357
 Unit D
Fields
INTRODUCTION
The introduction of fields created a mechanism for âaction at a distanceâ. A mass creates a gravitational field around it at every point in space. An electric charge creates an electric field. A mass placed at some point in space âreactsâ to the existence of the gravitational field by experiencing a gravitational force and similarly an electric charge experiences an electric force in the presence of an electric field. Maxwell, in his synthesis of electric and magnetic phenomena in the 19th century (one of the greatest unifications of knowledge in the history of science) wrote down the equations that describe the behaviour of electric and magnetic fields. It was a straightforward consequence of these equations that energy and momentum are carried by these fields in the form of electromagnetic waves propagating at the speed of light. If a charge were to suddenly appear, an electric field would be established in space at the speed of light. The âinformationâ about the existence of the charge is carried by electromagnetic waves travelling through vacuum at the speed of light. It is similarly believed that gravitational waves carry the âinformationâ about the existence of mass.
The idea of a field started as a classical concept but has found a smooth passage into quantum theory as well, at least for fields other than the gravitational field. According to Einstein, the presence of mass curves space-time. Another mass that finds itself in this curved space-time will follow a path of least length (in space-time). This geometric interpretation of gravity is unique to gravity which perhaps explains why the transition from the classical to the quantum field, while very successful for electromagnetic and other phenomena, has not been met with the same success for the gravitational field. Hence physicists have looked elsewhere for solutions to these problems, focusing on the idea that the elementary constituents of matter may not be point-like particles as field theory suggests but tiny one dimensional objects known as strings.