对未来的时间旅行者来说，有一个坏消息：即使能够回到过去，你也无法改变已经发生的事情。但这个消息也有好的一面，虽然研究结论粉碎了我们改变历史的念头，但它同时也消除了时间旅行的一个主要障碍。
A Major Time Travel Perk May Be Technically Impossible
A bit of sad news for prospective time travelers — your efforts to change the past are futile, according to mathematical modeling by a pair of researchers at the University of Queensland. The past is immutable, it seems. But that implies some good news, too, for even as these findings crush our hopes of changing history, they may also remove a barrier to journeying back in time.
对未来的时间旅行者来说，有一个坏消息：即使能够回到过去，你也无法改变已经发生的事情。这是澳大利亚昆士兰大学的两位研究人员数学建模的结论。但这个消息也有好的一面，虽然研究结论粉碎了我们改变历史的念头，但它同时也消除了时间旅行的一个主要障碍。
The rationale goes like this: If our actions in the past won’t alter anything, they also won’t produce the paradoxes that have prompted some experts to rule out time travel altogether. For example, we can’t endanger our own existence, à la Marty McFly in Back to the Future, by preventing our parents from falling in love and conceiving us. In fact, we may not be able to alter anything at all. Therefore, no logical conundrum stands between us and our temporal excursions (though they may not even be technically feasible).
其中的逻辑是这样的：如果我们回到过去后的行为不会改变任何事情，也就不会产生一些逻辑悖论，正是这些悖论让许多专家完全否定了时间旅行的可能性。例如，我们回到过去后，并不能阻止父母坠入爱河并生下我们，不然我们自身的存在性就没有根据了，就像《回到未来》中的马丁·麦克弗莱一样。事实上，我们可能无法改变任何已经发生的事情。因此，在今天的我们和回到过去的我们之间，并不存在什么逻辑上的冲突。时间旅行在逻辑上的障碍被扫清了，尽管技术上依然困难。
That line of thinking has existed for decades and is embodied in Russian astrophysicist Igor Novikov’s self-consistency principle, which states there is zero chance of an event occurring if it would change the past in any way — the universe simply forbids revision. But now it’s supported by the calculations of Fabio Costa, a theoretical physicist, and Germain Tobar, a Cambridge student pursuing a masters in mathematics, which were published last year in the journal Classical and Quantum Gravity. The results, Costa says, “are the stuff of science fiction.”
这种想法已经存在了几十年，俄罗斯天体物理学家伊戈尔·诺维科夫（Igor Novikov）早就提出过“自身一致性原则”，该原则认为，如果某一事件以任何方式改变过去，那么它发生的可能性为零：宇宙历史不允许修改。现在，理论物理学家法比奥·科斯塔（Fabio Costa）和正在剑桥大学攻读数学硕士学位的热尔曼·托巴尔（Germain Tobar）的计算结果支持了这一观点，这些计算结果去年发表在《经典与量子引力》（Classical and Quantum Gravity）杂志上。科斯塔说，他们的研究成果“就像科幻小说里的情节。”
The Perils of Paradox
回到过去的风险
Though no one knows whether time travel is physically possible, Einstein’s theory of general relativity proves the concept is at least theoretically sound. Specifically, his equations allow for closed timelike curves, or CTCs: loops in space-time that end where they begin. An object following one of these circular trajectories would eventually arrive back at the place (and time) it started, and could interact with its past self.
虽然没有人知道时间旅行如何从技术上实现，但爱因斯坦的广义相对论证明了这个概念至少在理论上是成立的。具体来说，广义相对论的方程允许封闭的类时曲线存在，这种曲线代表起点和终点在同一时空点上的循环。遵循这些封闭类时曲线运动的物体最终会回到它一开始所处的时间地点，并可能与过去的自己相互作用。
There’s no guarantee they actually exist. Many eminent physicists have argued against the possibility of CTCs, whether natural or engineered via time machines made by intelligent beings. “The laws of physics do not allow the appearance of closed timelike curves,” Stephen Hawking wrote matter-of-factly in 1992. He half-jokingly dubbed this idea the “chronology protection conjecture,” a feature of reality that “makes the universe safe for historians.”
然而没人能保证这种曲线真的存在。许多著名物理学家都否定封闭类时曲线的可能性——无论是自然生成的的还是由时间机器制造出来的。“物理定律不允许出现封闭类时曲线。”霍金在1992年实事求是地写道。他半开玩笑地称这个想法为“保护年表猜想”，这是一个“让历史学家安心的宇宙”所需要的特征。
But even as a pure thought experiment, time travel is plagued by challenges. The granddaddy of them all, aptly enough, is the grandfather paradox. The name originated with the famous scenario in which a person goes back in time to kill their own grandfather before he has children. If they succeed, they’ll never be born in the future; but in that case, who killed granddad? More abstractly, it refers to any change in the past that produces a logical inconsistency.
即使作为一个纯粹的思想实验，时间旅行也要面对许多挑战。这一切的根源就是“祖父悖论”。这个悖论的名字源于一个著名的场景：一个人回到过去，在自己的祖父留下后代前杀死了他。如果这个人成功了，他作为他祖父的后代理应不会在未来出现；但那样的话，是谁杀死了祖父？更抽象地说，祖父悖论指的是，对已发生事情的任何改动可能会产生逻辑上的不一致。
The simplest solution to the paradox is to contend that time travel just isn’t possible — we can’t defy logic if the laws of nature prevent us from doing so in the first place. But Costa and Tobar’s modeling suggests an alternative: Try as we might to create a paradox, events will always play out the same and reach the destined outcome.
