# General Theory Of Relativity Turns 100

100 years ago today, Albert Einstein first wrote down his general theory of relativity. It’s still the most plausible explanation of gravity and remains key to our understanding of how the universe works.

The work ultimately followed on from Gallileo’s principle of relativity that “It is impossible by mechanical means to say whether we are moving or staying at rest.”  In simplified terms, it means the perception of movement is relative, hence the way somebody standing on Earth can’t feel it moving around the Sun.

That in turn led to Newton’s first law of motion which is that a moving object continues to move with the same velocity until altered by an external force.

In 1905 Einstein developed the special theory of relativity, which tried to deal with the fact that Gallileo’s theories suggested the speed of light should vary depending on the speed the observer was travelling, but that experiments disproved this.

Einstein’s explanation was that both measurable length and time duration change as the Earth moves through space. The beauty is that the changes always balance out so that combining this length and duration creates the same figure for the speed of light. He explained that this wasn’t just a freaky coincidence, but rather a law of nature.

The problem was that this special theory didn’t account for the effects of gravity. That led to another 10 years work for Einstein, during which his teacher Hermann Minkowski created the model known as the space-time continuum. This was the idea that space and time are totally interconnected, something that’s usually represented by the idea of time being a fourth dimension alongside height, width and depth.

Einstein began working on an explanation for how relativity and space-time were connected and, by November 1915 had what he thought was the solution. He discovered his explanation perfectly explained why the change in Mercury’s orbit over time didn’t match the calculations based on Gallileo’s work.

On November 25, 1915 he wrote down his general theory of relativity which holds that moving objects such as planets cause both space and time to curve and thus distort the movement of other objects.

The theory meant some fundamental rethinks of how the universe operates. One of the simplest to comprehend is that gravity doesn’t make an object fall to earth by exercising a force similar to pulling something on a string. Instead the Earth curves space enough that the object effectively slides down an invisible slope.

For objects in orbit, it’s not a case of the Earth holding them in place through a pulling force. Instead the curvature at this point is weaker and the effect is instead that the object’s “forward” motion is simply taking it round in a circle.

On a more complex level, the theory helps explain matters as diverse as the expansion of the universe, the bending of light (shown in the apparent relocation of stars during an eclipse) and the existence of black holes.