第二公设在狭义相对论中的作用
The Function of the Second Postulate in Special Relativity
摘要：
许多作者指出，相对性原理和时空对称性足以推导出惯性系之间的类洛伦兹坐标变换。它们包含一个自由参数$k$（等于狭义相对论中的$c^{-2}$），它通常被认为是经验确定的，所以狭义相对论不需要光速恒定的假设。我分析了这一主张，发现所有注定要测量$k$的方法都失败了，没有进一步的假设，类似于第二个假设。具体地说，测量$k$需要一个在相反方向上相同传播的信号（这与同步的约定无关，因为一假设程序隐含地选择标准同步约定）。假设光的这种性质在逻辑上比爱因斯坦的第二个假设弱，但足以完全恢复狭义相对论。
英文摘要：
Many authors noted that the principle of relativity, together with space-time symmetries, suffices to derive Lorentz-like coordinate transformations between inertial frames. These contain a free parameter, $k$, (equal to $c^{-2}$ in special relativity) which is usually claimed to be empirically determinable, so that special relativity does not need the postulate of constancy of the speed of light. I analyze this claim and find that all methods destined to measure $k$ fail without further assumptions, similar to the second postulate. Specifically, measuring $k$ requires a signal that travels identically in opposite directions (this is unrelated to the conventionality of synchronization, as the one-postulate program implicitly selects the standard synchronization convention). Positing such a property about light is logically weaker than Einstein's second postulate but suffices to recover special relativity in full.
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