影片《星際穿越》中的科學(xué)家們通過(guò)蟲(chóng)洞實(shí)現(xiàn)了穿越，《回到未來(lái)》中的布朗博士乘著一輛跑車(chē)改造的時(shí)光機(jī)隨心所欲地回到過(guò)去和未來(lái)，《終結(jié)者》系列中約翰·康納把自己的父親送回過(guò)去孕育自己，這些穿越的場(chǎng)景在令人感到震撼的同時(shí)又令人十分迷?！鼈冋娴目尚袉幔繉?shí)際上，從科學(xué)角度而言，穿越還是需要遵守一定規(guī)則的，胡來(lái)可不行哦。

Time travel isn't magic； it may or may not be allowed by the laws of physics—we don't know them well enough to be sure—but we do know enough to say that if time travel were possible， certain rules would have to be obeyed. So if you wanted to create a fictional world involving travel through time， here are five rules by which you should try to play.

時(shí)間旅行并不神奇。在物理學(xué)法則內(nèi)，時(shí)間旅行可能行得通，也可能行不通，我們對(duì)物理學(xué)法則的認(rèn)識(shí)尚且不足，還不能確定。但我們目前所知確實(shí)足以讓我們肯定一點(diǎn)：如果時(shí)間旅行確有可能，那也必須遵守一定的規(guī)則。因此，你如果想創(chuàng)造一個(gè)涉及時(shí)間旅行的虛構(gòu)世界，那應(yīng)該盡量遵守以下五個(gè)規(guī)則。

Traveling into the future is easy 穿越到未來(lái)容易

We travel into the future all the time， at a fixed rate： one second per second. Stick around1）， you'll be in the future soon enough. You can even get there faster than usual， by decreasing the amount of time you experience elapsing2） with respect to3） the rest of the world—either by low-tech ways like freezing yourself， or by taking advantage of the laws of special relativity and zipping4） around near the speed of light. （Remember we're talking about what is possible according to the laws of physics here， not what is plausible or technologically feasible.） It's coming back that's hard. 我們一直在以固定的速度邁向未來(lái)：每秒向前邁進(jìn)一秒。就算待在原地不動(dòng)，你也會(huì)很快置身于未來(lái)。你甚至還能比通常更快地到達(dá)未來(lái)，方法是減少（相對(duì)于世界所有其他事物而言）你感知的時(shí)間流逝量，不管是通過(guò)像身體冷凍法這種低技術(shù)含量的方法來(lái)達(dá)成，還是利用狹義相對(duì)論的理論，以接近光速的速度移動(dòng)。（記住，我們現(xiàn)在談?wù)摰氖前凑瘴锢韺W(xué)法則可能出現(xiàn)的情形，而不是實(shí)際上可行的或是技術(shù)上可行的東西。）穿越到未來(lái)不難，難的是再穿越回來(lái)。

Traveling into the past is hard—but maybe not impossible 穿越到過(guò)去雖難，但也不是沒(méi)有可能

In Einstein's curved-spacetime5） universe， things are more flexible. From your own personal， subjective point of view， you always move forward in time—more technically， you move on a timelike curve through spacetime. But the large-scale curvature6） of spacetime caused by gravity could， conceivably7）， cause timelike curves to loop8） back on themselves—that is to say， become closed timelike curves—such that anyone traveling on such a path would meet themselves in the past. That's what respectable， Einstein-approved time travel would really be like. Of course， there's still the little difficulty of warping9） spacetime so severely that you actually create closed timelike curves. 在愛(ài)因斯坦的彎曲時(shí)空宇宙里，事物有更多的可能性。從你的個(gè)人主觀角度來(lái)講，你總是沿著時(shí)間前行——更嚴(yán)格地來(lái)說(shuō)，你是沿著一條類時(shí)曲線在時(shí)空里運(yùn)動(dòng)。然而，由重力引起的時(shí)空大幅彎曲有可能會(huì)導(dǎo)致類時(shí)曲線首尾相接，即成為閉合類時(shí)曲線——這樣一來(lái)，任何沿著此類曲線運(yùn)動(dòng)的人都會(huì)遇見(jiàn)過(guò)去的自己。這就是備受推崇的、愛(ài)因斯坦也認(rèn)可的時(shí)間旅行真正的情形。當(dāng)然，目前還存在一個(gè)小小的困難，那就是如何讓時(shí)空十分扭曲從而真正創(chuàng)造出閉合類時(shí)曲線。

