From the collision of galaxies to the birth of black holes, the creation of a multi-million dollar Hollywood movie to the dropping of fruits on unaware scientists, gravity governs the interaction between all sorts of massive objects. With its effects resonating throughout the fabric of our observable universe, it is surprising to acknowledge just how little we know about a force this dominant and palpable. 

Our present understanding of gravity tells us that it is an attractive force inherent and directly proportional to the mass of a body. Such was also dictated by Sir Issac Newton in 1687 when he mathematically translated this ubiquitous force in his Law of Universal Gravitation1 which goes as follows:

This is in accordance with his statement that goes “Every particle of matter in the universe attracts every other particle with a force that is directly proportional to the product of the masses of the particles and inversely proportional to the square of the distance between them.”

Now while Newton’s portrayal of gravity treasures practical applicability and adequate accuracy when viewing planetary systems such as our own, when translated to large dimensionless parameters, it is found to be insufficient to explain the high-velocity and high-gravitational-potential nature of bodies. Not just that, Newton’s theory fails to fully encapsulate the rate of the planetary shift in for orbit of Mercury2. The dilemma of the unexplained “wobble” of Mercury’s orbit was such that astronomer Urbain Le Verrier, who is credited to have purely mathematically predicted the existence and placement of Neptune, claimed that there existed a so-called “other” planet he named Vulcan whose gravitational attraction was responsible for the 43 arcsecond-per-century calculational discrepancy (which was obviously proven wrong). 

Both these problems were resolved by Albert Einstein’s Theory of General Relativity in which he described gravity as a warping of the fabric of spacetime instead of a force between two bodies. He claims that energy and momentum distort spacetime in their surroundings and other bodies move in trajectories defined by the geometry of the curved spacetime. 

Einstein even called gravity a pseudo-force that does not arise from any physical interaction between two objects. This is where we confront the puzzling nature of gravity. 

The art of pattern recognition has been the sole ability that drives advancements in human knowledge and technology. This dream of fitting things into patterns to understand them and arrive at a single unified theory of physics is hindered by the reluctance of gravity to comply with the standards set by its sibling forces. In comparison, gravity is weaker than the others by 36 orders of magnitude. Gravity is also unidirectional in being only attractive, which is in contrast to the nature of the other three fundamental forces where the electromagnetic forces have both positive and negative charges, weak nuclear forces have hypercharges, and strong forces have ‘colour’ with the latter two having multiple values.

Not just this, in quantum mechanics everything is described as a particle, even the three complying fundamental forces, be it using gluons, photons, or bosons. But gravity is not. The existence of a hypothetical ‘graviton’3 is merely conjecture that has not yet been experimentally validated. Plus, this quantum mechanical definition of gravity clashes with Einstein’s general relativity which is a theory that has evidence backing it. Maybe the only way we will be able to resolve this problem would be with a surprise visit to a supermassive black hole4 which, while surely does seem enticing, doesn’t look likely in the coming future. 

Maybe gravity is special and unlike any of the other fundamental forces we know so far. Or maybe it is not. Maybe the now-hypothetical graviton is merely waiting for us to discover and acknowledge its existence. Either way, an understanding of the nuances of gravity would drastically impact our understanding of the cosmos. Noting that gravity seems to be the only force acting on such dimensionally grand scales, it would prove to be critical in determining the fate of our universe. If one day, we ever go so far as to manipulate this force per our command, then travelling through folded spacetime into distant corners of our universe may not just be a dream. 

Our limited knowledge of gravity doesn’t always have to bring us down, it can also dramatically revolutionise our notions of reality. I guess we’ll just have to weight.

Who said gravitational forces always keep your feet on the ground?

Citations:

1. https://youtu.be/kxkFaBG6a-A
2. https://www.scienceabc.com/nature/universe/vulcan-the-planet-between-mercury-and-the-sun-that-was-never-there.html
3. https://www.quantamagazine.org/gravitons-revealed-in-the-noise-of-gravitational-waves-20200723/
4. https://youtu.be/Zo4gRhG90S4