光學方法幫助尋找其他太陽系的生命

Science and technology.

科技。

The search for alien life

尋找外星生物

Twinkle, twinkle, little planet

閃爍吧，小行星

An undervalued optical trick may help to find life in other solar systems

一個不被人重視的光學方法可能能幫助尋找其他太陽系的生命

MOST astronomical telescopes employ reflection to focus starlight. A concave mirror creates an image from this light using a design pioneered in the 17th century, by Sir Isaac Newton. Those telescopes that do not employ reflection use refraction. They have a system of lenses, an idea first used to look at the stars by Galileo.

大部分天文望遠鏡都是運用的焦點星光的反射原理。早在17世紀，艾薩克.牛頓就開創性的利用這個光，讓凹透鏡產生了一個圖像。那些不利用反射的望遠鏡利用的是折射。他們有透鏡系統。利用透鏡系統的想法最早是伽利略用來觀測星星的。

But there is a third way to focus light. A century and a half after Newton, and more than two after Galileo, a Frenchman called Augustin-Jean Fresnel worked out that you can do it using diffraction. A set of concentric rings, alternately transparent and opaque, will scatter and spread light waves in a manner that causes them to reinforce each other some distance away, and thus form an image. The rings are known as a zone plate. And Fresnel's countryman, Laurent Koechlin, of the Midi-Pyrenees observatory, thinks zone plates are the way to find out if there is life on other planets.

但還存在第三種聚焦光源的方法。在牛頓利用星光的150年後，伽利略的透鏡系統的兩百多年後，一個名叫奧古斯丁.讓.菲涅爾的法國人想到也可以利用衍射來達到目的。一組兼有透明和不透明的同軸環可以分散和傳播光波，並且在稍遠的地方可以再使他們重新聚焦，由此形成一個圖像。這些環被稱作波帶片。法國南比利牛斯天文臺勞倫.凱什蘭認爲波帶片可以用來尋找其他星球上是否存在生命。

Seeing oxygen in another planet's atmosphere would be a giveaway of biological activity because the gas is so reactive that it needs to be continuously renewed. That would almost certainly mean something akin to photosynthesis was going on, for no known non-biological process can produce oxygen from common materials in sufficient quantity. Looking at such an atmosphere, though, is tricky. Stars are so much brighter than the planets which orbit them that their light overwhelms the small amount reflected from a planet's surface. And this is where Fresnel comes in.

在其他星球的大氣層發現氧氣則表明這個星球上有生物活動，因爲氧氣是一種非常活躍的氣體，所以他需要不停的更新。而這也就基本上意味着星球上進行着某些類似於光合作用的活動，因爲在我們已知的非生物學過程中，沒有一種過程可以在普通材料供應充足的情況下產生氧氣。然而，觀測到這樣的大氣層也是非常難的。恆星比以他們爲軌道運行的行星亮得多。他們的光蓋過了從行星表面上反射過來的少量光。而這也就是菲涅爾的突破口。

Fresnel telescopes have not been developed in the past because the image formed by one that was large enough to rival a useful-sized reflecting telescope would be several kilometres from the zone plate. But Dr Koechlin does not worry about that, because his Fresnel telescope will be in space. Free of the confounding effects of the Earth's own atmosphere, it will be able to isolate images of alien planets, make spectra of the light from their air, and examine those spectra for the characteristic dark lines that are caused by part of the light being absorbed by particular gases-oxygen among them.

過去，菲涅爾設計的望遠鏡還製造不出來，因爲要想使衍射望遠鏡的大小和正常使用的反射望遠鏡大小相同，它所產生的圖像就會距離波帶片數千米遠。然而，凱什蘭博士並不爲此擔心，因爲他的菲涅爾望遠鏡將會在太空中。脫離了地球大氣層的混淆效應，望遠鏡將能夠分離外星生物的圖像，在他們的大氣中製作出來光譜，並且爲這些有特點的暗線檢查光譜。這些暗線部分是由特殊氣體——大氣中的氧氣——所吸收的光產生的。

Plate tectonics

行星構造地質學

Space telescopes are nothing new, of course, and several more are in the works (see article). But existing plans to photograph extrasolar planets in this way involve orbiting arrays of reflecting telescopes all pointing in exactly the same direction. An array is needed because a single mirror big enough to do the job of separating star from planet would be too large to launch. The problem is the word "exactly". It means just that. The formation would have to fly with a precision of a few billionths of a metre.

