内容来源：https://www.quantamagazine.org/how-do-you-take-a-picture-of-a-black-hole-20261029/

内容总结：

近日，哈佛大学天体物理学家谢普·多尔曼与来自全球的250名研究人员合作，通过将世界各地的射电望远镜连接成网络，成功拍摄到人类历史上首张黑洞照片。这一名为“事件视界望远镜”的网络项目，首次呈现了距地球约5500万光年的M87星系中超大质量黑洞的图像，以及银河系中心黑洞的首张照片。

多尔曼在播客节目中介绍，将地球变成一台足以观测黑洞的望远镜需要极高的技术精度和国际协作。研究团队利用原子钟对各地接收的射电波进行精确时间标记，再将波形进行比对合成，从而实现超长基线干涉测量。这一技术的角分辨率极高，相当于在纽约读取洛杉矶一枚硬币上的日期。

该项目始于上世纪90年代的构想，经过二十余年的技术攻关，事件视界望远镜于2016年正式成立。研究团队在2018年通过四组独立团队使用不同算法对数据进行处理，最终确认了环形图像的可靠性。观测结果验证了爱因斯坦广义相对论的预言——黑洞的阴影大小与其质量精确匹配。

目前，研究团队正致力于扩展望远镜网络，计划在智利、墨西哥、美国怀俄明州、加那利群岛和坦桑尼亚乞力马扎罗山等地增设站点，以实现黑洞的“动态影像”拍摄。多尔曼预计，到2030年代初，人类将能够制作黑洞的事件视界高清影像，从而进一步测量黑洞的自旋、研究磁場结构，并检验广义相对论之外的替代理论。

中文翻译：

如何拍摄黑洞的照片？

引言

黑洞是宇宙中最奇异、最难以捉摸的天体之一。当任何东西——甚至光——都无法逃脱其强大的引力束缚时，拍摄一张黑洞的照片似乎难以想象。但一张黑洞的图像——更准确地说，是它的阴影——在检验理论预测时，能提供仅靠理论无法给予的洞见。那么，如何才能做到呢？

哈佛大学天体物理学家谢普·杜勒曼与一支由250名研究人员组成的团队，通过将全球各地的射电望远镜联网，实现了这一非凡壮举。这个网络被称为“事件视界望远镜”，它拍摄到了室女座星系M87中一颗超大质量黑洞的首张图像——该黑洞距我们约5500万光年——以及我们银河系中心黑洞的首张图像。在本期节目中，杜勒曼向联合主持人詹娜·莱文讲述了他如何将地球变成一台能够看见黑洞的望远镜、实现这一目标所需的令人难以置信的科学水平和协作精神、这些图像如何加深了我们对这些奇异天体的理解，以及到2030年代初拍摄黑洞“电影”的可能性。

可在Apple播客、Spotify、TuneIn或你喜爱的播客应用上收听，也可以从Quanta网站流媒体播放。

文字记录

詹娜·莱文：好了。我是詹娜·莱文。

史蒂夫·斯特罗加茨：我是史蒂夫·斯特罗加茨。

莱文：这里是《为什么的乐趣》。

斯特罗加茨：一档来自Quanta杂志的播客，我们在这里探讨当今数学和科学中一些最重大的未解之谜。

莱文：欢迎收听新一期节目。

斯特罗加茨：很高兴见到你。

莱文：很高兴见到你。我对这一期相当兴奋，我无法否认也无法掩饰。我们谈论的不仅是我倾注了大量心血的黑洞——最伟大、最奇异的话题——而且我还有机会和一位老朋友交谈。所以这是一期有趣的节目，我希望如此。

斯特罗加茨：我非常期待听到这期节目。我知道你对黑洞一直有着特殊的感情。在之前某个版本的节目中，我有机会采访过你。

莱文：没错。

斯特罗加茨：你一次又一次地让我对黑洞惊叹不已。

莱文：是啊，它们需要一些辩护。它们名声不好，但并不完全是咎由自取。

斯特罗加茨：什么坏名声？因为它们喜欢吞噬东西？

莱文：对。你知道，黑洞，围绕黑洞总是有那种怪物卡车的语言。你知道，它们就像……但实际上，黑洞是非常优雅、干净、简单的系统。你不惹它们，它们就不惹你。相对于它们的重量来说，它们其实非常小。这差不多就是它们的整体形态——相对于它们的重量，它们其实很小。

而且，你知道，尽管它们是宇宙中最黑暗的现象，它们却设法成为我们在可观测边缘看到的最明亮灯塔。

斯特罗加茨：哇。

莱文：所以，它们非常了不起。我还认为它们对物理学来说具有根本性的意义，不同于你能想到的任何其他宏观物体。

斯特罗加茨：嗯。当我想到黑洞时，我倾向于关注它们对时空的影响。你知道，它们导致奇点。总是它们的几何结构吸引我。但这未必是思考黑洞的唯一方式。它们确实是天体。真正的参与者。

莱文：是的。我认为在人们度过了它们确实存在于那里、可以被间接探测到的兴奋之后，有一种感觉是它们不是宇宙中的主要角色。“呃，外面有些黑洞而已。”

恰恰相反，我认为我们已经逐渐认识到它们塑造了我们的历史和过去，它们在决定星系的形状和大小、甚至可能存在宜居区域的地方方面，扮演着非常调节性的重要角色。最终，我们注定要落入黑洞。所以，它们在某种程度上是我们的过去和我们的未来。

但是，可能会让人惊讶的是，我们实际上从未拍过黑洞的照片。直到大约10年前，我们所有的证据都是间接的，那个团队工作了十多年，有些人甚至长达二十年，才试图捕捉到黑洞的第一张照片。

斯特罗加茨：哈。真了不起。

莱文：这是一项巨大的成就，由谢普·杜勒曼领导，他就是我们的嘉宾。让我来介绍一下谢普。谢普是哈佛大学的高级研究员，哈佛黑洞计划的副主任，同时也是史密森天体物理观测台的天体物理学家。

他是事件视界望远镜联盟的创始主任，这是一个全球天文台网络，为我们提供了人类有史以来捕获的首批也是唯一的黑洞图像。

斯特罗加茨：嗯。这听起来像是一场关于科学最佳状态的精彩讨论。

莱文：是的，这是一个史诗般的故事。让我们听听谢普本人怎么说。

[音乐播放]

莱文：谢普，欢迎来到《为什么的乐趣》。

谢普·杜勒曼：很高兴来到这里，詹娜。

莱文：很高兴你能来做客。我很好奇，你是否还记得自己不仅是走向黑洞的道路，还有走向思考整个宇宙这一理念的道路？

杜勒曼：哇。嗯，每个人都会在某个时刻爱上天文学。不管你你是谁，你都会爱上天文学。无论是仰望夜空——你知道，在夏令营或者看到流星——对我来说，是看到一次日全食。那是1980年，我13岁。我父母带我去了俄勒冈州沙漠的中央，一开始我们以为看不到全食了，云层正在飘来，然后云层突然散开，我们看到了那次日食，太壮观了。真是了不起。

莱文：你当时有没有想，“这就是我余生要做的事”？还是说那段记忆后来回来了，我们重新构建了这个故事？

杜勒曼：这是个好问题。我觉得主要是它后来回来了。生活不是那样线性的。但我要说的是，这些巨大的天体在运动，它们能完美对齐，让你看到平时被隐藏的东西——比如日冕——这个想法让我感到震惊。我是说，事情不一定要这样。比如，月球的角直径不一定非得和太阳一样，才能完全遮挡住它。但这些巧合有时会给你一个窥视自然的特殊视角。

莱文：是啊。我喜欢这个想法：我们受到苍穹的启发——那是科学最古老的仪器——而我们所有人都共有这一点。

嗯，我觉得我们得坦白一件事，因为我们认识彼此很久了。我觉得我们可能是在麻省理工学院研究生第一周认识的。我想是在饼干桌旁。

杜勒曼：我想是的。

莱文：我记得你在研究生院总是尝试各种疯狂的想法。你就像一个充满创造力的好奇心在寻找问题。先跟我说说你是怎么落到这个想法上的——追求一张黑洞的肖像？

杜勒曼：黑洞的肖像，我喜欢这个说法。我很早就高中毕业了。当时15岁。上了大学，之后我相当疲惫，然后我看到一则广告说：“去南极洲，在那里运行所有这些实验。”我说，“这适合我。”我去面试了那个职位，不知怎么的他们给了这个19岁的孩子。他们需要一个人在南极照看宇宙射线实验、大气光度实验、电磁实验，我真切地体验到了在困难条件下做实验研究的感觉。我就是喜欢。然后回到麻省理工学院，1988年在那里遇见了你。

莱文：哦，你不必说年份的，老兄。那没必要[笑]。

杜勒曼：我知道，那时候地球还在冷却呢。但是，我摸索了一阵子。我开始在X射线组，然后做了一些神经科学方面的生物物理研究。然后我被介绍了这个把地球变成望远镜的想法，以及我们用来成像黑洞的这个实验，它立刻吸引了我。

莱文：好的。我们要谈论这个项目，但你在研究生院的时候，黑洞在科学上是什么状态？它们被探测到了吗？所有人都相信它们是真实的天体物理现象吗？

杜勒曼：确实有一种感觉，黑洞存在，或者有强有力的证据支持它们。当然，黑洞有两种类型。有恒星级质量的黑洞，它们在恒星死亡时形成。当一颗恒星发生超新星爆发时，它会炸掉外层壳，然后内部会坍缩，如果恒星质量足够大，就会变成黑洞，那个黑洞可能有几十个太阳质量。

但还有这些巨兽——这些超大质量黑洞，它们生活在星系中心。虽然一个星系中可能有很多恒星级质量的黑洞，但任何给定星系的核心可能只有一个，也许两个超大质量黑洞。

对于恒星级质量的黑洞，人们看到了这些X射线双星，他们看到这些恒星大小的源在X射线甚至射电波段有非常快速的波动。一切都符合这个想法：有一个黑洞在吞噬另一颗恒星，形成X射线双星。

莱文：那就是天鹅座X-1，在天鹅座方向的第一个X射线天体，对吧？

杜勒曼：没错，就是那类源。然后射电天文学家主要看到了一些分辨率非常高的图像，非常令人不安。所以你在光学波段看到，一个星系看起来就像普通星系，有大量来自恒星的光学光。

但如果你在射电波段看它，你会看到这些方向相反的喷流，全部追溯到星系中心。是什么造成的？

人们最好的想法是一个旋转的超大质量黑洞，它激发以接近光速从黑洞南北极喷射的喷流，穿透整个星系。

所以从两条完全不同的探究路线，你有一种黑洞被X射线怀疑存在，另一种主要被射电怀疑存在。

莱文：所以即使人们开始推测这些一定是黑洞，没有人拍过黑洞的照片——不是在70年代、80年代、90年代、00年代。我认为这让人们感到惊讶，因为我们把它们描述为宇宙中如此引人注目的居民、恒星的死亡状态，但我们实际上从技术上从未直接看到过一个。从未拍过肖像。而这正是你在很多方面着手去做的事情。

