有關時間旅行、黑洞、多元宇宙你需要知道的一切
1961年的今天，蘇聯宇航員加加林乘坐東方一號宇宙飛船首次飛入太空。2011年，為記錄這一偉大時刻，聯合國將每年的4月12日定為國際載人航天日。然而，載人航天也只是人類為宇宙探索所付出的努力中的一小部分，人類的歷史文獻中從不缺乏觀測星空的詳盡記述。2016年，引力波的成功探測使得天文觀測邁入新的時代。人類到底為什麼為星空而著迷？可能正如天體物理學家Neil deGrasse Tyson所說：儘管我們在宇宙中顯得無比渺小，我們卻也都是宇宙中重大事件的參與者。
《紐約時報》暢銷書，《福布斯》年度科普暢銷書
Welcome to the Universe以Neil deGrasse Tyson,Michael A. Strauss, 和J. Richard Gott三位教授最受歡迎的普林斯頓大學天體物理入門課程為基礎，為普通讀者呈現了一本裝幀精緻，插圖精美的宇宙科普書。今天，不用坐在普林斯頓大學課堂，我們也能上到精彩的天文物理學課。
這是由三位當今世界領軍天文物理學家親自帶領的宇宙之旅。Neil deGrasse Tyson是美國著名「科學網紅」，艾美獎獲獎紀錄片Cosmos: A Spacetime Odyssey的主持人。
下面請閱讀普林斯頓大學出版社與三位作者的獨家專訪。
MAS=Michael A. Strass
NDT=Neil deGrasse Tyson
JRG=J. Richard Gott
What is the Cosmic Perspective?
什麼是宇宙視角？
NDT:A view bigger than your own that offers a humbling, yet enlightening, and occasionally empowering outlook on our place as humans in time, space, on Earth and in the Universe. We devote many pages ofWelcome to the Universeto establishing our place in the cosmos – not only declarations of that place, but also the reasons and the foundations for how we have come to learn how we fit in that place. When armed with a cosmic perspective, many earthly problems seem small, yet you cultivate a new sense of belonging to the universe. You are, in fact, a participant in the great unfolding of cosmic events.
NDT:宇宙視角是一個比你個人的視角更廣闊的觀念，給我們這些存在於時間、空間、地球上和宇宙中的人類提供了一個令人敬畏，卻又有啟發性的、偶爾令人振奮的出發點。在Welcome to the Universe一書中我們花了大量篇幅來確立人類在宇宙中的位置——不僅表明了這是怎樣一個位置，而且也解釋了我們探索這個定位所依靠的依據和基本概念。即使許多塵世的問題看起來仍舊很渺小，當你擁有了一個宇宙視角，你就培養了一種對宇宙的新的歸屬感。事實上，你仍然是宇宙重大事件的參與者。
插圖：俯瞰銀河模擬圖
Photo credit: NASA Chandra Satellite
What are some of the takeaways from the book?
讀這本書會有什麼收穫？
NDT:If you read the entire book, and if we have succeeded as authors, then you should walk away with a deep sense of the operations of nature, and an appreciation for the size and scale of the universe; how and why planets form; how and why we search for planets orbiting around other stars, and alien life that may thrive upon them; how and why stars are born, live out their lives and die; what galaxies are and why they are the largest organizations of stars in the universe; the large scale structure of galaxies and space-time; the origins and future of the universe, Einstein』s relativity, black holes, and gravitational waves; and time travel. If that』s not enough, you will also learn about some of the continued unsolved mysteries in our field, such as dark matter, dark energy, and multiverses.
NDT:如果你讀了整本書，而且我們盡到了作者的職責的話，那麼你能學到的是：對自然運作的深刻理解；對宇宙尺寸和規模的領會；行星形成的方式和原因；我們為什麼以及如何在尋找著圍繞其他恆星運行的行星，和可能在它們上面繁衍生息的外星生命；恆星為什麼以及如何誕生、活著、死去；星系是什麼，為什麼它們是宇宙中最大的恆星組織；星系的大規模結構與時空；宇宙的起源和未來、愛因斯坦的相對論、黑洞和引力波；最後，還有時間旅行。如果這還不夠的話，你還將了解到我們領域中一些尚未解開的謎團，比如暗物質、暗能量和多元宇宙。
插圖：蟹狀星雲
Photo credit: Hubble Space Telescope, NASA
This book has more equations than do most popular books about astrophysics. Was that a deliberate decision?
