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Frank Wilczek
弗兰克·维尔切克是麻省理工学院物理学教授、量子色动力学的奠基人之一。因发现了量子色动力学的渐近自由现象，他在2004年获得了诺贝尔物理学奖。
感谢Frank夫人Betsy Devine女士为本专栏配音！
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▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▼▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲▲为了找到解决问题的最佳方案，科学家必须避免跌进“局部极值点”的陷阱。
假如一切可以从零开始，毫无疑问，我们应该使用公制单位而不是英寸和英尺，用十二进制而不是十进制，用德沃夏克键盘而不是QWERTY键盘（QWERTY键盘是指以QWERTY构成键盘字母区前6个字母的键盘 ；德沃夏克键盘采用了不同布局，将5个元音键和最常用的辅音键安排在键盘中间行）。但在这个世界的很多地方，英寸、十进制和QWERTY键盘已经成为了标准。人们围绕着这些标准形成了思维习惯，建造了配套设备。此时要做出改变是非常痛苦的。这些日常生活中的例子背后折射出的是一个深刻的问题——“局部极值”。这个问题如同一个幽灵，困扰着科学发展与人类生存。为了理解什么是局部极值，让我们设想一个登山者，她尝试登上一段山脉的最高峰。无论她从哪里出发，只要遵循一个简单的模式——始终往上攀登，一段时间内，她总能越爬越高并到达一个顶峰。可这样的攀登模式却不一定能保证她登上整段山脉的最高峰。换句话说，她到达了一个局部最大值，但不一定是全局最大值。更高的山峰可能在别的地方，被山谷和山口隔开了。如果要去那儿，她必须先往下走，或许还要再绕点儿弯路，然后才能继续登高。
在物理及其应用中，人们经常遇到这样的情况，即需要探索一系列可能出现的结果。比如，我们已经知道了某些蛋白质中氨基酸的序列，然后想去确定蛋白质在内部张力下可能折叠成的形状。我们可以先猜一个形状，然后加一些小的变化，最后得到一个稳定的结构。但是，如果我们从一个截然不同的起点开始，答案可能全然不同。在“疯牛病”和人类的库鲁病等疾病中，原本在正常情况下有用的蛋白质出现了构型的变异，从而导致蛋白斑块的形成，给细胞造成了损伤。如果从正常构型的蛋白质开始，这种变异是很难发生的。但只要出现了一个错误折叠的分子，它就可能作为模板产生另一个结构变异的分子。这种变异一旦形成就比较稳定，因为它是一个局部极值点。对于生物演化或者人类社会来说，要脱离局部极值点是困难的。如果没有小行星的毁灭性碰撞，恐龙可能还在称霸地球，而哺乳动物仍然是被它们统治的夜行物种。如果没有欧洲黑死病的爆发，文艺复兴和整个现代文明都可能被严重推迟。
在我们个人的生活中，放弃一个舒适的局部最佳点可能是很痛苦的选择。如果我的生活已经够好了，我还需要再去考虑换工作、移民或者重返学校吗？伏尔泰 (Voltaire) 告诫我们 ：“最好是进步的敌人。” 可是波斯诗人欧马尔 · 哈耶姆 (Omar Khayyam) 在诗里写道（摘自《鲁拜集》）：
啊，爱！你我应与命运同谋，
理解这令人憔悴的万物之道，
我们把它砸成碎片，然后，
再按我们的心愿重新筑构！
心理学家告诉我们，通常随着年龄增长，人们会变得更加知足。对此，一个可能的解释是，当强烈改善生活的希望随着光阴逐渐逝去，人们也不再需要为由此带来的付出与风险而焦虑。似水流年间，诗人欧马尔的一腔热血渐渐褪化为伏尔泰的云淡风轻。To find the best possible solution to a problem, scientists have to avoid the trap of the 'local maximum.'
If we could start from scratch, we'd be better off, no doubt, using metric units instead of inches and feet, duodecimal (base 12) numbers instead of decimals, and Dvorak keyboards instead of QWERTY. But in parts of the world where inches, decimals and QWERTY are the standard, people have built habits of thought and devices around them, so switching would be far from painless. These are homely examples of a profound issue that haunts both science and the human condition: the specter of local maxima.
To understand local maxima, imagine a climber who is trying to reach the highest point in a mountain range. No matter where she starts, she can get higher for a while by following a simple rule: always ascend. Eventually she will reach a peak. But there is no guarantee that the peak she reaches this way is the highest in the range. She has reached a local maximum but not necessarily the global maximum. Higher peaks may lie elsewhere, separated by valleys and mountain passes. To get to them, she’d have to go down for a while and maybe wander a bit, before ascending.
In physics and its applications, one often encounters the problem of exploring a landscape of possible outcomes. We might know the sequence of amino acids within some protein, for instance, and look to determine the possible shapes that the protein, responding to internal stresses, can fold into. By starting with some guess and considering small changes, we can home in on a stable structure. But a radically different starting point might have led to a different answer.
In "mad-cow disease" and the human disease kuru, among others, proteins that normally perform useful functions take on an alternative shape that leads to formation of plaques and devastating cell damage. The diseased form is difficult to reach, starting from the normal form, but one misfolded molecule can form a template for others. The alternative shape is effectively stable, once formed, because it is a sort of local maximum.
Local maxima can be difficult for evolution, or for human societies, to escape. Without the disruptive effect of an asteroid, dinosaurs might still be roaming the Earth, leaving mammals as an oppressed race of nocturnal vermin. Without the disruptive effect of the Black Death in Europe, the Renaissance and the modern world in general might have been substantially delayed.In our own lives, the choice of whether to abandon a comfortable local maximum can be wrenching. If my life is pretty good already, should I consider changing jobs, emigrating to a different country or going to back to school? Voltaire warned that “the best is the enemy of the good.” But Omar Khayyam urges us on:
Ah Love! could thou and I with Fate conspireTo grasp this sorry Scheme of Things entire,Would not we shatter it to bits—and thenRe-mould it nearer to the Heart’s Desire!
Psychologists tell us that, as a rule, people become more content as they age. A plausible explanation is that as the prospect of making dramatic improvements in one's life recedes, so does anxiety about taking on the associated labor and risks. Omar's fifiery resolve passes into Voltaire's cold comfort.
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