21. Where m _ 0 is invariant mass and invariant energy equals E _ 0 =-m _ 0 c ^ 2 > 0. 22. In such cases, the photons contribute invariant mass to the system, even though they individually have no invariant mass or rest mass. 23. In such cases, the photons contribute invariant mass to the system, even though they individually have no invariant mass or rest mass. 24. This causes the invariant mass of a non-isolated system to depend on one's choice of coordinates even in special relativity. 25. Again, neither the relativistic nor the invariant mass of totally closed ( that is, isolated ) systems changes when new particles are created. 26. This invariant mass is the same in all relativistic version of the Pythagorean theorem which has a different sign for the space and time dimensions. 27. Conservation of invariant mass also requires the system to be enclosed so that no heat and radiation ( and thus invariant mass ) can escape. 28. Conservation of invariant mass also requires the system to be enclosed so that no heat and radiation ( and thus invariant mass ) can escape. 29. The same is true for massless particles in such system, which add invariant mass and also rest mass to systems, according to their energy. 30. This is why physicists usually reserve the useful short word " mass " to mean rest mass, or invariant mass , and not relativistic mass.
