Predicting Gravitational Waves

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Newton's Theory of Gravity

Albert Einstein is probably the world’s most famous scientist. Although his fame is well deserved, Einstein’s work was largely based on the work of other great scientists. One of these scientists was Sir Isaac Newton, a man often called the founder of modern physical science.

 Sir Isaac Newton. Image courtesy of the AIP Emilio Segrè Visual Archives, Physics Today Collection

Newton made many important contributions to math, physics, and astronomy, but he is best known for discovering the law of gravity. Much of Einstein’s work, including his prediction of gravitational waves, was inspired by Newton’s theory of gravity.

An often told story recounts Isaac Newton sitting under an apple tree one day, thinking about the universe. Suddenly an apple fell from the tree and hit him on the head. The startled Newton realized what had happened and, in that split second, he understood how the force of gravity pulled objects to the earth.

This story probably isn’t true, but it does have some truth in it. Newton’s observations about nature led him to the law of gravity. He realized that the force that pulls apples to the ground could also be what keeps the moon orbiting around the earth. Newton realized there must be an attractive force between all masses and he called this force gravity.

From his observations, Newton noticed that all objects are attracted to one another. The strength of the attraction is larger for large masses, but decreases as the objects move farther apart. He called this the law of gravity.

In his theory of gravity, Newton combined the work of the great scientists Copernicus ( 1473-1543) , Kepler (1571-1630), and Galileo (1564-1642). Newton’s theory solved many puzzles of his time, including the cause of tides, the motion of the earth and moon, and the orbit of comets.

Although Newton’s theory explained what gravity does, the question of why it works this way remained a mystery for the next 300 years.