What is gravity?

Abstract

We review the historical pursuits and recent developments on the origin of gravity. We start with the Newtonian gravity and Einstein gravity to introduce the two major leaps of the historical pursuits on the origin of gravity. Then we introduce the recent developments on the origin of gravity from the point of view of modified gravity, quantum gravity and holographic gravity. For Newtonian gravity, we start with Keplers law on planetary motion to introduce the Newtons universal law of gravitation, and recent developments on the modified Newtonian dynamics and dark matter; for Einstein gravity, we explain the geometrization of gravity, and introduce the application of Einstein gravity on cosmology and gravitational waves; for modified gravity, we introduce it from the point of view of extra gravitational degrees of freedom, higher derivative gravity and higher dimensional gravity; for quantum gravity, we introduce it from the point of view of covariant quantum gravity, canonical quantum gravity and other scenarios of quantum gravity; for holographic gravity, we introduce its holographic picture, emergent properties and its connection with quantum information. However, the answer to the question of gravity origin still remains unknown. We point out here that three possible roads to approach the nature of gravity can be taken in future. The first one is the coming observations of gravitational waves from both ground-borne and space-borne gravitational-wave detectors. A system involving either black hole or neutron star has certainly run into the regime of strong gravity, which is quite different from previous tests for general relativity that is in the regime of weak gravity. The gravitational-wave observation will most likely provide us with clues on classical gravity with any possible deviations if Einstein gravity does modify itself at classical level. The second one is the understanding of the cosmic acceleration at both early-time and late-time. The cosmic acceleration at early-time is supposed to be driven by cosmic inflation, and the cosmic acceleration at late-time is supposed to be driven by the cosmological constant or alternatively dark energy. Future precise observations of radiation fluctuations and matter fluctuations will show us the way approach the nature of quantum gravity. The third one is the cognition of the relation of quantum gravity and quantum information. The past fifty years or so of pursuit on quantum gravity has leaded us to the path on the conjecture that quantum gravity might be emerged from quantum information. Although the perspective of this idea is not clear, it might be our best shot for now.