What is the nature of gravity?

Abstract

There are four conventionally accepted fundamental interactions in nature—gravitational, electromagnetic, strong, and weak forces. The gravitational force, based on Einstein’s general theory of relativity, is described as a continuous classical field. The other three, part of the standard model of particle physics, are described as discrete quantum fields, and their interactions are each carried by a quantum, an elementary particle. Two theoretical frameworks upon which all modern physics rests have been developed and have been able to withstand almost all the experimental tests so far individually, but they are mutually incompatible—they cannot both be right. It is a puzzle in physics that the two perfect theories are not compatible, and how to reconcile quantum theory with general relativity is still an open question. Our understanding on the nature of gravity is the key issue. In this article, we first introduce interpretation of gravity in the framework of general relativity, in which gravity is not a real force but a representation of the curved spacetime. On the other hand, the standard model gave a different interpretation of gravity, in which gravity is effects of exchange of gravitons between two masses. Graviton is the fundamental particle, whose mass is zero and spin is two, but it has not been found in any experiments. Then, the contradiction when scientists attempt to unify general relativity and quantum theory is introduced. The superstring theory, a quantum theory not of point particles and a possible candidate in pursuit of a theory of everything in nature, seems to give an unique solution to solve the contradiction, in which all of the particles and interactions of nature are modelled as the vibrations of tiny supersymmetric strings, and the price to pay is unusual features such as six extra dimensions of space in addition to the usual three. The size of the string is about 10-35m, the Plank scale. String and its vibration could construct everything, which also includes gravity, and the physical properties of forces and particles were decided by the frequencies of the string, such as spin and mass. The superstring theory had a puzzle that supersymmetry could be drew into string theory to form five different superstring theories by five different methods. M-theory was born with the development of superstring theory, which unified five different forms of superstring theory. Finally, the experimental tests of the nature of gravity is briefly introduced.