Bacteria on Mars, weird alien fish in the oceans of Europa,

火星上的細菌

and also ancient alien ruins on Titan.

木衛二海中怪異的外星魚

Wouldn't that be great?

以及泰坦(土衛六)上古老的異星遺跡。

Well, no. It would be horrible news,

這不是棒呆了嗎？

devastating even.

恩~~錯，這是個壞消息，或許還會是一場災難。

It could mean that the end of humanity is almost certain

這可能表示人類注定要滅亡了，

and that it might be coming soon.

甚至可能很快就會發生了!

Why?

為什麼？

Why would the most exciting discovery of our lifetime

為什麼這個也許是一生中最新奇的新聞會很糟？

be bad?

00:00:40,320 --> 00:00:44,740 讓我們想像生命從一開始到今日的發展

Let us imagine the development of life,

是一個階梯狀。

from its inception to us today,

第一層是無生命的化合物，必須自行組合

as a flight of stairs.

成能複製的結構，具穩定性和可修復性，

The first step is dead chemistry that needs to assemble itself

但又能改變和演化。

into self-replicating patterns, stable and resilient,

第二層的生命形式變得更複雜，

but also able to change and evolve.

能夠建造出更複雜的結構，

The second step is for our early life to become more complex,

更有效率地運用可得的能量。

able to build more complicated structures,

再來，這些細胞組合成多細胞生物，

and use the available energy much more efficiently.

使其發展出不可思議的多樣性和更高的複雜性。

On the next step, these cells combine to become multicellular beings,

最上層則是該物種演化出巨大的腦，

enabling unbelievable variety and further complexity.

讓他們能使用工具、產生文化並傳播知識，

The step above sees the species evolve big brains,

又創造出更高的複雜性。

enabling the use of tools, culture and shared knowledge,

於是該生命成為星球的主宰，

which creates even higher complexity.

能依其需求加以改造。

The species can now become the dominant lifeform on its planet,

現階段正在畏畏縮縮地試著離開行星，

and change it according to its needs.

這是我們目前的階段。

First shy attempts to leave its planet are happening.

我們知道生命的天性是去每個適合生活的地方，

This is where we are now.

但由於每個行星的承載量和它的壽命皆為有限，

It's in the nature of life as we know it

若ㄧ物種要長存，必定要尋找更多容身之處。

to reach out, to cover every niche it can.

因此下一個階段似乎很合理：

And since planets have a limited carrying capacity and lifespan,

殖民所在的行星系、

if a species wants to survive,

然後散播至其他恆星、

it will look for more places to spread to.

理論上，其最終將是橫跨整個星系的文明。

So the steps above the current ones seem logical:

這很可能是全宇宙文明的通則，

colonize your own solar system,

不論他們源自何處。

then spread further to reach other stars,

如果一個物種有足夠的競爭力和動機， 而能宰制其所在的行星，

to the possible final step: becoming a galaxy-wide civilization.

他們大概不會就此打住。

It's very likely that this is a universal principle for civilizations,

我們已知銀河系中行星可多達五千億顆星球，

no matter where they're from.

其中類似地球的至少一百億顆，

If a species is competitive and driven enough to take control over its planet,

很多星球比地球年長數十億年。

they'll probably not stop there.

但目前觀測到的星系文明數量是零。

We know that there are up to 500 billion planets in the Milky Way,

我們應該能看到些什麼，但沒有─

at least 10 billion Earth-like planets.

不論從何觀之，虛無的太空一片死寂。

Many have been around billions of years longer than Earth.

這表示有東西，阻擋著生命

But we're observing zero galactic civilizations.

爬上比我們如今所在更高的階梯，

We should be able to see something...

它使星系文明成為難如登天，甚至是永不可能的目標。

...but there's nothing.

這就是大過濾，一種挑戰或危機，

Space seems to be empty and dead.

危險得足以摧毀幾乎所有遭遇它的物種。

This means something is preventing living things from climbing the staircase,

有以下兩種情況：

beyond the step we're on right now.

其一，我們何其特殊而幸運；

...Something that makes becoming a galactic civilization

其二，我們大難臨頭且必死無疑。

extremely hard, maybe impossible.

這取決於「濾網」位於我們目前階段的前面或後面。

This is the Great Filter.

情況一：大過濾已過去。我們獨占鰲頭。

...A challenge or danger so hard to overcome,

如果濾網在我們後方，表示我們經歷過的某個步驟

that it eliminates almost every species that encounters it.

是幾乎無法越過的，

There are two scenarios:

那會是哪一步？

One means we are incredibly special and lucky,

生命本身是否極其罕見？

the other one means we are doomed and practically already dead.

