时光真的可以倒流？Indeed!
Who has no regrets about things done in the past?
有谁能会对过去做过的事情完全不感到遗憾，后悔呢？
Wouldn't it be nice if, somehow, we could go back to tweak a couple of bad decisions?
如果我们能够回到过去，改变一些不好的决定，会不会很棒呢？
This sounds (and as we will see, is, to a certain extent) like science fiction.
这听起来（从某种程度上我们将会看到）就像科幻小说。
The laws of physics prohibit traveling backwards in time for many reasons.
物理学原理会以多种理由禁止往回进行时间上的穿越。
If we did travel backwards in time and changed the course of events, we would be altering the course of history.
如果我们真的从时间上往回旅行，改变了过去的一些事情，我们就就会改变历史的轨迹。
An example often cited is the grandfather's paradox: If you traveled back in time and killed your grandfather when he was still a high school student, he wouldn't have met your grandmother and your father and you wouldn't exist.
经常被引用的一个例子就是爷爷悖论：如果你穿越时间回到过去，当你爷爷还在读高中的时候就杀死他，那么他就不会遇见你的奶奶，你的爸爸和你也就不复存在了。
A popular example of traveling back in time is the fascinating Canadian TV series Travelers: In a distant future, the Earth is in shambles; humans are controlled by a benevolent artificial intelligence that finds a way to project the consciousness of people into unaware hosts in the 21st century.
时光倒流的最有名的例子之一就是加拿大的精彩的电视剧系列《旅行者》：讲述在遥远的将来，地球陷入一片混乱；在21世纪的时候，人类被一个仁慈的人工智能所控制，它设法将人类的意识转移到那些不具意识的东西上面。
The idea is that travelers from the future take over the minds of people in the 21st century right before they are about to die.
其理念就是来自未来的穿越者们在21世纪的人们即将死去之前占领他们的心智。
There is some obscure talk about quantum entanglement of consciousness between traveler and host, but this is secondary.
关于旅行者和宿主之间的意识的量子纠缠有一些讲不清楚的讨论，但这还是次要的。
The point of the show is that the travelers go back to try to change the course of history — so that the future looks better.
这部剧最主要的观点是旅行者回到过去试图改变历史轨迹—这样将来会更美好。
Putting humans or consciousness traveling back in time aside for the moment, is there anything in science even remotely similar?
暂时先不管把人类或者意识穿越到过去 这回事，科学上有没有哪怕稍微与之类似的东西呢？
Surprisingly, yes.
结论竟然是，有！
At the level of quantum particles (we are talking individual photons, elementary particles or individual atoms), there is something called Wheeler's delayed-choice experiments that show that actions in the present can influence the past.
在 量子水平（我们讲的是单光子，基本粒子或者单原子），有一种叫做惠勒的延迟选择实验，展示了现在的行为可以影响到过去。
The experiments use something called the wave-particle duality of light and of matter, the fact that the physical nature of quantum objects is undetermined until it is measured.
这个使用用到了光和物质的波粒二象性，也就是量子态物质在被测量前具有不确定的物理特性的事实。
In other words, this means that a particle of light, or of matter, can behave either as a wave (spreading out in space, showing interference) or as a particle (staying together as a small bundle) depending on the measuring apparatus.
换句话说，这意味着一个光或者物质的粒子，既可以表现得像光（在空间中传播，产生影响），也可以表现得像粒子（紧紧聚成一小粒），这种区别取决于测量仪器的不同。
Long and ongoing discussions about the nature of quantum physics are still trying to work out what this actually means.
关于量子物理的本质的讨论由来已久并且还在进行，人们试图找到这究竟意味着什么。
Do our minds determine the nature of physical reality?
难道我们的思维能决定物理事实的本质吗？
But experiments measure — they don't ask questions of meaning.
但实验测量——它们不会问意义是什么的问题。
John Wheeler, the physicist who proposed such experiments in the 1970s, would have been amazed if he had seen the current results.
约翰.惠勒于1970年代提出的这种实验，他如果看到现在的结果肯定会很吃惊。
It does seem that the present can influence the past, at least at the level of quantum objects.
看起来确实现在可以影响过去，至少在量子体的水平可以。
Imagine that there is a source of photons (or other small quantum particle).
请想象有一个质子源（或者其他的量子微粒）。
The photons can pass through a double slit.
这些质子可以通过一个双缝。
Behind the double slit, there is a screen.
在双缝后面有一个屏幕。
If the photons hit the screen, those conducting the experiment observe an interference pattern of bright and dark fringes, typical of waves.
如果质子撞击到了屏幕，那么实验人员就会观测到干涉图案的明暗条纹，这是光的经典规律。
If the screen is not there, and there are photon counters aligned with the slits, the photons will hit either one or the other, behaving like little bullets (or particles).
如果那里没有屏幕，那马没有对齐缝隙的的质子就会相互碰撞，表现得像微小的子弹（或者粒子）一样。
So far, this is the typical setup for a double-slit experiment.
截至目前，这是双缝实验的经典设置。
The "mystery which cannot go away" (as physicist Richard Feynman famously remarked) of the double-slit experiment, is that the person performing the experiment determines the physical nature of the particle — i.e., whether it is a wave or a particle.
双缝实验被物理学家理查德.费曼经典地评价为“解不开的谜团” ，这谜在于进行实验的人决定了粒子的物理特性——也就是说，它到底是波还是粒子。
And, with Wheeler, the mystery deepens.
后来，随着惠勒的加入，谜团更深重了。
Imagine that the setup — having or not having the screen — is decided after the photon goes through the slits.
想象实验设置中—是否有这块屏幕—是在质子穿过双缝之后才决定的。
In 2007, a group in France did exactly that, letting a single photon pass through a double slit and then, after it passed through, having a random number generator choose whether the screen would be there or not to detect it.
在2007年，一支法国的研究团队就是这样做的，让一个单质子穿过双缝，然后，在它通过以后，让一个随机计数器来决定有还还是没有屏幕来检测它。
As Wheeler wrote, "Thus one decides the photon shall have come by one route or by both routes after it has already done its travel."
正如惠勒写道的那样，“这样我们就可以决定这个质子在完成穿过之后是从一条还是两条路径过来的。”
Since then, many other groups have performed refined versions of the experiment, confirming Wheeler's intuition.
从那以后，很多其他的团队也进行了很多个改良版本的实验，来确认惠勒的直觉。
Unfortunately, these experiments say very little about how we could interfere with the past in events relevant to the human scale.
遗憾的是，这些实验室并不能太多告诉我们在那些影响整个人类的历史事件中可以如何干涉。 Better to think carefully about decisions than to try to fix them backwards.
我们最好能在做决定之前想好，而不是事后再去补救。
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