In 1999 I finished my three volume book on the quantum theory of fields (..."QTF"), and... set... the task of learning... the theory underlying the g… - Steven Weinberg

The development of quantum mechanics in the 1920s was the greatest advance in physical science since the work of Isaac Newton. It was not easy; the ideas of quantum mechanics present a profound departure from ordinary human intuition. Quantum mechanics has won acceptance through its success. It is essential to modern atomic, molecular, nuclear, and elementary particle physics, and to a great deal of chemistry and condensed matter physics as well.

[T]o extend this to the geometry of spacetime... include a term... in the spacetime line element, with <math>a</math> now an arbitrary function of time (known as the Robertson-Walker scale factor):<math>d\tau^2 \equiv -g_{\mu\nu}(x) dx^\mu dx^\nu = dt^2-a^2(t)[d\mathbf{x}^2 + K \frac{(\mathbf{x} \cdot d\mathbf{x}^2)}{1-K\mathbf{x}^2}]</math>

Early speculation on about electric forces relied... on an analogy with Newton's theory of gravitational forces. At the end of Principia, Newton described gravitation as a cause that acts on the sun and the planets "according to the quantity of solid matter which they contain and propagates on all sides to immense distances, decreasing always as the inverse square of the distances." ...It was irresistible to guess that the electric force might obey a similar law, also proportional to the inverse square of the distance... with charge playing the role that mass plays...