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Garik Israelian談光譜學如何揭示外星生命

How spectroscopy could reveal alien life

 

Photo of three lions hunting on the Serengeti.

講者:Garik Israelian

20097月演講,200910月在TED上線

 

翻譯:洪曉慧

編輯:劉契良

簡繁轉換:陳盈

後製:洪曉慧

字幕影片後制:謝旻均

 

影片請按此下載

閱讀中文字幕純文字版本

 

關於這場演講
Garik Israelian是一位光譜學家,研究恆星發射的光譜,以瞭解其組成元素及其可能的演變。這是一場關於這門學科難得且平易近人的演講,它或許進一步接近了發現適合生存星球的夢想。

關於Garik Israelian

Garik Israelian在加那利群島觀星的工作促成關於太空大災難的知名探索,包括第一個超新星爆炸形成黑洞的證據。

 

為什麼要聽他演講:
Garik Israelian研究恆星和其他星體的光譜特徵,他是一位在加那利大型望遠鏡做研究的天文物理學家,也是加那利群島(世界上最大光學紅外線望遠鏡所在地)天體物理學研究所成員之一。他已發表超過150篇主題環繞在諸如太陽系外行星和黑洞二元體系之類的文章,他致力研究遙遠星系組成之光譜數據的觀測工作,導致了鋰同位素特徵的發現,這意味著太陽規模的恆星可能吞噬其所屬行星現象的存在。

1999年時,Israelian領導一項集體研究,發現首次超新星爆炸形成黑洞的觀測證據。他即將發佈更多的大新聞。(他也是說服皇后樂團吉他手Brian May,在30年的搖滾巨星生涯後,繼續完成博士學位的天文學家之一)。

 

Garik Israelian的英語網上資料

網站:El Instituto de Astrofisica de Canarias

 

[TED科技娛樂設計]
已有中譯字幕的TED影片目錄(繁體)(簡體)。請注意繁簡目錄是不一樣的。

 

Garik Israelian談光譜學如何揭示外星生命

 

我有個非常艱鉅的任務。我是一位光譜學家,我要談論天文學,卻不展示任何一張關於星雲和星系的影像。因為我的工作是光譜學,影像不是我的研究範圍。但我將盡力說服你,光譜學事實上可以改變世界。光譜學或許可以回答這個問題,「外太空是否有生命存在?」我們是宇宙單一的生命形式嗎?SETI(尋找外星智慧)。研究光譜學並不是很有趣的事。

 

我一位在保加利亞的同事,Iviana Marcos,花了約20年時間研究這些圖表。她發表的42篇文獻,就只專注於這個主題。你能想像嗎?日以繼夜,思考、觀察同一顆星星20年。真是令人難以置信。但我們的確很瘋狂,我們全都是這樣。(笑聲)

 

我還不到那個程度。我花了大約8個月研究這些圖表。因為我注意到一個非常小的對稱情況,在這顆行星的宿主恆星圖表上。於是我想,也許這顆恆星含有鋰–6同位素。表示這顆恆星曾吞噬過一顆行星。因為顯然脆弱的鋰–6同位素,不該存在於似太陽恆星的大氣中,但其卻會存在於行星和小行星中。所以,如果恆星吞噬過行星,或大量的小行星,就會有鋰–6同位素存在於恆星的光譜中。因此,我花了超過8個月的時間,就只有研究這顆恆星的圖表。

 

事實上令人驚訝的是,因為我接到了許多記者的電話,問說:「你親眼目睹行星被恆星吞噬嗎?」因為他們認為,如果你有一個望遠鏡,而又是一位天文學家,你所做的就是用望遠鏡實際觀測,你可能就會目睹行星被恆星吞噬。我說:「不,對不起,我看到的是這個。」(笑聲)這簡直令人難以置信,因為沒有人真正瞭解,我打賭只有極少數人真正理解我所說的事。因為這表示行星被恆星吞噬。這真是很神奇。

 