目前很難預測生命有多少機率從非生命中起源，

It depends on where the filter is on our staircase:

學界眾說紛紜。

behind, or ahead of us?

某些科學家認為只要環境具備，生命能源自任何一處，

Scenario 1:

其他人則主張地球可能為宇宙中唯一有生命之處。

Scenario 1: The filter is behind us.

另一個選項是複雜的動物細胞是非常特殊的，

We are the first.

這個階段發生了一件極特殊之事件，

If the filter is behind us,

據目前所知僅發生過一次。

that means that one of the steps we passed

一個原始的獵食細胞吞噬了另一個細胞，

is almost impossible to take.

卻結為盟友而非消化它。

Which step could it be?

大細胞提供防護，

Is life ITSELF extremely rare?

處理與外部環境的交互作用並提供資源，

It's very hard to make predictions about how likely it is

小細胞享受住所與免費資源，

for life to emerge from dead things.

專住於提供大量多餘能量給宿主。

There is no consensus.

擁有了更多能量，宿主細胞長得比以前更大，

Some scientists think it develops everywhere where the conditions are right;

產生更高成本的新結構以增進其生存，

others think that Earth might be the

而房客成為細胞的發電廠(粒線體)。

ONLY living place in the universe.

這種細胞組成了地球上所有動物。

Another candidate is the step of complex animal cells.

也許銀河中有億萬顆覆滿細菌的行星，

A very specific thing happened on this step,

卻沒有半個能達到地球生命的複雜度。

and as far as we know, it happened exactly once.

或者是智慧：

A primitive hunter cell swallowed another cell,

創造填字遊戲和言情小說的人類似乎相當聰明，

but instead of devouring it, the two cells formed a union.

但巨大的腦本身就是非常昂貴的演化投資。

The bigger cell provided shelter,

它如此脆弱，無法在和熊搏鬥時成為助力，

took care of interacting with the environment and providing resources,

又消耗大量能源，

while the smaller one used its new home and free stuff,

即使如此，現代智人也花上二十萬年

to focus on providing a lot of extra energy for its host.

從尖銳的棍棒到現代文明。

With the abundant energy, the host cell could grow more than before

智商不必然導致輕鬆獲勝。

and build new and expensive things to improve itself,

也許智慧本身其實不合適宜，只是我們僥倖成功。

while the guest became the powerhouse of the cell.

情況二：濾網在我們前面。

These cells make up every animal on the planet.

千萬的物種已經死亡，

Maybe there are billions of bacteria-covered planets in the Milky Way,

我們前方的大過濾，

but not a single one, apart from us,

比我們歷經的任何災難都艱險無數倍，

has achieved our level of complexity.

即使一次大災難殺死多數人，或讓文明倒退數千年，

...or intelligence.

我們仍能存活並東山再起，

We humans feel very smart and sophisticated

只要我們能再起，哪怕需要數百萬年，

with our crossword puzzles and romantic novels.

這也不是個大過濾，只是邁向星系文明的普通障礙。

But a big brain, is first and foremost,

以宇宙尺度而言，幾百萬年不過眨眼之間而已。

a very expensive evolutionary investment.

如果大過濾真的橫亙於眼前，

They are fragile,

它必然是如此艱難、絕望且強大

they don't help in a fistfight with a bear,

足以摧毀星系中數十億年來大多數(若非全部)先進文明。

they cost enormous amounts of energy,

一個令人膽寒且沮喪的假設是，

and despite them, it took modern humans,

每當一個物種主宰其母星，

200,000 years to get from sharp sticks to civilization.

它便已走上自我毀滅的道路。

Being smart does not mean you get to win automatically.

科技是達成這個目標的有效方式，

Maybe intelligence is just not so great,

它須是某些十分明顯的,讓幾乎所有人都能發現的東西,

and we're lucky that it worked out for us.

同時它要非常危險。只是它的問世就可以導致幾乎所有人都處於毀滅的災難中

Scenario 2:

大規模核戰

Scenario 2: The filter is ahead of us.

失控的奈米科技

Plenty of others died already.

基因工程產生的超級病菌

A Great Filter before us is orders of magnitude more dangerous

讓整個大氣層陷入燃燒的實驗

than anything we encountered so far.

也或許是超級人工智慧，意外或刻意消滅其創造者。

Even if a major disaster killed most of us or threw us back thousands of years,

各種我們不可預知的事物。

we would survive and recover. And if we can recover,

或者真相更加簡單：

even if it takes a million years, then it's not a Great Filter,

一個競爭性強到足以主宰其星球之物種，

but just a roadblock to an eventual galactic civilization.

