100 years of General Relativity
November 2015 marked a notable anniversary in the history of physics: one hundred years before, Albert Einstein presented his general theory of relativity, in the form of four papers, to the Prussian Academy of Sciences in Berlin on 4, 11, 18 and 25 November 1915. A revolutionary approach at the time, general relativity remains to date the best physical theory to describe gravity, particularly on cosmic scales.
A new frame of mind
Prior to general relativity, the leading theory of gravity was Isaac Newton's law of universal gravitation, which unified the description of moving massive bodies both on Earth and in the Solar System. In Newton's framework, gravity is a long-range attractive force, acting between any two massive objects. It is directly proportional to the product of their masses and inversely proportional to the square of their distance. While Newton's theory is still a great approximation to address gravity in a variety of situations, especially in our daily life, Einstein's novel approach achieved a more complete account of gravity's behaviour, especially in extreme cases.
Using the sophisticated mathematical formalism of non-Euclidean geometry, developed by Bernhard Riemann and other mathematicians in the 19th century, Einstein could describe spacetime in a more flexible way, identifying gravity as the source of its curvature. In general relativity, spacetime is not ‘flat’ but is curved by the presence of massive bodies and, as these move in spacetime, they continually change its curvature. Gravity then provides a description of the dynamic interaction between matter and spacetime.
From prediction to experiment
General relativity successfully explained the additional shift in the precession of Mercury's perihelion that could not be justified with Newton's gravity. In addition, it predicted a number of new physical phenomena that would later find experimental proof.
In the curved spacetime of general relativity, free-falling objects subject to gravity alone move along geodesics – the equivalent of straight lines in curved geometry. Photons, the massless particles of light, travel on special geodesics that trace the minimum path between two objects.
These trajectories are generally not straight, but can be bent if massive objects are present along the way. As seen by a distant observer, this distortion of the path of light is effectively similar to the focussing of light by an ordinary glass lens, and is thus known as gravitational lensing.
A certain degree of light distortion is predicted also within Newton's theory, and Einstein had obtained the same value in 1911, when his theory was not yet complete. With the full general relativity treatment, however, the amount of gravitational lensing exerted by a massive body on nearby passing light is twice as much.
Several teams of astronomers tried to measure during a solar eclipse how much the Sun causes the path of light from background stars to deviate, but with no success until 1919. Then, an expedition led by Arthur Eddington to Príncipe, a small island off the west coast of Africa, and a parallel expedition led by his colleague Andrew Crommelin to Sobral, in the north-east of Brazil, measured the positions of stars in a patch of the sky near the eclipsed Sun on 29 May 1919. The stars appeared to be shifted, with respect to their usual position, by the amount that Einstein had predicted with his general relativity.
The measurement of gravitational lensing during the 1919 eclipse was the first experimental proof of Einstein's theory of gravity. Gravitational lensing is now a thriving field in astrophysics, ever since the discovery, in 1979, of the first astronomical source whose light is distorted so much by the presence of a foreground galaxy that it appears as two distinct images.
Black holes, the expanding Universe and beyond
Other predictions of general relativity concern black holes, celestial bodies so dense that nothing, not even light, can escape their gravitational pull. These objects were hypothesised and studied theoretically for many decades, until the weight of observational evidence became overwhelming in the 1970s.
Numerous black holes have been discovered by studying the peculiar motion of stars and gas in their vicinity. On the one hand, there are relatively small black holes, with masses a few times more than that of the Sun, that derive from the collapse of massive stars; on the other hand, supermassive black holes, ranging from millions to billions of solar masses, are found at the centre of most massive galaxies, including our Milky Way.
But there is still one prediction from general relativity that eludes proof: the emission of gravitational waves – ripples in the fabric of spacetime – by any suitably accelerated massive body. While indirect evidence was found in the late 1970s by observing the feeble speeding up of two stellar remnants, a pulsar and a neutron star, in a binary system, physicists are still trying to directly detect gravitational waves with ground-based experiments and, in the future, space-borne observatories.
(Update 12 February 2016: high-frequency gravitational waves, emitted by a pair of merging black holes, were directly detected for the first time with the Advanced Laser Interferometer Gravitational-Wave Observatory.)
ESA's LISA Pathfinder mission is testing the technologies for detecting gravitational waves from space, joining the exciting quest to prove yet another prediction from Einstein's General Theory of Relativity.
Last Update: 12 February 2016For further information please contact: SciTech.firstname.lastname@example.org