光譜學的力量在1973年早已為人知曉,那就是Pink Floyd樂團。(笑聲)因為他們確實說過,你可以得到任何喜歡的顏色,就在光譜上。你需要的只是時間和金錢,來製造你的光譜儀。這是一個有最高解析度,世上最精確的光譜儀,叫做HARPS。這實際上是用於探測太陽系外行星,和恆星大氣中的聲波。

 

我們如何獲得光譜?我相信你們大多從學校的物理中學過。它基本上是將白光切成各種顏色。如果是熱的液態物質,將產生所謂的連續光譜。熱氣體只產生放射譜線,那是不連續的。如果你將冷氣體置於熱源前,你會看到一些特定圖形,稱之為吸收譜線。實務上,這用於辨識化學元素的存在。於冷物體中,它會在這些頻率產生吸收譜線。

 

我們可以用光譜來做什麼?我們實際上可以研究徑向速度,在宇宙物體上。我們也可以研究恆星的化學成份和物理參數,或是星系、星雲。恆星是最簡單的星體,其核心有10個單一核反應在進行,產生一些化學元素。我們有冷的大氣。對我來說稱得上冷,對我來說三千到四、五千度算冷的了。對我研究紅外線天文學的同事來說,絕對溫度零下200度才算冷。但你知道,一切事物都是相對的。因此,對我來說五千度算是相當冷的。(笑聲)

 

這是太陽的光譜,有兩萬四千條譜線。約百分之十五的譜線尚未鑑別出來。很令人驚訝,現在是21世紀,至今我們還不能正確瞭解太陽的光譜。有時我們只需研究一條小而微弱的光譜線,來測量大氣中的化學元素組成。例如,你看到的黃金譜線,是太陽光譜中唯一的譜線。我們利用這個微弱的特性,來測量太陽大氣中的黃金成份。

 

這是正在進行中的研究。我們一直研究一個類似的、非常微弱的特徵,就是鋨的譜線。這是一個重元素,產生於超新星的熱核爆炸。這也是唯一可以實際產生鋨的地方。只要比較鋨的成份,在行星的宿主恆星中。我們想瞭解,為什麼有那麼多的這種元素存在。我們甚至認為,或許是超新星爆炸,觸發了行星和恆星的形成。它是一個跡象。

 

某天,我來自柏克萊的同事Gibor Basri,發給我一封電子郵件。那是一張非常有趣的光譜。他問說:「你可以看看這個嗎?」接下來的兩週我難以成眠,當我看到有大量的氧和其他元素在這恆星的光譜中。我知道,在星系中不會觀測到像這樣的物質,令人難以置信,我們由此得出的唯一結論就是,有一個超新星爆炸的實證,在這個系統中,污染了這顆恆星的大氣。之後黑洞形成,以二元體系形式。它仍然停留在原處,其所帶有的質量約為太陽的5倍。這被認為是第一個證據,就是事實上,黑洞來自於超新星爆炸。

 

我的同事比較化學元素組成,在不同星系恆星中,確實發現了銀河系中有外來恆星。令人驚訝的是,能有這麼多的進展,僅僅靠研究恆星的化學組成。他們說,其實你在光譜中看到的恆星之一就是外來的,它來自不同的星系,有相互作用的星系。我們知道這一點,有時它們會捕捉恆星。

 

你們聽說過太陽耀斑。我們很驚訝地發現一個超級耀斑。耀斑爆發的力量,是千百萬倍於我們在太陽中所見的。在銀河系一個的雙星中,其名為FH Leo,我們發現了超級耀斑。之後我們研究恆星的光譜,以觀察這些星體有什麼異常之處。我們發現一切正常。這些恆星像太陽一樣正常,年代等,一切都正常。所以這是一個謎。超級耀斑是依然存在的不解之謎之一。還有六、七個類似的情況,在文獻報告中。

 

再來看看這個。我們真的需要瞭解宇宙的化學演化。這是非常複雜的,我並不是真的要你們現在就理解這些東西。(笑聲)但它能讓您們知道整個過程有多複雜,以產生化學元素。這裡有兩種反應途徑。大質量恆星和低質量恆星,生產和回收宇宙中的物質及化學元素。這個過程進行了140億年。我們以此圖片來做總結。這是一張非常重要的圖表,顯示化學元素的相對含量,在似太陽恆星中,以及星際介質間。

