Invariant energy of an arbitrary physical system is a positive quantity, which consists of all types of energies of the system, and is equal to the relativistic energy, measured by the observer who is fixed relative to the center of momentum of the system. The invariant energy usually includes the rest energy of the matter; the potential energy of the proper electromagnetic and gravitational fields associated with the system; the internal energy of the system’s particles; the energy of the system in external fields; the energy of emission interacting with the system. The invariant energy of a particle equals to its rest energy and due to the principle of mass–energy equivalence is associated with the invariant mass of the particle by the equation:
where is the speed of light.
The order of calculating the invariant energy through various types of energy of the system is determined by the principle of energies summation.
Relationship with other physical variables
The relation holds for the photon, so that the invariant energy of the photon is zero.
In four-dimensional formalism in Minkowski space the energy can be calculated through the 4-momentum of the particle:
where is the metric tensor of the Minkowski space, is 4-velocity, is the Lorentz factor.
As a result, 4-momentum can be represented using the invariant energy: 
where is the 3-vector of relativistic momentum.
In the curved spacetime with the metric tensor the invariant energy of the particle is found as follows:
If we take into account the definition of 4-velocity: , where is 4-displacement vector, is the differential of the proper time; and the definition of the spacetime interval: , then again we obtain the equality: .
The system of particles
In elementary particle physics the interaction of several particles, their coalescence and decay with formation of new particles are often considered. Conservation of the sum of 4-momenta of free particles before and after the reaction leads to the conservation laws of energy and momentum of the system of particles under consideration. The invariant energy of the system of particles is calculated as their total relativistic energy in the reference frame in which the center of momentum of the particle system is stationary. In this case can differ from the sum of invariant energies of the particles of the system, since the contribution into is made not only by the rest energies of the particles, but also by the kinetic energies of the particles and their potential energy.  If we observe the particles before or after the interaction at large distances from each other, when their mutual potential energy can be neglected, the invariant energy of the system is defined as:
where is the sum of relativistic energies of the system’s particles, is the vector sum of the particles’ momenta.
The massive body
In determining the invariant energy of a massive body in general relativity (GR) there is a problem with the contribution of the gravitational field energy,  since the stress-energy tensor of gravitational field is not clearly defined, and stress-energy-momentum pseudotensor is used instead. In case of asymptotically flat spacetime at infinity for the estimation of the invariant energy the ADM formalism for the mass-energy of the body can be applied.  For the stationary spacetime metric the Komar mass and energy are determined.  There are other approaches to determination of the mass-energy, such as Bondi energy,  and Hawking energy.
In the weak-field approximation the invariant energy of a stationary body in GR is estimated as follows: 
where the mass and charge of body are obtained by integrating the corresponding density by volume, is the energy of motion of particles inside the body, is the gravitational constant, is the radius of the body, is the electric constant, is the pressure energy.
For the masses, the relation is:
where the inertial mass of the system is equal to the gravitational mass , the mass denotes the total mass of the particles that compose the body.
Covariant theory of gravitation
In covariant theory of gravitation (CTG) in the calculation of the invariant energy the energy partition into 2 main parts is used – for the components of the energy fields themselves and for components associated with the energy of the particles in these fields. Calculation shows that the sum of the components of the energy of acceleration field, pressure field, gravitational and electromagnetic fields, for the spherical shape of the body is zero.  As a result there is only a sum of the energies of the particles in the four fields:
where is the Lorentz factor of particles, and is the scalar potential of pressure field at the surface of system.
The ratio of the masses is as follows:
In this case the inertial mass system should be equal to the total mass of particles , the mass equals the gravitational mass and excess over is due to the fact that particles move inside the body and are under pressure in the gravitational and electromagnetic fields.
A more accurate expression for the invariant energy is presented in the following article: 
This leads to a change in the ratio for the masses:
Here the gauge mass is related to the cosmological constant and represents the mass-energy of the matter’s particles in the four-potentials of the system’s fields; the inertial mass ; the auxiliary mass is equal to the product of the particles’ mass density by the volume of the system; the mass is the sum of the invariant masses (rest masses) of the system’s particles, which is equal in value to the gravitational mass .
In Lorentz-invariant theory of gravitation (LITG), in which CTG is transformed in the weak-field approximation and at a constant velocity of motion, for the invariant energy the following formula holds:
where is the relativistic energy of a moving body taking into account the contribution of the gravitational and electromagnetic field energy, is the total momentum of the system.
These formulas remain valid at the atomic level, with the difference that the usual gravity replaced by strong gravitation. In the covariant theory of gravitation based on the principle of least action is shown that the gravitational mass of the system increases due to the contribution of mass-energy of the gravitational field, and decreases due to the contribution of the electromagnetic mass-energy. This is the consequence of the fact that in LITG and in CTG the gravitational stress-energy tensor is accurately determined, which is one of the sources for the determining the metric, energy and the equations of motion of matter and field. The acceleration stress-energy tensor, dissipation stress-energy tensor and pressure stress-energy tensor are also identified in covariant form.
Vector fields such as the gravitational and electromagnetic fields, the acceleration field, the pressure field, the dissipation field, the fields of strong and weak interactions are components of general field. This leads to the fact that the invariant energy of the system of particles and fields can be calculated as the volumetric integral in the center-of-momentum frame: 
where and denote the time components of the 4-potential of general field and the mass 4-current , respectively, is the tensor of the general field.
- Invariant mass
- Principle of energies summation
- Field mass-energy limit
- General field
- Dissipation field
- Acceleration field
- Pressure field
- Field energy theorem
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