科学松鼠会

流体运动的多样性和不稳定性长期以来都是流体力学重要的话题。由于描述方程组往往是非线性的，对相当一部分情况，解析计算比较困难，因此通过实验或数值模拟来直接观察流体的行为也是重要的研究手段。下面这些图片都选自美国物理学会流体力学分会举办的年度流体运动图片展，有的是实验结果，也有计算机模拟图象。其意义不仅仅在于科学，更有着独特的欣赏性。

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无式镜

在从未被文字记录下来的那段历史中的某一天，一个腰上挂着树叶串、头上长发飘飘的人一脚飞起一块石子。他用类似于尖叫的语言说：“咦，这是什么东西亮闪闪在地下？”他捡起这块大致像颗棋子的透明石头瞅瞅，“石子对面的世界放大啦～”他的同类还试着用透明圆石头在炎炎烈日下长时间凝视地上一些烂草棍，结果草棍呼的一下烧着了！对大自然打磨的奇妙石头的记忆一直延续到公元1世纪初，在罗马哲学家的笔记中，它们被称为“放大器”（magnifier）或“点火石”（burning glasses）；直到13世纪，这些石头终于从脚下一路登鼻子上脸，被赐名透镜（lense），因为它们长得好像一颗小扁豆（lentil）。

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近日一遭奇遇，自觉颇有所得。且拿来与众位分享一通，聊充饭后谈资~

偶遇脑专家

大学里有一次去朋友那玩，饭前无聊的时候，大家比赛辨认CD音乐里的各种乐器。小D以前学过一阵钢琴，有点艺术细胞，多数都是他抢先辨认出来。我们都不服气地嚷嚷根本没听到，小D就把CD回放。还真邪门，每次重听的时候，当他提醒我们“注意小提琴加进来了”，大家就果真都听到了小提琴。唉呀呀，一屋子人，摸着脑袋自叹弗如。

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封面故事

追查宇宙前世

撰文/马丁·博约沃尔德（Martin Bojowald）

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(《百科知识》约稿，已发。)

人类用冰来“镇”食物的尝试从公元前就开始了，世界各地也早就有了萌芽状态的冰激凌。不过，真正意义上的冰激凌直到十八世纪才出现。在英语里，“冰激凌”是由“冰（ice）”和“奶油（cream）”两个词组成的。最早的冰激凌确实就是冰镇的奶油，里面也可能有一些糖或者水果。经过了两三百年的发展，现在的冰激凌已经不是吴下阿蒙，早已变得越来越复杂，越来越多样了。不过，对于冰激凌为什成为冰激凌，则直到最近几十年才有了比较深入的认识。这里，让我们一头扎进冰激凌的内部，看看那里是一个什么样的世界吧。 

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已发于《瞭望东方周刊》
对于全世界的粒子物理学家来说，2008是个相当不错的好年头：首先，他们等着用来保住饭碗的大型强子对撞机（LHC）耗资60亿欧元，在法国和瑞士边界的欧洲核子研究中心（CERN）宣告落成了；其次，不到一个月后，瑞典皇家科学院把今年度的诺贝尔物理奖颁给了三位在这一领域作出突出贡献的人物：南部阳一郎（Yoichiro Nambu）、小林诚（Makoto Kobayashi）和益川敏英（Toshihide Maskawa），其中南部独得奖金的1/2，后二人则分享另一半，他们都因在对称破缺（Broken Symmetries）方面的工作受到了评委的青睐。

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史上最著名的一道松鼠数学题是这样的：

上帝从伊甸园抓起一把土捏成松鼠亚当，又抽他一根肋骨变作松鼠夏娃。他们都有不死之躯，自由自在终日玩耍。由于太贪玩，二人从第二月开始每月生下兄妹一双。兄妹本着肥水不流外人田的精神，同样自二月大时生小兄妹一双并以每月2只的进度继续下去，小兄妹继续小小兄妹，然后小小生小小小，小小小再小小小小……这是一道天堂里的题，一切情况理想化，所以夫妻从来没有外遇。一年之后伊甸园里统共有几对松鼠呢？

