Invariant mass, or intrinsic mass and intrinsic mass, usually referred to as mass for short, refers to Lorentz invariants composed of total energy and momentum, which are the same in all reference frames.

If the system consists of a single particle, the constant mass is also called the static mass of the particle. Because the center of mass of an isolated system always keeps moving in a straight line at a uniform speed, the observer can always choose such a reference system that the total momentum of the system in this reference system is zero, that is, it is still relative to this reference system.

Applications in physics:

In particle physics, the invariant mass is a mathematical combination of energy and momentum of a particle, which is equal to the static mass of the particle in a static reference frame. Invariant mass has the same value in all reference frames (see special relativity). Or in natural units, such as the speed of light, this equation shows that the constant value of mass is equal to the modulus (length) of the four-dimensional momentum vector.

The calculation of the module here is based on the Pythagorean theorem of relativity, so that the space dimension and the time dimension have opposite signs. The modulus of this vector remains unchanged under any Lorentz transformation (rising or rotating) in four-dimensional space-time, just like the length of an ordinary three-dimensional vector remains unchanged after any rotating transformation in three-dimensional space.