Sophie thought deeply. Then she said, "Because the board was sloping, the marble was drawn toward the floor by the force of gravity."-"Yes, yes! This is nothing less than a sensation! Here I go, dragging a girl who's not yet fifteen up to my attic, and she realizes exactly the same thing Galileo did after one single experiment!"
He clapped his hands. For a moment Sophie was afraid he had gone mad. He continued: "You saw what happened when two forces worked simultaneously on the same object. Galileo discovered that the same thing applied, for instance, to a cannonball. It is propelled into the air, it continues its path over the earth, but will eventually be drawn toward the earth. So it will have described a trajectory corresponding to the marble's path across the inclined plane. And this was actually a new discovery at the time of Galileo. Aristotle thought that a projectile hurled obliquely into the air would first describe a gentle curve and then fall vertically to the earth. This was not so, but nobody could know Aristotle was wrong before it had been demonstrated."
"Does all this really matter?"
"Does it matter? You bet it matters! This has cosmic significance, my child. Of all the scientific discoveries in the history of mankind, this is positively the most important."
"I'm sure you are going to tell me why."
"Then along came the English physicist Isaac Newton, who lived from 1642 to 1727. He was the one who provided the final description of the solar system and the planetary orbits. Not only could he describe how the planets moved round the sun, he could also explain why they did so. He was able to do so partly by referring to what we call Galileo's dynamics."
"Are the planets marbles on an inclined plane then?"
"Something like that, yes. But wait a bit, Sophie."
"Do I have a choice?"
"Kepler had already pointed out that there had to be a force that caused the heavenly bodies to attract each other. There had to be, for example, a solar force which held the planets fast in their orbits. Such a force would moreover explain why the planets moved more slowly in their orbit the further away from the sun they traveled. Kepler also believed that the ebb and flow of the tides-- the rise and fall in sea level--must be the result of a lunar force."
"And that's true."
"Yes, it's true. But it was a theory Galileo rejected. He mocked Kepler, who he said had given his approval to the idea that the moon rules the water. That was because Galileo rejected the idea that the forces of gravitation could work over great distances, and also between the heavenly bodies."
"He was wrong there."
"Yes. On that particular point he was wrong. And that was funny, really, because he was very preoccupied with the earth's gravity and falling bodies. He had even indicated how increased force can control the movement of a body."
"But you were talking about Newton."
"Yes, along came Newton. He formulated what we call the Law of Universal Gravitation. This law states that every object attracts every other object with a force that increases in proportion to the size of the objects and decreases in proportion to the distance between the objects."
"I think I understand. For example, there is greater attraction between two elephants than there is between two mice. And there is greater attraction between two elephants in the same zoo than there is between an Indian elephant in India and an African elephant in Africa."
"Then you have understood it. And now comes the central point. Newton proved that this attraction--or gravitation--is universal, which means it is operative everywhere, also in space between heavenly bodies. He is said to have gotten this idea while he was sitting under an apple tree. When he saw an apple fall from the tree he had to ask himself if the moon was drawn to earth with the same force, and if this was the reason why the moon continued to orbit the earth to all eternity."
"Smart. But not so smart really."
"Why not, Sophie?"