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给《南方人物周刊》写的小文，其实，我不想去写科学，也不想去讨论“世界会不会毁灭”。整个采写过程中，我只是想感觉一下，花了这么多钱的粒子物理学家是怎么想的？结果，某些物理学家们的理想主义气质，让我很羞愧。在这个世界上，有理想，又努力把理想实现的人，还是令我有那么一点点崇敬的。——————————————————————-

一台机器，功效是让两个粒子经过周长为27千米的管道加速，速度都达到光速的99.9999991%，能量达到7万亿电子伏，然后迎头碰上。

除了物理学家，其他人会对此感兴趣吗？

答案却是肯定，据说，那是因为这台机器——LHC（大型强子对撞机）中这次碰撞的能量太大了。

欧洲和美国的反对人士分别向当地法院提出起诉，要求叫停或推迟这个项目，他们的理由是：这台机器能产生危险的粒子或者微型黑洞，从而毁灭整个地球。 Read the rest of this entry »

译自LHC Milestones, CERN

译序：大型强子对撞机（LHC）是西欧核子中心（CERN）迄今建造过的加速器中最大的一台。它周长27公里，可以把质子束加速到14 TeV的能量上，这一数字14倍于蚊虫在飞行中的动能。建造LHC的目的是探测更深层的微观世界，为理论提供检验，也可以研究早期宇宙的粒子物理现象。值得一提的是，为解决海量数据处理的难题，LHC采用了分布式运算，还推出了面向公众的LHC@Home项目。

现今LHC的建设已经完成，并于2008年9月10日启用。如CERN的宣传语所说，bigger is better when you are searching for smaller，单单凭借LHC的规模，就足以让无数粒子物理学家对其拭目以待。

本年表译自CERN官方网站，初稿译成于2007年8月，由于LHC最近已开始运转，故最近又将2007年底至2008年的内容补充。原文是按年度分页面介绍的，译稿将其合并，图片一律引用原文。

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（这是发在《牛顿科学世界》上那篇的原稿，刊发版被我弄丢了，原文更加冗长。请谨慎打开，阿弥陀佛~嘿嘿）

引言

脑电图是一项应用广泛的医学诊疗手段和脑功能研究方法。可除此之外，它对人类探索意识的本质也有着深远的影响。作为世界上第一个无伤害性测量活体大脑信号的方法，脑电图的历史和人们试图了解自身的努力交织在一起。而它和电子计算机的结合又为意识领域的新发现提供了广阔的前景。让我们一起走进脑电波的世界，开始一场探索意识之旅吧。 

贝格尔医生的秘密实验 

1924年夏天的一个普通的傍晚，德国医生贝格尔（Hans Berger）从心理诊所出来没有马上回家，而是来到附近一所不起眼的小房子里，进行他的“私人”实验。一个病人已经等在那儿了。“泽德尔（Zedel）先生，你好！”打过招呼，贝格尔让病人坐在椅子上，拿起两个连着电线的金属片，一片放在病人的前额，一片放在后脑勺上面，固定好。他让泽德尔闭上眼睛，然后打开电流计，密切注视着记录仪上的指针变化。指针开始缓缓地运动起来……

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如果比速度，大概不如田径，但是比谁穿的少，短池游泳大概要数一数二了吧？但是在北京奥运会的水立方里，我们恐怕会悲痛的发现，无论是青筋暴起的肌肉男们还是身材魔鬼的一塌糊涂的美女们，身上都将像粽子一样越包越严实，直接剥夺了观众们大好的养眼机会。是什么让他们不顾全球变暖的大趋势，逆流而上越穿越多的呢？

(上图：菲尔普斯展示鲨鱼皮四代（快速皮肤IV)——激光竞速者）

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公元02007年10月3日，一位叫做周正龙的农民声称拍到了一只野生华南虎，镇坪县林业局绕过安康市林业局，越级上报省林业厅，省林业厅当即决定奖给拍虎英雄2万元，并最终演变本世纪初中国一场啼笑皆非的‘年画虎事件’。公元01988年，在日本一个叫阿南的小城市里，一位普通的日亚公司职员厌倦了十年来生长一些磷化镓砷化镓单晶的活，也冒然越级走进公司董事长的办公室，提出了要制备氮化镓蓝光发光二极管，董事长当即决定资助500万美元的设备支持。三年后这位中村修二同学便在《应用物理快报》上发表了生平第一篇英文文章：一种用于生长氮化镓新颖的金属有机物化学气相沉积法。论文一发表便轰动了世界半导体产业界和科学界，要知道这个时候世界上有多少大公司、著名大学科研机构都在为半导体蓝光光源薄膜材料的制备工艺头痛不已，而氮化镓正是III-V族半导体材料中最具有希望的宽禁带光学材料。

