穿越有規矩，請君別胡來

影片《星際穿越》中的科學家們通過蟲洞實現了穿越，《回到未來》中的布朗博士乘著一輛跑車改造的時光機隨心所欲地回到過去和未來，《終結者》系列中約翰·康納把自己的父親送回過去孕育自己，這些穿越的場景在令人感到震撼的同時又令人十分迷茫——它們真的可行嗎？實際上，從科學角度而言，穿越還是需要遵守一定規則的，胡來可不行哦。

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.

時間旅行并不神奇。在物理學法則內，時間旅行可能行得通，也可能行不通，我們對物理學法則的認識尚且不足，還不能確定。但我們目前所知確實足以讓我們肯定一點：如果時間旅行確有可能，那也必須遵守一定的規則。因此，你如果想創造一個涉及時間旅行的虛構世界，那應該盡量遵守以下五個規則。

Traveling into the future is easy 穿越到未來容易

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：每秒向前邁進一秒。就算待在原地不動，你也會很快置身于未來。你甚至還能比通常更快地到達未來，方法是減少（相對于世界所有其他事物而言）你感知的時間流逝量，不管是通過像身體冷凍法這種低技術含量的方法來達成，還是利用狹義相對論的理論，以接近光速的速度移動。（記住，我們現在談論的是按照物理學法則可能出現的情形，而不是實際上可行的或是技術上可行的東西。）穿越到未來不難，難的是再穿越回來。

Traveling into the past is hard—but maybe not impossible 穿越到過去雖難，但也不是沒有可能

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. 在愛因斯坦的彎曲時空宇宙里，事物有更多的可能性。從你的個人主觀角度來講，你總是沿著時間前行——更嚴格地來說，你是沿著一條類時曲線在時空里運動。然而，由重力引起的時空大幅彎曲有可能會導致類時曲線首尾相接，即成為閉合類時曲線——這樣一來，任何沿著此類曲線運動的人都會遇見過去的自己。這就是備受推崇的、愛因斯坦也認可的時間旅行真正的情形。當然，目前還存在一個小小的困難，那就是如何讓時空十分扭曲從而真正創造出閉合類時曲線。

Traveling through time is like traveling through space 時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. 這話我只說一遍：時間旅行不會伴有閃光出現（編注：這是針對一些科幻作品中的設定來說的，例如在電影《回到未來》系列中穿越時就伴有閃光特效）。至少只有當你帶上一些頻閃燈并決定在沿著閉合類時曲線移動的同時打開頻閃燈，此時閃光才會出現。同樣地，你也不會在一股煙中消失不見，然后又在另一個時間點現身。時間旅行與空間旅行十分類似：你會沿著一定的路徑移動，（我們假定）這條路徑早由宇宙幫你安排妥當，因而你的旅行能將你帶回到過去的某個時間。不過，時光機看上去也不會像帶轉輪的轎廂那樣，于此時消失，并在另一時間重現，而會酷似火箭飛船，或者可能像德勞瑞恩汽車那樣——萬一你的閉合類時曲線以地球此處為起點，之后就一直沒離開路面呢。

Black holes are not time machines 黑洞不是時光機

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，假如你掉進一個黑洞，它壓根兒就不會將你拋出，而會吞噬你，并在此過程中略微膨脹一點。如果黑洞體積足夠大，你甚至可能會完全察覺不到自己越過了被稱為“事界”的有去無回的黑洞邊界。但一旦你靠近黑洞的中心，黑洞的旋渦力就會撕扯你，起初很柔緩，但最終卻能將你撕成碎片。蟲洞是一個更有希望的選擇。蟲洞是時空穿梭隧道，原則上可以連接相隔很遠的時空坐標點。蟲洞之于黑洞，猶如電梯之于遍布蟒蛇和毒刺的深井。問題是，不像黑洞，我們并不知道蟲洞是否存在（甚至蟲洞能否存在），不知道如何創造蟲洞，創造出來后又如何保留它。蟲洞易坍塌、消逝，而且保持蟲洞的暢通需要一種負能量。盡管人們有時會隨便將“奇異物質”這一名稱安在負能量這一概念上，并自以為負能量可能存在，但對于如何制造負能量，卻沒人知道。

If something happened， it happened 過去的事情無法改變

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. 人們想用時光機做的事情就是回到過去，改變從前。然而你不可能做到。過去的事情既然已經發生，就不可逆轉。盡管你可能會納悶，到底有什么能阻止你跳進時光機，找到高中時期的你，并說服他真的還是別去參加畢業舞會了，這樣你就能避免遭遇種種丟面子的事情。當然，一個簡單的解決辦法就是認定回到過去根本不可能。不過，即使能夠回到過去，你也無法改變已經發生的事情。時空之中的每個坐標點都是由正在發生的特定事情決定的，這些事情一旦發生，就永恒不變。