In 1967, researchers from around the world gathered to answer a long-running scientific question— just how long is a second? It might seem obvious at first.
1967年，來自世界各地的研究人員齊聚一堂，回答了一個長期存在的科學問題——一秒到底有多長？一開始似乎很明顯。
A second is the tick of a clock, the swing of a pendulum, the time it takes to count to one. But how precise are those measurements? What is that length based on? And how can we scientifically define this fundamental unit of time?
一秒鐘是時鐘的滴答聲，鐘擺的擺動，是數到一的時間。但是這些測量有多精確呢？長度依據是什麼？我們怎樣才能科學地定義這個基本的時間單位呢？
For most of human history, ancient civilizations measured time with unique calendars that tracked the steady march of the night sky. In fact, the second as we know it wasn’t introduced until the late 1500’s, when the Gregorian calendar began to spread across the globe alongside British colonialism.
在人類歷史的大部分時間裡，古代文明用獨特的曆法來測量時間，這些曆法追蹤著夜空的穩定行進。事實上，我們所知的第二個公曆直到1500年代末才被引入，當時公曆開始和英國殖民主義一起在全球傳播。
The Gregorian calendar defined a day as a single revolution of the Earth about its axis. Each day could be divided into 24 hours, each hour into 60 minutes, and each minute into 60 seconds. However, when it was first defined, the second was more of a mathematical idea than a useful unit of time.
格里高利曆法把一天定義為地球繞地軸旋轉一圈。每天可分為24小時，每小時可分為60分鐘，每分鐘可分為60秒。然而，當它第一次被定義時，第二個概念更多的是一個數學概念，而不是一個有用的時間單位。
Measuring days and hours was sufficient for most tasks in pastoral communities. It wasn’t until society became interconnected through fast-moving railways that cities needed to agree on exact timekeeping.
在牧區，測量天數和時間足以完成大多數任務。直到社會通過高速鐵路相互聯繫起來，城市才需要就精確的計時達成一致。
By the 1950’s, numerous global systems required every second to be perfectly accounted for, with as much precision as possible. And what could be more precise than the atomic scale? 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.
到了20世紀50年代，無數的全球系統要求每一秒都能得到完美的解釋，儘可能精確。還有什麼比原子尺度更精確呢？早在1955，研究人員就開始開發原子鐘，它依靠物理學不變的規律來建立一個新的計時基礎。
An atom consists of negatively charged electrons orbiting a positively charged nucleus at a consistent frequency. The laws of quantum mechanics keep these electrons in place, but if you expose an atom to an electromagnetic field such as light or radio waves, you can slightly disturb an electron’s orientation.
原子由帶負電荷的電子組成，這些電子以一致的頻率繞著帶正電荷的原子核運轉。量子力學的定律使這些電子保持在適當的位置，但是如果你把原子暴露在電磁場中，比如光或無線電波，你可以稍微幹擾電子的方向。
And if you briefly tweak an electron at just the right frequency, you can create a vibration that resembles a ticking pendulum. Unlike regular pendulums that quickly lose energy, electrons can tick for centuries.
如果你把一個電子簡單地調整到合適的頻率，你就能產生一種類似滴答作響的鐘擺的振動。與快速失去能量的常規擺不同，電子可以滴答作響幾個世紀。
To maintain consistency and make ticks easier to measure, researchers vaporize the atoms, converting them to a less interactive and volatile state. But this process doesn’t slow down the atom’s remarkably fast ticking.
為了保持一致性並使蜱蟲更容易測量，研究人員蒸發了原子，將它們轉換成一種互動性和不穩定的狀態。但是這個過程並沒有減緩原子非常快的滴答聲。
Some atoms can oscillate over nine billion times per second, giving atomic clocks an unparalleled resolution for measuring time. And since every atom of a given elemental isotope is identical, two researchers using the same element and the same electromagnetic wave should produce perfectly consistent clocks.
有些原子每秒可以振蕩90多億次，使原子鐘具有無與倫比的時間測量解析度。由於某一元素同位素的每一個原子都是相同的，所以兩個研究人員使用相同的元素和相同的電磁波應該產生完全一致的時鐘。
But before timekeeping could go fully atomic, countries had to decide which atom would work best. This was the discussion in 1967, at the Thirteenth General Conference of the International Committee for Weights and Measures. There are 118 elements on the periodic table, each with their own unique properties.
但在計時完全原子化之前，各國必須決定哪種原子能發揮最好的作用。這是1967年國際度量衡委員會第十三屆大會上的討論。元素周期表上有118種元素，每種元素都有其獨特的性質。
For this task, the researchers were looking for several things. The element needed to have long-lived and high frequency electron oscillation for precise, long-term time keeping.
對於這項任務，研究人員正在尋找一些東西。該元件需要有長壽命和高頻電子振蕩，以實現精確、長期的計時。
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— as well as a simple energy level structure— meaning the active electrons are few and their state is simple to identify. Finally, it needed to be easy to vaporize.
為了很容易地跟蹤這種振蕩，它還需要有一個可靠的可測量的量子自旋，也就是說電子旋轉軸的方向，以及一個簡單的能級結構，也就是說活躍的電子很少，它們的狀態很容易識別。最後，它需要易於蒸發。
The winning atom? Cesium-133. Cesium was already a popular element for atomic clock research, and by 1968, some cesium clocks were even commercially available. All that was left was to determine how many ticks of a cesium atom were in a second.
獲勝的原子？銫-133。銫已經是原子鐘研究中的一種流行元素，到1968年，一些銫鐘甚至可以在市場上買到。剩下的就是確定一秒鐘內銫原子的滴答數。
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. 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.
會議使用了當時最精確的一秒天文測量——從一年中的天數開始，再除以一秒。當與原子的滴答聲頻率相比較時，結果正式定義一秒鐘正好是銫-133原子的9192631770滴答聲。
Today, atomic clocks are used all over the Earth— and beyond it. From radio signal transmitters to satellites for global positioning systems, these devices have been synchronized to help us maintain a globally consistent time— with precision that’s second to none.
今天，原子鐘在地球上到處都在使用，甚至更遠。從無線電信號發射器到全球定位系統的衛星，這些設備已經同步，以幫助我們保持全球一致的時間-精度是首屈一指的。