人类能否实现穿越?

包括电影、电视剧以及文学作品在内的现代文化中充斥着各种有关时空旅行的故事，故事里的人们可以在时空中上下求索千年，来回穿越古今，仿佛物理意义上的时间与空间于他们来说毫无意义。当然大多数人都相信这只是人们的美好愿望，这样的故事不可能在现实中上演。但你真的那么确定吗？如果人类飞翔的梦想可以借助飞机实现，那么穿越的梦想是否也指日可待？甚或已在某种程度上实现？

From summer blockbusters to sensational science headlines, modern culture is constantly inundated1) with tales of time travel. But when you boil down2) the physics involved, is it possible to travel through time?

To answer this question, I tracked down3) theoretical physicist and cosmologist Paul Davies, author of How to Build a Time Machine.

We Are All Time Travelers

我们都是时空旅行者

“The short answer is that time travel into the future is not only possible, it’s been done, and we’ve known about it for over a century,” says Davies. “The reason that the public doesn’t seem to know about it is because the amount of time travel involved is so pitifully small that it doesn’t make for a Doctor Who4) style adventure.”

A phenomenon called time dilation5) is the key here. Time passes more slowly the closer you approach the speed of light—an unbreakable cosmic speed limit. As such, the hands of a clock in a speeding train would move more slowly than those in a stationary clock. The difference would not be humanly noticeable, but when the train is pulled back6) into the station, the two clocks would be off by billionths of a second. If such a train could attain 99.999 percent light speed, only 1 year would pass onboard for every 223 years back at the train station.

But speed isn’t the only factor that affects time. On a much smaller scale, mass also influences time. Time slows down the closer you are to the center of a massive object.

“Time runs a little bit faster in space than it does down on Earth,” Davies says. “It runs a little faster on the roof than it does in the basement, and that’s a measurable effect.”

A clock aboard an orbiting satellite experiences time dilation due to both the speed of its orbit and its greater distance from the center of Earth’s gravity.

“Both gravity and speed can give you a means of jumping ahead,” Davies says. “So in principle, if you had enough money, you could get to the year 3000 in as short a time as you like—one year, one month, whatever it takes. It is only a question of money and engineering.”

Forward, Not Back?

未来可行，过去不成？

Time travel into the future is an established and fundamental aspect of Einstein’s theory of relativity7). Scientists have tested and retested this in both experimental and practical settings. But what about time travel in the opposite direction?

“Going back in to the past is a whole different kettle of fish8).” Davies says. “There’s nothing in Einstein’s theory, which is the best theory that we have about the nature of time, which precludes9) it. There’s nothing in even his general theory of relativity, published in 1915, which precludes travel back into the past, but many scientists are deeply uneasy about it because of all the well-known paradoxes that it unleashes.”

For instance, imagine going back in time and killing your own mother. Then she’d never give birth to you, and just how would you have been able to travel back in time to commit matricide10) in the first place?

Wormholes11) As Space-time Shortcuts

虫洞是时空中的捷径

Davies surmises12) that, given our current understanding of the nature of time and physics, time travel into the past simply isn’t possible. But the universe is full of mysteries, and one of them—the hypothetical wormhole—might just permit such a journey.

“This is a little bit like a tunnel or shortcut between two distant points,” Davies says, “So for example, if I had a wormhole here in my hotel room and I jumped through it I wouldn’t come out on Pennsylvania Avenue, I’d maybe come out near the other side of the galaxy.”

Scientists have theorized that such a shortcut through time and space could be turned into a time machine.

“If a wormhole could exist and could be traversable13), then it would provide a means of going back in time,” Davies says. “So it all hinges on14) whether stable wormholes are a reality or if there’s some aspect of physics—not relativity, because there’s nothing wrong from that point of view—but some other aspect of physics might intercede and prevent the wormhole from forming. That’s an open question.”

World-famous theoretical physicist Stephen Hawking has proposed that wormholes occurring at a quantum level could theoretically provide a foothold for time travel, but University of California at Santa Barbara physicist Andrew Cleland urges caution on that front.

“I’m an experimentalist, and physics is ultimately an experimental science,” Cleland says. “Any predictions that are made based on mathematics or on philosophical or intellectual speculation have to pass the test of experiment, and I am certainly not aware of any experiment that demonstrated the possibility of traveling backward in time.”

Cause and Effect

因果关系

Cleland also points out that the fundamental principle of causality stands in the way15) of travel into the past. The entire universe, as we understand it, is beholden to this rule.