注定要彼此競爭資源而滅亡。

On universal timescales, even millions of years are just the blink of an eye.

或許每個生態系都有個連鎖反應，

If a Great Filter really lies before us,

一旦啟動便無可挽回。

it has to be so dangerous,

於是當一個文明強大到能改變其大氣組成，

so purely devastating and powerful,

即會將行星化為不毛之地。

that it has destroyed most, if not all,

來祈求這不是真的。

advanced civilizations in our galaxy

如果濾網在前方，我們存續的機率渺茫。

over billions of years.

我們能期望的：

A really daunting and depressing hypothesis

這就是為什麼發現外星生命恐怕是壞消息。

is that once a species takes control over its planet,

假若生命在宇宙中越普遍、越先進且越複雜，

it's already on the path to self-destruction.

大過濾就有越高的機率在我們眼前。

Technology is a good way to achieve that.

發現細菌很不好，小動物會更糟糕，

It needs to be something that's so obvious,

發現智慧生命則是重大警訊。

that virtually everybody discovers it,

古老的外星文明遺跡更是頭等悲劇。

and so dangerous, that its discovery leads almost universally

我們目前最好的情況，

to an existential disaster.

就是火星一片貧瘠，歐羅巴的海洋毫無生機，

A large-scale nuclear war,

銀河的廣大旋臂中，僅有空蕩的海洋圍繞寂靜的大陸。

nanotechnology that gets out of control,

億萬的行星等待我們發現以及充滿生命。

genetic engineering of the perfect super bug,

億萬的新家園，等待我們的到來。

an experiment that lights the whole atmosphere on fire.

億萬的新家園，等待我們的到來。

It might be a super-intelligent AI

發現和我們相近的外星人的機率到底有多大呢?

that accidentally (or purposely) destroys its creators.

這取決於在恆星附近的"適居帶"有多少星球

Or things that we can't even see coming right now.

"適居帶"就是和恆星有剛好的距離能夠讓液態水存在。

Or it's way simpler:

由於每顆恆星的大小和型態都不一樣

species competitive enough to take over their planet

他們的適居帶也不一樣， 大概只需要一點點的物理學去計算........吧?

necessarily destroy it while competing with each other for resources.

如果你對那一點點的物理學有興趣，這個網站可以跟你解釋這是如何計算的

Maybe there are runaway chain reactions in every ecosystem

Brilliant 是一個解決問題的網站， 可以引導你像科學家一樣的化解問題~

that once set in motion, are not fixable.

它們的內容像這樣~ 會分成很多細小的部分。

And so once a civilization is powerful enough to change the composition of its atmosphere,

讓你每個細節都很清楚之後 堆疊起來，得到有趣的結論~

they make their planet uninhabitable

如果你造訪這個網站~

100% of the time.

或是直接點下面的連結

Let's hope that that's not the case.

你就可以免費註冊並學習任何東西啦~

If the filter IS ahead of us,

而我們的觀眾有福拉~

our odds are really bad.

前688位觀眾在裡面可以享有20%的優惠喔!

What we can hope for.

而現在如果你真的發現了外星生命?

THIS is why finding life beyond Earth would be horrible.

也許現在暫時不理會它們是一個明智的選擇

The more common life is in the universe,

00:09:30,665 --> 00:09:31,665

and the more advanced and complex it is,

the more likely it becomes that a filter is in front of us.

Bacteria would be bad,

small animals would be worse,

intelligent life would be alarming.

Ruins of ancient alien civilizations...

would be horrible.

The best case scenario for us right now

is that Mars is sterile,

that Europa's oceans are devoid of life,

and the vast arms of the Milky Way

harbor only empty oceans hugging dead continents.

...That there are billions of empty planets waiting to be discovered

and to be filled up with life.

Billions of new homes...

waiting for us...

to finally arrive.

How likely is it that we'll find life outside of Earth that is similar to us?

Well, that depends on how many planets there are out there

in their star's Goldilocks Zone--

the area around the star where water can be liquid.

Because stars come in all sizes and configurations,

this zone is different for every star system

and requires a little bit of physics to figure out.

If that sounds like fun to you, this quiz from Brilliant

helps to break down the maths for exactly how this is calculated.

Brilliant is a problem-solving website

that teaches you to think like a scientist

by guiding you through problems.

They take concepts like these break them up, into bite-sized bits

present clear thinking in each part, and then build back up to an interesting conclusion.

If you visit brilliant.org/nutshell or click the link in the description,

you can sign up for free and learn

all kinds of things.

And as a bonus for Kurzgesagt viewers,

the first 688 people will also get 20% of their annual membership.

And if you DO find life on other planets,

it may be wise to leave them alone for a while.