Gravity, also called gravitation, is a force that exists among all material objects in the universe. For any two objects or particles having nonzero mass , the force of gravity tends to attract them toward each other. Gravity operates on objects of all sizes, from subatomic particles to clusters of galaxies. It also operates over all distances, no matter how small or great.
Gravitation is one of the most studied phenomena in physics. Isaac Newton defined its behavior with his famous Law of Universal Gravitation. Newton hypothesized that the force of gravity between any two spherical objects of uniform density is directly proportional to the product of their masses, and inversely proportional to the square of the distance between their centers. According to this law, if m1 and m2 are the masses of two objects, and if their centers are separated by a distance d, then the gravitational force f between them is given is given by the following formula:
f = km1m2/d2
where k is a constant that depends on the units specified for m1, m2, d, and f.
Newton's theories about gravitation were modified in the 20th century when Albert Einstein developed the general theory of relativity . Einstein saw that the nature of the force of gravity is the same as the nature of the force exerted on any object that undergoes acceleration. This is known as the Principle of Equivalence. He also demonstrated mathematically that gravitational fields cause distortion in space and time . These hypotheses have been validated by experimentation.
Every object in the universe is surrounded by a gravitational field. The strength of the field is directly proportional to the mass of the object, and inversely proportional to the square of the distance from its center (for any point outside the object itself). Thus, the gravitational field of the sun is stronger overall than that of the earth, and the gravitational field of the earth is stronger in the vicinity of a person standing on the surface than it is at the altitude of a satellite in orbit. Nevertheless, the gravitational field surrounding any object extends indefinitely in all directions. Every particle in the entire universe is gravitationally attracted to every other particle. This is an important consideration in the development of theories concerning how the universe formed, how it is evolving, and what fate ultimately awaits it.