21. People confuse relative mass with rest mass . 22. Rest mass increases according to velocity, with infinite mass at the speed of light.23. A hypothetical particle with Complex rest mass would always travel faster than the speed of light. 24. In this case, invariant mass is positive and is referred to as the rest mass . 25. This is true in special relativity theory, even though individually photons have no rest mass . 26. This is why the invariant mass is the same as the rest mass for single particles. 27. The problem comes from thinking of rest mass as the " real " mass. 28. Anything with a zero rest mass can " only " travel at light speed. 29. Any form of energy excerts gravitational pull and has inertia, not just the rest mass . 30. :: The physics behind this is very simple-it involves the rest mass energy.
