追寻时间之源_English Corner

In Search of Time’s Origin

Rearrange the following words to tell a coherent life story: A man dies, later he gets married, and finally he is born. Thanks to our built-in temporal sense, it’s pretty straightforward: Tomb always follows womb, it’s never the other way around.

Yet at a fundamental level, time’s origin remains a mystery. “It’s one of the deepest questions at the forefront of science, but when we ask, ‘What is time? Where does it come from?’ it’s not even clear the words make any sense,” says Nima Arkani Hamed, a physicist at the Institute of Advanced Studies (IAS) in Princeton, N.J. “We can barely articulate what a world without time, or physics without time, means.”

Confusing as the absence of time would be, there is mounting evidence that at the most basic level of reality, time is an illusion. Stranger still, laboratory tests with laser lights and advances in our understanding of string theory—the proposed framework positing that particles are composed of small threads of energy—independently point to the idea that time doesn’t really exist.

Little more than a century ago, our picture of time and space was far less complicated. Physicists happily tracked objects across a fixed background set by our three spatial dimensions and marked how fast they moved against a single clock—God’s proverbial stopwatch, that they believed ticked at the same rate no matter where you were in the cosmos. But in the early 20th century, two revolutions in physics disrupted this view.

In the first revolution, Einstein’s theory of relativity wove together time and space into a flexible four-dimensional fabric. That fabric, which Einstein called “spacetime,” could mold itself around massive objects, creating a curvature. Smaller objects could roll down those curves toward the larger masses, investing the universe with a force called gravity. In this new theory of the universe, time was no longer an immutable bystander, but an interconnected dimension enmeshed with space itself. Instead of being that unambiguous dimension against which others could be measured, time was now relative. Einstein’s relativity showed that clocks would tick at different rates depending on their motion through space and their proximity to massive objects that pulled them in with gravitational force.

The second development disrupting our view of time was quantum mechanics, the physics of the subatomic realm. Quantum mechanics revealed that on the smallest scales, reality was strange indeed. For instance, two particles can become “entangled” in such a way that they always act in tandem. An experiment carried out on one will immediately influence its partner, no matter how distant it may be. In other words, the distant particles communicate instantly, apparently defying the rule that nothing can travel faster than the speed of light and the very concept of time itself.

Mounting evidence shows that at the most basic level of reality, time is an illusion, and stranger still, that time doesn’t really exist.

But the real “problem of time,” as it has become known, arose in the 1960s as physicists struggled to combine these two frameworks—each successful at describing its own realm of the universe, either the very tiny or the large. The search for an overarching “theory of everything,” a set of rules that governed objects of all sizes, was on. One of the most famous but controversial hypotheses came from two New Jersey physicists: John Wheeler of Princeton University and Bryce DeWitt of the IAS. Wheeler and DeWitt tried to describe the whole universe through quantum mechanics—that is, they attempted to apply the physics of the very small to planets, galaxies, and other cosmic structures on a mass scale. Many questioned whether their tactics would work, because there had been no evidence to suggest that quantum laws held sway over cosmic distances, notes Marco Genovese, a quantum physicist at Istituto Nazionale di Ricerca Metrologica (INRIM) in Turin, Italy. But it seemed reasonable to at least try to unite the mathematics of the two theories and see what would happen.

When the two physicists tried to combine Einstein’s equations of relativity with quantum physics, they came up with a surprise. Both sets of laws independently featured time as a variable against which events evolved. But when the theories were combined into one, the time variable was literally cancelled out of the mathematical equation. The duo had derived a new equation for how the universe behaved, yet there was no longer a quantity in their mathematical description that could be used to mark out change or the passage of time. “The Wheeler-DeWitt equation says that the universe is stationary and that nothing evolves,” says Genovese. “But, of course, we all experience time and change.”

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The conclusion that the universe never changes was clearly wrong. Yet physicists could not find anything wrong with the mathematical steps that Wheeler and DeWitt had taken. At first, it seemed that the pair must have been mistaken to think that the whole cosmos could be described in quantum terms. But there was another intriguing possibility, proposed in the 1980s by physicists Don Page, now at the University of Alberta, in Edmonton, Canada, and William Wooters, at Williams College in Williamstown, Mass.