 

因此,這意味著不可能存在任何一個星體,其含硫量比含矽量多10倍,含鈣量比含氧量多5倍。這是不可能的。如果你真的發現了一個這樣的星體,我會說,這就和搜尋外星智慧有關了。因為在自然狀態下這是不可能的。都卜勒效應是一個很重要的概念,來自於基礎物理。這是關係到在移動源上的頻率變化。都卜勒效應通常用於探索太陽系外行星。

 

徑像速度精確度,用於探索一個似木星行星,繞行似太陽恆星,大約是每秒28.4米。而需每秒9厘米的精確度,以探索一個似地球行星。這可用未來光譜儀觀測到。我本身實際參與了開發codex的團隊,它是高解析度的未來新一代光譜儀,為42米超大型天文望遠鏡所開發的。這是一個儀器,未來將用於似地球行星,繞行似太陽恆星的探測工作。這是一個稱為星震儀的神奇工具。我們可以偵測聲波,於恆星的大氣中。

 

這是半人馬座α星發出的聲音。我們可以偵測聲波,於似太陽恆星的大氣中。這些波的頻率屬次聲域,是人類無法聽見的聲域。回到最重要的問題上:「外太空是否有生命存在?」這密切關聯於行星構造及其火山活動。生命與放射性核種間的關聯是很直接的。沒有構造活動就無法形成生命。沒有火山活動也無法形成生命。我們很清楚,主要地熱能源是來自於鈾、釷及鉀的衰變。

 

該如何測量呢?如果行星上這些元素的含量很少,行星的構造活動就會停滯,就不會有生命產生。如果有太多的鈾或鉀或釷,同樣的,也不可能有生命。你能想像整個星球沸騰的情況嗎?星球上有太多能量了。現在,我們已經測量到豐富的釷,存在於太陽系外行星之一的宿主恆星中。同樣的情形,未來依然很渺茫。

 

我們正試圖測量這個圖表,以偵測釷的存在。這是非常艱難的,確實非常艱難。你必須,首先你必須說服自己。然後,你必須說服你的同事。再來你必須說服整個世界,你的確偵測到像這樣的物質,在太陽系外行星的宿主恆星大氣中,在離這裡100秒差距的地方。這真的很難。但是,如果你想知道一個太陽系外行星上的生命,你就必須這麼做。因為你必須知道有多少放射性元素在這些系統中。

 

一個探索外星生命的方法是,調整你的電波望遠鏡,並聆聽訊號。如果您收到一些有趣的訊號,這就是SETI的工作,它已這樣進行了許多年。我認為最有希望的方法是使用生物標記。你可以看到地球的光譜,這是地球反照光譜。是一個非常明確的訊號。這個下降的坡,稱之為紅光臨界效應,可以偵測到含植物的區域。很神奇的,我們可以偵測植物,使用光譜技術。想像一下利用此技術來探索其它星球。

 

僅在不久前,就在最近,我是指最近六、七、八個月前。水、甲烷和二氧化碳已經被偵測到,就在太陽系系統外的行星光譜中。這是驚人的,這就是光譜學的力量。事實上,你可以探測及研究行星的化學成分,在離太陽系很遠、很遠的地方。我們必須偵測到氧或臭氧,才能確保它擁有一切必要條件,讓生命跡象得以存在。

 

宇宙奇蹟是某些與搜尋外星智慧有關的訊息。現在想像一個星體,令人訝異的星體,或某個我們無法解釋的狀況。我們只能站起來說:「瞧,我們放棄了,物理學行不通。」這就是你可以將其歸因於外星智慧的訊息,並說:「好吧,無論如何,總有人必須做這件事。」

 

由已知的物理學知識得知,事實上這一點是Frank Drake早已提及的,在很多年前[不清楚]。如果你看到,在行星的宿主恆星光譜中,如果看到奇怪的化學元素,它可能是來自另一個文明的訊號。他們意圖發出這樣的訊號,用訊號確實顯示出他們的存在。藉由這些恆星光譜中的譜線,或以其它不同方式傳遞。

