I was eight or nine,

我曾經看過一次幽浮。

playing in the street with a friend who was a couple of years older,

我當時約八、九歲，

and we saw a featureless silver disc hovering over the houses.

和一位朋友在街上玩耍， 他比我大幾歲，

We watched it for a few seconds,

我們看到了一個毫無特色的 銀色碟形物在房子上方盤旋。

and then it shot away incredibly quickly.

我們看了幾秒鐘，

Even as a kid,

接著，它以非常快的 速度瞬間飛走。

I got angry it was ignoring the laws of physics.

即使當時只是孩子，

We ran inside to tell the grown-ups,

我很氣它竟然可以不遵守物理定律。

and they were skeptical --

我們跑進屋內告訴大人，

you'd be skeptical too, right?

大人充滿懷疑——

I got my own back a few years later:

換成是你們也會懷疑，對吧？

one of those grown-ups told me,

幾年後我也找回了我的懷疑：

"Last night I saw a flying saucer.

其中一位大人告訴我：

I was coming out of the pub after a few drinks."

「昨晚我看到了飛碟。

I stopped him there. I said, "I can explain that sighting."

我在酒吧喝了幾杯， 當時正要離開。」

(Laughter)

我在那時打斷他。我說： 「我能解釋你的目擊。」

Psychologists have shown we can't trust our brains

（笑聲）

to tell the truth.

心理學家已經證明了 我們不能相信我們的大腦

It's easy to fool ourselves.

會告訴我們真相。

I saw something,

要愚弄自己是很容易的。

but what's more likely --

我看到某種東西，

that I saw an alien spacecraft,

但哪種情況比較有可能——

or that my brain misinterpreted the data my eyes were giving it?

是我看到了外星人的太空船？

Ever since though I've wondered:

或是我的大腦錯誤詮釋了 我的眼睛給它的資料？

Why don't we see flying saucers flitting around?

從那之後，我就一直納悶：

At the very least,

為什麼我們不會到處看到飛碟？

why don't we see life out there in the cosmos?

至少，

It's a puzzle,

為什麼我們不會看到 在宇宙中有生命？

and I've discussed it with dozens of experts

這是個謎，

from different disciplines over the past three decades.

在過去三十年，我已經

And there's no consensus.

和數十位不同領域的專家討論過。

Frank Drake began searching for alien signals back in 1960 --

沒有達成任何共識。

so far, nothing.

1960 年，法蘭克德雷克 就開始尋找外星人的線索——

And with each passing year,

目前，一無所獲。

this nonobservation,

隨著每一年過去，

this lack of evidence for any alien activity gets more puzzling

都沒人觀察到，

because we should see them, shouldn't we?

且缺乏外星人活動的證據， 讓這一切更令人迷惑不解，

The universe is 13.8 billion years old,

因為我們應該會看見他們，對吧？

give or take.

宇宙已經 138 億歲了，

If we represent the age of the universe by one year,

這是概略數字。

then our species came into being about 12 minutes before midnight,

若我們把宇宙的年齡比喻為一年，

31st December.

那麼我們這個物種大約是在 午夜前 12 分鐘才出現的，

Western civilization has existed for a few seconds.

12 月 31 日。

Extraterrestrial civilizations could have started in the summer months.

西方文明才幾秒鐘而已。

Imagine a summer civilization

外星文明可能在夏季就已經出現。

developing a level of technology more advanced than ours,

想像一個夏季文明，

but tech based on accepted physics though,

發展出了比我們更先進的科技水平，

I'm not talking wormholes or warp drives -- whatever --

不過，我是指以已經 被接受的物理為基礎，

just an extrapolation of the sort of tech that TED celebrates.

不是指蟲洞或曲速引擎—— 諸如此類——

That civilization could program self-replicating probes

只是 TED 所頌掦的 那種科技的推斷而已。

to visit every planetary system in the galaxy.

該文明有可能可以設計出 會自我複製的探測飛船，

If they launched the first probes just after midnight one August day,

去探訪銀河系中的每一個星系。

then before breakfast same day,

如果他們在八月某一天的午夜之後 發射了第一艘探測飛船，

they could have colonized the galaxy.

那麼在同一天的早餐時間之前，

Intergalactic colonization isn't much more difficult,

他們就已經殖民了整個銀河系。

it just takes longer.

跨銀河系的殖民並不會困難太多，

A civilization from any one of millions of galaxies

只是比較花時間。

could have colonized our galaxy.

數百萬銀河系中的任何一個文明

Seems far-fetched?

