原创翻译:龙腾网  翻译:飞雪似炀花



The Best Explanation for Everything in the Universe


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


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.


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?


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.


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.

然后,在1995年,物理学家Edward Witten发现了一切弦理论的根源。他发现不同的迹象表明这些摄动的弦理论与一个连贯的非摄动理论是相吻合,他称之为M理论。M理论在不同的物理环境中看起来就像各种弦理论,但是它本身却没有对其有效性状态的限制——这是对万有理论的一个主要要求。Witten的计算结果反映的现象大致如此。加州理工学院的理论物理学家David Simmons-Duffin解释称:“Witten可以在不写出M理论方程式的情况下提出这些观点,这令人印象深刻,但它也留下了许多问题。”

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.

紧随其后的两年后的另一场研究大爆炸,当时物理学家Juan Maldacena发现了反德西特空间和共形场论的对应:一种类似全息图的关系,它在一个被称为反德西特空间的时空区域中将引力与对在该区域的边界附近移动的粒子的量子描述(称之为“共形场论”)联系在一起。对于反德西特空间时空几何体的特殊案例而言,反德西特空间和共形场论给出了M理论的一个完整定义,它们被注入了负能量,使它们以不同于我们的宇宙的方式实现弯曲。对于这样的假想世界,物理学家可以描述所有能量的过程,原则上包括黑洞的形成和蒸发。在过去的20年里,引用Maldacena的1.6万篇论文主要就是为了进行这些计算,以便更好地理解反德西特空间和共形场论与量子引力。

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.


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.


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.



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