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Mark Buchanan　文 Shea 译

大约50年前，一个对量子力学标准观点不满的物理系学生提出了他自己的全新观点。

1957年，一位来自美国普林斯顿大学的年轻物理学家发表了他的第一篇论文——这篇论文当时被没有受到重要，然后他便从学术界消失了。直到他1982 年去世（终年51岁）为止，他一直是美国国防工业的一名工程师和分析师。但是一些物理学家却认为，休·埃弗雷特（Hugh Everett）对科学的深远贡献远远超出了他的论文所含盖的范围。他们说，他的第一篇论文为解开量子力学中最久远的谜体之一提供了新的方法。

量子理论已经取得了许多巨大的成就，但是物理学家们始终对它的逻辑自洽性表示不满。以薛定谔的波动方程为例，量子理论认为诸如电子这样的微观粒子会具 有奇怪的“叠加态”，这会使得它们可以同时处于两个地方。薛定谔方程帮助我们解释了原子的行为，但是对于例如椅子这样由微观粒子组成的宏观物体，为什么我 们从来不曾看到一把椅子可以同时出现在两个不同的地方呢？

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Max Tegmark　文    Shea 译

如果你认为量子物理是普适的真理，那你就应该相信有平行宇宙。

[图片说明]：是否只有在科幻小说中我们才能生活在平行的世界里？如果一个原子可以同时出现在两个地方，那么你也能。 Read the rest of this entry »

前些天和同实验室的同学聊天，无意间聊到了饮水机如何制冷的问题。要说也是，这东西一来价钱不算太贵，二来耗电也不算太大，不大可能是用压缩机，那么是用什么方式让水降温？于是google之，赫然发现用的是半导体制冷，原理就是所谓的帕耳帖效应。原来这东西跟业余天文用CCD的制冷是一个道理，于是连忙去翻实测天体物理的讲义……

帕耳帖效应其实是诸多热电效应的一种，算是Seebeck效应（也就是温差电效应）的逆过程。Seebeck效应是在两种导电材料的接合处通过温差来产生电压，帕耳帖效应就是让施加有电压的两种导电材料产生温差。具体计算也很简单，如下式所示：

其中是产热率，I是电流，指两种导体的帕耳帖系数，物理意义是单位电流在某种材料中携带的热流数量。由于两种材料连接处电流连续而帕耳帖系数不连续，此处就会有热量的积累或是损失。通过改变电流的方向，就可以决定让设备产热或是制冷。

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当我们把一条干毛巾的一端浸在水里，水会沿着毛巾往上走。那么，可不可以利用这个现象把水从低处吸到高处，然后收集起来呢？如果可以的话，就可以让高处的水流下来发电，然后再沿着毛巾爬上去。如此往复，不用外加能源，可以源源不断地发电了。实际上这是历史上一个永动机的设想，当然是不能成功。我们自然会问：水的确是爬到了高处，为什么就不能被收集起来呢？

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这两天真热闹，我也翻出一篇老帖来助助兴：

偶然看见一篇“從10億光年外看地球”的转帖，介绍不同尺度的世界图像，图片制作相当精细，中文解说也很很到位，这样的视角在这个普及了卫星地图、电脑动画、电子隧道扫描显微镜的时代的确算不上新鲜，但如果是三十年前呢？

原文链接

The strange thing about Einstein’s special relativity is that the theory was spot on – even though an essential assumption was way off-beam. Mark Buchanan discovers the dark side of E = mc2

IMAGINE you are on a bicycle, pedalling across the cosmos. A beam of light – perhaps sent off by a distant collapsing star – zings past you. How fast are you and the light approaching each other? You are travelling at hardly any speed, so the answer will be more or less exactly light^’s speed through the interstellar vacuum, around 300 million metres a second.

