In the seventeenth century, English physicist and mathematician Sir Isaac Newton's (1642–1727) Philosophiae Naturalis Principia Mathematica (Mathematical Principles of Natural Philosophy) accomplished a grand synthesis of physics that used three laws of motion and the law of gravity to explain motions we observe both on the Earth and in the heavens. These laws worked very well, and continue to be used in modern day engineering.
In eighteenth and nineteenth centuries prominent philosophical and religious thought led many scientists to accept the argument that seemingly separate forces of nature shared an absolute reference frame. Against this backdrop, nineteenth century experimental work in electricity and magnetism resulted in James Clerk Maxwell's (1831–1879) unification of concepts regarding electricity, magnetism, and light in his four famous equations describing electromagnetic waves. Prior to Maxwell's equations it was thought that all waves required a medium of propagation (i.e., an absolute reference frame akin to the ocean through which pressure waves prorogate). Maxwell's equations, however, established that electromagnetic waves do not require such a medium. Maxwell and others scientists were, however, not convinced of a lack of need for a propagating medium (this and he worked toward establishing the existence and properties of an "ether" or transmission medium.
The absence of a need for an ether for the propagation of electromagnetic radiation (e.g., light) was subsequently demonstrated by ingenious experiments of Albert Michelson (1852-1931) and Edward Morley (1838-1923). The importance and implications of the Michelson-Morley experiment was lost much of the scientific world. In many cases, the lack of determination of an ether was thought simply a problem of experimental design or accuracy. In contrast to this general dismissal, Einstein, then a clerk in the Swiss patent office developed a theory of light that incorporated implications of Maxwell's equations and demonstrated the lack of need for an ether.
Other important components of Einstein's special theory involved length contraction and time dilation for bodies moving near the speed of light. In separate papers published in 1889, both Irish physicist George Francis FitzGerald 91851-1901) and Dutch physicist Hendrik Antoon Lorentz (1853-1928) pointed out that the length of an object would change as they moved through he ether, the amount of contraction related tot he square of the ration of the object's velocity to the speed of light. Subsequently this was know as a FitzGerald-Lorentz contraction. Near the same time, French mathematician Jules-Henri Poincaré (1854-1912) pointed out problems with concepts of simultaneity and, just a year before Einstein published the special theory of relativity, Poincaré pointed out that observers in different reference frame would measure time differently. These anomalies led to the development of both relativity and quantum mechanics in the early part of the twentieth century.
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 constant regardless of the direction of propagation and independent of the velocity of the observer.
Key to the development of special relativity was Einstein's confidence in the results of the Michelson-Morley experiment. To understand this experiment, imagine a bored brother and sister on a long train ride. (Einstein liked thought experiments using trains.) To pass the time, they get up and start throwing a baseball up and down the aisle of the train. The boy is in the front and the girl in the back. The train is traveling at 60 MPH, and they can each throw the ball at 30 MPH. As seen by an observer standing on the bank outside the train, the ball appears to be traveling 30 MPH (60-30) when the boy throws the ball to the girl and 90 MPH (60 + 30) when the girl throws it back. The Michelson-Morley experiment was designed to look for similar behavior in light. The Earth orbiting the Sun takes the place of the train, and the measured speed of light (like the baseball's speed) should vary by the Earth's orbital speed depending on the direction the light is traveling. The experiment did not work as expected; the speed of light did not vary. Because Einstein took this result as the basic assumption that led to the special theory of relativity, the Michelson-Morley experiment is sometimes referred to as the most significant negative experiment in the history of science.
Editor's Note: The orbit of the planet Mercury around the Sun has some peculiarities that can not by explained by Newton's classical laws of physics. The general theory of relativity can explain these peculiarities, so they are described in the article on general relativity.