Gravitational Waves are much simpler to understand then perhaps at first thought so I will quickly explain and you will wonder why you weren’t sure.
Remember back to earlier days, perhaps reading of Einstein’s General Relativity theory, and looking at a drawing that represented the curvature of space. (such as the one directly below)
A a gravitational wave is little more than a ripple in the very fabric of time and space that is produced by really massive objects such as stars and planets, black holes,… Do you remember that ‘imagine-if-you-will’ moment of a tightly pulled rubber sheet and dropping a bowling ball on it. The point being that ripples will be created around the ‘sheet of rubber’ much as they are created around the Sun. The claim is that there is no actual force that pulls the objects (in this case the planets of the solar system) toward the Sun but rather it is the massive distortion in the space around them.
Consider that a gravitational wave is born whenever an object of considerable mass perturbs the space around it through the mass accelerating. This wave must be produced by supermassive object, such as the pair of black holes merging which cause LIGO to detect them. (It could also be something such as two Neutron stars moving around each other.)
Until this month, researchers were looking and hoping but were unable to locate Gravitational Waves through existing models. To better understand the difficulty, it has been described as being at a concert and over the crowd and the loud rock band hearing the voice of a person in the front row singing along when you are in the back row.
The fact that LIGO has detected gravitational waves is likely a consequence of the Lorentz invariance of general relativity since it brings the concept of a finite speed of propagation of the physical interactions with it. Of course, gravitational waves cannot exist in the Newtonian theory of gravitation, which postulates that physical interactions propagate at infinite speed. Imagine that, something considered Quantum has finally made it! (Okay, Einstein actually brought together Geometry and Matter, but the details scream Quantum!)
Where does it come from? It’s existence is a likely by-product of the Lorentz invariance of general relativity since it allows for the concept of a finite speed of propagation of the physical interactions with it. In Newtonian theory of gravitation, which postulates that physical interactions propagate at infinite speed, gravitational waves cannot exist.
Before the direct detection of gravitational waves, which were detected by LIGO, there was indirect evidence for their existence. For example, measurements of the Hulse–Taylor binary system suggested that gravitational waves are more than a hypothetical concept. Potential sources of detectable gravitational waves include binary star systems composed of white dwarfs, neutron stars, and black holes. Various gravitational-wave observatories (detectors) are under construction or in operation, such as Advanced LIGO which began observations in September 2015.
So what in the world, or Universe, is the “proof”, well, on February 11, 2016, the LIGO Scientific Collaboration and Virgo Collaboration teams announced that they had directly detected gravitational waves from a pair of black holes merging using the Advanced LIGO detectors.
Can LIGO even detect Gravitational Waves?
Good question and not going to make you happy. A Gravitational Wave is comprised of a rather large number of Gravitons – but measuring their individual membership is beyond current abilities. Consider a TV signal – it doesn’t come through if the signal were to come it individual pieces, it can’t resolve the images, but if it is swamped with a signal,… If gravitons exist, LIGO detects them, but it cannot distinguish the huge amount of gravitons from an unquantized gravitational wave; Therefore, LIGO cannot not tell us anything about the existence of gravitons.
A theoretical that most can agree on is that quantum gravitational effects should become large in regions of strong space-time curvature. But in the quantum gravity community, “strong curvature” means the curvature towards the center of black holes, not a curvature at the horizon, which by comparison is quite weak. A black hole merger, such as LIGO’s evidence, does not probe what happens in the black hole’s center, and therefore it does not test strong quantum gravitational effects.
*What am I trying to say since apparently I have shown you two sides of the coin? Well, while it is likely that Gravitational Waves exist, in my opinion, they haven’t been proven as yet. Do I believe they will be? Yes, but not using current observation methods.