See mass in special relativity for more information on this debate. A particle whose rest mass is zero is called "massless". Photons and gravitons are thought to be massless, and neutrinos are nearly so. There are a couple of (equivalent) ways to define momentum and energy in SR. One method uses conservation laws. If these laws are to remain valid in SR they must be true in every possible reference frame. However, if one does some simple thought experiments using the Newtonian definitions of momentum and energy, one sees that these quantities are not conserved in SR. One can rescue the idea of conservation by making some small modifications to the definitions to account for relativistic velocities. It is these new definitions which are taken as the correct ones for momentum and energy in SR. The four-momentum of an object is straightforward, identical in form to the classical momentum, but replacing 3-vectors with 4-vectors:  The energy and momentum of an object with invariant mass formula_11, moving with velocity formula_10 with respect to a given frame of reference, are respectively given by  The factor formula_21 comes from the definition of the four-velocity described above. The appearance of formula_21 may be stated in an alternative way, which will be explained in the next section. The kinetic energy, formula_23, is defined as  and the speed as a function of kinetic energy is given by  The spatial momentum may be written as formula_26, preserving the form from Newtonian mechanics with relativistic mass substituted for Newtonian mass. However, this substitution fails for some quantities, including force and kinetic energy. Moreover, the relativistic mass is not invariant under Lorentz transformations, while the rest mass is. For this reason, many people prefer to use the rest mass and account for formula_21 explicitly through the 4-velocity or coordinate time. A simple relation between energy, momentum, and velocity may be obtained from the definitions of energy and momentum by multiplying the energy by formula_10, multiplying the momentum by formula_29, and noting that the two expressions are equal. This yields  formula_10 may then be eliminated by dividing this equation by formula_32 and squaring,  dividing the definition of energy by formula_21 and squaring,  and substituting: This is the "relativistic energy–momentum relation". While the energy formula_37 and the momentum formula_38 depend on the frame of reference in which they are measured, the quantity formula_39 is invariant.