杜勒曼：是的。我认为最简单的说法是，在某种意义上，眼见为实。我的意思是，你可以从理论上推断某事物，但真正看到它赋予了事物一种否则你无法获得的存在感。而且还有一些非常重要的特征。不仅仅是看到它。而是测量它。测量这个东西不仅给你对其真实性的信心，而且你可以检验预测。

所以1915年，当爱因斯坦提出他的广义相对论——它支配着黑洞周围的时空——然后第二年施瓦西在一战的战壕里解出了它，这个故事我们很熟悉，即使在那之后，很明显，如果你走到那个解的结论处，你最终会看到黑洞周围的一个光环。

如果一堆气体被吸引到黑洞，它会加热到数千亿度，因为你把它限制在一个小体积内，它会发光。那光会被黑洞的引力弯曲。从你观察黑洞的任何方向，你都会看到一个光环。基本上，来自黑洞后方的辐射被引力透镜效应弯折成一个环。而那个环的大小你可以从爱因斯坦方程推导出来。

所以我们从理论上推断，如果我们能拍照，如果我们能看到这个环，我们就能测量黑洞的质量。而且仅仅这个环本身就是超大质量黑洞在那个尺度上存在的最佳证据。所以我们追求的不只是一本咖啡桌画册。我们追求的是对宇宙基本本质的真正测量。

莱文：现在，当人们听到你要试图把地球变成望远镜——或者一个地球大小的望远镜——听起来像是一个疯狂的想法。是什么让你认为这是可行的？这绝非易事。

杜勒曼：是的。嗯，把地球变成望远镜这个想法已经存在很长时间了，从60年代就有了。关键在于，望远镜的角分辨率——或者说放大天空中物体的能力——它与你观测的光的波长λ除以望远镜的尺寸成正比。

所以如果你想看到天空中非常小的东西，你要么减小光的波长，要么扩大望远镜的尺寸。所以你经常会看到山顶上那些巨大的玻璃块。那些巨大的光学镜面，它们大是有原因的。它们收集大量光线，而且它们越大，能看到的物体就越小。

莱文：分辨率越好。

杜勒曼：分辨率越好，正是如此。有些天体非常小——而黑洞是爱因斯坦引力理论预测的最小天体——你需要极大的望远镜、极精细的角分辨率才能看到它们，你必须把这个把地球变成望远镜的想法推到极致。

你必须在以前从未使用过的波长上运行它，因为你必须真正走向越来越小的波长来获得所需的角分辨率。这就是花了我们20年时间的事——把这项技术推到绝对的极限。

莱文：是的。所以有几件事。一个是很多人惊讶地听到黑洞很小。他们想象它们巨大无比。你能解释一下吗？为什么它们在空间上实际上很小，尽管它们重得难以置信？

杜勒曼：是的。简单地说，黑洞就是当你把足够的物质限制在足够小的体积内，引力失控，物质经历灾难性的引力坍缩成一个密度无限大的点。你会想，“好吧，那挺小的，”但实际上围绕着它的是一个叫做事件视界的膜，那里的引力如此之强，以至于光都无法逃脱。所以那真正定义了我们自己能观测到的空间区域。

然后天体物理黑洞并不是不可见的，因为你可能会想，“好吧，如果你看不到它，”因为它周围有这个事件视界，但它们吸引气体、尘埃以及你能想象的一切，你把所有这些都塞进一个非常非常小的空间，所以它会加热。因此，在它们自身引力的悖论中，这些黑洞可以发出极其明亮的光芒。

如果你有一个位于星系中心的黑洞，重量是太阳的数百万或数十亿倍，它可以比那个星系中所有恒星的总光度还要亮。所以你看到这些非常小的天体，但它们可以极其明亮。

莱文：那么如果这没有被预测到、没有被预期到，他们怎么推断出那是黑洞呢？

杜勒曼：是的，很大一部分是变化的时标，因为这告诉你你所看的东西的尺度。所以在天文学中通常，如果你看到某物在某个时标上变化，你就说，“嗯，我认为它不能比光速更快地运动。”所以这给了你一个概念：那个物体能有多大。

而对于它的光度，很难想象有什么比黑洞更大的东西能导致这种波动。然后还有释放的能量与物质落入引力势阱并释放能量有关。这是人们拥有的另一条线索。所以正如你所说，有这些间接方法来建模我们所看到的为黑洞，而黑洞是最符合的。

对于超大质量黑洞，人们可以看到这些喷流释放的能量，我们真正能想到的唯一机制是磁场在黑洞本身扭曲的时空中缠绕。就像打蛋器旋转一样，它们以接近光速抛出这些带电粒子。观测结果如此极端，那确实是唯一符合的模型。而奇迹般地，随着你获得越来越好的角分辨率，当你把喷流一路追溯到中心时，它一直看起来像这个喷流。

它没有打破这个模型。你一直看到这个喷流越来越近、越来越近，而我们有一个疯狂的想法：我们必须看到是什么造成了这一切。

莱文：让我们区分一下恒星的死亡状态——那是像天鹅座X-1那样产生信号的天体类型。两个恒星系统一起诞生、一起死亡。一个变成黑洞，另一个开始下降。它被蚕食。它产生这些X射线、这些喷流。这些超大质量黑洞，当时我们称它们为准恒星天体或类星体，对吧？当时人们不了解这些超大质量黑洞来自哪里，或者它们是否如此普遍，是否渗透了整个宇宙。我是说，这是一个惊喜，不是吗？那些类星体如此普遍，它们可能存在于每一个星系中。

杜勒曼：是的。有很多不同的证据线索。当然对于那些射电明亮的星系，你可以做干涉测量——沿着我们拍摄黑洞照片的思路。在那里，你最终会看到这些物质喷流，它们大约占我们所看到的星系的10%。对于其他星系，我们看到恒星族的尖峰，当你接近中心时急剧增加，所以你可以假设那里可能有一个超大质量黑洞。

现在有了更好的分辨率和更好的望远镜，我们已经观测到这些星系中心非常非常近的恒星的统计速度——在距这些星系几光年之内。现在变得清楚的是，所有星系似乎都在其核心有这些超大质量黑洞。

为什么有些射电明亮，为什么有些在射电波段辐射，为什么有些不是，可能很大程度上与你观察它们的方向有关。也与那个星系生命周期中的演化时刻有关。有些时候射电明亮，有些时候不。所以我们看到巨大的多样性，就像跨越宇宙时间的星系动物园，我们很幸运其中一些确实展示了这些极其明亮的射电结构，我们可以用地球大小的望远镜来成像。

而这真正推动了射电干涉测量领域的发展。我们需要制作一张比之前任何图像好一万倍的图像。干涉测量的魔力在于，你可以在智利探测来自黑洞的射电波，你可以用原子钟给它们打上时间标签，这样当这些射电波到来时你能看到所有的波峰和波谷。你可以在夏威夷做同样的事。然后你可以把这些波形带到一起，像它们被一台巨大的望远镜接收一样进行比较，它们将是相干的。你可以把它们相互回放，对齐波谷和波峰。这就是让你把地球变成望远镜的秘密。

莱文：所以这就是你说的甚长基线干涉测量——或者一般意义上的干涉测量。你说的是连接全球各地的这些天线。这项工作真正开始于想象、构思、执行，跨越了几十年。但是，事件视界望远镜作为一个项目是什么时候正式成立的？

杜勒曼：嗯，这要看情况。我在1995年写了论文，把技术推到了接近但还不足以做黑洞成像的波长。所以我们对前进方向有相当好的概念。然后我们真正开始认真考虑它大概是在1998年。我们开始做实验，把它推到这些前沿频率，比如230吉赫兹，高到足以看穿所有的气体和尘埃。

2008年，我们在银河系中心发现了事件视界尺度的结构。然后，我和少数同事为2010年十年回顾写了一份论文，在其中我们说我们将在下一个十年建造一台事件视界望远镜。所以那是我认为我们启动的时候。实际的事件视界望远镜作为一个机构在2016年成立，我是创始主任。

莱文：那么，你最初使用的望远镜在哪里？

杜勒曼：嗯，让我想想。南极点，智利有两台望远镜，墨西哥有一台，夏威夷有两台，亚利桑那有一台——现在亚利桑那有两台。西班牙有一台，法国有一台，这就足够了。

莱文：你知道，这个项目最迷人的地方在于，所有这些望远镜都已经存在了。想法是利用已经存在的、用于其他目的的望远镜，这跟建造一颗卫星从头开始完全不是一回事。

试图从所有这些不同的天文台建立一个联盟，这个想法进展如何？而且，这是一个真正的国际合作。我是说，数百人。

杜勒曼：我这样说吧。在我的团队中，我们花了很多时间研究仪器，使望远镜能够记录信息，然后与其他望远镜的记录进行比较。这就是干涉测量在全球范围内运作的方式。

但这不是这些望远镜在做的事，所以我们必须说服这些望远镜，我们可以把我们的仪器带过来，用螺栓固定上去，在某些情况下还要摆弄一些他们非常非常灵敏的设备，以它没有被设计的方式来操作望远镜，然后带着所有这些数据磁盘消失，然后也许几个月后告诉他们发生了什么。

这是关键，因为我们需要在这些望远镜上获得时间，我们必须说服他们这是一个值得的项目。我们已经做了足够的前期观测，我们在银河系中心探测到了事件视界尺度的结构——就是我们银河系中心那个四百万太阳质量的黑洞。我们探测到了那个，所以他们在听我们说，但他们还是有点怀疑我们能不能做到。需要大量的劝说。大量的说服。大量的申请时间。但我们很小心，一路走来我们都在产出结果。然后人们开始意识到你可能真的能做到。所以他们有点让我们进去了。他们给了我们天文台的钥匙……

莱文：嗯哼。

杜勒曼：……就像“午夜之前还回来”那种。

莱文：对。我还想回到原子钟的想法，因为这些原子钟如此精确，在宇宙年龄的时间里才损失几秒钟？

杜勒曼：呃，嗯，不，它们没那么好。我在宇宙的大部分年龄里都存在了，所以我知道。所以是的，关键是当你在任何事件视界望远镜站点记录射电波时，你需要信号不抖动。

它必须稳定，这样当你有另一个信号时，它们能以正确的方式对齐。如果信号因为你的时钟不完美而来回抖动，你就永远无法对齐来自地球不同部分的这些波形。

所以你需要一个超级稳定的时钟。我们使用氢脉泽钟，它们大约每一千万年损失一秒，这就是我们需要的稳定性来保持一切稳定。

莱文：我明白了。

杜勒曼：我记得在墨西哥的大毫米波望远镜那里，看着他们用绞盘把其中一个原子钟沿着螺旋楼梯的中央部分吊上去，就在想我在海拔一万五千英尺的地方看着这个精密计时器被吊上去安装到位，我在做什么。

