霍金睿思讲坛录之一

译者：郑中（Geongs Zhern）

题注：

虽然湖南科技出版社曾撞准了科学明星霍金的《时间简史》发了点财，而这下又想继续拿残疾科学明星来炒作赚钱了，连几页的睿思广播稿也不放过了。霍金老了，名气不减，名气可卖钱啊，自然主动有人找他凑书。不仅国内湖南科技出版商几乎专占霍金著作版权，国外电视台也在不断打主意（大家分钱，何乐不为），这下是BBC科学编辑戴维德·舒克曼（DavidShukman）写了点引言，加了几个注解，这竟然也成了炒作点。湖南科技出版社又在大吹“霍金又出书了”，为了凑页数，找人插了些图件，再补上霍金博士生吴忠超的一篇黑洞杂文，加上稀松的排版，凑成一本薄书又可卖钱了。我们可谅解霍金他老人家残疾五十年而不甘寂寞，可欣赏霍金屡次打赌认错，但不可忍受老霍金被炒干油了。读者为了这样一本无啥新的实质内容的商业包装书而掏几十元腰包，确实不值啊。

霍金在2015、2016年这两次睿思演讲，实际上是针对一般中小学生的，语言非常通俗，且较随意，大家可看下，但对于大学生及以上的人来说确实就没啥新意了。但其中有句话让我很感动“无论人生看起来多么困难，因为如果你不能嘲笑你自己的平凡人生，那么你就可能丧失所有希望”，我们现在就明白了为啥霍金敢于认错、从不绝望了，所以他从死神手里多赚了五十年！但为此，霍金的家人活得不容易。霍金犟得不怕出输，那些盲目吹捧霍金的商人，就显得虚伪无知透顶了。

其实，有些人在阅读霍金这两次演讲对话中，就会发现安迪•法卞、达拉•欧布瑞、克瑞斯•库克等在对话中表达出委婉的讽刺，而苏•劳丽有时在调和尴尬，有时也加入了委婉的讽刺，这就是幽默，而霍金也回答得很巧妙。对于中国的霍金粉丝面团们，晚年霍金这大咖又在说啥，到底有啥奇谈怪论，抑或老生常谈啊？还是“黑洞不黑”吗？晚年霍金到底是科学家还是幻想家，是残疾明星还是临终哲人呢？我们不妨了解下霍金这两次睿思演讲的内容。

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！

霍金睿思讲坛录之二

此图转自霍金营销团队搞的微博。类似这样简单被PS的图片被人当成插图或转过无数次。但博主认为这样形态的理论黑洞实际上是不存在的。侧看是吞噬一切的旋涡，轴向看是灰洞，即高能粒子射流而已；当灰洞超过某极限，就发生爆炸，而这就是所谓的白洞，天文学家可能误认为是超级的超新星爆发。至于那些幻想通过黑洞钻入一个宇宙的梦想家们，只有粉身碎骨化尘埃、至死不明真与幻了。真实黑洞可能像啥，请来看天文观测和数值模拟结果吧！点击黑洞不黑而且非洞。

霍金掉入灰洞咋办啊 http://blog.sciencenet.cn/home.php?mod=space&uid=289142&do=blog&id=1058967

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讲演1中文译本

2015年睿思演讲录（THE REITH LECTURES 2015）

睿思演讲录：斯蒂芬•霍金教授（Professor Stephen Hawking）

黑洞有毛吗？

苏•劳丽[1]：你好！欢迎来到BBC睿思讲坛[2]。我们是在伦敦西边的大不列颠皇家研究所，它创建于1799年，为了鼓励人们和我自己引述“更深刻地思考科学的神奇和应用”。有比本年度演讲者更合适的人来填补那个夙愿吗？

   当我们在广播上透露他的名字时，就有20000名听众申请了门票。于是我看着400名幸运者聚集在这历史性的讲坛剧场中。

   有一丁点了解那位男子汉，他具有这样的兴趣：他是一位富有才华的人，他认为自己是牛津大学的一位‘懒惰的’六零后物理学生（课程‘简单得可笑’）。（笑声）但是在随后的职业生涯中，他就像他如此热爱的恒星那样，从此明亮地闪耀了。尽管他21岁时被诊断为患有一种罕见的运动神经元病，从此导致他身体残废，但他对宇宙定律的研究开始取得突破。

   他的睿思演讲主题是——别的什么吗——黑洞。在过去的半个世纪，人们对他感到着迷。[黑洞]几乎不可见，[即使在]数十亿英里外，我们仍认为它们是恐怖的。我们的演讲者说，[这是]一个错误，——如果我们能理解黑洞，那么我们可能破解宇宙之谜。

   女士们先生们，请欢迎世界最著名科学家和BBC睿思演讲者斯蒂芬•霍金教授！

热烈喝彩

   欢迎斯蒂芬！上一次我们一起作广播，那是在1992年圣诞节的另一个BBC广播节目上，而你一直竭力在巧克力慕斯与奶油布丁之间做选择。是的，——这是“荒岛唱片”，对于你你说是奢侈品。我好奇的是你是否记得这事儿？你的母亲曾说，你总具有她所描述的那种强烈好奇心。她说：“我能理解恒星吸引他”。你记得那事儿吗？

