Can Time Travel Survive a Theory of Everything 時間旅行在萬有理論下可行嗎？
Tom Siegfried 湯姆·西格弗裡德
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.」
在許多宇宙，通常是影視劇中的宇宙，時間旅行並不比尖峰時段在任何一個大城市的鬧市區開車困難多少。
在現實生活中，時間旅行沒那麼容易。事實上，它很可能無法辦到，是比到訪愛麗絲仙境、在彩虹盡頭發現黃金或者清除臉書網站上所有仇恨言論更遙不可及的幻想。
不過時間旅行未必違背物理學定律。根據愛因斯坦的萬有引力理論——廣義相對論——空間和時間融合為時空，從而有可能形成可以折返至過去並回送到未來的路徑。這樣的通路被稱為閉合類時曲線。
即便廣義相對論中的時間閉環可能存在，也不會有人認為它們對時間旅行具有實用性。首先，正如數學家庫爾特·哥德爾在20世紀40年代所證明的，它們可能只在特定情況下存在——宇宙必須在旋轉並且不膨脹。但是宇宙正在膨脹，而且很可能並沒有在旋轉，這讓重訪石器時代或者得到一隻寵物恐龍的前景變得暗淡。
再說，即便真有這樣的路徑，建造一艘飛船來穿越它們所需的費用要高於過去製造出來的所有德羅寧汽車（以及其他所有交通工具）的成本之和（德羅寧汽車因出現在美國科幻電影《回到未來》中而聞名——譯者注）。它需要達到每秒14萬英裡的巡航速度。而且由於沒有地方讓它停下來加油（或添加其他什麼燃料），燃料箱的體積必須是油輪大小的一萬億倍以上。
因此，實際上，時間旅行的時機尚未到來。但就算它只是原則上有實現的可能，它對宇宙基礎物理學的潛在影響或許值得為此投入時間去研究。
第一步是嘗試弄清相關物理學定律到底是什麼。愛因斯坦的廣義相對論很偉大，但很顯然並非有關宇宙物理學的定論。畢竟，它與量子力學難以和諧共存，後者定義了亞原子世界的規則，而且由於一切都是由亞原子構成，量子力學可能也定義著宇宙其他組成部分的規則。量子-廣義相對論的結合體是否真的允許時間旅行存在也許取決於結合了兩者的終極正確理論是什麼樣。
為了將廣義相對論與量子力學合併為一套統一的理論，多種候選理論已出現。日內瓦大學哲學家克裡斯蒂安·武特裡希在新發表的一篇論文中指出，這些候選理論能否像廣義相對論所允許的那樣，讓時間旅行在某種環境下得以存在，這仍是一個值得探討的問題。
他說，有可能出現的情況是，替代廣義相對論的理論可能仍在某種程度上包含與廣義相對論的類時閉環相當的東西。
不過，即便時間閉環存在於基礎理論中，還是無法保證它們會在對於現實生活具有重要意義的演生大規模時空中保存下來。而且，武特裡希指出，考慮到時間旅行確實根本無法辦到的可能性很大，預測時間旅行閉環的存在可能被當做反駁這一理論的證據。
不管怎樣，對量子引力理論是否保留了廣義相對論在時間旅行方面的漏洞加以研究可以揭示許多棘手問題，只有解答了這些問題，才能形成一套成功的理論並理解該理論與廣義相對論之間的關係。武特裡希寫道：「僅憑這一點，超越了廣義相對論的時間旅行問題就值得我們花時間去研究。」（李鳳芹譯自英國科學新聞網站9月20日文章）