Every once in a while， a science-fiction book， movie or TV show will feature characters who can do what we all wish: Stop time.
But is such a thing possible? Answering that question requires a deep dive into the farthest corners of physics， philosophy and human perception.
First， we have to define time. “To a physicist， it's not that mysterious，” Sean Carroll， a theoretical physicist at the California Institute of Technology， told Live Science. “Time is just a label on different parts of the universe. It tells us when something is happening.”
For him， it makes little sense to talk about stopping time. We know that a car is moving because， at different moments of time， it's at a different location in space， he said. “Motion is change with respect to time，so time itself can't move.” In other words， if time stopped， all motion would stop too.
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While sci-fi has sometimes given us protagonists who can pause time for everyone else， such situations raise a great deal of questions. “Are you stopping the air from moving?” Carroll asked. “Because if so， then you're imprisoned by the air.”
A time-stopping character would also likely be unable to see anything， he added，because light rays would no longer reach their eyeballs. “There's not really any consistent scenario in which time stops.”
So much for physics. But time is more than just something read on a clock. It's also a feeling that we have in our heads and bodies， as well as the natural rhythms of the world. Yet in those cases， time can become something subject to personal whims.
“Thinking about the subjective impression of time gets interesting，” Craig Callender， a philosopher who specializes in time at the University of California， San Diego， told Live Science.
He described a well-known psychological illusion known as “chronostasis，” in which a person places a clock at the edge of their vision and then stares at something else for a moment. Glancing back at the timepiece and focusing on the second hand will make it pause.
The question then becomes， what is the relationship between our perceptions of time and the time physicists are talking about? Callender has written a number of books that attempt to explore the connection between the two， and as yet， there isn't much consensus on a final answer.
Regarding the ultimate flow of time， Callender favors a picture “where there's nothing flowing， but the story of yourself is flowing.”
And what does he believe regarding the possibility of stopping time? “If we think of our subjective sense of time， then we can stop portions of it with chronostasis，”Callender said. “But that's probably the closest we can do.”
隔三差五，科幻书籍、电影或电视剧会着力塑造一些角色能做到我们都想做的事情：停住时间的脚步。
不过这种事情可能吗?回答这个问题需要深入探究物理学、哲学及人类感知能力的最悠远角落。
首先，我们必须定义时间。加州理工学院理论物理学家肖恩·卡罗尔对趣味科学网站说：“在物理学家看来没那么神秘。时间不过是给宇宙不同组成部分贴的标签。告知我们某些事情的发生节点。”
对他来说，谈论让时间停止没什么意义。他说，我们之所以知道汽车在移动，是因为在不同时刻，它处在不同的空间位置。“运动是相对于时间的变化，因此时间本身是不动的。”换句话说，如果时间停止，所有的运动也会停止。
科幻作品有时为我们呈现了能够让其他人的时间暂时静止的主人公，不过这种情况会带来许多问题。卡罗尔问：“你要让空气停止流动吗?因为如果是的话，你会被空气禁锢。”
他还说，能让时间停止的人很可能也看不到任何东西，因为光线不再能够抵达他们的眼球。“其实在时间停止的情况下不存在任何连续的场景。”
从物理学上讲就是如此。不过时间不仅仅是在时钟上标示的东西。它还是我们头脑和身体中的一种感觉，也是这个世界的自然节律。不过，在这些情况下，时间会成为容易受个人心血来潮影响的东西。
加利福尼亚大学圣迭戈分校专门研究时间的哲学家克雷格·卡伦德对趣味科学网站说：“思考对时间的主观印象很有意思。”
他提到“停表错觉”这种著 名的心理错觉，具体来说是把时钟放在视线边缘，然后盯着别的东西看上一会儿。重新看向时钟并盯住秒针，会感觉它停了下来。
接下来的问题成了，我们对时间的感知和物理学家谈论的时间有什么关系?卡伦德著有大量论著，试图探究两者之间的关联，不过迄今为止，尚未就最终答案达成多少共识。
关于时间最终的流逝，卡伦德赞同这样一种描述：“什么都没有流逝，正在流逝的只是你自己的故事。”
至于他如何看待让时间停止的可能?卡伦德说：“想到自己对时间的主观感觉，那么我们可以借助停表错觉在某种程度上停住时间。不过这或许是我们所能做到的最接近于让时间停止的事。”
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