包括電影、電視劇以及文學(xué)作品在內(nèi)的現(xiàn)代文化中充斥著各種有關(guān)時(shí)空旅行的故事，故事里的人們可以在時(shí)空中上下求索千年，來(lái)回穿越古今，仿佛物理意義上的時(shí)間與空間于他們來(lái)說(shuō)毫無(wú)意義。當(dāng)然大多數(shù)人都相信這只是人們的美好愿望，這樣的故事不可能在現(xiàn)實(shí)中上演。但你真的那么確定嗎？如果人類(lèi)飛翔的夢(mèng)想可以借助飛機(jī)實(shí)現(xiàn)，那么穿越的夢(mèng)想是否也指日可待？甚或已在某種程度上實(shí)現(xiàn)？

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(shí)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(lái)可行，過(guò)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(chóng)洞是時(shí)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(guān)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(wú)論是暑期檔大片，還是賺人眼球的科學(xué)報(bào)道，現(xiàn)代文化中到處都充斥著有關(guān)時(shí)間穿越的故事。然而，如果你對(duì)其中包含的物理知識(shí)進(jìn)行總結(jié)和歸納，不禁會(huì)產(chǎn)生這樣的疑問(wèn)：人類(lèi)有可能實(shí)現(xiàn)時(shí)間穿越嗎？

為了解答這一問(wèn)題，我找到了理論物理學(xué)家、宇宙學(xué)家保羅·戴維斯，他也是《如何建造時(shí)光機(jī)》一書(shū)的作者。

“簡(jiǎn)單地說(shuō)，穿越到未來(lái)不僅有可能實(shí)現(xiàn)，而且已經(jīng)實(shí)現(xiàn)了。對(duì)此我們一個(gè)世紀(jì)以前就知道了，”戴維斯說(shuō)，“大眾之所以對(duì)此似乎不太了解，是因?yàn)闀r(shí)空穿越的時(shí)間實(shí)在是短得可憐，根本沒(méi)法進(jìn)行像電視劇《神秘博士》里那樣的冒險(xiǎn)旅行?！?/p>

要理解這一點(diǎn)，關(guān)鍵是要理解一種被稱(chēng)為“時(shí)間膨脹效應(yīng)”的現(xiàn)象。光速是一種不可超越的宇宙速度極限，運(yùn)動(dòng)速度越接近光速，你會(huì)發(fā)現(xiàn)時(shí)間過(guò)得越慢。因此，高速行駛的火車(chē)上的鐘表指針要比靜止?fàn)顟B(tài)下的鐘表指針走得慢。這其中的區(qū)別是肉眼無(wú)法察覺(jué)的，但是當(dāng)火車(chē)開(kāi)回車(chē)站，兩塊鐘表的指針將相差十億分之一秒。如果這列火車(chē)的行駛速度能達(dá)到光速的99.999%，那么這列火車(chē)上一年的時(shí)間就相當(dāng)于火車(chē)站223年的時(shí)光。

但速度并不是影響時(shí)間的唯一因素。從相對(duì)較小的范圍來(lái)說(shuō)，質(zhì)量也會(huì)影響時(shí)間。距離龐大物體的中心越近，時(shí)間就流逝得越慢。

“宇宙空間中的時(shí)間比地球上的時(shí)間走得快一點(diǎn)，”戴維斯說(shuō)，“屋頂上的時(shí)間比地下室的時(shí)間走得快一點(diǎn)。這個(gè)影響是可以測(cè)量的?！?/p>

將鐘表置于在軌衛(wèi)星上，鐘表就會(huì)出現(xiàn)時(shí)間膨脹效應(yīng)，這是由于在軌衛(wèi)星的運(yùn)行速度快，而且距離地球引力中心較遠(yuǎn)。

“重力和速度能為你開(kāi)辟一條穿越未來(lái)之路，”戴維斯說(shuō)道，“所以，原則上說(shuō)，如果你有充足的資金，你就能夠隨意在很短的時(shí)間里——比如一年、一個(gè)月或者更短的時(shí)間內(nèi)——穿越至3000年。這只是資金和工程的問(wèn)題?！?/p>

