# Relativity Theory

## Relativity Theory (World of Earth Science)

Relativity theory, a general term encompassing special and general relativity theories, sets forth a specific set of laws relating motion to mass, space, time, and gravity. Relativity theory allows calculations of the differences in mass, space, and time as measured in different reference frames.

At the start of the twentieth century the classical laws of physics contained in Sir Isaac Newton's (1642727) 1687 work, Philosophiae Naturalis Principia Mathematica (Mathematical Principles of Natural Philosophy) adequately described the phenomena of everyday existence. In accord with these laws, more than century of experimental and mathematical work in electricity and magnetism resulted in Scottish physicist James Clerk Maxwell's (1831879) four equations describing light as an electromagnetic wave. Prior to Maxwell's equations it was thought that all waves required a medium or ether for propagation. Such an ether would also serve as an absolute reference frame against which absolute motion, space and time could be measured. Ironically, although Maxwell's equations established that electromagnetic waves do not require such a medium, Maxwell and others remained unconvinced and the search for an elusive ether continued. For more than three decades, the lack of definable or demonstrable ether was explained away as simply a problem of experimental accuracy. The absence of a need for an ether for the propagation of electromagnetic radiation was demonstrated in late nineteenth century experiments conducted by Albert Michelson (1852931) and Edward Morley (1838923).

In 1904, French mathematician Jules-Henri PoincarĂ© (1854912) pointed out important problems with concepts of simultaneity by asserting that observers in different reference frames must measure time differently. In 1905, a German-born clerk in the Swiss patent office named Albert Einstein (1879955) published a theory of light that incorporated implications of Maxwell's equations, demonstrated the lack of need for an ether, explained FitzGerald-Lorentz contractions, and explained PoincarĂ©'s reservations concerning differential time measurement. Both Einstein and his special relativity theory went to revolutionize modern physics.

In formulating his special theory of relativity, Einstein assumed that the laws of physics are the same in all inertial (moving) reference frames and that the speed of light was measured as a constant regardless of its direction of propagation. Moreover, the measured speed of light was independent of the velocity of the observer.

Einstein's special theory of relativity also related mass and energy. Einstein published a formula relating mass and energy, E=mc2 (Energy=mass times speed of light squared). Einstein's equation implied that tremendous energies were contained in small masses. Along with advancements in atomic theory, Einstein's insights ultimately allowed the development of atomic weapons during World War II and the dawn of the nuclear age.

Special relativity also gave rise to a number of counterintuitive paradoxes dealing with the passage of time (e.g., the twin paradox) and with problems dependent upon an assumption of simultaneity. According to the postulates of special relativity, under certain conditions it would be impossible to determine when one event happened in relation to another event.

Although Einstein's special theory was limited to special cases dealing with systems in uniform nonaccelerated motion, the theory did away with the need for an absolute frame of reference. In addition, the implications of special relativity on the equivalence of mass and energy revolutionized classical laws regarding conservation of mass and energy. A more complete understanding of the conservation of mass and energy now relies upon mass-energy systems. Einstein's special relativity theory was also important in the development of quantum theory. German Physicist Max Planck (1858947) and others who were in the process of developing quantum theory, set out to reconcile (often unsuccessfully) relativity theory with quantum theory.

In 1915, a decade after publishing his special relativity theory, Einstein published his general theory of relativity that soon came to supplant well-understood Newtonian concepts of gravity. Although Newtonian theories of gravity hold valid for most objects, there were small but noticeable errors in calculations regarding the motion of bodies at high velocities or for description of motion in massive gravitational fields. These small errors were completely corrected by the general theory of relativity that described nonuniform, or accelerated, motion.

General relativity's impact on calculations regarding gravity sparked dramatic revisions in cosmology that continue today. The conceptual fusion of traditional three-dimensional space with time to create space-time also made observers more integral to measurement of phenomena.

In a sophisticated elaboration of Newton's laws of motion, in general relativity theory the motion of bodies is explained by the assertion that in the vicinity of mass, spacetime curves. The more massive the body the greater the curvature of space-time and, consequently, the greater the force of gravitational attraction.

It may be fairly argued that the most stunning philosophical consequence of general relativity was that space-time is a creation of the universe itself. Under general relativity, the universe is not simply expanding into preexisting space and time, but rather creating space-time as a consequence of expansion. In this regard, general relativity theory set the stage for the subsequent development of Big Bang theory.

Unlike the esoteric proofs of special relativity, the proofs of general relativity could be measured by conventional experimentation. General relativity's assertion that gravitational fields would bend light was confirmed during a 1919 solar eclipse. Other predictions regarding shifts in the perihelion of Mercury and in redshifted spectra also found confirmation. Using relativity based equations, German physicist Karl Schwarzschild (1873916) mathematically described the gravitational field near massive compact objects. Schwarzschild's work subsequently enabled the predication and discovery of the evolutionary stages in massive stars (e.g., neutron stars, black holes, etc.).

Relativity theory was quickly accepted by the general scientific community, and its implications on general philosophical thought were profound. Although Newtonian physics still enjoys widespread utility, along with quantum physics, relativity-based theories have replaced Newtonian cosmological concepts.