Section 1: Introduction
Any two objects, regardless of their composition, size, or distance apart, feel a force that attracts them toward one another. We know this force as gravity. The study of gravity has played a central role in the history of science from the 17th century, during which Galileo Galilei compared objects falling under the influence of gravity and Sir Isaac Newton proposed the law of universal gravitation, to the 20th century and Albert Einstein's theory of general relativity, to the present day, when intense research in gravitational physics focuses on such topics as black holes, gravitational waves, and the composition and evolution of the universe.
Figure 1: Portraits of Sir Isaac Newton (left) and Albert Einstein (right).
Source: © Image of Newton: Wikimedia Commons, Public Domain; Image of Einstein: Marcelo Gleiser. More info
Any study of gravity must accommodate two antithetical facts. In many ways, gravity is the dominant force in the universe. Yet, of the four forces known in nature, gravity is by far the weakest. The reason for that weakness remains a major unanswered question in science. Gravity also forms the central focus of efforts to create a "theory of everything" by unifying all four forces of nature. Ironically, gravity was responsible for the first unification of forces, when Newton identified the force that caused an apple to fall to Earth to be the same as the force that held the Moon in orbit.
Current research on gravity takes several forms. Experiments with ever-greater precision seek to test the foundations of gravitational theory such as the universality of free fall and the inverse square law. Other experimentalists are developing ways to detect the gravitational waves predicted by Einstein's general relativity theory and to understand the fundamental nature of gravity at the largest and smallest units of length. At the same time, theorists are exploring new approaches to gravity that extend Einstein's monumental work in the effort to reconcile quantum mechanics and general relativity.