然后我们把它放进一个木制外壳里，因为你知道，它需要保持在一定温度，我们在里面用灯泡把它加热到正好合适的温度。所以当你在山顶上时，你会变得非常有想象力——我该怎么把这个原子钟裹好？

莱文：对。而且你仍然需要自然配合，因为你需要天气顺着你的意思。你不能让雪积在望远镜里。如果你要指向它，它不能冻住。所以做观测确实有大量这方面的工作。

杜勒曼：嗯，每天傍晚在我们触发观测之前，我们必须确保各地的天气都很好。

莱文：现在，当你把所有这些仪器串在一起，你有这些极其精确的原子钟，所以你在对齐波峰，你现在有一只大眼睛在接收这些天体——看向某个方向，聚焦于某个特定天体——你获得了多好的分辨率？

杜勒曼：比如说，银河系中心的超大质量黑洞是太阳质量的四百万倍。在银河系中心的距离——大约27000光年——整个透镜化的事件视界部分张角大约50微角秒。

现在，那小得离谱。那就像，你知道，如果你在纽约市，你的朋友在洛杉矶拿着一枚25美分硬币，你能读出那枚硬币上的日期。或者如果月球上有一个橘子——或者任何品种的柑橘类水果——你能看到那个水果。甚至一个柚子，可能一个橘子。

莱文：咱们别太挑剔了。

杜勒曼：所以我们在试图从我们行星表面获得最高的角分辨率。这就是所需要的。

莱文：所以当我们谈论这些黑洞时，即使离我们最近的那个——你刚才提到的27000光年外我们银河系中心的那个——也勉强、勉强在可见度的边缘。我们还能看到哪些其他超大质量黑洞？

杜勒曼：嗯，我们很幸运，我们有一个——人马座A*——我们确信可以成像。然后还有另一个，M87，在室女座。那个黑洞大约是太阳重量的六十亿倍，但它距我们五千五百万光年。所以它大约远了两千倍，但也大约重了两千倍。事件视界随质量缩放，所以它的角大小与银河系中心的超大质量黑洞大致相同。

莱文：现在，我记得当你给我发消息说将要有一个新闻发布会。所以我赶到华盛顿特区的国家新闻俱乐部，因为我知道你不会召开新闻发布会来告诉我们你失败了。所以这就是那个时刻——你即将揭晓人类有史以来获得的第一张黑洞照片。我期待的是我们所说的 人马座A*——我们一直在讨论的银河系中心我们自己的黑洞。但结果是M87。我没料到这个。你对他首先分辨出这个感到惊讶吗？你知道像我这样的人会感到惊讶吗？

杜勒曼：你知道，回顾起来一切都说得通。当然，人马座A*是首选的候选者。我们都爱我们银河系中心的超大质量黑洞。

但人马座A有一个问题——尽管我们很爱它——因为它只有四百万太阳质量，物质绕人马座A轨道运行的时间只有大约半小时。这意味着，在一个晚上的观测中，你试图成像的东西外观在变化。东西在绕着它转，它在冒泡，它是湍流的，每一刻看起来都不同。总体上，平均下来，你会看到这个环，但每一刻它都在变化。

所以即使从基本原理出发，我们也应该意识到成像那个源会困难得多，而M87*是六十亿太阳质量，所以时标随质量缩放。所以在一個晚上的观测中，它基本上是静止的。它的外观完全不变，所以你可以把一个晚上观测中采集的所有数据——随着地球转动而变化的目光方向和变化的基线——加起来，构建一张非常非常好的图像。所以，那就是我们先拿出来的，因为它成像容易得多。但能够宣布那个发现，并且知道有专业的天文学家被惊到了，这实际上是整件事挺酷的部分。

莱文：[笑]是的。

杜勒曼：我们最终还是拿出了人马座A*的图像，但只有在我们克服了如何处理一个在你观察时正在变化的东西的算法问题之后。所以我们有两个这样的源，我们都用事件视界望远镜对它们成了像。

当我们在事件视界的时空上放了一把尺子，我们确实验证了对于它们的质量，它们的大小正好是爱因斯坦预测的，在我们测量的不确定度范围内，这是我们拥有的超大质量黑洞存在于星系中心的最佳证据。也是，爱因斯坦万岁。他一直在通过我们扔给他的所有测试。

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斯特罗加茨：听到这个故事很鼓舞人心——为了让十亿人（或者不管多少人）看到那张令人惊叹的图像，投入了多少科学。原子钟、干涉仪的想法、所有广义相对论——多少基础科学……

莱文：我完全同意，他们需要的时钟的那种非凡精度，以及在可观测边缘上的勤奋——我是说，那真的几乎是不可能的。你完全可以想象有人会说，“这太荒谬了。放弃吧。你需要一个地球大小的望远镜，所以换个事做吧。我们永远不会看到任何黑洞的照片。”

而相反，他们想，“好吧，我怎么做一台地球大小的望远镜，”并号召那些已经用于其他目的的天文台，而不是建造一个全新的仪器。

所以我太喜欢其中投入的一切：已有的天文台、计时变得如此精确并将其连接在一起的概念、恰好在可观测性的边缘。真的，他们只有几个像素。

斯特罗加茨：你提出了一个关于已有天文台的有趣观点。我之前没有意识到这一点——不像LIGO，雷·韦斯和那个愿景中你必须创造一整个新仪器来探测引力波，在这个案例中，天文台已经存在，但巧妙之处似乎是协调。但我相信肯定远不止这些。

莱文：是的，我是说，不，协调绝非易事。真的是。大量的协调。大量的工作。以及意识到它是可能的——仅仅是他们一开始就能足够准确地连接几台望远镜，就能做那个概念验证。

有一篇很棒的《纽约时报》文章报道了这件事，实际上远在他们成功之前，只是跟着他们上山，射电天线冻住了，里面堆满了雪，他们遇到地震，他们在那些早期运行中想做的只是证明这个概念是可能的。

所以很大一部分不涉及建造新仪器，但有些确实依赖于为更大目的、面向更大受众建造的新仪器，并在这台仪器上获得时间。

斯特罗加茨：好的。我好奇的一件事是，不是有很多年的黑洞存在的间接证据吗？

莱文：哦，是的。你可以看到东西以如此快的时标变化，以至于暗示一个非常小的天体，但输出的能量暗示一个非常大质量的天体，而这正是你在黑洞的组合中寻找的。

比如当黑洞蚕食邻近恒星时射出的X射线喷流。所以这些是恒星大小的黑洞，不是超大质量黑洞。但那是非常间接的。当你看到那些喷流时，你离得很远。你在收集X射线。喷流巨大。你离聚焦到那个阴影区域——黑洞的定义性特征——还差得远。

所以他们通过靠得如此之近所实现的是测试强场引力——一种甚至比你看到恒星被蚕食时更强的引力，令人惊讶。

所以这就是为什么人们把它视为对广义相对论的检验，因为广义相对论预测了事件视界。那就是预测。那就是金门票，你知道吧？而确实，那就是他们看到的。他们看到的东西稍微大一点，稍微在外面一点，因为光在稍微外面一点的地方绕行，但如此之近，以至于这确实是对那个预测成功的一次令人难以置信的强检验。

斯特罗加茨：嗯哼。所以这就是谢普说“爱因斯坦万岁”时我们应该听到的意思？是为了那种事？

莱文：爱因斯坦万岁。那里看起来确实有一个事件视界，你知道吧？而我们以前不能以那种信心说出来。

所以有一个诺贝尔奖被授予了观测绕人马座A*运行的恒星。他们耐心地观察那些恒星17年，从它们的轨道推断出我们银河系中心有一个超大质量黑洞。所以这要近得多。他用事件视界望远镜看到的东西要近得多，时标也短得多。

斯特罗加茨：嗯嗯。

莱文：但人马座A*令人兴奋的地方——以及它在如此短的时标上变化的事实——在于他们现在正在研究电影。

斯特罗加茨：哈。

莱文：我们可以期待的这个项目的下一个重大发布是，我们将实时观看物质绕黑洞运行，那太了不起了。但要听到那些，你得等到休息之后。

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莱文：欢迎回到《为什么的乐趣》。我们请到了谢普·杜勒曼来给我们讲述有史以来第一批黑洞图像。

所以你决定选M87，你做了这个宣布。全球十亿人观看了这个揭晓。经过二十年的工作，那是什么感觉？

杜勒曼：太神奇了。令人谦卑。有几次我差点流泪，因为你在职业生涯中为某件事工作了那么久，它最终如此有意义，然后看到它对他人有影响力，令人震惊。我是说，它登上了全球大多数报纸的头版，你意识到它触及了一个核心，因为你在第一次看到某样东西。

所以是带着敬畏、谦卑，但也带着惊叹，还有很多宽慰，因为20年之后，桥下流过大量的工作和努力。但还有聚在一起实现它的团队非常特别。我们需要250人才能做到。我们一起经历了很多。

莱文：是的。这个过程对你来说是什么样的？我是说，你是在一个瞬间看到的，还是跨越了几个月？

杜勒曼：在某种意义上是慢火。所以，你首先看到的只是一个图，一个二维图。你在Y轴上看到的是两台相距很远的望远镜接收到的能量。在X轴上，是望远镜之间的距离。发生的是，如果你看到一个环，你会在这个图中看到一个特征性的跳动，那就是我们看到的。所以，我们绘制了数据，还没有成像，我们就看到了这个特征性的跳动，我们意识到这些数据有非常特别的地方。

我想有一张我在一个正式晚宴上的照片。我打着领带，我的一个博士后第一次给我看数据，有一张照片上我的表情像是，“我在看什么？”那天晚上我回到办公室，根据这些跳动的位置做了计算，我意识到它正好是它应该的大小，对吧？

所以那是我们有了东西的第一个迹象。然后我们开始成像。我们一直在合成数据上完善这些成像算法。我们把编造的数据喂给这些算法。我们会拿一张我们可能在看的黑洞图像，用噪声破坏它，然后把它通过这些算法，我们不断完善这些算法，直到我们对它们相当有信心。

但在我们把这些算法放开到数据上之前，我们也意识到人为错误或人类群体思维可能是一个非常大的问题。所以我们把团队分成四组。每组都有数据，但他们不能互相交谈，因为我们要确保我们不是都在同一个房间里想，“我看到一个环。你看到环了吗？我看到一个环。”而它不是环，对吧？

