the principle of locality in physics and are expressed in terms of retarded time or the time at which the cause of a measured field originated given that the influence of field travelled at speed of light. The retarded time for a point particle is given as solution of: where is retarded time or the time at which the source's contribution of the field originated, is the position vector of the particle as function of time, is the point in space, is the time at which fields are measured and is the speed of light. The equation subtracts the time taken for light to travel from particle to the point in space from the time of measurement to find time of origin of the fields. The uniqueness of solution for for given , and is valid for charged particles moving slower than speed of light. Magnetic field of arbitrary moving point charge The solution of maxwell's equations for electric and magnetic field of a point charge is expressed in terms of retarded time or the time at which the particle in the past causes the field at the point, given that the influence travels across space at the speed of light. Any arbitrary motion of point charge causes electric and magnetic fields found by solving maxwell's equations using green's function for retarded potentials and hence finding the fields to be as follows: where and are electric scalar potential and magnetic vector potential in Lorentz gauge, is the charge of the point source, is a unit vector pointing from charged particle to the point in space, is the velocity of the particle divided by the speed of light and is the corresponding Lorentz factor. Hence by the principle of superposition, the fields of a system of charges also obey principle of locality. Quantum electrodynamics In modern physics, the electromagnetic field is understood to be not a classical field, but rather a quantum field; it is represented not as a vector of three numbers at each point, but as a vector of three quantum operators at each point. The most accurate modern description of the electromagnetic interaction (and much else) is quantum electrodynamics (QED), which is incorporated into a more complete theory known as the Standard Model of particle physics. In QED, the magnitude of the electromagnetic interactions between charged particles (and their antiparticles) is computed using perturbation theory. These rather complex