From an apple to black holes
“Nature and Nature's laws lay hid in night: God said, Let Newton be! and all was light.”
― Alexander Pope
In 1666, a 23 year old gentleman in England upon seeing an apple fall from a tree asked the question, "If an apple fall, does the moon fall too?" and in 1905, a 26 year old gentleman in Switzerland, watching a man on the top of a building, asked the question, "Does a man falling from a building feel weight?" Despite being separated both by time and space, the contribution of these two men to science and to mankind in general is incomparable. But for the time being, we are just going to look at their contribution to our knowledge of gravitation.
Let's start with the 23 year old gentleman, Sir Isaac Newton. The answer to his question is YES. The moon also falls if an apple falls and this is due to the effect of the force of gravity(or gravitation). Newton devised a simple law that can find the gravitational force between any two objects knowing their masses and the distance between them. With the law of Newton, one could calculate the motion of planets, predict eclipses and even discover planets. In fact, Neptune was first predicted theoretically and then discovered by astronomers. Even sending people on the moon required only the law of Newton. However, Newton's law of gravitation had a few limitations. It was hard to predict the motion of a system containing three or more bodies and the orbit of Mercury did not match the predictions. Other great physicists and mathematicians such as Joseph-Louis Lagrange and Leonhard Euler reformulated Newton's laws but still failed to correctly predict the orbit of Mercury.
In 1915, the second gentleman, Albert Einstein answered his question. NO, the man falling off a building does not feel any weight. In 1915, Einstein , who was already famous for special relativity and photoelecticity (We will come to these later), conceived a theory that could explain the motion of Mercury. According to his theory moving bodies cause four-dimensional space-time to warp and the warping of space-time causes bodies to move. Hence, Mercury's orbit was affected by the large warping of the space-time around it caused by the nearby Sun. Moreover, with Einstein's theory of general relativity, the notion of gravitational force disappeared and was replaced by the notion of gravitational field and since the man is falling freely in the gravitational field, he doesn't need to exert any reaction force on anything and feels no weight. It took Einstein 10 years to develop general relativity but this theory has been able to make predictions that are continuously being tested. This year, the existence of gravitational waves was confirmed. However, general relativity is an incomplete theory. It has made predictions which it cannot explain. One of these predictions are black holes.