所以我们有四个地理位置不同的小组，他们都使用不同的算法。然后我们在2018年7月聚集到我此刻坐的这栋楼——哈佛的黑洞计划——我们在这栋楼的地下室里，四个团队展示了他们的图像，它们都是环。

那是一个有很多哭泣和尖叫的时刻。呃，然后我们都——然后我们花了接下来六个月试图让那些环消失。我们会训练算法在所有东西上，除了环。

所以它们变得非常擅长成像双源，或者变得非常擅长成像填充的圆盘。然后我们会把它们放开到数据上，它们总是回到环。那时候我们也以一种更稳健的方式知道环的图像是真实的。

莱文：让我们再谈谈环的图像。你已经暗示了为什么它看起来是这样。物质散布在黑洞周围，在事件视界外面轨道运行——说清楚——但很近，非常近。你提到了一些绕黑洞运行的东西的观测，它们在数千倍事件视界之外，太远了，无法有任何信心。

这些就在事件视界上方，也许是事件视界的1.5倍，差不多那样，真的紧贴着它。所以黑洞只是这个明亮环中心的一个暗影，那就是你试图分离出来的。但如果我在看一个像土星环那样的环，那不会为我分离出阴影。

所以你能不能再给我们讲一遍，黑洞后面的光如何让我们把它包裹起来，以便我们实际上能看到投射的阴影？

杜勒曼：是的。原理是这样的：当你如此接近黑洞时，光的路径——通常沿直线传播——被迫沿着弯曲的时空传播。所以它实际上在追踪时空的度规。它在追踪它所处的引力势阱。

所以当光离开围绕星系中心旋转的热气体时——比如说——它开始出发，但不是走某条直线，它会弯曲并来到你这里。所以来自黑洞后面的所有光都被透镜效应弯折形成这个环。

而正是这个环的大小由爱因斯坦方程给出。你会得到这些疯狂的因子，比如2的平方根乘以27。G是牛顿引力常数，M是黑洞的质量，c是光速。但你意识到这个环的直径基本上只由黑洞的质量控制。

莱文：是的，那里没有任何不是自然基本事实的成分，除了黑洞的质量。我认为人们没有意识到黑洞在这种方式上的根本性。所有恒星都多多少少不同。不同的颜色，但你知道，黑洞本质上就是它们的质量。有时它们有电荷，有时它们旋转。但这就是关于它们如此惊人的全部故事。它们几乎是没有任何特征的。

杜勒曼：是的，是的。现在酷的是，想象在银河系另一边有一个天文学家。他们也会看到一个环，因为他们会看到从我们这里发出的所有光被透镜效应弯折到他们的方向。所以任何文明——如果你想的话我们可以谈论银河系中的不同文明——但银河系中的任何人都将看到和我们一样的东西。

所以如果任何人好奇检验爱因斯坦的理论或其他文明炮制的任何理论，他们会对我们正在看的同一个天体感到好奇。

莱文：你知道吗，你让我想到的是，当我们试图向外星文明——我们希望存在但尚未发现的——发送一条信息说“我们正在思考你们正在思考的同样的事情”时，我们有时使用有理数或无理数或特殊信号。如果我们给他们看一张黑洞的照片呢？因为他们看到同样的照片。

杜勒曼：我认为那将是一种非常好的沟通方式。我是说，我们在旅行者号飞船上发送了金唱片，我认为那很有趣。我认为给他们看一些他们可能已经知道的东西也会非常有趣。

莱文：那太疯狂了。我确实想回到某个会把我们推向未来的话题。你在描述人马座A*是如何活的。你知道，它在运动，它在变化，这很大程度上指向下一代。你已经提到了要达到更高的角分辨率。我会问你怎么做到。但这也暗示也许你可以拍电影。那么事件视界望远镜现在在哪里？那个项目现在往哪里去？下一代是什么？你确实叫它下一代，对吧？

杜勒曼：我知道。以最没有想象力、最没有创意的方式命名的。所以我们已经拍摄了黑洞的静态图像，而任何伟大的发现都会提出下一组问题。所以下一个自然要问的是，我们能拍一部黑洞的电影，不仅看到光弯曲，还能看到物质绕黑洞运行的运动吗？

因为这将是其他更灵敏的爱因斯坦理论检验的关键。但它也将解锁天体物理学——使这些超大质量黑洞发射我告诉你的那些喷流的过程。所以我们有充分的理由继续前进，关键是增加全球更多的射电天线。

如果我们能增加更多射电天线，我们将更快地填充地球大小的虚拟透镜。连接数按N乘以N减1增长，所以那里有一个二次因子。所以你不只是多了一台望远镜，你获得了到那台望远镜的所有连接。而且，拥有更多望远镜确保当地球转动时，你总是有临界数量的望远镜来制作电影的下一帧。

五个站点对实现这一点至关重要：一个在智利，一个在墨西哥，一个在怀俄明，一个在加那利群岛，第五个在坦桑尼亚乞力马扎罗山坡上也会非常非常好。如果我们有所有这些，并且我们在最高频率上观测，我们绝对可以拍摄黑洞的电影——高清晰度、高灵敏度、全彩的黑洞电影。

这是下一步。我只需说我们已经获得了在加那利群岛建造第一台望远镜的全部资金。我现在已经两次登顶乞力马扎罗山，在那里寻找建造望远镜的地点。

莱文：哇。

杜勒曼：这就是我为之而活的。我为把钢铁放在山顶而活。呃，而乞力马扎罗山是整個非洲大陆做这类测量的最佳站点，仅仅因为它的海拔。话虽如此，它是一个非常敏感的站点，所以我们三年多前开始与坦桑尼亚开放大学和坦桑尼亚的其他机构合作开发这个想法。我们离完成所有事情还有相当长的路要走。

最重要的是让技术可能性到位，同等重要——如果不是更重要的话——是确保在所有层面上受到影响的人——经济的、社会的、教育的、文化的——都批准它。我告诉团队的是，“‘不’是一个可接受的答案，到此为止。”但如果我们能做到，那么我认为这对国家、对社区、对教育、当然对天文学来说都将是一笔巨大的财富。所以到2030年代初，我认为我们将拥有真正的事件视界电影。

莱文：从科学上讲，一部电影的回报是什么？我是说，显然它会很激动人心。我会想看的。但除了激动之外，我能从看电影中获得什么是我从静态图像中得不到的？

杜勒曼：所以有几件事。一个是自旋是难以捉摸的。黑洞基本上只有几个定义它的特征。它的质量、它的自旋、也许它的电荷，但我们所知的大多数天体物理黑洞的电荷可能已经中和了，因为周围有等离子体会做到这一点。所以质量相当容易。这个环的大小直接与质量相关。我们已经勾掉了那一项。

但黑洞的自旋印刻着它的历史——比如它如何随时间增长——自旋也告诉你参考系拖曳，即它旋转时时空的扭曲。要掌握这一点，你需要真正观察物质的动力学。

给你一个概念，如果你有一个黑洞，它完全不旋转，你得到一个与质量相关的环的大小。如果它以最大速度旋转——它能旋转的最快速度——你看到同样的环，但它大约小4%，好吧？所以自旋不能通过看我们已经看到的黑洞阴影来很好地测量。

但物质绕黑洞运行所需的时间，对于人马座A*来说，从如果不旋转的半小时到如果最大旋转的四分钟不等。现在我们有了一个指标，现在我们有了一个可以进行测量来清楚地告诉我们自旋是多少的方法。所以，对天文学家和物理学家来说最大的回报是，我们可以通过观察绕黑洞运行的物质来精确测量自旋。

莱文：是的。自旋绝对是一个非常有趣的方面。我要说我不认同黑洞被中和了，但如果你有一个旋转的黑洞在磁性环境中——大概你确实有来自所有那些热等离子体的磁场——那么它实际的低能态是带电的。

杜勒曼：有趣，是的。

莱文：我想我不会太轻易排除电荷。

杜勒曼：我们不要排除它。我们就测量它。

莱文：咱们别，老兄。我多年来一直想跟你谈这个。

杜勒曼：但詹娜，你说了一些非常有趣的话。因为你说，“好吧，如果黑洞可以带电呢？”我完全同意。当我们通过这些电影解锁动力学时，我们不仅会看到非偏振光。我们会看到偏振光，它将追踪磁场。所以我们不仅会看到时空的形状，还会看到贯穿那个时空的磁场的形态、形状、配置。所以那将有助于理清你提出的那些非常酷的电池模型。

我想说的另一件事是，只想着广义相对论是我们的耻辱。我是说，那里还有其他不是黑洞的天体，对吧？我们所做工作的一个酷处是，我们开始进行一些首批测试来排除这些奇异的、可以模仿黑洞的天体。

这些天体刚好足够大，比事件视界稍大一点，所以它们有某种表面，但在所有其他方面模仿黑洞及其引力。我们现在已经到了可以把事件视界望远镜的观测与红外观测结合起来的程度——你会预测如果有表面，它会在红外波段发光——我们现在正在对此设定限制。所以通过事件视界望远镜的观测和其他波段的观测，我们

