Light reflected, absorbed or emitted is according to the Doppler principle of relative motion. If the light source relatively moves away from an object, then the light received by the object is relatively weaker. Consider also the principle of equivalence of general relativity theory: The inertia of mass equates to the gravity of mass. Consider, then, the internal energy of matter is such that it emits gravitational radiation according to the Doppler principle. The emission of radiation renders a vacuum effect in its wake. Since this emission of radiation is according to the Doppler principle, the cause of gravity is explained in a manner consistent with the principle of equivalence. Moreover, there is no violation of the law of conservation of momentum if the gravity caused by this emission is the result of the energy in space containing the internal energy of matter. The push cause of gravity is the a ripple effect of the surround space filling the otherwise vacuum, as the radiation emitted cannot continually attract other matter without a mutual change in momentum. However, conservation of energy further depends on a recycling process. Instead of the creation of gravity continually filling up space, consider it does change into other forms. However, this change in form is gradual and long range, as consistent with the force of gravity per mass being extremely weak in comparison to such other forces as electromagnetism. A possibility for a recycling process is similar to Hubble Constant of a red shift in light spectrum from light sources of longer distance. It so happens that the value of the Hubble Constant calculated from data obtained by the WMAP (deep space probe) equates exactly with ratio of gravitational to electromagnetic force of the hydrogen atom. This book thus explains gravity in a manner consistent with the principle of equivalence of general relativity, conservation of momentum, and conservation of energy. Moreover, it is also explained in a manner consistent with quantum physics, thermodynamics and electromagnetism. It is also explained in a mathematically consistent manner. The mathematics is simplified in the manner of algebra and geometry instead of more advanced calculus such that it can be understood by a high school student. The historical evolution of theoretical physics, as a step by step process, is also provided in support of this explanation, which seems to lead the way for a unified theory.