We live — and stop me if I'm going too fast — on a planet.
我們生活在（如果我講的太快可以隨時打斷我）一顆行星上。
I mean, sure, duh.
我是說，這不是很明顯嗎。
But this isn't the natural state of the Universe; or, at least, it's not the way things usually are.
不過，這並不是宇宙的自然狀態；或者說這不是事物通常存在的狀態。
Most of the Universe is pretty empty — that's why we call it "space" — and if I were to magically transport you someplace randomly in the cosmos, the chances are you'd be a million light years from the nearest substantial object.
大部分的宇宙空間都是空空蕩蕩的——這也是為什麼我們稱之為「太空」的原因——如果我們通過某種神奇的方式把你隨機傳送到宇宙中，你多半離最近的大型物體也有幾百萬光年那麼遠。
Evolving on a planet has warped our sense of physics.
在一個星球上進化已經扭曲了我們對物理的感官。
If I throw an object away from me, it comes back.
如果我把一個物體扔出去，它會回來。
That's bizarre! It should just keep going, moving away from me at a constant speed.
這非常詭異的好嗎！它應該繼續前進，以恆定的速度遠離我。
Instead though, it goes up, slows, stops, then falls back down toward me.
而實際上，它卻向上運動，逐漸減速，停止，然後回落到我這邊。
The difference between living on a planet and being in deep space is gravity.
生活在行星上和生活在深空之間的區別就是引力。
Gravity from an object goes on forever, but it gets weaker rapidly with distance.
物體的引力是無限的，但隨著距離的增加，引力就會迅速減弱。
A zillion light years away, the Earth's gravity is fantastically weak, but here on Earth it's literally a force to be reckoned with.
數不清的光年之外，地球的引力非常微弱，但在地球上，它確實是一種不可忽視的力量。
And in some places it can be a lot stronger than what we experience right here.
還有些地方，引力可能比我們在地球上體會的要大得多。
For most of history, gravity was just a fact of life, neither understood nor examined terribly closely.
在人類的大部分歷史中，引力是生活中的常態，沒有人想到要去仔細檢驗和理解它。
In the mid 1600s, scientists like Robert Hooke and Isaac Newton started investigating it using math - in fact, the two men got into a bitter feud over who thought of what first.
在17世紀中期，像羅伯特·胡克和艾薩克·牛頓這樣的科學家才開始運用數學工具來研究它——事實上，這兩個人就誰先想到了什麼還產生了讎隙。
But whoever it was who first got it right, now we have a much better understanding of how gravity works.
不過無論是誰先搞明白了引力，現在我們對引力的作用都有了更深入的理解。
One thing before we get to gravity.
在我們講引力之前還有一件事。
An important concept that comes up a lot is mass.
質量是一個經常被提起的重要概念。
It's a bit tricky to define, but you can think of it as how much stuff makes up an object.
定義它有點棘手，但你可以把它當做組成物體的物質的多少。
I know, that's not very scientific sounding, but it's not a bad way to think about it.
我知道，這聽起來不是很科學，但這是一個不錯的思考方式。
Something with more mass has more stuff in it.
質量更大的物體含有更多的物質。
Size doesn't really play into this; two objects can have the same mass, but one can be much larger than the other.
大小並不一定和質量相關；兩個物體質量相同，但其中一個可能比另一個大得多。
In that case, the bigger object's mass is more spread out, so we say it has lower density, where density is how much mass is inside a given volume.
在這種情況下，更大的物體的質量更分散，所以我們說它的密度更低，密度就是給定體積內的質量。
In science terms, mass tells us how much an object resists having its motion changed.
在科學術語中，質量告訴我們一個物體對其運動變化的抗性。
An object with more mass is harder to get moving than an object with less mass, which is pretty obvious if you've ever tried pushing on a toy car versus a real truck.
質量大的物體比質量小的物體更難移動，如果你試過推動玩具汽車和真正的卡車，就知道這一點是顯而易見的。
But mass is also defined using gravity.
