Two objects exert a force of attraction on one another known as “gravity.” Sir Isaac Newton quantified the gravity between two objects when he formulated his three laws of motion. Yet Newton’s laws assume that gravity is an innate force of an object that can act over a distance. In 1905, Albert Einstein determined that the laws of physics are the same for all non-accelerating observers, and that the speed of light in a vacuum was independent of the motion of all observers. This was the theory of special relativity. As a result, he found that space and time were interwoven into a single continuum known as space-time. Events that occur at the same time for one observer could occur at different times for another.
Einstein then spent ten years trying to include acceleration in the theory and published his theory of general relativity in 1915. In it, he determined that massive objects cause a distortion in space-time, which is felt as gravity. Einstein proposed that objects such as the sun and the Earth change the geometry. In the presence of matter and energy it can evolve, stretch and warp, forming ridges, mountains and valleys that cause bodies moving through it to zigzag and curve. So although Earth appears to be pulled towards the sun by gravity, there is no such force. It is simply the geometry of space-time around the sun telling Earth how to move.
Although instruments can neither see nor measure space-time, several of the phenomena predicted by its warping have been confirmed. Light around a massive object, such as a black hole, is bent, causing it to act as a lens for the things that lay behind it. Astronomers routinely use this method to study stars and galaxies behind massive objects. Einstein’s Cross, a quasar in the Pegasus constellation, is an excellent example of gravitational lensing. The quasar is about 8 billion light-years from Earth, and sits behind a galaxy that is 400 million light-years away. Four images of the quasar appear around the galaxy because the intense gravity of the galaxy bends the light coming from the quasar. The general theory of relativity also explains the motion of the planets and can also describe the history and expansion of the universe.