斯蒂芬•霍金：我记得有一天夜晚从伦敦回家。在那些日子，他们为了省钱，关闭了街灯。我看见了我以前从未见过的夜空，银河恰好横跨天空。在我[探险]的“荒岛”上，不会有街灯，所以我应能看清恒星。

苏•劳丽：嗯，那是在23年前，而你不曾想到遭难，我提醒：

斯蒂芬•霍金：你能听到我吗？

苏•劳丽：我们能听到。

斯蒂芬•霍金：在23年前，受困于荒岛上的想法使我充满荣誉感。在那时，我想要洞察作用的核心，那里事物正在发生，而不是受困在有些遥远的地点。现在我更苍老了，荒岛突然听起来相当吸引人。（笑声）我可能有更多得多的工作要做。但如果没有焦糖炖蛋，我仍不想去。（笑声）物理学毕竟是令人陶醉的，但你为了布丁就不能搞物理。

苏•劳丽：好，现在——物理学是主要课程。所以斯蒂芬请上菜吧，如果你愿意的话。女士们先生们——第一堂演讲的主题是“黑洞没毛吗？”

斯蒂芬•霍金：我的讲话是关于黑洞的。就是说，真相有时比科幻更奇怪，但无论如何，比黑洞那种情况更加真实。黑洞比科学作家梦想出来的任何物体都要更奇怪，但它们是稳固的科学事实问题。科学界逐渐意识到大质量恒星受它们自身的引力，可向自身内部坍缩，而留下的天体会继续活动。阿尔伯特•爱因斯坦曾在1939年写了一篇论文，声称恒星在引力作用下不会坍缩，因为物质不会被压缩超过某个临界点。许多科学家认同爱因斯坦的直觉。主要例外是美国科学家约翰•惠勒（JohnWheeler），他在许多方面是黑洞演化故事的英雄。他在1950年代和60年代的研究中，强调许多恒星会最终坍缩，这就向理论物理学提出了问题。他也预见到坍缩星变成黑洞这种天体的许多特性。

   正常恒星在大部分寿命期间，超过许多个十亿年，将反抗自身的引力，通过氢转化成氦的核过程所导致的热压，而维持自身。然而最终，恒星将耗尽它的核燃料。恒星将收缩。在有些情况中，它能使自身保持作为一颗白矮星。但钱德拉赛卡（Subrahmanyan Chandrasekhar）在1930年证明了白矮星的最大质量约为太阳质量的1.4倍。为了恒星产生全部中微子，苏联物理学家列夫•兰道（Lev Landau）也推算出类似的最大质量。

   那些比白矮星或中子星质量更大的无数恒星，当它们已耗尽核燃料时，其命运会如何呢？该问题被罗伯特•沃本海默（RobertOppenheimer；后来原子弹的声誉）研究过。在1939年的几篇论文中，他与乔吉•沃尔科夫（George Volkoff）和哈兰德•斯耐德（Hartland Snyder），证明了这种恒星不会被压力维持。而如果有人忽略了压强，一个标准的球对称恒星回收缩成一个无限大密度的单点。这个点叫做奇点。我们关于空间的所有理论被形式化，基于假设“时空是平滑而几乎平直的”，所以它们在奇点失败了，那儿的时空曲率是无限大。实际上，这标志着时间自身的终结。那就是爱因斯坦发现如此反感之处。

   后来战争打断了。绝大多数科学家，包括罗伯特•沃本海默，将他们的注意力转向核物理学，而引力坍缩问题被大加遗忘了。有趣的是，在复活的天体中，发现了所谓类星体的遥远天体。第一颗类星体3C273发现于1963年，后来迅速发现了许多其它类星体。它们是明亮的，尽管距离遥远。核过程没考虑它们的能量输出，因为它们仅将剩余质量的百分之一泄放出来作为纯能量。唯一候选是引力能量，引力坍缩导致能量泄放。

   恒星的引力坍缩被再次发现了。显然，一个标准的球状恒星会收缩成一个无限大密度的点，一个奇点。

   爱因斯坦方程在奇点处未被定义。这意味着在这个无限密度的点处，人们不能预测未来。这暗示无论恒星何时坍缩，怪事总会发生。如果奇点不是裸露的，即它们没受到外部的保护，那么我们就不会受预测失败所影响。当约翰•惠勒在1967年引入了术语“黑洞”，它代替了早期的名称“冻结星”（frozenstar）。惠勒强调坍缩恒星如何形成残留体，值得感兴趣。新名称很快被嘲笑。这说明事物是黑暗而神秘的，但法国就是法国，看见了一个更加淫邪的含义。（笑声）过了几年，他们恢复了名称trou noir（法语黑洞），并声称它是淫邪的。（笑声）但那有点像试图反对重温蜜月（Le Week-end）和其它异域英语（Franglais）。在最后，他们不得不放弃。谁能抗拒这样一个胜利者的称号呢？

   从外部看，你不能讲述黑洞内部是啥。你能将电视机、钻石戒指，甚或你最坏的敌人，都扔进黑洞，但所有黑洞将记住的是总质量和旋转状态。约翰•惠勒将这个原理表达为“黑洞无毛”而著名。到了法国，这恰好证实了它们的疑心。（笑声）