這本書中有許多物理學方程式，比大多數關於天體物理學的通俗書籍都多，這是刻意的安排嗎？
MAS:Yes. The book』s subtitle is 「An Astrophysical Tour,」 and one of our goals in writing it was to show how observations, the laws of physics, and some high school mathematics can combine to yield the amazing discoveries of modern astrophysics: A Big Bang that happened 13.8 billion years ago (we show you how that number is determined), the dominant role dark matter has in the properties of galaxies (we tell you how we came to that conclusion), even the fact that some planets orbiting other stars have conditions conducive for liquid water to exist on their surface, thought to be a necessary prerequisite for life.
MAS:是的。這本書的副標題是「天體物理之旅」，我們的寫作目標之一是展示實際觀察、物理定律和高中程度的數學如何能結合起來並推導出現代天體物理學的驚人發現，例如: 138億年前發生的大爆炸(我們向讀者展示了這個數字是如何計算得出的)；暗物質在星系屬性中的主導作用(我們告訴讀者這個結論是如何確定的)；我們還向讀者展示了一些有條件讓液態水存在於它們的表面的環繞其他恆星運行的行星——而能讓液態水存在被認為是生命產生的必要先決條件。
Our goal is not just to present the wonders of the universe to the reader, but to have the reader understand how we have determined what we know, and where the remaining uncertainties (and there are plenty of them!) lie.
我們的目標不僅僅是向讀者展示宇宙的奇觀，還要讓讀者了解我們是如何決定哪些事情是我們目前已知的，以及剩下的許多不確定性又在哪裡。
Is time travel possible?
時間旅行可能嗎？
JRG:In 1905 Einstein proved that time travel to the future is possible. Get on a rocket and travel out to the star Betelgeuse 500 light-years away and return at a speed of 99.995 % the speed of light and you will age only 10 years, but when you get back it will be the year 3016 on Earth. Even though we have not gone that fast or far, we still have time travelers among us today.
JRG:1905年，愛因斯坦證明了到達未來的時間旅行是可能的。如果你搭乘火箭以光速的99.995 %的速度前往距離地球500光年的恆星參宿四並從那裡返回地球，那麼你的年齡只會老去10歲，可你回來時地球上將會是3018年。（相比靜止的物體，運動物體的時間流逝會更慢，因此當你搭乘火箭高速飛行時，時間流逝的速率與地球上的並不相同）。即使我們還不能走得那麼快、那麼遠，在我們當中已經有過時間旅行者。
Our greatest time traveler to date is the Russian cosmonaut Gennady Padalka, who by virtue of traveling at high speed in low Earth orbit for 879 days aged 1/44 of a second less than if he had stayed home. Thus, when he returned, he found Earth to be 1/44 of a second to the future of where he expected it to be. He has time traveled 1/44 of a second to the future. An astronaut traveling to the planet Mercury, living there for 30 years, and returning to Earth, would time travel into the future by 22 seconds.
到目前為止，我們中最偉大的時間旅行者是俄羅斯宇航員根納季·帕達爾卡，當他在近地軌道上高速飛行879天時，比呆在家裡少變老1/44秒。因此，當他回來時，他發現地球比他所期望的未來要快1/44秒。所以，他已經向未來穿行1/44秒。如果一名宇航員前往水星並在水星上生活30年，當他返回地球時，他將向未來穿行22秒。
Einstein』s equations of general relativity, his theory of curved spacetime to explain gravity, have solutions that are sufficiently twisted to allow time travel to the past. Wormholes and moving cosmic strings are two examples. The time traveler can loop back to visit an event in his own past. Such a time machine cannot be used to journey back in time before it was created. Thus, if some supercivilization were to create one by twisting spacetime in the year 3000, they might use it to go from 3002 back to 3001, but they couldn』t use it go back to 2016, because that is before the time loop was created.
廣義相對論的方程有很多不同的解，在某些特定的解中，可以創造出回到過去的條件。蟲洞和移動的宇宙弦就是其中兩個例子。時間旅行者可以穿越時間參與他自己過去的事件，並創造出一個時間循環。這樣的一個時間機器不能回到時間機器被製造之前的時間。因此，如果某個超級文明通過扭曲時空在3000年製造了一個時間機器，那他們可以在3002年時使用時間機器從3002年回到3001年，但他們不能使用它回到2016年，因為那時候並沒有產生時間循環。
To understand whether such time machines can be realized, we may need to understand how gravity works on microscopic scales, which will require us to develop a theory of quantum gravity. Places to look for naturally occurring time machines would be in the interiors of rotating black holes and at the very beginning of the universe, where spacetime is strongly curved.
