact, the first experimental paper to test macrorealism, I think it was called No Moon There or something like that.
So, there’s a whole set of things here which I can try and sketch. So though on the one hand, these tests of macroscopic realism, it’s a version of what is classicality, and it’s designed for single systems that you measure sequentially in time. So when you measure something and the outcome you get is not the real outcome of what it actually is, but it might depend on what’s going on somewhere else.
This is what is called contextuality. Now, this is what happens in Leggett-Garg tests of macroscopic realism. What Leggett-Garg tests are doing is trying to eliminate classical explanations of quantum results, when you do a two-time measurement. When you’ve got a very quantum situation, you get something that would classically be quite paradoxical.
To go on to what macrorealism is, what we do is have a simple system with, say, it could be a spin, which just has two values, or it could be something more complicated by, you know, approximate position measurements of a particle like the moon, and then you just measure it a bunch of times and you compute average positions or average spins.
The important thing is you also have to compute correlations between different spins, like are two spins the same? And then from those you can actually try and assess are these correlations, are these measurements consistent with a classical model? And by a classical model, what you really mean is an underlying system in which the thing you measure has a definite value, so the spin is either plus or minus, or the position is definitely this or definitely not that.
And it seems like such a simple thing, but this is the idea of definiteness of value that actually is probably not true in quantum theory, and you prove that it’s not true by assuming that it is, and then deriving some contradictory consequences.
STROGATZ: So, when you speak about definite values, a particle or a quantum system having definite values, as opposed to what?
HALLIWELL: That’s an excellent question. In quantum physics, you have the possibility of a so-called superposition state, where a particle can, for instance, be in two places at the same time. So position, we would say, is not definite. But it’s a little bit more nuanced than that. I mean, it could be a probabilistic distribution of being here or here.
But quantum physics, you can do a measurement that shows it definitely is in two places at the same time, and then you can measure it in each place and find it’s definitely there and definitely there. So sometimes people say like misty states or indefinite states.
The only thing you can say is that if you assume it’s indefinite states and you assume also various measurements can’t influence each other, then you find a contradiction. Therefore, the assumption that it was in a definite state is false. So it’s indirect actually. You can only refute the worldview of definite states. It’s very elusive concept. And it’s the most important one in quantum physics, actually, I would say, conceptually.
STROGATZ: The idea of definite states versus superposition or…?
HALLIWELL: Versus superposition, but it goes even beyond that. Sometimes with a superposition, you can still find an underlying model which has definite states. It does come down to this notion of contextuality. Another way you can explain away indefinite states by saying when you make a bunch of measurements, if they depend on each other in ways that you hadn’t noticed, that’s what an indefinite state might look like.
Except that in quantum physics, we do these experiments in such a way that there isn’t a dependent on context. So two distant particles in an entangled state, you arrange it so they can’t possibly signal. It still looks like they’re talking to each other. I mean, that’s really the true mystery.
There is a quantum mechanical explanation. The quantum mechanical explanation is it’s described by a wave, and the wave is a non-local thing that spreads out over space. And it’s only when you try to think of it as two separate particles that it looks like two particles with different states. The wave is like a hologram, and every little bit of the hologram contains a whole. That’s the best analogy, actually.
STROGATZ: I’m just wondering when you earlier mentioned the double-slit experiment, if we were to say that the particle definitely did go either through the left slit or the right slit, if that were true, it would leave a two-humped probability distribution on the recording screen, not a set of interference fringes?
HALLIWELL: It would change the interference pattern. In fact, you can take a double slit experiment and turn it into one of these Leggett-Garg tests for macrorealism.
STROGATZ: Is that right?
HALLIWELL: So you make a measurement of the first time. Did it go through the left slit or the right? And then you look at the screen at some small localized area and say, “Did it arrive here or not?”
So, you’ve got simple histories at two times, and then you do a bunch of measurements of correlations and averages, and well, what you find is that if you assign definite values to these, then you end up with a contradiction, or more specifically, you end up with a probability which is negative.
That essentially, a Leggett-Garg inequality or a Bell inequality, they are essentially probabilities for something. But they actually turn out to be negative, which means that actually there isn’t an underlying probability, which means you were wrong to assume that they have definite values.
In Leggett-Garg, we’re trying to actually explain that sequential measurements obey very non-classical ideas as long as we can justify this idea about invasiveness. I’ve spent quite a lot of time in my book on this to try understand it, but it’s very, very hard. The whole area is just riddled with loopholes.
STROGATZ: It’s interesting this word invasiveness that you just used. What it calls to mind for me is like people used to explain things like the uncertainty principle by giving a picture of a measurement disturbing a system, like an electron or a photon would knock the system off course. The act of measurement could disturb the system, right?