英文来源：

How Do You Take a Picture of a Black Hole?
Introduction
Black holes are one of the universe’s most exotic and elusive objects. When nothing, not even light, can escape their powerful gravitational grasp, taking a picture of one seems unimaginable. But an image of a black hole, or more accurately its shadow, could offer insights that theory alone may not provide when testing predictions. So how can it be done?
Harvard astrophysicist Shep Doeleman and a team of 250 researchers achieved this extraordinary feat by linking a group of radio telescopes around the world. This network, called the Event Horizon Telescope, produced the first image of a supermassive black hole in the galaxy M87, around 55 million light-years away, and the first image of the black hole at the heart of our galaxy. In this episode, Doeleman tells co-host Janna Levin how he turned the Earth into a telescope capable of seeing a black hole, the incredible level of science and collaboration that was required to achieve this, how the images have furthered our understanding of these exotic objects, and how movies of black holes may be possible by the early 2030s.
Listen on Apple Podcasts, Spotify, TuneIn or your favorite podcasting app, or you can stream it from Quanta.
Transcript
JANNA LEVIN: Okay. I’m Janna Levin
STEVE STROGATZ: And I’m Steve Strogatz.
LEVIN: And this is The Joy of Why
STROGATZ: A podcast from Quanta Magazine where we look at some of the biggest unanswered questions in math and science today.
LEVIN: Welcome to another episode.
STROGATZ: Great to see you.
LEVIN: Great to see you. I’m pretty excited about this one. I cannot deny or hide it. We’re talking about not only an area where I’ve spent a lot of my life black holes — the greatest, strangest topic out there — but also I had a chance to talk to an old friend. So it’s a fun episode, I hope.
STROGATZ: I’m really looking forward to hearing it. I know you’ve always had a special affection for black holes. I’ve had a chance to interview you in some earlier version of this.
LEVIN: That’s right.
STROGATZ: And you blew my mind over and over about black holes.
LEVIN: Yeah, they require some defense. They’ve got a bad reputation, not entirely earned.
STROGATZ: What’s the bad reputation? Because they like to eat things?
LEVIN: Yeah. You know, black holes, kind of this monster truck language around black holes all the time. You know, they’re like… But actually, black holes are remarkably elegant, clean, simple systems. They leave you alone if you leave them alone. They’re actually quite small for their heft. That’s kind of their whole gestalt, that they’re actually tiny for their heft.
And, uh, you know, though they’re the darkest phenomenon in the universe, they figure out a way to become the brightest beacons that we see on the edge of what’s observable.
STROGATZ: Wow.
LEVIN: So, they’re, they’re pretty awesome. I also think that they’re fundamental to physics, unlike any other macroscopic object you can think of.
STROGATZ: Hmmm. When I think of black holes, I tend to focus on what they do to spacetime. You know, that they cause singularities. It’s their geometry that always grabs me. But that’s not really necessarily the only way to think about black holes. They’re really objects. Real players.
LEVIN: Yeah. I think after people got over the thrill that they actually existed out there and that they could indirectly be detected, there was a sense that they’re not major players in the universe. “Eh, so there’s some black holes out there.”
Quite to the contrary, I think we’ve come to understand that they sculpt our history and our past, that they have a very regulating and important role in determining the shape and size of galaxies and where there might be habitable regions even. And ultimately, we’re kinda doomed to fall into black holes eventually. So, they’re kind of our past and our future.
But, it might surprise people that we had never actually taken a picture of a black hole. All of our evidence had been indirect until about 10 years ago, and that group worked for well over a decade, for some people up to two decades, to try to capture the first picture of a black hole.
STROGATZ: Huh. Wild.
LEVIN: It was a tremendous accomplishment, led by Shep Doeleman, who is our guest. So let me tell you about Shep. Shep is a senior research fellow at Harvard University, the assistant director of Harvard’s Black Hole Initiative, also an astrophysicist at the Smithsonian Astrophysical Observatory.
And he was the founding director of the Event Horizon Telescope Consortium, which is a network of observatories around the world which gave us the first and only images humanity has ever captured of black holes.
STROGATZ: Hmm. This sounds like it’s gonna be a great discussion of science at its best.
LEVIN: Yeah, it’s an epic story. Let’s hear from Shep himself.
[Music plays]
LEVIN: Shep, welcome to The Joy of Why.
SHEP DOELEMAN: It’s good to be here, Janna.
LEVIN: It’s a pleasure to have you on the show. I’m really curious if you remember not just your way into black holes, but your way into the entire idea of thinking about the universe?
DOELEMAN: Wow. Well, everybody falls in love with astronomy at some point. I don’t care who you are, you fall in love with astronomy. And whether it’s staring up at the night sky, you know, at summer camp or seeing a meteorite, for me, it was seeing a total solar eclipse. I was 13, 1980. And my parents took me out to the middle of the desert in Oregon and at first we thought we weren’t gonna see totality, the clouds were moving in, and then the clouds just parted and we saw this amazing eclipse. It was just tremendous.
LEVIN: Did you think at that moment, “Well, this is what I’m gonna do for the rest of my life”? Or is it that the memory comes back and we reconstruct the story, you know?
DOELEMAN: That’s a great question. I think it mostly, it just comes back. Life isn’t linear like that. But I will say the idea that there would be these huge objects that were moving and they could align perfectly and allow you to see what would normally be hidden, like the corona of the sun, was startling to me. I mean, it didn’t have to be that way. Like, the angular diameter of the moon doesn’t have to be the same as the sun so they totally block each other out. But these coincidences sometimes give you a special view into nature.
LEVIN: Yeah. I, I love that there’s just this idea that we’re inspired by the firmament, science’s oldest instrument, and that we all have it in common.
Um, so I really feel we have to make a confession ’cause we’ve known each other a really long time. I feel like we met maybe the first week of graduate school at MIT. I think over a cookie table.
DOELEMAN: I think so.
LEVIN: So I remember just you were always trying these crazy ideas in graduate school. You were like a creative curiosity in search of a problem. Tell me a little bit first about how you landed on this idea of pursuing a portrait of a black hole?
DOELEMAN: Portrait of a black hole, I love that. So I graduated high school pretty early. I was 15 years old. And went to college, and then after that I was pretty burned out, and I saw an advertisement that said, “Go to Antarctica and run all these experiments down there.” And I said, “That’s for me.” I interviewed for the position, and somehow they gave it to this 19-year-old kid. They needed some warm body down there to look after the cosmic ray experiments, the aerophotometry experiments, the electromagnetic experiments, and I got a real taste for doing experimental research in difficult circumstances. I just loved it. And then came back to MIT, where I met you in 1988.
LEVIN: Oh, you didn’t have to say the year, man. That was unnecessary [laughs].
DOELEMAN: I know, when the Earth was cooling. But, I cast around a bit. I started in the X-ray group there, and then I did a little bit of neuroscience looking at biophysics. And then I was introduced to this idea of turning the Earth into a telescope, and this experiment that we use to image black holes and that immediately drew me in.
LEVIN: Yeah. So we’re gonna talk about this project, but when you were in graduate school, what was the status of black holes scientifically? Were they detected? Did everyone believe that they were real astrophysical phenomena?
DOELEMAN: There was really a sense that black holes existed or strong evidence for them. And of course, there are two flavors of black holes. There are stellar-mass black holes which form during star death. So when a star goes supernova, it will explode the outer shells off, and then the interior part will collapse and if the star is massive enough, it will turn into a black hole, and that black hole could be tens of solar masses.
But then there are these behemoths, these supermassive black holes that live in the centers of galaxies. And while there can be many stellar-mass black holes in a galaxy, there’s probably just one, maybe two, supermassive black holes at the core of any given galaxy.
And for the stellar-mass black holes, people had seen these X-ray binaries, and they were seeing very fast fluctuations in X-rays and even in the radio from these stellar-sized sources. And everything fit with this idea that there was a black hole eating another star that goes an X-ray binary.
LEVIN: And that was Cygnus X-1, a first X-ray object in the direction of the constellation Cygnus, right?
DOELEMAN: Exactly, it was those kinds of sources. And then the radio people predominantly had seen some very high-resolution images that were very troubling. So you would see in the optical, a galaxy would look like a normal run-of-the-mill galaxy with a lot of optical light coming from stars.
But then if you looked at it in the radio, you would see these oppositely directed jets of material all tracing to the center of the galaxy. What is causing that?
And the best idea that people had for that was a spinning supermassive black hole that was energizing jets that moved at near light speed from the north and south pole of that black hole, and it was piercing the entire galaxy.
So from two completely different lines of inquiry you had one flavor of black holes being suspected from the X-ray, and you had another suspected predominantly by the radio.
LEVIN: So even though people started to theorize that these must be black holes, nobody had taken a picture of a black hole, not in the ’70s, ’80s, ’90s, the noughts. I think that was a surprise to people because we talked about them as being such compelling inhabitants of the universe, death states of stars, but we had actually technically never seen one directly. Never taken a portrait. And that’s really what you set out to do in many ways.
DOELEMAN: Yeah. I think the simplest way of saying it is that seeing is believing in a certain way. I mean, you can theorize something, but actually seeing it confers a reality on things that you don’t get otherwise. And there were some really important signatures, too. It wasn’t just seeing it. It was measuring it. Taking the measure of the thing not only gives you confidence in its reality, but you can test predictions.
So in 1915, when Einstein came with his theory of general relativity, which governs the space time around a black hole, and then the year after that Schwarzschild solved it in the trenches of World War I, a story that we know well, even after that, it was pretty clear that if you went to the conclusion of that solution, you would wind up seeing a ring of light around the black hole.
That if a bunch of gas was drawn to the black hole, it heated up to hundreds of billions of degrees because you’re confining it to a small volume, and it emits light. That light would be curved by the gravitational force of the black hole. And from any direction that you look at the black hole, you would see a ring of light. Basically, the emission from behind the black hole lensed around into a ring. And the size of that ring you can derive from Einstein’s equations.
And so we theorized that if we could make a photo, if we could see this ring, we’d be able to measure the mass of the black hole. But also just the ring itself would be like the best evidence for the existence of supermassive black holes at that scale. So it wasn’t just a coffee table book we were after. We were after the real measurement of the fundamental nature of the universe.
LEVIN: Now, when people hear this idea that you’re gonna try to turn the Earth into a telescope or a telescope the size of the world, it sounds like such a crazy lark. What made you think it was viable? It was no mean feat.
DOELEMAN: Yeah. Well, so, so this idea of turning the Earth into a telescope had been around for a long time, since the ‘60s. And the key is that the angular resolution of a telescope or the ability to magnify things on the sky, it scales as the wavelength of light you’re observing, lambda, divided by the size of your telescope.
So if you wanna see things that are really small in the sky, you either reduce your wavelength of light or you expand the size of your telescope. So you’ll often see these huge blocks of glass on top of mountaintops. These huge optical dishes, and they’re big for a reason. They collect a lot of light, but also the bigger they are, the smaller the objects they can see.
LEVIN: The better the resolution.
DOELEMAN: The better the resolution, precisely. And so there are some objects that are so small, and black holes are the smallest objects predicted by Einstein’s theory of gravity, that you need extremely big telescopes, extremely fine angular resolution to see them, and you’ve gotta take this idea of turning the Earth into a telescope to the utmost extremes.
You have to operate it at wavelengths that had never been operated before ’cause you have to really go to smaller and smaller wavelengths to get the angular resolution you need. And that’s what took us 20 years, to push the technique to the absolute limit.
