What causes gravity remains a mystery. Einstein’s general theory of relativity states a massive object curves space-time, causing other bodies to move toward it. This theory helps explain why gravity can act at a distance and why the strength of its force lessens as distance increases. Many questions about the exact nature of gravity still need answers.

Newton famously described the mechanics of gravity using the inverse square law, stating that the attractive force between two masses is proportional to the square of the distance between them. His equation works well, but never addresses why gravity works, or why the distance between the two masses matters. Einstein’s theory addressed the distance issue, showing how gravity works using the curvature of space-time, and why it gets stronger as the two objects get closer. Einstein’s theory also predicts gravitational waves that would propagate the force, but these remain theoretical.

Gravity is one of the least understood of the four fundamental forces that make up the Standard Model. Unlike the strong and weak nuclear forces that hold atoms together, the range of gravity is infinite, yet it is the weakest of the four forces. Electromagnetism can easily overcome the attraction of gravity, but gravitational force pulls matter together to form planets, stars and galaxies. Electromagnetism also works at a distance, but it can be blocked or transformed, while gravity remains constant and propagates uniformly. Electromagnetic forces attract and repel, but gravity only attracts. So, while the mechanics of gravity are well understood, the true nature of gravity has yet to be explained.