If a photon is emitted by an electron inside of a clock on Earth and it travels to a clock four light years away, time stops for the clock on Earth and time jumps forward eight years for the distant clock, also the electron that will capture the photon becomes infinitely large, relative to the photon but the electron that emitted it does not become infinitely small therefore, time is not perfectly symmetrical.
R.A.Delmonico
relativity, space, time