According to Maxwell's Equations, electromagnetic waves in a vacuum travel at the speed of light, c. Since c is finite, a photon emitted when t = t0 takes a certain amount of time to reach an observer located at a distance r from the source, so that the observer only notices it when t = t1:
This can be rearranged as:
The time is defined as the retarded time and represents the delay between the photon emission and the moment at which it reaches the observer.
A theory which borrows heavily from the concept of retarded time is the Noron theory, proposed by Professor Richard Hills of Cambridge University.
