In the early 20th century, Berlin had become a fertile ground for the German Expressionist movement. In fields such as architecture, painting and cinema new forms of artistic styles were invented. At the end of the First World War in 1918, a republic was proclaimed by Philipp Scheidemann at the Reichstag building. In 1920, the Greater Berlin Act incorporated dozens of suburban cities, villages, and estates around Berlin into an expanded city. The act increased the area of Berlin from . The population almost doubled, and Berlin had a population of around four million. During the Weimar era, Berlin underwent political unrest due to economic uncertainties but also became a renowned center of the Roaring Twenties. The metropolis experienced its heyday as a major world capital and was known for its leadership roles in science, technology, arts, the humanities, city planning, film, higher education, government, and industries. Albert Einstein rose to public prominence during his years in Berlin, being awarded the Nobel Prize for Physics in 1921.