虽然平行宇宙是科幻作品的常见题材，但也有一些真实的科学理论为其提供支持。然而，如果平行宇宙真的存在，我们有可能穿越到那里吗？这肯定并非易事，不过让我们探讨一下这种可能性。

物理理论中有两个地方出现过平行宇宙。其一是我们的宇宙膨胀理论，即关于极早期宇宙的理论。在那些动荡时期，许多宇宙可能同时膨胀(并持续膨胀)并分化出数量庞大的独立宇宙。每个宇宙都有其独特的物理规律和物质排列方式。但要前往其他宇宙并非易事，因为它们远远超出了我们的可观测范围，且以超光速的速度远离我们。这可要耗费大量的“飞行里程”。

另一个潜在的多元宇宙存在于量子力学的多重世界诠释中。这种诠释认为，当某个随机的量子过程发生时，一个“宇宙”会得到其中一种可能的结果，其他宇宙则会得到其他结果。

但我们要如何才能进入其中一个平行宇宙呢？

关键是建造一台时间机器。具体方式并不重要，只要能回到过去就行。

一个可能的答案是，当你穿越回到过去时，进入的并非自己原本的过去。相反，你进入了平行历史。如果你回到过去并杀死希特勒，你杀死的并非你所在过去的那个希特勒，而是另一个希特勒。在那个平行宇宙中，希特勒总是遭到穿越时空而来的另一宇宙的刺客暗杀。当你回到未来时，回到的依然是自己所在的世界，过去并未改变。

多重世界诠释为创造这些替代历史提供了一个天然的平台。如果宇宙一直在不断分裂和分岔，那么穿越时空的旅行只不过是将你从其中一个分岔转移到另一个分岔。或者，还有一种可能，当你回到过去时，创建了一个之前不存在的新分岔。

通过研究量子力学解决此问题，其结果好坏参半。如果任由量子场自行发展，当涉及时间机器时，它们往往会发生混乱。你可以让它们稳定下来——但前提是放弃该理论的一些核心原理，比如对应原理或幺正性原理。这些原理认为量子过程最终会导致宏观行为，且基本反应可逆。实际没人愿意放弃这些原理，因为它们似乎是该理论的核心——所以在这方面我们陷入了困境。

此外，即使是经典的非量子系统也会遇到问题。假设你有一个开关，可以控制时间机器的启动与关闭，比如通过打开或关闭一个虫洞。我们不知道平行历史如何能适应这种时空结构的变化，且不论在亚原子层面发生了什么量子过程。

然而，如果我们能制造出一台时间机器，就可以轻松验证会否创造出平行历史。我们要做的只是改变你记忆中过去的某件事。如果你不可以这么做(比如说，无论你多么努力，就是杀不死希特勒)，那么你就会明白只有一条时间线，过去是一成不变的。但如果你完成了任务，那么你就会明白平行历史真实存在，且量子力学的多重世界诠释可能是正确的。

By Paul Sutter 

Let's explore the possibility of traveling to universes beyond our own — if they so exist, that is.

While parallel universes are a staple of science fiction, there are some real scientific theories to support them. But if parallel universes do exist, could we ever travel to them? It certainly wouldn't be easy, but let's explore this possibility.

Parallel universes crop up in two places in physical theories. One is in our conception of inflation, the theory of the extremely early universe. In those tumultuous times, many universes may have inflated all at once (and kept going) and branched out into a tremendous number of individual universes, each with their own kinds of physics and arrangements of matter. But traveling to the other universes wouldn't be easy, because they're far beyond our observable horizon and moving away faster than the speed of light. That would take a lot of frequent flyer miles.

The other potential multiverse is in the many-worlds interpretation of quantum mechanics. This interpretation says that when some random quantum process occurs, one "universe" gets one of the possible results, while other universes get the others. Thus, the multiverse is constantly being filled with every possible quantum possibility.

But how would we get to one of those parallel universes?

The trick is to build a time machine. It doesn't matter how you do it; you just need to go back in time. Normally, going back in time introduces all sorts of nasty paradoxes, like the infamous grandfather paradox, or, less violently, inconsistent histories. Try going back in time and destroying your time machine. Now it doesn't exist, which means you can't go back in time to destroy it, which means it should exist.

Perhaps time travel into the past is forbidden for exactly these reasons, according to Stephen Hawking's chronology protection conjecture. Or perhaps time travel into the past is allowed, but with one strict rule: You can't change the past. This is known as Igor Novikov's self-consistency conjecture. But how could you go into the past without changing it?

One possible answer is that when you travel into the past, you don't go into your own past. Instead, you slip into another history. If you go back in time and kill Hitler, you're not killing the Hitler of your past; you're killing somebody else's. And in that alternate universe, Hitler was always killed by a time-traveling assassin from another universe. When you return to the future, you come back home, with an unchanged past.

The many-worlds interpretation offers a natural platform for creating these alternate histories. If the universe is constantly splitting and branching anyway, then time travel simply moves you from one of those branches to another. Or, in another possibility, when you go back in time, you create a new branch that didn't exist before.

While this all sounds neat and tidy, it runs into the slight complication that nobody has ever gotten it to work. We don't know how this process actually unfolds or through what mechanism the alternate history emerges.

Attempts to navigate the issue by studying quantum mechanics have had mixed results. Left to their own devices, quantum fields tend to go haywire when time machines are involved. You can stabilize them — if you give up some of the core tenets of the theory, like the correspondence principle or unitarity, which say that quantum processes eventually lead to macroscopic behavior and that fundamental reactions are reversible. Nobody is really willing to give those up, since they seem central to the theory — so we're stuck on that front.

Besides, even classical, non-quantum systems run into issues. Let's say you have a switch that can turn your time machine on and off — say, by opening or closing a wormhole. We don't know how the alternate histories can accommodate changes in their space-time structures like that, regardless of whatever quantum processes are happening on a subatomic level.

However, if we could build a time machine, we could easily test whether alternate histories are created. All we'd have to do is change something in the past that you remember. If you're not allowed to do it (say, no matter how hard you try, you just can't kill Hitler), then you know there's only one timeline with a past that is locked in stone. But if you accomplish the mission, then you know that alternate histories are real and that the many-worlds interpretation of quantum mechanics might be valid.

We don't know if any of this is possible. On the other hand, we can't exactly rule it out. Time travel into the past seems forbidden but for reasons we can't readily discern. Our past seems to be gone forever — but it's also possible that it's just one branch of many and that visiting alternate universes is as easy as … Well, it's not easy at all.