In quantum electrodynamics (QED), the electromagnetic tensor can indeed vary in time. QED describes the interaction between charged particles and the electromagnetic field, and it allows for the description of processes involving the emission, absorption, and scattering of photons.
In general, the electromagnetic field can vary both in space and time. The electromagnetic tensor, also known as the electromagnetic field strength tensor or the electromagnetic field tensor, contains information about both the electric and magnetic fields. It encodes how the fields vary with respect to both space and time.
When considering the dynamics of charged particles interacting with the electromagnetic field, one typically takes into account the time dependence of the fields. For example, when analyzing the behavior of charged particles in the presence of an external electromagnetic field, such as in the case of radiation, it is important to consider the time-varying nature of the field.
In the case of radiation, the electromagnetic field is usually characterized by oscillating electric and magnetic fields, which propagate through space as electromagnetic waves. These waves carry energy and momentum, and they can interact with charged particles, leading to phenomena such as absorption, emission, and scattering of photons.
In summary, the electromagnetic tensor used in QED can vary in time, and it is essential to consider the time dependence of the electromagnetic field when studying processes involving the interaction of charged particles with the electromagnetic field, such as in the case of radiation.