The term "energy-momentum density" is not like an electron, nor is it an electron itself. Energy-momentum density refers to the distribution of energy and momentum in a physical system. It is a concept used in physics to describe how energy and momentum are distributed within a given region of space.
An electron, on the other hand, is a fundamental particle with specific properties, such as mass, charge, and spin. Electrons are one of the building blocks of matter and are classified as fermions. They have both energy and momentum, but they are not synonymous with the concept of energy-momentum density.
Energy-momentum density can be thought of as a quantity that characterizes the local distribution of energy and momentum within a system, whereas an electron is a specific particle with its own distinct properties. The energy-momentum density of a system can be calculated by considering the contributions from all the particles and fields present in that system, including electrons if they are part of it.
In summary, the term "energy-momentum density" is a concept used to describe the distribution of energy and momentum in a physical system, while an electron is a fundamental particle with its own set of properties. They are distinct concepts in physics.