不過質量還有另一種使用引力的定義。
Everything that has mass also has gravity and can inflict this force on another object.
任何有質量的物體都有引力，並可以把這種力施加到另一個物體上。
The amount of force you feel from the gravity of an object like a planet depends on three things: how much mass it has, how much mass you have, and how far away you are from it.
你所感覺到的物體的引力，比如行星，它的引力大小取決於三件事：它有多少質量，你有多少質量，以及你離它有多遠。
In fact, distance dominates here; the force of gravity weakens with the square of the distance.
實際上，距離在其中起主導作用；引力隨著距離的平方而減弱。
Double your distance from an object and the force of gravity drops by 2 x 2 = 4 times.
如果你和一個物體的距離加倍，引力就會減弱2×2 = 4倍。
Go 10 times farther away and the force drops by 10 x 10 = 100 times.
如果距離變為10倍，引力就會減弱10 x 10 = 100倍。
Gravity is also attractive: It can only draw things in, not repel them.
引力還有吸引的方向性：只能把物體拉向自己，而不是排斥它們。
But how it attracts things is where it gets fun.
不過它是如何吸引物體的才是真正有趣的地方。
If I hold up a rock and let go of it, it falls to the ground.
如果我舉起一塊石頭然後鬆手，它就會落向地面。
What might be hard to see is that it gets faster the longer it drops.
比較難以察覺的是，它下落的距離越長，速度就越快。
Forces accelerate objects, so the longer the force acts, the more the object's velocity changes — in this case getting faster.
力使物體加速，所以力作用的時間越長，物體的速度變化就越大——在這個例子中，就是速度越快。
If I drop a rock from higher up, it'll move faster when it hits the ground.
如果我從更高的地方扔下石頭，它落地時就會移動得更快。
Other forces act on moving objects, as well, like friction and air resistance, counteracting gravity, making this acceleration hard to see.
當然也有其他的力作用在移動的物體上，如摩擦力和空氣阻力，它們抵消了部分引力，使這種加速度很難被察覺。
But in space, the force of gravity becomes very clear.
但在太空中，引力的作用就非常明顯了。
Two objects that have mass will attract each other.
兩個有質量的物體會相互吸引。
If there are no other forces acting on them, they'll accelerate toward each other until they meet.
如果沒有其他的力作用在它們上面，它們就會相互加速，直至碰到一起。
Remember, though, that the force of gravity depends on those masses.
記住，雖然引力的大小取決於兩個物體的質量。
If one is really massive, and the other not so much, then in more practical terms the massive one will pull in the less massive one.
但如果疑中之疑質量巨大，而另一個比較小的話，更實際的情況就是質量大的會把小的拉向自己。
The more massive one does move, but much less than the other one.
質量更大的那個物體確實也會移動，但會比小的那個少很多。
When objects are free to move under the effects of gravity, we say they are in orbit.
當物體在引力作用下可以自由移動時，我們稱它們處於軌道運動中。
The simplest kind of orbit may not be what you think: It's actually just a line!
最簡單的軌道可能與你想的相差甚遠：它實際上就只是一條直線！
When you drop a rock, it's very briefly in orbit.
當你扔下一塊石頭時，它在很短的時間內也處於軌道運動中。
Ignoring things like the Earth's rotation (which adds a bit of sideways motion) it's close enough to say the rock just falls straight down, and is stopped because the Earth itself gets in the way.
如果不考慮地球的自轉的話(考慮自轉會加入一點側向運動)，這足以說明石頭是垂直下落的，它會停下僅僅是因為地球本身擋住了它的去路。
That's not a terribly interesting orbit!
這並不是什麼非常有趣的軌道！
So what if, instead of dropping the rock, we throw it?
所以如果不考慮扔石頭，而是拋出去呢？
That gives it a little bit of sideways motion, so instead of hitting the ground at my feet, it hits a bit farther away.
