地面是一个静止的立方体，这是Box2D已知的，因为这个立方体的质量为零(默认)，因此不会与其他物体相撞。
The ground is a box that is static, which Box2D knows, because the box has zero mass (by default) and therefore doesn't collide with other objects.
给出了由光子的波粒二象性推导光子静止质量为零的一种方法。
From the wave-particle properties of photon, the author presents a method with which the static mass of photon can be deduced to zero.
显然光子的一个主要区别于其它粒子的特征就在于它的静止质量为零。
Evidently one of the main distinguishing characteristics of photon is just its zest rest mass.
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