**Einstein Proposes His Theory of Relativity (1905):** In 1905, Albert Einstein was a 26-year-old young man working six days a week in the Swiss Patent Office in Bern. In between work and his family life (he had a wife and son), Einstein worked diligently on his scientific theories. Even with what seems like very little time, Einstein had his most productive and momentous year of revolutionary scientific theories that year.

In 1905, Einstein wrote five articles and had them published in the prestigious *Annalen der Physik* (*Annals of Physics*). In one of these papers, “Zur Elektrodynamik bewegter Koerper” (“On the Electrodynamics of Moving Bodies”), Einstein detailed his Special Theory of Relativity.

There were two main parts of his theory. First, Einstein discovered that the speed of light is constant. Secondly, Einstein determined that space and time are not absolutes; rather, they are relative to the position of the observer.

For example, if a young boy were to roll a ball across the floor of a moving train, how fast was the ball moving? To the boy, it might look like the ball was moving at 1 mile per hour. However, to someone watching the train go by, the ball would appear to be moving the one mile per hour plus the speed of the train (40 miles per hour). To someone watching the event from space, the ball would be moving the one mile per hour the boy had noticed, plus the 40 miles an hour of the speed of the train, plus the speed of the earth.

In a follow-up paper published that same year, “Ist die Traegheit eines Koerpers von seinem Energieinhalt abhaengig?” (“Does the Inertia of a Body Depend Upon Its Energy Content?”), Einstein determined the relationship between mass and energy. Not only are they not independent entities, which had been a long held belief, their relationship could be explained with the formula E=mc^{2} (E=energy, m=mass, c=speed of light).

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