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No. 3012:
Newton, God and Gravity

by Andrew Boyd

Today, we get pulled in. The University of Houston presents this series about the machines that make our civilization run, and the people whose ingenuity created them.

Sir Isaac Newton mechanized the world. His theory of gravity swept away the need for a divine hand guiding the planets. And he unleashed a fervent search for simple rules governing the universe — a search that transformed not only physics, but chemistry, biology — virtually every aspect of scientific inquiry.

But while Newton's work freed planetary motion from dependence on a deity, it solidified his own personal belief in God. How so?

Newton is depicted critically as a "divine geometer". Photo Credit: Wikimedia Commons

The best explanation comes from a series of letters written by Newton to Richard Bentley, a leading theologian and scholar of the day. Newton had no problem with the purely mechanical way in which gravity explained the ongoing motion of the planets. What troubled him were questions about how the planets got their motion in the first place. And not metaphysical questions, but scientific.

Newton considered various scenarios of how the solar system might have formed. If it was formed by gravity pulling together tiny bits of matter spread throughout the universe, why would there be planets? Specifically, why wouldn't all the matter get pulled into a single great mass? Further, why was the sun the one and only body in the solar system to give off heat and light?

Newton then considered the possibility that the planets were wandering comets trapped by the sun's gravity. But here, too, he quickly saw the problems. For one, the planets have very orderly orbits relative to one another. Mercury's orbit is inside Venus's, Venus's inside the earth's, and so on. The chances of random comets having this property defy all probability. Comets captured at random would almost certainly have crossing, highly eccentric orbits.

Comet orbit between Earth and Jupiter Photo Credit: Wikimedia Commons

Even more, all of the planets have orbits that lie in a plane. That just couldn't happen by chance alone. "The motions which the planets now have," wrote Newton to Bentley, "could not spring from any natural cause alone, but were impressed by an intelligent Agent." Here, Newton capitalizes the word "Agent." And as an unapologetic Christian, there's no doubt to whom he was referring.

Today we have answers for Newton's concerns. Astrophysicists tell us the solar system began as a whirling disk of small particles, like barley soup stirred in a shallow pan. Over time, most of the matter was attracted to the center and formed the sun. But some smaller quantities clustered together to form planets. Theories go so far as to explain the size and composition of planets in relation to their distance from the sun. The sun alone shines in the sky because of nuclear reactions brought about by tremendous gravitational forces inside the massive body.

And herein lies an inescapable irony. The root solution to everything that troubled Newton stems from one source: his very own theory of gravity.

Flybrid Systems
Artist's impression of the birth of the Solar System Photo Credit: Wikimedia Commons

I'm Andy Boyd at the University of Houston, where we're interested in the way inventive minds work.

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Newton's Philosophy of Nature: Selections from His Writings. H. Thayer., ed. Mineola, New York: Dover, 2005. Originally published by New York: Hafner, 1953. See, in particular, Section III: God and Natural Philosophy.

This episode was first aired on July 2, 2015