The Theory of Relativity

Two-dimensional projection of a three-dimensional analogy of spacetime curvature described in General Relativity

   The theory of relativity, or simply relativity, encompasses two theories of Albert Einstein: special relativity and general relativity. However, the word relativity is sometimes used in reference to Galilean invariance.

    The term "theory of relativity" was based on the expression "relative theory" used by Max Planck in 1906, who emphasized how the theory uses the principle of relativity. In the discussion section of the same paper Alfred Bucherer used for the first time the expression "theory of relativity".

    The theory of relativity enriched physics and astronomy during the 20th century. When first published, relativity superseded a 200-year-old theory of mechanics elucidated by Isaac Newton. It changed perceptions.

    For example, it overturned the concept of motion from Newton's day, into all motion is relative. Time was no longer uniform and absolute, as related to everyday experience. Furthermore, no longer could physics be understood as space by itself, and time by itself. Instead, an added dimension had to be taken into account with curved spacetime. Time now depended on velocity, and contraction became a fundamental consequence at appropriate speeds.

    In the field of microscopic physics, relativity catalyzed and added an essential depth of knowledge to the science of elementary particles and their fundamental interactions, along with introducing the nuclear age. With relativity, cosmology and astrophysics predicted extraordinary astronomical phenomena such as neutron stars, black holes, and gravitational waves.

    The theory of relativity was representative of more than a single new physical theory. It affected the theories and methodologies across all the physical sciences. However, as stated above, this is more likely perceived as two separate theories. There are some related explanations for this. First, special relativity was published in 1905, and the final form of general relativity was published in 1916.

   Second, special relativity fits with and solves for elementary particles and their interactions, whereas general relativity solves for the cosmological and astrophysical realm (including astronomy).

    Third, special relativity was widely accepted in the physics community by 1920. This theory rapidly became a notable and necessary tool for theorists and experimentalists in the new fields of atomic physics, nuclear physics, and quantum mechanics. Conversely, general relativity did not appear to be as useful. There had appeared to be little applicability for experimentalists as most applications were for astronomical scales. It seemed limited to only making minor corrections to predictions of Newtonian gravitation theory. Its impact was not apparent until the 1930s.

    Finally, the mathematics of general relativity appeared to be incomprehensibly dense. Consequently, only a small number of people in the world, at that time, could fully understand the theory in detail. This remained the case for the next 40 years. Then, at around 1960 a critical resurgence in interest occurred which has resulted in making general relativity central to physics and astronomy. New mathematical techniques applicable to the study of general relativity substantially streamlined calculations. From this, physically discernible concepts were isolated from the mathematical complexity. Also, the discovery of exotic astronomical phenomena in which general relativity was crucially relevant, helped to catalyze this resurgence. The astronomical phenomena included quasars (1963), the 3-kelvin microwave background radiation (1965), pulsars (1967), and the discovery of the first black hole candidates (1971).