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In 1967, researchers from around the world gathered to answer a long-running scientific question -- just how long is a second?
1967年，世界各地的科研人员齐聚一堂，共同研究一个困扰学界已久的问题--一秒究竟多长呢？
It might seem obvious at first. A second is the tick of a clock, the swing of a pendulum, the time it takes to count to one.
乍听之下似乎答案很明显。一秒不就是时钟滴答一声，钟摆摆动一下，数一个数花费的时间。
But how precise are those measurements? What is that length based on?
但这些测量方法精确吗？时长又是基于什么而定的呢？
And how can we scientifically define this fundamental unit of time?
我们如何科学地定义秒这个最基础的时间单位呢？
For most of human history, ancient civilizations measured time with unique calendars that tracked the steady march of the night sky.
人类历史上很长的一段时间，古代文明都在使用一种记录夜空稳步变化的特殊日历来计时。
In fact, the second as we know it wasn't introduced until the late 1500's,
事实上，直到16世纪，随着英国殖民主义发展，
when the Gregorian calendar began to spread across the globe alongside British colonialism.
公历得以在全球普及这才有了秒这个概念。
The Gregorian calendar defined a day as a single revolution of the Earth about its axis.
公历将一天定义为地球绕轴自转一周。
Each day could be divided into 24 hours, each hour into 60 minutes, and each minute into 60 seconds.
一天分为24小时，每小时60分钟，一分钟60秒。
However, when it was first defined, the second was more of a mathematical idea than a useful unit of time.
然而，最开始，秒更像一个数学概念，而非实用的时间单位。
Measuring days and hours was sufficient for most tasks in pastoral communities.
在乡村，靠天和小时计时就已经够用了。
It wasn't until society became interconnected through fast-moving railways that cities needed to agree on exact timekeeping.
直到四通八达的高速铁路将人类社会紧密联系起来，城市之间才需要在精准计时方面达成一致。
By the 1950's, numerous global systems required every second to be perfectly accounted for, with as much precision as possible.
到了20世纪50年代，无数的全球体系要求每一秒钟都要准确计算，每秒钟都要尽可能地精准。
And what could be more precise than the atomic scale?
那么还有什么能比原子标度更精准呢？
As early as 1955, researchers began to develop atomic clocks, which relied on the unchanging laws of physics to establish a new foundation for timekeeping.
早在1955年，科研人员就开始开发原子钟了，这种时钟基于物理学的不变性原理为计时打下了新的基础。
An atom consists of negatively charged electrons orbiting a positively charged nucleus at a consistent frequency.
原子内带负电荷的电子周期性地绕带正电荷的原子核转动。
The laws of quantum mechanics keep these electrons in place,
量子力学定律将电子保持在固定的距离，
but if you expose an atom to an electromagnetic field such as light or radio waves, you can slightly disturb an electron's orientation.
但如果原子暴露在电磁场中如光或无线电波，电子的朝向会受到轻微干扰。
And if you briefly tweak an electron at just the right frequency, you can create a vibration that resembles a ticking pendulum.
如果按照正确的频率，短暂地拉扯电子，就能创造出像嘀嗒摆动的钟摆一样的震动。
Unlike regular pendulums that quickly lose energy, electrons can tick for centuries.
一般的钟摆能量衰减地很快，但电子却能运转几百年之久。
To maintain consistency and make ticks easier to measure, researchers vaporize the atoms, converting them to a less interactive and volatile state.
为了保持一致，并更易于测量电子的摆动，科研人员将原子汽化把其转化为一种交互性低且稳定的状态。
But this process doesn't slow down the atom's remarkably fast ticking.
但这并未减缓原子惊人的运转速度。
Some atoms can oscillate over nine billion times per second, giving atomic clocks an unparalleled resolution for measuring time.
一些原子可以每秒振荡超90亿次，原子钟因而具备无与伦比的计时精准度。
And since every atom of a given elemental isotope is identical,
由于特定元素的同位素的每个原子完全相同，
two researchers using the same element and the same electromagnetic wave should produce perfectly consistent clocks.
两个科研人员使用相同元素和相同的电磁波应该可以制作出完全一致的钟表。
But before timekeeping could go fully atomic, countries had to decide which atom would work best.
但原子计时完全实现之前，各国首先要找出哪个原子最好用。
This was the discussion in 1967, at the Thirteenth General Conference of the International Committee for Weights and Measures.
1967年，第十三届国际度量衡委员会大会便是围绕这个问题展开的。
There are 118 elements on the periodic table, each with their own unique properties.
元素周期表上有118种元素，每种元素都有其独特的特性。
For this task, the researchers were looking for several things.
对于计时这项任务，科研人员的要求有如下几点。
The element needed to have long-lived and high frequency electron oscillation for precise, long-term timekeeping.
这种元素的原子振荡需要持久且高频，这样才能精准长期地计时。
To easily track this oscillation, it also needed to have a reliably measurable quantum spin -- meaning the orientation of the axis about which the electron rotates
为了便于追踪其振荡，这种元素的量子自旋--即电子旋转所绕的轴的方向，
as well as a simple energy level structure -- meaning the active electrons are few and their state is simple to identify.
和一种简单的能级结构--即活性电子少且状态易辨认，都需要可靠易测。
Finally, it needed to be easy to vaporize. The winning atom? Cesium-133.
最后，还要容易汽化。那么获胜的是哪种原子呢？铯-133。
Cesium was already a popular element for atomic clock research, and by 1968, some cesium clocks were even commercially available.
铯原子此前就已经是原子钟研究的大热元素之一，到了1968年，在市面上已经可以买到一些铯原子钟了。
All that was left was to determine how many ticks of a cesium atom were in a second.
最后要做的就是决定铯原子摆动多少下算作一秒钟。
The conference used the most precise astronomical measurement of a second available at the time -- beginning with the number of days in a year and dividing down.
大会使用了当时最精密的天文测量方法计算一秒的长度--由一年中的天数开始，往下进行时间分割。
When compared to the atom's ticking rate, the results formally defined one second as exactly 9,192,631,770 ticks of a cesium-133 atom.
对比原子的摆动速度，最终确定一秒钟为铯-133号原子正好摆动9,192,631,770的用时。
Today, atomic clocks are used all over the Earth -- and beyond it.
如今，原子钟风靡全球--甚至用到了太空。
From radio signal transmitters to satellites for global positioning systems,
从无线电信号发射器到定位卫星，
these devices have been synchronized to help us maintain a globally consistent time -- with precision that's second to none.
这些设备全部协调同步用于维持时间的全球一致性--且精准度无可比拟。
