“穿越”回过去真的有可能吗？

你是不是觉得虫洞很神奇呢？对你不稳定的电容器感到疑惑？那么就来学些关于时空之旅的基本知识吧。

You will need: a basic knowledge of the theory of relativity and an open mind.

你需要：相对论的基本知识和开阔的思维。

Optional: a PhD in Physics.

可选项：拥有物理系博士学位。

Time-travel theories are riddled³ with paradoxes⁵ that may make your brain explode.

时空之旅的理论充满着矛盾，这将会让你的思维爆炸！

Step 1:

Brush up on your Einstein: His special theory of relativity revolutionized physics by suggesting that traveling into the future is possible. The idea is that time slows down when you're traveling at high speeds relative to stationary⁶ objects. So if you're in a spaceship going very close to the speed of light for five years, you could return home to find that it's 50 years later on Earth.

重温一下你有关爱因斯坦的知识：他的相对论是物理学的一个革命性理论——穿越到未来是有可能的。这个理论是这样的，如果你的移动速度相对于静止的物体非常快，那么时间就会减缓。因此，如果你坐在一艘接近光速的宇宙飞船上行驶5年，当你回到家发现这已经是50年后的地球了。

Even on a normal transatlantic flight, time slows down with speed. In one experiment using two atomic clocks, the clock sent into space fell 40 nanoseconds behind the one left on the ground.

即使在普通的飞行器上，时间也会随着速度的增加而减缓。用两个原子表做实验，送入太空的原子表比在地球上的这个原子表时间要落后400亿分之一秒。

Step 2:

Going back in time is a far trickier⁷ proposition – but might be possible if you could travel faster than the speed of light. Theoretically, if you were moving faster than light speed, you could receive a signal from Earth before it was even sent. Upon returning home, you’d find yourself back in time. The only problem is that traveling that fast is scientifically impossible – at least for now.

穿越回过去是一个很棘手的课题，但是如果你行驶的速度能超过光速这就有可能。理论上讲，如果你的移动速度大于光速，即使地球还没有发出信号，你都能接收到。当你回到家，你就会发现你回到了过去。唯一的问题就是移动速度超过光速在科学理论中是不可能的——至少现在不可能。

Step 3:

Make the quantum leap. Some quantum physicists⁹ believe that traveling back in time could be possible on a subatomic level. But unless you’re a subatomic particle, you’re out of luck.

让量子论飞越吧。一些量子物理学家相信在亚原子水平上穿越回过去是有可能的。但是除非你是一个亚原子颗粒，但很不幸你并不是。

Step 4:

Currently, the most popular time-travel theory involves wormholes, or tunnels through the fabric¹⁰ of space and time. If you were to tow one end of a wormhole into space close to the speed of light, then return that end to Earth, you would theoretically be able to enter the wormhole today and come out of the other end in yesterday.

当今最流行的时间旅行理论要属虫洞了，也就是构造空间和时间物质的小缝隙。如果你以接近光速被拖入虫洞的一端，然后从另一端回到地球，理论上你可以是今天进入虫洞但在昨天从虫洞里出来。

Be skeptical¹¹. Wormholes are so tiny and close so rapidly that it would take a great leap of science to be able to harness them for time travel.

值得怀疑的是虫洞如此小而且关闭的如此迅速，要驾驭虫洞去进行时间之旅，当今科学还需要很大的飞跃啊！

Step 5:

Get to know your granddad. The so-called Grandfather Paradox⁴ – going back in time and killing¹² your grandfather, thus preventing your own birth and making it impossible for you to travel back in the first place – has been used to help illustrate¹³ why backward time travel is impossible.

想去了解下你的祖父？所谓的祖父悖论——回到过去杀掉你的祖父，因此阻止了你的出生，因此让你不可能穿越回过去——这个悖论帮助我们阐明为什么穿越回到过去是不可能的。

Step 6:

Understand what's done is done. Most ideas about backward time travel include the “chronology protection conjecture,” meaning that nature would prevent you from changing the past. For example, you could try to go back in time to shoot your grandfather, but the gun wouldn't fire. It also means that all the embarrassing moments you'd love to erase¹⁴ are going to happen all over again.

要了解做过的事就是做过了。大部分有关于回到过去的理论，包括“时序保护假说”，意思就是大自然会组织你去改变过去。例如，你可能终于回到了过去，想杀了你的祖父，但是枪却没有开火。也就是说所有你想抹去的窘迫的瞬间依然会重新上演。

Step 7:

Open your mind to the “many-worlds” interpretation¹⁵. Some theorists have gotten around the rigid¹⁶ “whatever happened, happened” idea by claiming that every different action you take in the past splits off into a parallel universe. So there might be hope yet for preventing those embarrassing moments, at least in one universe.

开阔你的思维，试着想像“多重世界”的解释。有些理论在“无论发生了什么，已经发生了”的思想上非常严格，它们主张你在过去所有的动作会分裂进一个平行宇宙中。因此至少在我们的宇宙中，阻止这些窘迫的时刻发生可能是大家所希望的。

Did you know? Theoretical physicist⁸ Stephen Hawking¹⁷ once said, “Time travel might be possible, but if that is the case why haven’t we been overrun by tourists from the future?”

你知道吗？理论物理学家史蒂芬·霍金曾经说过：“时间旅行是有可能的，但是如果真能如此为什么还没有从未来来的游客来拜访我们呢？”

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