Einstein's Relativity
In 1905, a 26-year-old patent clerk in Bern, Switzerland, published a paper that overturned long-held assumptions about space and time. Albert Einstein's special theory of relativity, set out in "On the Electrodynamics of Moving Bodies," showed that measurements of time and distance depend on the observer's motion, and that the speed of light is the same for everyone. It became one of the two pillars of modern physics, alongside quantum theory, and reshaped how scientists understand the universe.
The problem Einstein addressed had been building for decades. James Clerk Maxwell's equations of the 1860s described light as an electromagnetic wave, and physicists generally assumed it traveled through an invisible medium called the ether. Yet experiments, most famously the Michelson-Morley experiment of 1887, failed to detect Earth's motion through this ether. Hendrik Lorentz and Henri Poincaré had developed mathematical fixes to explain the puzzle, and Poincaré had also questioned the nature of simultaneity. Einstein's contribution was to rebuild the foundations rather than patch the old structure.
He started from two postulates: the laws of physics are the same for all observers moving at constant velocity, and the speed of light in empty space is constant regardless of the motion of its source or the observer. From these, he showed that the ether was unnecessary and that simultaneity is relative: two events that seem simultaneous to one observer may not be to another. Moving clocks tick more slowly, and moving objects contract in the direction of travel, effects too small to notice at everyday speeds but real at speeds approaching that of light.
Later in 1905, Einstein published a short follow-up paper arguing that the energy of a body is tied to its mass, a relationship later summarized in the famous equation E = mc². The idea implied that a small amount of mass corresponds to an enormous amount of energy. That same year, often called his "miracle year," he also published work on the photoelectric effect and Brownian motion.
The paper did not win instant fame, but leading physicists such as Max Planck took notice quickly, and Hermann Minkowski soon recast the theory as a unified four-dimensional "spacetime." Einstein extended his ideas to gravity in the general theory of relativity, completed in 1915. Special relativity became essential to particle physics and nuclear physics, and its principles are built into technologies such as particle accelerators. Satellite navigation systems also depend on relativistic corrections, though those involve general relativity as well. Einstein received the 1921 Nobel Prize in Physics, notably for the photoelectric effect rather than relativity, which some committee members then considered still too controversial.