Traveling through time is like traveling through space 時(shí)間旅行和空間旅行非常相似

I'm only going to say this once： there would be no flashing lights. At least， there would only be flashing lights if you brought along some strobes10）， and decided to start them flashing as you traveled along your closed timelike curve. Likewise， there is no disappearance in a puff11） of smoke and re-appearing at some other time. Traveling through time is just like traveling through space： you move along a certain path， which （we are presuming） the universe has helpfully arranged so that your travels bring you to an earlier moment in time. But a time machine wouldn't look like a booth12） with spinning wheels that disappears now and appears some other time； it would look like a rocket ship. Or possibly a DeLorean13）， in the unlikely event14） that your closed timelike curve started right here on Earth and never left the road. 這話我只說(shuō)一遍：時(shí)間旅行不會(huì)伴有閃光出現(xiàn)（編注：這是針對(duì)一些科幻作品中的設(shè)定來(lái)說(shuō)的，例如在電影《回到未來(lái)》系列中穿越時(shí)就伴有閃光特效）。至少只有當(dāng)你帶上一些頻閃燈并決定在沿著閉合類時(shí)曲線移動(dòng)的同時(shí)打開(kāi)頻閃燈，此時(shí)閃光才會(huì)出現(xiàn)。同樣地，你也不會(huì)在一股煙中消失不見(jiàn)，然后又在另一個(gè)時(shí)間點(diǎn)現(xiàn)身。時(shí)間旅行與空間旅行十分類似：你會(huì)沿著一定的路徑移動(dòng)，（我們假定）這條路徑早由宇宙幫你安排妥當(dāng)，因而你的旅行能將你帶回到過(guò)去的某個(gè)時(shí)間。不過(guò)，時(shí)光機(jī)看上去也不會(huì)像帶轉(zhuǎn)輪的轎廂那樣，于此時(shí)消失，并在另一時(shí)間重現(xiàn)，而會(huì)酷似火箭飛船，或者可能像德勞瑞恩汽車(chē)那樣——萬(wàn)一你的閉合類時(shí)曲線以地球此處為起點(diǎn)，之后就一直沒(méi)離開(kāi)路面呢。

Black holes are not time machines 黑洞不是時(shí)光機(jī)

Sadly， if you fell into a black hole， it wouldn't spit you out at all—it would gobble you up15） and grow slightly more corpulent16） in the process. If the black hole were big enough， you might not even notice when you crossed the point of no return defined by the event horizon17）. But once you got close to the center of the hole， tidal forces would tug at you—gently at first， but eventually tearing you apart. Wormholes18）—tunnels through spacetime， which in principle can connect widely-separated events19）—are a more promising alternative. Wormholes are to black holes as elevators are to deep wells filled with snakes and poisoned spikes. The problem is， unlike black holes， we don't know whether wormholes exist， or even whether they can exist， or how to make them， or how to preserve them once they are made. Wormholes want to collapse and disappear， and keeping them open requires a form of negative energies20）. Nobody knows how to make negative energies， although they occasionally slap21） the name \"exotic matter22）\" on the concept and pretend it might exist. 很不幸，假如你掉進(jìn)一個(gè)黑洞，它壓根兒就不會(huì)將你拋出，而會(huì)吞噬你，并在此過(guò)程中略微膨脹一點(diǎn)。如果黑洞體積足夠大，你甚至可能會(huì)完全察覺(jué)不到自己越過(guò)了被稱為“事界”的有去無(wú)回的黑洞邊界。但一旦你靠近黑洞的中心，黑洞的旋渦力就會(huì)撕扯你，起初很柔緩，但最終卻能將你撕成碎片。蟲(chóng)洞是一個(gè)更有希望的選擇。蟲(chóng)洞是時(shí)空穿梭隧道，原則上可以連接相隔很遠(yuǎn)的時(shí)空坐標(biāo)點(diǎn)。蟲(chóng)洞之于黑洞，猶如電梯之于遍布蟒蛇和毒刺的深井。問(wèn)題是，不像黑洞，我們并不知道蟲(chóng)洞是否存在（甚至蟲(chóng)洞能否存在），不知道如何創(chuàng)造蟲(chóng)洞，創(chuàng)造出來(lái)后又如何保留它。蟲(chóng)洞易坍塌、消逝，而且保持蟲(chóng)洞的暢通需要一種負(fù)能量。盡管人們有時(shí)會(huì)隨便將“奇異物質(zhì)”這一名稱安在負(fù)能量這一概念上，并自以為負(fù)能量可能存在，但對(duì)于如何制造負(fù)能量，卻沒(méi)人知道。

If something happened， it happened 過(guò)去的事情無(wú)法改變

What people want to do with time machines is to go into the past and change it. You can't. The past already happened， and it can't un-happen. You might wonder what's to stop you from jumping in your time machine， finding your high-school self， and convincing them that they really shouldn't go to the senior prom after all， thereby saving yourself all sorts of humiliation. The simple way out， of course， is to suppose that travel into the past is simply impossible. But even if it's not， you can't change what already happened； every event in spacetime is characterized by certain things occurring， and those things are fixed once and for all23） once they happen. 人們想用時(shí)光機(jī)做的事情就是回到過(guò)去，改變從前。然而你不可能做到。過(guò)去的事情既然已經(jīng)發(fā)生，就不可逆轉(zhuǎn)。盡管你可能會(huì)納悶，到底有什么能阻止你跳進(jìn)時(shí)光機(jī)，找到高中時(shí)期的你，并說(shuō)服他真的還是別去參加畢業(yè)舞會(huì)了，這樣你就能避免遭遇種種丟面子的事情。當(dāng)然，一個(gè)簡(jiǎn)單的解決辦法就是認(rèn)定回到過(guò)去根本不可能。不過(guò)，即使能夠回到過(guò)去，你也無(wú)法改變已經(jīng)發(fā)生的事情。時(shí)空之中的每個(gè)坐標(biāo)點(diǎn)都是由正在發(fā)生的特定事情決定的，這些事情一旦發(fā)生，就永恒不變。