Page and Wooters decided to apply the controversial concept that the universe as a whole could be treated as a giant quantum object—subject to the same physical laws as electrons, protons, and other tiny particles of the subatomic world. They imagined splicing the contents of the cosmos into two pieces. Because quantum laws prevailed, the pieces would be entangled. Scientists have found that two entangled particles measured in the lab can have equal but opposite values. If one is spinning clockwise, for instance, the other will be spinning counter-clockwise so that, when summed together, the properties cancel each other out. Page and Wooters argued that in similar fashion, each section of their divided cosmos could independently evolve, but because they were entangled, the changes in one would be counter-balanced by the changes in the other. To someone inside one of the sections, time would appear to pass. But to the outside observer, the overall universe would appear static.

While Page and Wooters had offered a theoretical sketch, based on quantum entanglement, for how the cosmos might appear to be stationary to someone peering in from the outside, there seemed to be no way to confirm or rule out their idea. But, in 2013, Genovese and his colleagues performed an experiment to test whether—at least in the lab—it is possible to create a model of the universe in miniature, with just two particles of light, or photons, generated from a laser. The aim of the experiment was to prove that it is possible to create a situation in which a quantum system, when viewed from outside, appeared unchanging, but when observed from within appeared to evolve.

To do the experiment, Genovese set out to monitor the photons’ polarizations—the directions in which they vibrated. If a polarized particle could be made to rotate at a constant rate, then its position at any moment could be used to mark out intervals in time, just like a second-hand on a clock. The team entangled the two photons together, in such a way that their polarizations took on opposing traits. For instance, if the polarization of one was measured to move up and down, the other would vibrate from side-to-side.

“What we are seeing is that at the start of the universe, the notion of time ceases to make sense.”

In order to set their photons’ “second-hands” in motion, the team passed both particles through quartz plates, causing their polarizations to rotate. The amount of rotation was related to the actual time spent within the plates, giving physicists a means of measuring the passage of time. They carried out their experiment repeatedly and in each run they stopped at a different moment and measured the polarization of one of the photons. “By measuring the first clock photon, we became entangled with it,” says Genovese. “That means we became part of that universe and can register the evolution of the second photon against our clock photon.” Vested with this ability, the team confirmed that one photon appeared to change when measured against its partner, in the same way that Wooters and Page believed one part of the universe could be seen to evolve if measured against another portion of the cosmos.

However, Genovese still had to confirm the second part of the hypothesis: that when the entire entangled system was monitored as a whole, from the outside, it would appear static. In this part of the experiment, the team took the point of view of a “super observer” standing outside the universe. This external watcher could never look at the individual state of either photon because by doing so he would become entangled with them, becoming an internal observer. Instead, the observer could only measure the joint state of the pair of photons. The team ran the test many times, stopping at different points. They looked at the two photons as a combined whole and measured their joint polarization. Each time, they ascertained that the two entangled photons were polarized in equal but opposite ways. No matter how much time passed, the two photons were always poised in exactly the same “embrace.” The mini-universe appeared to be static from the outside and completely unchanged. It turns out the so-called “problem of time,” discovered by Wheeler and DeWitt, can be resolved if time is an artifact of quantum entanglement.

Over the past few decades, support for the illusory nature of time has also emerged from string theory, developed in the 1960s to help describe the strong nuclear force that binds elementary particles together within atoms. As they studied the strong force, physicists came up with the idea that subatomic particles, then thought to be the smallest objects in the universe, were in fact themselves composed of tiny vibrating strings.

This new way of perceiving the basic objects in nature had far-reaching consequences. It turned out that string theory was extremely helpful for those like Wheeler and DeWitt, who wanted to unite general relativity with quantum mechanics. Such a unifying framework is needed to explain what the universe was like soon after the Big Bang, when all cosmic matter was squashed into a tiny volume. A unified theory could also reveal what happens at the cores of black holes—the corpses of stars that have collapsed under the force of gravity, compressing matter into a small central point.

Before the discovery of string theory, physicists ran into trouble whenever they tried to combine the equations of general relativity with those of quantum mechanics. The combined mathematics appeared to tell them that infinitely small points in space all around us should contain infinitely large amounts of energy—essentially predicting that we are surrounded by black holes everywhere we turn, which is not true. String theory sidestepped this problem, however, by positing that nothing can be smaller than the size of a string. That meant that its equations never had to worry about regions of space that were smaller than this fundamental limit, eliminating the messy math with its predictions of infinite energies and other impossible results. With string theory, the physics of the very large and the very small appeared as if they could coexist—at least once string theory was finessed.

