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Natural Science - Year I

Unit 32: Newton and Universal Law

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Unit 32: Universal Gravitation

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Lecture outline:

Universal Law

We come now to one of the foundations of modern science: the concept of universal laws. Newton's work with gravity convinced many people that Nature followed certain immutable laws, many of which are very simple when expressed mathematically but which have profound implications for many events and situations. A universal law describes a relationship which applies at all times in all places the same way to all objects. We consider the law of gravity to be such a law, and conclude that the galaxies move the way that they do because they are mutually attracted to each other according to the set formula F = GMm/r2.

To some degree, Newton is inverting the assumptions Aristotle and the Greeks made about the relationship of mathematics to the physical world. Aristotle believed that mathematics dealt only with abstractions, and through the middle ages, philosophers debated on whether it was correct to create a mathematical description of the physical world. In his Principia Mathematica, Newton declares that geometry procedes from mechanical (that is, the forces resulting in motions of bodies), and therefore it is appropriate to use mathematical propositions to describe physical entities and their movements.

Read the preface to Newton's Principles of Mathematics.

  • What does Newton consider the difficulty of philosophy?
  • What motions has he been able to explain based on his mathematical principles?
  • How was Newton's theory first published? To what group?

One of the results of the concept of universal law is the idea of determinism, which says that if we could just discover all the laws and knew all the starting conditions, we could predict outcomes of situations, and if we could alter initial situations, we could control the outcomes.

In many ways, the technologies that we build on scientific principles bear out this conviction. Planes fly, televisions broadcast pictures, brakes stop cars within a safe distance, genes control protein breakdown. This facility makes our "modern life" possible but it also raises challenges. How much does the current situation control the future? Do we have free will to make other choices?

The scientists who followed Newton debated these ideas at some length. Some moved toward complete determinism, others believed in a deterministic universe but not in our ability to observe it in all its complexity with sufficient accuracy to predict outcomes very far into the future. In the nineteenth century, however, the mechanical view of the universe began to break down....but that's a story for another year.

    According to the Wikipedia article, modern physicists Feynman and Davies characterize physical laws or laws of nature as true, universal, simple, absolute, stable, eternal, omnipotent, conservative (generally), often expressing symmetry and are reversible in time.
  • Do you agree that a physical law can have all these characteristics? Are there problems with this definition?
  • How are physical laws different from theories?
  • If a physical law describes an approximation, how is it true?

Study/Discussion Questions:

Further Study/On Your Own