Relativistic dynamics and Lorentz contraction

Regarding the structure of a bound system of two particles in relativistic quantum mechanics we raise the following question: Does the shape of (i.e., the density distribution in) a bound system in motion always conform to Lorentz contraction? A negative possibility is illustrated by means of the Bakamjian-Thomas model which is relativistically covariant in the sense that the generators of the Poincaré algebra for the model can be constructed …