对于祖父悖论，最简单的解决办法是否定时间旅行的可能性——如果自然法则一开始就阻止我们回到过去，我们的行为就不会产生违背逻辑的后果。但科斯塔和托巴尔的模型提出了另一种解决办法：一旦回到了过去，无论我们如何改变已经发生的事情，尝试创造一个悖论，事情的发展总会是相同的，并最终走向注定的结果。
Time Without Beginning
没有起点的故事
If that last sentence sounds bizarre, it’s because we tend to get these mind-bending causal sequences backward. As Costa explains, “everything that has ever happened has happened only once,” and in precisely one way. Our contrary intuition (that the past is changeable) stems in part from the fact that we imagine time travel as external to the mainline of history. In reality, it’s just as entangled in that history as any other event.
如果这个结论听起来很奇怪，那是因为我们倾向于把这些令人费解的因果顺序颠倒过来。正如科斯塔所解释的，“所有发生过的事情都只发生过一次”，并且是以一种精确的方式发生的。我们直觉上认为过去是可以改变的，部分源于这样一个观点，即我们把时间旅行想象成历史主线之外的东西。但实际上，时间旅行就像其他事件一样，也纠缠在历史中。
If you visit a previous era, then you have always visited and always did whatever you did there. There was no “first time around” in which it occurred differently. “If a time traveller is going to travel to some past time, then she has already been there,” as the Australian philosopher Nicholas J. J. Smith wrote in a 1997 paper titled “Bananas Enough for Time Travel?” She can affect the past, certainly, but not change it. Her actions will never alter the course of events — it’s just the way things went the only time around.
如果你回到了过去，你会一直旁观，一直做你已经做过的那些事情。现实不像游戏，不会给你那么多“从第一轮”开始的机会。澳大利亚哲学家尼古拉斯·J。 史密斯（Nicholas J。 Smith）在1997年的一篇论文《为时间旅行准备的香蕉皮够多吗？》中写道，“如果一个时间旅行者要回到过去的某个时间，那么历史上他已经在那里出现过了。”他可以影响过去，但不能改变最后的结局。他的行为永远不会改变事情的发展轨迹——这是事情唯一的发展方式。
It may help to realize that although we’re accustomed to thinking of time as linear, this is an inherent human bias. It suits our standard notions of chronology, but “when there is time traveling involved, this way of reasoning doesn’t really work anymore because you cannot have a story with a beginning and an end,” Costa says. The frames in the movie reel of life are no longer side-by-side, but overlapping. “We tell the story all at once,” he says.
尽管我们习惯于认为时间是线性的，但这是人类固有的偏见。这种观念符合我们对时间顺序的认识，但“涉及时间旅行时，这种推理方式就不再有效了，因为你的故事并没有一个明确的开始和结束。”科斯塔说。生活如电影胶片上的画面，不再是并排呈现的，而是重叠的。他说：“我们一次讲述了整个故事。”
For physicists like Costa, there is a practical upshot here: You can’t study events in a time loop the same way you would in our familiar world by extrapolating an outcome based on initial conditions. There are no “initial” conditions, and the word loses meaning. So Costa and Tobar shifted the perspective. Their model doesn’t proceed from some imaginary beginning but rather from a set of fixed variables that you can imagine as human choices. From there, Costa says, “the physics writes the rest of the story.”
对科斯塔这样的物理学家来说，有一个实用的结论值得牢记：如果在一个时间循环中研究事件，你不能像在我们熟悉的世界中那样，根据初始条件推断出结果。这个时间循环中没有“初始”条件，甚至“初始”这个词都已经失去了意义。所以科斯塔和托巴尔改变了视角。他们的模型不是从某个假想的开端出发，而是从一组固定的变量出发，你可以把这些变量理解成人的选择。以此出发，“物理学会写出剩下的故事。”科斯塔说。
And what is the story? It’s one in which people are still “free,” in some sense, to act as they will. In the sacrosanct procession of time, Costa says, “the universe is just doing the only thing it can do, which is to be consistent with itself.”
所以剩下的故事是什么？在某种意义上，人们仍然是“自由”的，可以按照自己的意愿行事。科斯塔说，在神圣的时间长河中，“宇宙只是在做它唯一能做的事，那就是与自身保持一致。”
And if visiting the past is possible, it could come with other caveats. First off, hypothetical designs for time machines typically require either infinite mass or negative energy, a power source we’re nowhere near harnessing. Second, due to the nature of closed timelike curves, a time machine can venture back no farther than the moment of its creation. So if we do manage to build or discover one, our voyages will still be limited. You’ll likely have to abandon those dreams of riding a brontosaurus.
如果时间旅行是可行的，它可能会带来其他麻烦。首先，建设时间机器可能需要无穷大的质量或负能量，而这种能量源我们还远不能利用。其次，由于封闭类时曲线的性质，时间机器只能回到它诞生的那一刻。所以，如果我们真的设法建造或发现了一个时间机器，我们的航程将仍然是有限的。你可能要放弃骑雷龙的梦想了。
The subject comes with enough complications to deter many scholars from tackling it. “Time travel isn’t the most studied subject,” Costa says, “because most likely it doesn’t exist.” But it remains a perennial subject of wonder and fascination, and not without reason: “The tantalizing aspect,” he adds, “is that we can’t prove it’s impossible.”
时间旅行的复杂性足以让许多学者望而却步。“时间旅行并不是研究最多的学科，”科斯塔说，“因为它很可能根本不存在。”但它仍然是一个永恒的话题，令人惊叹和着迷，这并不是没有理由的。“时间旅行吸引人的地方在于，”他补充说，“我们无法证明它是不可能的。”