“Something occurs first and the outcome of that occurrence happens afterward,” Cleland says, “and there has never to my knowledge been an experiment that came out different from that. I am not aware of any experimental tests of quantum mechanics that have shown any violation of causality16), in spite of the fact that many experiments could reveal such a violation.”

Still, in the same way that time dilation isn’t flashy enough to seem like time travel into the future, the public often overlooks a very common means of “traveling” into the past.

“In a sense, astronomers are always traveling backward in time, but it is in a way that most people are not so excited about,” Cleland says. “When we measure the cosmic microwave background17), we’re looking back more than 10 billion years in time. That’s how long it took for the light18) to reach us.”

A number of questions about time travel remain unanswered. Will time tourists from the future ever show up to help us out? We’ll just have to wait and see. But if they come here using a wormhole time machine, we’ll have to build one first. After all, you couldn’t cross a bridge if only one side had been completed, right?

“Theoretically, it would take more than 100 years to create a 100-years’ time difference between the two ends of a wormhole,” Davies says, “so there’s no way that our descendants could come back and tell us we’re wrong about this.”

无论是暑期档大片，还是赚人眼球的科学报道，现代文化中到处都充斥着有关时间穿越的故事。然而，如果你对其中包含的物理知识进行总结和归纳，不禁会产生这样的疑问：人类有可能实现时间穿越吗？

为了解答这一问题，我找到了理论物理学家、宇宙学家保罗·戴维斯，他也是《如何建造时光机》一书的作者。

“简单地说，穿越到未来不仅有可能实现，而且已经实现了。对此我们一个世纪以前就知道了，”戴维斯说，“大众之所以对此似乎不太了解，是因为时空穿越的时间实在是短得可怜，根本没法进行像电视剧《神秘博士》里那样的冒险旅行。”

要理解这一点，关键是要理解一种被称为“时间膨胀效应”的现象。光速是一种不可超越的宇宙速度极限，运动速度越接近光速，你会发现时间过得越慢。因此，高速行驶的火车上的钟表指针要比静止状态下的钟表指针走得慢。这其中的区别是肉眼无法察觉的，但是当火车开回车站，两块钟表的指针将相差十亿分之一秒。如果这列火车的行驶速度能达到光速的99.999%，那么这列火车上一年的时间就相当于火车站223年的时光。

但速度并不是影响时间的唯一因素。从相对较小的范围来说，质量也会影响时间。距离庞大物体的中心越近，时间就流逝得越慢。

“宇宙空间中的时间比地球上的时间走得快一点，”戴维斯说，“屋顶上的时间比地下室的时间走得快一点。这个影响是可以测量的。”

将钟表置于在轨卫星上，钟表就会出现时间膨胀效应，这是由于在轨卫星的运行速度快，而且距离地球引力中心较远。

“重力和速度能为你开辟一条穿越未来之路，”戴维斯说道，“所以，原则上说，如果你有充足的资金，你就能够随意在很短的时间里——比如一年、一个月或者更短的时间内——穿越至3000年。这只是资金和工程的问题。”

穿越到未来是爱因斯坦相对论中既定且基本的一个方面。科学家们在实验和应用环境中对此进行过反复验证。但如果是往相反的方向穿越时空，是否可行呢？

“回到过去就完全是另一码事了，”戴维斯解释道，“在我们所知的有关时间本质的理论中，爱因斯坦的理论是最好的。他的理论并没有排除回到过去的可能性。即便在他1915年发表的广义相对论中也没有排除这种可能性。不过，很多科学家对这种可能性深感不安，因为很多著名的悖论都是因此而产生的。”

比如，你可以设想自己穿越到过去，杀死自己的母亲。但如果这样的话，她就不可能把你生下来，那你又怎么能回到过去犯下弑母的罪行呢？

戴维斯推测，从我们目前对时间本质的理解和所掌握的物理知识来看，回到过去的穿越是不可能实现的。但宇宙中充满了各种难以理解的现象，理论上存在的虫洞便是其中之一。也许虫洞可以实现通往过去的时空旅行。

“虫洞有点像是相距遥远的两个点之间的通道或者捷径，”戴维斯说道，“举个例子，如果我的宾馆客房里有个虫洞，我从这个洞穿过去后可能不会到达宾夕法尼亚大街，而是跑到了银河系的另一端附近。”

根据科学家们的推理，这种时空上的捷径可以转化为时间机器。

“如果虫洞确实存在，且可以穿越，它就有可能提供一条穿越时空、回到过去的道路，”戴维斯说，“所以这完全取决于是否存在稳定的虫洞，或者是否有某种物理学理论阻碍虫洞的形成。这种物理学理论是指除相对论以外的其他理论，因为从相对论的角度看，虫洞一说是没有问题的。这是个开放式命题。”

