Collectively, their brilliant contributions formed the two intellectual pillars of modern physics and profoundly influenced the trajectory of our global civilization. Einstein was born on March 14, 1879, into a middle-class Jewish family in Ulm, Germany. He graduated with a degree in physics and mathematics from the prestigious Federal Polytechnic Academy in Zurich, Switzerland, in 1900. Shortly thereafter, he became a clerk in the Swiss patent office in Berne. This rather unchallenging job allowed him the time he needed to continue his world-changing theoretical work in physics.
In 1905, while working in the patent office, Einstein presented a paper entitled “Zur Elektrodynamik bewegter Körper” (“On the Electrodynamics of Moving Bodies”). In it, he described the special theory of relativity, which deals with the laws of physics as seen by observers moving relative to one another at constant velocity, that is, by observers in nonaccelerating or inertial reference frames. In formulating special relativity, Einstein proposed two fundamental postulates:
First postulate of special relativity: The speed of light (c) has the same value for all [inertial-reference-frame] observers, independent and regardless of the motion of the light source or the observers. Second postulate of special relativity: All physical laws are the same for all observers moving at constant velocity with respect to each other.
From the theory of special relativity, Einstein concluded that only a zerorest-mass particle, such as a photon, could travel at the speed of light. Another major consequence of special relativity is the equivalence of mass and energy—which is expressed in Einstein’s famous formula: E = mc2, where E is the energy equivalent of an amount of matter (m) that is annihilated or converted completely into pure energy and c is the speed of light. Among many other important physical insights, this equation was the key that nuclear physicists needed to understand energy release in such nuclear reactions as fission, fusion, and radioactive decay. Einstein’s special relativity theory became one of the foundations of modern physics.
Einstein published several other important papers during 1905 and received his doctoral degree in physics from the University of Zurich that year as well. Nevertheless, he could not obtain a university teaching job until 1909, when the University of Zurich finally offered him a low-paying position. He became a special professor at the Kaiser Wilhelm Physical Institute in Berlin in 1913.
Two years later, Einstein introduced his general theory of relativity. He used this development to describe the space-time relationships of special relativity for cases where there was a strong gravitational influence, such as white dwarf stars, neutron stars, and black holes. One of Einstein’s conclusions was that gravitation is not really a force between two masses (as Newtonian mechanics suggests) but, rather, arises as a consequence of the curvature of space and time. In a four-dimensional universe (described by three spatial dimensions [x, y, and z] and time), space-time becomes curved in the presence of matter, especially large concentrations of matter. The fundamental postulate of general relativity states that the physical behavior inside a system in free-fall is indistinguishable from the physical behavior inside a system far removed from any gravitating matter (i.e., the complete absence of a gravitational field). This very important postulate is also called Einstein’s principle of equivalence.