It became a kind of credo of old-fashioned Copenhagen interpretation of quantum theory. And it seems like the Leggett-Garg tests, if I’m understanding you, are maybe saying something deeper, that even if we take a lot of care to measure a system without disturbing it, that we do a non-invasive measurement, the classical picture could still fail. Am, am I getting that right?
HALLIWELL: It is something like that. So, in the Bell case, if the particles are quite close together, you can always say, “Well, you got this quantum result, but actually a classical system that just signals really quickly would give the same.”
Now, in Leggett-Garg tests, you could do two sequential measurements, and if you get the expected quantum result, you could always say, “Well, the first measurement you actually, you were clumsy.” That’s the word we use.
STROGATZ: It’s a useful one.
HALLIWELL: In Bell tests, it’s called the signaling loophole. In Leggett-Garg we call the clumsiness loophole, where if you do a classical clumsy measurement it will actually explain the quantum results. So that is true. You can cook up models that actually do that.
So it’s not particularly a quantum issue here. It’s really, can you eliminate a classical model which has some invasive measurement?
But there is a quantum measurement aspect also to do with the wave function that’s affected, but you can actually factor that out. So I mean classically supposing, uh, you’re speeding down the highway and a policeman has a radar gun, and they bounce the radar off your car and that’s how they work out the speed.
The bounce actually slows the car down a tiny, tiny little bit. There’s a change of momentum but it’s absolutely minuscule. But when you do that for a quantum particle, then the bouncing off of a photon can actually be quite significant.
There is a way that Leggett and Garg first suggested for creating this non-invasiveness, and they called it an ideal negative measurement, where supposing this particle has two options at the first time, then you attach a detector on option A but not to option B.
And if the detector doesn’t click, you say, “Well, option B happened,” but the detector wasn’t attached to B, so we didn’t actually disturb anything. Classically, that would actually be non-invasive. But there’s a tricky debate there saying, “Well, it’s really quantum, and it’s actually a wave function, and the wave function goes through both slits at the same time,” so it was still invasive at a quantum level.
So a certain number of pirouettes have to be performed in order to actually get past that.
STROGATZ: But so how far have experiments pushed these kinds of ideas? Like, are we still in the microscopic world of photons and atoms and tiny crystals?
HALLIWELL: Sure. I mean, what has been done is interferometry experiments for pretty big systems. I think Vlatko Vedral from Oxford, who’s an expert on this thing, he talked about Schrödinger’s virus as the sort of scale we might be getting up to.
Those experiments simply confirm there’s an interference pattern, when you do a double slit experiment. Leggett-Garg tests they do more than that. They not only confirm quantum, but they rule out alternative classical explanations, which is a stronger thing to do.
This is sort of interesting. You can still get an interference pattern, but there’s a classical model for it actually, if the interference isn’t too big. The thing is, interference is created by the fact that the wave goes through two holes, and what come out is two waves. They’re not probabilities, they’re waves that can be all positive and negative, and they can constructively interfere, so they get bigger. And that’s like a classical effect actually, ’cause if you add two probabilities, they get bigger.
But they can destructively interfere, so they cancel each other out. So you can get two waves coming in and you get nothing, which can’t happen with probabilities. Now, if the destructive interference isn’t too big, you can still model them as like classical probabilistic flows, and that would satisfy the Leggett-Garg inequalities, which actually means that there is a classical model of interference patterns in a certain limited regime.
So for one of these big interferometers experiments with big particles, to really say this is definitely quantum and not pseudo-classical, you’d need to violate a Leggett-Garg inequality, basically. That can all be done in principle. I mean, even just with the data from existing experiments, basically.
STROGATZ: Let me start backing away from some of the more theoretical points and get into some more personal things. One of them is just a matter of opinion, really, but I bet you have a strong opinion, which is so far no experiment has contradicted quantum mechanics or quantum field theory. I’m not sure how far we should push this, so why do we keep testing it? What are we hoping to learn by pushing these quantum tests to larger and larger systems?
HALLIWELL: Yes, that is a very good point. It is very, very striking indeed that there isn’t, in terms of experimental predictions that have been measured to, you know, many, many, many decimal places, there are no contradictions in the formalism. To the point actually that I know experts in quantum foundations who, who’s even said to me, “Why do you work on Leggett-Garg inequalities which confirm quantum behavior when,” in their words, “we know quantum physics is true anyway?”
Um, I have a problem with that. I mean, it’s this bordering on a kind of fundamentalism about accepting something as truth, that could be a wider discussion. But whatever theories are, they must be exposed to experimental tests and confirmation. I mean, really, absolutely.
I mean, it would be a wonderful thing if we started seeing differences. But it is still a really striking thing that there have been no differences with theory ever observed. I think behind this it is about what is the true interpretation of quantum physics, behind all these investigations, Bell inequalities, Leggett-Garg inequalities. What these things do, they eliminate certain very natural worldviews.
