霍金睿思讲坛录之二

原英文演讲稿发布于02.02.2016

郑中（Geongs Zhern）译于2017年7月7日

译者注：为庆祝被科学网编辑用那点权力而禁言一个星期（因本人有篇博文涉及“批驳封建儒家”，本人当天不知科学网才发了三条红线，其中包括敏感问题，幸好其它网站容纳得下），特发布霍金睿思讲谈录之二。霍金第二次睿思演讲后面有点新意了，总结为三个关键词“超译过程”、“黑洞软毛”和“黑洞奇环”。霍金在演讲最后一语双关的道出了一句人生哲理“若你活在黑洞中，那么别放弃，会有一条出路的。”这句话更像是霍金对自己一生的自励共勉。但当霍金说“我毕竟可能获得一个诺贝尔奖”的时候，又让人感到他的自信乐观得有点好笑。

   一天，我在书店翻到湖南科技出版社的小书《霍金里斯演讲录》，为了掩盖书太薄，他们居然大大加厚了前后封面，依然是吴忠超翻译的。本书开头致辞就是霍金的话“随机应变是一种智慧”，为啥将此句开头了，目的是掩盖过去一而再、再而三的科学打赌失败。随后翻到，吴教授将“a singularity in the form of a ring”译为“环状的奇性”，如此措辞显得拙劣，不如译为“奇环”（与奇点相对）简明，况且物理学中使用过。此书最后吴教授拟了一个对不起的对联，导致这本又小又薄的书收尾又露丑。此书主要由霍金这两篇讲演稿组成，每页段落稀松，竟然排版成80几页，标价39元，谁买啊？典型商业运作而已。霍金若以此书结束写作，就太不精彩了。

严正声明：此译稿谢绝转载，禁止抄袭，严禁用于商业利益，违者必究！

霍金睿思讲坛录之一，请点击：http://blog.sciencenet.cn/home.php?mod=space&uid=289142&do=blog&id=1062388

  附有BBC科学编辑戴维德•舒克曼（DavidShukman）的注释。“世界最出名的科学家”斯蒂芬•霍金正将进行本年度BBC睿思演讲。作为一名“有趣而困惑的”向导，我按最近那个星期采用的相同方式，将几个注释（见下面的意大利字体，在译文中为斜体）添加到了霍金教授的第二次演讲稿中。

演讲正文

   在我前一次演讲中，我给你们留下了一个悬念：关于黑洞（恒星坍缩产生的、难以置信的致密天体）本性的悖论。一种理论认为，同等质量的黑洞可由无限多种恒星形成。另一种认为，这种数量是有限的。这是一个信息问题，即宇宙内的每种粒子和每种自然力都蕴含着信息，是对是非问题的一种含蓄回答。

   因为黑洞没有毛发，当科学家约翰•惠勒提出这点，人从外侧就不知晓黑洞内部如何，除其质量、电荷和旋转以外。这意味着从外侧世界来说，黑洞含有大量信息被隐藏起来。如果黑洞内部隐藏的信息量取决于黑洞的径尺，那么我们就可根据普适原理预测：黑洞会具有温度，而且会像一个块热金属那样发着辉光。但那是不可能的，因为如每个人都知道，没有任何东西可脱离黑洞。大致就是这么想的。

   这个问题保留到1974年初，当时我根据量子力学，正在研究黑洞附近的物质行为如何。

注释：量子力学是关于极小的科学，它探究解释最微小粒子的行为。这些极小粒子不遵循统治大得多的物体（如行星）的运动定律，即艾萨克•牛顿首先建立的定律。将关于很小的科学应用于关于很大的科学，这是斯蒂芬•霍金的开创性成就之一。

   令我大吃一惊的是，我发现黑洞看来以稳定的速度在发射出粒子。就像当时其它每个人一样，我接受了那句名言：黑洞不会发射任何东西。我因此付出了大量的努力，试图革除这个让人难堪的影响。但我越思考它，就越舍不得，乃至在最后，我必须理解它。最后，使我确信的是一个真实的物理过程，即外逃粒子产生波谱，严格地说是热学的。我的推算预测到：黑洞产生并发射粒子，而发生辐射，就好像是一个普通的炽热天体，具有的温度正比于表面引力，而反比于质量。

注释：这些推算首次显示出，黑洞无需一条通往衰亡终结的单行道。该理论提出的发射作用成为著名的“霍金辐射”。

   自那时以来，黑洞发出热辐射的数学证据已被许多其它人用各种方式加以证实。一种理解辐射的方式如下。量子力学暗示，全部空间就是虚-反粒子对，充满虚粒子和反粒子的对偶，不断成对的物质化、分离化，然后再次碰撞一起而彼此湮灭。

注释：这个概念链接着如下思想：真空非绝对虚空。根据量子力学的不确定性原理，粒子总有一定概率可变成存在，但是在一瞬间。而这总会涉及具有相反特征的粒子对，不断出现，不断消失。

   这些粒子被称为虚的，因为不像实粒子那样，它们不能被粒子探测器直接观测到。但它们的间接效应可被观测到，而且它们的存在性已被小小的朗伯移位（Lamb shift））所证实，这是从激发的氢原子以光谱能的形式产生的。现在对于存在黑洞的情况，虚粒子对中的一个粒子可落入洞内，并留下另一个没有伴儿的粒子有待湮灭。被抛弃的粒子，或者说反粒子，可在它的伴粒子之后而落入黑洞，但它也可逃逸到无限远处，在那里看起来黑洞就发生了辐射。

