Assume that there is a dielectric material with a mass density of ρ(adsbygoogle = window.adsbygoogle || []).push({}); _{0}observed in the dielectric-rest frame. And further, it is assumed that observed in the lab frame,v(x,y,z,t) is the velocity distribution,β=v/c is the normalized velocity, and γ=(1-β^{2})^{-1/2}is the relativistic factor, with c the vacuum light speed.

In my opinion, ρ_{0}is a Lorentz scalar, and γ(v,c) is a Lorentz 4-velocity, and thus ρ_{0}γ(v,c) also is a 4-vector.

My question is:

Can ρ_{0}γ(v,c) be defined as the momentum density 4-vector of the dielectric material?

What I mean is: If ρ_{0}γ(v,c) is defined as the momentum density 4-vector, is it compatible with the principle of relativity?

Note: I do not claim ρ_{0}is a constant, otherwise the material would be rigid, not consistent with the relativity.

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# What is the dielectric momentum density 4-vector?

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