Sir Isaac Newton and the
Unification of Physics & Astronomy

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Sir Isaac Newton (1642-1727) was by many standards the most important figure in the development of modern science. Many would credit he and Einstein with being the most original thinkers in that development.

The Accomplishments of Newton

Newton's accomplishments were of astonishingly broad scope. For example, as a sidelight to his fundamental contributions in physics and astronomy, he (in parallel with Liebnitz) invented the mathematical discipline of calculus, so if you have to take both physics and calculus courses, you have Newton to blame! No survey course such as this one can possibly do justice to what Newton accomplished. The poet Alexander Pope was moved to pen the lines

Nature and Nature's laws
lay hid in night;
God said, Let Newton be!
and all was light

and a study of Newton's discoveries suggests that Pope was indulging only slightly in hyperbole. We shall concentrate on three developments of most direct relevance to our discussion: (1) Newton's Three Laws of Motion, (2) the Theory of Universal Gravitation, and (3) the demonstration that Kepler's Laws follow from the Law of Gravitation.

The Great Synthesis of Newton

Kepler had proposed three Laws of Planetary motion based on the systematics that he found in Brahe's data. These Laws were supposed to apply only to the motions of the planets; they said nothing about any other motion in the Universe. Further, they were purely empirical: they worked, but no one knew a fundamental reason WHY they should work.

Newton changed all of that. First, he demonstrated that the motion of objects on the Earth could be described by three new Laws of motion, and then he went on to show that Kepler's three Laws of Planetary Motion were but special cases of Newton's three Laws if a force of a particular kind (what we now know to be the gravitational force) were postulated to exist between all objects in the Universe having mass. In fact, Newton went even further: he showed that Kepler's Laws of planetary motion were only approximately correct, and supplied the quantitative corrections that with careful observations proved to be valid.