Now imagine you abandon pedal power for the day. Bowling along in your spaceship at half light speed, you meet another light pulse head-on. What is your speed of approach now? Surely it is just your speed plus that of the light: in total, one and a half times light speed.

Wrong. Your speed of approach will be the speed of light, no more – and that^’s true however fast you are travelling. Welcome to the weird world of Einstein^’s special relativity, where as things move faster they shrink, and where time gets so distorted that even talking about events being simultaneous is pointless. That all follows, as Albert Einstein showed, from the fact that light always travels at the same speed, however you look at it.

Really? Mitchell Feigenbaum, a physicist at The Rockefeller University in New York, begs to differ. He^’s the latest and most prominent in a line of researchers insisting that Einstein^’s theory has nothing to do with light – whatever history and the textbooks might say. ^“Not only is it not necessary,^” he says, ^“but there^’s absolutely no room in the theory for it.^”

What^’s more, Feigenbaum claims in a paper on the arXiv preprint server that has yet to be peer-reviewed, if only the father of relativity, Galileo Galilei, had known a little more modern mathematics back in the 17th century, he could have got as far as Einstein did (www.arxiv.org/abs/0806.1234). ^“Galileo^’s thoughts are almost 400 years old,^” he says. ^“But they^’re still extraordinarily potent. They^’re enough on their own to give Einstein^’s relativity, without any additional knowledge.^”

The claim has got other physicists thinking. Take Feigenbaum^’s argument a step further, some say, and we might long ago have seen our way not only to Einstein’s relativity but also to the idea of an expanding universe –even one whose expansion is accelerating – without the intellectual upheavals that have led us to those conclusions today.

The discussion centres on two assumptions that Einstein made when formulating his special theory of relativity in 1905. The first is uncontroversial: that the laws of physics should look the same to anyone at rest or moving steadily. Say I am standing motionless and you are moving past on a train travelling at a constant speed in a straight line – in other words, at a constant velocity. To you on the train, I am the one who seems to be moving. But it does not actually matter who is ^“really^”moving relative to whom: although perceived velocities depend on one^’s point of view, the physical laws governing motion stay the same.

This is the principle of relativity proposed by Galileo in A Dialogue Concerning the Two Chief World Systems, his treatise of 1632 that got him into hot water with the Catholic church for discussing Copernicus^’s idea that Earth goes round the sun. Galileo writes of a passenger inside a ship who cannot tell if it is moving or standing still ^“so long as the motion is uniform and not fluctuating this way and that^”. The analogy was aimed at those sceptics who believed that Earth could not be moving because they could not feel it.

Galileo^’s relativity served well for almost 250 years. But when Scottish physicist James Clerk Maxwell derived his theory of electricity and magnetism in the late 19th century, it hit a snag. Maxwell^’s equations make clear that light is a wave travelling at a constant speed. But oddly, they do not mention from whose point of view this speed is measured.

This was a problem if Maxwell^’s theory, like all good physical theories, was to follow Galileo^’s rule and apply for everyone. If we do not know who measures the speed of light in the equations, how can we modify them to apply from other perspectives? Einstein^’s workaround was that we don’t have to. Faced with the success of Maxwell^’s theory, he simply added a second assumption to Galileo^’s first: that, relative to any observer, light always travels at the same speed.

This ^“second postulate^” is the source of all Einstein^’s eccentric physics of shrinking space and haywire clocks. And with a little further thought, it leads to the equivalence of mass and energy embodied in the iconic equation E = mc². The argument is not about the physics, which countless experiments have confirmed. It is about whether we can reach the same conclusions without hoisting light onto its highly irregular pedestal.

According to David Mermin, who has been teaching relativity at Cornell University in Ithaca, New York, for 30 years, a consensus has emerged that we can, although this shift has yet to filter through to a wider audience. ^“All the textbooks teach relativity based on Einstein^’s principles,^” he says. ^“And there^’s an extremely widespread misunderstanding that relativity is somehow tied up with light.^”

Two years ago, Feigenbaum^’s puzzlement with relativity^’s logic led him to Galileo^’s Dialogue. ^“The book is quite a knockout,^” he says. ^“When I finished reading, I wondered, if you take what he says seriously, what can you produce?^” So he sat down and started calculating as Galileo might have, but using today^’s more sophisticated mathematics.

