*This story came from our special January issue, "The Beginning and the End of the Universe.” Click here to purchase the full issue.*

In ancient times, scholars such as Aristotle thought that heavy objects would fall faster than lightweight objects under the influence of gravity. About four and a half centuries ago, Galileo Galilei decided to test this assumption experimentally. He dropped objects of different masses from the Tower of Pisa and found that gravity actually causes them all to fall the same way. More than 300 years later, Albert Einstein was struck by Galileo’s finding. He realized that if all objects follow the same trajectory under gravity, then gravity might not be a force but rather a property of space-time — the fabric of the universe, which all objects experience in the same way.

In one of the most important advances in modern physics, Einstein recognized that when space-time is curved, objects do not follow straight lines. He reckoned that Earth, for example, orbits the Sun in a circle because the Sun curves space-time in its vicinity. This is similar to the path of a ball on the surface of a trampoline whose center is weighed down by a person.

In November 1915, Einstein published the mathematical equations that established the foundation for his general theory of relativity. These equations describe the link between matter and the space-time in which it resides, showing that mass deforms space-time and influences the path of matter. In the words of physicist John Wheeler: “Space-time tells matter how to move and matter tells space-time how to curve.”

## JOIN THE DISCUSSION

Read and share your comments on this article