超越100年前的廣義相對論

You might think that physicists would be satisfied by now. They have been testing Einstein’s theory of general relativity, which explains what gravity is, ever since he first described it 100 years ago this year. And not once has it been found wanting. But they are still investigating its predictions to the nth decimal place, and this centenary year should see some particularly stringent tests. Perhaps one will uncover the first tiny flaw in this awesome mathematical edifice.

你可能以爲，物理學家現在已經滿意了。他們一直在對愛因斯坦的廣義相對論進行檢驗。愛因斯坦在整整100年前第一次提出了廣義相對論，它解釋了引力是什麼。科學家們一直沒有發現它存在任何不足之處，但卻仍在調查根據它做出的預測，精確到第n位小數。在該理論100週年之際，科學家會做一些特別嚴格的驗證。也許會有人發現這座非凡數學大廈的第一個微小缺陷。

Stranger still is that, though general relativity is celebrated and revered among physicists like no other theory in science, they would doubtless react with joy if it is proved to fail. That’s science: You produce a smart idea and then test it to its breaking point.

更爲奇怪的是，雖然在物理學家中，廣義相對論獲得的讚頌和尊崇超過了所有其他科學理論，但如果驗證證明它站不住腳，他們無疑會感到欣喜。這就是科學：你提出了一個聰明的想法，然後檢驗它至極限。

But this determination to expose flaws isn’t really about skepticism, far less wanton nihilism. Most physicists are already convinced that general relativity is not the final word on gravity. That’s because the theory, which is applied mostly at the scale of stars and galaxies, doesn’t mesh with quantum theory, the other cornerstone of modern physics, which describes the ultra-small world of atoms and subatomic particles. It’s suspected that underlying both theories is a theory of quantum gravity, from which general relativity and conventional quantum theory emerge as excellent approximations just as Isaac Newton’s theory of gravity, posed in the late 17th century, works fine except in some extreme situations.

但是揭示該理論缺陷的這種決心，其實無關乎懷疑主義，和肆意的虛無主義更是遠遠扯不上關係。大多數物理學家已經確信，廣義相對論並不是引力的最終定論。這是因爲該理論主要應用在恆星和星系的規模，和量子理論沒有交集。量子理論是現代物理學的另一塊基石，針對的是原子和亞原子粒子級別的微觀世界。科學家們覺得，這兩個基本理論的依託是一個量子引力理論，廣義相對論和常規量子理論是它的絕佳近似值，這就像艾薩克·牛頓在17世紀後期提出的萬有引力理論，除某些極端情況外，應用起來通常都沒問題。

The hope is, then, that if we can find some dark corner of the universe where general relativity fails, perhaps because the gravitational fields it describes are so enormously strong, we might glimpse what extra ingredient is needed — one that might point the way to a theory of quantum gravity.

科學家的希望是，如果能找到廣義相對論站不住腳的一些黑暗角落——這有可能是因爲它描述的引力場如此強大——那麼我們或許會發現它欠缺了哪些成分，而這可能會指明通向量子引力理論的道路。

General relativity was not just the last of Einstein’s truly magnificent ideas, but arguably the greatest of them. His “annus mirabilis” is usually cited as 1905, when, among other things, he kick-started quantum theory and came up with special relativity, describing the distortion of time and space caused by traveling close to the speed of light. General relativity offered a broader picture, embracing motion that changes speed, such as objects accelerating as they fall in a gravitational field. Einstein explained that gravity can be thought of as curvature induced in the very fabric of time and space by the presence of a mass. This, too, distorts time: Clocks run slower in a strong gravitational field than they do in empty space. That’s one prediction that has now been thoroughly confirmed by the use of extremely accurate clocks on space satellites, and in fact GPS systems have to adjust their clocks to allow for it.

