Einstein would often wrap up his work as soon as possible to contemplate the truths of the universe in his free time.

愛因斯坦經常會盡快結束工作，在空閒時間思考宇宙的真理。

It was one of these thought experiments he devised on that tram car that revolutionized modern physics forever. While receding away from the Zeitglaube clock tower, Einstein imagined what would happen if the tram car were receding at the speed of light.

正是他在電車上設計的這些思想實驗中的一個，徹底改變了現代物理學。當電車駛離 Zeitglaube 鐘樓時，愛因斯坦想象瞭如果電車以光速後退會發生什麼。

He realized that if he were to travel at 186,000 miles per second, the clock's hands would appear to completely freeze.

他意識到，如果他以每秒 186 000 英里的速度飛行，時鐘的指針就會完全凝固。

At the same time, Einstein knew that, back at the clock tower, the hands would tick along at their normal pace.

與此同時，愛因斯坦知道，回到鐘樓後，指針會以正常的速度滴答作響。

For Einstein, time had slowed down.

對愛因斯坦來說，時間已經慢了下來。

This thought blew his mind. Einstein concluded that the faster you move through space, the slower you move through time.

這個想法讓他大吃一驚。 愛因斯坦得出結論：在空間中移動得越快，在時間中移動得就越慢。

How is this possible?

這怎麼可能？

Einstein's work was heavily influenced by two of the most iconic physicists of all time.

愛因斯坦的研究深受兩位最具代表性的物理學家的影響。

First, there were the laws of motion, discovered by his idol, Isaac Newton, and second were the laws of electromagnetism, laid down by James Clerk Maxwell. Newton's laws insisted that velocities are never absolute, but always relative, so that their magnitudes must be appended by the phrase "with respect to".

首先是他的偶像牛頓發現的運動定律，其次是詹姆斯-克拉克-麥克斯韋制定的電磁學定律。 牛頓定律堅持認為，速度永遠不是絕對的，而總是相對的，是以速度的大小必須加上 "相對於 "一詞。

For instance, a train travels at 40 kilometers per hour with respect to someone at rest.

例如，相對於處於靜止狀態的人來說，火車的時速為 40 公里。

However, it only travels 20 kilometers per hour with respect to a train traveling 20 kilometers per hour in the same direction, or it travels 60 kilometers per hour with respect to another train traveling in the opposite direction at 20 kilometers per hour.

然而，相對於同方向以每小時 20 公里速度行駛的火車，它只能以每小時 20 公里的速度行駛，或者相對於反方向以每小時 20 公里速度行駛的另一列火車，它只能以每小時 60 公里的速度行駛。

This is also true for the velocities of Earth, the Sun, and the entire Milky Way galaxy. On the other hand, Maxwell found that the speed of an electromagnetic wave, such as light, is fixed, at an exorbitant 299,792,458 meters per second, regardless of who observes it.

地球、太陽和整個銀河系的速度也是如此。另一方面，麥克斯韋發現，電磁波（如光）的速度是固定的，每秒 299 792 458 米，無論誰來觀測都是如此。

However, Maxwell's notion seems incompatible with Newton's notion of relative velocities.

然而，麥克斯韋的概念似乎與牛頓的相對速度概念不相容。

If Newton's laws are truly universal, why should the speed of light be an exception?

如果牛頓定律真的具有普遍性，為什麼光速會是個例外呢？

This presented Einstein with a daunting dilemma. This conflict between the ideas of Newton and Maxwell can be demonstrated with another of Einstein's brilliant thought experiments.

這給愛因斯坦帶來了一個令人生畏的難題。 牛頓和麥克斯韋思想之間的衝突可以通過愛因斯坦的另一個傑出的思想實驗來證明。

Einstein imagined himself on a train platform witnessing two lightning bolts strike on either side of him.

愛因斯坦想象自己站在火車月臺上，目睹兩道閃電從兩側劈來。

Now, because Einstein stands precisely in the middle of the two strikes, he receives the resulting beams of light from both sides at the same time. However, things get more complicated when someone on a passing train views this event while whizzing past Einstein at the speed of light.

