Einstein publishes the special theory of relativity
Like Max Planck, Albert Einstein first studied mathematics because he was told that everything interesting in physics had already been discovered. But in 1905, Einstein, a young cleark at the Swiss patent office, published several ideas that led to a total rethinking of physics and shook the foundation of science.
The publication that had the most profound effect was not the most ballyhooed at the time. Einstein received the Nobel Prize for one of the other ideas, but the special theory of relativity has since become the most well-known, perhaps of his career or even the entire field of physics! When first published, the article didn't include the now-famous equation E=mc2, but later in 1905, he added it to another publication. The theory stated that the speed of light is constant and absolute -- it always goes the same speed, and nothing can go faster than that. In fact, as things travel at speeds approaching the speed of light, strange things happen to them. They get shorter in the direction of travel, their mass increases, and time passes more slowly for them.
This was incredibly shocking to a world that saw Newton's laws of physics as sacrosanct. Everyone knew that space had three dimensions, and time only one. But Einstein put them together in one four-dimensional system where space and time cannot be separated or viewed independently. In that system, energy and mass are really the same. And that is the fundamental point of E=mc2: Energy equals matter times the speed of light squared. You only need a minute amount of matter, say, an atom, to create a tremendous amount of energy.
The implications of Einstein's theory were strange, but proved true mathematically and in the real world. Experimenters have carried extremely accurate atomic clocks on high-speed jets on around-the-world journeys. And when they compared these clocks to the extremely accurate clocks they left at home, the traveling clock had indeed gone slower and lost time. But by very little. Researchers calculated that going about a quarter of the speed of light would only change mass, length, and time measurements by 1 percent. That means that Newton's ideas and "laws" still work well for local measurements at ordinary speeds.This changes drastically the closer one goes to the speed of light: For an object moving at 6/7 the speed of light, its mass would be doubled.
The special theory of relatively left Einstein with many questions still unanswered. He didn't let them go, however, and in answering them he developed the general theory of relativity ten years later.