穿越到未來(lái)是愛(ài)因斯坦相對(duì)論中既定且基本的一個(gè)方面?？茖W(xué)家們?cè)趯?shí)驗(yàn)和應(yīng)用環(huán)境中對(duì)此進(jìn)行過(guò)反復(fù)驗(yàn)證。但如果是往相反的方向穿越時(shí)空，是否可行呢？

“回到過(guò)去就完全是另一碼事了，”戴維斯解釋道，“在我們所知的有關(guān)時(shí)間本質(zhì)的理論中，愛(ài)因斯坦的理論是最好的。他的理論并沒(méi)有排除回到過(guò)去的可能性。即便在他1915年發(fā)表的廣義相對(duì)論中也沒(méi)有排除這種可能性。不過(guò)，很多科學(xué)家對(duì)這種可能性深感不安，因?yàn)楹芏嘀你Ｕ摱际且虼硕a(chǎn)生的?！?/p>

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

戴維斯推測(cè)，從我們目前對(duì)時(shí)間本質(zhì)的理解和所掌握的物理知識(shí)來(lái)看，回到過(guò)去的穿越是不可能實(shí)現(xiàn)的。但宇宙中充滿(mǎn)了各種難以理解的現(xiàn)象，理論上存在的蟲(chóng)洞便是其中之一。也許蟲(chóng)洞可以實(shí)現(xiàn)通往過(guò)去的時(shí)空旅行。

“蟲(chóng)洞有點(diǎn)像是相距遙遠(yuǎn)的兩個(gè)點(diǎn)之間的通道或者捷徑，”戴維斯說(shuō)道，“舉個(gè)例子，如果我的賓館客房里有個(gè)蟲(chóng)洞，我從這個(gè)洞穿過(guò)去后可能不會(huì)到達(dá)賓夕法尼亞大街，而是跑到了銀河系的另一端附近。”

根據(jù)科學(xué)家們的推理，這種時(shí)空上的捷徑可以轉(zhuǎn)化為時(shí)間機(jī)器。

“如果蟲(chóng)洞確實(shí)存在，且可以穿越，它就有可能提供一條穿越時(shí)空、回到過(guò)去的道路，”戴維斯說(shuō)，“所以這完全取決于是否存在穩(wěn)定的蟲(chóng)洞，或者是否有某種物理學(xué)理論阻礙蟲(chóng)洞的形成。這種物理學(xué)理論是指除相對(duì)論以外的其他理論，因?yàn)閺南鄬?duì)論的角度看，蟲(chóng)洞一說(shuō)是沒(méi)有問(wèn)題的。這是個(gè)開(kāi)放式命題?！?/p>

世界知名的理論物理學(xué)家史蒂芬·霍金曾經(jīng)提出，發(fā)生在量子能級(jí)的蟲(chóng)洞能夠從理論上為時(shí)間穿越提供支撐，但是加州大學(xué)圣巴巴拉分校的物理學(xué)家安德魯·克萊德呼吁對(duì)這一看法要謹(jǐn)慎對(duì)待。

“我是一個(gè)實(shí)驗(yàn)主義者，物理學(xué)說(shuō)到底也是一門(mén)實(shí)驗(yàn)科學(xué)，”克萊德說(shuō)道，“任何基于數(shù)學(xué)、哲學(xué)思考或理性思維的預(yù)測(cè)都需要經(jīng)過(guò)實(shí)驗(yàn)的證明。而我的確不知道有哪個(gè)實(shí)驗(yàn)證明了回到過(guò)去的可能性。”

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

“一件事只有先發(fā)生，然后才會(huì)有結(jié)果，”克萊德說(shuō)，“據(jù)我所知，還沒(méi)有哪個(gè)實(shí)驗(yàn)得出過(guò)與此不同的結(jié)論。我還沒(méi)發(fā)現(xiàn)有哪個(gè)量子力學(xué)實(shí)驗(yàn)出現(xiàn)過(guò)違背因果關(guān)系的現(xiàn)象——盡管有許多實(shí)驗(yàn)可能會(huì)揭示這一現(xiàn)象?！?/p>

