A Closer Look at Newton's Gravity

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Newton found that his gravity law is obeyed everywhere in the universe and could explain Kepler's three laws of orbital motion. Newton's development of the unifying law of gravity was also the culmination of a process of Occam's Razor in action. From Ptolemy to Newton, the theories of how the planets move got simpler and more powerful as time went on. Ptolemy's model had become extremely complicated by the time of the Renaissance and Copernicus reduced the number of circular motions to around 50 so it was simpler to use. Kepler vastly simplified the theory of planet motion by reducing the number of essential parts to just three laws. Newton unifed all of those laws to the ONE unifying law of gravity. This law was so simple and elegant that it could also explain motions on the Earth.

But what is gravity? Newton understood how the gravity force affected the motion of objects but not why gravity worked the way it did. Recognizing the limits of his knowledge, he adopted an instrumentalist view: the scientist's job is to capture observations in precise mathematical equations; explain the ``how'' not the ``why''. Only things verified by our experience of the world are admissible into science. Though the ``why'' question is intriguing and a few scientists will spend years trying to answer it, most scientists share Newton's instrumentalist view.

With Newton, there was no longer a hierarchical-teleological universe (one designed by God for some purpose with man playing a crucial role in the plan). The universe was now a perfect machine, based on mathematics, set in motion by God long ago. God is the reference point for absolute space and time. Newtonian mechanics requires an absolute coordinate system to keep things sensible (according to Newton this also gave God something to do).

With the success of Newton's ideas, a major change occurred in how people viewed the world around them. Reality was completely reduced to material objects. Ideas, thought, feelings, and values were secondary. What is real does NOT depend on us---this is probably the actual completion of the Copernican revolution and was soon so widely accepted that it became ``common sense'' (how about that for a paradigm shift!). Newtonism seemed to undercut the role of God and religion and the validity of science: science became just a subjective perspective of the machine universe.

Decades before Newton, Descartes saw the need to rescue thoughts, ideas and values. Descartes developed a mind-body dualism: a world of thought and spirit exists independent of, but parallel to, the material world. There is a correspondence between the God-inaugurated, mathematical thoughts of scientists and the motions in the physical world. Descartes said that mathematical ideas work so well because there is a pre-established parallelism between the physical world and the human mind.

Newton's machine universe with God establishing the reference system of absolute space and time gave Newton and those that followed the conviction that we could eventually understand the universe. In fact, some (such as as Spinoza) saw our attempt to understand that universe through the tools of science and mathematics as a way of studying God Himself. Whether or not a scientist sees science as a religious endeavor, the emphasis is on an external material world that is independent of us. There is an objective reality and our thoughts and values do not affect this reality.

Review Questions

  1. What important discoveries and ideas did Newton make?
  2. How does Occam's Razor relate to the progress of planet motion theory from Ptolemy to Newton?
  3. How can Newton's work be considered the completion of the process started by Copernicus almost 120 years earlier?

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last updated: June 12, 2014

Is this page a copy of Strobel's Astronomy Notes?

Author of original content: Nick Strobel