It's always running forward without waiting for anyone or anything.

它總是向前奔跑，不等待任何人或任何事。

But is time really like this?

但時間真的是這樣的嗎？

Special relativity theory tells us that time is really not like this.

狹義相對論告訴我們，時間其實並非如此。

Time is not the same for all across the whole universe.

在整個宇宙中，所有人的時間都是不一樣的。

Different observers feel different times at different places.

不同的觀察者在不同的地點感受到不同的時間。

So why is this?

為什麼會這樣？

How is this possible?

這怎麼可能？

Let's see.

讓我們看看

Einstein's special relativity theory starts with two basic ideas.

愛因斯坦的狹義相對論始於兩個基本思想。

The first postulates that the laws of physics are the same in all frames of reference.

第一個假設是，物理定律在所有參照系中都是相同的。

Imagine your friend is standing at the platform of a train station and you are in a train traveling at a constant velocity.

想象一下，你的朋友站在火車站的月臺上，而你在一列勻速行駛的火車上。

Constant velocity means an object is moving in a fixed speed and fixed direction.

勻速運動是指物體以固定的速度和方向運動。

This motion doesn't have any acceleration, deceleration, and does not change direction.

這種運動沒有任何加速度、減速度，也不改變方向。

So your friend is on the platform.

所以你的朋友在平臺上。

From his perspective, he is at rest and you are heading forward.

從他的角度來看，他在休息，而你在前進。

But from your perspective, who is traveling at a constant speed, does not feel any motion.

但從你的角度來看，以恆定速度行進的你並沒有感覺到任何運動。

And so you also can claim to be at rest.

是以，你也可以說自己是在休息。

The rest of the world can appear to be running towards you.

世界上的其他事物似乎都在向你奔來。

If the train windows are closed, there is no outside reference point.

如果火車窗戶關閉，就沒有外部參照點。

Thus, you believe you are at rest.

是以，你認為自己處於靜止狀態。

As all the laws of physics will apply to both your friend and you, both of you can be considered to be at rest.

由於所有物理定律都適用於你和你的朋友，所以你們都可以被視為處於靜止狀態。

To explain more simply, let's take a look at a mosquito in a flying plane.

為了更簡單地解釋，讓我們來看看飛行飛機上的蚊子。

Now that the mosquito is sitting on the back seat, it wants to fly to the pilot room.

現在蚊子坐在後座上，它想飛到駕駛室去。

If the laws of physics is not the same for a moving plane, it must travel above the flight speed.

如果物理定律對飛行中的飛機來說不一樣，那麼它的飛行速度一定高於飛行速度。

But there is nothing like that and the mosquito feels at rest on a moving plane.

但是，沒有這樣的感覺，蚊子在移動的飛機上感覺是靜止的。

So the normal effort is enough to reach the pilot's room.

是以，正常的努力就足以到達駕駛室。

Now let's come back to the train scenario.

現在讓我們回到火車上的情景。

You are in a moving train and your friend is on the platform.

你在一列行駛中的火車上，你的朋友在站臺上。

Whatever your friend is doing on the platform, you can also do in the moving train.

無論你的朋友在站臺上做什麼，你都可以在動車上做。

However, the speed is not the same for all observers.

然而，所有觀測者的速度並不相同。

Let's say the train is moving at a constant speed of 100 km per hour and you throw a football at the speed of 20 km per hour.

假設火車以每小時 100 千米的速度勻速行駛，而你以每小時 20 千米的速度投擲足球。

From your perspective, the ball is traveling 20 km per hour because you are at rest inside the train.

從你的角度看，球的時速為 20 公里，因為你在火車內處於靜止狀態。

But from your friend's perspective who is on the platform watching the train, he feels the ball is traveling at a whopping 120 km per hour.

但從你朋友的角度來看，他在站臺上看著火車，他覺得球的時速高達 120 公里。

Let's say you have another friend who is running on the track towards your train at 10 km per hour.

假設你有另一個朋友，他正在軌道上以每小時 10 公里的速度向你的火車跑來。

Now if he measured the ball speed, he would judge it to be 130 km per hour.

現在，如果他測量球速，就會判斷為每小時 130 公里。

How can the same moving object be showing different speeds?

同一個運動物體怎麼會有不同的速度？

Because speed is relative, it can vary from one to another.

因為速度是相對的，所以會因人而異。

Different observers can measure different speeds on the same moving object.

