Thursday, March 6, 2008

Newton’s Law is not just a good idea

In Principia, Newton proposed that the force of gravity exerted by objects upon one another is proportional to the mass of the two objects, and weakens as the square of the distance between those objects.

Specifically, he postulated that the gravitational force between two objects is directly proportional to the product of their masses (mass of object A times mass of object B). So two objects, one very massive and the other with very little mass, will “feel” the same mutual attraction. In addition, he claimed that the force between two objects will decrease in proportion to the square of the distance. This “inverse-square law” means that the force of gravity mutually exerted by two objects, say 10 units of distance apart, is 100 times (102) weaker than that exerted by objects only 1 unit apart—yet this force never reaches zero.

The most distant galaxies in the universe exert a gravitational pull on one another. These relations between mass, distance, and force comprise what we call Newton’s Law of Universal Gravitation. Consider the solar system, with the planets moving in elliptical orbits around the sun. Newton’s Principia explained not only what holds the planets in their elliptical orbits (an “inverse-square” force called gravity), but also predicted that the planets themselves (massive Jupiter in particular) would have a small but measurable effect on each other’s orbits.

Like any good scientific theory, Newton’s laws not only explained what was already observed (the motion of the planets), but was able to make testable predictions. The orbit of Saturn, for example, was known to deviate slightly from what one would expect if it were simply in orbit around the sun (with no other planets present). The mass of Jupiter has a small, but measurable, effect on its orbital path. Newton noted with a sense of humor that the effect of Jupiter on Saturn’s orbit made so much sense (according to his theory) that “astronomers are puzzled with it.” For the first time, a scientist had claimed that the rules of motion on the earth were no different from the rules of motion in the heavens.

The moon was just a big apple, much farther away, falling to the earth in its own way. The planets orbit the sun following the same rules as a baseball thrown up into the air, and the pocket watch of the earth is held in its orbit by a chain called gravity. Did Newton bring the celestial sphere down to Earth, or elevate us all to the status of planets? Whatever you think, we have never looked at the solar system or the universe in the same way since.

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