都有可能在我們的銀河殖民。

Maybe it is,

似乎很牽強？

but wouldn't aliens engage in some recognizable activity --

也許是吧，

put worldlets around a star to capture free sunlight,

但外星人不會從事一些 可辨識的活動嗎？——

collaborate on a Wikipedia Galactica,

比如在一顆恆星的周圍建立 小世界來捕捉免費的陽光、

or just shout out to the universe, "We're here"?

合作撰寫銀河維基百科，

So where is everybody?

或是對著宇宙大叫： 「我們在這裡！」

It's a puzzle because we do expect these civilizations to exist, don't we?

所以，他們在哪裡？

After all, there could be a trillion planets in the galaxy --

這是個謎，因為我們確實預期 會有這類文明存在，對吧？

maybe more.

畢竟，銀河系中 可能有一兆個星球——

You don't need any special knowledge to consider this question,

甚至更多。

and I've explored it with lots of people over the years.

不需要任何特殊知識， 你也會去思考這個問題，

And I've found they often frame their thinking

這些年來，我已經和許多人 一起探索了這個問題。

in terms of the barriers that would need to be cleared

我發現，他們通常 在表述他們的想法時，

if a planet is to host a communicative civilization.

會用需要被清除的四個障礙

And they usually identify four key barriers.

來思考一個星球是否會有 能夠溝通的文明存在。

Habitability --

他們通常會去確認四項關鍵障礙。

that's the first barrier.

可居住性——

We need a terrestrial planet in that just right "Goldilocks zone,"

那是第一個障礙。

where water flows as a liquid.

我們需要有一個剛好屬於 「適居帶」的外星星球，

They're out there.

也就是說，水要以液體形式流動。

In 2016, astronomers confirmed there's a planet in the habitable zone

外面有這種星球存在。

of the closest star,

2016 年，太空人確認了 在最近的恆星適居帶上

Proxima Centauri --

有一個星球，

so close that Breakthrough Starshot project plans to send probes there.

這個恆星是比鄰星——

We'd become a starfaring species.

它非常近，近到「突破攝星」 計畫打算要送探測飛船過去。

But not all worlds are habitable.

我們成了在星際旅行的物種。

Some will be too close to a star and they'll fry,

但並非所有世界都可居住。

some will be too far away and they'll freeze.

有些會太靠近恆星，會被烤熟，

Abiogenesis --

有些則太遙遠，會被冰凍。

the creation of life from nonlife --

無生源論——

that's the second barrier.

從無生命到創造生命——

The basic building blocks of life aren't unique to Earth:

那是第二個障礙。

amino acids have been found in comets,

生命的基礎建材並非地球獨有：

complex organic molecules in interstellar dust clouds,

已經發現慧星上有氨基酸，

water in exoplanetary systems.

行星際塵雲中有複雜的有機分子，

The ingredients are there,

行星系統中有水。

we just don't know how they combine to create life,

原料已經存在，

and presumably there will be worlds on which life doesn't start.

我們只是不知道它們 要如何結合起來創造生命，

The development of technological civilization is a third barrier.

很有可能，這些世界的 生命跡象尚未開始。

Some say we already share our planet with alien intelligences.

第三個障礙是科技文明的發展。

A 2011 study showed that elephants can cooperate to solve problems.

有些人說，我們已經在 和外星智慧共享我們的星球。

A 2010 study showed

一篇 2011 年的研究指出， 大象能夠合作來解決問題。

that an octopus in captivity can recognize different humans.

一篇 2010 年的研究指出，

2017 studies show that ravens can plan for future events --

被囚禁的章魚能夠認出不同的人類。

wonderful, clever creatures --

2017 年的研究則指出， 渡鴉會規劃未來事件——

but they can't contemplate the Breakthrough Starshot project,

很美好、聰明的生物——

and if we vanished today,

但牠們無法去思考 「突破攝星」計畫，

they wouldn't go on to implement Breakthrough Starshot --

如果今天我們都消失了，

why should they?

牠們也不會去執行 「突破攝星」計劃——

Evolution doesn't have space travel as an end goal.

為什麼牠們要去執行？

There will be worlds where life doesn't give rise to advanced technology.

演化的最終目標並不是太空旅行。

Communication across space -- that's a fourth barrier.

有一些世界中的生命 不會去發展先進科技。

Maybe advanced civilizations choose to explore inner space

跨越太空的溝通—— 那是第四個障礙。

rather than outer space,

也許先進文明會選擇探索內太空，

or engineer at small distances rather than large.

而不是外太空，

Or maybe they just don't want to risk an encounter

或做短距離的工程建設， 而非長距離的。

with a potentially more advanced and hostile neighbor.

或許，他們只是單純 不想冒險，以免遇到

There'll be worlds where, for whatever reason,

可能更先進且有敵意的鄰居。

civilizations either stay silent or don't spend long trying to communicate.