這就給了它一點側向運動，所以它不會落在我腳下，而是落在更遠一點的地方。
If I throw it harder, it moves horizontally even more before it hits.
如果我拋得更用力，它在落地前就會水平移動更遠的距離。
What if I throw it really hard?
如果我拋得非常用力呢？
This is where Newton's genius comes in.
這就是牛頓的天才之處了。
He realized that if you throw the ball hard enough sideways, it will fall at the exact same rate the Earth would curve away underneath it.
他意識到，如果你把球側向扔得足夠用力，它的下落曲線就和它下方的地球弧度完全一致了。
As Douglas Adams said in "Hitchhiker's Guide to the Galaxy," flying is just falling and missing the ground.
正如道格拉斯·亞當斯在《銀河系漫遊指南》中所說的那樣，飛翔就是在掉落的過程中錯過地面。
It turns out, that's exactly what orbiting is, too.
確實，這正是軌道運動。
A rock thrown hard enough sideways will fall toward the Earth, but always miss it, going instead into a circular path around it, guided only by gravity.
一塊被用力拋出的石頭總是在朝向地球墜落，並且總是錯過地面，於是它就只由引力引導，在一個環形軌道上運動。
It will orbit the Earth in a circle, taking about 90 minutes to go around the planet once.
它將會沿著環形軌道繞著地球轉，花上大約90分鐘來繞地球一圈。
Circles are simple orbits.
圓形軌道是很簡單的軌道。
The speed at which the orbiting satellite travels depends on the mass of the object it's orbiting, and its distance from it.
物體在軌道中的速度取決於它所環繞的物體的質量以及它與物體之間的距離。
The farther it is, the weaker gravity is, so it doesn't have to travel as quickly to maintain the orbit.
距離越遠，引力就越弱，它保持軌道運動所需的速度也越低。
Roughly 400 years ago, the astronomer Johannes Kepler realized that there can be other shapes of orbits as well.
大約400年前，天文學家約翰尼斯·克卜勒意識到，可能還有其他形狀的軌道。
He discovered the planets orbit the Sun on ellipses, when previously it was thought they orbited in perfect circles.
他發現繞著太陽的行星軌道是橢圓形的，而以前人們都認為這些軌道是完美的圓形。
An elliptical orbit happens when you throw the rock sideways even harder than it takes for a circular orbit; it goes up higher on one end of the orbit than on the other.
要形成橢圓形軌道，扔出石塊的力量要比圓形軌道更大；因為在橢圓軌道一端的物體的位置比圓形軌道更高。
In fact, the harder you throw the rock, the more elongated the orbit gets.
實際上，你扔的力氣越大，軌道就更扁更長。
An orbit like this is still closed; that is, the orbit repeats itself and the rock is still bound to the Earth by gravity.
這樣的軌道仍然是閉合的；也就是說，軌道會周期性重複，石頭仍然受地球的引力的束縛。
At some point, though, if you throw the rock hard enough, an amazing thing happens: it can escape.
直到你扔出的力道足夠大，一件神奇的事情就會發生：石頭可以逃逸。
Remember, gravity gets weaker with distance.
記住，引力隨著距離的增加而變弱。
If you throw a rock hard enough, while gravity can slow it down, the gravity gets weaker the farther away the rock is.
如果你扔得足夠用力，儘管引力會讓它減速，但引力本身隨著它的距離變遠而減弱。
If the rock has enough velocity, gravity weakens too quickly to stop it.
如果巖石有足夠的速度，而引力減弱太快，不足以使它停止。
The rock can escape, moving away forever, so we call this the escape velocity.
石頭就會逃逸，越來越遠，永不停止，我們稱這個速度為逃逸速度。
The escape velocity of an object like a planet or star depends on how much mass it has and how big it is.
行星或恆星等物體的逃逸速度取決於它的質量和大小。
For the Earth, that turns out to be about 11 kilometers per second, for Jupiter, it's about 58 kilometers per second, and for the Sun it's a whopping 600 kilometers per second.
