History of general relativity

General relativity is a theory of gravitation that was developed by Albert Einstein between 1907 and 1915, with contributions by many others after 1915. According to general relativity, the observed gravitational attraction between masses results from the warping of space and time by those masses.

Before the advent of general relativity, Newton's law of universal gravitation had been accepted for more than two hundred years as a valid description of the gravitational force between masses, even though Newton himself did not regard the theory as the final word on the nature of gravity. Within a century of Newton's formulation, careful astronomical observation revealed unexplainable differences between the theory and the observations. Under Newton's model, gravity was the result of an attractive force between massive objects.

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. General relativity also predicts novel effects of gravity, such as gravitational waves, gravitational lensing and an effect of gravity on time known as gravitational time dilation. Many of these predictions have been confirmed by experiment or observation, while others are the subject of ongoing research.

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. 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). General relativity is also part of the framework of the standard Big Bang model of cosmology.

Creation of general relativity

Early investigations

The first idea for a theory about gravity that included ideas from special relativity was suggested by Henri Poincaré in 1905. He thought gravity might be carried by waves that move at the speed of light.

Albert Einstein started working on his theory of gravity while he was still working at a patent office in 1907. He had an important idea he called his "happiest thought." He realized that the rules of special relativity could also apply to gravity. This led him to write an article in 1907 where he explained that falling freely feels the same as moving without any forces, a concept called the equivalence principle. In this article, he also guessed that time might pass differently depending on how strong gravity is.

In 1911, Einstein wrote another article where he imagined a box that was being pushed upward. He thought that this would feel the same as sitting still in a place with gravity. Using special relativity, he figured out that clocks higher up would tick faster than clocks lower down. This meant that time passes at different speeds depending on where you are in a gravitational field.

Einstein also guessed that big objects like the Sun or Jupiter could bend light. He asked scientists to look for this during solar eclipses when stars become visible near the Sun.

Developing general relativity

In 1912, Einstein returned to Switzerland and met his old friend Marcel Grossmann. Grossmann introduced Einstein to some new math that helped him describe curved space. By late 1915, Einstein finished his big theory, called the general theory of relativity. This theory says that gravity comes from how matter changes the shape of space and time.

In May 1919, a team led by Arthur Eddington took pictures during a solar eclipse. They found that light from stars near the Sun did bend, just as Einstein had predicted. This helped prove that Einstein's theory was right.

General covariance and the hole argument

In 1912, Einstein started trying to explain gravity as something about the shape of space. He worked with a mathematician named Tullio Levi-Civita. But Einstein thought there was a problem with this idea, called the "hole argument." Later, he realized he had made a mistake and went back to his original idea.

The development of the Einstein field equations

When Einstein finished his theory, he created some equations to describe it. At first, he made a small mistake, but he fixed it by November 1915. These equations help us understand how matter and energy change the shape of space and time.

Einstein and Hilbert

In the final year of his work on general relativity, Einstein met with a mathematician named David Hilbert. Hilbert was also working on similar ideas around the same time.

Sir Arthur Eddington

After Einstein published his theory, Sir Arthur Eddington helped explain it to others. Because the theory was very complicated, some people said only a few people in the world really understood it.

Solutions

The Schwarzschild solution

Einstein thought the equations were too complex to solve, but Karl Schwarzschild found a solution in 1915. He showed how space and time change around a big, round object. This is called the Schwarzschild solution. Many more solutions have been found since then.

The expanding universe and the cosmological constant

Main article: Cosmological constant

In 1922, Alexander Friedmann discovered that the universe could be growing or shrinking. Later, Georges Lemaître also found a way for the universe to expand. At first, Einstein thought the universe stayed the same size. To make his ideas work, he added something called the cosmological constant. But in 1929, Edwin Hubble showed the universe was actually expanding. Einstein then removed the cosmological constant.

More exact solutions

More work has been done to solve these equations. For example, a solution for a charged object was found and named the Reissner–Nordström solution. In the 1960s, Roy Kerr found a solution for a spinning object, called the Kerr solution. Later, the Kerr–Newman solution was found for a spinning, charged object.

Testing the theory

Main articles: Tests of general relativity and Gravitational wave observation

The first proof that general relativity worked came from its correct prediction of how Mercury's orbit moves in a special way. Later, an expedition led by Arthur Stanley Eddington in 1919 showed that light from the Sun bends just as Einstein predicted during a total solar eclipse. Since then, many observations have matched what general relativity predicts. These include studies of spinning stars called pulsars, observations of radio signals near the Sun, and the way our global positioning system works.

The theory also predicts that space and time can ripple, creating waves called gravitational waves. The first time we saw these waves was on September 14, 2015, when two black holes merged. This discovery happened 100 years after Einstein first shared his theory. In 2019, we also got the first picture of a huge black hole at the center of the galaxy Messier 87.

Alternative theories

Main article: Alternatives to general relativity

Scientists have tried to create new ideas about gravity that are different from Einstein's theory. Two well-known ideas are the Brans–Dicke theory and Rosen's bimetric theory. These ideas change how we think about gravity, but they have problems. Observations of stars called binary pulsars showed that Rosen's theory does not work. For Brans–Dicke, there are limits to how much it can differ from Einstein's theory.

Einstein's theory also does not fit well with quantum mechanics, which explains how tiny particles behave. Scientists are still trying to find ways to bring these two ideas together. Some of these ideas are called quantum gravity, and examples include string theory and loop quantum gravity.

Golden age

Kip Thorne called the time from 1960 to 1975 the "golden age of black hole research." During this time, the study of general relativity became very important in theoretical physics. Many exciting ideas, like black holes and gravitational singularities, were introduced. Around the same time, the study of the universe, called physical cosmology, also became very popular. The idea of the Big Bang became widely accepted.

Fulvio Melia often talks about the "golden age of relativity" in his book Cracking the Einstein Code. In 1962, Andrzej Trautman held a meeting in Warsaw that helped start this exciting time. After this meeting, general relativity entered a period of many new discoveries that lasted until the mid-1970s. Roy Kerr, a main character in Melia’s book, said the book beautifully captures this wonderful time in science.