Two famous scientists did research on how gravity works.

兩位著名科學家對重力的作用原理進行了研究。

Newton was the foremost of them.

牛頓就是其中的佼佼者。

What Newton is explaining about gravity is that all things in the universe have mass.

牛頓對萬有引力的解釋是，宇宙萬物都有品質。

There will always be an attraction between two objects, or every mass attracts every other mass.

兩個物體之間總是存在吸引力，或者說，每個品質都會吸引其他品質。

The gravitational force of an object also varies according to its mass.

物體的引力也隨其品質而變化。

That is, an object with a greater mass pulls a lighter object to its side.

也就是說，品質較大的物體會將品質較輕的物體拉到它的一側。

The greater the gravitational force decreases.

引力越大，引力越小。

Force is inversely proportional to the distance to the power of two.

力與距離的平方成反比。

This means that if the distance increases by two times, the force will decrease by four times.

這意味著，如果距離增加 2 倍，力將減小 4 倍。

If the distance increases by three times, the force decreases by nine times.

如果距離增加 3 倍，力就會減少 9 倍。

This is what Newton explained about gravity.

這就是牛頓對萬有引力的解釋。

Everything happens just as Newton said.

一切都如牛頓所說的那樣發生了。

The earth orbits the sun, the moon orbits the earth, and the universe itself works according to Newton's gravity.

地球繞著太陽轉，月亮繞著地球轉，宇宙本身就是按照牛頓的萬有引力運轉的。

If the sun's gravity is so strong, why didn't the earth smash into it?

如果太陽的引力如此強大，為什麼地球沒有撞上去？

Distance and angular momentum are relevant factors in this context.

在這種情況下，距離和角動量是相關因素。

Well, everything's all right.

一切都很好

This was accepted by everyone who was there then because it fits perfectly according to physics.

當時在場的所有人都接受了這一說法，因為它完全符合物理學原理。

But here comes the biggest confusion.

但最大的困惑來了。

The distance between the sun and the earth is about 150 million kilometers.

太陽和地球之間的距離約為 1.5 億公里。

Will there be a force for such a distance?

這樣的距離會有力量嗎？

For example, if I kick a ball, there is and the ball.

例如，如果我踢球，就會有 和 球。

So there is physical contact that makes the ball move.

是以，有了身體接觸，球才會移動。

But what lies between the sun and the earth is the greatest distance.

但太陽和地球之間的距離才是最遠的。

That being the case, scientists are gradually confused about what the kind of force that causes binding between such objects.

在這種情況下，科學家們逐漸搞不清楚是什麼樣的力導致了這些物體之間的結合。

And this is where Einstein joins.

這就是愛因斯坦的加入。

According to him, gravity is not a force, but it doesn't make sense.

根據他的說法，重力不是一種力，但這並不合理。

Why does it happen if there is no gravity?

如果沒有重力，為什麼會發生這種情況？

What's making it happen then?

是什麼讓它發生的呢？

Which mysterious thing causes it?

是什麼神祕的東西導致了這種情況？

But his explanation for it, though not understood by anyone at the time, was understood a few years later.

但他對此的解釋，雖然當時沒有人理解，但幾年後卻被理解了。

And it was that discovery that elevated him to an important scientist in the world.

正是這一發現使他晉升為世界上重要的科學家。

So what was his discovery?

那麼，他的發現是什麼呢？

Let's see how that works.

讓我們拭目以待。

Our universe looks like it is empty, but in reality it is not empty.

我們的宇宙看起來是空的，但實際上並不空。

Empty spaces are not really empty.

空洞並非真的空洞。

The whole universe is like a fabric sheet.

整個宇宙就像一塊布。

That sheet combined with fourth dimension time becomes space-time.

這張紙與四維時空相結合，就成為了時空。

He says curvatures in space-time are responsible for gravity.

他說，時空曲率是引力的原因。

Let's take a fabric mat, for example, and make a fat man sit in the middle of it and make a skinny man sit on the edge of the mat.

例如，讓一個胖子坐在布墊中間，讓一個瘦子坐在布墊邊緣。

Now we will see that the mat bends inside where the fat man is.

現在我們會看到，墊子在胖子所在的位置向內彎曲。

So the skinny man is automatically pulled towards the fat man.

於是，瘦子就會被自動拉向胖子。

The fat man does not exert any force, and even if he doesn't touch him, the skinny man is drawn towards him.

胖子沒有施加任何力量，即使他沒有碰到瘦子，瘦子也會被他吸引過去。

Similarly, gravity occurs for everything in the universe.

同樣，宇宙萬物都有引力。

As the sun bends space-time, the earth is attracted towards it.

當太陽彎曲時空時，地球就會被吸引過來。

It is not a force.

它不是一種力量。

As space bends, it has an impact on the objects on the curve.

空間彎曲時，會對曲線上的物體產生影響。

The mass tells space-time how to curve.

品質告訴時空如何彎曲。

Space-time tells the mass how to move.

時空告訴品質如何運動。

Now it's easy to explain how Einstein came to realize this.

現在，我們不難解釋愛因斯坦是如何意識到這一點的。

He found out the truth through thought experiments.