So Bell inequalities shows that quantum physics and experiment are consistent with the worldview of local realism. So the idea is that a particle has definite properties independent of what distant particles are doing. Similarly, Leggett-Garg is testing macroscopic realism. A particle has definite properties irrespective of measurements in the past or measurements in the future, for that matter. So we know what reality is not.
But there are other worldviews, if you like, other interpretations. One of the most famous, complements to these ideas is Bohmian mechanics, if you’re familiar with that one, or de Broglie–Bohm theory, which basically does take standard quantum mechanics, which is based on a wave function, but says the particle has a wave function, and it also has a position trajectory. Then you don’t need to worry about is the particle here or there? The particle definitely has a trajectory. It’s definitely somewhere. The wave function has a back seat as a kind of, they call it a guidance field, like a fluid that sort of flows along and tells the particle what to do, but the particle always has a definite position. And that’s very appealing, and it was invented actually shortly after quantum physics was invented.
And a lot of people like it because you know where you are, basically. You’ve got a definite path through. But it has a feature, which is that it’s non-local. Because there’s a wave function and a particle, that where the particle goes, the wave function can be spread out all over the place. So the guidance field for the particle can depend on very distant observers. It basically says that in an entanglement situation with distant particles, that one particle really can depend on what the other one is doing or what you measure on the other particle. But that might be it. I mean, you can have a world in which everything is definite, but it’s non-local. And that might also be true, actually. So that’s why I don’t believe in truth. I believe in multiple perspectives.
STROGATZ: Do you wanna expand on that idea? I, in some of my preparation for our conversation, I was told you have an interest and have practiced things like yoga, meditation. Is that related to the sorts of perspectives you’re describing here?
HALLIWELL: It’s not directly related, but in terms of the sort of attitude and perspective I take towards physics and scientific ideas, it sort of helps me in that sense. So I think in the early days when I wanted to figure out, you know, how does the universe actually start? We really want to know. Now I’m a lot happier to sit with the mystery.
When I’ve given popular talks, I have the sense that the general public interested in physics, feel discomfort at the lack of understanding. That they want the physicist to deliver something mythologically meaningful to them in terms of the Big Bang, and they’re dissatisfied. For me, I found a sort of other perspective in a more, let’s say, spiritual side, where I’m just happier to sit with the mystery of multiple perspectives.
There was an American cosmologist who’s now more of a sort of mythologist and writer called Brian Swimme, S-W-I-M-M-E, who you may have come across. And he’s basically… starts with the idea that all societies have creation myths of one form or another, and modern society doesn’t necessarily have that.
But he said, “Well, look, let’s take the Big Bang cosmology as a modern myth for our time. We don’t have to take it as true, but we can take it as a meaningful mythological tale from which we can draw a sense of guidance or purpose or perspective and so on.”
And I felt in popular talks I’ve given, I felt that actually filled a gap in things where people have the Big Bang, but it’s presented to them as, almost as truth, but it doesn’t really fulfill people to a degree that they actually want.
STROGATZ: It’s interesting to me the analogy between the perspective you’ve described with holding different possibilities in mind at the same time and being comfortable I wanna say sort of as a superposition. That you seem to resist collapse in your own thinking.
HALLIWELL: Yes. I mean, some of my non-physics friends, or even physics friends, they say, “Well, how is you as a sort of, you know, hard-nosed theoretical physicist, how can you entertain all of these more elusive things that are spiritual or mythological or whatever?”
Well, here’s the thing. You see in physics, of course, we had two major revolutions at the beginning of the 20th century, one in relativity, one in quantum theory. In relativity, suddenly space and time are not even anything like what you think it is. In quantum theory, matter is not what you think it is. So physicists had to get used to adjusting enormously their worldviews.
There’s always new perspectives coming along. For sure the next generation of scientists will have new ways of thinking about these things.
Quantum information is a very current vogue, and a lot of people turn the question of interpretation into quantum informational notions. So I, I think what we will see is kind of multiple ways of actually saying the same thing about the sort of bizarreness and mystery of quantum mechanics.
So I think at the end of the day, I believe in mystery. Sitting with the mystery and the uncertainty, and that, that is the most comfortable position. In fact, one of my meditation teachers said, I think, quoting from thousands of years, that a lot of these things are about the infinite one way or another, the beginning of the universe.
And instead of worrying about “What it is”, you can ask, “Well, who am I in relation to that?” That’s the simple question, I think, which is there in a lot of spiritual traditions. So that’s where I am with physics. You know, how do I feel in relation to all this uncertainty and immensity?
STROGATZ: This has been a real pleasure. Thank you for spending time with us here on The Joy of Why. This has really been fascinating.
