人類能否實現穿越?

包括電影、電視劇以及文學作品在內的現代文化中充斥著各種有關時空旅行的故事，故事里的人們可以在時空中上下求索千年，來回穿越古今，仿佛物理意義上的時間與空間于他們來說毫無意義。當然大多數人都相信這只是人們的美好愿望，這樣的故事不可能在現實中上演。但你真的那么確定嗎？如果人類飛翔的夢想可以借助飛機實現，那么穿越的夢想是否也指日可待？甚或已在某種程度上實現？

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

我們都是時空旅行者

“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?

未來可行，過去不成？

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

蟲洞是時空中的捷徑

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

因果關系

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.”

無論是暑期檔大片，還是賺人眼球的科學報道，現代文化中到處都充斥著有關時間穿越的故事。然而，如果你對其中包含的物理知識進行總結和歸納，不禁會產生這樣的疑問：人類有可能實現時間穿越嗎？

為了解答這一問題，我找到了理論物理學家、宇宙學家保羅·戴維斯，他也是《如何建造時光機》一書的作者。

“簡單地說，穿越到未來不僅有可能實現，而且已經實現了。對此我們一個世紀以前就知道了，”戴維斯說，“大眾之所以對此似乎不太了解，是因為時空穿越的時間實在是短得可憐，根本沒法進行像電視劇《神秘博士》里那樣的冒險旅行。”

要理解這一點，關鍵是要理解一種被稱為“時間膨脹效應”的現象。光速是一種不可超越的宇宙速度極限，運動速度越接近光速，你會發現時間過得越慢。因此，高速行駛的火車上的鐘表指針要比靜止狀態下的鐘表指針走得慢。這其中的區別是肉眼無法察覺的，但是當火車開回車站，兩塊鐘表的指針將相差十億分之一秒。如果這列火車的行駛速度能達到光速的99.999%，那么這列火車上一年的時間就相當于火車站223年的時光。

但速度并不是影響時間的唯一因素。從相對較小的范圍來說，質量也會影響時間。距離龐大物體的中心越近，時間就流逝得越慢。

“宇宙空間中的時間比地球上的時間走得快一點，”戴維斯說，“屋頂上的時間比地下室的時間走得快一點。這個影響是可以測量的。”

將鐘表置于在軌衛星上，鐘表就會出現時間膨脹效應，這是由于在軌衛星的運行速度快，而且距離地球引力中心較遠。

“重力和速度能為你開辟一條穿越未來之路，”戴維斯說道，“所以，原則上說，如果你有充足的資金，你就能夠隨意在很短的時間里——比如一年、一個月或者更短的時間內——穿越至3000年。這只是資金和工程的問題。”

穿越到未來是愛因斯坦相對論中既定且基本的一個方面。科學家們在實驗和應用環境中對此進行過反復驗證。但如果是往相反的方向穿越時空，是否可行呢？

“回到過去就完全是另一碼事了，”戴維斯解釋道，“在我們所知的有關時間本質的理論中，愛因斯坦的理論是最好的。他的理論并沒有排除回到過去的可能性。即便在他1915年發表的廣義相對論中也沒有排除這種可能性。不過，很多科學家對這種可能性深感不安，因為很多著名的悖論都是因此而產生的。”

比如，你可以設想自己穿越到過去，殺死自己的母親。但如果這樣的話，她就不可能把你生下來，那你又怎么能回到過去犯下弒母的罪行呢？

戴維斯推測，從我們目前對時間本質的理解和所掌握的物理知識來看，回到過去的穿越是不可能實現的。但宇宙中充滿了各種難以理解的現象，理論上存在的蟲洞便是其中之一。也許蟲洞可以實現通往過去的時空旅行。

“蟲洞有點像是相距遙遠的兩個點之間的通道或者捷徑，”戴維斯說道，“舉個例子，如果我的賓館客房里有個蟲洞，我從這個洞穿過去后可能不會到達賓夕法尼亞大街，而是跑到了銀河系的另一端附近。”

根據科學家們的推理，這種時空上的捷徑可以轉化為時間機器。

“如果蟲洞確實存在，且可以穿越，它就有可能提供一條穿越時空、回到過去的道路，”戴維斯說，“所以這完全取決于是否存在穩定的蟲洞，或者是否有某種物理學理論阻礙蟲洞的形成。這種物理學理論是指除相對論以外的其他理論，因為從相對論的角度看，蟲洞一說是沒有問題的。這是個開放式命題。”