他通過思想實驗發現了真相。

Then the mathematical equations were also confirmed.

隨後，數學公式也得到了確認。

He is sitting in his office one day when he sees a man cleaning the windows of a building close by.

一天，他坐在辦公室裡，看到一個人正在擦拭附近一棟大樓的窗戶。

He starts to think about what would happen if that person fell down.

他開始思考，如果那個人摔倒了會怎麼樣。

While it may seem strange to us to hear this, Einstein describes it as the happiest thought in my life.

雖然我們聽到這句話可能會覺得很奇怪，但愛因斯坦卻把它描述為我生命中最快樂的想法。

Let's look at the things that crossed his mind.

讓我們看看他想到了什麼。

We will lock the man who was falling from the top of the building into an elevator.

我們將鎖定那個從樓頂墜入電梯的人。

A man who is falling will not feel any force, since he is inside the elevator, and there is no reference point.

墜落的人感覺不到任何力，因為他在電梯內，沒有參照物。

He can't realize if he falls down or floats.

他不知道自己是摔倒了還是飄起來了。

The phenomenon of falling down as a free fall without any reference point is like floating in space.

沒有任何參照物的自由落體墜落現象就像漂浮在太空中一樣。

That is, according to physics, there is no difference between a person who falls from the top of a building and floats in space.

也就是說，根據物理學原理，一個人從樓頂墜落和漂浮在太空中是沒有區別的。

Both of them wouldn't feel any gravitational force.

兩人都感覺不到任何引力。

So empty space weightlessness is equal to free fall on earth.

是以，太空失重相當於地球上的自由落體。

Let's move on to the next step.

讓我們進入下一步。

Now I am floating in space inside the lift, and there is a ball also floating with me.

現在，我漂浮在電梯內的太空中，有一個球也和我一起漂浮著。

A motor engine has been fit into the bottom of my lift, and I started that engine and accelerated the lift upwards at a speed of 9.8 per second square. 9.8 is 1g, the gravitational force for earth.

我在升降機底部安裝了一個發動機，啟動發動機後，升降機以每秒 9.8 平方英寸的速度向上加速。9.8 等於 1 克，即地球引力。

Now what happens when I accelerate at this speed is that the ball floating with me will fall down on the lift floor, and I will also be pushed towards the is accelerating at a speed of 9.8 meters per second square in space.

現在，當我以這個速度加速時，與我一起漂浮的小球會掉落在電梯地板上，而我也會被推向以每秒 9.8 米的速度加速的空間。

Whatever can be done with the help of gravity on earth can also be done in this lift.

在地球上可以藉助重力完成的事情，在這個電梯裡也可以完成。

I don't feel any difference, whether in space or on earth.

無論是在太空還是在地球上，我都感覺不到任何區別。

So free fall on earth is equal to free movement in space.

是以，地球上的自由落體等同於太空中的自由運動。

Gravity on earth is equal to 9.8 meters per second square.

地球上的重力等於每秒 9.8 米見方。

Acceleration in space.

空間加速度

That is, acceleration causes gravity.

也就是說，加速度導致重力。

Now I am in space inside a lift, and I turn off that engine, and the lift is made to float steadily in one place.

現在，我在太空中的升降機裡，我關掉髮動機，升降機就會穩定地漂浮在一個地方。

What will happen if I throw the ball in straight?

如果我把球直接扔進去會怎麼樣？

When I throw a ball, it goes in a straight line.

當我拋球時，球會沿著一條直線前進。

What if I throw that ball at an accelerating lift?

如果我把球扔向一個加速上升的物體呢？

The ball will go down in a curved path and hit onto the floor.

球會以彎曲的軌跡下墜，然後砸在地板上。

So far everything is going well.

到目前為止，一切進展順利。

Now I have switched from the ball to a light beam.

現在，我把球換成了光束。

How will it go if I torch the light in the accelerating lift?

如果我在升降機加速過程中用火把照明，情況會怎樣？

Light is known to always in a straight line at constant velocity.

眾所周知，光總是以恆定的速度沿直線傳播。

If you throw a ball on the earth, it will travel in a curved path, and then fall to the ground.

如果你把一個球扔到地球上，它會沿著彎曲的軌跡飛行，然後落到地面上。

If the ball is thrown from an acceleration lift in space, it will also travel in a curved path.

如果球從太空中的加速升降臺拋出，它也會沿著彎曲的路徑飛行。

Does that mean light itself will be able to bend?

這是否意味著光本身能夠彎曲？

Can light be bent even though it has no mass and always moves in a straight line?

儘管光沒有品質，而且總是直線運動，但它會彎曲嗎？

Einstein says it will bend.

愛因斯坦說它會彎曲。

Yes, it is possible, if there is something called space curvature, as he claims.

是的，這是可能的，如果像他所說的那樣存在一種叫做空間曲率的東西的話。

How do we find it?

我們如何找到它？

The lift that is accelerating at just 9.8 meters per second square does not have enough opportunity to bend the light.

加速度僅為 9.8 米/秒的升力沒有足夠的機會讓光發生彎曲。

So now, I change the acceleration speed from 9.8 to 98.