 

可以用許多不同方式來進行。一個實例是鎝,一種放射性元素,其衰變時間約420萬年。如果你突然觀測到鎝存在於似太陽恆星中,你可以肯定,一定是有人將這個元素放到大氣中的,因為在自然狀況下這是不可能的。現在,我們正在審查大約300顆太陽系外行星的宿主恆星光譜。我們自2000年就開始做這些,這是一個工作量很繁重的計畫,我們一直非常努力。我們有一些有趣的案例,或說「可能案例」等,一些我們無法真正解釋的事。希望在不久的將來我們可以證實這一點。

 

因此,主要問題是:「我們是宇宙單一的生命形式嗎?」我認為它不是來自幽浮。不是來自無線電訊號。我認為它將來自像這樣的光譜。這是似地球行星的光譜,顯示出氮氧化物的存在。一個生命形式的明確訊號。還有氧和臭氧。如果有一天,我想大約是從現在起1520年內。如果我們探索到像這樣的光譜,就可以肯定那個星球有生命存在。再過大約5年,我們將會發現像地球一樣的行星,繞著似太陽恆星運轉,有著與從地球到太陽相同的距離,這過程大約需要5年時間。然後,我們需要有1015年的太空計劃,以獲得似地球行星的光譜,就像我向您顯示的一樣。如果我們看到了氮氧化物還有氧,我認為我們就找到了真正的外星人。非常感謝。(掌聲)

 

 

 

 

以下為系統擷取之英文原文

About this talk

Garik Israelian is a spectroscopist, studying the spectrum emitted by a star to figure out what it's made of and how it might behave. It's a rare and accessible look at this discipline, which may be coming close to finding a planet friendly to life.

About Garik Israelian

Garik Israelian's stargazing on the Canary Islands has led to high-profile discoveries about space's big disasters -- including the first evidence that supernova explosions make black holes.
Full bio and more links

Transcript

I have a very difficult task. I'm a spectroscopist. I have to talk about astronomy without showing you any single image of nebulae or galaxies, etc. because my job is spectroscopy. I never deal with images. But I'll try to convince you that spectroscopy is actually something which can change this world. Spectroscopy can probably answer the question, "Is there anybody out there?" Are we alone? SETI. It's not very fun to do spectroscopy.

One of my colleagues in Bulgaria, Neviana Markova, spent about 20 years studying these profiles. And she published 42 articles just dedicated to the subject. Can you imagine? Day and night, thinking, observing, the same star for 20 years is incredible. But we are crazy. We do these things. (Laughter)

And I'm not that far. I spent about eight months working on these profiles. Because I've noticed a very small symmetry in the profile of one of the planet host stars. And I thought, well maybe there is Lithium-6 in this star, which is and indication that this star has swallowed a planet. Because apparently you can't have this fragile isotope of Lithium-6 in the atmospheres of sun-like stars. But you have it in planets and asteroids. So if you engulf planet or large number of asteroids, you will have this Lithium-6 isotope in the spectrum of the star. So I invested more than eight months just studying the profile of the lithium line of this star.

And actually it's amazing because I got phone calls from many reporters asking, "Have you actually seen the planet going into a star?" Because they thought that if you are having a telescope, you are an astronomer so what you are doing is actually looking in a telescope. And you might have seen the planet going into a star. And I was saying, "No, excuse me. What I see is this one." (Laughter) It's just incredible. Because nobody understood really. I bet that there were very few people who really understood what I'm talking about. Because this is the indication that the planet went into the star. It's amazing.

The power of spectroscopy was actually realized by Pink Floyd already in 1973. (Laughter) Because they actually said that you can get any color you like in a spectrum. And all you need is time and money to make your spectrograph. This is the number one high resolution, most precise spectrograph on this planet, called HARPS, which is actually used to detect extrasolar planets and sound waves in the atmospheres of stars.

