 Einstein's theory uncovered the nature of space and time, or spacetime, which is the fabric the universe is made of. Mass deforms this fabric, and the curvature of this deformation gives rise to gravity. As an analogy, think of spacetime as a trampoline. If you place masses onto the trampoline, the fabric will tend to deform around the objects. If you place the objects close to one another, then they'll tend to roll toward one another, and if there's a light mass and a heavy mass, then the trampoline will curve more around the heavier mass, and the lighter mass will tend to fall into the heavier mass's deformation. Obviously the heavier mass also moves toward the lighter mass, but to a smaller extent. This matches the behavior we intuitively associate with gravity, and the effects propagate through spacetime at the speed of light in a vacuum, as we might expect from a thought experiment previously. One area in which Einstein's theory of general relativity differs from Newton's law of universal gravitation is that light, like everything, travels along the curvature of spacetime. We have actually measured light deflection around our sun as it travels through spacetime. Since light has no mass, Newton's law of universal gravitation could never predict that light would actually experience a gravitational force. In most circumstances, instead of general relativity, we can rely on special relativity, which is valid for weak gravitational fields, but breaks down in strong gravitational fields. This is similar to how Newtonian or classical mechanics breaks down at very high speeds. So when would we need to understand general relativity? While one of the results from general relativity that couldn't be predicted by Newton's law of universal gravitation, or by special relativity, is gravitational waves. It was announced in 2016 that these had been directly detected by the Advanced LIGO project. Gravitational waves are like ripples in spacetime that are radiated from objects with particular motion characteristics. These waves travel at the speed of light and literally stretch and contract the fabric of space. However, they generate only tiny changes in space. Even extremely large masses giving rise to the strongest gravitational wave sources only produce gravitational waves that stretch and contract spacetime by an amount far smaller than the width of an electron. Currently, we've only just proved that we can directly detect gravitational waves from Earth. In the future, we can build bigger and better detectors, including by improving the Advanced LIGO detectors and by building detectors such as ELISA in space. These detectors can give us information about our universe that isn't possible to get from other sources. For example, if there was an exploding supernova behind a gas cloud, then the light from the supernova may get scattered in the cloud. In this case, we wouldn't be able to study the explosion very easily using light or electromagnetic radiation that was scattered. However, if we had an amazing gravitational wave detector, then we could still see the supernova exploding, just using a different sense from our usual sight. This could be done using gravitational waves, which are an active area of research in the physics community.