【诺奖得主Wilczek科普专栏】冷而酷的物理
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。
作者 | Frank Wilczek
翻译 | 胡风、梁丁当
现代制冷技术在很多方面改变了我们的生活。借助冷藏技术，我们得以品尝到来自全球各地的食物——无论是当季的还是反季节的；有了空调，家和汽车也变成了三伏天的避暑场所。
Modern cooling technology shapes our lives in many ways. Refrigeration gives us access to food from all over the world, in and out of season; airconditioning turns homes and cars into places of refuge from sweltering summers.
Illustration: Tomasz Walenta
相比于古老的冰块制冷，这些技术无疑是重大的进 步。在用冰制冷的年代，曾形成过工业规模的冰块开采。我经常散步的地方——马萨诸塞州剑桥的弗雷什塘（Fresh Pond），就曾经是一个冰矿。
These are big improvements over the time-honored method of using ice, which used to be mined on an industrial scale. Fresh Pond in Cambridge, Mass., where I often take walks, was once an ice mine.
将物体放到冰上冷却，其物理原理是较简单直接的。由于能量总是从高能态往低能态流动，热量会从较热的物体传递到冰上，结果就是前者变冷、后者融化。
Putting something on ice to cool it is straightforward physics. Since energy flows downhill from a higher-energy state to a lower-energy one, heat is transferred from a warm object to the cold ice, causing the former to become cooler and the latter to melt.
更先进的制冷方法则更难实现，因为这需要从物体中抽取能量并转移到周围环境中，从而保持物体比周围环境温度更低。要实现这一点，就必须输入能量。这一点看似矛盾，但借助热力学定律确实能实现。较为廉价的能源，再加上出色的工业设计，注定了冰工业的衰落。
More advanced methods of cooling are harder, because they require pumping energy out of a body to keep it cooler than its surroundings. Paradoxically, this can only be done by investing energy, as well as taking advantage of the laws of thermodynamics. Reasonably cheap energy, plus some brilliant engineering, doomed the ice industry.
极低温仍然是发现新物理的重要领域。物体在极低的温度下呈现出明显的量子效应。量子力学的一个主要特征是能量只能取离散的值，即某个能量单位的整数倍。要观测到这种量子性， 这个倍数必须足够小：1000002个能量单位和1000003个能量单位没有本质不同，但如果是2个和3个能量单位，就会存在根本性的差别了。超冷材料的能量极低，在这种场景中量子力学的神奇定律能够充分发挥它的魔力。
Extremely low temperature is still a major discovery zone in physics. It is where quantum mechanics comes into its own. The distinguishing feature of quantum mechanics is that energy comes in discrete units. To see the effects of this discreteness, you’ve got to keep the number of units small: There’s no qualitative difference between 1,000,002 and 1,000,003 units of energy, but there is between two and three units. Ultracold materials are energy-starved, allowing the strange laws of quantum mechanics to work their magic.
在接近绝对零度时，很多金属和一些其他物质会突然变成超导态。超导材料中的电流没有电阻，所以只需要很少的能耗——甚至零能耗——就能维持。这个美丽的性质赋予了超导材料很多用途，比如，它可以用于制造核磁共振成像（MRI）所需的强力电磁铁。
Close to absolute zero, many metals and some other substances suddenly become superconductors. Electric currents flow through superconductors smoothly, without friction, so that it takes little or no energy to maintain them. Among many other applications, that beautiful fact makes it possible to create the powerful electromagnets used in magnetic resonance imaging (MRI).
类似的，液氦在低温下会转变成超流体，从而在没有摩擦阻力的理想情况下传输质量。因此，液氦是制 造低温冰箱的理想材料，而液氦超流已经成为了现代低温技术的主力军。
Similarly, liquid helium becomes a superfluid at low temperature, able to transport heat without friction. This makes it ideal for low-temperature refrigerators, and superfluid helium is the workhorse of cryogenics.
信息处理是另一个非常活跃的低温物理前沿。现代计算机是通过电流工作的，而电流的热效应是制约计算机发展的一个主要因素。目前研究人员正在研究如何用信息“超流” 在没有热损耗的情况下传输数据。
Information processing is another active frontier of low-temperature physics. The heat generated by modern computers, which are powered by electric currents, is a major limitation on computing technology. Researchers are working on ways to transfer data without generating heat, using “supercurrents” of information flow.
此外，计算中的许多重要问题，比如如何优化配电网络或者航线规划系统，都可以转化为“制冷”的问题来思考。将一个问题数字化后，问题和（可能的）答案都成了一长串由0和1组成的编码。我们可以把这些二进制码翻译成一个物理系统：0和1分别对应一个开关的“关”和“开”，或者是电子的“自旋向上”和“自旋向下”两种状态。
In addition, many important problems in computing, such as finding efficient distribution networks or airline routing systems, can be thought of in terms of making things cold. Whenever a problem is posed digitally, both the question and the potential answers are long strings of zeros and ones. These binary numbers can be translated into physical systems: Zero and one can become the “off” and “on” positions for a switch, or “spin up” and “spin down” for an electron.
通过这种“物理化”的编程，我们可以把问题和答案分别对应到一个物理系统和它的状态上。其中，能量最低的那个态对应的就是最佳答案。
This kind of physical programming allows us to map questions and answers into states of physical systems, where the most efficient solution will be the one that contains the least heat.
制冷需要考虑多方面的因素，例如温度的高低、系统是处于大自然还是人工环境中。这些都不断地挑战着人们的聪明才智。发明新的冰箱和空调或许是计算机技术取得新突破的关键。这让我在夏天游泳的时候有了新的思考目标。
The multifaceted problem of refrigeration, at high or low temperatures, in natural or engineered environments, continues to challenge human ingenuity. There are strong incentives to invent new kinds of ice and airconditioners—refreshing things to envision during my long summer swims.
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