為理解這種時間機器能否實現，我們可能需要了解重力在微觀規模上是如何運作的，這就需要我們發展量子重力理論。若我們要尋找自然產生的「時間機器」，我們需要探索時空被強烈扭曲的地方，比如轉動的黑洞的中心。
插圖：史瓦西黑洞模擬圖
Photo credit: Andrew Hamilton (using Milky Way background image adapted from Axel Mellinger)
Do we live in a multiverse?
我們生活在多元宇宙中嗎？
JRG:A multiverse seems to be a natural consequence of the theory of inflation. Inflation explains beautifully the pattern of slightly hotter and colder spots we see in the Cosmic Microwave Background Radiation. It explains why the universe is so large and why it is as smooth as it is and still has enough variations in density to allow gravity to grow these into galaxies and clusters of galaxies by the present epoch. It also explains why the geometry of the universe at the present epoch is approximately Euclidean.
JRG:多元宇宙似乎是宇宙暴脹理論的自然結果。暴脹理論很好地解釋了我們在宇宙微波背景輻射中看到的稍熱的點和稍冷的點的規律。它解釋了為什麼宇宙如此之大又如此平滑，卻仍然有足夠的密度變化，使得在現今這個紀元里星系和星系團仍然可以通過引力而形成。它還解釋了我們現今所處的宇宙為什麼大致符合歐氏幾何規律。
Inflation is a period of hyperactive accelerated expansion occurring at the beginning of our universe. It is powered by a large vacuum energy density and negative pressure permeating empty space that is gravitationally repulsive. The universe doubles in size about every 3 10-38seconds. With this rate of doubling, it very quickly grows to enormous size: 1, 2, 4, 8, 16, 32, 64, 128, 256, 512, 1024… That explains why the universe is so large.
宇宙暴脹是宇宙早期發生的一個極度活躍的加速膨脹時期。它是由一個大的真空能量密度和負壓滲透的空間之間的引力相斥所驅動的。宇宙的體積大約每隔3x10-38秒就翻一番。以這樣的速度翻倍，它很快就增長到巨大的規模: 1，2，4，8，16，32，64，128，256，512，1024…這就是宇宙如此之大的原因。
When the high density vacuum state decays, it doesn』t do so all at once. Like water boiling in a pot, it does not turn into steam all at once, but should form bubbles. Each expanding bubble makes a universe. The inflationary sea should expand forever, creating an infinite number of bubble universes, ours being one of them. Other distant bubble universes are so far away, and the space between us and them is expanding so fast, that light from them may never reach us. Nevertheless, multiple universes seem a nearly inevitable consequence of inflation.
當高密度真空狀態衰減時，它不會一次性地膨脹。就像水在鍋里沸騰一樣，它不會一下子變成蒸汽，而是會形成氣泡。每個膨脹的氣泡構成一個宇宙。膨脹的海洋應該永遠膨脹，創造出無數的泡沫宇宙，我們的宇宙就是其中之一。其他遙遠的氣泡宇宙離我們如此之遠，我們和它們之間的空間擴張得如此之快，以至於來自它們的光可能永遠不會到達我們。然而，多元宇宙似乎是宇宙暴脹幾乎不可避免的結果。
插圖：風車星系
Photo credit: NASA/HST
What discovery about the universe surprises or inspires you the most?
關於宇宙的什麼發現最讓你吃驚或受到鼓舞？
JRG:Perhaps the most amazing thing about the universe is that it is comprehensible to intelligent, carbon-based life forms like ourselves. We have been able to discover how old the universe is (13.8 billion years) and figure out many of the laws by which it operates. The object of this book is to make the universe comprehensible to our readers.
JRG:也許宇宙中最令人驚奇的事情是，它對於像我們這樣有頭腦的碳基生命來說是可以理解的。我們已經能夠發現宇宙的年齡(138億歲)，並弄清它運行的許多規律。這本書的目的是使我們的讀者也能去理解這個宇宙。
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