He starts with a simple scenario. You are standing watching a friend, Frank, moving past you on a train at 50 kilometres per hour towards the east. Frank, on the other hand, has his eyes on Kate, whom he sees receding from him at 50 km/h towards the north. Feigenbaum asks a simple question: how do you see Kate moving?

It seems natural that Kate^’s velocity relative to you should in some sense be the sum of Frank^’s velocity relative to you and Kate^’s relative to Frank. The fact that Frank sees Kate both receding to the north and keeping up with his eastbound motion implies that, from your stationary point of view, her motion is towards the north-east.

But now swap Frank and Kate^’s motions. Frank is travelling at 50 km/h northwards relative to you, and Kate at 50 km/h eastwards relative to Frank. This should not affect how Kate is travelling relative to you – you will still see her heading off towards the north-east.

Galileo would certainly have said so. Only with Einstein^’s introduction of a space-time warped, as he thought, by a universal speed of light did it become clear that the rules of adding motions were not quite so simple as all that. But in fact, says Feigenbaum, both Galileo and Einstein missed a surprising subtlety in the maths – one that renders Einstein’s second postulate superfluous.

It is this: if Frank^’s world is aligned with yours – if the north and east of both you and Frank point in the same direction – and Kate^’s world is similarly aligned with Frank^’s, you might think that Kate^’s is aligned with yours. The problem is, mathematical logic alone does not permit that conclusion. Strange as it may seem, it in fact allows a distinct possibility that Kate^’s world could be rotated with respect to yours – even if she is perfectly aligned with Frank and Frank is perfectly aligned with you.

This means that, while still seeing Kate careering off towards the north-east, you might also see her skewed slightly to the left or right relative to her direction of motion (see diagram, right). The direction of the rotation, and thus Kate^’s motion as seen by you, would depend on what the relative motions of you and Frank and of Frank and Kate are.

^“Not only is light not necessary in relativity – there^’s no room in the theory for it^”

The possibility of such rotations turns out to have far-reaching consequences. Ignore them, and Galileo^’s relativity pops out. Allow them, and the algebra works out very differently: the mangled space-time of Einstein^’s relativity emerges, complete with a definite but unspecified maximum speed that the sum of individual relative speeds cannot exceed. ^“These rotations are hard to understand,^” Feigenbaum says, ^“but they^’re the wellspring of physics.^”

Feigenbaum emphasises that he is not the first person to question Einstein^’s second postulate or arrive at the idea of such bizarre rotations. Even so, Mermin is impressed.

^“Mitch^’s way of deriving the theory is quite complicated,” he says, “but the rotations come up in a very natural and beautiful way.^”

The result turns the historical logic of Einstein^’s relativity on its head. Those contortions of space and time that Einstein derived from the properties of light actually emerge from even more basic, purely mathematical considerations. Light^’s special position in relativity is a historical accident: it was just the first (and is still the most obvious) phenomenon we have encountered that travels at the universal maximum speed.

The idea that Einstein^’s relativity has nothing to do with light could actually come in rather handy. For one thing, it rules out a nasty shock if anyone were ever to prove that photons, the particles of light, have mass. We know that the photon^’s mass is very small – less than 10^-49 grams. A photon with any mass at all would imply that our understanding of electricity and magnetism is wrong, and that electric charge might not be conserved. That would be problem enough, but a massive photon would also spell deep trouble for the second postulate, as a photon with mass would not necessarily always travel at the same speed. Feigenbaum^’s work shows how, contrary to many physicists^’ beliefs, this need not be a problem for relativity.

^“Feigenbaum’s ideas could be very helpful in correcting this misconception,^” says physicist Sergio Cacciatori of the University of Insubria in Como, Italy. He suggests that further thinking along similar lines could reveal much more about the universe. Together with his colleague Vittorio Gorini, and Alexander Kamenshchik of the Landau Institute for Theoretical Physics in Moscow, Russia, he has explored what would happen if you took Feigenbaum^’s conclusions about adding motions and applied them to changes in position (www.arxiv.org/abs/0807.3009). What if where you ended up after two consecutive displacements depended on the order of their occurrence?

