  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.
