说“铯原子”大约需要一秒钟。而近60年来，一秒钟有多长正是由铯原子来定义的。这是因为铯-133（铯的一种同位素）在受到微波辐射时会在两个能态之间跃迁，其频率极其规律，你甚至可以用它来校准手表。事实上，原子钟正是利用这种稳定的节律来计量时间。根据国际单位制，一秒钟等于9,192,631,770个周期。
IT TAKES ABOUT a second to say “caesium atoms”. And, for nearly 60 years, caesium atoms have said how long a second lasts. That is because caesium-133, an isotope of caesium, switches between two energy states when exposed to microwave radiation, at a rate so regular you could set your watch by it. Indeed, atomic clocks use this steady rhythm to measure time. According to the International System of Units, a second is 9,192,631,770 cycles.
但这种情况不会持续太久了。10月13日，全球顶尖的计量学家齐聚凡尔赛，参加国际计量大会（CGPM）——自1889年以来，他们大约每四年聚会一次，讨论测量科学。经过三天的辩论，他们为自己设定了一个最后期限：到2030年，他们将决定如何重新定义秒。
But not for much longer. On October 13th the world’s top metrologists convened in Versailles for the General Conference on Weights and Measures (CGPM), as they have done roughly once every four years since 1889, to discuss the science of measurement. After three days of debate, they set themselves a deadline: by 2030 they will decide how to redefine the second.
国际计量大会负责监督国际计量局（BIPM），后者制定全球时间流逝的标准。其做法是汇总世界各地实验室中数百台原子钟的测量数据，以修正可能导致每台单独读数出现误差的微小波动。最终得出的是一个统一的标准化时间，用于协调从金融交易到卫星导航的一切事务。
The CGPM oversees the International Bureau of Weights and Measures (BIPM), which sets the global standard for the passage of time. It does so by combining measurements from hundreds of atomic clocks based in laboratories around the world, to account for small fluctuations which can cause errors in each individual reading. The result is a single standardised time that co-ordinates everything from financial transactions to satellite navigation.
但国际计量局面临一个问题。为了核验其标准化时间的准确度，它使用了一套基于不同化学元素的新型时钟，这些时钟比最好的铯钟测量时间更为精确。而要核验这些新型时钟的准确度，又需要秒本身的定义。用铯钟给出的秒定义来做这件事，就像用一把刻度比头发还宽的尺子去量头发的粗细。要让这套钟表系统像钟表一样精准运转，国际计量大会需要把秒与铯解绑。
But the BIPM has a problem. To check the accuracy of its standardised time it uses a set of newer clocks, based on different chemical elements, which can measure time more precisely than the best caesium clocks can. Checking the accuracy of the newer clocks requires a definition of the second itself. Using that from a caesium clock is like trying to measure the width of a hair using a ruler whose divisions are wider than the hair. To run the clockwork like clockwork the CGPM needs to untie the second from caesium.
新加坡国立大学物理学家凯尔·阿诺德是一个正在开发基于镥离子时钟的团队成员。铯钟利用微波辐射触发原子能态之间的跃迁，而阿诺德的设备则使用激光。这种光学时钟“滴答”得更快，从而可以更精细地测量时间。今年9月，阿诺德的团队在《自然》杂志上撰文称，他们的镥钟滴答规律性比最好的铯设备高出约1000倍。
Kyle Arnold, a physicist at the National University of Singapore, is part of a team developing a clock based on lutetium ions. While caesium clocks use microwave radiation to trigger transitions between the atom’s energy states, Dr Arnold’s device uses lasers. Such optical clocks “tick” more quickly, allowing time to be measured more finely. Writing in Nature, in September, Dr Arnold’s group reported its lutetium clock had a tick about 1,000 times more regular than the best caesium devices.
原子钟必须在世界各地都能可靠运行。然而，阿诺德的设备有房间那么大，尚未离开他在新加坡的实验室。其他更便携的竞争者包括基于镱原子和锶原子的光学时钟，这些原子由激光悬浮。这些方案也有各自的问题。今年7月发表在《物理评论快报》上的一项研究发现，比较基于这些元素的光学时钟的测量结果与早期结果存在分歧，差异超出了已知不确定性所能解释的范围。
Atomic clocks must work reliably in different places around the world. Dr Arnold’s device, however, is the size of a room and yet to leave his Singapore lab. Other, more portable, contenders include optical clocks based on ytterbium and strontium atoms suspended by lasers. These come with their own issues. A study published in Physical Review Letters, in July, found that measurements comparing optical clocks based on these elements disagreed with earlier results by more than their known uncertainties could explain.
计量学家现在必须决定将哪种原子跃迁提升为官方标准。这需要在另一个“时间领主”团队中达成共识：时间频率咨询委员会，他们正在就秒的命运向国际计量局和国际计量大会提供建议。截止日期设定为2030年，他们大约有1亿秒的时间来达成协议。■
Metrologists must now decide which atomic transition is promoted to the official standard. That requires consensus among yet another team of time lords: the Consultative Committee for Time and Frequency, who are advising the BIPM and CGPM on the second’s fate. With the deadline set for 2030, they have about 100m seconds to reach an agreement. ■