A rough equivalent: "momentum is 'just' the relativistic correction to energy between masses observed in different relativistic frames of reference". It might b

A rough equivalent: "momentum is 'just' the relativistic correction to energy between masses observed in different relativistic frames of reference". It might be vaguely trueish if we're very generous in interpreting the "just", but it's absolutely the wrong way to think about things. The electromagnetic field tensor is the real object here. The electric and magnetic fields are artifacts of a particular choice of coordinate system.