**Einstein’s theory of relativity** was beyond any doubt lovely, different from any other theory ever imagined by mankind. Newton described gravity merely as a force acting over a distance attracting any 2 masses, the force proportional to the product of the two masses and inversely proportional to the square of the distance between them. This simple straight forward theory worked well for over two hundred years.

The theory is deceptively simple. First, there is no “absolute” frame of reference. Every time you measure an object’s velocity, or its momentum, or how it experiences time, it’s always in relation to something else. Second, the speed of light is the same no matter who measures it or how fast the person measuring it is going. Third, nothing can go faster than light.

**The Problem**

The mass that determined the strength of the gravitational force was the same mass that appeared in Newton’s second law of motion, F = ma; gravitational mass was the same as the “inertial mass.” There was no apparent reason this had to be true, it simply was.

Einstein didn’t think this was mere coincidence. He insisted that the two types of mass are identical because of a fundamental symmetry in nature; that the laws of physics must take the same form whether one is in a gravitational field or in a region of space with no gravity (say, in a spaceship undergoing an equivalent acceleration). Carrying this principle to its logical conclusions eventually led to the equations of general relativity, the theory considered by many to be “probably the most beautiful of all existing physical theories.”

General relativity, also known as the general theory of relativity, is the geometric theory of gravitation published by Albert Einstein in 1915. 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.

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. The predictions of general relativity have been confirmed in all observations and experiments to date. Although general relativity is not the only relativistic theory of gravity, it is the simplest theory that is consistent with experimental data.

**The Unanswered Questions**

How general relativity can be reconciled with the laws of quantum physics to produce a complete and self-consistent theory of quantum gravity?

The problem of quantum gravity and the question of the reality of spacetime singularities remain open. Observational data that is taken as evidence for dark energy and dark matter could indicate the need for new physics. Even taken as is, general relativity is rich with possibilities for further exploration. Mathematical relativists seek to understand the nature of singularities and the fundamental properties of Einstein’s equations,and increasingly powerful computer simulations (such as those describing merging black holes) are run.

So, in a nutshell, **Einstein’s Theory of Relativity is not complete**. More needs to be added, more explanations to be given and more observations to be endorsed.

Ref: http://www.gallup.unm.edu/~smarandache/UnsolvedProblemsRelativity.pdf