當然，太空望遠鏡並不是什麼新事物，並且已經有幾個已經在使用中了（見文章）。但是在現在的計劃中，利用這種方法給太陽系以外的行星拍照就需要讓多組的反射望遠鏡在軌道運行的時候全部精準的朝向同一個方向。由於一個體積足夠大到能夠將恆星與行星分開的單一鏡面將會由於體積太龐大而無法發射，因此一組反射望遠鏡就是必須的。而問題就出在"精準"上。它就如字面意思一樣，要精準到十億分之一米。

Using a zone plate instead of a mirror gets around this. Because the plate is flat, it can be made of plastic and folded up for launch. Size thus ceases to be an issue. And although a second satellite containing the "eyepiece" (a special lens that also uses Fresnel optics, and a camera to record the image) must fly at the focus, the accuracy required is only hundredths of a metre, not billionths. That, Dr Koechlin reckons, gives Fresnel optics a big advantage over Newtonian ones.

用波帶片代替鏡面在軌道運行。因爲波帶片表面是平的，他可以用塑料製作而成，然後摺疊起來發射。而尺寸大小就不再是問題。並且，儘管還必須有一個含有"目鏡"（也是一種運用菲涅爾視覺的特殊透鏡，也是一種記錄圖像的相機）的衛星在焦點上運行，所需要的精準度也只是百分之一米。凱什蘭博士認爲，這將是菲涅爾視覺超越牛頓的設計的一大優點。

To test the idea, he and an international consortium of his colleagues have built a ground-based prototype. This is a piece of copper foil 20cm square that has 696 rings, a portion of which is reproduced above. Because it is this small, its focal length is only 18 metres. In order that the foil does not fall apart, each transparent ring is actually a series of curved slots in the copper rather than a continuous gap. This, though, does not affect the system's optical properties and it can, indeed, see small, faint objects that are near large, bright ones.

爲了檢測這一想法，他和他各國的同事建立了一個陸基的雛形。這是一片20平方釐米的銅箔，它有696個環。銅箔的一部分是再生的。因爲它體積偏小，所以它的聚焦只有18米。爲了不讓銅箔散開，每一個透明的環實際上都是銅箔裏的一系列的曲線輪槽，而不是連續的空隙。儘管如此，這並不影響整個系統的視覺特性，並且，它也確實能夠看見巨大且明亮的恆星旁邊那些小型且微弱的物體。

When Dr Koechlin and his team pointed it at Mars they could distinguish that planet's two tiny moons-a task which would require a Newtonian telescope with a mirror at least 30cm across. And when they aimed at Sirius they could see the dim white-dwarf which orbits what is the brightest star in the night sky. Extrapolating from these results, they think that an orbiting zone plate measuring somewhere between 15 metres and 40 metres across will be enough to distinguish the spectrum of an Earthlike planet at a distance of 30 light-years. With that, they should be able to find out if mankind really does have any next-door neighbours, and Fresnel will have come into his own at last.

當凱什蘭博士和他的團隊觀測火星時，他們可以分辨火星的兩個微小衛星——這樣的任務如果是利用牛頓的望遠鏡則需要一個直徑最少長達30釐米的鏡面。並且，當他們觀測天王星時，他們能夠看見暗淡的白矮星。它圍繞着夜空中最亮的那顆星運行。從這些結果推測，他們認爲，一個直徑在15到40米、並且在軌道上運行的波帶片足夠在30光年以外的距離分辨一個與地球相似的行星的光譜。如此一來，他們就應該能夠找到人類是否還有其他鄰居，而菲涅爾也最終能實現自己的想法。