General relativity and the equivalence principle
General relativity and the equivalence principle
General relativity (GR) is a theory of gravitation that was developed by Albert Einstein between 1907 and 1915. According to general relativity, the observed gravitational attraction between masses results from the warping of space and time by those masses. General relativity has developed into an essential tool in modern astrophysics. It provides the foundation for the current understanding of black holes, regions of space where gravitational attraction is so strong that not even light can escape.
As Albert Einstein later said, the reason for the development of general relativity was that the preference of inertial motions within special relativity was unsatisfactory, while a theory which from the outset prefers no state of motion (even accelerated ones) should appear more satisfactory.[105] Consequently, in 1908 he published an article on acceleration under special relativity. In that article, he argued that free fall is really inertial motion, and that for a freefalling observer the rules of special relativity must apply. This argument is called the Equivalence principle. In the same article, Einstein also predicted the phenomenon of gravitational time dilation. In 1911, Einstein published another article expanding on the 1907 article, in which additional effects such as the deflection of light by massive bodies were predicted.
Hole argument and Entwurf theory
While developing general relativity, Einstein became confused about the gauge invariance in the theory. He formulated an argument that led him to conclude that a general relativistic field theory is impossible. He gave up looking for fully generally covariant tensor equations, and searched for equations that would be invariant under general linear transformations only.
In June 1913, the Entwurf ("draft") theory was the result of these investigations. As its name suggests, it was a sketch of a theory, with the equations of motion supplemented by additional gauge fixing conditions. Simultaneously less elegant and more difficult than general relativity, after more than two years of intensive work Einstein abandoned the theory in November 1915 after realizing that the hole argument was mistaken.[106]
Cosmology
In 1917, Einstein applied the General theory of relativity to model the structure of the universe as a whole. He apprehended that his equations predicted the universe to be either contracting or expanding. He wanted the universe to be eternal and unchanging, but this type of universe is not consistent with relativity. To fix this, Einstein modified the general theory by introducing a new notion, the cosmological constant, which he called ''Lambda''.[107] The purpose of Lambda was to rectify the effects of gravity and allow the whole system to stay balanced. With a positive cosmological constant, the universe could be an eternal static sphere. However, in 1929, Edwin Hubble confirmed that the universe is expanding, Einstein exclaimed after his Mount Wilson visit with Hubble: "If there is no quasi-static world, then away with the cosmological term!"[108][109] and Einstein supposedly discarded the cosmological constant.
Einstein believed a spherical static universe is philosophically preferred, because it would obey Mach's principle. He had shown that general relativity incorporates Mach's principle to a certain extent in frame dragging by gravitomagnetic fields, but he knew that Mach's idea would not work if space goes on forever. In a closed universe, he believed that Mach's principle would hold. Mach's principle has generated much controversy over the years.
In many of Einstein biographies, writers claim that he called the creation of Lambda his "biggest blunder". Recently, astrophysicist Mario Livio showed that Einstein possibly never said that.[110] Instead of discarding Lambda, Einstein was continually experimenting with it.[111]
In late 2013, Irish physicist Cormac O'Raifeartaigh, happened to discover a handwritten manuscript by Einstein which was since then overlooked by other scientists. The research paper was titled ''"Zum kosmologischen Problem"'' ("About the Cosmological Problem").[112][113] And Einstein proposed a revision of his model, still with a cosmological constant, but now the constant was responsible for the creation of new matter as the universe expanded. Thus, the average density of the system never changed. He stated in the paper, ''"In what follows, I would like to draw attention to a solution to equation (1) that can account for Hubbel's [sic] facts, and in which the density is constant over time." And: "If one considers a physically bounded volume, particles of matter will be continually leaving it. For the density to remain constant, new particles of matter must be continually formed in the volume from space."''
This is consistent with the now-obsolete Steady State model of cosmology, proposed later in 1949, and with today's modern understanding of dark energy.[114]