Yet string size raised new questions about the reality of space, and, in turn, of time itself. This is because string theory says that no experiment, no matter how elaborate, will ever be able to show us what happens at distances smaller than the size of a single string. “What happens at short distances,” explains IAS string theorist Nathan Seiberg, “is an ill-defined concept—maybe space exists, but we can’t measure it, or perhaps there is nothing there to measure at all.” That meant that space may simply not exist below a certain limit. Since Einstein had already shown with his theory of relativity that time is just another dimension, like space, then “if space becomes ambiguous, time must do so too,” says Seiberg. “People often ask: ‘What happened before the Big Bang?’ But what we are seeing is that at the start of the universe, the notion of time ceases to make sense.”

When it comes to cosmic ingredients, quantum entanglement is more fundamental than space and time.

This ambiguity gave string theorists their first inkling that time might not exist at a fundamental level, notes Seiberg. Instead, our experience of time might be constructed from underlying building blocks, much like temperature, which arises from the motion of a collection of atoms. An individual atom does not have a temperature; the concept of hot or cold only has meaning when you measure the average speed of a large number of atoms: Fast moving particles have a higher temperature than slow atoms. In a similar way, there may be fundamental grains that together generate our experience of time. But just what those grains may be, well, “that’s the $64,000 question,” says Seiberg.

Stranger still, later advances in string theory suggest that time’s seeds are sown at the very edges of reality. This idea has its roots in an odd model of a hypothetical universe devised in the late 1990s by string theorist Juan Maldacena, then at Harvard University, who was searching for a mathematical relationship that might connect quantum mechanics and general relativity. He decided that he could get there by using strings.

Maldacena’s imaginary cosmos was shaped like a soup can, but with walls that are infinitely far away. Inside his can, he placed strings and black holes, whose behavior was governed by gravity. On the surface of the can he placed normal subatomic particles that interacted through the laws of quantum mechanics. Although Maldacena’s soup-can universe did not sound much like ours, it helped him visualize how the deepest laws of nature could be connected.

In the model, general relativity held sway in the vast three-dimensional space within the can, while quantum mechanics ruled the particles lining the two-dimensional surface. Maldacena’s hunch was that the two sets of laws were somehow equivalent, and that gravitational events unfolding inside the can would correspond to quantum processes on the surface, like a shadow projected onto the can’s walls. Using this mathematical model, Maldacena indeed found that for every quantum process on the surface, an equivalent event unfolded within the can. Theoretical models developed by Maldacena and others indicate that quantum particles entangled on the surface of the soup can rewrite their patterns by creating tunnels, or “wormholes,” within the inner realm. This suggests that entanglement itself is the fundamental cosmic process generating the emergent properties of space and time.

The idea that both space and time are created by quantum entanglement has been independently bolstered by string theorist Mark van Raamsdonk at the University of British Columbia in Vancouver, who also investigated Maldacena’s soup can model. Using a mathematical model, he found that by gradually eroding particle entanglement on the can’s surface, the spacetime fabric within the can starts to disintegrate as well. This implies that quantum entanglement somehow plays a role in tying the threads of space and time together; without it, the fabric of spacetime itself could not exist.

Maldacena’s model provides more support than ever for the claim that, when it comes to cosmic ingredients, entanglement is more fundamental than space and time. Time, it turns out, is not present at the most basic layer of reality; it springs from fundamental seeds. But while emerging physics suggests time is an illusion, the forces that conjure it remain at large. “My intuition is that it will take more than just a re-working of quantum physics, it will require a breakthrough that will come totally out of left field,” Seiberg says. “Only time will tell what that revolution will be.”

