Gravity through the Ages

 

Aristotle’s Gravity, or Gravitas?

Way back in the day, thousands of years ago, Aristotle thought that there was a reason for motion, that something was either heavy and fell or light and floated. Consequently, he believed that heavy objects experience greater acceleration than light ones. This idea prevailed until the Renaissance.

It took a man of talent in experimentation to combat this ancient idea: Galileo Galilei. Legend tells of Galileo refuting Aristotle’s idea with a demonstration of an orange and a grape falling in unison from the top of the Leaning Tower of Pisa. Although, like all legends, the demonstration might not have taken place, but we like to imagine it did. Not so legendary, the unsuspecting tourist that was hit with two pieces of fruits while visiting the Leaning Tower of Pisa. Worst. Vacation. Ever.

Our “modern” understanding of gravity began with Nicolaus Copernicus, who, in 1514, published his calculations on planetary motion. Copernicus’ work marked a departure from the then-accepted view of the universe as being Earth-centric. Copernicus proposed the idea of heliocentrisim, the idea that all the known planets orbit around the sun.

Such a novel idea at the time paved the way for further investigation into the real, perhaps misunderstood nature of these orbits and the forces that produced them. That’s how Galileo found himself dropping fruit from very high places, possibly onto innocent tourists. Galileo championed Copernicus’ theory and built on his work.

Galileo also gave the world the telescope. A contemporary of Galileo, Johannes Kepler, used the telescope to directly observe celestial bodies and expand upon Copernicus’ new sun-centric model. We celebrate Kepler for his laws of planetary motion, which say that planets follow elliptical, rather than circular, orbits. Mercury’s was the only orbit left not entirely explained.

In 1687, Sir Isaac Newton took the matter further with the inverse-square law for gravitational force. Oh, yeah, and his gravitational constant G is quite useful too. In other words, he’s the brilliant human that generalized gravity mathematically. Don’t hate him. We all have our own talents.

Fast forward a little over a hundred years. In 1797, Henry Cavendish used a clever torsion balance apparatus to directly measure the gravitational attraction between two (very carefully) suspended balls. The gravitational constant G was repeatedly measured and refined from there, with more and more accuracy as increasingly modern instrumentation allowed.

It was not until 1915, however, that Albert Einstein turned our understanding of gravity on its head with General Relativity, and the anomaly within Mercury’s orbit, which doesn’t follow Kepler’s laws, was explained at last. General Relativity continues to provide our best understanding of gravity.