1. What is Gravity?

Gravity is one of the four fundamental interactions in the universe. This chapter deals with the Theory of Relativity, a non-classical description of gravity. Understanding Relativity is necessary for understanding frontier astrophysical phenomena such as black holes, pulsars, quasars, stars, and modern cosmology. Relativity also supports informational technology by calculating the errors which Newtonian physics cannot interpret. As one of the four fundamental interactions, understanding gravity is necessary for the theory of everything, and Relativity is the key to reaching this final answer.

 

Research on gravity is divided into two big parts. The methodology for studying microscopic and macroscopic gravity is different from its fundamental theoretical basis. This chapter only discusses macroscopic and astrophysical scale gravity but not the microscopic scale gravity often referred to as quantum gravity.

 

These four properties of gravity are important for understanding gravity. First, gravity is a universal interaction between all masses. Gravity works with earthworms crawling on the ground and black holes gobbling up the entire galaxy. Second, gravity is unscreened. Gravity only works as an interaction, there is no repulsion or negative gravitational charges to cancel out positive ones. Third, gravity is a long-range interaction that affects everything in the universe. There is no set range for gravity. With the first property, it is why gravity is the fundamental force supporting the major macrostructures of the universe. Last, gravity is weak. Compared to other interactions, gravity is notably weaker than the others. Richard Feynman once said "Gravity is weak. in fact, it is damned weak." 

 

These four properties allow gravity to exist as the giant holding the enormous structure of the universe and prevents it from crushing the universe into a ball of fire. Electromagnetism, weak force, and strong force cannot be such giants but manipulates the balance in the microscopic world. Understanding gravity brings us a more complicated understanding of the structure of the universe. Advancement in the theory of gravity is advancement in astrophysics.

 

The origins of gravity can be traced to the Laws of Isaac Newton. According to Newton, gravity is the force pulling two masses together defined as below.

 

$$ \vec{F}_{G}=G\frac{m_{1}m_{2}}{\vec{r}^{2}} $$

2.1.1.1. Newtonian Gravity

 

One of the most essential properties of Newtonian gravity is non-locality. The magnitude of gravity converges to zero as it gets far, and it propagates to every point in space instantaneously.

 

Einstein's special theory of relativity published in 1905 stated that non-local and instantaneous propagation of information is impossible. Since every piece of information cannot be sent faster than light, gravity had to be sent not faster than light. Therefore, Newtonian gravity is not the precise theory describing the essence of gravity but its approximation.

 

Einstein's general relativity published in 1915 defined gravity as the geometric curvature of spacetime and recognized matter as the major agent of curving. It is one of the largest footsteps mankind has left on the history of science and Einstein's methodology of research became the role model for future physicists. We will dissect Einstein's gravity in this chapter and try to explore the depths of relativity.