不同的觀察者可以測量出同一運動物體的不同速度。

Special Relativity's other postulate is that the speed of light is the same for all observers.

狹義相對論的另一個假設是，所有觀測者的光速都是相同的。

The speed of light is constant and it always shows the same value for all frames of reference.

光速是恆定的，它在所有參照系中始終顯示相同的值。

Let's take a simple example.

讓我們舉一個簡單的例子。

You and your friend are standing opposite each other.

你和你的朋友相對而立。

He has a flashlight and your friend flashes the light.

他拿著手電筒，而你的朋友在閃燈。

If you could measure the speed of light quick enough, you would measure it at around 300,000 km per second.

如果你能足夠快地測量光速，你就能測出它的速度約為每秒 30 萬公里。

No surprises yet, as you are both at rest.

還沒有驚喜，因為你們都在休息。

But now you are running towards your friend at the phenomenal speed of 10,000 km per second and he flashes the light.

但現在，你正以每秒一萬公里的驚人速度向你的朋友跑去，而他卻閃了一下光。

You would measure it still showing the same value of 300,000 km per second instead of showing 310,000 km per second.

你測量到的數值仍然是每秒 30 萬公里，而不是每秒 31 萬公里。

This is due to the fact that the speed of light is constant.

這是因為光速是恆定的。

It's the same for all the observers, no matter where that observer is at rest or in motion.

無論觀察者處於靜止還是運動狀態，對所有觀察者來說都是一樣的。

In order to maintain the constant nature of the speed of light, space and time experience weird features.

為了保持光速的恆定性，空間和時間會出現一些奇怪的特徵。

So what is time?

那麼，什麼是時間？

Even today we still don't know exactly what time is, but we do have some basic understanding.

即使在今天，我們仍然不知道時間到底是什麼，但我們已經有了一些基本的認識。

We've all agreed that all the clocks in our world tick at the same rate.

我們都同意，世界上所有的時鐘都以相同的速度滴答作響。

We've all agreed on how long some things take to happen and what things happen at the same moment.

我們都同意，有些事情需要多長時間才能發生，有些事情會在同一時刻發生。

These basic things of time we experience in our everyday life.

我們在日常生活中體驗到這些基本的時間。

But the constant nature of light says that it's all wrong.

但光的恆定性表明，這一切都是錯誤的。

How?

怎麼做？

Imagine there are two nations, nation A and nation B, and they want to sign an agreement.

設想有兩個國家，A 國和 B 國，他們想要簽署一項協議。

The problem is that they both want to sign the agreement at the same moment, and they found the procedure too.

問題是，他們都想在同一時刻簽署協議，而且他們也發現了這一程序。

The procedure is that the leader of both nations need to sit at the opposite sides of the table, with a light bulb fixed over the middle of the table.

程序是，兩國領導人需要坐在桌子的兩邊，桌子中間固定一個燈泡。

Turn on the light bulb, and when the light reaches both leaders' eyes, they can both sign simultaneously, because they already know that light always travels at the same speed.

打開燈泡，當光線到達兩位領導者的眼睛時，他們可以同時做出手勢，因為他們已經知道光總是以相同的速度傳播。

Both nations' leaders decide to sit exactly the same distance from the bulb.

兩國領導人決定坐在距離燈泡完全相同的位置上。

So the event begins.

活動開始了。

They turn on the light, and the light reaches both leaders at the same moment.

他們打開燈，燈光在同一時刻照亮了兩位領導人。

The agreement is signed simultaneously by leaders, and everyone is happy with the result.

雙方領導人同時簽署協議，結果皆大歡喜。

A few months later, they decide to sign another agreement, but this time, both leaders want to do it differently.

幾個月後，他們決定簽署另一份協議，但這一次，雙方領導人都希望以不同的方式行事。

Even though they have many different options, both leaders have an extreme love of trains.

儘管他們有很多不同的選擇，但兩位領導人都對火車情有獨鍾。

So they set up the same scenario in a moving train.

於是，他們在一列行駛中的火車上設置了同樣的場景。

The train goes on.

火車繼續前進。

The light flashes and goes to each leader, reaching them simultaneously.

燈光閃爍，同時照射到每個領導者身上。

They sign, and again everybody in the train is happy with the result.

他們簽了字，火車上的每個人再次對結果感到滿意。

But the people standing on the platform started fighting, because the people from nation B complained that nation A leader signed the agreement first.

