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Einstein’s theory of special relativity states that it is impossible for anything to travel faster than light.
爱因斯坦的狭义相对论指出，任何物体的速度都不可能超过光速。
In a vacuum, light travels at a constant speed of 299,792,458 metres per second.
在真空中，光以每秒299,792,458米的恒定速度传播。
Scientists use the speed of light to measure distances between stars in space.
科学家利用光速来测量太空中恒星之间的距离。
A light-year is a unit that represents the distance that light can travel in a year.
光年是一个距离单位，代表光在一年内所能传播的距离。
Even the closest stars and exoplanets are removed from our solar system by several light-years.
即使是离我们最近的恒星和系外行星也离太阳系有好几光年的距离。
Proxima Centauri, the closest star to the Sun, is about 4.25 light-years away.
比邻星是离太阳最近的恒星，距离太阳约4.25光年。
Some visible stars are thousands of light-years away.
一些我们能看到的恒星离我们有几千光年之远。
This means that the night sky is an image of the past.
这意味着夜空呈现的是过去太空的景象。
In fact, it is possible that some of the stars we can see no longer exist.
事实上，我们能看到的一些恒星可能已经不存在了。
