时空旅行在万有理论下可行吗？

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Can Time Travel Survive a Theory of Everything 时空旅行在万有理论下可行吗？

Tom Siegfried 汤姆·西格弗里德

In many universes， typically those on TV shows or in movies， time travel is not much more difficult than driving downtown in any major city during rush hour.

In real life， time travel isn't so easy. In fact， it's probably impossible， a fantasy more farfetched than visiting Alice's Wonderland， finding gold at the end of a rainbow or cleansing all the hate speech off of Facebook.

Yet time travel does not necessarily violate the laws of physics. In Einstein's theory of gravity — general relativity — space and time are merged as spacetime， which allows for the possibility of pathways that could bend back to the past and loop back to the future. Such paths are known as closed timelike curves.

Nobody thinks that general relativity's time loops would be practical for time travel even if they are possible. For one thing， they might exist only under certain circumstances — the universe would have to be rotating， and not expanding — as the mathematician Kurt G del showed in the 1940s. But the universe is expanding， and probably isn't rotating， so that dampens the prospects for revisiting the Stone Age or acquiring a pet dinosaur.

Besides， even if such pathways did exist， building a ship to traverse them would cost more than all the DeLoreans （and all other transportation vehicles） ever made. It would need a cruising speed of 140，000 miles per second. And with no place to stop for gas （or whatever）， the fuel tank would have to be more than a trillion times the size of an oil tanker.

So for practical purposes， time travel's time has not yet arrived. But even if it's possible only in principle， the potential ramifications for the basic physics of the universe might make it worth the time to investigate it.

A first step would be to attempt to figure out exactly what the relevant laws of physics really are. Einstein's general relativity is great， but indubitably not the last word about the physics of the universe. After all， it coexists uneasily with quantum mechanics， which rules the subatomic world and presumably， since everything is made of subatomic stuff， the rest of the universe as well. Whether the quantum–general relativity combo truly permits time travel might depend on what the ultimate correct theory combining the two turns out to be.

Several candidate theories have been developed for merging general relativity and quantum mechanics into a unified theory. It's an open question whether these candidates would allow time travel in something like the way general relativity does， philosopher Christian Wüthrich of the University of Geneva notes in a new paper.

It's possible， he says， that a theory that supersedes general relativity might still in some way include the equivalent of general relativity's timelike loops.

Even if time loops exist in the fundamental theory， though， there's still no guarantee that they would be preserved in the emergent large-scale spacetime that would be relevant in real life. For that matter， Wüthrich points out， predicting the existence of time travel loops might be taken as evidence against the theory， considering the serious likelihood that time travel really isn't possible at all.

In any case， investigating whether quantum gravity theories retain general relativity's time travel loophole can illuminate many tough questions that must be answered to develop a successful theory and understand how it relates to general relativity. “For this reason alone，” Wüthrich writes， “the question of time travel beyond general relativity is worth our while.”

在许多宇宙，通常是影视剧中的宇宙，时空旅行并不比高峰时段在任何一个大城市的闹市区开车困难多少。

在现实生活中，时空旅行没那么容易。事实上，它很可能无法办到，是比到访爱丽丝仙境、在彩虹尽头发现黄金或者清除脸书网站上所有仇恨言论更遥不可及的幻想。

不过时空旅行未必违背物理学定律。根据爱因斯坦的万有引力理论——广义相对论——空间和时间融合为时空，从而有可能形成可以折返至过去并回送到未来的路径。这样的通路被称为闭合类时曲线。

即便广义相对论中的时间闭环可能存在，也不会有人认为它们对时空旅行具有实用性。首先，正如数学家库尔特·哥德尔在20世纪40年代所证明的，它们可能只在特定情况下存在——宇宙必须在旋转并且不膨胀。但是宇宙正在膨胀，而且很可能并没有在旋转，这让重访石器时代或者得到一只宠物恐龙的前景变得暗淡。

再说，即便真有这样的路径，建造一艘飞船来穿越它们所需的费用要高于过去制造出来的所有德罗宁汽车（以及其他所有交通工具）的成本之和（德罗宁汽车因出现在美国科幻电影《回到未来》中而闻名——译者注）。它需要达到每秒14万英里的巡航速度。而且由于没有地方让它停下来加油（或添加其他什么燃料），燃料箱的体积必须是油轮大小的一万亿倍以上。

因此，实际上，时空旅行的时机尚未到来。但就算它只是原则上有实现的可能，它对宇宙基础物理学的潜在影响或许值得为此投入时间去研究。

第一步是尝试弄清相关物理学定律到底是什么。爱因斯坦的广义相对论很伟大，但很显然并非有关宇宙物理学的定论。毕竟，它与量子力学难以和谐共存，后者定义了亚原子世界的规则，而且由于一切都是由亚原子构成，量子力学可能也定义着宇宙其他组成部分的规则。量子-广义相对论的结合体是否真的允许时空旅行存在也许取决于结合了两者的终极正确理论是什么样。

为了将广义相对论与量子力学合并为一套统一的理论，多种候选理论已出现。日内瓦大学哲学家克里斯蒂安·武特里希在新发表的一篇论文中指出，这些候选理论能否像广义相对论所允许的那样，让时空旅行在某种环境下得以存在，这仍是一个值得探讨的问题。

他说，有可能出现的情况是，替代广义相对论的理论可能仍在某种程度上包含与广义相对论的类时闭环相当的东西。

不过，即便时间闭环存在于基础理论中，还是无法保证它们会在对于现实生活具有重要意义的演生大规模时空中保存下来。而且，武特里希指出，考虑到时空旅行确实根本无法办到的可能性很大，预测时空旅行闭环的存在可能被当做反驳这一理论的证据。

不管怎样，对量子引力理论是否保留了广义相对论在时空旅行方面的漏洞加以研究可以揭示许多棘手问题，只有解答了这些问题，才能形成一套成功的理论并理解该理论与广义相对论之间的关系。武特里希写道：“仅凭这一点，超越了广义相对论的时空旅行问题就值得我们花时间去研究。”（李凤芹译自英国科学新闻网站9月20日文章）

原文地址：今日头条

17109324198

17109324198

17109324198

17109324198