I HAVE DONE SOME HARDWORK FROM MY MIND AND IN SEARCHING STUFF TO DESCRIBE THE THEORY OF RELATIVITY SO YOU NEED TO APPROVE MY ANSWER PLEASE fREIND...!!

THEORY OF RELATIVITY  has come from einstein''s two bold postulates

1:Special relativity

2:General relativity

1:Special relativity :It extends galileo principle of relativity which states that there is no inertial frame, i.e no object can be said at rest, it may be rest at an inertial frame may not be rest in another frame of reference, example: when you are in moving train you see the poles moving opposite in the direction of train, while for the person standing on ground the poles are at rest. 

And also states that thus there is no inertial frame for measuring speed of light, i.e for every frame the speed of light is a constant whether you''re moving away from it or towards it.

The two postulates are:

Relativity of simultaneity

Two events happening in two different locations that occur simultaneously in the reference frame of one inertial observer, may occur non-simultaneously in the reference frame of another inertial observer.

Time dilation

The time lapse between two events is not invariant from one observer to another, but is dependent on the relative speeds of the observers'' reference frames,i.e if two clocks are traveling with the speed close to speed of light with respect to each other, and you are sitting on one clock observing the time on another you''ll find the time over another clock is much slower than you one''s.

Equivalence of mass-energy.

As an object''s speed approaches the speed of light from an observer''s point of view, its relativistic mass increases thereby making it more and more difficult to accelerate it from within the observer''s frame of reference

the position of body in space-tine form a 4-vector in relativity, and this relates the time component (the energy) to the space components (the momentum) in a nontrivial way. For an object at rest, the energy-momentum four-vector is (E, 0, 0, 0): it has a time component which is the energy, and three space components which are zero. By changing frames in the x direction with a small value of the velocity v, the energy momentum four-vector becomes (E, Ev/c², 0, 0). The momentum is equal to the energy multiplied by the velocity divided by c². As such, the Newtonian mass of an object, which is the ratio of the momentum to the velocity for slow velocities, is equal to E/c².

Something interesting..!!

How far can one travel from the Earth?

Since one can not travel faster than light, one might conclude that a human can never travel further from Earth than 40 light years if the traveler is active between the age of 20 and 60. One would easily think that a traveler would never be able to reach more than the very few solar systems which exist within the limit of 20-40 light years from the earth. But that would be a mistaken conclusion. Because of time dilation, a hypothetical spaceship can travel thousands of light years during the pilot''s 40 active years. If a spaceship could be built that accelerates at a constant 1g, it will after a little less than a year be traveling at almost the speed of light as seen from Earth. Time dilation will increase his life span as seen from the reference system of the Earth, but his lifespan measured by a clock traveling with him will not thereby change. During his journey, people on Earth will experience more time than he does. A 5 year round trip for him will take 6½ Earth years and cover a distance of over 6 light-years. A 20 year round trip for him (5 years accelerating, 5 decelerating, twice each) will land him back on Earth having traveled for 335 Earth years and a distance of 331 light years. A full 40 year trip at 1 g will appear on Earth to last 58,000 years and cover a distance of 55,000 light years. A 40 year trip at 1.1 g will take 148,000 Earth years and cover about 140,000 light year

The theory of relativity, or simply relativity, generally encompasses two theories of Albert Einstein: special relativity andgeneral relativity.^[1] (The word relativity can also be used in the context of an older theory, that of Galilean invariance.)

Concepts introduced by the theories of relativity include:

• Measurements of various quantities are relative to the velocities of observers. In particular, space and time can dilate.
• Spacetime: space and time should be considered together and in relation to each other.
• The speed of light is nonetheless invariant, the same for all observers.

The term "theory of relativity" was based on the expression "relative theory" (German: Relativtheorie) used by Max Planckin 1906, who emphasized how the theory uses the principle of relativity. In the discussion section of the same paper Alfred Bucherer used for the first time the expression "theory of relativity" (German: Relativitätstheorie).