请将下列语句排列成一个合乎情理的人生故事：一个人死了，然后他结婚了，最后他诞生了——好在我们有天生的时间感，做这种问题完全是手到擒来：欲入坟冢，先出子宫，从未有过其它情况。

不过，在基础层面上，时间之源仍然是一个谜。新泽西普林斯顿高等研究所（Institute of Advanced Studies, IAS）的一名物理学家，尼玛·阿卡尼·哈姆德（Nima Arkani Hamed）说：“它是科学前沿最深刻的问题之一，当我们问起：‘时间是什么？它来自何处？’时，我们都不清楚这些问题是不是有意义。我们很难说明白一个没有时间的世界、或一个没有时间的物理学意味着什么。”

尽管除去时间会让人困惑不已，但越来越多的证据表明，在实在（存在）的最基本的层面上，时间只是一个幻象。更奇怪的是，激光实验测试和弦论（一个认为粒子是由细小的能量纤维组成的理论框架）的进展都不约而同地指向一条思路：时间并不存在。

一个世纪多点前，我们认识中的时间与空间图景远没有这么复杂。物理学家愉快地在一个固定的三维空间背景下追踪物体的运动，并用一个独立的时钟（上帝之秒表）来标记它们运动的快慢。人们认为，不论身处宇宙何处，上帝之表都以相同的速度滴答走秒。但到了20世纪初，两大物理革命撼动了这种观念。

第一场革命，是爱因斯坦的相对论将时间与空间编织成了随动的四维结构。爱因斯坦将这种结构称为“时空”，它能依随周围大质量的物体而变形，产生弯曲。质量小的物体则会沿着这些弯曲“滚向”大质量物体，这让宇宙产生了一种称为“引力”的作用力。在这一新的宇宙论中，时间不再是千年旁观者，而成为与空间相融合、相联系的一个维度。时间维和那些毫不含糊、可以测定的不同，它现在变成相对的了。相对论说明，时钟的快慢取决于物体穿过空间的运动快慢以及它们靠近通过引力牵引它们的大质量物体的程度。

撼动我们对时间认识的第二大发展是量子力学。它是运用于亚原子领域的物理学。量子力学显示，在最微观的尺度下，事物的实质与存在变得很奇怪。比如，两个粒子可以以某种方式“纠缠”起来，这样它们就总会同时运动和变化。对其中一个进行的实验会立即影响到另一个，且不论两者距离多远都是如此。换言之，相距甚远的粒子对能够即时“交流”，这明显与“任何物体都不能超光速运动”及时间本身的概念相左。

但随着这一问题越来越多地为人所知，真正的“时间问题”在上世纪60年代产生了。当时物理学家为结合上面两大理论框架而焦头烂额——它们各自在其适用范围内成功地描述了宇宙：一个是在极小尺度下，一个则是在大尺度下。向着一个囊括一切的“万有理论”（一套规范各种尺度下物质的规则）的探索启程了。其中最知名、但也饱受争议的假说是有两位新泽西的物理学家提出的，他们是普林斯顿大学的约翰·惠勒（John Wheeler）和IAS的布莱斯·德维特（Bryce DeWitt）。惠勒和德维特试着用量子力学来描述整个宇宙——即，他们将适用于小尺度物质的理论应用到大尺度的行星、星系以及其它宇宙结构上。很多人对于惠勒他们的方式是否可行都表示质疑。意大利都灵的意大利国家计量院（Istituto Nazionale di Ricerca Metrologica, INRIM）的量子物理学家马可·吉诺维斯（Marco Genovese）称，这是因为没有迹象表明量子定律能够延伸到宇宙范围中。不过试着联合两大理论的数学表述，看看结果如何的做法起码还是合乎逻辑的。

二人将爱因斯坦的相对论方程与量子力学理论结合后，他们都惊呆了。两套法则本都独立地将时间视为表征事件的一个变量，但在将二者结合后，时间因子在数学方程中被完全抵消。两套方程得出了一套描述宇宙行为的新方程，不过此时在其数学表述中，已经没有哪个量可以标志变化或时间流逝了。吉诺维斯说：“惠勒-德维特方程表明，宇宙是静态的，没有任何东西在演化。不过呢，我们也当然都感觉得到时间与变化。”

宇宙从未变化的结论明显是错的。但物理学家在惠勒和德维特的数学推导中却找不到任何错误。起先，人们认为二人的错误似乎是在于认为整个宇宙都可以用量子的方式加以描述。不过我们还有另一种有意思的可能性，它是上世纪80年代由物理学家唐·佩奇（Don Page，现在在加拿大埃德蒙顿的亚伯达大学）和威廉·沃特斯（William Wooters，在马塞诸塞州威廉姆斯镇的威廉姆斯学院）提出的。

