100 Years of General Relativity

On 4 November 1915 Einstein wrote to his elder son Hans Albert Einstein, “In the last days I completed one of the finest papers of my life; when you are older I’ll tell you about it” (1). He referred to the first out of four papers Einstein wrote in November 1915 where he finally developed his General Theory of Relativity. Today, November 25th, marks the 100th anniversary of the presentation of the Einstein field equations on the Prussian Academy of Sciences, which is one of the anniversaries celebrated during the International Year of Light and Light-based Technologies 2015 (IYL 2015).


The General Theory of Relativity provides the law of gravitation and its relation to the other forces of nature. From Newton we knew about the “strength” of gravity, but his theory did not tell us how gravity pulls on things. Einstein’s General Theory of Relativity was a game-changer. In the general theory of relativity the doctrine of space and time no longer figures as a fundamental independent of the rest of physics. The geometrical behaviour of bodies and the motion of clocks rather depend on gravitational fields, which in their turn are produced by matter.

To obtain this result he spent ten years (from 1905 to 1915) completely dedicated to obtain a more “general” application of what he developed with the special theory of relativity. He noticed that for the purpose of describing nature the choice of the state of motion of an arbitrary coordinate system should have no restrictions.

In 1907, he had what he defined as “the happiest thought of his life”, a milestone that paved the way for the development of his theory (2). He imagined a man falling from the roof of a house and he thought that man would not feel his own weight; he would feel weightless. He realized that there is a link between gravity and motion and after working out this idea for the next eight years, Einstein concluded that space is like a stretchy sheet of fabric that warps when objects are on it. Bigger objects like the Sun warp space more than objects like the Earth. This is also true for time: clocks run slower in a strong gravitational field than they do in empty space.

Confirmation of the theory

Negative of the 1919 solar eclipse taken from the report of Sir ArthurEddington with the title "A Determination of the Deflection of Light by the Sun's Gravitational Field, from Observations Made at the Total Eclipse of May 29, 1919". Credit: Wikimedia Commons.

Negative of the 1919 solar eclipse taken from the report of Sir ArthurEddington with the title “A Determination of the Deflection of Light by the Sun’s Gravitational Field, from Observations Made at the Total Eclipse of May 29, 1919”. Credit: Wikimedia Commons.

The scientific community received the theory with scepticism and its complexity did not help either. It needed to be confronted with experiments and Einstein proposed three tests, the so-called classical tests of general relativity: the perihelion precession of Mercury’s orbit, the deflection of light by the Sun and the gravitational redshift of light. But it was the second one that made Einstein and his theory world famous.

Newtonian gravity had predicted that starlight will bend around a massive object such as the Sun. Einstein used general relativity to calculate how much the light would bend. However, to test this calculation, they have to measure how much stars would change their position as they passed near the Sun. This was done in 1919 during a total solar eclipse (3), when the British scientist Arthur Edddington led two simultaneous observations in Brazil and São Tomé and Príncipe to test General Relativity. The size of the deflection measured during the eclipse was on agreement of Einstein’s predictions and next day Einstein’s life changed forever.

Einstein was always thankful to the astronomers and physicists of England for testing the implications of a theory that was perfected and published during the First World War in Germany, the land of their enemies (4). It was a way to prove of the power of science on overcoming the darker pages of human history.

The Nobel Prize that never came

Einstein gave us a way to understand the universe as a whole and made one of the most beautiful, complex theories on the history of science that, after confirmation, would have given him immediately the Nobel Prize in Physics on any period of history. But the world was a mess, to say the least, in 1919.

Before 1919, the Nobel committee decided against his work on the grounds that relativity was unproven. But in 1919 he was snubbed again. On one hand, some people argued that it was due to the fact that on the aftermath of World War antisemitism was on the rise on Germany, other sources claimed that the complexity of relativity did not help. On the other hand, some people agreed that the Eddington’s observations had not been perfect and he had discarded data he considered poor from his final analysis so a better test to prove general relativity was still missing (later tests in another Solar eclipses agreed with the results from 1919) (5).

In 1921 the Nobel Committee even decided it was better not to award the prize at all. At the end, the following year Einstein received the deferred 1921 prize but not for general relativity. Instead the prize was awarded for his work on the photoelectric effect in 1905. The prize was awarded for “services to theoretical physics, and especially for his discovery of the law of the photoelectric effect” (6). One might argue that the reference to theoretical physics was a way to acknowledge relativity or that the doors were still open for a second Nobel prize on the future but this never happened and became one of the biggest injustices on the history of the Nobel prizes.

The Consequences of General Relativity

Albert Einstein’s general theory of relativity fundamentally changed the way we understand gravity and the Universe in general. For instance, the German astronomer Karl Schwarzschild first used general relativity to predict the existence of black holes, which are regions of the spacetime exhibiting such strong gravitational effects that nothing – not even light  – can escape from it. General relativity has also helped to study other dense objects on the universe such as quasars or explained effects such a gravitational lensing, where massive structures in the Universe exert such a powerful gravitational pull that they can warp the spacetime around them and act as cosmic lenses that can magnify, distort and bend the light behind them.

