【英语时差8,16】光速与星际高速公路-上

（单词翻译）

If you’re driving on an interstate highway, the only thing keeping you from going faster than the speed limit is your sense of civic¹ duty, or perhaps that police car up ahead. There’s one speed limit, however, that most scientists agree nothing will ever break. That’s the speed of light: 186,280 miles per second. As it travels through deep space, light moves at exactly this speed, no more and no less. The speed of light is so reliable², we even use it to measure distances between the stars. If it takes starlight fifty years to reach us, we say that this star is fifty light-years away. You might be surprised to learn, however, that light itself does not always travel at the speed of light. Our interstate highways generally have a higher speed limit, but once you enter a city or residential³ street, the speed limit lowers⁴.