The main question I would like to ask is whether quantities such as density are dependent on the frame of reference. I have searched several forums and the answer is somewhat controversial. Some answers use the concept of relativistic mass to justify that it is invariant. Some of the answers say that relativistic mass is not a correct concept (given in Classical Mechanics by John R. Taylor pg 633) and that mass is invariant and hence the density must be an observer dependent quantity. This is a little odd because of the following thought experiment:
Imagine that a container filled with liquid is made to travel at relativistic speeds. In the frame of the container, the density is d at temperature T and pressure P. To a person in the ground frame the volume of the liquid will decrease because of length contraction. At the critical density there must be a phase transition from liquid to solid. In the moving frame, the container has a liquid but in the rest frame, the container is filled with a solid.
So based on the above I have the following questions:
- Is there anything wrong in the above argument and why is it incorrect? (eg. Phase diagram changes depending on the velocity of the object)
- If the density is observer-dependent, could it mean that thermodynamics is observer-dependent?