Monday, November 11, 2013

General Relativity

Published in 1916, the general theory of relativity is often regarded as Albert Einstein's greatest achievement and one of the most remarkable scientific contributions of the 20th century. It is known for redefining gravity beyond the previous Newtonian interpretation and describing it as a geometric property of space-time. General relativity is viewed as an expansion of the special theory of relativity and it differs from the special case because it takes into account the motion due to gravitational fields and other accelerating reference frames, thought to be similar according to the equivalence principle. Along with quantum theory, general relativity is considered to be a central pillar of modern physics and is currently accepted as the leading theory for gravitation.

Earth's mass warps space-time and shapes the Moon's geodesic orbit (Image: NASA).

General relativity describes the attractive force of gravity as being the result of an acceleration produced when something interacts with curved space-time, usually there because of an object with a very large mass. The more mass an object has, the more it warps the space-time around it and the larger its gravitational field is. Any object with some mass or just energy is thought to have a gravitational influence as well but will always experience an attractive pull when it is close enough to another object with a sufficient amount of gravity. Even rays of light that approach a gravitational field will bend towards it in a phenomenon known as gravitational lensing. This effect was confirmed to exist when the light from stars behind the Sun was observed to travel around it during the total solar eclipse of 1919.

Other tests of general relativity include the observation of a gravitational redshift in visible light emerging from a gravitational field, which loses energy and increases its wavelength, shifting towards a redder color. When entering a gravitational field, it behaves in the opposite way by gaining a shorter wavelength and a bluer appearance. Gravitational redshift was first measured in the light emitted by white dwarf star Sirius B in 1925. When a very massive object rotates on its axis, it will also twist the space-time around it as it spins. This is known as frame-dragging and it too was confirmed to exist by observing the orbital shifts that emerged from satellites traveling around our planet. General relativity predicts the existence of gravitational waves, which are ripples in space-time that propagate at the speed of light and are caused by sudden changes in an object's gravitational field, but their direct detection is currently a subject of on-going research.

From the smallest interaction of an elementary particle to the behavior of the cosmos itself, Albert Einstein's ideas have altered the way we view our physical surroundings on many scales. While general relativity accurately describes the gravitational attraction between celestial objects, it is additionally useful for describing other processes such as the expansion or contraction of a universe. Exotic objects such as black holes, white holes, and wormholes are also predicted to exist according to the equations of general relativity. Einstein's dream was to find a theory of everything that could combine the fundamental interactions of nature, specifically gravity and electromagnetism, to show that they were all facets of the same unified force. The search for a theory of everything continues to this day with the goal of bringing together Einstein's laws of general relativity with those of the quantum world in order to develop a consistent theory for quantum gravity.

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