1. The problem statement, all variables and given/known data Hello, I have probably quit easy task, but I dont know how show that d3x d3p is a Lorentz invariant. 2. Relevant equations 3. The attempt at a solution I mean I have to show that d3x d3p = d3x' d3p', where ' marks other system. I can prove ds2=ds'2, but I am not sure what with p? Assume boost in x-way: x'=g(x-vt) => dx'=g(dx-vdt) , where g=1/(1-(v/c)2)...is it right? what about p?