Einstein first proposed his special theory of relativity in 1905. Special relativity rests on two ideas: the speed of light is the same for any observer, and the laws of physics are the same in every frame of reference that is not speeding up or slowing down. It is impossible for any matter to travel faster than the speed of light. Special relativity revealed that space and time are united in a fabric known as space-time. According to special relativity, the faster you move, the slower time passes for you. Muons made by cosmic rays reach Earth's surface because their time runs about 40 times slower than an observer on Earth experiences.

In 1905, Einstein published four groundbreaking science articles while he worked at the patent office. A fourth 1905 paper by Einstein was about the relationship between mass and energy, which had been viewed as completely separate. The equation E=mc2 says that energy equals mass times the speed of light squared. In nuclear fusion, which happens inside the Sun and other stars, two nuclei merge into a heavier nucleus that has less mass than the two lighter nuclei, and the leftover mass becomes energy. The mass of a helium nucleus is less than the sum of the masses of its building blocks, which E=mc2 explains as binding energy. Special relativity has played a key role in the design and operation of particle accelerators for many decades.

Einstein published his general theory of relativity in 1915. General relativity says that gravity is how mass warps space and time. According to Einstein's equivalence principle, gravity's pull in one direction is equivalent to an acceleration in the opposite direction, so light bent by acceleration must also be bent by gravity. In 1919, astronomers photographed stars during a total solar eclipse to test whether the Sun's gravity bends starlight. Some astronomers questioned the 1919 eclipse results, but many observations over the years have since confirmed the gravitational deflection of light that general relativity predicts. Gravitational lensing happens when a massive object warps space and time so that light bends, distorts and magnifies as it passes around the object. Hubble astronomers measured the mass of a lone white dwarf, LAWD 37, by seeing how much its gravity deflected light from a background star; it is 56 percent the mass of the Sun. In 2018, scientists confirmed that light from a star passing close to a black hole is stretched to longer wavelengths, as general relativity predicts.

Scientists have compared atomic clocks on airplanes, satellites and skyscrapers with clocks on the ground, and each test confirmed Einstein's predictions. When the GPS satellites were launched, engineers realized they needed to correct for relativity to get accurate time on Earth. For GPS satellites, special relativity says their clocks should fall behind by 7 microseconds per day, while the weaker gravity in orbit speeds them up by 45 microseconds per day. As of November 2026, atomic clocks on GPS satellites run 38 microseconds per day faster than clocks on Earth. In 2010, two aluminum-ion optical clocks at heights about a foot apart ticked at slightly different rates because of the small difference in gravity. In 2024, an optical lattice clock measured a gravitational effect at the submillimeter scale, about the thickness of a human hair.

Einstein's mathematics showed that massive accelerating objects, such as neutron stars or black holes orbiting each other, would send waves of undulating space-time in all directions. Einstein predicted gravitational waves in his general theory of relativity, about a hundred years before they were first observed. In 1974, Hulse and Taylor found the first proof that gravitational waves exist. Gravitational waves were first observed directly on September 14, 2015. The first gravitational waves seen came from two black holes of about 29 and 36 times the mass of the Sun that collided 1.3 billion years ago. A change thousands of times smaller than an atomic nucleus was measured as the gravitational wave passed Earth. About 3 times the mass of the Sun was converted into gravitational waves in a fraction of a second in the first event.

In 1933, Einstein began work at the Institute for Advanced Study in Princeton, New Jersey. He lived and worked in Princeton in his later years. Einstein wrote that in Princeton he had created an atmosphere that helped him study and kept him free from distraction. He spent his later years looking for a unified field theory, one set of ideas to join the laws of physics. After retiring, Einstein kept working toward the unification of the basic concepts of physics.

In the photoelectric effect, light that shines on a material makes the material give off tiny charged particles. Einstein explained the photoelectric effect by proposing that light comes in small packets of energy called quanta. Einstein proposed that a packet of light must have a certain minimum frequency before it can liberate an electron. Werner Heisenberg introduced a new way of treating the mechanics of the atom in 1925; Einstein was impressed, but felt the theory stood on a weak foundation. A paper that arose in an Institute for Advanced Study teatime conversation introduced the concept of quantum entanglement. As of November 2026, quantum entanglement is used in quantum computers.

Einstein at first did not accept that general relativity predicts an expanding or contracting universe and held to a fixed, static universe. Edwin Hubble discovered that galaxies are moving away from one another, which Einstein's theory explains as the fabric of space itself expanding. Einstein looked through the 100-inch Hooker telescope at Mount Wilson Observatory with Edwin Hubble in January 1931. Einstein thought gravitational waves were too weak to detect, and he did not believe in black holes. Analysis of the properties of black holes using Einstein's theory began about half a century before astronomers found good evidence for them in galaxies in the 1980s.

When Albert was four or five years old, his father showed him a compass. The compass deeply impressed Albert as a little boy. Albert could not see what made the compass needle move. Einstein later wrote that seeing the compass as a boy made a deep and lasting impression on him. At age 16, Einstein imagined chasing after a beam of light, a thought experiment that played a role in his later work on special relativity. As of November 2026, the Einstein Papers Project presents 700 writings by Einstein and more than 4,500 letters written by and to him as annotated full text. Einstein is quoted as saying: "The important thing is not to stop questioning."

Words to know

• unified: joined together