(Music) The basic question is, does life exist beyond Earth? Scientists who are called astrobiologists are trying to find that out right now. Most astrobiologists are trying to figure out if there's microbial life on Mars, or in the ocean under the frozen surface of Jupiter's moon Europa, or in the liquid hydrocarbon lakes that we've found on Saturn's moon Titan. But one group of astrobiologists works on SETI. SETI is the Search for Extraterrestrial Intelligence, and SETI researchers are trying to detect some evidence that intelligent creatures elsewhere have used technology to build a transmitter of some sort. But how likely is it that they will manage to find a signal? There are certainly no guarantees when it comes to SETI, but something called the Drake equation, named after Frank Drake, can help us organize our thinking about what might be required for successful detection. If you've dealt with equations before, then you probably expect that there will be a solution to the equation, a right answer. The Drake equation, however, is different, because there are so many unknowns. It has no right answer. As we learn more about our universe and our place within it, some of the unknowns get better known, and we can estimate an answer a bit better. But there won't be a definite answer to the Drake equation until SETI succeeds or something else proves that Earthlings are the only intelligent species in our portion of the cosmos. In the meantime, it is really useful to consider the unknowns. The Drake equation attempts to estimate the number of technological civilizations in the Milky Way Galaxy -- we call that N -- with whom we could make contact, and it's usually written as: N equals R-star multiplied by f-sub-p multiplied by n-sub-e multiplied by f-sub-l multiplied by f-sub-i multiplied by f-sub-c and lastly, multiplied by capital L. All those factors multiplied together help to estimate the number of technological civilizations that we might be able to detect right now. R-star is the rate at which stars have been born in the Milky Way Galaxy over the last few billion years, so it's a number that is stars per year. Our galaxy is 10 billion years old, and early in its history stars formed at a different rate. All of the f-factors are fractions. Each one must be less than or equal to one. F-sub-p is the fraction of stars that have planets. N-sub-e is the average number of habitable planets in any planetary system. F-sub-l is the fraction of planets on which life actually begins and f-sub-i is the fraction of all those life forms that develop intelligence. F-sub-c is the fraction of intelligent life that develops a civilization that decides to use some sort of transmitting technology. And finally, L -- the longevity factor. On average, how many years do those transmitters continue to operate? Astronomers are now almost able to tell us what the product of the first three terms is. We're now finding exoplanets almost everywhere. The fractions dealing with life and intelligence and technological civilizations are ones that many, many experts ponder, but nobody knows for sure. So far, we only know of one place in the universe where life exists, and that's right here on Earth. In the next couple of decades, as we explore Mars and Europa and Titan, the discovery of any kind of life there will mean that life will be abundant in the Milky Way. Because if life originated twice within this one Solar System, it means it was easy, and given similar conditions elsewhere, life will happen. So the number two is a very important number here. Scientists, including SETI researchers, often tend to make very crude estimates and acknowledge that there are very large uncertainties in these estimates, in order to make progress. We think we know that R-star and n-sub-e are both numbers that are closer to 10 than, say, to one, and all the f-factors are less than one. Some of them may be much less than one. But of all these unknowns, the biggest unknown is L, so perhaps the most useful version of the Drake equation is simply to say that N is approximately equal to L. The information in this equation is very clear. Unless L is large, N will be small. But, you know, you can also turn that around. If SETI succeeds in detecting a signal in the near future, after examining only a small portion of the stars in the Milky Way, then we learn that L, on average, must be large. Otherwise, we couldn't have succeeded so easily. A physicist named Philip Morrison summarizes by saying that SETI is the archaeology of the future. By this, he meant that because the speed of light is finite, any signals detected from distant technologies will be telling us about their past by the time they reach us. But because L must be large for a successful detection, we also learn about our future, particularly that we can have a long future. We've developed technologies that can send signals into space and humans to the moon, but we've also developed technologies that can destroy the environment, that can wage war with weapons and biological terrorism. In the future, will our technology help stabilize our planet and our population, leading to a very long lifetime for us? Or will we destroy our world and its inhabitants after only a brief appearance on the cosmic stage? I encourage you to consider the unknowns in this equation. Why don't you make your own estimates for these unknowns, and see what you come up with for N? Compare that with the estimates made by Frank Drake, Carl Sagan, other scientists or your neighbors. Remember, there's no right answer. Not yet.
