北京時間2019年4月10日，天文學家召開全球新聞釋出會，宣佈首次拍攝到黑洞附近的照片。黑洞是廣義相對論的特殊預言，近數十年來的天文觀測對其存在性提供了廣泛的證據。
那麼關於“黑洞”，我們還有哪些不知道的事情呢？黑洞有一天會吞噬銀河系嗎? 我們的宇宙就是一個黑洞? 黑洞是永恆存在的嗎? 太陽會變成黑洞嗎？來聽聽Frank Wilczek是如何詮釋“黑洞”的吧！
Frank Wilczek
弗蘭克·維爾切克是麻省理工學院物理學教授、量子色動力學的奠基人之一。因在夸克粒子理論（強作用）方面所取得的成就，他在2004年獲得了諾貝爾物理學獎。
宇宙奇蹟零距離
作者 | Frank Wilczek
翻譯 | 胡風、樑丁當
配音 | 李娜（蔻享學術）
幾十年來，物理學家一直在理論上研究黑洞，而今我們終於成功看到了它。
Physicists have been theorizing about black holes for generations. Now science has made it possible to see one.
4月10日，天文學家利用事件視界望遠鏡拍攝到了一張黑洞的“照片”，這個巨大的黑洞位於M87星系的中心，距離地球5400萬光年。僅僅當作一張照片來看的話，它既平淡無奇又不怎麼清晰：照片中是一個毫無特色、模模糊糊、半圈發光的圓環——它展示的並不是黑洞本身，而是黑洞扭曲時空，在周圍的發光背景中留下的“影子”。但是，喜歡思考的人知道，這張照片代表了人類智慧的非凡成就，而擅長想象的人則會意識到它打開了通向空間、時間及宇宙久遠歷史的全新視窗。
On April 10, astronomers working with the Event Horizon Telescope unveiled a “photograph” of the monstrous black hole that sits at the center of the M87 galaxy, 54 million light years away. Viewed simply as an image, it is neither impressive nor straightforward: It presents a sort of nondescript, blurry, half-glazed doughnut, showing not the black hole itself but the shadow-like distortion it carves in the surrounding illumination. Yet, to a thinking mind, the image reflects the glory of understanding, and to an alert imagination it opens new portals into space, time and deep history.
這張照片可以說是當代科技的偉大結晶。照片中的黑洞非常巨大，半徑約有150億千米——相當於地球到太陽距離的100倍，品質是地球的2000萬億倍。但因為太遙遠，它只佔據了極小的一塊天空，所以我們必須用一個很大的、不 同尋常的望遠鏡來觀察它。實際上，事件視界望遠鏡並非一臺單獨的儀器，而是由分別位於夏威夷、亞利桑那、西班牙、 墨西哥、智利和南極洲的8臺射電望遠鏡共同構成。天文學家利用精度高達萬億分之一秒的原子鐘同步來自不同望遠鏡的資料，再通過超級計算機把它們整合起來。
The making of the image was a tour de force of science and technology. The black hole is enormous, with a radius of roughly 9 billion miles (or one hundred times the distance from the Earth to the sun) and a mass equivalent to two quadrillion Earths. But because it is so far away, it occupies only a tiny portion of the sky, so we must use a very large, exotic telescope to see it. In fact, the Event Horizon Telescope isn’t a single instrument but a system of eight radio dishes at six far-flung locations in Hawaii, Arizona, Spain, Mexico, Chile and Antarctica. Astronomers used precise atomic clocks, accurate within a trillionth of a second, to synchronize data from all these places and then stitched it all together using supercomputers.
黑洞的概念可以追溯到18世紀，當時英國的天文學家兼牧師約翰·米歇爾（John Michell）通過計算髮現：如果一顆恆星具有足夠大的品質，它將不會發光，因為脫離恆星引力所需的逃逸速度超過了光速。然而，由於18世紀的物理學對光、引力或者恆星都缺乏足夠的認知，米歇爾的猜想遠遠超越了他所在的時代。
The concept of a black hole goes back to the 18th century, when the English astronomerclergyman John Michell calculated that a sufficiently large star couldn't shine because light wouldn’t move fast enough to “lift off” and escape the star’s gravity. But Michell’s conjecture outran the physics of its time, which didn’t understand light, gravity or stars well enough to support it.