How we get spectra? I'm sure most of you know from school physics that it's basically splitting a white light in to colors. And if you have a liquid hot mass, it will produce something which we call a continuous spectrum. A hot gas is producing emission lines only, no continuum. And if you place a cool gas in front of a hot source, you will see certain patterns which we call absorption lines. Which is used actually to identify chemical elements in a cool matter, which is absorbing exactly at those frequencies.

Now, what we can do with the spectra, we can actually study a line of sight velocities of cosmic objects. And we can also study chemical compostion and physical parameters of stars, galaxies, nebulae. A star is the most simple object. In the core we have thermonuclear reactions going on, creating chemical elements. And we have a cool atmosphere. It's cool for me. Cool in my terms is three or four or five thousand degrees. My colleagues in infra-red astronomy call minus 200 Kelvin is cool for them. But you know, everything is relative. So for me 5,000 degrees is pretty cool. (Laughter)

This is the spectrum of the sun. 24 thousand spectral lines, and about 15 percent of these lines is not yet identified. It is amazing. So we are in 21st Century. And we still can not properly understand the spectrum of the sun. Sometimes we have to deal with just one tiny, weak spectral line to measure the composition of that chemical element in the atmosphere. For instance you see the spectral line of the gold is the only spectral line in the spectrum of the sun. And we use this weak feature to measure the composition of gold in the atmosphere of the sun.

And now this is a work in progress. We have been dealing with a similarly very weak feature, which belongs to osmium. It's a heavy element produced in thermonuclear explosions of supernovae. It's the only place where you can produce, actually, osmium. Just comparing the composition of osmium in one of the planet host stars, we want to understand why there is so much of this element. Perhaps we even think that maybe supernova explosions trigger formations of planets and stars. It can be an indication.

The other day my colleague from Berkeley, Gibor Basri, emailed me a very interesting spectrum, asking me, "Can you have a look at this?" And I couldn't sleep, next two weeks, when I saw the huge amount of oxygen and other elements in the spectrum of the stars. I knew that there is nothing like that observed in the galaxy. It was incredible. The only conclusion we could make from this is clear evidence that there was a supernova explosion in this system, which polluted the atmosphere of this star. And later a black hole was formed in a binary system, which is still there with a mass of about five solar masses. This was considered as first evidence that actually black holes come from supernovae explosions.

My colleagues, comparing composition of chemical elements in different galactic stars actually discovered alien stars in our galaxy. It's amazing that you can go so far simply studying the chemical composition of stars. They actually said that one of the stars you see in the spectra is an alien. It comes from a different galaxy. There is interaction of galaxies. We know this. And sometimes they just capture stars.

You've heard about solar flares. We were very surprised to discover a super flare, a flare which is thousands of millions of times more powerful than those we see in the sun. In one of the binary stars in our galaxy called FH Leo, we discovered the super flare. And later we went to study the spectral lines to see is there anything strange with these objects. And we found that everything is normal. These stars are normal like the sun. Age, everything was normal. So this is a mystery. It's one of the mysteries we still have, super flares. And there are six or seven similar cases reported in the literature.

Now to go ahead with this, we really need to understand chemical evolution of the universe. It's very complicated. I don't really want you to try to understand what is here. (Laughter) But it's to show you how complicated is the whole story of the production of chemical elements. You have two channels -- the massive stars and low-mass stars -- producing and recycling matter and chemical elements in the universe. And doing this for 14 billion years, we end up with this picture. Which is a very important graph, showing relative abundances of chemical elements in sun-like stars and in the interstellar medium.

So which means that it's really impossible to find an object where you find about 10 times more sulfur than silicon, five times more calcium than oxygen. It's just impossible. And if you find one I will say that this is something related to SETI. Because naturally you can't do it. Doppler effect is something very important from fundamental physics. And this is related to the change of the frequency of a moving source. The Doppler effect is used to discover extrasolar planets.