世界知名的理论物理学家史蒂芬·霍金曾经提出，发生在量子能级的虫洞能够从理论上为时间穿越提供支撑，但是加州大学圣巴巴拉分校的物理学家安德鲁·克莱德呼吁对这一看法要谨慎对待。

“我是一个实验主义者，物理学说到底也是一门实验科学，”克莱德说道，“任何基于数学、哲学思考或理性思维的预测都需要经过实验的证明。而我的确不知道有哪个实验证明了回到过去的可能性。”

克莱德也指出，因果关系的基本原则阻碍了穿越至过去的道路。众所周知，整个宇宙都遵循因果关系的基本原则。

“一件事只有先发生，然后才会有结果，”克莱德说，“据我所知，还没有哪个实验得出过与此不同的结论。我还没发现有哪个量子力学实验出现过违背因果关系的现象——尽管有许多实验可能会揭示这一现象。”

不过，就像时间膨胀不够快，看起来不像是穿越到了未来那样，大众也经常忽略“穿越”到过去的一种常见方法。

“从某种意义上来说，天文学家一直都能穿越回过去，但其穿越方式并不会让大多数人感到兴奋，”克莱德说，“当我们测量宇宙微波背景的时候，我们就是在往回追溯到一百多亿年以前，这是微波到达我们的观测视野所需花费的时间。”

很多关于时间穿越的问题仍悬而未决。来自未来的时空旅行者是否会出现来为我们答疑解惑呢？我们只能拭目以待。但如果他们要乘坐虫洞时光机而来，我们必须得先为他们建造一台。毕竟，只有一边建成了通道，他们还是无法穿越到现在的，对吧？

“理论上说，要建造一个跨越百年的双向虫洞通道，我们需要用一百多年的时间，”戴维斯总结道，“所以，我们的子孙无论如何都不可能穿越回来告诉我们，说我们关于往回穿越的想法是错误的。”

1.inundate [ˈɪnʌndeɪt] vt. 淹没

2.boil down：归纳，简化

3.track down：(在长时间的搜寻后)找到某人(或某物)

4.Doctor Who：《神秘博士》，由BBC制作的英国科幻电视剧。该剧的主人公是一个名为“博士”的神秘外星时间旅行者，他与他的时空机器Tardis一起展开了一系列冒险故事。

5.time dilation：时间膨胀效应，一种物理现象，两个完全相同的时钟之中，拿着甲钟的人会发现乙钟比自己的走得慢，该现象常被描述成对方的钟“慢了下来”。但这种描述只有在观测者的参考系上才是正确的。任何本地的时间(也就是位于同一个坐标系上的观测者所测量出的时间)都以同一个速度前进。时间膨胀效应适用于任何解释时间速度变化的过程。

6.pull back：把……向后拉，使回到从前的状态

7.theory of relativity：相对论，关于时空和引力的基本理论，主要由阿尔伯特·爱因斯坦创立，其基本假设是相对性原理，即物理定律与参照系的选择无关。相对论分为狭义相对论(special relativity)和广义相对论(general relativity)。下文中提到的general theory of relativity即为广义相对论，是爱因斯坦于1915年以几何语言建立而成的引力理论，统合了狭义相对论和牛顿的万有引力定律，将引力改述成因时空中的物质与能量而弯曲的时空，以取代“引力是一种力”的传统看法。

8.different kettle of fish：(等于a very different matter)另一码事

9.preclude [prɪˈkluːd] vt. 排除

10.matricide [ˈmeɪtrɪsaɪd] n. 弑母

11.wormhole：虫洞，由阿尔伯特·爱因斯坦提的理论。简单地说，“虫洞”就是连接宇宙遥远区域间的时空细管。暗物质维持着虫洞出口的敞开。虫洞可以把平行宇宙和婴儿宇宙连接起来，并提供时间旅行的可能性。虫洞也可能是连接黑洞和白洞的时空隧道，所以也叫“灰道”。

12.surmise [sə(r)ˈmaɪz] vt. 推测

13.traversable [trəˈvɜː(r)ˈsəb(ə)l] adj. 可以穿过的

14.hinge on：因……而定

15.in the way：挡道的

16.causality [ˈkɔːsælɪti] n. 因果关系

17. cosmic microwave background：宇宙微波背

景，散布于宇宙空间的微波辐射

18. the light：指上文提到的cosmic microwave background

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