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我在 SETI 的四十年
文: Philip Morrison

提出微波搜尋的提案

1950年代中期,在結束了戰時服務期間我所擔任的的中子開發工程師之後,我投入了高能天文學(high-energy astronomy), 由於無線電天文學的成功,連帶使得開闢新領域的念頭變得極具有吸引力。 在1958年,我想看看伽瑪射線是否有指望開闢出新的領域,而結果就像原先預測的一樣。 其中一點是它們可以輕易橫穿行星遍佈的銀河的一個特性,不像以光速行進的直線。 我的一個足智多謀的朋友同時也是我在康乃爾大學的同事,Giuseppe Cocconi,跑來向我提出一個問題: 「我們已經製造出伽瑪射線了(在當時,電子同步速器, electron synchrotron 還是嶄新的科技),為什麼我們不試著將它們射向太空,然後看看是否會被其他"人"偵測到?」 這實在是一個令人驚異的問題,卻也另人感到興奮。我的回答是我們應該先看看整個伽瑪射線的光譜(spectrum)、無線電,然後決定該訊號最合適的頻帶(band)。

到了1959年,我們已獲得的足夠的知識,得以提出微波無線電為最適合接聽的頻帶;但尚未對外做傳送--因為受限於我們自已的能力。 康乃爾大學在接近波多黎各的阿雷西伯望遠鏡附近所擁有無線電碟盤,在當時,即將有能力可以偵測 到像它一樣大的碟盤在星際間的距離所發出的訊號,前提是方位要對準、且具有足夠強度。 而能和阿雷西伯望遠鏡相對應的發送源,則甚至可以遠至銀河的另一端。 伽瑪射線相較之下難以掌控得多,而普通光線則受限於其他星球產生的亮光。 阿雷西伯發出的微波,擁有筆直的電波束而且頻率非常的精準,阿雷西伯發出的微波即使面對太陽、銀河所散射出來的微波 都將使它們相形失色!它位於1420兆赫(21公分)的氫線(hydrogen line)上,乃最具研究的自然無線電放射。

整個銀河系在40年內沒什麼大改變,但我們的科技和眼光卻有長足的進展。 在地球上的許多無線電碟盤中,阿雷西伯一直保持它的「最大片」冠軍寶座,而且搜尋能力也可說是最好的。 雖然微波接收器的技術不斷在進步,不過卻沒有人可奪走阿雷西伯的地位。 現今的人為干擾訊號和以前相比真是多上太多了。不過經驗告訴我們有許多方法可以減少它們的影響(或許哪一天我們也會有能力接收月球背面坑泂底部傳出的訊號--被最大干擾訊號遮蔽的情況下)。

搜尋

在搜尋之前必須先決定要對向哪個方位、在什麼時間對準、每個選定方位要對準的時間長短、涵蓋的頻率範圍、要找尋多麼微弱的訊號、以及訊號的型式? 由於此工作仍有許多不明確的界線,故只有少部份系統性的搜尋已使用最低限度的的資源完成。 我們最主要受限兩項因素,一是於自然界的可知性質、一是發送訊者可能做的各種難以猜測的決定。 第一個對 SETI 所做的努力由Green Bank 的 Frank Drake 完成,而他也是一位如假包換的無線電天文學家。 他使用了一個大小適中的碟盤,對準一些像是太陽的星球,他一個一個的搜尋 -- 並找到來源不明的人為瞬間干擾訊號。


Frank Drake

不過由於此工作的簡陋規格,若想搜尋天空中比較寬廣的掃瞄區,也可能不用大片碟盤就可辦到,不過 傳送端將需要難以置信的的功率,且必須同時向多個方向發射。 阿雷西伯的微電波接收束可以在天空一度的掃瞄區中區別出數千個方位。而在天空中,有40,000個像這樣 的掃瞄區(別忘了還有南半球的天空呢!我們最近也掃瞄了南半球有一陣子了)您會選擇觀測所有的 方位,亦或是只選選一些特定的點呢? 到目前為止,兩者看來都很重要:一個個的窄頻電波束增加了像是在我們附近的星球一樣的可辨識目標, 而覆蓋整個天空的範圍,則必須一個點一個點搜尋敏感度較低的部分。 要想預知未知的彼方所使用的傳輸設定強度真的很不容易。 大部分的銀河空間都非常遙遠,在這麼多星球中,當然,也可能會有發送源很罕見的會擁有很強的訊號。 在我們附近的行星比較起來可能性比較小,且也不需要這種罕見的能力。

物理學告訴我們,在銀河系星球之間所漂流的稀薄氣體意味著,即使是當最強烈的訊號在行進開始時, 它的頻率也會被搞模糊掉。 perhaps to acquire an enforced width of 0.1 hertz; if that holds, it is not much use to look for still narrower ones. 即使我們接受先前天真的 1420 兆赫頻帶推論,上千萬種調頻方式對於這種搜尋單頻帶的工作其實都還不多。 再乘上取決於方向的頻率數,結果是一個完整的搜尋會需要上兆次的短期監聽。 當然我們將會考慮用紅外線、光學頻帶的雷射訊號 -- 目前正順著整個光譜進行審慎的試驗。

Only one visit to each possible choice? A sender sweeping the stars to economize on power has to face the fatal chance that his blind choice of time at the right direction and the right frequency has ended any chance of success. The answer is to repeat, repeat, repeat, or burn power steadily. These alternatives have been studied, and plausible choices made—doubtless to be remade as we learn more.

Multiplicity

SETI@home participants themselves demonstrate the major change in technology since 1960, not new dishes, new receivers, or even new knowledge of stars and their medium. It is the multiplicity of choice implied by the amazing rise in computer power. The early proposals expected a thousand channels at once to be recorded during the search , a good start at shortening the search time. Today we operate rather inexpensive systems that can receive data in one hundred million channels all at once. Nor is the limit clearly at hand. The million and more volunteer CPUs put to use now to help analyze a backlog of recent search data from Arecibo is only a sign of what lies ahead in the next century of signal processing.

