And that's really neat to think about, because photons, of course, are massless particles, they have no mass, so it's neat to think about something that has no mass, but that actually does have a momentum.

而且那真的不容易想明白，因为光子，当然是无质量的粒子，它们没有质量，所以这个真的不容易想明白,一些物体没有质量，但是它们事实上确实有动量。

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