   黑洞具有一个边界，叫做事件视界。在那儿，引力恰好强到足以将光拉回来，而防止光逃逸。因为没有任何事物传播快于光，所以其它万物也将被拉回来。坠落穿过事件视界，有点像乘着一叶独木舟渡过尼亚加拉瀑布。如果你在瀑布上面划桨足够快的话，那么你可飞渡而去；但如果你悬在深渊边缘，那么你将丧生。没法回去了。当你逐渐接近瀑布，水流变得更快。这意味着在独木舟前面比在后面的拉引力更强，而危险的是独木舟将被分崩离析。这类似于黑洞情况。如果你先向黑洞坠落一英尺，那么引力拖拉你的脚部比拖拉你的头部更强，因为它们更接近黑洞。结果是你将在纵向被拉伸，而在横向被挤压。如果黑洞具有几倍于我们太阳的质量，那么你会被撕心裂肺，而制成意大利面条，然后你到达视界。然而，如果你落入一个更大得多的黑洞，具有百万倍太阳质量，你会毫无困难地到达视界。所以，如果你想要探测黑洞内部，那你必须确信你选了一个大大的黑洞。（笑声）有一个黑洞具有大约四百万倍于太阳质量，位于我们银河系的中央。

   当你落入一个黑洞时，虽然你不会注意到任何特别的事情，但从一定距离观测你的某个人绝不会看见你穿过事件视界。反之，你会显得是在缓慢下降，而盘旋在黑洞外面。你的影像会变得越来越暗淡，而且越来越红，直到你实际上消失于视野。对外部世界而言，你会永远消失。

   在1970年数学发现，使我们对这些神秘现象的理解有了戏剧性进展。这就是作为黑洞边界的事件视界的表面积，具有这种特性：当另外的物种或辐射落入黑洞时，事件视界的面积总在递增。这些特性说明，黑洞事件视界的面积与传统牛顿物理学之间具有相似之处，特别是热力学中的熵概念。熵可被认为是系统混乱程度的测度，或者换言之，是对系统精确状态的知识缺乏程度的测度。著名的热力学第二定律说，熵总随时间而递增。这个发现是二者重要关系的第一线索。

   虽然熵与事件视界面积之间具有明显的相似性，但对于我们来说，面积怎么作为黑洞自身的熵的标志，这还不显明。黑洞熵意味着什么？在1972年，雅各布•贝肯斯坦（Jacob Bekenstein）作了至关重要的说明，他是普林斯顿大学的一名研究生，而后来在耶路撒冷希伯莱大学。于是变得就像这样。当黑洞由引力坍缩而产生时，黑洞快速变迁到一种定态，仅具有三个特征参量：质量、角动量和电荷。除这三个特征参量之外，黑洞没保存坍缩天体的其它细节。

   他的定理对信息（宇宙学意义的信息）具有重要涵义：宇宙内的每种粒子和每种自然力对于一个是非问题作了一个隐式回答。该定理暗示，大量信息丢失于引力坍缩中。比如，最终的黑洞状态，无关乎坍缩天体是否是由物质还是反物质组成，是球状还是高度不规则状。换言之，给定质量、角动量和电荷的黑洞，可被任何一种不同组合的大量物质坍缩而成。这样同样的黑洞，看起来可由大量不同类型的恒星的坍缩而形成。实际上，如果量子效应被忽视，组合的数目是无限的，因为黑洞曾被无数质量不明确的粒子云坍缩而形成。但组合的数目真是无限的吗？

   量子力学的不确定性原理暗示，只有波长小于黑洞本身的粒子，才可形成黑洞。这意味着波长会受限制：这就不可能无限了。因此，看来虽然可形成一定质量、角动量和电荷的黑洞的组合数很大，但可能也是有限的。雅各布•贝肯斯坦认为，根据有限的组合数，我们可解释黑洞熵。这就是当黑洞产生时，在坍缩过程中不可避免地丢失的信息量的一种测度。

   贝肯斯坦提议中的明显致命缺陷在于，如果黑洞具有有限熵值（正比于黑洞事件视界的面积），那么它也应具有有限温度（正比于黑洞表面引力）。这暗示黑洞与热辐射保持平衡，处在大于零的某个温度。仍然根据经典概念，没有这种平衡是可能的，因为黑洞会吸收照射到它上面的任何热辐射，但显然不能反向发射任何东西。黑洞不可能辐射任何东西，它不可能辐射热能。

   这就是悖论。而这就是我要回来进行下一堂演讲的主题，那时我将探讨黑洞如何挑战关于宇宙的可预测性、历史确定论的最基本原理，并追问如果你曾陷入黑洞，那么会发生什么？谢谢大家！

掌声

苏•劳丽：谢谢您！由衷感谢您，斯蒂芬。好啦，现在我们问了想问您的听众，他们的问题洪水般涌来，有几百个。我们已选择一个有代表性的问题，并邀请了一些听众来到现场当面提出他们的问题。你会感激观众，我们不得不提前给斯蒂芬问题，这样他就可以将他的问题编码输入他的电脑了。更好的办法是通过他的面肌运动，当红外探测器扫描那些运动时，你将听到微小的哔哔声。这不是一个迅速的过程，大约一个单词一分钟，但现在回答都在那儿。所以，如果你准备好了，那就让我开始问安迪•法卞（Andy Fabian），他是一位天文学家和天体物理学家，是剑桥大学天文学研究所所长，因此是斯蒂芬的一位同事——安迪，提你的问题，请提吧。