Searching in Time

SETI seeks one day to search the space of this Galaxy, a home to a few hundred million suitable suns. An extragalactic reach opens so many possibilities that an experimenter is daunted, even though his pencil beam covers a large area of stars all at once in a distant galaxy.. All of them are millions of light years away. Any sender way out there has to wait out the round trip as a minimum time for an answer. So much does this transcend our idea of history—our own species is maybe 100,000 years old-- that we find it hard to plan. One brief search was made at Arecibo years ago of the nearest big galaxy, the Andromeda spiral ; it brought no signal. How little we understand of what to expect and how to act over such depths of time!


Andromeda Galaxy

But there is a much smaller time delay --still more than we ordinarily face in experiment design--when we restrict our search within our own home spiral, the Milky Way. First of all the nearest 100 stars, most of them faint red dwarf stars much dimmer than the sun, occupy a sphere about 50 light years across. Before 1960 or so, we had no way to know whether or not every nearby star was actively sending our way. We are less worried now about a crowded dial. Those common fainter stars do not promise much; they seem unlikely to warm a planet steadily and safely, and there are sun-like stars by the billion farther out .

The marvelous 1999 discovery of a planet in orbit around a distant sun-like stable star has shown that planets resembling our own neighbor Jupiter accompany a few percent of all the sun-like stars we have examined near us, out as far as 150 light years or so. We know about two dozen such sun-planet systems, though not yet one earth-like planet, for our present methods are too crude to detect such small rocky planets as Earth even if they are present. We can presently find only the gas giants. That should change within the next decade or so, once we have launched new space-borne instruments able to detect earths-- if they are there.

Looking for Peers

Begin with symmetry, which promises the possibility of life and astronomers resembling ourselves in goal, if not in appearance. To find 100 candidates we need to examine many planetary systems, even if we make the most optimistic guess that earths –so far not seen near any star but our Sun —will be found, and not merely the hot Jupiters in tight orbits about their star that now fill our initial lists. An optimist would propose looking among the billion stars up to 1000 light years away, our near galactic neighbors. It follows at once that we should listen for hundreds of years before we need try to send, on the good grounds that we are not likely to be the first astronomers among hundreds of star-warmed earths. Even though we cannot exclude our priority , we cannot claim any evidence for it, save our own single example. It makes sense to listen for a century or two before we enter after consensus on a serious phase of transmission on our own —systematic sending is far more costly than listening . So we say "keep on listening", and improve our efforts, possibly with other signal types, until some day, maybe in the year 2100, the issue of sending might be raised.

But we know this: we did not begin radio astronomy (or lasers or gammas or neutrinos or what you will) on a new planet. Rather, life grew here on Earth for about half the age of the galaxy before we humans even knew that the sun was a star among the stars. Our single species itself was five hundred centuries old before we knew our place in the sky. Reflection has led me to argue that we had to number on the order of a billion thinking human beings before our earth could become home to such devices as sensitive microwave receivers, longer still for the alternatives. Only a population at such scale can have given rise to the innumerable special discoveries, skills, insights and resources that comprise modern technology: from copper to mathematics, with theory and practice worldwide that underlie all of astronomy and its imaginative dreams. But can that specialization appear if many more people still have not long been growing crops, digging in the mines, voyaging, writing, drawing-- yes, and dreaming. A hunting band, however wise its individuals, is not persuasive as a realistic basis for interstellar signaling, our SETI. A billion humans lived on earth around 1800 (of course, I mean only that magnitude, not a precise figure). Technology is a social phenomenon spread among billions whose diverse lifework led to what we now can do. IQ does not alone create means for detecting signals from the stars. Intelligence is necessary but it is insufficient. The first firemakers of the caves, the Cro-Magnon flint knappers, the Europeans around Galileo and Newton—admirable discoverers all, but none could make an interstellar search with any chance of success.

A rough estimate of one social, adept, thoughtful species of a billion members implies a few billion years of evolution as far as we now know—true, though from only one example. That tentatively defines the time scale. Without a synchronizing feature we do not notice at all in the Galaxy, we cannot expect a good match to our quite new status, for we have known of radio astronomy for less than one century out of all earth history. One sees at once that detectable counterparts are likely to be ahead of our level of technology, while the rest of them, still silent and undetectable, are far behind on the human scale of time.

What we see as possible is an ambitious project, old by any human standards, defined by a timescale we do not know, undertaken perhaps intermittently by some curious, effective, but by no means all-powerful, species of billions of beings of our technological kind dwelling somewhere out there among the many, many stars. They too must pay the energy bills and await an answer, perhaps not for the first time. Their presence is a conjecture only. Most likely they reside among the tens of millions of planetary systems which we now expect through a very small, close-by sample of two dozen gaseous planets. A planet near a stable sun is the lowest rung of a ladder of still only conjectural nature that may follow the one case we now know by upward rung after rung to another earth, to life, to evolution, and eventually—perhaps-- to sentience, curiosity, and capability. SETI is an audacious but direct search for the top rung of the ancient ladder.

Digging for more evidence, finally the physical signals we hope for, SETI is the task of our time. It will last many years, until we succeed or at least until we accept the other amazingly unlikely case, that we are the very first to attempt distant exchange among the 400 billion stars of the Milky Way. After all, if we are to hear any authentic signal among the stars that functionally resembles what we do, such conjectures must have been true and enacted in reality somewhere out there in earlier times.

Close with a salutation very old among our clever forebears: Good hunting!

Philip Morrison


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