he classical Newton formula 
reveals the law of gravity of celestial bodies and everything, but does not explain the cause of gravity. General relativity says that gravity doesn't exist, but is a curvature of spacetime. Quantum mechanics suggests the existence of so-called gravitons, but so far none have been found. Quantum mechanics can't explain gravity, relativity can't explain quantum mechanics, and gravity and electromagnetic theory are still not unified. It is pointed out that the gravitational constant 
is PI, c is numerical value of velocity of light. The essence of gravity is the fluctuating pressure of particles in a vacuum.This paper is also an attempt to unify the theories of gravity and electromagnetism.