安迪•法卞：斯蒂芬，你和他人的许多研究主题，诸如关于信息消失于黑洞内，以及关于黑洞辐射，但这是理论性的，迄今缺乏观测支持。你看见用许多各种观测成果（现在一般认为吸积黑洞遍布宇宙）来改变这种境况的方法了吗？

苏•劳丽：斯蒂芬？

斯蒂芬•霍金：我假设你正在引用黑洞视界面积递增律。检验这个问题的最佳方式是黑洞碰撞而不是吸积作用。

苏•劳丽：好，你获得了个漂亮的忽视，我不得不说。（笑声）也许你应该告诉我们吸积黑洞与黑洞碰撞之间有啥差异？

安迪•法卞：好，黑洞碰撞发生在当你有两个黑洞彼此对撞并合并一起的时候。吸积只是物质平稳地泄漏进入黑洞，这产生巨量的能量泄放，是宇宙内很辉煌的事件。

苏•劳丽：你预见将有斯蒂芬正在谈论的证据吗？正如所说，此刻这全是理论性的。

安迪•法卞：寻找黑洞的观测证据，这会是让人痴迷的，但我自己没明白如何做它。

苏•劳丽：好吧，让我们追究这个主题“黑洞如何追上我们观看它们的时候？”而实际上当我们不观测它们时，就转向这里来自西中陆郡（West Midlands）的一群年青爱好者吧。他们是来自沃索尔（Walsall）巴尔灯塔（Barr Beacon）中学的徒儿们，他们年龄在十二三岁左右。凯特•哈瑞斯（KateHarris），你是他们的教师。你如何让他们对宇宙学和其它一切感兴趣的？

凯特•哈瑞斯：好，我是他们的正式家庭教师，所以我每天照看他们。我收到了一个电子邮件，来自和我们一起的科学教师之一，巴特沃斯（Butterworth）博士，他将斯蒂芬演讲消息告诉了他们。而他们如此强烈，想了解更多，因为他们曾在“大爆炸理论”中已见过他…（笑声）

苏•劳丽：这是电视台…还是美国情景剧？

凯特•哈瑞斯：是的。而他们也是强烈的科学热爱者。他们只想了解更多。

苏•劳丽：是的，如此有趣无休。好，让我们从他们之一提出一个问题。阿如尼亚•穆拉立达然（Aruniya Muraleedaran），你十二岁了吧。你的问题是啥？

阿如尼亚•穆拉立达然：如果某个黑洞与另一个发生碰撞时，会发生何种事情？

斯蒂芬•霍金：如果两个黑洞碰撞并合并成一个单独的黑洞，那么在形成的黑洞附近的事件视界面积就大于当初黑洞附近的事件视界面积之和。

苏•劳丽：你理解那个吗，阿如尼亚？你把你的头搞晕了吗？（笑声）好，正如我的理解，那就是当两个黑洞碰撞时，总周长大于两个部分之和。总面积增加了。理解了吗？

阿如尼亚•穆拉立达然：我现在理解了。（笑声）你呢？你…你想要说什么吗？

学生：是的，我理解了。

苏•劳丽：你理解了吗？

学生：一点点。（笑声）

掌声

苏•劳丽：好，这可能因为我对你解释它。（笑声）现在到了提一个更个性的问题——我们实际上收到了他们许多邮件。这是一位来自广播电台4的17岁听众。他的名字叫盾勘•迈克金朗（Duncan McKinnon）。你告诉我们，盾勘，斯蒂芬曾对你是一种鼓舞。用什么方式？

盾勘•迈克金朗：好，观看他的片子，这是真正的鼓舞，他如何管控实现他的梦想和目标。

苏•劳丽：你意思是“万有理论”影片…

盾勘•迈克金朗：是的是的。

苏•劳丽：…他们关注电影明星埃迪·雷德梅尼（Eddie Redmayne）

盾勘•迈克金朗：是的。

苏•劳丽：…当然因为他赢得了奥斯卡。大影片，是吗？好，喜欢你提问题？

盾勘•迈克金朗：我想问，尽管您一生坎坷不平，但什么激励着您坚持进行？

苏•劳丽：斯蒂芬？

斯蒂芬•霍金：我想我的研究工作和幽默感使我坚持进行。当我21岁时，我的期望值降到了零。你或许已经知道这个，因为关于此，已有一部电影。在这种情况，重要的是我逐渐欣赏我曾做了什么。虽然我不幸患了运动神经元病，但我几乎其它每件事情已很幸运了。我庆幸在风华正茂的年代研究理论物理，而这是我的残疾不是一种严格障碍的少数几个领域之一。不生气，这也重要。无论人生看起来多么困难，因为如果你不能嘲笑你自己的平凡人生，那么你就可能丧失所有希望。