注释：这儿关键在于这些粒子的形成和消逝一般不被察觉地进行着。但如果该过程恰发生在黑洞边缘，你那么粒子对之一可被拖入，而另一个粒子则不被拖住。于是，逃逸的粒子会看起来就像被黑洞正在发射出来一样。

   太阳质量的黑洞会以一种很低的速度而发泄出粒子，探测到它是不可能的。然而，有许许多多更小的微黑洞具有所谓山岳般的质量。山岳般尺寸的黑洞会以约千万兆瓦的功率发射出X射线和伽马射线，足以满足世界电力供给。但控制一个微黑洞不可能是容易的。你不可能在一个电站维持住微黑洞，因为它会落下穿过地面，而最终抵达地球核心。如果我们具有这样一个黑洞，持续控制住它的唯一方式就是把它送入地球周围的轨道。

   人们探寻过了这种质量的微黑洞，但迄今还没发现任何结果。这是一件憾事，因为如果他们发现了，我会获得诺贝尔奖。但另一种可能性就是，我们可能在时空额外维中创造微黑洞。

注释：他采用“额外维”，意思是高于我们日常生活中都熟悉的三维空间以及第四维的时间。该思想试图解释为何引力比其它自然力（如磁力）更微弱得多——可能这也必须在平行维度进行。

   根据某些理论，我们经历的宇宙只是在十维或十一维空间内的一张四维曲面。电影《星际》关于宇宙像啥给出了一些想法。我们不可能看见这些额外维，因为光不能沿它们传播，而仅仅传播通过我们宇宙的四维。然而，引力会影响额外维，可比在我们宇宙内更强得多。这可导致在额外维内更容易形成一个小黑洞。用LHC（在瑞士的CERN大型强子对撞机，由环形隧道构成，周长27千米）或许可观测到这种小黑洞。两道粒子束沿反方向在这个隧道中传播，从而产生碰撞。有些碰撞可能产生微黑洞。这会以一种容易识别的型式而辐射出粒子。所以，我毕竟可能获得一个诺贝尔奖。

注释：当一种理论“已受时间检验”时，实际上意味着被坚实的证据所证实，就可获得物理学诺贝尔奖。比如，皮特尔•希格斯（Peter Higgs）在在科学家们中，他曾在1960年代就提出存在一种粒子可将其质量赋予其它粒子。近50年之后，在大型强子对撞机上两次不同的探测发现著名的希格斯玻色子的迹象。这是科学和工程学的胜利，也是聪明理论和难得证据的胜利。而皮特尔•希格斯和一位柏林科学家甫然索瓦•英格勒特（Francois Englert）共同获得了该奖。还没有发现关于霍金辐射的任何物理学证据。

   当粒子从黑洞逃逸出来，黑洞将丢失质量而收缩。这将增大粒子发射的速度。最终，黑洞将丧失它所有的质量而消失。那全部粒子和不幸的宇航员落入黑洞，会发生什么呢？当黑洞消失时，这些不可能再现。看来落入其中的信息丢失了，只剩下总质量和辐射量。但如果信息丢失了，那么这就上升为一个严肃问题，重击了我们的科学解释的核心。

   两百年以来，我们信任科学确定论，即科学定律确定了宇宙的演化。这被拉普拉斯（Pierre-Simon Laplace）形式化了，他说如果我们已知宇宙在某一刻的状态，那么科学定律就可确定宇宙所有的未来和过去。传说拿破仑曾问拉普拉斯上帝如何放入这张图像中。拉普拉斯答道，“先生，我不需要那个假设。”我认为拉普拉斯不会声称上帝不存在。这就是说，他不妨碍打破科学定律。这必须是每位科学家的立场。科学定律不是那种科学定律——它仅当某些超自然存在决定让万物运行而不加干涉的时候才成立。

   在拉普拉斯的确定论中，我们需要知道所有粒子在某一刻的位置和速度，以预测未来。但存在不确定性关系，由沃尔特•海森堡（WalterHeisenberg）在1923年发现，它位于量子力学的核心。

   这就是说，你知道粒子的位置越精确，你知道它们的速度就越不精确；反之亦然。换言之，你不可能都精确知道[粒子的]位置和速度。那么你如何精确地预测未来呢？答案就是，虽然我们不能分别预测位置和速度，但我们可预测所谓的量子态是什么。这因为位置和速度都可被计算到一个确定的精确度。如果我们知道宇宙在某一刻的量子态，那么在这种意义上说，我们仍然可预测宇宙是确定的；科学定律应能使我们预测宇宙在任何其它时刻[的量子态]。

注释：为何从解释“在事件视界处发生什么”开始，已深入对一些最重要的科学哲理的探究——从牛顿的钟表宇宙，到拉普拉斯定律，到海森堡不确定性——而它们挑战了黑洞之谜。根据爱因斯坦广义相对论，进入黑洞的信息实际上应被毁坏，而量子理论说信息不能被破坏，而这留存了一个未解决的问题。

   如果信息丢失于黑洞内，那么我们不可能预测未来，因为黑洞可发射出任何粒子集合。黑洞可发射出一台正在工作的电视机，或一个莎士比亚全集的封面皮边，虽然如此奇异的发射概率是很低的。这或许看起来，如果我们不可能预测从黑洞出来什么，那么这就完全不是问题。我们附近不存在任何黑洞。但这是一个原理问题。如果确定论（宇宙的可预测性）在黑洞失效了，在其它情况也失效了。甚至更糟糕的是，如果确定论失效了，那么我们也不能确定我们的往昔历史。历史书和我们的记忆可能就是幻觉。正是有过去，才告诉我们自己是谁。没有了过去，我们就失去我们的身份。

