© David Kaplan.
The theory of relativity states that the results of a scientific experiment (or really, any cause-and-effect process), cannot be used to distinguish whether the experiment was done at rest or moving at a constant velocity. The fields we discuss in this unit pervade all space. If a field was something like a gas or a fluid, one could tell the difference between the two experiments because the scientist on the rocket would be plowing through this material. Thus, a field has to be special enough (have the symmetry properties) such that one cannot tell the difference! (Unit: 2)