苏•劳丽：我们让斯蒂芬的女儿露茜•霍金（Lucy Hawking）到这儿，正在朝前台那儿走去。露茜，你已看了斯蒂芬超过四十年了，如果你不介意我说那个。（笑声）

露茜•霍金：那是有点隐私，苏女士。（笑声）

苏•劳丽：我道歉，露茜。

露茜•霍金：那就好吧。这是广播，苏女士。

斯蒂芬•霍金：（插话）我会把爱因斯坦带来的。（笑声）

苏•劳丽：感叹我是正确的。露茜，你理解他天气有些相当艰难的时期。你如何使他恢复活力，这种决心消沉过吗？

露茜•霍金：我认为他是大大的顽固（笑声），有很值得羡慕的希望而坚持生活，有能力唤起他所有的潜能、能力、毅力，并将它们全都倾注到那个坚持进行的目标。但不只是为了活着的目标而坚持进行，而是超越了这个目标，通过非同寻常的研究、写书、做演讲、激励患有神经退化症和其它疾病的他人，而且作为一个家庭男人，对于那么多人是一个朋友和同事…并与世界各地的朋友保持联络。所以，我想有…有很多很多因素，但我认为这种犟劲，生活的毅力，以及——像他说他自己——嘲笑自己的幽默感，每天结束时就是那样…

斯蒂芬•霍金：（插话）我会回想爱因斯坦。（笑声/鼓掌）他会惊讶广义相对论已推进了我们对世界的理解有多大。

露茜•霍金：我想那就是他要我停止说话的方式。（笑声）

苏•劳丽：而我们必须到此结束了。感谢你做了这个值得怀念的事情。在BBC睿思网站上有大量的科学主题，包括斯蒂芬提及的罗伯特•沃本海默——原子弹之父，天体物理学家马廷•瑞斯（Martin Rees），射电天文学家伯纳德•罗福尔（Bernard Lovell）以及更多。有一个1948年的录音记录的档案，大家看一看：

   谨此感谢伦敦皇家研究所的东道主，当然非常感谢我们的睿思讲坛，斯蒂芬•霍金教授

掌声

郑中译于2016年某日

[1]译注：访谈者苏•劳丽（SUE LAWLEY），全名苏圣•劳丽（Susan Lawley），生于1946年，英国资深广播员，在2001年获得官佐勋章（OBE）。

[2]译注：睿思讲坛（Reith Lecture）是英国的一个年度广播讲坛系列，由当代领导人物作讲谈，受BBC和BBC广播四频道和BBC世界服务台的委托。该讲坛由BBC创始于1948年，以纪念对公共服务广播作出历史性贡献的约翰•睿思先生（曾为该机构的第一总干事）。睿思讲坛的目的是推进公众理解力，并争论当代有趣的重要问题。第一次睿思演讲者是哲学家和诺贝尔获得者伯特兰•罗素（BertrandRussell）。第一位女性演讲者是达穆•马杰丽•裴翰（Dame Margery Perham）。

演讲1英文原本

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.

THEREITH LECTURES 2015

ReithLecturer: Professor Stephen Hawking

Lecture1: Do black holes have no hair?

SUE LAWLEY: Hello and welcome to the BBCReith Lectures. We’re at the Royal Institution of Great Britain in the West End of London. It was founded in 1799, to encouragepeople (and I quote) ‘to think more deeply about the wonders and applicationsof science’. Can there be a better person to fulfil that ambition than thisyear’s lecturer?

When we advertised his name on theairwaves, 20,000 listeners applied for tickets. So I am looking at 400 luckypeople gathered here in this historic lecture theatre.

A little bit first about the man whocommands such interest: He was a brilliant, though by his own admission ‘lazy’physics student at Oxfordin the 1960s (the course was ‘ridiculously easy’ he said). (laughter) But thecareer which followed, like the stars he loves so much, has shone brightly eversince. Despite being diagnosed with a rare form of Motor Neurone Disease whenhe was 21 and the physical deprivations it has caused, his work on the lawswhich govern the universe have been ground-breaking.

The subject of his Reith Lectures is – whatelse – Black Holes. They’ve held a fascination for him for the past half century.Virtually invisible and billions of miles away, we nevertheless regard them asmenacing. A mistake, says our lecturer – if we could understand them, then wecould possibly unlock the secrets of the Universe.  

Ladies and Gentlemen, please welcome theworld’s most famous scientist and the BBC Reith Lecturer, Professor StephenHawking.

APPLAUSE

Stephen, welcome. The last time webroadcast together, it was Christmas 1992 and it was on another BBC radioprogramme, and you were struggling to choose between chocolate mousse and crèmebrulee. Yeah – it was Desert Island Discs and that was to be your luxury. Iwonder if you remember this? Your mother has said that you always had what she describedas a strong sense of wonder. “I could see that the stars could draw him”, shesaid.

Do you remember that?

STEPHEN HAWKING: I remember coming homelate one night from London.In those days they turned the street lights out at midnight to save money. Isaw the night sky as I had never seen it before, with the Milky Way going rightacross. There won’t be street lights on my desert island, so I should get agood view of the stars.  

SUE LAWLEY: Well, that was 23 years ago andyou didn’t really fancy being castaway, I recall:

STEPHEN HAWKING: Can you hear me?

SUE LAWLEY: We can.

STEPHEN HAWKING: 23 years ago, the idea ofbeing stuck on a desert island filled me with horror. At that time, I wanted tobe in the heart of the action, where things were happening, not stuck in someremote quiet spot. Now that I am older, a desert island suddenly sounds quiteappealing. (laughter) I might get much more work done. But I still don't wantto go if there's no crème brulee. (laughter) Physics is fascinating but afterall, you can't have it for pudding.