The precision which we need to discover a Jupiter-like planet around a sun-like star is something like 28.4 meters per second. And we need nine centimeters per second to detect an Earth-like planet. This can be done with the future spectrographs. I, myself, I'm actually involved in the team which is developing a codex, high resolution, future generation spectrograph for the 42 meter ELT telescope. And this is going to be an instrument to detect Earth-like planets around sun-like stars. It is and amazing tool called Astroseismology where we can detect sound waves in the atmospheres of stars.

This is the sound of an Alpha Centauri. We can detect sound waves in the atmospheres of sun-like stars. Those waves have frequencies in infrasound domain, the sound actually nobody knows, domain. Coming back to the most important question, "Is there anybody out there?" This is closely related to tectonic and volcanic activity of planets. Connection between life and radioactive nuclei is straightforward. No life without tectonic activity, without volcanic activity. And we know very well that geothermal energy is mostly produced by decay of uranium, thorium, and potassium.

How to measure, if we have planets where the amount of those elements is small, so those planets are tectonically dead, there can not be life. If there is too much uranium or potassium or thorium, probably, again, there would be no life. Because can you imagine everything boiling? It's too much energy on a planet. Now we have been measuring abundancy of or thorium in one of the stars with extrasolar planets. It's exactly the same game. A very tiny future.

We are actually trying to measure this profile and to detect thorium. It's very tough. It's very tough. And you have to, first you have to convince yourself. Then you have to convince your colleagues. And then you have to convince the whole world that you have actually detected something like this in the atmosphere of an extrasolar planet host star somewhere in 100 parsec away from here. It's really difficult. But if you want to know about a life on extrasolar planets you have to do this job. Because you have to know how much of radioactive element you have in those systems.

The one way to discover about aliens is to tune your radio telescope and listen to the signals. If you receive something interesting, well that's what SETI does actually, what SETI has been doing for many years. I think the most promising way is to go for biomarkers. You can see the spectrum of the Earth, this Earthshine spectrum, and that is a very clear signal. The slope which is coming, which we call a Red Edge, is a detection of vegetated area. It's amazing that we can detect vegetation from a spectrum. Now imagine doing this test for other planets.

Now very recently, very recently, I'm talking about last six, seven, eight months, water, methane, carbon dioxide have been detected in the spectrum of a planet outside the solar system. It's amazing. So this is the power of spectroscopy. You can actually go and detect and study a chemical composition of planets far, far, far from solar system. We have to detect oxygen or ozone to make sure that we have all necessary conditions to have life.

Cosmic miracles are something which can be related to SETI. Now imagine an object, amazing object, or something which we can not explain when we just stand up and say, "Look, we give up. Physics doesn't work." So it's something which you can always refer to SETI and say, "Well, somebody must be doing this, somehow."

And with the known physics etc, it's something actually which has been pointed out by Frank Drake, many years ago [unclear]. If you see, in the spectrum of a planet host star, if you see strange chemical elements, it can be a signal from a civilization which is there and they want to signal about it. They want to actually signal their presence through these spectral lines, in the spectrum of a star, in different ways.

There can be different ways doing this. One is for instance technetium is a radioactive element with a decay time of about 4.2 million years. If you suddenly observe technetium in a sun-like star, you can be sure that somebody has put this element in the atmosphere. because in a natural way it is impossible to do this. Now we are reviewing the spectra of about 300 stars with extrasolar planets. And we are doing this job since 2000 and it's a very heavy project. We have been working very hard. And we have some interesting cases, candidates, so on, things which we can't really explain. And I hope in the near future we can confirm this.

So the main question, "Are we alone?" I think it will not come from UFOs. It will not come from radio signals. I think it will come from a spectrum like this. It is the spectrum of a planet like Earth showing a presence of nitrogen oxide, as a clear signal of life, and oxygen and ozone. If, one day, and I think it will be within 15 years from now, or 20 years. If we discover a spectrum like this we can be sure that there is life on that planet. In about five years we will discover planets like Earth, around sun-like stars, the same distance as the Earth from the sun. It will take about five years. And then we will need another 10, 15 years with space projects to get the spectra of Earth-like planets like the one I showed you. And if we see the nitrogen oxide and oxygen, I think we have the perfect E.T. Thank you very much. (Applause)


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