   因此，很重要的是，确定信息是否真丢失在黑洞中，或在原则上说，信息是否被遮蔽了。许多科学家觉得信息不应丢失，但没一个人认为信息通过一种机制而被保存起来。争论进行了数年。最后，我发现我所想的就是答案。这依靠瑞恰德•费曼（RichardFeynman）的理念，即不存在个体的历史，而存在许多不同的可能性历史，每个历史具有它们自己的概率。在这种情况，存在两种历史。在一种历史中，存在一个黑洞，粒子可落入其中；但在另一种历史中，不存在黑洞。

   关键点在于，从外侧看，我们不能确定是否存在黑洞。所以总有不存在黑洞的可能。这种可能性足以保存信息，但信息没以很有用的形式而被返回。这就像烧一本百科全书，如果你保存所有的烟雾和灰烬，那么信息就没丢失，但却难以解读。科学家基普•索恩（Kip Thorne）和我与另一位物理学家约翰•佩斯奇尔（John Preskill）曾打过一次赌：信息会丢失于黑洞。当我发现信息如何被保存时，我就承认赌败了。我送给约翰•佩斯奇尔一本百科全书，或许我应该只送他一撮灰烬。

注释：理论上讲，根据纯粹的宇宙确定论观念，你可烧一本百科全书而后重构它，如果你知道组成每页墨汁的每个分子的每个原子的特征和位置，并保存它们在所有时刻的墨迹。

    当前我正与剑桥学院的马尔科穆•裴瑞（Malcolm Perry）和哈佛大学的安德濡•斯卓明格（Andrew Strominger）一起研究一种新理论，根据所谓的超译（supertranslation）数学理念，来解释哪些信息从黑洞返回的机制。信息可从黑洞视界上被解译出来，观测这个空间吧。

注释：自睿思演讲被记录后，霍金教授及其同事已发表了一篇论文，举了信息可储存在事件视界上的一个数学实例。该理论涉及信息通过著名的超译过程而转化入一张二维全息图像中。该论文题名为“黑洞上的软发”，可下载看看高该领域的深奥语言，科学家面临试图解释它的挑战。

   这告诉我们什么，是否可能落入黑洞而从另一个宇宙内出来？黑洞的候选历史的存在性认为，这或许是可能的。黑洞要够大，且在旋转，那么或许有一条通往另一个宇宙的通道。但你不可能返回我们宇宙了。所以，即使我在太空飞船上痛哭，也不会尝试那种事。

注释：如果黑洞正在旋转，那么它们的核心可能不含有无限致密点意义上的奇点。反之，可能存在一个奇环(a singularity in the form of a ring)。而那导致猜测存在不仅落入并穿过黑洞的可能性。这意味着离开我们所知的宇宙。而斯蒂芬•霍金用一种急切思想推断：[信息]可能存在于另一侧。

   本次演讲的主旨是“黑洞不是他们画的那么黑，黑洞不是他们曾想的永久监狱”。事物可逃脱黑洞，既可能通往黑洞的外部，也可能通往另一个宇宙。所以，如果你觉得你活在黑洞中，那么[请]别放弃，总会有一条出路的。非常谢谢你们！

原文地址：bbc.co.uk/reithlectures

第二次演讲后的听众问答

苏•劳丽：霍金教授，由衷非常感谢您！于是，我们已被带上旅程，通往宇宙外部区域，通往人类理解等前缘。听众已向教授发了数百个问题，现在他们其中有些人和我们就在伦敦皇家研究所的演讲大厅这里，提出他们的个人问题。我们的第一位提问者，她是玛丽•格瑞菲斯（MarieGriffiths），来自萨里（Surrey）的戈德尔明，是教育部门的一名公务员，总对物理学感兴趣。玛丽，请提你的问题。

玛丽•格瑞菲斯：大爆炸开始了仅仅一个宇宙，还是所有的多宇宙？

苏•劳丽：斯蒂芬？

斯蒂芬•霍金：某些关于大爆炸的理论允许产生很大的复杂宇宙，甚至可能产生许多宇宙。然而，即使存在其它宇宙，我们也不了解它们。我们连通的时空分域就是我们所能知道的全部。

苏•劳丽：这就是我们所能知道的一切，玛丽。而且听起来，它相当足够了。让我们看看下一个问题——这个问题来自中堡市[1]的约翰•布克麦瑞（John Brookmyre），他把自己描述为一个普通的工作家伙和长期学习者。他今天有幸到了这儿，但让我把他的问题转告您，斯蒂芬。如果你是时间之主，你对哪个时刻感兴趣，为什么？

斯蒂芬•霍金：我愿意会见伽利略。他是第一位现代科学家，意识到观测的重要性。伽利略是挑战古希腊人接受的智慧的第一人，而亚里斯多德实际上是科学的最高权威。伽利略之处，简单的观测，比如从高处坠落重物，显示物体不遵循亚里斯多德所说的方式。这必曾被许多人看见过，但他们将其归因为不完美的观测，或者其它原因。但是，伽利略说古人实际上是错误的，而开始从观测推出正确的定律。那使得他成为现代科学之父。他特立独行，有点像一个造反者。（笑声）

苏•劳丽：一个造反者当然只有被迫放弃了。好吧，我将来到正义凝然地坐在那儿的达拉•欧布瑞（DaraO’Briain），他是演艺人，还是科学研究生，在都柏林学院研究纯数学和理论物理学，他准备当一名“单人转”演员（stand-up comedy，又译脱口秀、单口相声）。（笑声）所以，你是一名身兼物理学和幽默剧的两栖专家，是吗，达拉？

