The General Relativity Theory was developed by Albert Einstein in 1916. It describes the behavior of mass in a four-dimensional curved space-time.* While the ►Special Relativity Theory is based entirely on the principle of constant light speed, the General Relativity Theory has two fundamental principles from which everything else in the theory is derived:
Matter or energy causes space-time nearby it to curve locally.
An object not affected by any forces will always move along the shortest route in space-time, which is called a geodesic.
This shortest route is not always a straight line. It is determined by a ►definition of distance, a so-called metric of space. Since mass causes a curvature in space, inert objects in the vicinity of other mass volumes move on curved paths that in this area of space represent the geodesics. We experience this phenomenon as gravitational force, which is the force that keeps the Earth on a circular path around the sun. In reality, however, gravitation is not really a force at all but a property of the curvature of spacetime.
Thus, the "curved" path of the Earth is in fact the shortest route that it can take within the portion of spacetime curved by the sun's mass. That this path is circular (or more precisely, elliptical) does not mean, however, that the sun curves space into a circular shape. Such extensive curvature can only occur in the vicinity of a ►black hole. Instead, the sun's mass curves space only slightly; yet we need to consider that the curvature does not just affect space alone but the entire four-dimensional space-time. And in space-time the Earth's route is indeed only slightly curved; the Earth actually travels a very gradual screw-shaped path that is hardly distinguishable from a straight line. When projected into our three spatial dimensions, however, the four-dimensional screw appears as a circle.
Like the sun, the Earth also curves its surrounding space, though to a much smaller extent. The curvature of space caused by the Earth alters the latter's diameter by only about one and a half millimeters. As you can see from this, general relativity affects only large amounts of mass in large spaces.
Heavier Clocks Run More Slowly
Special cases of general relativity include not only special relativity, but also Newton's laws of gravity, as well as the principle -- first noted by Galileo -- that all objects fall at the same rate. But general relativity also explains other phenomena that are less accessible to everyday observation:
Time runs more slowly in the vicinity of mass volumes. For example, a clock located in a valley runs more slowly than one located on a mountain. This effect can be directly measured with the help of precise atomic clocks.
When the mass of an object reaches a certain density, it curves its surrounding space-time to such an extent that the latter isolates itself from the rest of the universe and creates a ►black hole.
Motions involving large volumes of mass -- supernova explosions, for example, or double stars circling around each other at a high velocity -- create gravitational waves. These are deformations of space-time that expand at the speed of light. It is difficult to obtain experimental evidence of these waves; all attempts to date have been unsuccessful. Currently, gigantic detectors are being built all over the world in the hope that they can eventually provide experimental data on gravitational waves.
Light rays are diverted by mass. This is why large mass volumes such as galaxies can act as gravitational lenses. This effect is utilized in astronomy to determine the ►distance of quasars.
Confirmation from Africa
In 1919, shortly after its initial publication, the General Relativity Theory received its first empirical confirmation. This confirmation did not come by way of a planned experiment in a laboratory, however; instead, it occurred by chance during an expedition to a volcanic island in the Gulf of Guinea in West Africa. There, the astronomer Arthur Stanley Eddington was observing the solar eclipse of May 29, 1919, when he noticed that light rays from stars well within the region around the shaded sun were shifted by precisely 1.75 arcseconds,** just as Einstein's theory had predicted. This match between prediction and confirmation at once made the theory famous. Since then numerous additional experiments have been conducted in order to test the validity of the Theory of General Relativity. According to what we now know, the theory does indeed describe the exact properties of the space that contains our universe.
* One might think that the curvature of four-dimensional space-time would create a fifth dimension into which space-time is curved. However, such a fifth dimension would be neither mathematically required nor physically accessible. For no experiment is conceivable that could establish any hypotheses about a fifth dimension. This is why natural science does not assign any reality to this dimension -- as opposed to the "involuted" space-time dimensions of string theory.
** 1 arcsecond = 1/3600 angular degree.