但是，站在平臺上的人們開始爭吵，因為 B 國人抱怨 A 國領導人先簽署了協議。

How did they come up with that conclusion?

他們是怎麼得出這個結論的？

Now take a look at that event again.

現在再來看看這個事件。

The train is moving.

火車開動了

The light flashes.

燈光閃爍。

Leader A is coming towards the flash.

領隊 A 正朝閃光燈走來。

Leader B is moving away from the flash.

領導人 B 正在遠離閃光燈。

So they saw that the flash first reached leader A, because he was moving towards the flash.

是以，他們看到閃光首先到達了領導者 A，因為他正朝著閃光移動。

They already know that light always travels at constant speed.

他們已經知道光總是以恆定的速度傳播。

Leader B was moving away from the flash, so it took a longer time to reach leader B compared with leader A.

領導者 B 正在遠離閃光燈，是以與領導者 A 相比，到達領導者 B 所需的時間更長。

From the perspective of those watching the event from the platform, both leaders didn't sign at the same moment.

從站臺上觀看活動的人的角度來看，兩位領導人並不是在同一時刻簽署的。

But from the leader's perspective, who were in the moving train at constant velocity, claimed that they signed at the same moment.

但從領導者的角度來看，他們在勻速行駛的列車上，聲稱他們是在同一時刻簽字的。

We already know that when an object travels at a constant speed from the perspective or frame of reference, it can claim it is at rest.

我們已經知道，當一個物體從視角或參照系中以恆定的速度運動時，它可以聲稱自己處於靜止狀態。

Now let's move on from the platform perspective.

現在，讓我們從平臺的角度出發。

Let's make a simple to understand event.

讓我們做一個簡單易懂的活動。

We will draw a line when the flash takes place, and draw a line when the flash reaches leader A versus leader B.

我們將在閃光發生時畫一條線，並在閃光到達領導者 A 和領導者 B 時畫一條線。

So from this line, we can clearly see that the flash reaching leader B needs to travel a longer distance compared to leader A.

是以，從這條線上我們可以清楚地看到，與領隊 A 相比，到達領隊 B 的閃光需要走更長的距離。

Light always travels at the same speed.

光總是以相同的速度傳播。

If it has to travel further, it takes a longer time.

如果要走得更遠，則需要更長的時間。

So who is right here?

那麼，這裡誰是對的？

The leaders inside the train or the people watching from outside?

是列車內的上司，還是列車外的觀眾？

The reality is that both are right.

現實情況是，兩者都是對的。

So what is it telling us?

那麼，它在告訴我們什麼呢？

The constant nature of speed of light means that the events take place at the same time even though from the perspective of one group of people, the events don't take place at the same time as from the perspective of other groups.

光速的恆定性意味著事件發生在同一時間，即使從一組人的角度來看，事件發生的時間與從其他組人的角度來看並不相同。

One group of people don't realize any time difference and the other group of people witness a time difference on the same event.

一組人沒有意識到任何時差，而另一組人則目睹了同一事件的時差。

It is not an optical illusion.

這不是錯覺。

It is reality.

這就是現實。

The thing that happened from one person's perspective does not happen at the same moment as from the other person's perspective if they are in relative motion.

如果兩個人處於相對運動中，從一個人的角度看發生的事情與從另一個人的角度看發生的事情並不在同一時刻。

Now we move on to the next step.

現在我們進入下一步。

Let's take a clock.

我們來看看時鐘。

Even though we have many different types of clocks to explain time dilation, we will take a light clock to understand this more easily.

儘管我們有許多不同類型的時鐘來解釋時間膨脹，但我們還是以光鍾為例，以便更容易地理解這一點。

The beauty of the light clock is that its mechanism is simple.

光鐘的美妙之處在於它的機械裝置非常簡單。

However, it is not different from any other clock we have.

不過，它與我們擁有的其他時鐘並無不同。

In a light clock, there are two mirrors with a light ball bouncing between them.

在光鍾中，有兩面鏡子，光球在兩面鏡子之間跳動。

Let's consider the light ball as light.

讓我們把光球看作光。

Every time the light ball goes up and down or tick-tocks, it counts as one second.

光球每上下移動或滴答作響一次，都算一秒鐘。

Now we take another light clock.

現在我們再看一個光鍾。

We will keep one clock in stationary and we will put another clock into motion.

