The most fundamental laws of physics are conservation laws, which tell us that we can’t create or destroy “stuff,” where “stuff” could mean quantities such as electric charge or energy-momentum. Since...The most fundamental laws of physics are conservation laws, which tell us that we can’t create or destroy “stuff,” where “stuff” could mean quantities such as electric charge or energy-momentum. Since charge is a Lorentz invariant, it’s an easy example to start with. Because charge is invariant, we might imagine that charge density ρ was invariant. But this is not the case, because spatial (3-dimensional) volume isn’t invariant; in 3 + 1 dimensions, only four -dimensional volume is an invariant.