达拉•欧布瑞：是的是的，我们在某些方式上是共通的。假如斯蒂芬在“辛普森斯一家”[2]露面两次，那么他会取得比我更成功的喜剧生涯。（笑声）

苏•劳丽：但他是你的少年英雄，不是吗？

达拉•欧布瑞：有一个大故事…是的，我记得我在大约16岁生日时收到了《时间简史》的复本。我这一年感到愉快，遇见他并签了名，还与斯蒂芬相处了一段时间。这是一种光荣。

苏•劳丽：好，问他另一个问题吧。

达拉•欧布瑞：好，实际上给了个机会，我把这个提问机会转给我所知道的一些物理学家——特别是吉穆•艾尔卡里（JimAl-Khalili）。吉穆•艾尔卡里教授想问一个科学群体内的问题。如他所说，物理学群体内的绝大多数人实际上可理解霍金辐射的证实，它是霍金教授在1974年首次提出的，值得诺贝尔奖，因为这是综合量子力学和相对论的首次理论预测。霍金教授认为霍金辐射将在他在世期间被观测到吗？而如果被观测到，那么他认为这种实验证据将会来自哪儿？

斯蒂芬•霍金：我被接受了如下事实——我不会直接见到霍金辐射的证据，虽然存在黑洞的固态类似体和旋转效应，但诺贝尔委员会或许理解为证据。（笑声）但存在另一种霍金辐射，来自早期膨胀宇宙的宇宙学事件视界。我现在正在研究我们是否可用原始引力波探测霍金辐射。所以，我毕竟可能获得诺贝尔奖。

苏•劳丽：（笑声）那么，一种新类型的霍金辐射来自更早的光年。那使你激动吗，达拉？

达拉•欧布瑞：然而，这说的是一件事——霍金教授一直在做的理论研究，已开始进行了？？迄今已溜到我们能做的实验的前头了，将长期有人们来争取保持这种研究。

苏•劳丽：所以，我敢说你会那么想：无论发生什么，他是应该获得诺贝尔奖哈？

达拉•欧布瑞：如果被公众称赞的话，如果电话选票的话，（笑声）但瑞典人声名狼藉，对那种材料是粘乎乎的。所以是，但我相信-是该获。

苏•劳丽：好。克瑞斯•库克（Chris Cooke）是一位25岁的产品设计师，来自萨斯克斯（Sussex）的柯饶利（Crawley）。克瑞斯学过机械工程学，所以他总对物理学感兴趣。在他空余时间里，他支持喜剧演员，达拉。他说，‘尽管我含蓄的…（笑声），尽管我含蓄的个性’。克瑞斯，你的问题呢？

克瑞斯•库克：你觉得用传声器来交流，无论如何也已改变了你的个性吗？作为一名含蓄者，这使你更含蓄了吗？

苏•劳丽：斯蒂芬？

斯蒂芬•霍金：好，我不确信我曾经被称为一名含蓄者。（笑声）恰恰因为我花了大量时间来思考，这不意味着我不喜欢约会并陷入麻烦中。（笑声）我喜欢交流，我喜欢给大众作关于科学的演讲。为此，我的语音合成器很重要，即使我的尾声带有美国口音。（笑声）在我失声之前，我的语音是含糊的，所以只有那些关系近的人才可理解，但我发现用计算机声音，我可向每个人讲话，而无需帮助。所以，它允许我表达我的个性，而不愿改变它。

苏•劳丽：非常感谢您回答那个问题。另一个提问者是帕垂克•朵纳修（Patrick Donaghue）。他是一名布景师，在伦敦生活工作。你的问题呢，帕垂克？

帕垂克•朵纳修：霍金教授，你认为世界将自然终结，还是人类将首先破坏它？

苏•劳丽：霍金教授，只是一个小问题。（笑声）

斯蒂芬•霍金：我们的生存面临大量威胁，如核战争、全球气候灾变和基因工程病毒。威胁的数量可能与日俱增，随着新技术开发，新事物可能走错。虽然行星地球在一定时期内的灾难概率是相当低的，但它随时间在增加，并在下一个千年或万年间变得几乎是必然的。到那时，我们应该已播迁入太空内，抵达其它恒星，所以地球上的灾难不意味着人类的终结。但至少在下一个百年间，我们不会建立自给自足的太空殖民地，所以我们不得不在当前这个时期内谨慎从事。（笑声）我们面临的大多数威胁来自我们科学技术领域的进步。我们不打算停止进步，或者开倒车，所以我们不得不认识危害，并控制它们。我是一名乐观主义者，我相信我们能。

苏•劳丽：好，我不了解世界，但我们的时间确实快要耗完了。我们已收到最后一个问题，来自塔拉•斯查特斯（TaraStruthers），她最初在欧克尼郡（Orkneys）工作，这说明她可能一生对天文学感兴趣，现在供职于制片公司。

塔拉•斯查特斯：如果你不得不给未来的科学家（即物理学家和宇宙学）提出一点建议，那会是什么呢？

斯蒂芬•霍金：科学是一件伟大的事业，我想分享我成功的激动和热忱。根据自己的认识，我曾有一段辉煌的活跃时期，做理论物理学研究，[虽然]没有像“我找到啦”那种空前发现的时刻。所以，我给年青科学家的建议是好奇，试图理解你的所见。我们生活在一个理性定律统治的宇宙之内，我们能发现并理解。尽管有了当前的成就，但仍有许多深奥的新秘密有待你们解决。而对我们浩瀚而复杂的宇宙保持好奇感，它为何存在。但你们也必须记住，科学技术正在戏剧性地改变我们的世界，所以重要的是确保这些改变领导着正确的方向。在一个民主社会中，这意味着每个人需要具有基本的科学理解力，才能对未来做出明智的决策。所以通俗地讲，你打算在科学中做啥，而谁知道呢，你自己或曾完成了对科学的理解。（笑声）

