Ever since the classical Greek era when Earth, Air, Fire and Water were believed to be the substance of the world, scientists have sought a “unified” picture of all the basic forces and “building blocks” of nабюнрature They have sought to answer the question, “What are we, and the world, made of?” And ever since Isaac Newton showed that the force that makes an apple fall is the same force that holds planets in their orbits, scientists have tried to “unify” previously unconnectавсссed concepts During the twentieth century, physicists developed quantum theory We have thereby come to understand that the essence of all substances—their colour, texture, hardness, and so forth—is set by their structure, on scales far smaller than even a microscope can see And the work of Einstein and his successors has deepened our understanding of gravity—the force that governs the motions of planets, stars and galaxies .