History of general relativity

However, experiments and observations show that Einstein's description accounts for several effects that are unexplained by Newton's law, such as minute anomalies in the orbits of Mercury and other planets.

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.

Their strong gravity is thought to be responsible for the intense radiation emitted by certain types of astronomical objects (such as active galactic nuclei or microquasars).

He published a Lorentz invariant theory on four-dimensional spacetime, where gravity is transmitted by gravitational waves that travel at the speed of light.

[2] 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.

Einstein urged astronomers to attempt direct observation of light deflection of fixed stars near the Sun during solar eclipses when they would be visible.

[4] In October 1911, Freundlich contacted astronomer Charles D. Perrine in Berlin to inquire as to the suitability of examining existing solar eclipse photographs to prove Einstein's prediction of light deflection.

In 1912 Freundlich asked if Perrine would include observation of light deflection as part of his program for the solar eclipse of October 10, 1912, in Brazil.

[6] Two years later, the three observatory directors, Perrine, Freundlich, and Campbell included light deflection in their expeditions to the Russian Empire for the solar eclipse of August 21, 1914.

The amount of deflection that he calculated in 1911 was too small (0.83 seconds of arc) by a factor of two because the approximation he used does not work well for things moving at near the speed of light.

When Einstein completed the full theory of general relativity in 1915, he rectified this error and predicted the correct amount of light deflection caused by the Sun (1.75 seconds of arc).

Another of Einstein's notable thought experiments about the nature of the gravitational field is that of a rotating disk (a variant of the Ehrenfest paradox).

For a while, Einstein thought that there were problems with the approach, but he later returned to it and, by late 1915, had published his general theory of relativity in the form in which it is used today.

After the war, Einstein maintained his relationship with Leiden University, accepting a contract as an Extraordinary Professor; for ten years, from 1920 to 1930, he travelled to the Netherlands regularly to lecture.

The Mount Wilson Observatory in California, United States, published a solar spectroscopic analysis that showed no gravitational redshift.

[15] However, in May 1919, a team led by the British astronomer Arthur Stanley Eddington claimed to have confirmed Einstein's prediction of gravitational deflection of starlight by the sun while photographing a solar eclipse with dual expeditions in Sobral, northern Brazil, and Príncipe, a west African island.

When that approach was proven to be inconsistent, Einstein revisited the concept of general covariance and discovered that the hole argument was flawed.

In the early years after Einstein's theory was published, Sir Arthur Eddington lent his considerable prestige in the British scientific establishment in an effort to champion the work of this German scientist.

Because the theory was so complex and abstruse (even today it is popularly considered the pinnacle of scientific thinking; in the early years it was even more so), it was rumored that only three people in the world understood it.

to the field equations, which became: This permitted the creation of steady-state solutions, but they were unstable: the slightest perturbation of a static state would result in the universe expanding or contracting.

However, in 1957 (two years after Einstein's death), Martin Kruskal published a proof that black holes are called for by the Schwarzschild solution.

The first piece of evidence in support of general relativity came from its correct prediction of the anomalous rate of precession of Mercury's orbit.

Subsequently, Arthur Stanley Eddington's 1919 expedition confirmed Einstein's prediction of the deflection of light by the Sun during the total solar eclipse of 29 May 1919, which helped to cement the status of general relativity as a viable theory.

These include studies of binary pulsars, observations of radio signals passing the limb of the Sun, and even the global positioning system.

The first observation of gravitational waves, which came from the merger of two black holes, was made on 14 September 2015 by the Advanced LIGO team, corroborating another prediction of the theory 100 years after it was published.

[31][32][33] The first image of a black hole, the supermassive one at the center of galaxy Messier 87, was published by the Event Horizon Telescope Collaboration on 10 April 2019.

Both of these theories proposed changes to the field equations of general relativity, and both suffer from these changes permitting the presence of bipolar gravitational radiation.

There is a great deal of speculation in the physics community as to the modifications that might be needed to both general relativity and quantum mechanics in order to unite them consistently.

At the same time, in a closely related development, the study of physical cosmology entered the mainstream and the Big Bang became well established.