苏•劳丽：于是，我们须要结束了。牛顿曾问，他如何设法更多理解宇宙的定律，而他回答：‘继续思考这些事物吧。’我们那些依靠他人来为他们思考的人很高兴的是，我们有像斯蒂芬•霍金这样的人。他的演讲将发布在BBC睿思网站上，你们将在那儿找到录音、讲稿和视频——1948年以来的全部67个睿思讲坛系列的档案。现在，我们感谢来自伦敦皇家研究所的BBC睿思演讲者斯蒂芬•霍金教授。再见！

掌声

[1] Middlesbrough，旧译米德尔斯布勒，英国英格兰东北部港市。

[2]《辛普森一家》(TheSimpsons)，是美国福克斯广播公司出品的一部动画情景喜剧，由马特·格勒宁创作。该剧通过展现霍默、玛姬、巴特、丽莎和麦琪一家五口的生活，讽刺性地勾勒出了居住在美国心脏地带人们的生活方式。该片从许多角度对美国的文化与社会、人的条件和电视本身进行了幽默的嘲讽，是一部家庭喜剧幽默片。

霍金睿思讲坛录之一

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演讲2英文原本

Transcript of Stephen Hawking’s second Reith lecture

Lecture broadcast on 02.02.2016

Withannotations by BBC Science Editor David Shukman Stephen Hawking, the"world's most famous scientist" is giving this year's BBC ReithLectures. As a guide for the "interested but perplexed", I have addeda few notes (in italics below) to the transcript of Prof Hawking's secondlecture, in the same way I did last week.

_____________  

In my previous lecture I left you on acliffhanger: a paradox about the nature of black holes, the incredibly denseobjects created by the collapse of stars. One theory suggested that black holeswith identical qualities could be formed from an infinite number of differenttypes of stars. Another suggested that the number could be finite. This is aproblem of information, that is the idea that every particle and every force inthe universe contains information, an implicit answer to a yes-no question.

Because black holes have no hair, as thescientist John Wheeler put it, one can't tell from the outside what is inside ablack hole, apart from its mass, electric charge, and rotation. This means thata black hole contains a lot of information that is hidden from the outsideworld. If the amount of hidden information inside a black hole depends on thesize of the hole, one would expect from general principles that the black holewould have a temperature, and would glow like a piece of hot metal. But thatwas impossible, because as everyone knew, nothing could get out of a blackhole. Or so it was thought.

This problem remained until early in 1974,when I was investigating what the behaviour of matter in the vicinity of ablack hole would be, according to quantum mechanics.  

DS: Quantum mechanics is the science of theextremely small and it seeks to explain the behaviour of the tiniest particles.These do not act according to the laws that govern the movements of much biggerobjects like planets, laws that were first framed by Isaac Newton. Using thescience of the very small to study the very large was one of Stephen Hawking’spioneering achievements.

To my great surprise I found that the blackhole seemed to emit particles at a steady rate. Like everyone else at thattime, I accepted the dictum that a black hole could not emit anything. Itherefore put quite a lot of effort into trying to get rid of this embarrassingeffect. But the more I thought about it, the more it refused to go away, sothat in the end I had to accept it. What finally convinced me it was a realphysical process was that the outgoing particles have a spectrum that isprecisely thermal.  My calculationspredicted that a black hole creates and emits particles and radiation, just asif it were an ordinary hot body, with a temperature that is proportional to thesurface gravity, and inversely proportional to the mass.  

DS: These calculations were the first toshow that a black hole need not be a one-way street to a dead end. No surprise,the emissions suggested by the theory became known as Hawking Radiation.

Since that time, the mathematical evidencethat black holes emit thermal radiation has been confirmed by a number of otherpeople with various different approaches. One way to understand the emission isas follows. Quantum mechanics implies that the whole of space is pairs ofvirtual and anti particles, filled with pairs of virtual particles andantiparticles, that are constantly materializing in pairs, separating, and thencoming together again, and annihilating each other.  

DS: This concept hinges on the idea that avacuum is never totally empty. According to the uncertainty principle ofquantum mechanics, there is always the chance that particles may come intoexistence, however briefly. And this would always involve pairs of particles,with opposite characteristics, appearing and disappearing.

These particles are called virtual becauseunlike real particles they cannot be observed directly with a particledetector. Their indirect effects can nonetheless be measured, and theirexistence has been confirmed by a small shift, called the Lamb shift, whichthey produce in the spectrum energy of light from excited hydrogen atoms. Nowin the presence of a black hole, one member of a pair of virtual particles mayfall into the hole, leaving the other member without a partner with which toannihilate. The forsaken particle or antiparticle may fall into the black holeafter its partner, but it may also escape to infinity, where it appears to beradiation emitted by the black hole.  

DS: The key here is that the formation anddisappearance of these particles normally passes unnoticed. But if the processhappens right on the edge of a black hole, one of the pair may get dragged inwhile the other is not. The particle that escapes would then look as if it’sbeing spat out by the black hole.

