General Relativity

Abstract

General Relativity—Einstein’s theory of gravity—is the most beautiful and elegant of physical theories. It is the foundation of cosmology—the subject that traces the evolution of the universe from its first intensely hot and dense beginning to its possible futures.¹ General Relativity is also the foundation for our understanding of compact stars. Neutron stars and black holes Can be understood correctly only in General Relativity as formulated by Einstein [15,16]. Dense objects like neutron stars could also exist in Newton’s theory, but they would be very different objects. Chandrasekhar found (in connection with white dwarfs) that all degenerate stars have a maximum possible mass [17, 18]. In Newton’s theory such a maximum mass is attained only asymptotically when all Fermions, whose pressure supports the star, are ultra relativistic. Under such conditions, stars populated by the three heavy quarks—known as charm, truth, and beauty—would exist. However, such stars do not occur in Einstein’s theory because the maximum-possiblemass star is not sufficiently dense, even at its center; therefore they cannot exist in nature.

Einstein, himself, never did apply his theory to the evolution of the cosmos. Indeed, when he discovered the theory (1915), it was a canon of western thought that the world lasted from “everlasting to everlasting.” Edwin Hubble’s 1927 discovery of universal expansion shook that belief.