我們將讓一個時鐘保持靜止，讓另一個時鐘開始運轉。

So with the clock in motion, we can clearly see the light ball is moving at a diagonal path.

是以，隨著時鐘的運動，我們可以清楚地看到光球以對角線軌跡運動。

Also, we can see the counts on both clocks are different.

此外，我們還可以看到兩個時鐘的計數是不同的。

Why?

為什麼？

Look at the trajectory on the moving clock.

看看移動時鐘的軌跡。

It's double diagonal.

是雙對角線。

The amount of one full up and down movement takes longer compared to a stationary clock.

與靜止的時鐘相比，一個完整的上下運動所需的時間更長。

This is because light needs to travel a longer distance, but the speed of light is constant and its speed always is the same for all observers.

這是因為光需要傳播更長的距離，但光速是恆定的，對於所有觀測者來說，光速總是相同的。

So in the moving clock, each tick-tock happens at a slower rate.

是以，在移動的時鐘中，每一個滴答聲都以較慢的速度發生。

Slower tick-tocks mean time runs slowly.

滴答聲變慢意味著時間流逝緩慢。

In the moving clock, one second takes longer than the stationary clock.

在運動的時鐘中，一秒鐘的時間比靜止的時鐘長。

Time runs slowly on the moving clock only for the outside observer who is watching this event.

時間在移動的時鐘上緩慢流逝，這只是對於正在觀看這一事件的外部觀察者而言。

But for whoever is inside the clock, they don't feel any time difference because they also travel along with the light clock, so they are unable to feel the light ball take the diagonal path.

但對於在時鐘內的人來說，他們感覺不到任何時間差，因為他們也跟著光鍾一起移動，所以他們無法感覺到光球走的是對角線軌跡。

So they don't feel time slowing down.

是以，他們感覺不到時間在變慢。

As we already know, when an object moves at constant velocity, it can claim to be at rest.

我們已經知道，當一個物體以恆定的速度運動時，它可以說是處於靜止狀態。

So when an object is in motion, its time slows down.

是以，當物體運動時，它的時間會變慢。

But why don't we see time dilation in our everyday lives?

但為什麼我們在日常生活中看不到時間膨脹呢？

Let's take a look at the time dilation formula.

讓我們來看看時間膨脹公式。

Let's take two clocks, one on Earth and another on a rocket in space.

讓我們來看看兩個時鐘，一個在地球上，另一個在太空火箭上。

Let's make the rocket move at 10% of the speed of light.

讓火箭以 10%的光速前進吧。

At this speed, we can't really see the time difference.

在這種速度下，我們真的看不出時間差。

There is a time difference, but it is still hard to notice.

雖然有時間差，但還是很難察覺。

Now increase the velocity of the rocket to 70% of the speed of light, and we can really see the time difference.

現在把火箭的速度提高到光速的 70%，我們就能真正看到時間差了。

The elapsed time on the rocket ship is less than from the perspective of those of us here on Earth.

從我們地球人的角度來看，火箭飛船上的時間要短一些。

Now let's move the rocket 98% of the speed of light, and now we can really experience the extreme difference between two clocks.

現在，讓火箭以 98% 的光速前進，我們就能真正體驗到兩個時鐘之間的巨大差異了。

So from our view who are on the Earth, the clock in motion time is ticking off very, very slowly.

是以，從我們地球人的角度來看，運動時間的時鐘走得非常非常慢。

To understand more deeply, we will take another example.

為了更深入地理解，我們再舉一個例子。

The light clock is placed on the moving train.

燈鐘被放置在行駛的列車上。

Now our train velocity is at 10% of the speed of light.

現在我們的列車速度是光速的 10%。

At this velocity, there is some time difference, but not much.

在這個速度下，會有一些時間差，但不會太大。

It looks like it is almost following the exact path of the light ball going straight up and down.

看起來，它幾乎是沿著光球的軌跡直上直下。

Then we increase the velocity to 70% of the speed of light.

然後，我們將速度提高到光速的 70%。

At this point, we can clearly see the diagonal path of the light ball.

此時，我們可以清楚地看到光球的對角線軌跡。

Now if we move the train extremely near the speed of light at 99.99%, there is a really big difference in the diagonal path.