In the centre of this image there are two faint galaxies that seem to be smiling. You can make out two orange eyes and a white button nose. In the case of this “happy face”, the two eyes are the galaxies SDSSCGB 8842.3 and SDSSCGB 8842.4 and the misleading smile lines are actually arcs caused by an effect known as strong gravitational lensing. Credit: NASA/ESA Hubble Space Telescope.

In the centre of this image there are two faint galaxies that seem to be smiling. You can make out two orange eyes and a white button nose. In the case of this “happy face”, the two eyes are the galaxies SDSSCGB 8842.3 and SDSSCGB 8842.4 and the misleading smile lines are actually arcs caused by an effect known as strong gravitational lensing.  Credit: NASA/ESA Hubble Space Telescope.

The theory also has important implications for cosmology, the study of the structure of the Universe. One of the consequences of the theory is that not only the universe is finite but should be contracting or expanding. Einstein thought that the Universe might be static so introduced the so-called cosmological constant to adjust his equations so they predict a static universe. Years later, the U.S. astronomer Edwin Hubble discovered that the Universe was indeed expanding so Einstein, later in his life, called the cosmology constant the greatest mistake of his life. However, observations at the end of the last century showed that the universe’s rate of expansion is accelerating (7) and scientists explained this acceleration with the presence of a negative “force” (dark energy) that pushes matter away rather than attracting it (8). The presence of Einstein cosmological constant helped reconcile theory with observations and scientists are reconsidering its importance.

But there is still one prediction that have not been observed, the gravitational waves. According to general relativity, ripples known as gravitational waves can propagate across space in much the same way that ripples spread across the surface of a pond. So far, researchers have yet to detect such changes. Today’s best instrument for its detection is the Advanced LIGO experiment, a U.S big interferometer that represents for the search of gravitational waves what the LHC was for the hunt of the Higgs Boson. But efforts to find these waves are not happening only down here on Earth, the European Space Agency (ESA) will launch on 2 December 2015 the LISA Pathfinder satellite that will try to catch the gravitational waves from space.

What’s next?

Even though general relative has passed every test and have helped scientist go well beyond what Newton dreamed, many scientists think is incomplete. This comes from the fact that scientists haven’t found a way yet to reconcile gravity and quantum physics (9) or that some scientists argue that phenomena such as dark matter could be actually failures of general relativity. This have led scientists to look for alternative explanations such as modifications of the General Relativity (10) but no explanation has been good enough to circumvent these problems and many questions are still open.

It took more than two hundred years and many struggles until Einstein showed us how gravity and the Universe works. A hundred years later we celebrate worldwide this beautiful theory with the same exciment as Einstein wrote to his son Hans to tell him the good news. And I wonder what beutiful explanation to understand better the Universe would come next.

More Information

1 – The Collected Papers of Albert Einstein – Princeton University Press. Volume 8: The Berlin Years: Correspondence, 1914-1918 (English translation supplement) Page 140 http://einsteinpapers.press.princeton.edu/vol8-trans/168
2 – Thoughts experiments in Einstein’s work – John Norton http://www.pitt.edu/~jdnorton/papers/TE_in_AE.pdf
3 – Solar Eclipse Observations proved that Einstein was right – IYL 2015 blog
4 – What is the Theory of Relativity? –  The Collected Papers of Albert Einstein – Princeton University Press. Volume 7: The Berlin Years: Writings, 1918-1921 (English translation supplement) Page 100
5 – D. Kennefick, “Testing relativity from the 1919 eclipse- a question of bias”, Physics Today, March 2009, pp. 37–42
6 – http://www.nobelprize.org/nobel_prizes/physics/laureates/1921/
7 – Distant Supernovae Indicate Ever-Expanding Universe http://www.eso.org/public/news/eso9861/
8 – http://science.nasa.gov/astrophysics/focus-areas/what-is-dark-energy/
9 – A. Ashtekar et al., “From General Relativity to Quantum Gravity” – General Relativity and Gravitation: A Centennial Survey, commissioned by the International Society for General Relativity and Gravitation and to be published by Cambridge University Press http://alpha.sinp.msu.ru/~panov/Lib/Papers/QGL/1408.4336v1.pdf
10 – T. Clifton et al.,  “Modified Gravity and Cosmology” http://arxiv.org/pdf/1106.2476.pdf

jorge_riveroJorge Rivero González (@jorgegrivero) was born in Las Palmas de Gran Canaria, Spain. He is a science communicator working at the European Physical Society (EPS) as the EPS IYL 2015 Outreach Officer. He is also IYL 2015 Press Officer and editor of the IYL 2015 blog. Before that he spent four years living in Munich while he was doing his PhD in Astrophysics at the University Observatory Munich. Since 2009 he is a member of the science outreach programme GalileoMobile.

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