November marks 100 years of Einstein's Theory of General Relativity

A follow-up to a big idea 10 years earlier.

After years of work, on 25 November 1915, Albert Einstein published an article with a new theory of gravity, which he called "the General Theory of Relativity". It was a revision of his first theory of relativity (known as "special relativity") published 10 years earlier. Unlike special relativity, which applied only to "special" conditions that didn't involve acceleration, the new theory explained many things that weren't previously explainable.

For more than 200 years, Newton's law of universal gravitation was accepted as an explanation of the interaction between masses such as objects falling down and planets going around the Sun. Newton's explanation was the existence of a attractive force between objects that draws them toward one another. This force was known as the force of gravity.

However, there was no explanation of the mechanisms by which this force caused interaction between objects separated by space. Also, this theory did not explain discrepancies of some astronomical observations. The common understanding of space was that of emptiness. The discovery of electromagnetism in the 19th century brought a realization that the emptiness wasn't as empty as previously thought. Einstein believed that gravity, similar to the electromagnetic field, was also a field. He later proposed that the gravitational field wasn't simply something that was dispersed through the space, but was the space itself. Gravitational attraction was the result of massive objects warping the space-time. The idea of space-time continuum was born.

Empirical confirmations of the new theory followed. In 1919, it was observed during a total solar eclipse that a massive body can bend the path of traveling light. In 1959, gravitational red shift, another prediction of general relativity, was observed. In 1968, it was confirmed that a massive body can slow down a beam of light (the Shapiro delay). In 1976, NASA's Gravity Probe A precisely measured time flow in the gravitational field, observing that a clock on Earth ran slower than the one on the orbit. Thirty years later, in mid-2000s, NASA's Gravity Probe B confirmed that Earth's axial rotation drags space-time with it. Both observations agree with Einstein's theory of general relativity.

One of theory's predictions was that, just as vibrating electrons give off energy in the form of electromagnetic radiation (such as visible light or radio waves), an accelerating mass gives off energy in the form of gravitational radiation. Recall that gravity is thought of as a space-time continuum; gravitational radiation can be imagined as ripples traveling through the space-time. These ripples are gravitational waves. Compared to other forms of radiation, gravity is very weak, and Einstein doubted that detecting these waves was possible.

A strong evidence of their existence came in 1974. Russell Hulse and Joseph Taylor observed a binary pulsar (two rotating neutron stars orbiting each other) and discovered that the two stars were gradually spiralling faster and closer to each other. This was due to the loss of energy to gravitational radiation in the form of gravitational waves. At the time, it was the strongest, if indirect, of gravitational waves' existence. Gravitational waves haven't been directly observed yet. Their direct detection is what LIGO aims to accomplish. By doing so, LIGO will provide the most convincing confirmation of Einstein's theory of general relativity.

How is all this relevant to you? Well, if it weren't for one of Einstein's most important contributions to modern science, many conveniences of our modern life wouldn't work. The theory's predictions are used in many modern technologies, from GPS to airplane landing systems to communication satellites.

Still curious? Read more and more and more, watch, flip through a slide show or listen to a podcast. Don't miss this interactive comic!

Image: Wikimedia Commons

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