現在，由於愛因斯坦正好站在兩次撞擊的中間，他同時接收到了來自兩側的光束。然而，當有人在一列經過的火車上以光速從愛因斯坦身邊呼嘯而過時看到這一事件，事情就變得複雜了。

If the speed of light conforms to the rules of relativity, then the person on the train wouldn't witness the lightning strike simultaneously.

如果光速符合相對論規則，那麼火車上的人就不會同時目睹雷擊。

Logically, the light closer to the man on the train would reach him first.

從邏輯上講，離火車上的那個人更近的光會先照到他。

A measurement of the speed of light made by both men would differ in magnitude.

兩人對光速的測量結果在大小上是不同的。

This would contradict an apparently fundamental truth of the universe. Einstein had to make a difficult choice.

這顯然與宇宙的基本真理相矛盾。愛因斯坦不得不做出艱難的選擇。

Either Newton's laws were incomplete or the speed of light was not a universal constant.

要麼牛頓定律不完整，要麼光速不是一個普遍常數。

Einstein realized that the two notions could coexist with a small tweak in Newton's laws.

愛因斯坦意識到，只要對牛頓定律稍作調整，這兩個概念就可以共存。

To get rid of the discrepancy in the measurements, Einstein suggested that time itself, for the man on the train, must slow down to compensate for the decrease in speed, such that the magnitude remains a constant.

為了消除測量中的差異，愛因斯坦建議，對於火車上的人來說，時間本身必須減慢，以補償速度的降低，從而使時間的大小保持不變。

Einstein called this absurdity time dilation and his newfound theory special relativity. Newton believed that time moved unflinchingly in a single direction, forward.

愛因斯坦將這種荒謬的現象稱為時間膨脹，並將他新發現的理論稱為狹義相對論。牛頓認為，時間堅定不移地沿著單一方向前進。

Einstein, however, had just realized that time stretches and contracts, varying with velocity.

然而，愛因斯坦剛剛意識到，時間會隨著速度的變化而拉伸和收縮。

Due to its malleability, time, like space, deserved its own dimension.

由於其可塑性，時間和空間一樣，理應有自己的維度。

In fact, Einstein claimed that the two were one and the same.

事實上，愛因斯坦聲稱這兩者是同一回事。

Together, they formed a four-dimensional fabric or continuum called spacetime, upon which the mundane events of the universe would unfold. Einstein suggested that massive objects like the Sun didn't pull bodies like Earth with a mysterious, inexplicable tug, but rather curved the fabric of spacetime around them, forcing Earth to fall down into this steep valley.

它們共同構成了一個名為時空的四維結構或連續體，宇宙中的世俗事件將在這個結構或連續體上展開。愛因斯坦認為，像太陽這樣的大品質天體並沒有用神祕莫測的拉力牽引像地球這樣的天體，而是使它們周圍的時空結構彎曲，迫使地球墜入這個陡峭的山谷。

A highly simplified analogy is the dip in a trampoline made by the bowling ball.

一個高度簡化的比喻是保齡球在蹦床上產生的凹陷。

If a marble were placed on that trampoline, the marble would immediately roll towards the bowling ball in the center.

如果在蹦床上放一個彈珠，彈珠會立即滾向中心的保齡球。

This is also true for Earth's gravity.

地球引力也是如此。

We are pinned to the ground because space, so distorted by the Earth's mass, pushes us down from above. However, the slump in the fabric around Earth is not uniform, and Earth's gravity grows more intense as we move towards its center, where the curvature is at a maximum.

我們被釘在地面上，是因為空間被地球的品質扭曲，把我們從上往下推。然而，地球周圍結構的傾斜度並不均勻，當我們向地球中心移動時，地球引力會變得越來越大，因為那裡的曲率是最大的。

Therefore, like the marble on the trampoline, an object that falls towards the Earth accelerates as it races towards the center of the planet.