不過(guò)，就像時(shí)間膨脹不夠快，看起來(lái)不像是穿越到了未來(lái)那樣，大眾也經(jīng)常忽略“穿越”到過(guò)去的一種常見(jiàn)方法。

“從某種意義上來(lái)說(shuō)，天文學(xué)家一直都能穿越回過(guò)去，但其穿越方式并不會(huì)讓大多數(shù)人感到興奮，”克萊德說(shuō)，“當(dāng)我們測(cè)量宇宙微波背景的時(shí)候，我們就是在往回追溯到一百多億年以前，這是微波到達(dá)我們的觀測(cè)視野所需花費(fèi)的時(shí)間?！?/p>

很多關(guān)于時(shí)間穿越的問(wèn)題仍懸而未決。來(lái)自未來(lái)的時(shí)空旅行者是否會(huì)出現(xiàn)來(lái)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(chóng)洞時(shí)光機(jī)而來(lái)，我們必須得先為他們建造一臺(tái)。畢竟，只有一邊建成了通道，他們還是無(wú)法穿越到現(xiàn)在的，對(duì)吧？

“理論上說(shuō)，要建造一個(gè)跨越百年的雙向蟲(chóng)洞通道，我們需要用一百多年的時(shí)間，”戴維斯總結(jié)道，“所以，我們的子孫無(wú)論如何都不可能穿越回來(lái)告訴我們，說(shuō)我們關(guān)于往回穿越的想法是錯(cuò)誤的。”

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

2.boil down：歸納，簡(jiǎn)化

3.track down：(在長(zhǎng)時(shí)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(guó)科幻電視劇。該劇的主人公是一個(gè)名為“博士”的神秘外星時(shí)間旅行者，他與他的時(shí)空機(jī)器Tardis一起展開(kāi)了一系列冒險(xiǎn)故事。

5.time dilation：時(shí)間膨脹效應(yīng)，一種物理現(xiàn)象，兩個(gè)完全相同的時(shí)鐘之中，拿著甲鐘的人會(huì)發(fā)現(xiàn)乙鐘比自己的走得慢，該現(xiàn)象常被描述成對(duì)方的鐘“慢了下來(lái)”。但這種描述只有在觀測(cè)者的參考系上才是正確的。任何本地的時(shí)間(也就是位于同一個(gè)坐標(biāo)系上的觀測(cè)者所測(cè)量出的時(shí)間)都以同一個(gè)速度前進(jìn)。時(shí)間膨脹效應(yīng)適用于任何解釋時(shí)間速度變化的過(guò)程。

6.pull back：把……向后拉，使回到從前的狀態(tài)

7.theory of relativity：相對(duì)論，關(guān)于時(shí)空和引力的基本理論，主要由阿爾伯特·愛(ài)因斯坦創(chuàng)立，其基本假設(shè)是相對(duì)性原理，即物理定律與參照系的選擇無(wú)關(guān)。相對(duì)論分為狹義相對(duì)論(special relativity)和廣義相對(duì)論(general relativity)。下文中提到的general theory of relativity即為廣義相對(duì)論，是愛(ài)因斯坦于1915年以幾何語(yǔ)言建立而成的引力理論，統(tǒng)合了狹義相對(duì)論和牛頓的萬(wàn)有引力定律，將引力改述成因時(shí)空中的物質(zhì)與能量而彎曲的時(shí)空，以取代“引力是一種力”的傳統(tǒng)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(chóng)洞，由阿爾伯特·愛(ài)因斯坦提的理論。簡(jiǎn)單地說(shuō)，“蟲(chóng)洞”就是連接宇宙遙遠(yuǎn)區(qū)域間的時(shí)空細(xì)管。暗物質(zhì)維持著蟲(chóng)洞出口的敞開(kāi)。蟲(chóng)洞可以把平行宇宙和嬰兒宇宙連接起來(lái)，并提供時(shí)間旅行的可能性。蟲(chóng)洞也可能是連接黑洞和白洞的時(shí)空隧道，所以也叫“灰道”。

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

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