大西洋月刊:對宇宙萬物的最佳闡釋

原文地址：https://www.theatlantic.com

原創翻譯：龍騰網  翻譯：飛雪似煬花

正文翻譯：

The Best Explanation for Everything in the Universe

大西洋月刊：對宇宙萬物的最佳闡釋

String theory is considered the leading 「theory of everything,」 but there’s still no empirical evidence for it.

弦理論被認為是最重要的「萬有理論」，但它還沒有任何實證證據。

It’s not easy being a 「theory of everything.」 A TOE has the very tough job of fitting gravity into the quantum laws of nature in such a way that, on large scales, gravity looks like curves in the fabric of space-time, as Albert Einstein described in his general theory of relativity. Somehow, space-time curvature emerges as the collective effect of quantized units of gravitational energy—particles known as gravitons. But na?ve attempts to calculate how gravitons interact result in nonsensical infinities, indicating the need for a deeper understanding of gravity.

要成為一種「萬有理論」並不容易。正如阿爾伯特·愛因斯坦在他的廣義相對論中所描述到的，萬有理論是將引力融入到自然的量子法則中去的一項異常艱難的工作，通過這種方式，在大尺度上，引力就像時空結構中的曲線一樣。在某種程度上，時空曲率是作為被稱作引力子的引力能量量子化單位的集體效應而出現的。但試圖計算引力子之間的相互作用的天真想法卻導致了毫無意義的無限，這表明我們需要對引力有更深入的理解。

String theory (or, more technically, M-theory) is often described as the leading candidate for the theory of everything in our universe. But there’s no empirical evidence for it, or for any alternative ideas about how gravity might unify with the rest of the fundamental forces. Why, then, is string/M-theory given the edge over the others?

弦理論(或者更加技術性地稱之為M理論)經常被描述為我們宇宙萬有理論的主要候選者。但是它還沒有任何實證證據，也沒有任何關於重力如何與其他基本力相統一的替代觀點。那麼，為什麼會認為弦理論或M理論相對於其他理論擁有優勢呢？

The theory famously posits that gravitons, as well as electrons, photons, and everything else, are not point particles but rather imperceptibly tiny ribbons of energy, or 「strings,」 that vibrate in different ways. Interest in string theory soared in the mid-1980s, when physicists realized that it gave mathematically consistent descriptions of quantized gravity. But the five known versions of string theory were all 「perturbative,」 meaning they broke down in some regimes. Theorists could calculate what happens when two graviton strings collide at high energies, but not when there’s a confluence of gravitons extreme enough to form a black hole.

這個理論的一個著名觀點是：引力子，以及電子、光子和其他一切，都不是點粒子，而是一種不可感知的微小的能量帶，或者是以不同方式振動的「弦」。上世紀80年代中期，人們對弦理論的興趣大增，當時物理學家們意識到，它與數學上對量子化引力的描述是一致的。但是，五個已知的弦理論版本都是「攝動的」，這意味著它們在某些狀態下會崩潰。理論學家可以計算當兩個引力子弦在高能量狀態下相撞時會發生什麼，但卻無法計算出當有足夠多的引力子聚集到一起形成黑洞時會發生什麼。

Then, in 1995, the physicist Edward Witten discovered the mother of all string theories. He found various indications that the perturbative string theories fit together into a coherent non-perturbative theory, which he dubbed M-theory. M-theory looks like each of the string theories in different physical contexts but does not itself have limits on its regime of validity—a major requirement for the theory of everything. Or so Witten’s calculations suggested. 「Witten could make these arguments without writing down the equations of M-theory, which is impressive but left many questions unanswered,」 explained David Simmons-Duffin, a theoretical physicist at the California Institute of Technology.

