Traveling in Time
You cannot change the laws of physics …… but could physics actually enable us to travel through time？
It might sound crazy， but according to Einstein's theories， there's no logical reason why time travel isn't possible――and there are now scientists looking into it as a serious scientific proposal， at least at a theoretical1 level.
Many science-fictional2 ideas from the past have subsequently became scientific realities. Take space exploration3， for example. In the 1950s this was just a fantastical idea that few people would have dreamt could actually happen. Now it's such a part of modern life that the latest blastoff4 of a spacecraft， or the discovery of a new planet often doesn't even make the national news.
Time travel is clearly a trickier5 proposition than space travel，though. And prior to Einstein， it would have been deemed6 utterly7 impossible！ That's because the old idea about time was that it was like a cosmic8 metronome9 keeping a regular and constant beat throughout the universe. And it was thought to move in one direction only （this is the idea of " time's arrow"――time can go forwards but never backwards）。
However， what physicists now know is that time is rather more flexible than the old " Clockwork10 Universe" ideas they had it. And it was Albert Einstein who set the cat among the pigeons11.
Put simply， Einstein's idea was that every object in the universe has it's own " time" ， and these vary as objects move. The faster an object moves， the slower its time is， compared to the time of a slower moving object. The extreme situation would be if an object could move as fast as the speed of light， its time would be completely halted. But whatever an object――or person's――time is， it's only evident in contrast to other objects. In other words， it's all relatives.
Einstein's theories about time and space were revolutionary. He became a celebrity――and not just in scientific circles. It's only since he published his theories that scientists have been able to demonstrate that space and time really behave the way he said they did.
In 1971， after Einstein's death， two scientists were able to carry out a crucial12 experiment. They used two atomic clocks， synchronized13 them， and placed one on a plane， while the other stayed in the same location on Earth. The plane then flew around the world for 80 hours. According to Einstein's theory， the clock on the plane would be expected to have lost time， due to being in motion over 80 hours compared to the clock on the ground. When they brought the clocks together and made a comparison， the clock on the plane was indeed a few nanoseconds14 slower than the other clock. The experiment was replicated15 in 1996 with advanced technology， and it was proved again――with an even bigger time difference this time. Which proves that not only is time " warp-able" ， but Einstein was arguably the greatest thinker the world has ever seen.
So， time is warped――and this opens up the possibility that we might be able to somehow manipulate16 it as a mode of travel one day.Already one scientist has published a paper in which he detailed how to build a time machine.
If it were possible， however， it would present some pretty knotty paradoxes17…… For example， what if someone or something traveled back in time and changed the ensuing18 future？ And have you heard the one about the time traveller who dots back and forward in time and by means of various medical technologies is able to be his own father AND mother？！ And besides， if time travel is possible， where are all the people from the future――surely they'd want to come and meet us poor stranded19 21st century beings？
时间旅行
你无法改变物理学定律……但是物理学能否使我们在时间中旅行呢？
这听上去或许像个痴想。但是根据爱因斯坦的理论，时间旅行在逻辑上不是不可能的。现在科学家正把它视为一个严肃的科学命题，至少在理论上如此。
以前科学小说中提出的很多想法已经变成了科学的现实。拿太空探险来说，50年代，这只不过是一种科学狂想，很少人相信有一天会变成现实。可是现在它已成为现代生活的一个组成部分，以至于一次最新的宇航器发射或者一个新行星的发现已不再列为国内新闻。
与太空旅行相比，时间旅行很显然是一个更为复杂的命题。在爱因斯坦之前，这个想法肯定会被认为是无稽之谈！这是因为传统上人们认为时间就如同一个巨大的节拍器，在宇宙空间中持续着同一种节奏，而且只沿着一个方向运动。（这就是"时间箭"的概念，即时间一直向前永不后退。）
但是物理学家现在所知道的事实是："精确宇宙"观念并不正确，时间其实是可伸缩的。而正是艾伯特・爱因斯坦掀起了这一轩然大波。
简而言之，爱因斯坦的观点就是，宇宙中每一物体都有其自身的"时间"，并随其运动的不同而不同。物体运动越快，其时间越慢（与运动较慢的物体相比而言）。最极端的假设是，如果一个物体运动达到光速，那么其时间就会完全停止。但无论一个物体或者一个人的时间为何，它只能参照其他物体才能得以体现。换句话说，万物皆相对。
爱因斯坦关于时间和空间的理论具有革命性。他成了名人，而且不仅局限于科学界。正是从他的理论发表以后，科学家才得以有机会证实时空规律确实如其所述。
爱因斯坦去世后，1971年，两名科学家完成了一项重要实验。他们使用两个原子钟，将时间调成一致。其中一个被放在飞机上，而另一个则置于地球上的原来地方。随后飞机环绕全球飞行了80个小时。按照爱因斯坦理论，相对于地面上的时钟，飞机上的时钟会因为80个小时的运动而减少时间。当他们将两个时钟拿到一起比较时，飞机上的时钟确实较另一只慢了几毫微秒。1996年，科学家用先进的技术重复了同样的实验，也证实了该理论，时间差距甚至更大了。这一实验不仅验证了时间的可扭曲性，还证明爱因斯坦可能是人类史上最伟大的思想家。
所以说，时间是可弯曲的。这也使我们或许有朝一日有可能利用它作为某种形式进行旅行。有一位科学家已经发表了一篇论文，详细地描述如何建造一部时间机器。
然而，如果这一切成为可能的话，那么将会出现一些非常令人困惑的逻辑混乱……比如，如果某人或某物逆时间旅行并改变随之发生的情况，那会发生什么情况呢？又比如说，你是否曾听说过这样的事情――一位时间旅行者在时间隧道中穿梭往返，利用各种医学技术使自己成为自己的父亲和母亲？还有，如果时间旅行成为可能，未来的所有人都会在哪里呢？他们肯定想来见见我们这些滞留在21世纪的可怜人。
注释：
1.theoretical adj.理论上的
2.science-fictional adj.科学幻想小说的
3.exploration n.探险，探测
4.blastoff n.（火箭、导弹、宇宙飞船等）发射
5.tricky adj.难对付的，需慎重对待的
6.deem vt.认为，视为，相信
7.utterly adv.完全地，彻底地
8.cosmic adj.宇宙的，无比巨大的
9.metronome n.节拍器
10.clockwork adj.有规律的
11.set the cat among the pigeons （英口）引起麻烦（或纷扰）
12.crucial adj.至关重要的
13.synchronize vt.校准，
14.nanosecond n.毫微秒
15.replicate vt.重复
16.manipulate vt.使用
17.paradox n.有明显的矛盾特点的事
18.ensue vi.接踵而来
19.stranded adj.未得到帮助的
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