A black hole of the mass of the sun, wouldleak particles at such a slow rate, it would be impossible to detect. However,there could be much smaller mini black holes with the mass of say, a mountain.A mountain-sized black hole would give off x-rays and gamma rays, at a rate ofabout ten million megawatts, enough to power the world's electricity supply. Itwouldn't be easy however, to harness a mini black hole.  You couldn't keep it in a power station,because it would drop through the floor and end up at the centre of the Earth.If we had such a black hole, about the only way to keep hold of it would be tohave it in orbit around the Earth.

People have searched for mini black holesof this mass, but have so far not found any. This is a pity, because if theyhad I would have got a Nobel Prize. Another possibility, however, is that wemight be able to create micro black holes in the extra dimensions of spacetime.  

DS: By ‘extra dimensions’, he meanssomething beyond the three dimensions that we are all familiar with in oureveryday lives, plus the fourth dimension of time. The idea arose as part of aneffort to explain why gravity is so much weaker than other forces such asmagnetism – maybe it’s also having to operate in parallel dimensions.

According to some theories, the universe weexperience is just a four dimensional surface in a ten or eleven dimensionalspace. The movie Interstellar gives some idea of what this is like. We wouldn'tsee these extra dimensions because light wouldn't propagate through them butonly through the four dimensions of our universe. Gravity, however, wouldaffect the extra dimensions and would be much stronger than in our universe.This would make it much easier to form a little black hole in the extradimensions. It might be possible to observe this at the LHC, the Large HadronCollider, at CERN in Switzerland.This consists of a circular tunnel, 27 kilometres long.  Two beams of particles travel round thistunnel in opposite directions, and are made to collide. Some of the collisionsmight create micro black holes. These would radiate particles in a pattern thatwould be easy to recognize. So I might get a Nobel Prize after all.

DS: The Nobel Prize in Physics is awardedwhen a theory is “tested by time” which in practice means confirmation by hardevidence. For example, Peter Higgs was among scientists who, back in the 1960s,suggested the existence of a particle that would give other particles theirmass. Nearly 50 years later, two different detectors at the Large HadronCollider spotted signs of what had become known as the Higgs Boson. It was atriumph of science and engineering, of clever theory and hard-won evidence. AndPeter Higgs and Francois Englert, a Belgian scientist, were jointly awarded theprize. No physical proof has yet been found of Hawking Radiation.

As particles escape from a black hole, thehole will lose mass, and shrink. This will increase the rate of emission ofparticles. Eventually, the black hole will lose all its mass, and disappear.What then happens to all the particles and unlucky astronauts that fell intothe black hole? They can't just re-emerge when the black hole disappears. Itappears that the information about what fell in is lost, apart from the totalamount of mass, and the amount of rotation. But if information is lost, thisraises a serious problem that strikes at the heart of our understanding of science.  

For more than 200 years, we have believedin scientific determinism, that is, that the laws of science determine theevolution of the universe. This was formulated by Pierre-Simon Laplace, whosaid that if we know the state of the universe at one time, the laws of sciencewill determine it at all future and past times. Napoleon is said to have asked Laplace how God fitted into this picture.  Laplacereplied, “Sire, I have not needed that hypothesis.” I don't think that Laplace was claiming that God didn't exist. It is justthat he doesn't intervene to break the laws of science. That must be theposition of every scientist. A scientific law is not a scientific law if itonly holds when some supernatural being decides to let things run and not intervene.

In Laplace'sdeterminism, one needed to know the positions and speeds of all particles atone time, in order to predict the future. But there's the uncertaintyrelationship, discovered by Walter Heisenberg in 1923, which lies at the heartof quantum mechanics.  

This holds that the more accurately youknow the positions of particles, the less accurately you can know their speeds,and vice versa. In other words, you can't know both the positions and thespeeds accurately. How then can you predict the future accurately? The answeris that although one can't predict the positions and speeds separately, one canpredict what is called the quantum state. This is something from which bothpositions and speeds can be calculated to a certain degree of accuracy. Wewould still expect the universe to be deterministic, in the sense that if weknew the quantum state of the universe at one time, the laws of science shouldenable us to predict it at any other time.

DS: What began as an explanation of whathappens at an event horizon has deepened into an exploration of some of themost important philosophies in science - from the clockwork world of Newton to the laws of Laplaceto the uncertainties of Heisenberg – and where they are challenged by themystery of black holes. Essentially, information entering a black hole shouldbe destroyed, according to Einstein’s Theory of General Relativity whilequantum theory says it cannot be broken down, and this remains an unresolvedquestion.  

If information were lost in black holes, wewouldn't be able to predict the future, because a black hole could emit anycollection of particles. It could emit a working television set, or aleather-bound volume of the complete works of Shakespeare, though the chance ofsuch exotic emissions is very low. It might seem that it wouldn't matter verymuch if we couldn't predict what comes out of black holes. There aren't anyblack holes near us. But it is a matter of principle. If determinism, thepredictability of the universe, breaks down with black holes, it could breakdown in other situations. Even worse, if determinism breaks down, we can't besure of our past history either. The history books and our memories could justbe illusions. It is the past that tells us who we are. Without it, we lose ouridentity.  

It was therefore very important to determinewhether information really was lost in black holes, or whether in principle, itcould be recovered. Many scientists felt that information should not be lost,but no one could suggest a mechanism by which it could be preserved. Thearguments went on for years. Finally, I found what I think is the answer. Itdepends on the idea of Richard Feynman, that there isn't a single history, butmany different possible histories, each with their own probability. In thiscase, there are two kinds of history. In one, there is a black hole, into whichparticles can fall, but in the other kind there is no black hole.  

