What is Theory of General Relativity ?

hi

The general theory of relativity describes the force of
gravity.

Einstein wasn't the first to come up with such a theory —
back in 1686 Isaac Newton formulated his famous inverse square law of
gravitation.

Newton's law works perfectly well on small-ish scales: we can use it to calculate how fast an object
dropped off a tall building will hurtle to the ground and even to send
people to the Moon.

But when distances and speeds are very large, or very massive objects are involved, Newton's law
becomes inaccurate.

It's a good place to start though, as it's easier
to describe than Einstein's theory.

Suppose you have two objects, say the Sun and the Earth, with masses respectively. Write for the distance between the two objects. Then Newton’s law says that the gravitational force between them is..

Newton's and Coulomb's formulas are nice and neat, but there is a
problem. Going back to Newton's law, suppose you took the Earth and
the Sun and very quickly moved them further apart.

This would make the
force acting between them weaker, but, according to the formula, the
weakening of the force would happen straight away, the instant you move the
two bodies apart.

The same goes for Coulomb's law: moving the charged
particles apart very quickly would result in an immediate weakening of
the electrostatic force between them.

But this can't be true. Einstein's special theory of relativity,
proposed ten years before the general theory in 1905, says that
nothing in the Universe can travel faster than light — not even the
"signal" that communicates that two objects have moved apart and the
force should become weaker.

A charged particle gives rise to an electric field, which is
"felt" by
another particle (which has its own electric field).

One particle will move in response to the other's electric field — that's what
we call a force.

When one particle is quickly moved away from the other, then this
causes ripples in the first particle's electric field.

The ripples travel through space, at
the speed of light, and eventually affect the other particle.

In fact, the
particle that is moved also generates a magnetic field and emits
electromagnetic radiation.

The end result is a complex interaction of rippling fields..

but the point is that the force is really one particle being
affected by ripples propagating through the field of the other.

It took scientists a long time to fully develop this field picture
of electromagnetism.

The main credit goes to the Scottish scientist James Clerk
Maxwell...

General relativity (GR, also known as the general theory of relativity or GTR) is the geometric theory of gravitation published by Albert Einstein in 1915 and the current description of gravitation in modern physics.

General relativity generalizes special relativity and Newton's law of universal gravitation, providing a unified description of gravity as a geometric property of space and time, or spacetime.

In particular, the curvature of spacetime is directly related to the energy and momentum of whatever matter and radiation are present. The relation is specified by the Einstein field equations, a system of partial differential equations.

Some predictions of general relativity differ significantly from those of classical physics, especially concerning the passage of time, the geometry of space, the motion of bodies in free fall, and the propagation of light.

Examples of such differences include gravitational time dilation, gravitational lensing, the gravitational redshift of light, and the gravitational time delay.