From its conception in 1916 by physicist Albert Einstein, the theory of relativity has been the cornerstone of modern-day physics.
General relativity is the idea that space and time are not separate. Time being a physical dimension and thus both forming a single geometry of space-time. From this emerged a new branch of mechanics known as relativity mechanics and thus leading to various hypotheses concerning the origins of gravity (the conservative force acting on objects in a gravitational field).
The concept of Gravity, its laws, and its causes are thus key features that are defined in General Relativity. The idea that all objects in space are relative to each other establishes that the concept of motion is only applied to objects moving relative to each other. For instance, generalizing the belief that the velocity of Earth is 30 kilometers per second can only be held true when it is taken relative to that of the sun, and its position as a center for each consecutive revolution.
Just like species on Earth have lifespans that defer from species to species - that is one species such as that of humans may outlive a horse’s life span by almost double; similarly, time around various planetary species in a galaxy is dilated as a result of the bending of space-time and therefore time itself. Thus, time being a physical constant will pass faster when closer to planetary species of greater magnitude of mass and slower as the distance from the planetary species increases.
To address what causes this disturbance in space and time (the bending effect of space-time), the belief that space-time is curved in all concerning dimensions needs to be eluded. Just like a spherical object such as a ball is generally flat when observed closely, curved space-time is also locally flat. Although space in 3Dimention is flat, in order for space-time to obey such restrictions it is necessary for it to obey the rules of special relativity.
The idea of special relativity is rather a body experiencing or being observed as part of a much larger planetary species instead of looking at the respective planetary species and the object. Mathematicians refer to this as the “Pseudo-Riemannian Manifold with Lorentzian Signature”.
Curvature in space-time doesn’t explain gravity on its own. It must be remembered that it is not necessary for an object of a mass to be smaller than the planetary species, such as, Earth, to follow a specific path relative to it.
From this emerge two presumptions: general relativity is the culmination that space-time is curved and that objects part of this space-time traveling at speeds way below the speed of light (3.0x10^8 m/s) obey the elementary laws of motion put forth by sir Isaac Newton.
An object in motion stays in its state of uniform motion following a straight path such as that on the surface of Earth even though it is curved in nature. In order to determine what causes this curvature and why objects tend to follow a straight path, general relativity is actually the idea that components of space: particles, momentum, pressure, radiation, etc. together with space-time itself obey a set of equations known as Einstein’s field equations.
Field equations may seem very straight forward when put forward, but they are derived from 10 different nonlinear equations in order to predict how space and time curves and how objects relative to it move.
So general relativity is the theory that the universe can be defined by a “Pseudo-Riemannian Manifold” and the laws of motion that governs objects in motion in space-time. Observations such as that of the deviation of the moon from orbit, to the gravitational lensing and red shift of starlight, and the time dilation around black holes explain the phenomenon of curved space-time and the changes in it as objects encounter gravitational fields.
But how does this explain gravity we experience on Earth? Consider this example: a vehicle turns around a roundabout, causing us to feel a sudden force compelling us to lean/accelerate in the opposite direction (inertia) of the path of motion of the car. In general relativity the initial straight path of an object is the centre of Earth and since the surface of the earth prevents us from falling towards it, we feel this acceleration as gravity. Objects in space on the other hand follow a straight path in curved space-time that is they are not accelerating so they are said to float in space.