So far, science has discovered four fundamental secrets of the nature, the four fundamental forces. Among those, science has defined theortically, mathematically and also practically explained the first three forces, so called, 'Strong', 'Weak', 'Electromagnetic' forces. Can this be interpreted as science has understood 75% of the nature? It seems like it did but if you look further into facts, it hasn't done and science has long way to go.
Gravity by Newton:
In classical physics, gravity is the most famous topic and also it is the most familiar force among laymen, as compared to other three fundamental forces. Around 300 years ago, Sir Isaac Newton described gravity as the character of an object that has a non zero mass. He is the father of the definition of gravitational force (g) and also for the gravitational constant (G).
According to Newton, Gravity is always a force that exists between two objects. Please don't forget that 300 years ago there was no atoms, no photons and no quantums. Everything is just an object. So, he came up with a formula that explains the gravitational force as it is directly proportional to the masses of two interacting objects and inversely proportional to the square of the distance between those objects; and also introduces the gravitational constant.
G, the gravitational constant is nothing but the gravitational force of the Earth and it's a constant value because on earth most of the objects have very less mass as compared to the mass of the earth and the distance is almost equal to the earth's radius. Hence the final outcome of the formula, approximately, stays as a constant. If m1 is the mass of object1, m2 is the mass of object 2 andd is the distance between the objects1 & 2. Then, the gravitational force F, acting between those two objects, can be calculated as F = m1. m2/d*d. In other words, on earth F will be g * m2 or F = mg. This is how gravity was defined 300 years ago.
Gravity by Einstein:
In 1916, Einstein re-defined gravity in his general theory of relativity in a brand new dimension. He was the person who also introduced Time as the 4th dimension. So, his perspective and mathematical theories are defined in his three dimensional space with Time as a 4th dimension. So, his models are viewed in the space-time matrix or fabric. The way how he had done mathematical models and defined the gravity is an amazing and interesting one.
According to Einstein, universe is nothing but the fabric which is made up of 3dimensional space and time. When we talk about gravity, it just remains the property or attribute of the planets in our galaxy. So, he demands that the planets resting on the fabric and rotating on their positions. Planets are massive and so due to their heavier masses, they push the fabric down at their positions. This creates slope around the heavier planet and it in turn makes the other smaller planets to roll around the heavier planet.
Hence in our solar system, Due to Sun's heavier mass, all other planets and satellites revolve around the Sun. As per Newton's formula, Sun pulls each planet with a unique amount of force based on the distance and the mass of the planet and make them revolving around.
If you take a closer look, such model demand and at the same time proves few other facts as follows:
1. Empty space or vacuum is not really empty in the outer space.
2. When Sun rays come through outer space, it carries the electromagnetic force, which creates energy and forms the fabric of 3-dimensional space.
3. So when Sun dies after burning all of its fuel, the fabric will no longer exist between Sun. In other words, there won't be any gravitational force from Sun and eventually other planets would be free from their path and slip through outer space until it's acceleration comes to zero; but this means earth may even stop rotating. If that happens then that would destroy everything on earth as everything on earth is pushed in opposite direction due to the acceleration that the objects gained from earth's rotational force. This is similar to the effect that the passenger is pushed front when brake is applied.
Quantum Gravity:
When we say quantum gravity, we still mean the gravitational force. Then what is the difference? While gravity has always been described as the property of macroscopic world, such as planets and solar system, quantum gravity is explained in terms of objects and entities from the microscopic world, in other words, quantums. Either in classical physics or quantum physics, all the phenomena have been described not only in theories but also mathematically and proved experimentally.
Quantum mechanics claims that gravity is another fundamental force caused by new set of sub-atomic particles, named as gravitons. While there is no concrete proof of the existence of gravitons, quantized theories of matter necessitate their existence. Supporting this theory is the observation that all other fundamental forces have one or more carrier particles, except gravity, leading researchers to believe that at least one most likely does exist; they have dubbed these hypothetical particles gravitons.
Many of the accepted notions of a unified theory of physics since the 1970s, including string theory, super-string theory, M-theory, loop quantum gravity, all assume, and to some degree depend upon, the existence of the graviton. It is clear that the detection of the graviton as vital to validating all of these theories up to the decent extent. While quantum mechanics demands that the gravitons would be theoretical spin-2 mass less particles, quantum physicists are trying to define gravity in quantum theories. In order to succeed, they also need to resolve gravity in their mathematical models!
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