在某些世界，不論是什麼理由，

As for the height of the barriers,

文明可能會保持沉默，或不會 花費心力去試圖與遠方溝通。

your guess is as good as anyone's.

至於障礙有多高，

In my experience,

大家都只能猜測。

when people sit down and do the math,

依我的經驗，

they typically conclude there are thousands of civilizations in the galaxy.

當大家坐下來算數學時，

But then we're back to the puzzle: Where is everybody?

他們的結論通常是， 在銀河系中有數千個文明存在。

By definition,

但，我們又回到 原本的謎：他們在哪裡？

UFOs -- including the one I saw --

就定義來說，

are unidentified.

幽浮（不明飛行物）—— 包括我看見的那一個——

We can't simply infer they're spacecraft.

是「不明」的。

You can still have some fun playing with the idea aliens are here.

我們不能很簡單地說 它們是太空船。

Some say a summer civilization did colonize the galaxy

你仍然可以為了好玩 而想像有外星人來到這裡。

and seeded Earth with life ...

有人說，夏季文明 的確已經殖民了銀河系，

others, that we're living in a cosmic wilderness preserve --

且在地球播下生命的種子……

a zoo.

其他人說，我們是住在 宇宙的荒野保護區——

Yet others --

一個動物園。

that we're living in a simulation.

還有其他人——

Programmers just haven't revealed the aliens yet.

說我們住在模擬世界中。

Most of my colleagues though argue that E.T. is out there,

只是程式設計師尚未 讓外星人現身而已。

we just need to keep looking,

不過，我大部分的同事 主張有 E.T. 存在，

and this makes sense.

只是我們得要繼續尋找，

Space is vast.

這是合理的。

Identifying a signal is hard,

太空很大。

and we haven't been looking that long.

找到一個訊號是很困難的，

Without doubt, we should spend more on the search.

且我們才開始找沒有很久。

It's about understanding our place in the universe.

無疑的，我們應該 投入更多心力去搜尋。

It's too important a question to ignore.

重點在於了解我們 在宇宙中的定位。

But there's an obvious answer:

這個問題太重要了，不能無視它。

we're alone.

但，有一個很明顯的答案：

It's just us.

我們是孤單的。

There could be a trillion planets in the galaxy.

只有我們。

Is it plausible we're the only creatures capable of contemplating this question?

在銀河系可能有一兆個星球。

Well, yes, because in this context,

若說我們是唯一會思忖 這個問題的生物，聽起來合理嗎？

we don't know whether a trillion is a big number.

是的，因為在這個情況中，

In 2000, Peter Ward and Don Brownlee proposed the Rare Earth idea.

我們不知道一兆是否 算是很大的數目。

Remember those four barriers

2000 年時，彼得沃德和唐布朗利 提出了「地球殊異」的想法。

that people use to estimate the number of civilizations?

還記得人們用來

Ward and Brownlee said there might be more.

估計文明數目的那四個障礙嗎？

Let's look at one possible barrier.

沃德和布朗利說，可能還有更多。

It's a recent suggestion by David Waltham,

咱們來看看一個可能的障礙。

a geophysicist.

這是地球物理學家大衛沃爾瑟姆

This is my very simplified version

在近期提出來的。

of Dave's much more sophisticated argument.

這是我的極簡化版，

We are able to be here now

大衛的原版是更精密許多的主張。

because Earth's previous inhabitants enjoyed

我們現在能夠在這裡，

four billion years of good weather --

是因為地球先前的居民享受了

ups and downs but more or less clement.

40 億年的好天氣——

But long-term climate stability is strange,

雖然起起落落， 但總的來說仍然是溫和的。

if only because astronomical influences

但長期的穩定氣候是很奇特的，

can push a planet towards freezing or frying.

因為天文氣候的影響是有可能

There's a hint our moon has helped,

將一個星球推向結冰或烤熟狀況的。

and that's interesting

我們的月球似乎是有幫上忙，

because the prevailing theory is

那很有趣，

that the moon came into being when Theia,

因為普遍流行的理論是

a body the size of Mars,

月球會出現是因為忒伊亞，

crashed into a newly formed Earth.

和火星一樣大的天體，

The outcome of that crash could have been a quite different Earth-Moon system.

撞上了剛形成的地球。

We ended up with a large moon

撞擊的結果本來有可能 是個很不同的地球—月球系統。

and that permitted Earth to have both a stable axial tilt

我們最後得到的是一個大月球，

and a slow rotation rate.

那讓地球有一個穩定的 軸向傾斜角度，

Both factors influence climate

和緩慢的轉速。

and the suggestion is that they've helped moderate climate change.

這兩個因子影響了氣候，

Great for us, right?

意味著它們協助調節了氣候改變。

But Waltham showed that if the moon were just a few miles bigger,

對我們很好，對吧？

things would be different.