現在，如果我們以 99.99% 的速度讓列車以極其接近光速的速度行駛，對角線軌跡就會有很大的不同。

So here, the light ball doesn't make a single tick while the stationary clock continues at its normal rate and completes a lot of tick-tocks.

是以，在這裡，光球沒有發出一聲嘀嗒聲，而靜止的時鐘則以正常的速度繼續運轉，並完成了大量的嘀嗒聲。

Therefore, time dilation is a universal phenomenon.

是以，時間膨脹是一種普遍現象。

However, its effects become significantly noticeable only when moving near the speed of light.

不過，只有在接近光速時，它的影響才會變得明顯。

At everyday speeds, time dilation does occur, but its impact is extraordinarily small.

在日常速度下，時間膨脹確實會發生，但其影響非常小。

We have seen so far everything is theoretical and formula-based, thus we can't actually conclude time dilation is happening when an object is in motion.

到目前為止，我們所看到的一切都是基於理論和公式的，是以我們無法真正得出物體運動時會發生時間膨脹的結論。

So are there any experimental things that have been proven by real life?

那麼，有沒有經過現實生活驗證的實驗性東西呢？

There are many experiments proving that time dilation really exists.

有許多實驗證明時間膨脹確實存在。

Let's take two simple experiments.

讓我們做兩個簡單的實驗。

The first example is the most straightforward experiment that verified time slows down for the moving clock.

第一個例子是最直接的實驗，它驗證了移動時鐘的時間變慢了。

In 1971, scientists Heffele and Keating took two synchronized atomic clocks.

1971 年，科學家赫費爾和基廷製作了兩個同步原子鐘。

One was left stationary, and another one was placed on a plane.

一個靜止不動，另一個被放在飛機上。

The plane flew all around the world a few times.

飛機在世界各地飛過幾次。

When the plane landed, they compared the two clocks.

飛機著陸後，他們比較了兩個時鐘。

They found that a different amount of time had elapsed on each clock.

他們發現每個時鐘上的時間都不一樣。

The difference was 60 nanoseconds, in fact.

事實上，兩者相差 60 納秒。

The time difference between them is exactly what Einstein had predicted.

它們之間的時間差正是愛因斯坦所預言的。

So this experiment established very directly that time on moving clocks ticks at a very different rate from stationary ones.

是以，這個實驗非常直接地證明，運動時鐘上的時間滴答作響的速度與靜止時鐘上的時間滴答作響的速度截然不同。

Another experiment was conducted with the lifetime of a muon.

另一項實驗是針對μ介子的壽命進行的。

Muons are particles similar to electrons, but with a lifetime of only 2.2 x 10-6 seconds.

μ介子是一種與電子類似的粒子，但壽命只有 2.2 x 10-6 秒。

After that, it breaks apart.

之後，它就會碎裂。

So after 2.2 x 10-6 seconds, its lifetime ends.

是以，在 2.2 x 10-6 秒之後，它的壽命就結束了。

Scientists found that sometimes muons take a little extra time to disintegrate.

科學家們發現，有時μ介子的分解需要額外的時間。

That's when muon particles are in motion, and they take longer to break apart.

這時μ介子粒子處於運動狀態，它們需要更長的時間才能分裂。

However, muons themselves always take the same time to disintegrate.

然而，μ介子本身的分解時間總是相同的。

But our perception as people who are watching this event experiences muon particles take a longer time to disintegrate.

但是，作為觀看這一事件的人，我們的感覺是，μ介子粒子需要更長的時間才能解體。

But muon particles still claiming that they disintegrated exactly after 2.2 x 10-6 seconds.

但μ介子粒子仍然聲稱，它們在 2.2 x 10-6 秒後正好解體。

However, this observation proved that time really slows down when an object is in motion.

然而，這一觀察結果證明，當物體運動時，時間確實會變慢。

So feel this.

所以，請感受這一點。

You and your friend are at the same age, and both of you don't know about special relativity theory.

你和你的朋友年齡相仿，都不知道狹義相對論。

You left your friend on Earth, and you are traveling in a rocket near the speed of light after traveling and return back to the Earth and meet your friend.

你把你的朋友留在了地球上，你乘坐火箭以接近光速的速度旅行後返回地球，與你的朋友見面。

Now you say that only 6 months passed for me.

現在你說我只過了 6 個月。

But your friend replies, but 40 years have already passed here on Earth.

但你的朋友回答說，但地球上已經過去 40 年了。

This is how time dilation works.