why gravity exists? [closed]

Question

I always have a doubt on gravity , why does gravity exists ?

is there any scientific explanation on gravity ?

I know that Gravity exists , i can feel it. But Who is creating all these rules ?

secondly i am curious about the bigbang theory , how its possible From Nothing TO Something

I know the stack rules , but curiosity is the main reason.

Thanks

Answer 1

The first good theory of gravity was Newton's Law of Gravitation, which was proposed in the 1600's. It says that any two masses are attracted to each other by a force which follows the following equation:

$$F=G\frac{m_1 m_2}{r^2}$$

where $G$ is the gravitational constant and $r$ is the distance between the two masses. Along with Newton's Second Law, $F=ma$, this tells us how masses will behave when they are placed near each other.

As time went on, it was observed that Newton's Law appeared to fail in certain circumstances, for example with the precession of the planet Mercury. Fast forward a few hundred years, and Einstein proposes General Relativity. This theory says that gravity is a consequence of the geometry of space and time. It says the metric of spacetime is related to the energy and momentum density in spacetime by the Einstein Field Equations:

$$R_{\mu \nu}-\frac{1}{2}g_{\mu \nu}R=\frac{8\pi G}{c^4}~T_{\mu \nu}$$

Along with the geodesic equation, a sort of generalization of Newton's First Law to curved spaces, this tells us how objects will move. (As a technical aside, the geodesic equation is actually derivable from the Field Equations.) General Relativity accounts for phenomena which is unexplainable by Newton's theory, such as the precession of Mercury and gravitational time dilation.

We now know that the world is, as far as we can tell, fundamentally quantum mechanical. General Relativity is a classical theory, so a theory of quantum gravity is required to explain phenomena where there is a strong gravitational field yet things are really small, such as in the singularity of a black hole. Unfortunately, gravity is very difficult to quantize unlike the other three fundamental forces. We currently have no complete and accepted theory of quantum gravity. String Theory provides probably the best lead so far.