Theory of Gravity: The Next Chapter

**Understanding the True Nature of Gravity**

How the Theory of Gravity ended up being the biggest mystery in Modern Physics

It's safe to say that the theory of gravity is one of the oldest universal theories ever to be studied, and ironically it is the only scientific theory whose progress has been such an emotional rollercoaster for the scientific community! 

To be more clear, currently, we're at that point where we know absolutely everything and nothing about gravity, and it has been this way since the introduction of a very successful giant branch in physics - Quantum Mechanics. 

Not only does gravity not fit within the framework of the quantum principles, but just trying to make it fit in seems to bring down the most successful explanation of gravity given by Einstein through his Theory of Relativity, which means that we're back at square one in figuring out the final theory of gravity! Before we jump right to the possibilities of finding an ultimate theory of gravity, let's discuss the timeline and evolution of gravity starting from the 17th century.

A Brief History of Gravity

Right from its introduction through the Universal Law of Gravitation by Newton in 1687, the notion of gravity still remains one of the most intriguing and mysterious features of our universe. From Newtonian gravity to Einstein's General Relativity to ideas like Quantum Gravity, it is obvious that the theory of gravity is still one of the most debated ideas in science, and unlocking the true nature of gravity would potentially lead us to decode the Ultimate Theory of the Universe! 

Up until the introduction of General Relativity in 1935, it was an established fact that gravity (or gravitation) was a universal force that obeyed the inverse square law formulated by Newton. This Newtonian interpretation of gravity seemed to be working just right within the limits of our local neighbourhood, by accurately predicting the orbital path of the moon and the extent of attraction between massive bodies. 

Image by Johnson Martin - Pixabay

But Newton did not have an actual reason as to why there was this attractive force between bodies in the first place. Did it arise out of something we can't observe? And why does this force act in an instantaneous manner that exists between two bodies? These questions remained unanswered for a while until the mid-1900s, when Einstein showed the world that gravity is not just a force, rather, it is a consequence of space-time curvature caused by massive objects in space. Due to this curvature in spacetime, objects tend to follow this curved path, called orbital paths, and every object tends to have its own effect on the fabric of spacetime by creating curvatures that its neighbouring objects tend to follow. 

This notion of gravity laid out by Einstein's Theory of Relativity was (and still is) considered the greatest, most elegant theory of gravity, mostly because it offers explanations to the unanswered questions posed by the Newtonian gravitational theory, and it perfectly explains the anomalies in the orbit of mercury, also which Newton failed to explain through his theory. That being said, Einstein's Theory of Relativity emerged as the center point of classical physics, which deals with the physics of the macro-universe. 

Image by Hans - Pixabay

Why a Final Theory of Gravity would potentially bring down Einstein's General Relativity

With the development of quantum mechanics in the 20th century, which deals with the physics of subatomic particles (or the micro-universe), the theory of gravity seemed to be out of the picture, the reason being that gravity is only explainable within the principles set under classical mechanics since gravity is a long-range (and the weakest) universal force that acts on massive bodies, unlike the other fundamental forces - the electromagnetic force, strong force and the weak force. 

Moreover, all of the fundamental forces in the universe, except gravity, seem to perfectly fit in the Standard Model of Particles, which explains the particles that constitute the building blocks of the entire universe, be it forces or matter. 

We know from the Standard Model that photons constitute the electromagnetic force, gluons make up the strong force and W and Z bosons form the weak force. And all these particles obey quantum mechanical principles and predict phenomena with great accuracy, unlike gravity, which, as far as we've observed, isn't made of any particles that possess quantum parameters like spin or charge (at least for now). 

This means that to make gravity fit in with the rest of the gang, we have to figure out a new boson (a force particle) that possesses its unique quantum parameters, by maintaining the same (or higher) level of accuracy that the classical interpretation of gravity holds. To be more clear, forging up a final theory of gravity rooted in quantum mechanics, one way or the other, will mark the end of Einstein's Theory of Relativity for good, thereby formulating the functioning of the universe governed entirely by quantum principles! 

But setting all these possibilities aside, are we close to developing a quantum theory of gravity? And can ideas like quantum gravity and string theory be ever observed by us in the near future? We can't exactly tell for sure...What we do know is that gravity will always remain the main character in the universe. Seriously, gravity gives the perfect rendition of main character energy - beautifully mysterious, complicated, leads the plot (the plot being the spacetime - *pun intended), and above all, just like every other main character, we think we have them all figured out all this time, but eventually showing us its most nerve-wracking and complex character development! 

Title of the movie you ask? 

Theory of Gravity: The Next Chapter

For more info on the topic, check out:

A Brief History Of Time - Stephen Hawking, Chapters - 5 & 11

The Grand Design - Leonard Mlodinow and Stephen Hawking, Chapter - 5