We critically revisit the definition of thermal equilibrium, in its operational formulation, provided by standard thermodynamics. We show that it refers to experimental conditions which break the covariance of the theory at a fundamental level and that, therefore, it cannot be applied to the case of moving bodies. We propose an extension of this definition which is manifestly covariant and can be applied to the study of isolated systems in special relativity. The zeroth law of thermodynamics is, then, proven to establish an equivalence relation among bodies which have not only the same temperature, but also the same center of mass four-velocity.

中文翻译：

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狭义相对论中的热力学第零定律

我们批判性地重新审视热平衡的定义，在其操作公式中，由标准热力学提供。我们表明，它指的是在基本水平上打破理论协方差的实验条件，因此，它不能应用于移动物体的情况。我们建议对这个定义进行扩展，它显然是协变的，可以应用于狭义相对论中孤立系统的研究。于是，热力学第零定律被证明在具有相同温度和相同质心四速的物体之间建立了等价关系。