廣義相對論不僅僅是愛因斯坦最後一個宏偉想法，而且可以說是他最偉大的構想。他的“奇蹟年”通常被認爲是1905年，這一年他開始構想量子理論，並提出了狹義相對論，描述了接近光速的運動導致的時空扭曲。廣義相對論則描繪了更加廣闊的畫面，探討了變速運動，比如物體在進入引力場時出現的加速。根據愛因斯坦解釋，引力可以看成是由於質量的存在，時間和空間結構中出現的彎曲。這也扭曲了時間：與沒有引力場的空間相比，時鐘在一個強大的引力場中走得慢一些。利用在空間衛星上極其精確的時鐘，科學家們徹底證實了這個預測的正確性。事實上，GPS系統必須考慮到這種影響，來調整自己的時鐘。

Einstein presented his theory of general relativity to the Prussian Academy of Sciences in 1915, though it wasn’t officially published until the following year. The theory also predicted that light rays will be bent by strong gravitational fields. In 1919 the British astronomer Arthur Eddington confirmed that idea by making careful observations of the positions of stars whose light passes close to the sun during a total solar eclipse. The discovery assured Einstein as an international celebrity. When he met Charlie Chaplin in 1931, Chaplin is said to have told Einstein that the crowds cheered them both because everyone understood him and no one understood Einstein.

愛因斯坦1915年向普魯士科學院(Prussian Academy of Sciences)提交了廣義相對論的論文，不過正式發表是在第二年。該理論還預測，強大的引力場會導致光的彎曲。在1919年，英國天文學家亞瑟·愛丁頓(Arthur Eddington)通過仔細觀察一次日全食中一些恆星的位置，證實了這一預測，這些恆星的光線會通過臨近太陽的區域。愛因斯坦自此成爲國際名人。當他在1931年與查理·卓別林(Charlie Chaplin)見面時，據說卓別林對他說，公衆爲他們兩人喝彩，是因爲每個人都理解自己的電影，但沒有一個人理解愛因斯坦的理論。

General relativity predicts that some burned-out stars will collapse under their own gravity. They might become incredibly dense objects called neutron stars only a few miles across, from which a teaspoon of matter would weigh 10 billion tons. Or they might collapse without limit into a “singularity” — a black hole from whose immense gravitational field not even light can escape, since the surrounding space is so bent that light just turns back on itself.

廣義相對論預言，一些燃料耗盡的恆星將因自身引力而崩塌。它們被稱爲中子星，其密度可能會變得非常之大，直徑只有幾英里，但一小勺就有100億噸。或者可能會無限地崩塌下去，變成“奇點”，也就是一個黑洞，其巨大引力場甚至連光都無法逃逸，因爲周圍的空間太過彎曲，光會直接轉彎回到原處。

Many neutron stars have been seen by astronomers: Some, called pulsars, rotate and send out beams of intense radio waves from their magnetic poles, beams that flash on and off with precise regularity. Black holes can only be seen indirectly from the X-rays and other radiation emitted by the hot gas that surrounds and is sucked into them. But astrophysicists are certain that they exist.

自那之後，天文學家發現了很多中子星：有些被稱爲脈衝星，它們旋轉運動，從磁極發射出強烈的電波，發射和停止存在着精準的規律性。黑洞只能通過X射線和熱氣體散發的其他輻射被間接看到，黑洞被這些熱氣體包圍着，並將它們吸入。但是天體物理學家堅信黑洞是存在的。

While Newton’s theory of gravity is mostly good enough to describe the motions of the solar system, it is around very dense objects like pulsars and black holes that general relativity becomes indispensable. That’s also where it might be possible to test the limits of the theory with astronomical investigations. Last year, astronomers at the National Radio Astronomy Observatory in Charlottesville, Virginia, discovered the first pulsar orbited by two other shrunken stars, called white dwarfs. This situation, with two bodies moving in the gravitational field of a third, should allow one of the central pillars of general relativity, called the strong equivalence principle, to be put to the test by making very detailed measurements of the effects of the white dwarfs on the pulsar’s metronome flashes as they circulate. The team hopes to carry out that study this year.

