equation E=mc² (where E is energy, m is mass, and c is the speed of light). This theory would overturn centuries of scientific thinking and orthodoxies.
But of course, Einstein did not develop this theory in a vacuum, and the road that led him to conclude that time and space were relative to the observer was long and winding. Einstein’s eventual hypothesis of relativity was in large part an attempt to reconcile Newton’s laws of mechanics with the laws of electromagnetism (as characterized by Maxwell’s equations and the Lorentz force law).
For some time, scientists had been grappling with the inconsistencies between these two fields, which were also reflected in Newtonian physics. Whereas Isaac Newton had subscribed to the idea of an absolute space and time, he also adhered to Galileo’s principle of relativity – which states that: “any two observers moving at constant speed and direction with respect to one another will obtain the same results for all mechanical experiments.”
As of 1905, when Einstein published his seminal paper “On the Electrodynamics of Moving Bodies“, the working consensus among scientists held that light traveling through a moving medium would be dragged along by the medium. This, in turn, meant that the measured speed of the light would be a simple sum of its speed through the medium plus the speed of that medium.
This theory also held that space was filled with a “luminiferous aether”, a hypothetical medium that was believed to be necessary for the propagation of light throughout the universe. In accordance, this aether would either be dragged by, or transported within, moving matter. However, this consensus resulted in numerous theoretical problems which by Einstein’s time, had remained unresolved.
For one, scientists had failed to find an absolute state of motion, which indicated that the relativity principle of motion (i.e. that only relative motion is observable, and there is no absolute standard of rest) was valid. Second, there was also the ongoing problem posed by “stellar abberation”, a phenomenon where the apparent motion of celestial bodies about their locations was dependent on the velocity of the observer.
In addition, tests performed on the speed of light in water (the Fizeau experiment) indicated that light traveling through a moving medium would be dragged along by the medium, but not nearly as much as expected. This supported other experiments – such as Fresnel’s partial aether-drag hypothesis and the experiments of Sir George Stokes – which proposed that the aether is either partially or entirely carried along by matter.
Einstein’s theory of special relativity was groundbreaking in that he argued that the speed of light is the same in all inertial reference frames, and introduced the idea that major changes occur when things move close the speed of light. These include the time-space frame of a moving body appearing to slow down and contract in the direction of motion when measured in the frame of the observer.
Known as Einstein’s Theory of Special Relativity, his observations reconciled Maxwell’s equations for electricity and magnetism with the laws of mechanics, simplified the mathematical calculations by doing away with extraneous explanations used by other scientists, and made the existence of an aether entirely superfluous. It also accorded with the directly observed speed of light and accounted for the observed aberrations.
Naturally, Einstein’s theory met with mixed reactions from the scientific community, and would remain controversial for many years. With his one equation, E=mc², Einstein had vastly simplified the calculations necessary to understand how light propagates. He also suggested, in effect, that space and time (as well as matter and energy) were merely different expressions of the same thing.
Between 1907 and 1911, while still working at the patent office, Einstein began considering how special relativity could be applied to gravity fields – what would come to be known as the Theory of General Relativity. This began with an article titled, “On the Relativity Prin