There have been several theories postulated and formulated to define and explain the natural phenomenon now known as gravity. Two of these is the Law of Universal Gravitation introduced by Sir Isaac Newton in 1687 and the General Relativity Theory developed by Albert Einstein between 1907 and 1915. Note that Newton and Einstein had different approaches in defining and explaining gravity.
The Difference Between the Law of Universal Gravitation and the General Relativity Theory
Gravity According to Sir Isaac Newton
The Law of Universal Gravitation introduced in the Philosophiæ Naturalis Principia Mathematica describes gravity as a force. Also known as Newtonian Gravity, it specifically explains that every particle attracts every other particle with a force that is directly proportional to the product of their masses and inversely proportional to the square of the distance between their centers
Hence, according to Newton, gravity is a force that causes any two bodies to be attracted to one another and that this gravitational force has an inverse-square property. It is also one of the four fundamental forces of nature that include electromagnetism, the strong force, and weak force.
For Newton, all objects with mass are drawn to one another. The greater the mass, the stronger the gravitational pull. He explained that this pull was responsible for the formation of objects in the universe such as the earth and other planets, moons, as well as the stars.
Gravitational pull is also responsible for keeping the earth and other planets in the solar system to remain in orbit around the sun, and for the low tide and high tide of seas due to the interaction of the moon with the earth.
Gravity According to Albert Einstein
Einstein had a different view. In his General Relativity Theory, he explained that gravity is not a force but a property of spacetime geometry. Note that the theory is also known as the Geometric Theory of Gravitation.
To be more specific, Einstein noted that gravity is a consequence of the curvature of space and time caused by the uneven distribution of mass. This curvature is directly related to the energy and momentum of whatever matter and radiation that are present.
General Relativity also predicts that all objects fall in the same way and at the same time, regardless of their mass and in the absence of obstructions, notably air resistance. Thus, in a vacuum, a feather and a block of concrete dropped at the same time would fall at the same rate.
Other predictions have been confirmed through observations and experiments. These include gravitational time dilation, gravitational lensing, the gravitational redshift of light, and the gravitational time delay. Note that the Geometric Theory of Gravitation also implies the existence of black holes.
In Summary: Newtonian Gravity vs. the Geometric Gravity of Einstein
For Newton, especially according to his Universal Law of Gravitation, gravity is one of the fundamental forces of nature. His theory explains that all objects with mass are naturally drawn to one another.
The greater this mass, the stronger the pull.
On the other hand, for Einstein, particularly according to his General Relativity Theory or the Geometric Theory of Gravitation, gravity is not a force but a geometric property of space and time in which an uneven distribution of mass causes the curvature of spacetime.