SUE LAWLEY: Well right now – physics is themain course. So serve it up if you would, Stephen.  Ladies and Gentlemen – Lecture Number One isentitled Do Black Holes Have no Hair?

STEPHEN HAWKING: My talk is on black holes.It is said that fact is sometimes stranger than fiction, and nowhere is thatmore true than in the case of black holes. Black holes are stranger thananything dreamed up by science fiction writers, but they are firmly matters ofscience fact. The scientific community was slow to realize that massive starscould collapse in on themselves, under their own gravity, and how the objectleft behind would behave. Albert Einstein even wrote a paper in 1939, claimingstars could not collapse under gravity, because matter could not be compressedbeyond a certain point. Many scientists shared Einstein's gut feeling. Theprincipal exception was the American scientist John Wheeler, who in many waysis the hero of the black hole story.  Inhis work in the 1950s and ‘60s, he emphasized that many stars would eventuallycollapse, and the problems that posed for theoretical physics. He also foresawmany of the properties of the objects which collapsed stars become, that is,black holes.

During most of the life of a normal star,over many billions of years, it will support itself against its own gravity, bythermal pressure, caused by nuclear processes, which convert hydrogen intohelium. Eventually, however, the star will exhaust its nuclear fuel. The starwill contract. In some cases, it may be able to support itself as a white dwarfstar. However Subrahmanyan Chandrasekhar showed in 1930, that the maximum massof a white dwarf star, is about 1.4 times that of the Sun. A similar maximummass was calculated by Soviet physicist, Lev Landau, for a star made entirelyof neutrons.

What would be the fate of those countlessstars, with greater mass than a white dwarf or neutron star, when they hadexhausted nuclear fuel? The problem was investigated by Robert Oppenheimer, oflater atom bomb fame. In a couple of papers in 1939, with George Volkoff andHartland Snyder, he showed that such a star could not be supported by pressure.And that if one neglected pressure, a uniform spherically systematic symmetricstar would contract to a single point of infinite density. Such a point is calleda singularity. All our theories of space are formulated on the assumption thatspace-time is smooth and nearly flat, so they break down at the singularity,where the curvature of space-time is infinite. In fact, it marks the end oftime itself. That is what Einstein found so objectionable.  

Then the war intervened. Most scientists,including Robert Oppenheimer, switched their attention to nuclear physics, andthe issue of gravitational collapse was largely forgotten.  Interest in the subject revived with thediscovery of distant objects, called quasars. The first quasar, 3C273, was discovered in 1963. Many otherquasars were soon discovered. They were bright, despite being at greatdistances. Nuclear processes could not account for their energy output, becausethey release only a percent fraction of their rest mass as pure energy. Theonly alternative was gravitational energy, released by gravitationalcollapse.  

Gravitational collapses of stars werere-discovered. It was clear that a uniform spherical star would contract to apoint of infinite density, a singularity.  

The Einstein equations can't be defined ata singularity. This means at this point of infinite density, one can't predictthe future. This implies something strange could happen whenever a starcollapsed. We wouldn't be affected by the breakdown of prediction, if thesingularities are not naked, that is, they are not shielded from the outside.When John Wheeler introduced the term black hole in 1967, it replaced theearlier name, frozen star. Wheeler's coinage emphasized that the remnants ofcollapsed stars are of interest in their own right, independently of how theywere formed. The new name caught on quickly. It suggested something dark andmysterious, But the French, being French, saw a more risque meaning. (laughter)For years, they resisted the name trou noir, claiming it was obscene.(laughter) But that was a bit like trying to stand against Le Week-end, andother Franglais. In the end, they had to give in.  Who can resist a name that is such a winner?

From the outside, you can't tell what isinside a black hole.  You can throwtelevision sets, diamond rings, or even your worst enemies into a black hole,and all the black hole will remember is the total mass, and the state of rotation.John Wheeler is known for expressing this principle as “a black hole has nohair”. To the French, this just confirmed their suspicions. (laughter)

A black hole has a boundary, called theevent horizon. It is where gravity is just strong enough to drag light back,and prevent it escaping. Because nothing can travel faster than light,everything else will get dragged back also. Falling through the event horizonis a bit like going over Niagara Fallsin a canoe. If you are above the falls, you can get away if you paddle fastenough, but once you are over the edge, you are lost. There's no way back. Asyou get nearer the falls, the current gets faster. This means it pulls harderon the front of the canoe than the back. There's a danger that the canoe will be pulled apart. It is the samewith black holes. If you fall towards a black hole feet first, gravity willpull harder on your feet than your head, because they are nearer the blackhole. The result is you will be stretched out longwise, and squashed insideways. If the black hole has a mass of a few times our sun you would be tornapart, and made into spaghetti before you reached the horizon. However, if youfell into a much larger black hole, with a mass of a million times the sun, youwould reach the horizon without difficulty. So, if you want to explore theinside of a black hole, make sure you choose a big one. (laughter) There is ablack hole with a mass of about four million times that of the sun, at thecentre of our Milky Way galaxy.

Although you wouldn't notice anythingparticular as you fell into a black hole, someone watching you from a distancewould never see you cross the event horizon. Instead, you would appear to slowdown, and hover just outside.  Your imagewould get dimmer and dimmer, and redder and redder, until you were effectivelylost from sight. As far as the outside world is concerned, you would be lostfor ever.  