地球的速度是每秒11千米，木星大約是每秒58千米，太陽是每秒600千米。
Whatever the particular escape velocity for your cosmic location is, if you fling a rock away from it faster than that, I hope you kissed it goodbye first, because it ain't coming back.
無論你在宇宙中身處之地的逃逸速度是多少，如果你用高於它的速度扔出一塊石頭，那我希望你先和它吻別過了，因為它不會再回來了。
One way to think of it is that the rock is always slowing, getting ever closer to stopping, but it never actually stops.
有一種理解是，想像那塊石頭逐漸減速，越來越接近於靜止，但它永遠不會真的完全停下來。
If it could travel infinitely far away, it would stop, but that's kind of a long trip.
如果它能飛到無限遠的地方，它確實會停止，但那是一段很長的旅程。
This works in reverse, too.
反過來也一樣。
If I go way far away from the Earth and drop a rock, it'll accelerate.
如果我們在離地球很遠的地方扔下一塊石頭，它就會逐漸加速。
When it hits the planet it'll be moving at escape velocity, that same 11 kilometers per second.
當它落到地球上時的速度就是逃逸速度，同樣是11公裡每小時。
And if I give it a little sideways kick, it'll miss the Earth but still pass us at escape velocity.
如果我稍微側向扔一點，它就會錯過地球，但仍然以逃逸速度經過我們身邊。
An escape orbit is open — it doesn't come back — and is shaped like a parabola.
逃逸軌道是開放的——它不會回來——形狀就像拋物線。
What if you throw the rock even harder than that?
如果你扔得更用力呢？
The rock doesn't come back, and moves away even faster.
石頭不會回來，而是以更快的速度離開。
The orbit is now a hyperbola, which is similar to a parabola, but is even more open.
這種軌道是條雙曲線，類似於拋物線，但更加開放。
The rock never stops, even at infinity.
石塊即使在無窮遠處也不會停止。
It just keeps moving on.
它會一直前進。
Like all forces, gravity gets weaker with distance.
像所有的力一樣，引力會隨著距離的增加而變弱。
But its force never quite drops to zero; it just gets smaller and smaller as you get farther and farther away.
但是它的影響力永遠不會完全降到零；只是隨著距離變遠而變得越來越小。
So why then are astronauts on the space station "weightless"?
那麼為什麼空間站上的太空人會「失重」呢？
Gravity is still pulling on the astronauts!
太空人其實仍然受到引力影響！
In fact, at the height of the station, Earth's gravity has only decreased by a little bit; it's still about 90% as strong as it is on the Earth's surface.
事實上，在空間站那種高度，地球的引力只減少了一點點；它的強度仍然是地表引力的90%。
If they were in a tower 320 kilometers high they'd weigh 90% of what they do on the Earth's surface.
如果他們在一座320千米高的塔中，它們的重量會是在地球表面的90%。
But the big difference is that the astronauts are in orbit, falling around the Earth.
但最大的不同是，太空人是在軌道運動中，圍繞地球下落。
Weight is actually not just the force of gravity on a mass, but how hard a surface pushes back on that mass.
重量實際上不僅僅是引力對質量的作用，還是一個物體表面反作用於另一質量物體的大小。
For example, when you stand on the ground, the ground pushes back.
比如說，當你站在地上時，地面對你有反作用力。
Otherwise you'd fall through!
否則你就掉下去了！
The force of the ground back on you is what causes you to have weight.
地面對你的反作用力是你有重量的原因。
In free fall, there's nothing pushing back.
而在自由落體中，沒有什麼東西會帶來反作用力。
You're falling freely, and so you have no weight.
你只是在自由下落，所以你是沒有重量的。
NASA likes to call this condition "microgravity," since there are subtle forces acting on you.
美國航天局（NASA）喜歡稱之為「微重力」，因為其實還有一些微小的力作用在你身上。
This actually highlights the difference between mass and weight.
