Who decides how long a second is? - John Kitching

譯者: 柏陞 郭 審譯者: Amanda Zhu

In 1967, researchers from around the world

1967 年，來自世界各地的 研究者齊聚一堂，

gathered to answer a long-running scientific question—

試圖解答一個 長久以來存在的科學問題：

just how long is a second?

究竟一秒鐘有多長?

It might seem obvious at first.

起初答案似乎看起來很明顯。

A second is the tick of a clock,

一秒鐘是時鐘滴答一聲、

the swing of a pendulum, the time it takes to count to one.

鐘擺擺動一下， 或是從零數到一的時間。

But how precise are those measurements?

但是這些測量方法真的準確嗎？

What is that length based on?

一秒鐘的長度以什麼為基準？

And how can we scientifically define this fundamental unit of time?

而我們該如何以科學方法 定義這個時間的基本單位?

For most of human history, ancient civilizations

在人類歷史中，許多古文明 都曾透過觀察星辰軌跡

measured time with unique calendars

that tracked the steady march of the night sky.

做出獨一無二的曆法來測量時間。

In fact, the second as we know it wasn’t introduced until the late 1500’s,

事實上，秒數的觀念 始於 16 世紀末，

when the Gregorian calendar began to spread across the globe

隨著英國在全球各地殖民， 格里曆也傳播至世界的每個角落。

alongside British colonialism.

The Gregorian calendar defined a day as a single revolution of the Earth

格里曆，即西曆，將一天定義為：

地球繞著其軸心自轉一圈。

about its axis.

Each day could be divided into 24 hours, each hour into 60 minutes,

每一天劃分為 24 個小時， 每個小時分為 60 分鐘，

and each minute into 60 seconds.

每一分鐘又分為 60 秒。

However, when it was first defined,

然而，秒的概念在剛定義時，

the second was more of a mathematical idea than a useful unit of time.

與其說是一組實用的時間單位， 不如說它是個數學概念。

Measuring days and hours was sufficient for most tasks in pastoral communities.

鄉村社會裡大部分的差事 以日和小時測量時間就能應付。

It wasn’t until society became interconnected

但是當社會因快速的鐵路 而使得來往更密切，

through fast-moving railways that cities needed to agree on exact timekeeping.

每個城市都需要一致的準確時間。

By the 1950’s, numerous global systems required every second

到了 1950 年代，全球化的系統

都要求儘可能精準，一秒也不能浪費。

to be perfectly accounted for, with as much precision as possible.

And what could be more precise than the atomic scale?

那麼還有什麼能比 原子尺度還要更精準的呢?

As early as 1955, researchers began to develop atomic clocks,

早在 1955 年，研究者 就開始研發原子鐘，

which relied on the unchanging laws of physics

靠著永恆不變的物理定律

to establish a new foundation for timekeeping.

為計時建立一個全新的基礎。

An atom consists of negatively charged electrons

一個原子包含了許多帶負電的電子，

orbiting a positively charged nucleus at a consistent frequency.

以規律的頻率環繞帶正電的原子核。

The laws of quantum mechanics keep these electrons in place,

依照量子力學的定律， 這些電子會在固定的軌道環繞。

but if you expose an atom to an electromagnetic field

但如果你將原子暴露在電磁場下，

such as light or radio waves,

像是光或是無線電波，

you can slightly disturb an electron’s orientation.

就能夠稍微影響電子的移動軌道。

And if you briefly tweak an electron at just the right frequency,

假使你以一個特定的頻率 稍微影響一個電子，

you can create a vibration that resembles a ticking pendulum.

就能使它如鐘擺般振動。

Unlike regular pendulums that quickly lose energy, electrons can tick for centuries.

但與快速失去能量的鐘擺不同， 電子能夠持續振動幾個世紀。

To maintain consistency and make ticks easier to measure,

為了保持振動一致和更容易測量，

researchers vaporize the atoms,

研究者將原子汽化，

converting them to a less interactive and volatile state.

使之轉換為互動性 和揮發性較低的狀態。

But this process doesn’t slow down the atom’s remarkably fast ticking.

但這個程序並不會使原本 出奇快速的電子慢下來。

Some atoms can oscillate over nine billion times per second,

有些原子能在一秒內擺動 90 億次以上，

giving atomic clocks an unparalleled resolution for measuring time.

讓原子鐘能夠以 前所未有的準確度測量時間。

And since every atom of a given elemental isotope is identical,

以某個元素而言， 同一種同位素的原子都相同，

two researchers using the same element and the same electromagnetic wave

兩位研究者認為若用相同的元素 和相同的電磁波

should produce perfectly consistent clocks.

應該就能製造出完全精確的時鐘。

But before timekeeping could go fully atomic,

但在計時能夠完全原子化之前，

countries had to decide which atom would work best.

各國必須先決定該用哪一種原子最好。

This was the discussion in 1967,

這也是 1967 年

at the Thirteenth General Conference of the International Committee

在國際度量衡委員會的 第十三屆大會上討論的議題。

for Weights and Measures.

There are 118 elements on the periodic table,

在元素週期表上的 118 個元素，

each with their own unique properties.

各自都有獨一無二的特性。

For this task, the researchers were looking for several things.

為了達成這項任務，研究者的考慮 是根據幾項必要的條件。

The element needed to have long-lived

首先，這個元素必須能夠長期存在，

and high frequency electron oscillation for precise, long-term timekeeping.

並具有高擺動頻率的電子， 計時才能準確、持久。

To easily track this oscillation,

要讓此擺動更易於追蹤，

it also needed to have a reliably measurable quantum spin—

這個元素也要有 可精準量測的量子自旋，

meaning the orientation of the axis about which the electron rotates—

也就是要知道電子 繞著軸心轉動的方向。

as well as a simple energy level structure—

這個元素也要有一個 簡單的能階結構，

meaning the active electrons are few and their state is simple to identify.

意思是活動的電子數量很少， 且能夠輕易辨認它們的狀態。

Finally, it needed to be easy to vaporize.

最後一點，這個元素必須易於汽化。

The winning atom? Cesium-133.

最後是哪個原子獲選呢?

答案是銫-133。

Cesium was already a popular element for atomic clock research,

在原子鐘的研究中， 銫早已是十分熱門的元素。

and by 1968, some cesium clocks were even commercially available.

到了 1968 年，一些原子鐘 甚至能在市面上買到。

All that was left was to determine how many ticks of a cesium atom

最後就要找出銫原子 能在一秒內振動多少下。

were in a second.

The conference used the most precise astronomical measurement

大會決定使用當時 測量時間最精確的科學方法

of a second available at the time—

來測量一秒鐘的時間：

beginning with the number of days in a year and dividing down.

由一年的天數開始往下劃分。

When compared to the atom’s ticking rate,

在比較原子的振動速率後，

the results formally defined one second

正式將一秒鐘定義為

as exactly 9,192,631,770 ticks of a cesium-133 atom.

銫原子-133振動

9,192,631,770 下。

Today, atomic clocks are used all over the Earth— and beyond it.

現在，原子鐘適用於整個地球， 甚至是外太空。

From radio signal transmitters to satellites

從無線電訊號傳送器

到全球定位系統的衛星，

for global positioning systems,

these devices have been synchronized

這些裝置都已經和原子鐘同步，

to help us maintain a globally consistent time—

從而幫助我們維持全球一致的時間，

with precision that’s second to none.

靠的是它無與倫比的準確度。