但沃爾瑟姆指出， 如果月球再大個幾英哩，

Earth's spin axis would now wander chaotically.

一切就會不同。

There'd be episodes of rapid climate change --

地球的轉軸現在就會 混亂地偏離正軌。

not good for complex life.

就會發生快速氣候變遷的情形——

The moon is just the right size:

對複雜的生命來說不是好事。

big but not too big.

月球的大小剛剛好：

A "Goldilocks" moon around a "Goldilocks" planet --

夠大但不會太大。

a barrier perhaps.

在一個「適居」的星球周圍 有個「適居」的月球——

You can imagine more barriers.

也許也是個障礙。

For instance,

你們可以想像出更多障礙。

simple cells came into being billions of years ago ...

比如，

but perhaps the development of complex life

數十億年前， 簡單細胞就出現了……

needed a series of unlikely events.

但也許要發展出複雜的生命，

Once life on Earth had access to multicellularity

還需要一連串 不太可能發生的事件。

and sophisticated genetic structures,

一旦地球上的生命能夠有多細胞

and sex,

以及精密的基因結構，

new opportunities opened up:

還有性，

animals became possible.

就會有新的機會產生：

But maybe it's the fate of many planets

可能會出現動物。

for life to settle at the level of simple cells.

但，也許，許多星球的命運就是

Purely for the purposes of illustration,

只會有簡單細胞等級的生命存在。

let me suggest four more barriers to add to the four

純粹只是為了說明清楚，

that people said blocked the path to communicative civilization.

讓我再提出四個 可能的障礙，來補充

Again, purely for the purposes of illustration,

前面已提到大家認為會阻擋 與外星文明溝通的四個障礙。

suppose there's a one-in-a-thousand chance of making it across each of the barriers.

我再說一遍，純粹只是為了 補充說明清楚，

Of course there might be different ways of navigating the barriers,

假設跨越每一個障礙的 機會是千分之一。

and some chances will be better than one in a thousand.

當然，可能會有不同的 方式來通過障礙，

Equally, there might be more barriers

有些機會可能會高於千分之一。

and some chances might be one in a million.

同樣的，可能會有更多障礙

Let's just see what happens in this picture.

且當中某些的機會 只有百萬分之一。

If the galaxy contains a trillion planets,

咱們來看看，在這狀況下 會發生什麼事。

how many will host a civilization capable of contemplating like us

如果銀河系有一兆個星球，

projects such as Breakthrough Starshot?

有多少個星球上會有文明存在， 且像我們一樣能夠去思忖

Habitability --

像「突破攝星」這樣的計畫？

right sort of planet around the right sort of star --

可居住性——

the trillion becomes a billion.

在對的恆星周圍有對的星球——

Stability --

一兆剩下了十億。

a climate that stays benign for eons --

穩定性——

the billion becomes a million.

氣候要長久都能很宜人——

Life must start --

十億剩下了一百萬。

the million becomes a thousand.

生命必須要能開始——

Complex life forms must arise --

一百萬剩下了一千。

the thousand becomes one.

必須要有複雜的生命形式出現——

Sophisticated tool use must develop --

一千剩下了一個。

that's one planet in a thousand galaxies.

必須要能夠去使用精密的工具——

To understand the universe,

一千個銀河中只有 一個星球能符合。

they'll have to develop the techniques of science and mathematics --

要了解宇宙，

that's one planet in a million galaxies.

他們就得要發展出 科學和數學的技術——

To reach the stars, they'll have to be social creatures,

一百萬個銀河中只有 一個星球能符合。

capable of discussing abstract concepts with each other

要去接觸其他恆星， 他們必須要是社交型的生物，

using complex grammar --

有能力和彼此討論抽象的概念，

one planet in a billion galaxies.

使用複雜的文法——

And they have to avoid disaster --

十億個銀河中只有 一個星球能符合。

not just self-inflicted but from the skies, too.

他們還得要避開災難——

That planet around Proxima Centauri,

不只是自己造成的災難， 還有來自天空的災難。

last year it got blasted by a flare.

繞著比鄰星的那個星球，

One planet in a trillion galaxies,

在去年被太陽耀斑給轟擊了。

just as in the visible universe.

一兆個銀河中只有 一個星球能符合。

I think we're alone.

這是在可見宇宙中的假設。

Those colleagues of mine who agree we're alone

我想我們是孤單的。

often see a barrier ahead --

我那些認同我們是孤單的同事

bioterror,

通常能夠先看到障礙——

global warming, war.

生物災難、

A universe that's silent

全球暖化、戰爭。

because technology itself forms the barrier

一個沉默的宇宙，

to the development of a truly advanced civilization.

因為科技本身就會形成障礙，