###### Blog

26 February 2015

Gravity is perhaps the best known of the four fundamental forces. It’s also the one that’s easiest to understand. At a basic level, gravity is simply the mutual attraction between any two masses. It’s the force that lets the Sun hold the planets in their orbits, and the force that holds the Earth to you. The force is always attractive, and the strength of the force between two masses depends inversely on the square of their distances, making it an inverse square force. But gravity’s simplicity is just a veneer that hides a deeply subtle and complex phenomenon.

When Newton proposed his model of universal gravity, one criticism of the model was how gravity could act at a distance. How does the Moon “detect” the presence of Earth and “know” to be pulled in Earth’s direction? A few ideas were proposed, but never really panned out. Since Newton’s model was so incredibly accurate, the action-at-a-distance problem was largely swept under the rug. Regardless of how masses detected each other, Newton’s model let us calculate their motion. Another difficulty came to be known as the 3-body problem. Calculating the gravitational motion of any two masses was straight forward, but the motion of three or more masses was impossible to calculate exactly. The motion could be approximated to great precision, and was even used to discover Neptune, but an exact, general solution for three masses would never be found. Newton’s idea was simple, but it’s application was complex.

In the early 1900s, we found that gravity wasn’t a force at all. In Einstein’s model, gravity isn’t a force, but rather a warping of spacetime. Basically, mass tells space how to bend, and space tells mass how to move. General relativity isn’t just a mathematical trick to calculate the correct forces between objects, it makes unique predictions about the behavior of light and matter, which are different from the predictions of gravity as a force. Space really is curved, and as a result objects are deflected from a straight path in a way that looks like a force.