Until the end of the 19th century, physicists believed that all physical phenomena, ranging from the motion of atoms to that of celestial bodies, were governed by one set of laws: the laws of motion formulated by Newton in the Principia. Newton’s theory implied that these laws were also valid for systems that move at constant speed relative to each other. That laws stay the same for such systems is known as the principle of relativity. It is impossible to find from within if a system is uniformly moving or not by performing mechanical experiments. A fundamental concept of Newtonian physics is the existence of absolute space. During the 19th century, when Thomas Young showed that light is a wave phenomenon, an invisible substance called ether, linked to absolute space, was believed to be the medium that carried these waves.

During the 1880s Albert A. Michelson and Edward Williams Morley attempted to measure the velocity of Earth relative to the ether by measuring the velocity of light. Their experiments showed that the velocity of light is exactly the same in every direction and thus does not depend on the proper motion of Earth. Some physicists argued that this result showed that the principle of relativity does not apply to electromagnetic radiation.

Dutch physicist Hendrik Antoon Lorentz and Irish physicist George FitzGerald tried to explain that the velocity of light is independent from the motion of Earth by assuming that everything contracts in the direction in which one is moving. They argued that the instrument that Michelson and Morley had used contracted imperceptibly in the direction of Earth’s motion, thus falsifying the measurement of the velocity of light. Perhaps the most important aspect of their theory is that they held on to the idea of an ether.

In 1905 Einstein published a theory based on the notion that it is impossible to determine the absolute motion of a moving object. Einstein’s concern, however, was not the failure of Michelson and Morley to measure the motion of Earth relative to the ether, but the validity of James Clerk Maxwell’s electromagnetic theory in systems that move at speeds close to the velocity of light. Einstein’s theory did not require the presence of an ether and was based on the following assumptions: (1) absolute speed cannot be measured, only speed relative to some other object; (2) the measured value of the speed of light in a vacuum is always the same no matter how fast the observer or light source is moving; and (3) the maximum velocity that can be attained in the universe is that of light.

Einstein’s theory is called the “special” theory of relativity because it applies the principle of relativity only to systems in uniform motion relative to each other. Because of the principle of relativity, passengers who are traveling smoothly in a train cannot tell whether they are moving or not unless they look out of a window. The situation becomes different if the train is uniformly accelerated. The passengers will feel a slight push in the direction opposite to that in which the train is moving. This is termed acceleration force: Because of this extra force it appears that the laws of physics would be different for bodies accelerated with respect to each other.

In 1916 Einstein published his general theory of relativity, which he based on the assumption that the laws of physics are also the same in systems that are accelerated relative to each other. To formulate this theory, he introduced the principle of equivalence: Acceleration forces and gravitational forces are not distinguishable from each other. Einstein argued that if one were in a closed elevator that is uniformly accelerated upward, one would perceive a force that is indistinguishable from gravitation.

The principle of equivalence can also be expressed by saying that the inertia of an object (its reluctance to be set in motion) is proportional to its mass. The principle of equivalence was already known to Galileo, who had deduced it from his experiments with wooden balls rolling down sloping planes. In 1891 Hungarian scientist Roland Eötvös made precise measurements and established that inertial and gravitational mass are equivalent to an extremely high degree. Because acceleration forces and gravitational forces are equivalent, they should not be distinguishable, but viewed as a property of space.

In a formulation of the special theory, mathematician Hermann Minkowski had introduced a four-dimensional space in which the fourth dimension is time. In this space-time continuum as adapted to the general theory, gravitation corresponds to the amount of curvature in a non-Euclidean, four-dimensional space.

Near a large mass, space becomes more curved, and objects moving near that mass will follow the curvature of space. One of the interesting consequences of the equivalence of gravitation and acceleration force is the bending of light rays by the presence of a large mass, such as a star or planet. For example, if light enters through a small hole on one side of a spaceship that is accelerated, the light ray will reach the other side after the spaceship has moved. The same effect would exist if the spaceship were to come close to a massive planet or star. To an observer in the spaceship, the light ray will appear curved in either case. But the observer cannot tell whether the spaceship is being accelerated or is near a planet or star.

In 1919, during a solar eclipse, Arthur Eddington showed that a star whose light passed close to the Sun appeared to be displaced by a minute amount that corresponded to the value calculated by Einstein. This was the first experimental proof of the general theory of relativity. Several other experiments since then have eliminated most doubts about both theories of relativity among physicists.