In the world around us, it is obviously a pretty good approximation to reality that, if you take 20 paces forward and 10 to the left, you end up in the same place as if you had taken 10 equally sized places to the left and then 20 forward. On the vast scales of the cosmos, however, the same assumption might be dangerously misleading. It amounts to requiring the universe to have a flat, Euclidean geometry – one like that in our immediate environment, in which parallel lines never cross and the inside angles of triangles add up to 180 degrees.

Abandon that assumption, Cacciatori and his colleagues show, and things look very different. The universe is not flat and Euclidean, but curved in on itself, creating a geometry akin to that of the surface of a sphere such as Earth – where the parallel lines of longitude converge at the poles and the internal angles of triangles add up to more than 180 degrees.

That discovery is significant because it feeds into a long-running debate about the shape – and fate – of the universe. Back in 1916, Einstein fused special relativity with Newton^’s ideas of gravitation to create a universal theory of gravity, known as the general theory of relativity. General relativity predicts that mass and energy warp space and time, and that the distribution of mass and energy determines the universe^’s geometry.

Predictable fate

When Einstein applied these ideas to calculate the dynamics of the universe, the outcome was decidedly odd: the gravity of the universe warps its fabric so much that it becomes unstable and collapses in on itself. To avoid this dispiriting and apparently nonsensical conclusion, Einstein added to his general relativistic brew a new quantity, the ^“cosmological constant^”, to counteract gravity and create a stable, static universe. This universe^’s geometry was curved and closed in on itself – rather like the three-dimensional surface of a four-dimensional sphere.

Einstein^’s constant was short-lived. In 1929, Edwin Hubble found evidence that distant galaxies were receding from the Earth, implying that the universe was expanding dynamically. Faced with this evidence against a static universe, Einstein famously decried the constant as his ^“greatest blunder^”.

Just recently, however, the cosmological constant has come back into fashion. The reason is the evidence accumulated by astronomers over the past decade – in the unexpected dimness of some extremely distant supernovae, and in the cosmic microwave background, the still-reverberating echo of the big bang – that the expansion of the universe is accelerating. That acceleration seems to require just the kind of anti­gravitational effect that motivated Einstein^’s constant in the first place.

The irony, says Gorini, is that we could have seen all along that something like the cosmological constant makes sense. Follow the mathematics of relativity through to its logical conclusion, allowing for displacements that add up in different ways, and you find that space must have just the curvature that a cosmological constant helps to produce. That has nothing to do with the distribution of mass, but follows from mathematical logic alone.

So had we put our faith in mathematical reasoning, might we have been able to predict the dynamics of the universe much sooner? Gorini thinks so. He claims that the German mathematician Hermann Minkowski came close to exploring these displacement effects in lectures on relativity he delivered in 1908. ^“Had he done it,^” Gorini says, ^“physicists would have known that the universe expands, and that its expansion is accelerated, well before the development of general relativity or even Hubble^’s observation of the recession of galaxies.^”

As it happens, in 1968, physicists Henri Bacry and Jean-Marc Levy-LeBlond of the University of Nice in France predicted the existence of a cosmological constant from first principles (Journal of Mathematical Physics, vol 9, p 1605). Their work presages that of Feigenbaum and of Gorini and colleagues, but remained unnoticed – largely because the constant was out of fashion at the time.

Against that prevailing scientific wind, it would have been a bold leap for those researchers to have predicted the dynamics of the universe; for Galileo, centuries before, even more so. Einstein, the ultimate physics revolutionary, probably would have afforded himself a wry smile at the picture that is now emerging. The startling edifice of the new physics he built remains undisturbed, even as its logical foundations are being greatly strengthened. Meanwhile, the power of mathematical reasoning in unlocking the secrets of the universe continues to amaze: what physicist Eugene Wigner once called ^“the unreasonable effectiveness of mathematics^” is one of the deepest mysteries of them all. ~