# Energy momentum tensor - general properties

1. Oct 14, 2014

### TSC

It is often stated and proved in textbooks that the momentum density is also the energy flux.
The explanation is often done using the dust model.
However, it is possible that in a real fluid, there is heat conduction via particle collision. There is energy flux, but since no molecules are ever transported in the IRF, how come there is momentum density?

Last edited: Oct 14, 2014
2. Oct 14, 2014

### pervect

Staff Emeritus
Energy and momentum in special relativity are related by the fact that their combination $(E, \vec{P})$ transforms as a 4-vector, via the Lorentz transform.

This means that what appears as "just energy" in one frame of reference, appears as energy and momentum in another. A transfer of energy in one frame of reference is a transfer of energy and momentum in another frame of reference, moving relative to the first.

This is similar to the way that time and space form a 4-vector, Time and space 'mix together" in just the same way as energy and momentum due, the mathematical formalism that describes this mixing together is the 4-vector formalism.