LEVIN: Yeah. So there’s a couple of things. One is a lot of people are surprised to hear that black holes are small. They imagine that they’re enormous. Can you explain that to us? Why they’re actually spatially tiny even though they’re incredibly hefty?
DOELEMAN: Yeah. Black holes simply put are when you confine enough matter into a small enough volume that the force of gravity runs away and the material undergoes catastrophic gravitational collapse into a point of infinite density. And you’d think, “Okay, that’s pretty tiny,” but what happens is that around that is a membrane called the event horizon, and that’s where the gravitational force is so strong that light can’t escape. So that really defines the region of space that we can observe ourselves.
And then astrophysical black holes are not invisible, because you would think to yourself, “Well, if you can’t see it,” because it’s got this event horizon around it, but they attract gas and dust and everything you can imagine to them, and you’re cramming it all into a very, very small space so it heats up. And so in a paradox of their own gravity, these black holes can shine extremely brightly.
And if you get one that’s at the center of a galaxy that weighs millions or billions of times what our sun does, it can outshine all the combined light of the stars in that galaxy. So you see these very small objects, but they can be extremely bright.
LEVIN: So how did they deduce that that’s a black hole if that wasn’t predicted, if that wasn’t expected?
DOELEMAN: Yeah, a lot of it was the timescale of the variations because that tells you the scale of what you’re looking at. So typically in astronomy, if you see something that is varying on a certain timescale, you just say, “Well, I think it can move faster than the speed of light.” And so this gives you an idea of how big that object can be.
And for its luminosity, it was very difficult to think that there could be anything bigger than a black hole that was causing these kinds of fluctuations. And then there was the amount of energy coming out was related to it falling into this gravitational well and energy being liberated. That was another clue that people had. So as you say, there were these indirect methods of modeling what we were seeing as black holes, and the black holes fit the best.
For the supermassive black holes, there people could see the energy coming out of these jets, and really the only mechanism that we could think of was magnetic fields twisting in the warped space time of the black hole itself. And like egg beaters twisting around, they fling these charged particles out at near the speed of light. And the observations were so extreme that really that’s the only model that fit. And miraculously, as you got better and better angular resolution, as you trace the jet all the way down to the center, it kept looking like this jet.
It didn’t break this model. You kept seeing this jet getting closer and closer and closer, and there was this mad idea that we had to see what was causing this.
LEVIN: So let’s distinguish death states of stars, which are the kinds of objects that were creating signals like Cygnus X-1. Two star systems born and died together. One becomes a black hole, the other starts to descend. It gets cannibalized. It creates these X-rays, these jets. These supermassive black holes, at the time we were calling them quasi-stellar objects or quasars, right? It was not understood where these supermassive black holes came from or if they were so prevalent, if they had permeated the entire universe. I mean, it was a surprise, wasn’t it, that those quasars were commonplace, that they probably exist in every single galaxy.
DOELEMAN: Yeah. There’s a lot of different lines of evidence for this. Certainly for the galaxies that are radio loud and where you can do interferometry along the lines of what we did to take the picture of the black hole. There, you wind up seeing these jets of material, and those account for maybe 10% of the galaxies that we see. For the other galaxies, we see these cusps of stellar population that increase dramatically as you get to the center, and so you can postulate that there could be a supermassive black hole there.
And now with better resolution and better telescopes, we’ve seen the statistical velocities of stars very, very close to the center of these galaxies within a couple of light years of these galaxies. And it’s now become clear that all galaxies seem to have these super massive black holes at their cores.
And why some are radio loud, why some emit in the radio waves, why some don’t probably has a lot to do with the direction that you’re looking at them. It also has to do with the evolutionary moment in the lifespan of that galaxy. Some radio loud at some times and some not at other times. So we’re seeing the great variety, like the zoo of galaxies across cosmic time, and we’re fortunate that some of them really exhibit these tremendously illuminating radio structures that we can image with an Earth-sized telescope.
And that’s really what drove the field of radio interferometry. We needed to make a picture that was, let’s say, 10,000 times better than anything that we had made before. And the magic of interferometry is that you can detect radio waves from a black hole, let’s say in Chile, and you can time tag them with an atomic clock so you see all the crests and the troughs of these radio waves as they’re coming in. And you can do the same thing in, let’s say Hawai‘i. And then you can bring those waveforms together and compare them as though they had been received by one huge telescope, and they will be coherent. You can play them back against each other, and you can line up the troughs and the crests. Now this is the secret that allows you to turn the Earth into a telescope.
LEVIN: So this is what you mean when you say very long baseline interferometry or interferometry in general. You’re talking about connecting these dishes around the globe. Now, it’s over decades that this work really begins in imagination, conception, execution. But, when was the Event Horizon Telescope as a project officially founded?
DOELEMAN: Well, it depends. I’d wrote my thesis in 1995 and pushed the technique up to wavelengths that were close but not quite sufficient to do this black hole imaging. So we had a pretty good idea of where we were going. And then we really started in earnest thinking about it in maybe 1998. We started doing experiments that were gonna push it to these frontier frequencies, like 230 gigahertz, that were high enough that it would be able to see through all the gas and dust.
In 2008, we discovered event horizon scale structure in the galactic center. And then, I wrote a paper along with a small number of colleagues for the 2010 Decadal Review, in which we said we’re building an event horizon telescope in the next decade. So that’s when I kinda think we kicked things off. The actual Event Horizon Telescope formed as a body in 2016, and I was the founding director of that.
LEVIN: So, where were the telescopes originally that you were using?
DOELEMAN: Well, let’s see. South Pole, there were two telescopes in Chile, a telescope in Mexico, two telescopes in Hawai‘i, telescope in Arizona — two telescopes now in Arizona. One in Spain, one in France, and that was just enough.
LEVIN: You know, what’s so fascinating about this project, was that all of these telescopes existed. The idea was to use telescopes that already existed that were being used for other purposes, not the same idea at all as building a satellite and starting from scratch.
How did that go over, this idea of trying to build a coalition out of all of these different observatories? And, it’s a really international collaboration. I mean, hundreds of people.
DOELEMAN: I’ll, I’ll put it to you this way. In my group, we spent a lot of time working on instrumentation that would allow a telescope to record information that could then be compared to recordings made at other telescopes. This is how interferometry works on a global scale.
But this was not what these telescopes were doing, so we had to convince these telescopes that we could bring our instrumentation, bolt it in, in some cases manhandle some of their very, very sensitive equipment, operate the telescope in a way that it had not been designed to be operated in, and then take all these disks with data and disappear, and then maybe tell them months later what happened.
And that’s the key thing because we needed to get time on these telescopes, and we had to convince them that this was a worthy project. And we had done enough precursor observations, and we detected event horizon-scale structure in the galactic center in the Milky Way black hole, this is the four million solar mass black hole at the center of our Milky Way galaxy. We detected that, and so they were listening to us, but they were still kinda skeptical that we were gonna be able to pull this off. And it was a lot of cajoling. There was a lot of convincing. There was a lot of applying for time. But we were careful, and we produced results all along the way. And then people began to realize you might actually pull this off. And so they kinda let us in. They gave us the keys to the observatory…
LEVIN: Uh-huh.
DOELEMAN: … Like “Have it back by midnight.” You know, that kind of thing.
LEVIN: Right. I also wanna return to the idea of the atomic clock, because these atomic clocks are so precise that, what you lose a few seconds over the age of the universe?
DOELEMAN: Uh, well, no, they’re not, they’re not that good. And I’ve been around for most of the age of the universe, so I know. So yeah, so the key thing is that when you record the radio waves at any of the Event Horizon Telescope sites, you need for the signal not to jitter.
It has to be stable so that when you have another one, they line up in just the right way. And if the signal is jittering back and forth because your clock is imperfect, you’ll never be able to line up these waveforms from different parts of the Earth.
So you need a super stable clock. We use hydrogen maser clocks, and these lose about one second every 10 million years or so, and that’s the stability that we need in order to keep everything stable.
LEVIN: I see.
DOELEMAN: I remember being at the Large Millimeter Telescope in Mexico and watching as they winched one of these atomic clocks up the central part of a helical staircase, and just like wondering what I was doing at 15,000 feet watching this precision timepiece being winched up and into place.
And then we put it into a wooden enclosure because, you know, it needed to be at a certain temperature, and we used light bulbs inside to heat it to just the right temperature. So you get very imaginative when you’re at the top of a mountain, how am I gonna keep this atomic clock you know, swaddled?
LEVIN: Right. And you still need for nature to comply, ’cause you need the weather to go your way. You can’t have snow accumulating in the telescope. It can’t freeze if you’re trying to point it. And so there is genuinely a lot of that in making an observation.
DOELEMAN: Well, so every evening before we trigger the observations, we have to make sure that the weather is good everywhere.
LEVIN: Now, when you’re stringing all these instruments together and you have these incredibly precise atomic clocks, and so you’re lining up the crest so you’re now have one big eye taking in these objects that looking in a certain direction and focusing on a particular object, how good a resolution have you gotten?
DOELEMAN: So for example, the supermassive black hole in the center of the Milky Way galaxy is 4 million times the mass of the sun. At the distance of the center of the galaxy, about 27,000 light years, the whole lensed part of the event horizon subtends about 50 microarcseconds.
Now, that’s insanely small. That’s like, you know, if you’re in New York City and your friend is holding a quarter in Los Angeles, you can read the date on that quarter. Or if there’s a tangerine or really a citrus fruit of any variety on the moon, you could see that fruit. Even a pomelo, possibly a tangerine.
LEVIN: Let’s not be picky.
DOELEMAN: And, so we’re, we’re trying to get the highest angular resolution possible from the surface of our planet. That’s what’s needed.
LEVIN: So when we’re talking about these black holes, even the one that’s closest to us, which you just mentioned at 27,000 light years away in the center of our own Milky Way galaxy, is barely, barely on the edge of visibility. What other supermassive black holes could we possibly see?
DOELEMAN: Well, so we’re lucky in the sense that we have one, Sagittarius A, that we felt for sure we could image. And then there was another one, M87, which is in the constellation Virgo. That black hole is about six billion times what our sun weighs, but it’s fifty-five million light years away. So it’s about two thousand times farther away, but it’s also about two thousand times more massive. And the event horizon scales with mass, so its angular size is about the same as the supermassive black hole in the center of the Milky Way galaxy.
LEVIN: Now, I remember when you sent me a note saying there’s going to be a press conference. So I come racing out to the National Press Club in Washington, DC ’cause I know you’re not holding a press conference to tell us you failed. So here is the moment, where you’re about to reveal the first picture humanity has ever procured of a black hole ever. And I’m expecting what we call Sagittarius A, our own black hole in the center of the Milky Way that we’ve been discussing. But instead it’s M87. I did not expect this. Were you surprised that that’s what you were able to resolve first? Did you know people like me would be surprised?
DOELEMAN: You know, it all makes sense when you look back on it. Of course, Sagittarius A was the preferred candidate. We’re all in love with our own supermassive black hole in the center of the Milky Way galaxy.
