Modernism is usually thought of as a shock wave of innovations hitting art, architecture, music, cinema and literature - the work of Picasso, Joyce, Schoenberg, movements like Futurism and Dada, the architecture of Le Corbusier, T.S. Eliot's The Wasteland and the avant-garde theatre of Bertolt Brecht or Samuel Beckett. But what really defines modernism? Why did it begin and how long did it...