直到20世紀初期，詹姆斯·克拉克·麥克斯韋（James Clerk Maxwell）的電磁場理論和愛因斯坦的相對論才奠定了現代黑洞物理學的基礎。1939年，J·羅伯特·奧本海默（J. Robert Oppenheimer）和哈特蘭·斯 奈德（Hartland Snyder）撰寫了黑洞研究史上最重要的論文《關於持續的引力收縮》（On Continued Gravitational Contraction）。這篇論文探討了一團任意大小、密度均勻的塵埃雲在自身引力作用下發生塌縮的結果。作者發現，如果觀察者處於塵埃雲中，他將發現自己被物質包圍著，而他眼中的宇宙與我們的宇宙大致相似，最終以大擠壓（big crunch）的方式終結。但是，外面的觀察者會看到塵埃雲在某個時刻突然失去光芒變成一個黑洞。這是因為塵埃雲邊界的引力變得非常強大，以至於光——或者說任何物質——都無法逃逸。
he foundations for the modern understanding of black holes weren’t laid until the early 20th century, building on James Clerk Maxwell’s theory of electromagnetism and Albert Einstein’s theory of relativity. In 1939, J. Robert Oppenheimer and Hartland Snyder wrote “On Continued Gravitational Contraction,” the most important paper in the history of black holes. They considered what would happen to a dust cloud of any size and uniform density as it collapses through the force of gravity. Observers within the dust cloud would find themselves surrounded by matter, and they would experience a universe broadly resembling our own, which would eventually end in a “big crunch.” But observers outside the collapsing cloud would see it wink out into a black hole, as gravity at its boundary increases beyond the ability of light—or any form of matter—to escape.
奧本海默和斯奈德的研究清晰地表明：黑洞的形成是已知物理過程的合理結果。他們還大膽地提出，黑洞本身可能就是一個宇宙，宇宙也可能是一個黑洞，只是觀察角度不同而已。時至今日，我認為物理學也還沒能充分消化這個思想。
Oppenheimer and Snyder’s work made it clear that black holes were a plausible outcome of known physical processes. It also makes the astonishing suggestion that a black hole can be a universe and vice versa, differently viewed. I don’t think physics has fully digested this idea, even today.
如果M87的黑洞揭示了一個全新的物理過程，那將是令人驚訝的。這類超大黑洞雖然品質巨大，但體積更大，因此密度反而很低。它們對外部施加的作用力盡管影響範圍很 廣，但非常微弱。但是，通過對比不同星系的中心黑洞，我們可以了解星系是如何形成和演化的。我們的銀河系也有一個巨大的中心黑洞，它籠罩在塵埃中，難以直接觀測。
It would be surprising if M87’s black hole revealed fundamentally new physical processes. Such gigantic black holes have low density and exert only weak forces on the outside, albeit on a grand scale. But by comparing and contrasting the central black holes in different galaxies, we will learn about how galaxies form and evolve. Our own Milky Way galaxy also harbors a central, giant black hole, hidden from direct observation by enshrouding dust.
看著黑洞的照片，我腦海裡浮現出更早的一張標誌性照片——阿波羅8號在月球軌道上拍到的“地球升起”。作為物理實體，黑洞比地球更大更壯觀，但也更加簡單，對生命更不友好。而我們的宇宙竟然同時擁有兩者，這實在是太美妙了！
Seeing the black hole image, my mind flashed back to an earlier iconic image, the “Earthrise” captured by Apollo 8. The black hole is much bigger and more imposing, as a physical object, but also much less complex—not to mention less user-friendly—than our Earth. It’s a wonderful world that is home to both.