How many times have you heard a jet plane going over, looked up to see it, but not been able tofind it even though you were looking toward where the sound was coming from?
有多少次你听到一架喷气式飞机飞过，抬起头去看它，即使你朝着声音传来的地方望去，却仍然无法找到它？
Well, thismoment of science is going to tell you why you couldn't find it?
那么这次科学时刻将会告诉你为什么你找不到它。
Sometime during your life you might have been told that the plane wasn't with the sound becauseit was traveling faster than the speed of sound.
在你生活中的某个时候，你可能被告知看到的飞机和听到的声音是不在同时出现的，这是因为光的传播速度比声音快。
It might have been, but it didn't have to be goingthat fast in order to appear to be separated from the sound.
它可能是这样的，但它没有必要为了同声音分离而飞的那么快。
The reason the jet appeared to be apart from the noise it was making was simply that soundtravels a lot more slowly than light-about a million times more slowly.
喷气式飞机之所以和它产生的噪音相分离，是因为声音传播速度比光速慢一百万倍。
For quick review, at sea levelsound travels about seven-hundred-fifty miles per hour, light travels at 186,000 miles per second.
简单回顾一下，在海平面上，声音的传播速度大约是每小时750英里，而光速则是每秒186，000英里。
Commercial jets normally fly at about thirty-thousand feet.
商用喷气式飞机一般在30，000英尺的高空飞行。
From that height it takes sound aboutthirty seconds to reach earth.
从那个高度，声音传送到地面需要30秒的时间，
Light gets here in only about four one-hundred-thousandths of asecond, or for practical purposes, instantaneously.
而光到达地面仅仅需要4/100，000秒，实际上就是瞬间到达。
A plane travelling at six-hundred miles an hour goes five miles in the thirty seconds it takes for thesound to reach you.
一架每小时飞行600英里的飞机在30秒中飞行了5英里，而声音传到你所在的地方需要30秒。
By the time the sound from the plane reaches your ears, the plane is fivemiles from where that sound originated.
在这30秒的时间里，声音从飞机传到你的耳朵里，而飞机离声音发出的地方已经有了5英里的距离。
So, when you look toward the sound, the plane is notthere anymore.
所以，当你朝声音发出的地方望去的时候，飞机早已不在那里了。