(音樂) 基本的問題是: 地球之外是否有生命存在? 被稱為「太空生物學家」的科學家們 正在嘗試找出答案 大多數太空生物學家想要瞭解 是否有微生物存在於火星上 或是在木星的衛星「木衛二」 冰層下的海洋 或是在土星的衛星「土衛六」 那裡已經被發現的液態烴湖泊中 但是,有一群太空生物學家 正在參與 SETI 計畫 SETI 的全名是 「尋找外星智慧計畫」 SETI 的研究人員正試圖找出證據 來說明外星球的智能生物 已經能用科技 建造某種信號發射器 但是他們有多少機率 能夠偵測到信號? 沒有人能夠保證 SETI 會成功 但是藉由弗蘭克·德雷克之名所命名的 德雷克公式 可以幫助我們整理思緒 並瞭解什麼是成功探測的必要條件 如果你曾經解過其他方程式 那麼你可能會期望 方程式有解 也就是可以求出正確答案 然而,德雷克公式是截然不同的 因為它包含太多未知數 它並沒有正確答案 當我們對於宇宙 以及我們所處的土地 有更多的學習 我們對於一些未知數 也有更多的瞭解 我們可以估計出更好的答案 但是德雷克公式 並不會有明確的答案 除非等到 SETI 計畫成功 或是其他事物來證明 在我們所處的宇宙中 地球人是唯一的智慧生物 在此同時 研究未知數也具有其意義 德雷克公式嘗試估計出 在銀河系科技文明的數量 我們把這個數量稱之為 N 代表我們有能力 進行接觸的文明數量 公式通常表示為: N = R* 乘以 fp 乘以 ne 乘以 fl 乘以 fi 乘以 fc 最後,乘以大寫 L 將所有因數相乘 可幫助我們估計 我們也許能立刻偵測到的 科技文明數量 R* 代表的是 過去幾十億年以來 恆星在銀河系中形成的速率 所以它是指每年恆星誕生的數量 我們所處的銀河系 已經有一百億年以上了 但是在它的歷史初期 恆星以不同的速率形成 所有的 f 因數 都是以分數表示可能性 每一個因數都必須小於或等於一 fp 是指恆星擁有行星的可能性 ne 是指 在任何一個星系中 適合居住行星的平均數量 Fl 是指 行星發展出生命的可能性 而 fi 是指 生物能夠發展出高等智慧的可能性 fc 是指高等智慧的生物 能夠發展出文明 並且決定運用某項通訊技術 與外太空聯繫的可能性 最後 L 是代表壽命的因數 平均來說,就是那些通訊傳輸設備 能夠持續運作多少年? 天文學家現在 幾乎已經能告訴我們 前三項的乘積是多少 我們目前正在 四處尋找系外行星 德雷克公式中關於孕育生命 發展高等智慧 與技術文明的分數 目前有許許多多的專家 正在仔細探究 但是沒有人知道確切的數字 直到目前為止 我們知道宇宙之中 唯一有生物存在的地方 毫無疑問的,就是地球 在接下來的幾十年之中 隨著我們對火星 木衛二和土衛六的探索 只要我們能在那些地方 發現任何物種的生命 就代表著銀河系之中 有著豐富多樣的生命存在 因為如果在這個太陽系之中 出現了兩種不同的生命起源 那麼這就表示 生命起源很容易發生 因此可以假設 如果別的地方也有相似條件 就會產生生命 因此,「二」這個數字 在這裡非常重要 科學家,包括 SETI 計畫的研究人員 經常試著作粗略的估計 希望能取得進展 但是也必須承認 這些估計仍然有很大的不確定性 我們認為已知的 R* 和 ne 這兩個變數 都是比較接近 10 而不是 1 而所有的 f 因數都小於 1 其中某些因數還可能遠小於 1 但在所有的未知數之中 最大的變數是 L 所以在德雷克公式中 可能最有用的版本 簡單來說是 N 大約等於 L 這個等式表達出 一個很清楚的訊息 除非 L 很大 否則 N 就會很小 但是你也可以把它倒過來看 如果 SETI 計畫在不久之後 成功偵測到了信號 而我們又已經能夠證明 在銀河系中 只有少數行星具有生命 於是我們就可以得知 平均來說, L 是很大的 否則,我們不會那麼容易 偵測到外星智慧 有一位物理學家菲力普·莫里森 提出一句總結: SETI 計畫是針對未來的考古學 這句話他想表達的是: 因為光的傳播速度是有限的 任何我們所偵測到 來自遠方科技的信號 當信號到達地球時 所能告訴我們的一切 都已經成為過去 但由於 L 必須是很大的數字 對外星智慧的偵測才能成功 所以可以了解到我們的未來 尤其是我們可能 還有很長的未來要走 我們已經開發出 能夠將信號送入太空 以及將人類送上月球的科技 但我們也開發了破壞環境的科技 以及利用武器和生物恐怖主義 發動戰爭的科技 在未來 我們的科技是否能協助穩定 我們的星球與全體民眾 讓人類可以長久存在? 或者是,我們將會在 短暫出現於宇宙的舞台之後 便毀掉我們的世界和居民呢? 我鼓勵大家思考 德雷克公式中的未知數 你們何不自己預測這些未知數 然後看看如何解決 N? 將你的預測結果和法蘭克·德雷克 卡爾·薩根或是其他科學家 甚至和你的鄰居作比較 記得,並沒有標準答案 至少目前還沒有