Time Dilation and the Speed of Light

In special relativity, a clock moving relative to an observer runs slower in that observer’s frame; the effect increases with speed and has been measured. But an object with mass cannot accelerate to the speed of light. The phrase “time stops for an object moving at light speed” is a popular simplification: a photon has no inertial rest frame, so it is not meaningful to assign it its own clock or viewpoint. In the formalism, proper time along a lightlike path is zero.