  We discuss a paradox from the field of relativistic thermodynamics: Two heat
reservoirs of the same proper temperature move against each other. One is at
rest in reference frame SA, the other in reference frame SB. For an observer,
no matter in which of the two reference frames he is at rest, the temperatures
of the two reservoirs are different. One might therefore conclude that a
thermal engine can be operated between the reservoirs. However, the observers
in SA and SB do not agree upon the direction of the entropy flow: from SA to
SB, or from SB to SA. The resolution of the paradox is obtained by taking into
account that the drive of an entropy current is not simply a temperature
difference, but the difference of a quantity that depends on temperature and on
velocity.
