41. For a particle of rest mass , the " total " angular momentum tensor is: 42. Only " rest mass " is loosely equated with matter ( since it can be weighed ). 43. Photons are uncharged particles with zero rest mass , and hence travel at the speed of light. 44. Moreover, under Lorentz transformations, this relativistic mass is not invariant, while the rest mass is. 45. Allowing to be expressed in terms of the energies and rest masses , or momenta and rest masses. 46. Allowing to be expressed in terms of the energies and rest masses, or momenta and rest masses . 47. Note how the final result depends only on the rest masses which are independent of the referencial used. 48. Where m _ 0 is the rest mass ( which, by definition, is a constant ). 49. Empirically, since the light quarks actually have minuscule nonzero masses, the pions also have nonzero rest masses . 50. Measuring the mass ( by measuring gravitational attraction ) of the objects near them gives their individual rest mass .
