来自《耶鲁大学开放课程：基础物理 第14集 第14集》

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  now, which is the time
  这是随质点运动的时钟
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  according to a clock moving with the particle.
  显示的时间
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  Everybody agrees on that,
  对所有人都一样
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  because we're not asking how much time elapsed
  因为我们没有问这段时间是
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  according to you or according to me.
  相对于你还是相对于我
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  We are going to quarrel about that indefinitely.
  我们不去讨论不确定的事
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  We are asking,
  我们要问
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  how much time passed according to the particle itself?
  对于质点自身过了多长时间
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  So, no matter who computes the time,
  无论谁计算这个时间
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  you will get the same answer,
  都会得到相同的结果
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  and that's what you want to divide it by.
  这就是你要除以的数
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  So, I'm going to form a new quantity called velocity,
  我要构造一个叫做速度的新的量