It has been a century since Albert Einstein published his theory of general relativity. It was November 25, 1915 when Einstein, in a series of lectures at the Prussian Academy of Sciences, presented a theory that would change the way the world viewed physics.

Just to be clear, this was nearly ten years after he published his theory of special relativity that proclaimed that energy was equivalent to mass multiplied by the speed of light (multiplied by itself) - E = mcÂ˛

In doing so, he had disproved Sir Isaac Newton's theory which stood valid for two centuries.

His theory of general relativity was subsequently confirmed when it was shown to fit with Mercuryâ€™s unusual orbit and the bending of all light around the sun.

A report in the New York Times adds:

Einstein went to the blackboard. And on November 25, 1915, he set down the equation that rules the universe. As compact and mysterious as a viking rune, it describes space-time as a kind of sagging mattress where matter and energy, like a heavy sleeper distort the geometry of the cosmos to produce the effect we call gravity, obliging light beams as well as marbles and falling apples to follow curved paths through space.

Here's a video explaining the theory.

**History**

Einstein laid the foundation for his theory of general relativity with his theory of special relativity, which dealt with the laws of physics in the absence of gravity.

The common element between both the theories was that they dealt with a different description of space and time from what common sense would suggest.

However, it was no easy task for the Nobel prize winner who took nearly a decade to formulate the equation.

A report in The Conversation adds:

To describe curved space-time, Einstein called upon the earlier work of Bernhard Riemann, a 19th century mathematician. And with the help of his friend Marcel Grossman, also a mathematician, Einstein spent several exhausting years learning the mathematics of curved spaces, or what mathematicians call â€śdifferential geometryâ€ť. As Einstein commented, â€ścompared with understanding gravity, special relativity was mere childâ€™s playâ€ť.

"In all my life I have not laboured nearly so hard, and I have become imbued with great respect for mathematics, the subtler part of which I had in my simple-mindedness regarded as pure luxury until now," he is said to have written a few months later. â€śI was beside myself with joyous excitement,â€ť he added.

The theory has stood the test of time over the past century with general relativity tested to remarkable accuracy each time.

**Relevance**

Just as there has been a lot of praise for Einstein, his theory also had a few flaws. One of the biggest ones being that it works perfectly on a massive scale of planets but not so much on the quantum level to describe the forces between atoms and sub-atomic particles.

A report in BBC titled 'Does Einstein's general theory of relativity still matter?' explores some of the other flaws with the theory including how it fails to explain the expanding universe, black holes and the gravity of tiny particles.

However, that's where quantum physics fits in. The present challenge for physicists is to find a 'theory of everything'.

As Robin Tucker, a professor at Lancaster University puts it:

General relativity is unparalleled when it comes to describing the world on a large scale, such as planets and galaxies, while quantum mechanics perfectly describes physics on the smallest scale, such as the atom or even parts of the atom. Uniting the two into a consistent â€śtheory of everythingâ€ť is the single biggest challenge in physics today â€“ and progress is slow.