MAKING SENSE OF GENERAL THEORY OF RELATIVITY

We, on this Earth, come across so many wonderful phenomena, where we admire the universe. The events that we observe form the basis of physics. After all, it's all about our perception, and for me, it's ALL ABOUT PHYSICS. 
Laws of nature describe the phenomena as they appear in nature. They are universal in their acceptability, their applicability, their translation, and their philosophical basis and are therefore considered to be most legitimate.

Today, I am here to share the facts about the most apparent phenomenon. Let's gather, and let's try to resolve the mystery of gravity together. 

Our God has blessed us with big brains and opposable thumbs, and there are some traits of ours that no other creature on the Earth can match. We observe the events and actions happening around us. And on the basis of the events occurring in nature, rules and principles are formulated.

Falling apples, tidal energy, the existence of the solar system, our body weight, projectiles, the round shape of the Earth, specific gravity, positive and negative geotropism, and problems faced by astronauts in space like bone and muscle loss, lengthening of the spine, and back pain. According to evolution health, gravity compresses our spine so that we can become smaller in height as we get older. 

Before proceeding to the general theory of relativity, I would like to throw light on the history of the theory of gravitation.

Galileo Galilei discovered that bodies fall at a rate independent of masses. He demonstrated that the acceleration due to gravity of a celestial body is independent of the mass of the falling body.

Let’s take an example: suppose a feather and a steel ball both have unequal masses. They are released from a certain height (in a vacuum) on the surface of a planet. They will reach at the same time on the surface of that planet because the gravitational effect exerted on both the objects is the same.

Next came Newton; he concluded that force exerted by gravitation is proportional to mass of the body on which it acts. Force acting on a falling body is proportional to its mass

Then came Kepler, Kepler’s laws were in direct support to Newton’s laws. He said that planets actually move on ellipses, not circles. And no one knew how to calculate centripetal acceleration at that time.

Einstein was very impressed with the observed equality of gravitational and inertial mass. It served him as a signpost towards the principle of equivalence.

Now the question is, what is this principle of equivalence??

The principle of equivalence says that at any point in space-time, we may erect a locally inertial coordinate system in which matter satisfies the laws of special relativity.

Einstein tried to develop non-Euclidean geometry for gravity.

Non-Euclidean Geometry: Euclidean geometry, we all are aware of. For locating the position of an object, we need three coordinate systems. Three position coordinates are required to locate the position of any object. Non-Euclidean geometry includes time also to locate the position of an event.

An event is described by the place where it occurred and the time when it occurred. Thus an event occurring in a certain material particle is defined by the three position coordinates and the time when the event occurs. This four-dimensional geometry is for an event (the position coordinates will be assigned for locating the position of the event and the time when the event is occurring). Three coordinates for position are required and one for time. So, four-coordinate geometry was introduced by H. Minkowski in connection with the theory of relativity. This geometry is called pseudo-Euclidean in contrast to ordinary Euclidean geometry.

Anything or any event occurring in the universe can be described by spatial coordinates or space-time coordinates of that event.

Our universe can be considered as a fabric of spacetime. Space and time both. Space describes the position of the event, and the time coordinate tells when the event occurs.

So, our universe can be considered as a fabric of space-time, a plain fabric of space-time. But our universe consists of so many bodies, astronomical objects, and heavenly bodies. They are all placed in this space-time fabric.

Let’s take an example of our solar system. Consider an object with a very heavy mass as our sun. when we place this heavy object on the spacetime fabric. We can see warping of the space-time fabric and curvature in the space-time fabric. It is this curvature that allows smaller, lighter heavenly objects to revolve around it. 

Here comes the principle of causality.

Principle of causality—The principle of causality says that if there is an effect due to some cause. The cause is the Sun, that heavy mass, and the effect is the revolution of the planets or bodies all around it. So, if there is an effect due to some cause, then necessarily the effect will be followed by the cause. If there exists an effect caused by something else. Then the effect has to be greater than the cause.

The effects must always follow the cause.

Albert Einstein's general theory of relativity explains that what we perceive as the force of gravity in fact arises from the curvature of space and time. He proposed that objects such as the Sun and the Earth change this geometry. So, although Earth appears to be pulled towards the Sun by gravity, there is no such force. It is simply the geometry of space-time around the Sun telling Earth how to move. The general theory of relativity has far-reaching consequences. It not only explains the motion of the planets; it can also describe the history and expansion of the universe, the physics of black holes, and the bending of light from distant stars and galaxies.

As the sun is the most massive object in our solar system, its curvature of space-time would be the most noticeable example in the local universe

All stellar/luminous objects exert radiation pressure on other astronomical/nonluminous bodies, which is meant to be responsible for the stability of nonstellar objects around stellar objects. 

Now the question is how do black holes exist on the spacetime fabric of the universe? Because as we are all aware, black holes are the kind of celestial bodies that nothing can escape from inside. They have sufficiently compact mass and a strong gravitational effect. I should rather use the word "curvature" for gravitational effect. So, spacetime curvature becomes infinite for such bodies...they can be considered as a hole in the fabric of space-time.