然後，在1995年，物理學家Edward Witten發現了一切弦理論的根源。他發現不同的跡象表明這些攝動的弦理論與一個連貫的非攝動理論是相吻合，他稱之為M理論。M理論在不同的物理環境中看起來就像各種弦理論，但是它本身卻沒有對其有效性狀態的限制——這是對萬有理論的一個主要要求。Witten的計算結果反映的現象大致如此。加州理工學院的理論物理學家David Simmons-Duffin解釋稱：「Witten可以在不寫出M理論方程式的情況下提出這些觀點，這令人印象深刻，但它也留下了許多問題。」

Another research explosion ensued two years later, when the physicist Juan Maldacena discovered the AdS/CFT correspondence: a hologram-like relationship connecting gravity in a space-time region called anti–de Sitter (AdS) space to a quantum description of particles (called a 「conformal field theory」) moving around on that region’s boundary. AdS/CFT gives a complete definition of M-theory for the special case of AdS space-time geometries, which are infused with negative energy that makes them bend in a different way than our universe does. For such imaginary worlds, physicists can describe processes at all energies, including, in principle, black-hole formation and evaporation. The 16,000 papers that have cited Maldacena’s over the past 20 years mostly aim at carrying out these calculations in order to gain a better understanding of AdS/CFT and quantum gravity.

緊隨其後的兩年後的另一場研究大爆炸，當時物理學家Juan Maldacena發現了反德西特空間和共形場論的對應：一種類似全息圖的關係，它在一個被稱為反德西特空間的時空區域中將引力與對在該區域的邊界附近移動的粒子的量子描述(稱之為「共形場論」)聯繫在一起。對於反德西特空間時空幾何體的特殊案例而言，反德西特空間和共形場論給出了M理論的一個完整定義，它們被注入了負能量，使它們以不同於我們的宇宙的方式實現彎曲。對於這樣的假想世界，物理學家可以描述所有能量的過程，原則上包括黑洞的形成和蒸發。在過去的20年裡，引用Maldacena的1.6萬篇論文主要就是為了進行這些計算，以便更好地理解反德西特空間和共形場論與量子引力。

This basic sequence of events has led most experts to consider M-theory the leading TOE candidate, even as its exact definition in a universe like ours remains unknown. Whether the theory is correct is an altogether separate question. The strings it posits—as well as extra, curled-up spatial dimensions that these strings supposedly wiggle around in—are 10 million billion times smaller than experiments like the Large Hadron Collider can resolve. And some macroscopic signatures of the theory that might have been seen, such as cosmic strings and supersymmetry, have not shown up.

這個基本的事件序列使得大多數專家都認為M理論是最重要的萬有理論候選者，儘管它在我們這樣的宇宙中確切的定義仍然是未知的。這個理論是否正確是一個完全不同的問題。它所設想的這些弦——以及這些弦在其中振動的額外的、彎曲的空間維度——要比大型強子對撞機能實現的實驗要小1億億倍。這一理論的一些可能已經被觀測到的宏觀特徵，如宇宙弦和超對稱，並沒有出現。

Other TOE ideas, meanwhile, are seen as having a variety of technical problems, and none have yet repeated string theory’s demonstrations of mathematical consistency, such as the graviton-graviton scattering calculation. (According to Simmons-Duffin, none of the competitors have managed to complete the first step, or first 「quantum correction,」 of this calculation.) One philosopher has even argued that string theory’s status as the only known consistent theory counts as evidence that the theory is correct.

與此同時，其他的萬有理論也被認為存在著各種各樣的技術問題，而且還沒有一種重複了弦理論的數學一致性表徵，比如引力子散射計算。(根據Simmons-Duffin的說法，沒有一個競爭者成功地完成了第一步，或者說這一計算的首次「量子校準」。)一位哲學家甚至認為，弦論的地位是唯一已知的一致理論，它被認為是理論正確的證據所在。

The distant competitors include asymptotically safe gravity, E8 theory, noncommutative geometry, and causal fermion systems. Asymptotically safe gravity, for instance, suggests that the strength of gravity might change as you go to smaller scales in such a way as to cure the infinity-plagued calculations. But no one has yet gotten the trick to work.

遠遠落後於它的競爭對手包括了漸近安全引力、E8理論、非交換幾何和因果費米體系。例如，漸近安全引力表明，當你進入小尺度的時候，引力的強度可能會發生改變，就像治癒被無窮所傳染的計算結果一樣。但是還沒有人獲得了這麼做的竅門。