Massless particles move in straight lines in spacetime, called geodesics, and gravitational lensing relies on spacetime curvature. This is because the momentum of a massless particle depends only on frequency and direction, while the momentum of low speed massive objects depends on mass, speed, and direction (see energy–momentum relation). The component of force parallel to the motion still affects the particle, but by changing the frequency rather than the speed. Thus, perpendicular components of forces acting on massless particles simply change their direction of motion, the angle change in radians being GM/ rc 2 with gravitational lensing, a result predicted by general relativity. Massless particles are known to experience the same gravitational acceleration as other particles (which provides empirical evidence for the equivalence principle) because they do have relativistic mass, which is what acts as the gravity charge. Rest mass is the same for all observers with any reference frames. In special relativity, rest mass means invariant mass.
In this context, they are sometimes called luxons to distinguish them from bradyons and tachyons. For example, these particles must always move at the speed of light.
The behavior of massless particles is understood by virtue of special relativity.
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