The point is that from the outside, onecan't be certain whether there is a black hole or not. So there is always achance that there isn't a black hole. This possibility is enough to preservethe information, but the information is not returned in a very useful form. Itis like burning an encyclopaedia. Information is not lost if you keep all the smoke and ashes, but it isdifficult to read. The scientist Kip Thorne and I had a bet with anotherphysicist, John Preskill, that information would be lost in black holes. When Idiscovered how information could be preserved, I conceded the bet. I gave JohnPreskill an encyclopaedia. Maybe I should have just given him the ashes.

DS: In theory, and with a purelydeterministic view of the universe, you could burn an encyclopaedia and thenreconstitute it if you knew the characteristics and position of every atommaking up every molecule of ink in every letter and kept track of them all atall times.  

Currently I'm working with my Cambridge colleagueMalcolm Perry and Andrew Strominger from Harvard on a new theory based on amathematical idea called supertranslations to explain the mechanism by whichinformation is returned out of the black hole. The information is encoded onthe horizon of the black hole. Watch this space.  

DS: Since the Reith Lectures were recorded,Prof Hawking and his colleagues have published a paper which makes amathematical case that information can be stored in the event horizon. The theoryhinges on information being transformed into a two-dimensional hologram in aprocess known as supertranslations. The paper, titled Soft Hair on Black Holes,offers a highly revealing glimpse into the esoteric language of this fieldhttp://arxiv.org/pdf/1601.00921v1.pdf and the challenge that scientists face intrying to explain it.  

What does this tell us about whether it ispossible to fall in a black hole, and come out in another universe? Theexistence of alternative histories with black holes suggests this might bepossible. The hole would need to be large, and if it was rotating, it mighthave a passage to another universe. But you couldn't come back to our universe.So although I'm keen on space flight, I'm not going to try that.

DS: If black holes are rotating, then theirheart may not consist of a singularity in the sense of an infinitely densepoint. Instead, there may be a singularity in the form of a ring. And thatleads to speculation about the possibility of not only falling into a blackhole but also travelling through one. This would mean leaving the universe aswe know it. And Stephen Hawking concludes with a tantalising thought: thatthere may something on the other side.

The message of this lecture is that blackholes ain't as black as they are painted. They are not the eternal prisons theywere once thought. Things can get out of a black hole, both to the outside, andpossibly to another universe. So if you feel you are in a black hole, don'tgive up. There's a way out. Thank you very much.

bbc.co.uk/reithlectures

Transcript of audience Q and A after the secondlecture

SUE LAWLEY: Professor Hawking, thank youvery much indeed. So we’ve been taken on a trip to the outer regions of theuniverse, to the brink of human understanding and beyond. Listeners have sentin hundreds of questions for the professor and some of them are here with usnow in the lecture theatre of the Royal Institution in London to put their questions in person. Canwe have our first questioner, please? She’s Marie Griffiths who comes fromGodalming in Surrey, a civil servant at theDepartment for Education and has always been interested in physics. Yourquestion, please, Marie?  

MARIE GRIFFITHS: Did the Big Bang startjust one universe or all the multiverses?

SUE LAWLEY: Stephen?  

STEPHEN HAWKING: Some theories about theBig Bang allow for the creation of a very large and complex universe, maybeeven many universes. However, even if there were other universes, we wouldn’tknow about them. Our connected component of space time is all we can know.  

SUE LAWLEY: It’s all we can know, Marie.And it’s quite enough, by the sound of it. Let’s have our next question – aquestion from John Brookmyre from Middlesbroughwho describes himself as an ordinary working bloke and a lifelong learner. Hecouldn’t unfortunately get here today, but let me put his question to you forhim, Stephen. If you were a time lord, what moment in time would interest youand why?  

STEPHEN HAWKING: I would like to meetGalileo. He was the first modern scientist, who realized the importance ofobservation. Galileo was the first person to challenge the received wisdom thatthe ancient Greeks, and Aristotle in particular, were the ultimate authority inscience. Galileo pointed out that simple observations, like dropping weightsfrom a height, show things do not work the way Aristotle said. This must havebeen seen by many people, but they had put it down to imperfect observations,or other reasons. But Galileo said the ancients were actually wrong and startedto work out the correct laws from the observations. That makes him the fatherof modern science. He followed his nose, and was a bit of a rebel. (laughter)  

SUE LAWLEY: A rebel who was forced torecant, of course. Right I’m going to come to Dara O’Briain over here on theright. Dara, the entertainer and science graduate. He studied pure mathematicsand theoretical physics at University College Dublin in preparation for hiscareer as a stand-up comic. (laughter) So you’re an expert, are you Dara, onboth physics and humour?  

DARA O’BRIAIN: Yes, yeah, we overlap insome ways. Given that Stephen has appeared twice in The Simpsons, he has a moresuccessful comedy career than I do. (laughter)

SUE LAWLEY: But he was your boyhood hero,wasn’t he?  

DARA O’BRIAIN: There was a huge … Yes Iremember receiving a copy of A Brief History of Time for my Christmas when Iwas about 16. I had the pleasure this year of meeting him and having itautographed as it were and spending some time with Stephen this year. It was anhonour.  

SUE LAWLEY: Okay ask him anotherquestion.  

DARA O’BRIAIN: Well actually given thechance, I turned the opportunity of this question over to some physicists Iknow – in particular Jim Al-Khalili. Professor Jim Al-Khalili wanted to ask aquestion from within the scientific community. As he said, most of the peoplein the physics community would indeed see the confirmation of Hawkingradiation, which Professor Hawking invented in 1974, as being worthy of a NobelPrize since it would have been the first theoretical prediction that requiredboth quantum mechanics and relativity. Does Professor Hawking believe thatHawking radiation will be observed in his lifetime? And if it is observed,where does he think this experimental evidence will come from?  