HALLIWELL: It has for me too. Well, thank you very much indeed.
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LEVIN: I’m glad Jonathan mentioned meditation. I wasn’t sure if I could mention it if he didn’t mention it. Jonathan used to do a lot of yoga. I remember this. This was an era when I was also doing a lot of yoga, and we would talk about that connection too. This kind of attempt to observe without tackling with hypotheses and conjectures and theories, right?
This dual modes that we were living. One is theorizing about the world precisely in an attempt to understand it. And one is this other mode of meditating on thoughts and not trying to understand it. And it seems to me that that’s kind of what he’s describing, this duality that he’s comfortable entertaining or or, you know, just experiencing.
STROGATZ: Mmm-hmm. I have to say, as someone who has not practiced meditation, I found myself wrestling with the word myth when he discusses creation myths. So we hear the word myth used a few different ways. Sometimes it’s used like, “Oh, that’s just a myth,” as a dismissive term. He’s clearly not using it that way. He says a creation myth gives people meaning, gives them purpose, helps them orient themself in the vast infinite mystery of the universe that we find ourselves in. So am I hearing him right, do you think? I mean, what do you hear when you hear creation myth?
LEVIN: Yeah, I was very surprised by that, too, even with what I just said, the idea of observing and tolerating mystery and a lack of understanding. But that is not the same as myth. Myth seems to me to be doing something very strong, which is making an unprovable declaration about something. Anyway, I can see why it’s a little uncomfortable to hear the word.
I mean, it was interesting to hear him talk about that. I’m uncertain, even in that conversation, if he’s suggesting that this is part of what people are looking for when they’re looking to understand the Big Bang, is they’re looking for a replacement or the myth itself. And whether or not people are searching for it, that’s certainly not what we’re doing when we’re practicing scientists trying to understand the origin of the universe.
You know, if you destroy all myths today and all memory of them and rebuild civilization, there’s gonna be different myths. But quantum mechanics is still gonna be there you know? Whether it’s described by matrices or anything else, quantum mechanics is still discoverable in a way that myth is not. I don’t know that… what do you think, Steve?
STROGATZ: Hmm. Well, okay, here was my question, when Jonathan speaks about multiple perspectives, it’s something that comes up a lot in quantum theory itself. So like for instance, going back to the very beginning, Schrödinger’s approach with wave mechanics. Before that, there was Heisenberg and Born and Jordan had this approach based on matrix mechanics. Dirac later had his way of doing it. Feynman eventually has his sum-over-histories way of doing it. I mean, in the case of those first two, matrix mechanics and wave mechanics, they’re two perspectives, but they turn out to be mathematically equivalent. There’s one quantum mechanics, right?
And so when Jonathan talks about the multiple perspectives, there’s really the one universe, okay, leaving aside the multiverse, the universe we seem to be in, our observable part of the whatever multiverse, there’s one thing. And if we have multiple perspectives, don’t they have to agree?
LEVIN: Yeah. Well, it’s really interesting because his early work on decoherence in quantum cosmology is precisely to formalize how a closed quantum system, there’s nothing external to the universe. It’s all within the closed system, and his careful and thoughtful work on that is exactly what made his early reputation, that you could do all of that within the universe itself. And, I thought that was quite profound, actually.
STROGATZ: Thank you. That’s a beautiful summary. I think we should leave it there.
LEVIN: There we go. Off into the wild blue yonder. Okay. Till next time.
STROGATZ: See you next time.
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LEVIN: If you’re enjoying The Joy of Why and you’re not already subscribed, hit the subscribe or follow button wherever you’re listening. You can also leave a review for the show. It helps people find this podcast. Find articles, newsletters, videos and more at quantamagazine.org.
STROGATZ: The Joy of Why is a podcast from Quanta Magazine, an editorially independent publication supported by the Simons Foundation. Funding decisions by the Simons Foundation have no influence on the selection of topics, guests, or other editorial decisions in this podcast or in Quanta Magazine.
The Joy of Why is produced by PRX Productions. The production team is Caitlin Faulds, Jade Abdul-Malik, Genevieve Sponsler, and Merritt Jacob. The executive producer of PRX Productions is Jocelyn Gonzales. Edwin Ochoa is our project manager.
From Quanta Magazine, Simon Frantz and Samir Patel provided editorial guidance with support from Samuel Velasco, Kit Sudol, Simone Barr, and Michael Kanyongolo. Samir Patel is Quanta’s editor-in-chief. The episode art is by Chanelle Nibbelink, and our logo is by Jaki King and Kristina Armitage. Special thanks to Garth Avery at the Cornell Broadcast Studio.
I’m your host, Steve Strogatz. If you have any questions or comments, please email us at [email protected]. Thanks for listening.
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