世界知名的理論物理學家史蒂芬·霍金曾經提出，發生在量子能級的蟲洞能夠從理論上為時間穿越提供支撐，但是加州大學圣巴巴拉分校的物理學家安德魯·克萊德呼吁對這一看法要謹慎對待。

“我是一個實驗主義者，物理學說到底也是一門實驗科學，”克萊德說道，“任何基于數學、哲學思考或理性思維的預測都需要經過實驗的證明。而我的確不知道有哪個實驗證明了回到過去的可能性。”

克萊德也指出，因果關系的基本原則阻礙了穿越至過去的道路。眾所周知，整個宇宙都遵循因果關系的基本原則。

“一件事只有先發生，然后才會有結果，”克萊德說，“據我所知，還沒有哪個實驗得出過與此不同的結論。我還沒發現有哪個量子力學實驗出現過違背因果關系的現象——盡管有許多實驗可能會揭示這一現象。”

不過，就像時間膨脹不夠快，看起來不像是穿越到了未來那樣，大眾也經常忽略“穿越”到過去的一種常見方法。

“從某種意義上來說，天文學家一直都能穿越回過去，但其穿越方式并不會讓大多數人感到興奮，”克萊德說，“當我們測量宇宙微波背景的時候，我們就是在往回追溯到一百多億年以前，這是微波到達我們的觀測視野所需花費的時間。”

很多關于時間穿越的問題仍懸而未決。來自未來的時空旅行者是否會出現來為我們答疑解惑呢？我們只能拭目以待。但如果他們要乘坐蟲洞時光機而來，我們必須得先為他們建造一臺。畢竟，只有一邊建成了通道，他們還是無法穿越到現在的，對吧？

“理論上說，要建造一個跨越百年的雙向蟲洞通道，我們需要用一百多年的時間，”戴維斯總結道，“所以，我們的子孫無論如何都不可能穿越回來告訴我們，說我們關于往回穿越的想法是錯誤的。”

1.inundate [#712;#618;n#652;nde#618;t] vt. 淹沒

2.boil down：歸納，簡化

3.track down：(在長時間的搜尋后)找到某人(或某物)

4.Doctor Who：《神秘博士》，由BBC制作的英國科幻電視劇。該劇的主人公是一個名為“博士”的神秘外星時間旅行者，他與他的時空機器Tardis一起展開了一系列冒險故事。

5.time dilation：時間膨脹效應，一種物理現象，兩個完全相同的時鐘之中，拿著甲鐘的人會發現乙鐘比自己的走得慢，該現象常被描述成對方的鐘“慢了下來”。但這種描述只有在觀測者的參考系上才是正確的。任何本地的時間(也就是位于同一個坐標系上的觀測者所測量出的時間)都以同一個速度前進。時間膨脹效應適用于任何解釋時間速度變化的過程。

6.pull back：把……向后拉，使回到從前的狀態

7.theory of relativity：相對論，關于時空和引力的基本理論，主要由阿爾伯特·愛因斯坦創立，其基本假設是相對性原理，即物理定律與參照系的選擇無關。相對論分為狹義相對論(special relativity)和廣義相對論(general relativity)。下文中提到的general theory of relativity即為廣義相對論，是愛因斯坦于1915年以幾何語言建立而成的引力理論，統合了狹義相對論和牛頓的萬有引力定律，將引力改述成因時空中的物質與能量而彎曲的時空，以取代“引力是一種力”的傳統看法。

8.different kettle of fish：(等于a very different matter)另一碼事

9.preclude [pr#618;#712;klu#720;d] vt. 排除

10.matricide [#712;me#618;tr#618;sa#618;d] n. 弒母

11.wormhole：蟲洞，由阿爾伯特·愛因斯坦提的理論。簡單地說，“蟲洞”就是連接宇宙遙遠區域間的時空細管。暗物質維持著蟲洞出口的敞開。蟲洞可以把平行宇宙和嬰兒宇宙連接起來，并提供時間旅行的可能性。蟲洞也可能是連接黑洞和白洞的時空隧道，所以也叫“灰道”。

12.surmise [s#601;(r)#712;ma#618;z] vt. 推測

13.traversable [tr#601;#712;v#604;#720;(r)#712;s#601;b(#601;)l] adj. 可以穿過的

14.hinge on：因……而定

15.in the way：擋道的

16.causality [#712;k#596;#720;saelig;l#618;ti] n. 因果關系

17. cosmic microwave background：宇宙微波背

景，散布于宇宙空間的微波輻射

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