Photon’s Billion-Ly Journey Ends in Zero Time

A photon crossing a billion light-years experiences no elapsed time—time is relative to speed and path, a consequence of special relativity that makes moving clocks run slower (time dilation). The article explains this with light clocks and the Hafele–Keating experiment, notes how GPS corrections rely on the same physics, and shows that curved spacetime preserves zero proper time along null paths, meaning light’s journey can span ages for Earthbound observers while remaining instantaneous for the photon itself.