On photon trajectories and electromagnetics near strongly gravitating cosmic sources

We see and understand the Universe around us through the medium of the electromagnetic spectrum, ranging from the low-frequency radio waves extending up to the high-frequency γ-rays. These electromagnetic waves, generated by different processes within or nearby the various astronomical bodies get modified in their transit through the vast distances of spacetime and its contents that make up the Universe. Depending upon the various physical features of the media they are transiting through, characteristics such as dispersion, polarization and frequency modulation can occur by the intervening forces and fields. While electric and magnetic fields could be strong sources of influence, the weaker force gravity also can be a modifying factor both in the generation and propagation of the electromagnetic waves from distant cosmos. In fact, it is well known that the influence of gravity on light, making it bend while passing near Sun, was the predicted and observed experimental fact that confirmed Einstein’s theory of general relativity as the most successful theory of the twentieth century. It is well known that the passage of electromagnetic waves through plasma leads to several changes in the dispersed waves, a systematic study of which could give information about the characteristics of the medium through which they arrived. Over the past century, several detailed studies have been carried out to analyse the structure and properties of electromagnetic fields of cosmic objects and their behaviour in the presence of gravitational fields. We give a brief review of the subject, starting with the gravitational bending of light and its consequences, viz., gravitational lensing and then consider the influence of gravity on the electromagnetic fields close to compact objects and the behaviour of charged particles in such electromagnetic fields. The changes in their trajectories, that occur because of the curved geometry, can have direct influence on the electromagnetic and plasma processes near bodies like neutron stars and black holes. Further, introduction of a possible non-minimal coupling of electromagnetism and gravity seems to be a possible source for the generation of the cosmic primordial magnetic field, a precursor to the presently observed magnetic field in the Universe.