But there’s a problem with Sagittarius A as much as we love it, because it’s only 4 million solar masses, the time it takes matter to orbit Sagittarius A is only about half an hour. Now, what that means is that during a night of observing, the thing you’re trying to take an image of is changing in its appearance. Things are orbiting around it, it’s burbling, it’s turbulent, and it looks different from one moment to the next. In aggregate, averaged, you would see this ring, but from moment to moment it would change.
So even from first principles, we should have realized that imaging that source would’ve been much more difficult, whereas M87, is 6 billion solar masses, so the timescales scale with mass. So during a night of observing, it’s essentially stationary. It’s not changing its appearance at all, so you can add up all the data you take during a night of observing with all the changing look directions and the changing baselines as the Earth is turning to build up a very, very good image. So, that’s the one we came with first because it was much easier to image. But being able to announce that discovery, and knowing that there was astronomers who were experts who were taken by surprise, that was actually kind of cool about the whole thing.
LEVIN: [Laughs] Yeah.
DOELEMAN: We finally did come with an image of Sag A, but only after we had overcome the algorithmic problems of how to deal with something that’s changing while you’re looking at it. So we have two of these sources, and we’ve imaged both of them with the Event Horizon Telescope.
And when we laid a ruler across the spacetime of the event horizon, we did verify that for their mass, they’re exactly the size that Einstein predicted, to within the uncertainty of our measurements, which is the best evidence that we have for the existence of supermassive black holes in the centers of galaxies. And also, yay for Einstein. He keeps passing all these tests that we throw at him.
[Music plays]
STROGATZ: It’s very inspiring to listen to the story of how much science goes in to make the stunning image that a billion people or whatever got to see. How much fundamental science between the atomic clocks, the idea of interferometers, all the general relativity…
LEVIN: I totally agree that this remarkable precision of the clock that they needed, and the industriousness of being on the edge of observability, I mean, it really was almost impossible. You could totally have imagined that somebody would’ve said, “This is ridiculous. Give this up. You need a telescope the size of the Earth, so move on to something else. We’ll never see a picture of any black hole ever.”
And instead, they thought, “Well, how would I make a telescope the size of the Earth,” and calling on observatories that had been in use for other purposes, as opposed to building a fundamentally new instrument.
So I just loved so much that went into it: the preexisting observatories, the notions of the timing having become so precise and linking it together just on the edge of observability. Really, it’s just a few pixels that they have.
STROGATZ: You raise an interesting point about existing observatories. I hadn’t appreciated that, that unlike LIGO, Ray Weiss and that vision where you had to create a whole new instrument to get the gravitational wave detection, in this case, the observatories existed, but the ingenuity seems to be the coordination. But I’m sure there must be much more to it than just that.
LEVIN: Yeah, I mean, but no, the coordination was no mean feat. It really was. It was a lot of that. It was a lot. And realizing it was possible, just the fact that they could accurately enough link up just a couple of telescopes in the beginning, that they could do that proof of concept.
There’s a great New York Times article about it actually long before they succeeded, just following them up a mountain and the radio dish freezes and there’s piles of snow in it, and they have earthquakes and all they’re trying to do in those early runs was just prove the concept was possible.
And, so a lot of it did not involve building new instruments, but some of it did rely on new instruments being built for a larger purpose with a larger audience in mind and getting time on that instrument.
STROGATZ: Okay. One thing I was wondering about it, wasn’t there indirect evidence for the existence of black holes going back many years?
LEVIN: Oh, yeah. You could see things changing on a timescale that was so fast it suggested a very small object, but the amount of power coming out suggested a very massive object, and that’s exactly what you’re looking for in the combination of a black hole.
So like X-ray jets coming out when a black hole’s cannibalizing a neighboring star. So these are star-sized black holes, not supermassive black holes. But it’s very indirect. You’re quite far out when you’re seeing those jets. You’re collecting X-rays. The jets are huge. You’re nowhere near honing into that shadowy region, which is the defining character of a black hole.
And so what they’re achieving by getting in so close is testing strong field gravity, a gravity where it’s even stronger than what you see when you’re looking at a star being cannibalized, surprisingly.
And so, that’s why people look at it as tests of general relativity because general relativity predicts the event horizon. That’s the prediction. That’s the golden ticket, you know? And indeed, that’s what they’re seeing. They’re seeing something a little bit bigger, a little bit outside because the light is orbiting a little bit further out, but so close that it is really an incredibly strong test of the success of that prediction.
STROGATZ: Uh-huh. And so that’s how we should hear when Shep says, “Yay, Einstein.” It’s for that kind of thing?
LEVIN: Yay, Einstein. There sure looks like there’s an event horizon, you know? And that we couldn’t say with that kind of confidence.
So there was a Nobel Prize awarded for observing stars orbiting Sagittarius A. And they were patiently watching those stars for 17 years, and from their orbits deducing that there was a supermassive black hole at the center of our galaxy. And so this is much closer in. He’s seeing stuff with the Event Horizon Telescope that’s much, much closer and shorter timescales.
STROGATZ: Mmm-hmmm.
LEVIN: What’s exciting though about Sagittarius A and the fact that it is changing on such a short timescale, is that they’re now working on movies.
STROGATZ: Huh.
LEVIN: The next big release that we can anticipate from this project is that we’ll watch the stuff orbit around the black hole in real time, and that’s remarkable. But to hear about that, you’ll have to wait until after the break.
[Music plays]
LEVIN: Welcome back to The Joy of Why. We have Shep Doeleman here telling us about the first images ever taken of a black hole.
So you decide to go for M87, and you make this announcement. A billion people around the globe watch this reveal. What is that like after two decades of working on this?
DOELEMAN: It was pretty amazing. It was humbling. There were times when I was close to tears during part of this because you work for something so long in your career, and it winds up being so meaningful, and then to see that it’s impactful for others is startling. I mean, it was on the front page of most newspapers around the globe, and you realize it touches a core because you’re seeing something for the first time.
And so it was with awe, humility, but also wonder, a lot of relief because after 20 years, it’s a lot of work and effort under the bridge. But also the team that came together to realize it was very special. We needed 250 people to pull this off. And we’d been through a lot together.
LEVIN: Yeah. How is this process for you? Are you, I mean, is it in a moment that you see it or is it over months?
DOELEMAN: It’s a slow burn in a certain way. So, the first thing you see is just a graph, a two-dimensional graph. And you see on the Y-axis, it’s the amount of energy the two telescopes that are widely separated are seeing. On the X-axis, it’s the distance between the telescopes. And what happens is that if you see a ring, you’d see a characteristic bounce in this graph, and that’s what we saw. So, we plotted up the data, and without making an image, and we saw this characteristic bounce, and we realized there’s something really special about these data.
I think there’s a picture of me at a fancy dinner. I’m wearing a tie and one of my post-docs is showing me the data for the first time, and there’s a picture of me going like, “What am I, am I looking at?” And I came back to my office that night, and I went through the calculations based on where these bounces were, and I realized it’s exactly the size it’s supposed to be, right?
So that was the first indication that we had something. And then we started imaging. So we had been perfecting these imaging algorithms on synthetic data. We’d fed these algorithms made-up data. So we would take a black hole image that we might be looking at. We’d corrupt it with noise, and we’d send it through these algorithms, and we’d refine these to the point where we were pretty confident in these algorithms.
But before we set these loose on the data, we also realized that human error or human groupthink could be a really big problem. So we split our teams up into four groups. Each group had the data, but they could not talk to each other because we wanted to make sure that we all weren’t in the same room thinking like, “I see a ring. Do you see a ring? I see a ring.” And it’s not a ring, right?
So we had four different geographically separated groups, and they all used different algorithms. And then we all came together to the building where I am sitting right now, the Black Hole Initiative at Harvard in July of 2018, and we were in the basement of this building, and the four teams put up their images, and they were all rings.
That was a moment where there was a lot of crying and screaming. Uh, and, and then we all, and then we spent the next six months trying to make those rings go away. So we would train the algorithms on everything except for rings.
So they became very good at imaging double sources, or they became very good at imaging filled disks. And then we would set them loose on the data, and they would always come back to the ring. And that’s when we also knew in a more robust way that the ring image was real.
LEVIN: Let’s talk about the ring image again. You’ve alluded to why it looks this way. There’s material splattered around the black hole that’s orbiting outside of the event horizon, to be clear, and but close, really close. You mentioned some observations of things in orbit around the black hole were thousands of times the event horizon away, way too far to have any confidence.
These are right on top of the event horizon, maybe 1.5 the event horizon, something like that, really up against it. And, and so really the black hole is just a dark shadow in the center of this bright ring, and that’s what you’re trying to isolate. But if I was looking at a ring like the rings of Saturn, that’s not gonna isolate the shadow for me.
So can you talk us through again how the light from behind the black hole allows us to have it enveloped so that we can actually see the shadow cast?
DOELEMAN: Yeah. The way this works is that when you’re so close to the black hole, the paths of light, which normally would travel in a straight line, is forced to travel along the curved spacetime. And so it’s really tracing the metric of spacetime. It’s tracing the gravitational well that it finds itself in.
So when light leaves hot gas that’s swirling around the galactic center, let’s say, it starts off, but then instead of going in some straight line, it’ll curve around and come to you. So all the light from behind the black hole is being lensed around to form this ring.
And it’s the size of this ring that is given by Einstein’s equations. And you get these crazy factors like two the square root of 27. G is Newton’s gravitational constant, M is the mass of the black hole, c is the speed of light. But you realize that the diameter of this ring basically just controlled by the mass of the black hole.
LEVIN: Yeah, there’s nothing, there are no ingredients in there that aren’t fundamental facts of nature except the mass of the black hole. And that’s, I think people don’t appreciate how black holes are fundamental in that way. There’s… all stars are kinda different. Different colors, but, you know, black holes are essentially their mass. Sometimes they have electric charge, sometimes they spin. But that is the whole story that is so stunning about them. They’re almost featureless.
DOELEMAN: Yeah, yeah. Now the cool thing is that imagine there was an astronomer on the other side of the galaxy. They would see a ring too, because they would see all the light from where we are lensed around in their direction. So any civilization, and we can talk about different civilizations in the Milky Way if you want to, but anyone in the Milky Way galaxy would be seeing the same thing we are.
So if anyone’s curious about testing Einstein’s theories or whatever theories have been concocted by another civilization, they would be curious about this same object that we’re looking at.
LEVIN: Do you know what you made me think of is that when we try to send alien civilizations, that we hope exist but have not yet discovered, a message that says, “We’re thinking about the same things you’re thinking about,” we sometimes use like rational numbers or irrational numbers or special signals. What if we showed them a picture of the black hole? Because they see the same picture.
DOELEMAN: I think that would be a very good way to communicate. I mean, we sent out the golden record in the Voyager spacecraft, and I think that’s interesting. I think showing them something that they may already know would also be very interesting.