There was a dramatic advance in ourunderstanding of these mysterious phenomena with a mathematical discovery in1970. This was that the surface area of the event horizon, the boundary of ablack hole, has the property that it always increases when additional matter orradiation falls into the black hole. These properties suggest that there is aresemblance between the area of the event horizon of a black hole, andconventional Newtonian physics, specifically the concept of entropy inthermodynamics. Entropy can be regarded as a measure of the disorder of asystem, or equivalently, as a lack of knowledge of its precise state. Thefamous second law of thermodynamics says that entropy always increases withtime. This discovery was the first hint of this crucial connection.  

Although there is clearly a similaritybetween entropy and the area of the event horizon, it was not obvious to us howthe area could be identified as the entropy of a black hole itself. What wouldbe meant by the entropy of a black hole? The crucial suggestion was made in1972 by Jacob Bekenstein, who was a graduate student at Princeton University,and then at the Hebrew University of Jerusalem.It goes like this. When a black hole is created by gravitational collapse, itrapidly settles down to a stationary state, which is characterized by onlythree parameters: the mass, the angular momentum, and the electric charge.Apart from these three properties, the black hole preserves no other details ofthe object that collapsed.  

His theorem has implications forinformation, in the cosmologist's sense of information: the idea that everyparticle and every force in the universe has an implicit answer to a yes-noquestion. The theorem implies that a large amount of information is lost in agravitational collapse. For example, the final black-hole state is independentof whether the body that collapsed was composed of matter or antimatter, orwhether it was spherical or highly irregular in shape. In other words, a blackhole of a given mass, angular momentum and electric charge, could have beenformed by the collapse of any one of a large number of different configurationsof matter. So what appears to be the same black hole could be formed by thecollapse of a large number of different types of star. Indeed, if quantumeffects are neglected, the number of configurations would be infinite, sincethe black hole could have been formed by the collapse of a cloud of anindefinitely large number of particles, of indefinitely low mass. But could thenumber of configurations really be infinite?

The uncertainty principle of quantummechanics implies that only particles with a wavelength smaller than that ofthe black hole itself, could form a black hole. That means the wavelength wouldbe limited: it could not be infinite. It therefore appears that the number ofconfigurations that could form a black hole of a given mass, angular momentumand electric charge, although very large, may also be finite. Jacob Bekensteinsuggested that from this finite number, one could interpret the entropy of ablack hole. This would be a measure of the amount of information that wasirretrievably lost during the collapse when a black hole was created.  

The apparently fatal flaw in Bekenstein'ssuggestion was that if a black hole has a finite entropy that is proportionalto the area of its event horizon, it also ought to have a finite temperature,which would be proportional to its surface gravity. This would imply that ablack hole could be in equilibrium with thermal radiation, at some temperatureother than zero. Yet according to classical concepts, no such equilibrium ispossible, since the black hole would absorb any thermal radiation that fell onit, but by definition would not be able to emit anything in return. It cannotemit anything. It cannot emit heat.

This is a paradox. And it's one which I amgoing to return to in my next lecture, when I'll be exploring how black holeschallenge the most basic principle about the predictability of the universe,and the certainty of history, and asking what would happen if you ever gotsucked into one. Thank you.

APPLAUSE

SUE LAWLEY: Thank you. Thank you very muchindeed, Stephen. Well now we asked listeners what they’d like to ask you andtheir questions came flooding in, hundreds of them. We’ve chosen arepresentative selection of topics and invited some of those listeners to comeand put their questions in person. You’ll appreciate, audience, that we had togive Stephen the questions beforehand, so that he could programme his answersinto his computer. This is done letter by letter through the movement of hisfacial muscles and you’ll hear little tiny bleeps as the infrared detectorpicks up those movements. It’s not a speedy process, round about a word aminute, but the answers are all in there now. So if you’re ready, Professor,let me begin by asking Andy Fabian, who’s an astronomer and astrophysicist –he’s the Director of the Institute of Astronomy at Cambridgeand therefore a colleague of Stephen’s – Andy, your question please?  

ANDY FABIAN: Stephen, much of the work byyourself and others on issues such as the potential loss of information inblack holes and on radiation from black holes is theoretical and lacksobservational support so far. Do you see ways to change that situation usingthe many and varied observations now routinely being made of accreting blackholes throughout the cosmos?  

SUE LAWLEY: Stephen?

STEPHEN HAWKING: I assume you are referringto the area increase law for black hole horizons. The best way of testing thisis black hole collisions rather than accretion.

SUE LAWLEY: Well you got pretty short shriftthere, I have to say. (laughter) Perhaps you should tell us what the differenceis between accreting black holes and black hole collisions? ANDY FABIAN: Wellblack hole collisions are when you have two black holes colliding with eachother and merging together. Accretion is just matter dribbling into the blackhole steadily and it produces enormous amounts of energy release, very luminousthings in the universe.  

SUE LAWLEY: Do you foresee that there willbe evidential proof of what Stephen is talking

about? As you say, it’s all theoretical atthe moment.

ANDY FABIAN: It would be fantastic to findobservational proof of it, but I myself don’t see how to do it.