佩奇和沃特斯决定采用一个颇有争议的观点：宇宙整体可以看成是一个巨大的量子对象，服从于电子、质子和其它亚原子世界中的微小粒子所遵循的物理规律。他们设想将宇宙分为两大块，按照量子力学定律，这两块是相互纠缠的。科学家已经在实验中发现，两个纠缠中的粒子具有相同但相反的值。例如，如果一个顺时针旋转，那么另一个就是逆时针旋转。这样，相加起来整体的这一性质就抵消了。佩沃二人称，按类似的思路，分开的每块宇宙将独立地演化，但因为它们是相纠缠的，一块中的变化会被另一个块中的变化中和。对在其中一块中的某人来说，时间在流逝，但对于一个宇宙之外的观察者来说，整个宇宙是静态的。

尽管佩沃二人基于量子纠缠提出了一个让宇宙在一个从宇宙外部向内窥视的人认为宇宙是静态的理论图景，但却似乎没法证实或证伪他们的想法。不过，到了2013年，吉诺维斯和他的同事进行了一项实验，至少在实验室中验证了制造一个这种宇宙的小型模型的可能。他们仅用了激光器产生的两个光粒子（光子）。实验的目的，是证明可以发生某种情形，让一个量子体系在从外部看时处于不变、而从内部看时却在演化。

在实验中，吉诺维斯检测光子的偏振，即光子振动的方向。如果让一个偏振粒子以一匀速旋转，那么它在任何时刻的指向就能像时钟的秒针那样用来标记时间间隔了。他的团队让两个光子发生纠缠，使它们的偏振处于相对的模式。比如，如果一个的方向是上下振动，另一个就是左右振动。

该团队让光子穿过石英片，使之偏振发生旋转，让光子“秒针”运动起来。转动量与穿越晶片所用的实际时间有关，这给了物理学家一种测定时间经过的方法。他们多次重复实验，每次都在一个不同的时刻终止，然后测量其中一个光子的偏振。吉诺维斯说：“在测量第一个光子钟时，我们也与它发生的纠缠。也就是说，我们变成了那个小宇宙的一部分，并且能够通过相比我们的光子钟而记录另一个光子的变化。”这样，该团队证实，在测量某光子的对子时，该光子会发生变化。同理，沃特斯和佩奇相信，如果测量宇宙的另一部分，那么这部分的宇宙看上去就在演化。

不过，吉诺维斯仍需证明假设的另一部分：如果从外部检测作为一个整体的纠缠体系，它应该是静态的。在这一部分的实验中，团队采用了宇宙外“超然观察者”的视角。这个外部的观察者不能查看任何一个光子单独的状态，因为这样就会让他也与之纠缠，而变成内部观察者。反过来说，这个观察者只能测量光子对的结合态。团队进行了多次测试，每次在不同的时刻终止。他们将两个光子视为一个结合的整体，测量它们的联合偏振。每次实验他们都确保两个纠缠光子以相同程度偏振，但方式相左。不论实验时间经历多长，两个光子总是保持完全相同的“抱团”。从外部看，这个迷你宇宙是静态的，并且完全不发生变化。它表明，如果时间是量子纠缠的产物，那么惠勒和德维特发现的所谓的“时间问题”就迎刃而解了。

在过去的几十年中，弦论也表现出支持时间本质虚幻这一点的。弦论于上世纪60年代开始发展，用于描述将原子内的基本粒子束缚在一起的强核力。在物理学家研究强作用力的过程中，他们冒出了一个想法，即当时被认为是宇宙中最小物质的亚原子粒子，实际上是由一些振动的“细弦”组成的。

这一看待自然基本对象的新方式产生了影响深远的结果。人们发现弦论对那些像惠勒和德维特那样希望结合广义相对论与量子力学的人非常有用。人们需要这种统一的理论框架来解释大爆炸后瞬间宇宙的样子，此时所有的宇宙物质都被挤压在一个极小的体积内。统一理论可以说出黑洞内核处发生着什么。（黑洞是恒星坍缩的结果：恒星在引力作用下收缩，将其物质压入一个很小的中心中。）

