ABSTRACT
As a result of Einstein’s theories of special and general relativity, it became an important requirement for the establishment of physical laws that they should be presented in mathematical terms that did not depend on the coordinate frame in which they were expressed. This is known as invariance. In this chapter, the meaning of invariance is discussed, and it is shown how quantities are tested for invariance. The significance is that expressions for a quantum field are later given in an invariant form.