雖然牛頓的引力理論基本上足以描述太陽系的運動，但對於密度極大的物體，比如脈衝星和黑洞，廣義相對論就不可或缺了。這也是用天文研究檢驗這個理論的侷限的地方。去年在弗吉尼亞州夏洛茨維爾，國家射電天文臺(National Radio Astronomy Observatory)的天文學家發現了一顆脈衝星，繞着它運動的另外兩顆縮小的恆星被稱爲白矮星，而這一現象是前所未見的。在這種情況下，有兩個星體在第三個的引力場中運動，如果在白矮星繞脈衝星運動的時候，非常細緻地測量它們對脈衝星電波發射規律的影響，應該可以檢驗廣義相對論的核心支柱之一“強等效原理”。該團隊希望今年開展這項研究。

But the highest-profile test of general relativity is the search for gravitational waves. The theory predicts that some astrophysical processes involving very massive bodies, such as supernovae (exploding stars) or pulsars orbited by another star (binary pulsars), should excite ripples in space-time that radiate outwards as waves. The first binary pulsar was discovered in 1974, and we now know the two bodies are getting slowly closer at just the rate expected if they are losing energy by radiating gravitational waves.

但最引人注目的廣義相對論檢驗是對引力波的尋找。該理論預測，一些非常龐大的星體，比如超新星（爆炸的恆星）或者被另一顆恆星圍繞盤旋的脈衝星（脈衝雙星），和它們有關的天體物理過程應該在時空中激發漣漪，像波一樣向外輻射。第一個脈衝雙星是在1974年發現的，科學家假設兩個星體輻射了引力波，因而損耗了能量，計算出了它們靠攏的速率，我們現在已經知道，它們確實在以這個速率慢慢靠攏。

The real goal, though, is to see such waves directly from the tiny distortions of space that they induce as they ripple past our planet. Gravitational-wave detectors use lasers bouncing off mirrors in two-kilometer-long arms at right angles, like an L, to measure such minuscule contractions or stretches. Two of the several gravitational-wave detectors currently built — the American LIGO, with two observatories in Louisiana and Washington, and the European VIRGO in Italy — have just been upgraded to boost their sensitivity, and both will start searching in 2015. The European Space Agency is also launching a pilot mission for a space-based detector, called LISA Pathfinder, this September.

不過，真正的目標是，當這些波經過我們的星球時，直接從它們導致的微小空間扭曲中看到它們。引力波探測器讓激光在長兩公里、擺成L形的干涉臂上來回反射，從而對這種微小收縮或擴張進行測量。目前世界上許多臺引力波探測器，其中兩臺——美國的LIGO，在路易斯安那州和華盛頓有兩個觀察站；以及歐洲的VIRGO，位於意大利——剛剛對靈敏性進行了升級，它們都將在2015年開始尋找引力波。去年9月，歐洲航天局還用太空中的LISA Pathfinder探測器開展了一個試點任務。

If we’re lucky, then, 2015 could be the year we confirm both the virtues and the limits of general relativity. But neither will do much to alter the esteem with which it is regarded. The Austrian-Swiss physicist Wolfgang Pauli called it “probably the most beautiful of all existing theories.” Many physicists (including Einstein himself) believed it not so much because of the experimental tests but because of what they perceived as its elegance and simplicity. Anyone working on quantum gravity knows that it is a very hard act to follow.

幸運的話，2015年就會是我們確認廣義相對論優勢和侷限性的一年。但這不會對它受到的推崇產生太大影響。奧地利-瑞士物理學家沃爾夫岡·泡利(Wolfgang Pauli)稱廣義相對論“可能是現有理論中最美的”。很多物理學家（包括愛因斯坦本人）相信它，並不是因爲它經過了實驗的檢驗，而是因爲他們認爲它簡潔優雅。每個在量子引力領域工作的人都知道，簡潔優雅是多麼難以達到。