In quantum electrodynamics (QED), the electromagnetic field is described by the electromagnetic tensor, which includes both electric and magnetic fields. The values of the electric and magnetic fields can vary both in space and time.
In general, the electromagnetic field can change over time. The time variation of the electromagnetic field is crucial in understanding phenomena such as radiation. When charged particles accelerate or undergo transitions between different energy states, they can emit or absorb electromagnetic radiation. This radiation carries energy and propagates through space as electromagnetic waves.
In the case of radiation, the electromagnetic field varies both in space and time. The electromagnetic waves associated with radiation have a time-varying electric and magnetic field. These waves propagate outward from the source, carrying energy and momentum with them.
In the framework of QED, the interactions between charged particles and the electromagnetic field are described by Feynman diagrams, which involve the exchange of virtual photons. These interactions can occur both in the presence of a fixed external electromagnetic field and in the context of radiation.
It is important to note that the electromagnetic field can have complex and dynamic behavior in different situations. In some cases, it is convenient to approximate the electromagnetic field as a fixed, external field, especially when studying the interactions of particles with a well-defined background field. However, when considering phenomena such as radiation or the behavior of the electromagnetic field in a dynamic environment, the time variation of the field becomes crucial.
Overall, the electromagnetic tensor in QED allows for variations in both space and time, and the understanding of its dynamics is essential for describing the behavior of charged particles and their interactions with the electromagnetic field.