引力的本质

What is gravity?

回顾了关于引力本质的历史探索和最新进展.从牛顿引力和爱因斯坦引力出发,介绍了关于引力本质历史探索上的两次重大飞跃.从修改引力、量子引力和全息引力三个方面,介绍了关于引力本质的最新进展.对于牛顿引力,从开普勒行星运动定律出发,介绍了牛顿万有引力定律.介绍了最近关于修改牛顿力学和暗物质的进展;对于爱因斯坦引力,阐释了引力的几何化,然后介绍了爱因斯坦引力在宇宙学和引力波方面的应用;对于修改引力,从额外的引力自由度、高阶导数引力和高维引力三个方面介绍;对于量子引力,从协变量子引力、正则量子引力和其他量子引力三个方面介绍;对于全息引力,介绍了它的全息图像、呈展性质以及它与量子信息之间的关系.但是截至目前,关于引力本质问题的答案依然是一个谜.

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 Kepler＇s law on planetary motion to introduce the Newton＇s 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.