這實際上突出了質量和重量之間的區別。
In space you have the same mass as you do on Earth, but no weight.
在太空中你仍然有跟在地球上相同的質量，但沒有重量。
If another astronaut pushed on you, they'd have to exert a force, but if you stood on a scale in space it wouldn't register anything.
如果另一個太空人想推動你，他們必須施加一個力，但如果你站在太空中的體重秤上，它就不會有任何顯示。
Space is weird.
太空真實太詭異了。
Well, compared to Earth.
好吧，這是和地球相比起來。
One more thing, and this is truly weird: photons, particles of light, have no mass, yet they can be affected by gravity, too, bending their direction of flight as they pass a massive object!
另一點非常奇特的是：光子，即光的粒子，它沒有質量，但是它們也會受到引力的影響，當它們經過一個巨大的物體時，它們的飛行方向會被改變！
It turns out gravity can actually warp space!
原來引力可以彎曲空間本身！
Light travels along the fabric of space like a truck on the road, and if the road curves, so does the truck.
光線在空間結構中穿行，就像是公路上的卡車一樣，如果公路彎曲，卡車也會轉彎。
I know this is an odd concept, and we'll be dealing with it later in more detail when we push escape velocity to its limits. . . with black holes.
我知道這是一個奇怪的概念，在後面的課程中，逃逸速度的極限，也就是討論黑洞時，我們會詳細解釋的。
Today you learned that gravity is a force, and everything with mass has gravity.
今天你學到了引力是一種力，所有有質量的物體都有引力。
Gravity accelerates object with mass, changing their speed and/or direction.
引力會使物體加速，改變其速度和方向。
An object moving along a path controlled by gravity is said to be in orbit, and there are many different kinds: straight lines, circles, ellipses, parabolae, and hyperbolae.
在引力控制下運動的物體被稱為在進行軌道運動，而軌道有許多不同的種類：直線、圓、橢圓、拋物線和雙曲線。
You can't ever escape gravity, but if you travel faster than escape velocity for an object you'll get away from it without falling back.
你永遠無法逃脫地心引力，但如果你的運動速度比物體的逃逸速度快，你就會持續遠離它而不落回去。
And if you're in orbit, in freefall, you have no weight, but you still have mass.
另外，如果你在進行自由落體的軌道運動，你就不會有重量，但仍然有質量。
This episode is brought to you by Squarespace.
本集由 Squarespace 為您呈現。
The latest version of their platform, Squarespace Seven, has a completely redesigned interface, integrations with Getty Images and Google Apps, new templates, and a new feature called Cover Pages.
他們最新的平臺 Squarespace 7經過了全新交互界面設計，加入了 Getty 圖片社、谷歌應用、新模板和封面頁的新功能。
Try Squarespace at squarespace. com, and enter the code Crash Course at checkout for a special offer.
你可以在 squarespace.com 試用，付款時輸入優惠碼 Crash Course 可享受特價。
Squarespace. Start Here. Go Anywhere.
Squarespace，從此始，任你行。
Crash Course Astronomy is produced in association with PBS Digital Studios, and you can head over to their channel and find more awesome videos.
Crash Course 是與 PBS 數字工作室聯合製作的，你們可以去看看，還有更多精彩的視頻。
This episode was written by me, Phil Plait.
我是菲爾·普萊特，這一集是由我創作的。
The script was edited by Blake de Pastino, and our consultant is Dr. Michelle Thaller.
腳本由 Blake de Pastino 編輯，我們的顧問是 Michelle Thaller 博士。
It was co-directed by Nicholas Jenkins, and Michael Aranda, edited by Nicole Sweeney, and the graphics team is Thought Café.
本片由尼古拉斯·詹金斯(Nicholas Jenkins)和麥可·阿蘭達(Michael Aranda)聯合執導，視頻畫麵團隊是 Thought Cafe。