Gravitation

What moves: Newton or the apple?

Late in the summer of 1666, a seemingly insignificant event occurred that has shaped our thinking about the universe ever since. A young man named Isaac Newton was sitting under an apple tree pondering the various mysteries of physics and astronomy that might lead to the ultimate truths of Natural Philosophy. Suddenly, an apple ripened by the summer sun broke loose from its stem directly over his head. Approximately one second later, a collision occurred between the apple and Newton’s head.

Newton immediately jumped to the conclusion that the apple had moved downward from the tree limb and struck him on the head. Perhaps he never even considering the opposite possibility that he had moved upward and struck the stationary apple. He went on to make the metaphysical assumption that the cause of this change in motion was a force field, or mutual attraction that existed between the earth and the apple, and therefore between all other bodies of matter in the universe.

Newton then used this assumption as the basis for his Principle of Universal Gravitation. This theory stated that all bodies in the universe are pulled toward all other bodies with a force that is directly proportional to the product of their masses and inversely proportional to the square of the distance between them. The force was seen as a perfect link of infinite reach between each particle in the universe. From this, Newton concluded that if such a force did exist, it could be used to account for the motions of heavenly bodies by acting in conjunction with his three laws of motion.

Once constituted, Newton’s “law” of Universal Gravitation worked very well, and could be used to predict the motions of heavenly bodies with near perfection. Although Newton was unable to account for this apparently infinite “action at a distance,” the idea worked so well for explaining celestial mechanics that he probably never seriously questioned his fundamental assumption that gravity was caused by an “attraction.”

However, if an experiment was performed to determine what really happened under the apple tree, the evidence would not necessarily confirm Newton’s conclusion. Let’s consider an experiment in which accelerometers are used to measure the absolute motion that takes place in this event. One accelerometer is attached to the apple and a second accelerometer is placed on the ground beside Newton. Before the apple breaks loose, both accelerometers register an upward acceleration of 9.83m/s2. At the moment that the apple breaks loose from its stem, its accelerometer registers virtually no acceleration at all. Then as the distance between the apple and Newton’s head begins to decrease, the apple’s accelerometer begins to register a small upward acceleration that gradually increases until the moment just before the apple and Newton’s head collide. At the moment of the collision, the apple’s accelerometer registers a short burst of upward acceleration considerably greater than the steady upward acceleration of 9.83m/s2 registered by the accelerometer at Newton’s side.

If we simply accept the most basic interpretation of this experiment, then we can easily determine what really happened in that apple orchard over three hundred years ago. After breaking loose from its stem, the apple remained motionless (except for a slight upward acceleration caused by air resistance) while the earth, the air, the tree, and Isaac Newton accelerated upward at the rate of 9.83m/s2, until Newton’s head struck the nearly motionless apple as it floated upward on the rising column of air.

If this explanation of the event is accepted, then we must conclude that gravity is just what it appears to be, a phenomenon of constant upward acceleration that produces real motion through absolute space. Furthermore, if the earth’s surface is constantly moving away from its center, we must conclude that the earth, as well as all other matter, is constantly expanding in size; and it is this expansion that causes the phenomenon we know as gravity.


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