General Relativity

Einstein liked his theory of special relativity, but he wasn’t content with it. Special relativity was only concerned with inertial reference frames, and Einstein wanted to make a theory that was compatible with all reference frames. In other words, he sought to include acceleration into his theory. He began his quest for a new, more generalized theory in 1907, and after eight years of blood, sweat, and tears, he published his theory of general relativity, a theory which has served as one of the pillars of modern physics ever since.

The mass of the Earth bending space-time around it, creating gravity. Graphic created by Wikimedia user Mysid.

In his law of universal gravitation, Isaac Newton stated that any two objects in the universe attract each other, and that attraction is due to the intrinsic mass of the objects. He didn’t know why¬†they attracted each other, but he just knew that they did, and that the more massive the object and the closer the object was to neighboring objects, the stronger the attraction was.

When coming up with his theory of general relativity, Einstein actually apologized to Newton in his notebook, writing “Newton, forgive me. You found the only way which, in your age, was just about possible for a man of highest thought and creative power.” In his theory, Einstein finally had an explanation for why¬†gravity existed. In Einstein’s theory of general relativity, gravity is not simply an innate force between two objects. Instead, gravity is a consequence of the influence of mass on space-time. Mass, he proposed, curves space-time, and this curvature is felt as the “force” of gravity.

Take a look at the diagram of the Earth above. Due to its mass, it bends space-time, with the most bend occurring closest to the Earth. Now, imagine that you have a ball rolling on the space-time surface. It will roll towards the Earth, and will roll fastest when it is closest to the Earth, where the slope of space-time – and therefore, the force of gravity – is the steepest/strongest. Of course, this ball, which has mass, will also bend space-time. Technically, this bend extends throughout the entire universe, but the effect of the Earth’s mass on the curvature of space-time is negligible once you get out of our solar system, so it’s really negligible in galaxies billions of light-years away.

The gravitational fields of different objects. Credit: NASA

The gravitational fields of different objects. Credit: NASA

The fact that mass bends space-time has some important implications, with two of the most important being the theorized existence of black holes and gravitational waves. Black holes are regions where the curvature in space-time, and by association, the gravitational force, is so drastic that nothing, not even light, can escape past a certain point, known as the event horizon. In fact, all the mass of the black hole is located at the “singularity,” which has no volume, infinite density, and infinite space-time curvature. Black holes had been theorized and indirectly observed but never observed directly until September 2015 via the first direct observation gravitational waves.

To learn about gravitational waves, click the picture below of gravitational waves created by two black holes orbiting each other.

Gravitational waves produced by two black holes orbiting each other Credit: NASA (retrieved from

Gravitational waves produced by two black holes orbiting each other
Credit: NASA (retrieved from

Written by Charlie Phillips – Last updated 12/1/2017