SUE LAWLEY: Okay well let’s pursue thistheme of what black holes get up to when we’re looking at them, and indeed whenwe’re not, and turn to a group of young enthusiasts here from the West Midlands. They’re pupils from BarrBeacon Secondary School in Walsall and they’reaged around 12 or 13. Kate Harris, you’re their teacher. How have you got theminterested in cosmology and everything else?

KATE HARRIS: Well I’m their form tutor, soI look after them every morning. I received an email from one of the scienceteachers who’s with us, Dr Butterworth, about Stephen’s lecture and told themabout it. And they were so keen to know more because firstly they’ve seen himin ‘The Big Bang Theory’… (laughter)  

SUE LAWLEY: This is the television … theAmerican sitcom?

KATE HARRIS: Yes. And also they are keenscience enthusiasts. They just wanted to know more.      

SUE LAWLEY: Yeah, so endlessly interested.Well let’s have a question from one of them. Aruniya Muraleedaran, you’re aged12. What’s your question?  

ARUNIYA MURALEEDARAN: What kind of thingswould happen if one black hole collided with another one?

STEPHEN HAWKING: If two black holes collideand merge to form a single black hole, the area of the event horizon around theresulting black hole is greater than the sum of the areas of the event horizonsaround the original black holes.

SUE LAWLEY: Did you understand that,Aruniya? Did you get your head round it? (laughter) Well, as I understand it,it’s when two black holes collide, the total circumference is greater than thesum of the two parts. The whole area increases. Got it?

ARUNIYA MURALEEDARAN: I’ve got it now.(laughter) What about you? Did you … Do you want to say something?

STUDENT: Yeah, I did understand it.  

SUE LAWLEY: You did?  

STUDENT: A bit, a bit. (laughter)  

APPLAUSE

SUE LAWLEY: Well maybe it was because Iexplained it to you. (laughter) Now to a rather more personal question – and wereceived many of them actually. This is from a 17 year old Radio 4 listener.His name is Duncan McKinnon. You told us, Duncan,that Stephen had been an inspiration to you. In what way?

DUNCAN McKINNON: Well watching the filmwith him, it was really inspirational how he managed to carry on with hisdreams and goals.

SUE LAWLEY: You mean the film ‘The Theoryof Everything’ …

DUNCAN McKINNON: Yeah, yeah.      

SUE LAWLEY: … which starred Eddie Redmayne…

DUNCAN McKINNON: Yeah.

SUE LAWLEY: … for which he won an Oscar ofcourse. Great film, wasn’t it? Okay, like to put your question?

DUNCAN McKINNON: I would like to ask youwhat inspired you to keep on going despite all the rough times in your life?  

SUE LAWLEY: Stephen?  

STEPHEN HAWKING: I think my work and asense of humour have kept me going. When I turned 21 my expectations werereduced to zero. You probably know this already because there’s been a movieabout it. In this situation, it was important that I came to appreciate what Idid have. Although I was unfortunate to get motor neurone disease, I have beenvery fortunate in almost everything else. I have been lucky to work intheoretical physics at a fascinating time, and it’s one of the few areas inwhich my disability was not a serious handicap. It’s also important not tobecome angry, no matter how difficult life may seem, because you can lose allhope if you can’t laugh at yourself and life in general.

SUE LAWLEY: We have here Lucy Hawking,Stephen’s daughter, just towards the front there. Lucy, you have seen Stephenover the past four decades if you don’t mind my saying that. (laughter)

LUCY HAWKING: That’s a little personal,Sue. (laughter)

SUE LAWLEY: I apologise, Lucy.

LUCY HAWKING: That’s okay. It’s radio,Sue.  

STEPHEN HAWKING: (interjecting) I wouldbring back Einstein. (laughter)

SUE LAWLEY: Interjection from my right.You’ve seen him, Lucy, weather some pretty rough times. What do you put hisresilience and this determination down to?

LUCY HAWKING: I think he’s enormouslystubborn (laughter) and has a very enviable wish to keep going and the abilityto summon all his reserves, all his energy, all his mental focus and press themall into that goal of keeping going. But not just to keep going for thepurposes of survival, but to transcend this by producing extraordinary work,writing books, giving lectures, inspiring other people with neurodegenerativeand other disabilities, and being a family man, a friend and a colleague to somany … so many people and keeping up with friends across the world. So I thinkthere … there are lots and lots of elements there, but I do think thestubbornness, the will to live and – like he says himself – the sense of humourto laugh at it, at the end of the day is what has …

STEPHEN HAWKING: (interjecting) I wouldbring back Einstein. (laughter/applause) He would be amazed at how much generalrelativity has advanced our understanding of the world.

LUCY HAWKING: I think that was his way ofasking me to stop talking. (laughter)  

SUE LAWLEY: And there we must end. Thankyou for making this a memorable event. There’s a mass of science on the BBCReith website, including Robert Oppenheimer, whom Stephen mentioned – one ofthe fathers of the atom bomb; the astrophysicist Martin Rees; the radioastronomer Bernard Lovell and many more. There’s an archive of recordings andtranscripts going back to 1948, so do have a look.  

For now our thanks to our hosts here at the Royal Institution in London and of course hugethanks to our Reith Lecturer, Professor Stephen Hawking.

APPLAUSE