現在，我把加速度從 9.8 改為 98。

According to Einstein's prediction, the bend of light in acceleration at 98 meters per second square is very small.

根據愛因斯坦的預測，光在每秒 98 米見方的加速度下的彎曲非常小。

That is, it bends only the size of an electron.

也就是說，它只彎曲了一個電子的大小。

This means that if we increase gravity or acceleration, light can bend more and more.

這意味著，如果我們增加重力或加速度，光就會越來越彎曲。

Well, how do we prove this?

那麼，我們如何證明這一點呢？

In those days, it was impossible to build an engine that had such acceleration and implement it.

在那個年代，不可能製造出具有這種加速度的發動機，也不可能將其付諸實施。

In fact, not even a rocket was invented at the time, but we have gravity.

事實上，當時連火箭都沒有發明出來，但我們卻有了萬有引力。

The sun was the strongest gravity when it comes to our nearby things.

說到我們身邊的事物，太陽的引力是最強的。

Its gravity is 28 times stronger than earth's because its mass is 330,000 times bigger.

它的重力是地球的 28 倍，因為它的品質是地球的 33 萬倍。

If we calculate what will gravity will bend the light to 1.75 arc seconds.

如果我們計算一下，重力將把光彎曲到 1.75 弧秒。

An arc second is 360 degrees in a circle, and if you take one degree of it, it has 60 minutes within it, and 60 seconds per minute.

一弧度秒是 360 度的圓，如果取其中的一度，則其中有 60 分鐘，每分鐘有 60 秒。

Of these, 1.75 arc seconds is very small.

其中，1.75 弧秒是非常小的。

In 1915, Einstein published his theory with equations.

1915 年，愛因斯坦發表了他的理論方程。

Everyone was confused when they saw this theory.

看到這個理論，大家都很困惑。

What is this?

這是什麼？

What is he trying to say?

他想說什麼？

No one at the time.

當時沒有人。

Then four years later, Arthur Eddington, the greatest astronomer at the time from England, set out with his team to experiment with this.

四年後，當時英國最偉大的天文學家阿瑟-愛丁頓（Arthur Eddington）帶著他的團隊開始了這方面的實驗。

At that time, they knew the exact location of some of the stars, and if Einstein's theory was correct, the star behind the sun should be visible somewhere else because the light should bend and be visible to our eyes.

當時，他們知道一些恆星的確切位置，如果愛因斯坦的理論是正確的，太陽背後的恆星應該在其他地方可見，因為光線應該彎曲，我們的眼睛可以看到。

They were waiting for a total solar eclipse because the sun's brightness makes it hard to see nearby stars.

他們在等待日全食的到來，因為太陽的亮度讓人很難看到附近的星星。

When the day of the solar eclipse came, Arthur Eddington's team started taking pictures.

日食當天，亞瑟-愛丁頓的團隊開始拍照。

To their surprise, the star showed up in the photos in a place they hadn't expected.

出乎他們意料的是，照片中的明星出現在了一個他們意想不到的地方。

Gravity had bent light itself because space-time had been bent.

引力使光本身彎曲，因為時空被彎曲了。

For example, I go straight from one place to the North Pole.

例如，我從一個地方直接去北極。

As far as I'm concerned, I'm going in a straight line, but to an outside observer, I appear to be taking a curved path.

在我看來，我走的是一條直線，但在外人看來，我走的卻是一條彎路。

Similarly, light always travels straight, and because its path is curved, it also bends.

同樣，光總是直線傳播的，由於其路徑是彎曲的，所以它也會彎曲。

So the shortest path of light can be a curved one.

是以，光的最短路徑可能是彎曲的。

Now you may have a doubt.

現在你可能有疑問了。

If the space at the bottom of a massed object only bends, does space bend at the top of that object?

如果品質物體底部的空間只會彎曲，那麼該物體頂部的空間會彎曲嗎？

All the space-time curvatures we see in the video are shown in 2D visuals so that we can easily understand them.

我們在視頻中看到的所有時空曲率都以二維視覺效果呈現，這樣我們就能很容易地理解它們。

Its 3D form is hard to understand, but that's exactly the case.

它的三維形式很難理解，但事實正是如此。

It was only after this space-time curvature was demonstrated that Einstein was celebrated as the greatest scientist.

在時空曲率被證明之後，愛因斯坦才被譽為最偉大的科學家。

This thought experiment is not a simple one.

這個思想實驗並不簡單。

It is the greatest thought in human history.

這是人類歷史上最偉大的思想。

This simply explained not only gravity, but also showed the world how the entire universe works.

這不僅簡單地解釋了萬有引力，還向世界展示了整個宇宙是如何運轉的。

This is called the General Theory of Relativity.

這就是廣義相對論。

That's what space-time curvature is, but how did this time come in it?

這就是時空曲率，但這個時間是怎麼來的呢？

How does space-time explain time deletion?

時空如何解釋時間刪除？

The Special Relativity Theory provides an explanation, and we'll see it in detail in the next video.

狹義相對論提供了一種解釋，我們將在下一個視頻中詳細瞭解。