在弦论出现前，物理学家在试图联立广义相对论和量子力学时总是会遇到麻烦。联立后的数学告诉他们，我们身边空间中的无限小空间点内包含着无限大的能量——这基本上就是说我们不管在哪，都被黑洞包围，这显然是错的。但是，弦论则认为任何东西都不能小过弦，因而回避了这个问题。这是说，它的方程无需担心小于这一基本下限的空间区域，这就消除了那些会得出无限能量及其它不可能结果的难缠的数学。有了弦论，超大尺度与极小尺度的物理看来就可以共存了——至少弦论一度是成功了的。

不过弦的大小又引出了关于空间实质、随之又引出时间实质的新问题。这是因为弦论认为，不论如何精心地设计，都没有实验能够向我们展示在小于单个弦的尺度下发生着什么。IAS的弦论家内森·塞伯格（Nathan Seiberg）解释说：“‘在小距离内发生着什么’是一个错误定义的观念——那里空间也许存在，但我们无法测量它；也许那里根本就没有可以测量的东西。”这意味着在某个极限下，空间也许不存在。因为爱因斯坦已经在他的相对论中表示过，时间不过是和空间类似的另一个维度，那么“如果空间概念在小间隔内变得暧昧，那么时间也会如此，”塞伯格如是说道，“人们经常会问：‘大爆炸前都发生了什么？’但我们看到的是，在宇宙创生之时，时间才开始有意义。”

塞伯格指出，这种模糊性让弦论家隐隐感觉时间在基本层面上也许不存在，而我们对时间的感觉可能是由一些隐含的“基建材料”构成的，就像温度是来自一群原子的运动那样。一个单独的原子并不具有一个温度，热和冷的概念只在你测量大量原子的平均速度时才有意义：速度快的粒子团比慢的具有更高的速度。类似地，也许存在某种基本“颗粒”，共同让我们产生了时间体验。但至于这种颗粒可能是什么，呃，塞伯格如是说：“那就是‘六万四千美金问题’[1]了。”

更奇怪的是，弦论后续的发展显示，时间之种播撒在实在（存在）的最边缘。这一思想的根源来自于一个奇特的假想的宇宙模型。这个模型是上世纪90年代末由弦论家胡安·马尔达西那（Juan Maldacena，当时在哈佛大学）提出的。他是在寻找一个可以联结量子力学和广义相对论的数学关系，他认为可以运用弦论来达到这一目的。

马尔达西那设想的宇宙形似一个罐头，不过它的边界设在无穷远处。罐内的是弦和黑洞，其行为受引力控制；罐面放置的是一般的亚原子粒子，它们通过量子力学定律而相互作用。尽管马尔达西那的罐头宇宙听上去和我们的不太一样，但它让他直观地看出最深层的自然律是如何联系在一起的。

在这一模型中，广义相对论支配着罐内巨大的三维空间，而量子力学则控制着二维表面上排列的粒子。马尔达西那的看法是，两套定律在某种方式下是等效的，罐内展开的引力事件可以对应表面上的量子过程，后者就像前者投射到罐面的影子一样。利用这一数学模型，马尔达西那确实发现，对表面的每个量子过程，在罐内都有一个对应的事件。马尔达西那等人发展的理论模型表明，纠缠在罐面的量子粒子可以通过在内部空间制造通道，或称“虫洞”，来改变它们的模式。这就说明了纠缠本身是产生空间和时间性质的基本宇宙过程。

空间和时间都是由量子纠缠产生的这一想法，也独立地由温哥华的不列颠哥伦比亚大学的弦论家马克·范·拉姆斯敦克（Mark van Raamsdonk）做出了改进，他也研究了马尔达西那的罐头模型。他借助一个数学模型发现，如果逐渐削弱罐面粒子间的纠缠，那么罐内的时空结构也会开始退联结。这意味着量子纠缠以某种方式扮演了让空间和时间之线交织在一起的角色。没有它，时空结构就会不复存在。

马尔达西那的模型为“涉及到宇宙组分时，纠缠比空间和时间更基本”的论述提供了前所未有的支撑。它表明，时间不是出现在实在的最基层，而是生发于其中。但尽管物理学越来越多地显露出时间是一场幻觉，将时间变化出来的作用力却仍旧无从知晓。塞伯格说：“我的直觉是，我们需要的不只是将量子物理重做一遍，更需要一个乍一看完全荒诞不经的突破。只有时间才能告诉我们会发生怎样的变革。”

[Free Talk] 追寻时间之源