STEPHEN HAWKING: I am resigned to the factthat I won't see proof of Hawking radiation directly, though there are solidstate analogues of black holes and cyclotron effects that the Nobel committeemight accept as proof. (laughter) But there's another kind of Hawking radiationcoming from the cosmological event horizon of the early inflationary universe.I’m now studying whether one might detect Hawking radiation in primordialgravitational waves. So I might get a Nobel Prize after all.  

SUE LAWLEY: (laughter) A new kind ofHawking radiation then from light years earlier. Does that excite youDara?  

DARA O’BRIAIN: It does say one thing,however – that the work that Professor Hawking’s been

doing, theoretically and has been doing??,has skipped so far ahead of what we can do experimentally that there will befor a long time people racing to keep up with this work.  

SUE LAWLEY: So I dare say you think that,whatever happens, he should get the Nobel Prize, huh?  

DARA O’BRIAIN: If it was done by publicacclaim, if it was a phone vote, (laughter) but the Swedes are notoriouslysticky about that kind of stuff. So yeah, but I do believe - yes.  

SUE LAWLEY: Okay. Chris Cooke, a 25 yearold product designer from Crawley in Sussex. Chris

studied mechanical engineering, so he’salways been interested in physics. In his spare time, he does stand-up comedy,Dara, “despite my introverted … (laughter) despite my introverted personalitytraits”, he says. Chris, your question?  

CHRIS COOKE: Do you feel that using aspeech device to communicate has changed your personality in any way? As anintrovert, has it made you more extroverted?

SUE LAWLEY: Stephen?  

STEPHEN HAWKING: Well I am not sure I haveever been called an introvert before. (laughter) Just because I spend a lot oftime thinking doesn’t mean I don’t like parties and getting into trouble.(laughter) I enjoy communicating and I enjoy giving popular lectures aboutscience. My speech synthesizer has been very important for this, even though Iended up with an American accent. (laughter) Before I lost my voice, my speechwas slurred, so only those close to me could understand, but with the computervoice I found I could talk to everyone without help. So it has allowed me toexpress my personality rather than changing it.

SUE LAWLEY: Thank you very much for thatquestion. Another questioner, Patrick Donaghue. He’s a set designer who livesand works in London.Your question, Patrick?  

PATRICK DONAGUE: Professor Hawking, do youthink the world will end naturally or will man destroy it first?  

SUE LAWLEY: Professor Hawking, just a smallquestion. (laughter)  

STEPHEN HAWKING: We face a number of threatsto our survival from nuclear war, catastrophic global warming, and geneticallyengineered viruses. The number is likely to increase in the future, with thedevelopment of new technologies, and new ways things can go wrong. Although thechance of a disaster to planet Earth in a given year may be quite low, it addsup over time, and becomes a near certainty in the next thousand or ten thousandyears. By that time we should have spread out into space, and to other stars,so a disaster on Earth would not mean the end of the human race. However, wewill not establish selfsustaining colonies in space for at least the nexthundred years, so we have to be very careful in this period. (laughter) Most ofthe threats we face come from the progress we have made in science andtechnology. We are not going to stop making progress, or reverse it, so we haveto recognize the dangers and control them. I'm an optimist, and I believe wecan.  

SUE LAWLEY: Well I don’t know about theworld, but we’re definitely running out of time. We’ve got one last questionfrom Tara Struthers who’s originally from the Orkneys, which may account forher lifelong interest in astronomy. These days she works for a film productioncompany.  

TARA STRUTHERS: If you had to offer onepiece of advice for future generations of scientists, namely physicists andcosmologists, what would it be?  

STEPHEN HAWKING: Science is a greatenterprise and I want to share my excitement and enthusiasm about its success.From my own perspective, it has been a glorious time to be alive and doingresearch in theoretical physics. There is nothing like the Eureka moment of discovering something thatno one knew before. So my advice to young scientists is to be curious, and tryto make sense of what you see. We live in a universe governed by rational lawsthat we can discover and understand. Despite recent triumphs, there are manynew and deep mysteries that remain for you to solve. And keep a sense of wonderabout our vast and complex universe and what makes it exist. But you also mustremember that science and technology are changing our world dramatically, soit’s important to ensure that these changes are heading in the rightdirections. In a democratic society, this means that everyone needs to have abasic understanding of science to make informed decisions about the future. Socommunicate plainly what you are trying to do in science, and who knows, youmight even end up understanding it yourself. (laughter)  

SUE LAWLEY: And there we must end. Newton was once asked howhe’d managed to understand so much about the laws of the universe and heanswered: "by thinking of these things continually." Those of us whorely on others to do their thinking for them, are very glad that we have menlike Stephen Hawking. His lectures will be available on the BBC Reith websitewhere you’ll find recordings, transcripts and videos - an archive of all 67series of Reith Lectures going back to 1948. For now, from the RoyalInstitution in London,our thanks to the BBC Reith Lecturer Professor Stephen Hawking. Andgoodbye.  

APPLAUSE

THIS TRANSCRIPT IS ISSUED ON THEUNDERSTANDING THAT IT IS TAKEN FROM A LIVE PROGRAMME AS IT WAS BROADCAST. THENATURE OF LIVE BROADCASTING MEANS THAT NEITHER THE BBC NOR THE PARTICIPANTS INTHE PROGRAMME CAN GUARANTEE THE ACCURACY OF THE INFORMATION HERE.

bbc.co.uk/reithlectures