In 1913, Einstein urged astronomers to measure the effect of gravity on starlight. 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. Einstein published his general theory of relativity in 1915. 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. At the 1922 solar eclipse in Western Australia, observers saw the effect for 15 stars and took it as verification of general relativity. Astronomers could not demonstrate the bending of light by gravity beyond reasonable doubt until the 1960s, when methods improved vastly.
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 predicted gravitational waves in his general theory of relativity, about a hundred years before they were first observed. 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.
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. Special relativity says the speed of light in a vacuum is the same for any observer. According to special relativity, the faster you move, the slower time passes for you. Einstein recognized that if you move away from a clock near the speed of light, the clock you see will be slow. It is impossible for any matter to travel faster than the speed of light. When the GPS satellites were launched, engineers realized they needed to correct for relativity to get accurate time on Earth. Scientists have compared atomic clocks on airplanes, satellites and skyscrapers with clocks on the ground, and each test confirmed Einstein's predictions. In 2018, scientists confirmed that light from a star passing close to a black hole is stretched to longer wavelengths, as general relativity predicts.
By the 1920s, special and general relativity were widely accepted by physicists. In the 1920s, Einstein traveled to give lectures in Europe, America and the Far East. Albert Einstein became famous around the world. His ideas changed how scientists think about time, space and the universe. In 1933, Einstein left Germany for America. He never returned to Germany. He began work at the Institute for Advanced Study in Princeton, New Jersey. Einstein became a United States citizen in 1940. Einstein wrote that in Princeton he had created an atmosphere that helped him study and kept him free from distraction.
