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Theories About Gravity

Although gravitational attraction can be demonstrated on many different scales, from everyday objects to planets and galaxies, no explanation for the root cause of gravity has been found. Scientists often use theories to try to explain why laws work in the ways that they do, and theories of gravity attempt to explain the force of attraction between objects.

Newtonian Gravity
- A portrait of Isaac Newton
  
  Isaac Newton's law of universal gravitation, which was published in 1687, describes the gravitational attraction between all objects. Newton's law of gravitation has enabled scientists to make accurate predictions, such as the discovery of Neptune in 1864, but it does not attempt to explain how gravity actually works. Newton theorized a force similar to magnetism, holding planets in orbit around the sun, but he refrained from forming any specific hypotheses.
Einstein's General Theory of Relativity
- Manuscript of Einstein's general theory of relativity
  
  In 1916, Albert Einstein published his general theory of relativity, redefining science's understanding of space, time and gravity. General relativity describes the universe as a four-dimensional structure called "spacetime." Massive objects warp the fabric of spacetime, and gravity is the manifestation of those distortions. Because of the enormous scale of the objects and phenomena described by general relativity, it is one of the least tested of Einstein's theories. Nevertheless, it is central to the understanding of gravity within modern physics.
String Theory
- String theory adds corrections to Einstein's theory of general relativity.
  
  String theory modifies Einstein's concept of spacetime by positing a surface called the "string worldsheet," a two-dimensional space defined by the motion of oscillating one-dimensional strings. According to string theory, theoretical particles called "gravitons" carry the gravitational force. While string theory predicts a curved spacetime consistent with general relativity's equation relating to the distribution of matter and energy, as distance gets smaller, Einstein's theory of gravity is no longer accurate.
Scalar-Tensor-Vector Gravity
- STVG incorporates quantum effects with Einstein's theory of gravity.
  
  The theory of scalar-tensor-vector gravity (STVG) attempts to explain galaxies and galaxy clusters, as well as accounting for other mysterious astronomical observations. With the addition of quantum effects -- involving graviton particles -- to Einstein's theory of gravity, STVG argues that gravity is stronger at the centers of galaxies. Moving away from galaxies, where stars are more sparse, the behavior of gravity is consistent with Newton's Laws, according to STVG's advocates.

Nature