Newton's law of gravitation states that "Every particle of matter in the universe attracts every other particle with a force that is directly proportional to the product of the masses of the particles and inversely proportional to the square of the distance between them." Here, the proportionality constant G is known as the universal gravitational constant. Its value is 6.67 × 10−11 newton meter squared per kilogram squared. Gravitational forces always act along the line joining the centers of two particles and form an action-reaction pair having equal magnitude. For example, the force that the Earth exerts on the apple has the same magnitude as the force that the apple exerts on the Earth. The Earth's mass is much greater than that of the apple. Therefore, its motion towards the apple is negligible compared to the apple's motion towards the Earth. Gravitational force acts at very long distances without any direct contact between the bodies. It is this force that is responsible for holding the entire expanding universe together.