LEVIN: That is wild. I did wanna return to something which is gonna push us into the future. You were describing how Sag A was alive. You know, it’s moving, it’s changing, and this very much speaks to the next generation. You’ve already mentioned going to even higher angular resolutions. I’ll ask you how you’re gonna do that. But also it suggests that maybe you can take movies. And so where is Event Horizon Telescope? Where is that project going now? What’s the next gen? And you do call it next gen, don’t you?
DOELEMAN: I know. It’s in the most unimaginative, unoriginal way possible. So we’ve taken still images of black holes, and with any great discovery, they just raise the next set of questions. So the next natural thing to ask is, can we make a movie of the black hole and see not just light bending around, but the motion of matter as it orbits the black hole?
Because this will be the key to other even more sensitive tests of Einstein’s theory. But it will also unlock the astrophysics, the processes that allows these supermassive black holes to launch those jets that I was telling you about. So there’s every reason for us to keep going, and the key is adding more radio dishes around the globe.
If we can add more radio dishes, we’ll fill in the Earth-sized virtual lens much faster. The number of connections goes as N times N minus one, so there’s a quadratic factor in there. So you don’t just get one more telescope, you get all the connections to that telescope. And also, having more telescopes ensures that as the Earth turns, you always have a critical number to make the next frame in a movie.
Five sites that are critical to allowing this to happen: One is in Chile, one is in Mexico, one is in Wyoming, and one is in the Canary Islands, and a fifth in Tanzania on the slopes of Mount Kilimanjaro would also be very, very nice to have. If we had all those, and we were observing at the highest frequencies, we can absolutely make movies of black holes, and high definition, high sensitivity, full color movies of black holes.
And this is the next step. I’ll just say we’ve secured all the funding to build the first telescope in the Canary Islands. I’ve now summited Mount Kilimanjaro twice, scouting for locations to build a telescope up there.
LEVIN: Wow.
DOELEMAN: This is what I live for. I live for putting steel on mountaintops. uh, and Kilimanjaro by far is the best site in the entire African continent to make these kinds of measurements just because of its altitude. Now that being said, it’s a very sensitive site, so we started over three years ago working with The Open University of Tanzania and other institutes in Tanzania to develop the idea. We’re still quite a ways off from doing everything.
The most important thing is to get the technical possibilities in line, and equally important, if not more important, is to make sure that everyone who’s affected at all levels, economic, social, educational, cultural, are all approving it. And what I tell the team is, “No is an acceptable answer, full stop.” But if we can, then I think it would be a tremendous asset for the country, for the community, for education, and of course for astronomy. So by the early 2030s, I think we’ll have real movies of an event horizon.
LEVIN: And what’s the payoff for a movie scientifically? I mean, obviously it’s gonna be thrilling. I’m gonna wanna see it. But beyond just the thrill of it, what can I extract from watching a movie that I can’t from a still image?
DOELEMAN: So there’s a couple things. So one is that spin is elusive. So, a, a black hole basically has only a few characteristics that define it. Its mass, its spin, maybe its charge, but most of the astrophysical black holes that we know of probably have charge equalized, because there’s plasma around it that would do that. So the mass is pretty easy. The size of this ring is directly related to mass. We’ve checked that one off.
But the spin of the black hole imprints its history, like how it grew over time, and the spin also tells you about the frame dragging, the warping of spacetime as it spins. And to get a handle on that, you need to really look at dynamics of matter.
To give you a sense of the problem, if you have a black hole and it’s spinning not at all, you get one size of the ring related to its mass. If it’s spinning maximally, the fastest it can ever spin, you see the same ring, but it’s just about 4% smaller, okay? So spin is not measured well by looking at the shadow of the black hole that we’ve already seen.
But the time it takes matter to orbit the black hole, that for Sagittarius A ranges from half an hour if it’s not spinning to four minutes if it’s maximally spinning. Now we have an indicator, now we have a measurement we can make that would tell us cleanly what the spin is. So, the biggest payoff for astronomers and physicists is that we can make a precise measurement of spin by looking at matter orbiting the black hole.
LEVIN: Yeah. Spin is definitely a very interesting aspect. I would say I don’t buy that black holes are neutralized, but if you have a spinning black hole in a magnetic environment, which presumably you do have magnetic fields from all of that hot plasma, then its actual low energy state is charged.
DOELEMAN: Interesting, yeah.
LEVIN: I think I wouldn’t exclude charge too out of hand.
DOELEMAN: Let’s not exclude it. Let’s just measure it.
LEVIN: Let’s not, man. I’ve been trying to talk to you about this for years.
DOELEMAN: But Janna, you said something really interesting. ’Cause you said, “Well, what if the black hole could be charged?” And I couldn’t agree more. When we unlock the dynamics with these movies, we’re not just gonna see the unpolarized light. We’ll see polarized light, which will trace magnetic fields. So we’ll see not just the shape of the spacetime, but also the morphology, the shape, the configuration of the magnetic fields threading that spacetime. So that would help disentangle these battery models that you’ve come up with which are really cool.
The other thing I would say is that shame on us for just thinking about GR. I mean, there are other objects out there that are not black holes, right? And one of the cool things about the work we’re doing is that we’re beginning to make some of the first tests to rule out these exotic objects that can be black hole mimickers.
These are objects that are just large enough so that they’re a little bit bigger than the event horizon, so they have a surface of some kind, but in all other ways mimic the black hole and its gravity. And we’re at the point now where we can start to combine the EHT observations with infrared observations, where you would predict that if you had a surface, it would glow in the infrared, and we are now putting limits on that. So with the EHT observations and observations from other wavebands, we’re starting to rule out even non-black hole solutions.
LEVIN: So, you have two things on the horizon, so to speak, which include making movies of the objects that you’ve already resolved, but also new candidate black holes. That’s really thrilling. You were just saying, well, what’s beyond GR? What other objects can you look at besides black holes?
DOELEMAN: Oh yeah. So, even now, the EHT, Event Horizon Telescope, is a bit democratic, so other researchers who are not in the collaboration can still apply for time on it, and there’s been like one or two cases I think where that’s happened. But also more to your point about what else can we look at, we train the Event Horizon Telescope not just on Sag Aand M87, but on many radio loud quasars. So these are supermassive black holes that produce these jets of oppositely directed matter, and we’re able to get the best angular resolution that you can get on those sources. So we’re studying the propagation of jets, the collimation of those jets.
They’re very far away, but we can get the best most detailed images of them using the EHT. So it’s advanced our understanding of how jets are produced and how they maintain their shape very far from the black hole. And then there’s also a couple of other objects we can look at.
When stars die, they release some of their atmosphere, and so you can wind up with a red giant star with a molecular soup around it, and the light from the star pumps that molecular soup, and you can get effectively a laser the size of the star.
And around the edge of the star, around the limb of the star, you can get pinpoints of laser light in the radio, and these can be now observed with the Event Horizon Telescope too. So we may be able to see stars breathe in and out by using the Event Horizon Telescope with these molecular lines.
LEVIN: Now, you said the timescale is 2030s?
DOELEMAN: Early 2030s?
LEVIN: Early 2030s. And then you think you’ll be online, then it’ll take a couple of years to collect data?
DOELEMAN: Actually, I think we’ll be able to produce images very quickly. So when the first time we imaged a black hole, we had no idea what we were doing. Well, we knew what we were doing, but we had never imaged a black hole before, so we proceeded extremely carefully.
Now that we understand the instrument much better, now that we’ve imaged these black holes multiple times, now that we have great confidence in our instrument. We’re developing imaging techniques now that will process the data and image it within days.
LEVIN: Wow.
DOELEMAN: So, I don’t think we’re gonna have to wait another two years as we did the first time.
LEVIN: So, you described this childhood experience of seeing an eclipse, something essentially rendering the sun nearly invisible, right? But then the moon renders the corona, the outer atmosphere of the sun, visible. And in some ways, this has been a thread through a lot of your work and your curiosity since then. Do you still find that same joy?
DOELEMAN: I really do. The ability to see something for the first time, the ability to see something hidden, the ability to coax this image from the black hole when it does everything in its nature not to be seen is really special. It’s the work of a lifetime, of a career and more than that, it just- it fills me with an intense joy to be able to do this.
I feel as though the team here is doing something almost sacred. We’re getting to the heart of something, and we’re bringing it to the planet. You can get the best people working on something from across borders, and you are fixated on a common science vision, and together you can do what individually you can’t. That’s a lesson I think that we can take broadly across the globe.
LEVIN: Hmm. Well, that’s beautiful, Shep, and I’m grateful and I’m looking forward with anticipation to see what’s next. Thanks so much for joining us on The Joy of Why.
DOELEMAN: Oh, it’s a great pleasure, Janna. Thanks for having me.
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STROGATZ: I love hearing that. This image of the scientists from all over the planet and also the cool places that you get to go if you’re in this line of work, the Canary Islands, Mount Kilimanjaro.
LEVIN: Yeah. It’s kind of funny they had to explore here on Earth so that they could explore way out there.
STROGATZ: And so what is it about seeing an image emotionally, do you think? I mean, I realize this is getting into psychology rather than physics, but why is it so important for us to have that gorgeous picture of the ring with the blackness inside it?
LEVIN: I mean, it’s thrilling. We are observers of the sky. We look at the sky and it was our first instrument in many senses, and our first clock and our first recognition that, wow, there’s something beyond this Earth. And so I think just the idea that here we are looking at something — literally looking at it two million light years away — that is extending our cosmos even further out. I just think that is a human disposition to be thrilled by that.
STROGATZ: Uh-huh. You guys in your conversation mention that the moon just happens to occlude the sun. It just happens that the moon is the right size, given its distance and given the sun’s distance and given the sun’s size, that it can block the sun, but not totally. Lets you see the corona, okay.
Do you think that this might have something to do with the human belief in miracles and gods? That because early people might have observed solar eclipses, that it’s such a bizarre coincidence and so, so what? So scary that the sun goes black at some point. Like maybe that gave us a sense of wonder as a species, and if the astronomical coincidence hadn’t been there, maybe we wouldn’t have ever started asking why?
LEVIN: Hmm, that’s so interesting. There’s so much mysterious about the universe, I feel like even just the sun rising and setting day after day. But it is remarkable the moon is 400 times smaller than the sun, but 400 times closer, and that crazy accident gives us the best eclipses in the solar system, if you ask me.
But, famously there was an eclipse that speaks to your question. It was something like 500 in the Common Era, I think, where there was a war that had fully broken out and somewhere around now modern-day Turkey. And in the middle of a battle there was a total solar eclipse, and apparently both sides put their weapons down and made peace. [Laughs] Fearing, fearing the rage of the gods. They thought better of their circumstances.
STROGATZ: We need more of those eclipses. Well, this has been a really interesting discussion, and I really appreciate your expertise walking all of us listeners through this topic of black holes. and it really is a very uplifting subject.
LEVIN: Well, It was quite remarkable. I think there’s more ahead.
STROGATZ: Looking forward to the next encounter.
LEVIN: To the next encounter.
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STROGATZ: If you’re enjoying The Joy of Why and you’re not already subscribed, hit the subscribe or follow button where 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.
LEVIN: 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, Janna Levin. If you have any questions or comments, please email us at [email protected]. Thanks for listening.
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