Inverse Temperature 4-vector in Special Relativity
  There exist several prescriptions for identifying the notion of temperature in special relativity. We argue that the inverse temperature 4-vector $\bf \beta$ is the only viable option from the laws of thermodynamics, and $\bf \beta$ is a future-directed timelike 4-vector. Using a superfluidity thought experiment, one can show that $\bf \beta$ is not necessarily along the time direction of the comoving frame of the system, as is usually thought. It is conjectured that, for an isolated system, the 4-vector is determined from the entropy-maximum principle. 
Zhong Chao Wu