After that, scientists wanted to determine the value of the gravitational constant, in which the English scientist Henry Cavendish calculated as G= 6.754 x 10 Nm /Kg.
For hundreds of years, Newton’s laws where considered as the basis of modern physics, and one of the things that proved Newton’s theories was the discovery of the planet Neptune but there were also many important issues that were not discovered by Newton such as, that there is no way to describe mass except with reference to acceleration, another issue is that no force moves without acceleration, a third one is that when we mention acceleration we have to say with respect to what?
Einstein who wondered why the inertial mass is proportional to the gravitational mass studied all the previously mentioned theories. After experimenting, he made his theory of equivalence, which stated that if we had two systems, the first has acceleration with no gravitational field while the second has acceleration with no gravitational mass and while the other is at rest and has a gravitational field, the results would be equivalent. By the year 1916, Einstein had completed the mathematical theory of gravitation, which was the general theory of relativity. Einstein’s theories opposed those of Newton’s in at least two major and measurable issues that were the curvature of light and the shifting of the wavelength of the light.
In conclusion, the issue was a subject of research done by the greatest scientists ever known as Newton and Einstein, and it will always be a subject of research, as the gravitational mystery still might not be totally unravelled.