是以，就像蹦床上的彈珠一樣，落向地球的物體在飛向地球中心時會加速。

It falls faster when just above the surface than it does, say, when it is slightly above the atmosphere. But hey, according to special relativity, the faster you move through space, the slower you move through time.

它在地表上方的下落速度要快於略高於大氣層時的下落速度。 但是，根據狹義相對論，在空間中移動得越快，在時間中移動得就越慢。

This means that time runs slower on Earth's surface than it does above the atmosphere.

這意味著，地球表面的時間比大氣層上的時間要慢。

Now, because different planets have different masses and thus different gravitational strengths, they also accelerate objects at different rates.

現在，由於不同行星的品質不同，是以引力強度也不同，它們對物體的加速度也不同。

As we have learned, this means a variable passage of time. This is what happens in the movie Interstellar when the protagonists land on a planet in the proximity of a black hole.

我們已經知道，這意味著時間的流逝是不固定的。這就是電影《星際穿越》（Interstellar）中主角們在黑洞附近的星球上著陸時發生的情況。

The gravity on the planet is so severe that one hour on the surface is equivalent to seven years on Earth.

地球上的重力非常大，在地表上的一小時相當於地球上的七年。

To understand how motion affects time, let's consider the simplest timekeeping mechanism.

要了解運動如何影響時間，讓我們來看看最簡單的計時裝置。

A second passes each time the photon is reflected. Let's imagine two people, one in a spaceship slightly above Earth's atmosphere, and the second on top of a small hill just above the Earth's surface.

光子每反射一次，就會過去一秒鐘。 讓我們假設有兩個人，一個在略高於地球大氣層的宇宙飛船中，另一個在地球表面上方的小山頂上。

Both are watching a man fall from space towards the ground.

兩人都在看著一個人從太空墜落地面。

Let's say that the falling man is carrying the photon clock like explained a moment ago.

假設墜落者攜帶的是光子鍾，就像剛才解釋的那樣。

What do each of the two men observe as the man falls past them? What they observe is eerily similar to what a stationary person would observe when watching a ball bounce in a moving train.

當這個人從他們身邊落下時，這兩個人分別觀察到了什麼？ 他們所觀察到的，與一個靜止的人在觀看一個球在行駛的火車上彈跳時所觀察到的非常相似。

As the man falls from space, the light in his clock would appear to move in triangles to the two observers.

當這個人從太空墜落時，在兩個觀察者看來，他的時鐘中的光會呈三角形移動。

This would mean that the light travels a longer distance, consequently stretching the duration of a second. It is obvious that the length of triangles the light traces, and therefore the duration of a second, is proportional to the velocity of the falling man.

這意味著光線傳播的距離更長，從而延長了一秒的持續時間。 很明顯，光線所經過的三角形長度，也就是一秒的持續時間，與墜落者的速度成正比。

When we recall that objects closer to the center of the planet fall faster, we can determine that time would appear to pass slower to the man on the hill than it does to the man in the spaceship above. Of course, the difference is infinitesimal.

當我們回想離地球中心越近的物體下落得越快時，我們就可以確定，對於山丘上的人來說，時間的流逝似乎比上面飛船裡的人要慢。 當然，兩者之間的差別是微乎其微的。

The difference between the time measured by clocks at the tops of mountains and at the surface of Earth is a matter of nanoseconds.

山頂上的時鐘和地球表面的時鐘所測得的時間僅相差納秒。

Time dilation affects every clock, whether it relies on basic electromagnetism or a complex combination of electromagnetism and Newton's laws of motion.

時間膨脹會影響每一個時鐘，無論它是依靠基本電磁學還是電磁學與牛頓運動定律的複雜組合。

In fact, even biological processes are slowed down.

事實上，連生物過程都會減慢。

Yes, that's right.

是的，沒錯。

Your head is slightly older than your feet.

你的頭比你的腳稍大一些。