LIGHT VS GRAVITY- CELEBRATING 100 YEARS OF GROUNDBREAKING EDDINGTON EXPERIMENT
Sir Arthur Stanley Eddington (28 December 1882 – 22 November 1944) was an English astronomer, physicist, and mathematician of the early 20th century who did his greatest work in astrophysics.
On 29 May 2019, exactly 100 years have passed since the confirmation of Einstein’s General Theory of Relativity, an event that fundamentally changed our understanding of physics and astronomy. Astronomer Sir Arthur Eddington organized two expeditions to observe a total solar eclipse – when the Moon appears to completely block out the Sun – with teams of scientists travelling to the island of Principe off the coast of West Africa and to Sobral in Brazil. This joint expedition was led by the Royal Astronomical Society and the Royal Society, with staff and equipment from the Royal Observatory Greenwich, and the universities of Cambridge and Oxford.
Photographs taken during the eclipse showed the apparent deflection of stars from their normal position, confirming Einstein’s theory. The famous eclipse expedition of 1919 to Sobral, Brazil, and the island of Principe, in the Gulf of Guinea, led by Dyson, Eddington and Davidson was a turning point in the history of relativity, not only because of its importance as a test of Einstein’s General Theory of Relativity, but also because of the intense public interest which was aroused by the success of the expedition.
Fig 1.1 (a) Albert Einstein (b) Arthur Eddington
- 1704 – Newton, Optiks: “Do not Bodies act upon Light at a distance, and by their action bend its rays; and is not this action strongest at the least distance?” δ = 2GM/Rc2
- 1784 – Henry Cavendish calculates deflection angle of corpuscular light ray in Newtonian gravity
- 1804 – von Soldner calculates deflection of 0.84 arc sec for stars close to the limb of the sun. Value considered too small to be observationally relevant.
- 1800’s – Corpuscular theory of light is disfavored for wave theory. General confusion over whether deflection is expected for a light wave.
- As early as 1912, Albert Einstein predicted that a geometric theory of gravity would exhibit this phenomenon.
- 1915 – Einstein publishes theory of GR and predicts new value for deflection angle solar deflection angle is calculated to be 1.75 arcsecs – double the Newtonian (corpuscle) value δ = 4GM/Rc2
Eddington’s enthusiasm and promotion of the work in the Royal Astronomical Society in 1917 pushed him to be granted permission to test the theory of bending light “out in the field” and an eclipse chaser. He travelled through the world and chased many eclipses and was unsuccessful many times due to cloudy skies and other problems. Since he is a ceaseless and motivational person, he continued his experiments. The eclipse on May 29th, 1919 was chosen as at the time the sun would be right in front of a prominent group of stars known as the Hyades. Two expeditions would be sent. Eddington would lead one to the island of Principe off the coast of West Africa and the second would travel to Sobral in Northern Brazil under the watch of Andrew Crommlin (an astronomer at the Royal Greenwich observatory).
Fig 2.1 Photograph of the solar eclipse by Arthur Eddington and Edwin Cottingham, Principe, 29 May 1919 [Image credit: Royal Astronomical Society]
3.1 GRAVITATIONAL LENSING – EXPLAINED
A gravitational lens is a distribution of matter (such as a cluster of galaxies) between a distant light source and an observer that is capable of bending the light from the source as the light travels towards the observer. This effect is known as gravitational lensing, and the amount of bending is one of the predictions of Albert Einstein’s general theory of relativity.
In simple terms, if a large gravitational mass (the sun) is in the line of sight between an observer and a light emitting body (a star) the gravitational field of the sun bends the light ray as shown in the diagram below towards the observer. As we observe light in straight lines the apparent position of the star (as we see it) is not the actual position of said star.
Fig 3.1 Schematic Illustration of gravitational lens
3.2 TYPES OF GRAVITATIONAL LENS
(i) Strong lensing
(ii) Weak lensing
(iii) Micro lensing
4.1 ARTHUR EDDINGTON’S GRAVITATIONAL LENSING EXPERIMENT
The first observation of light deflection was performed by noting the change in position of stars as they passed near the Sun on the celestial sphere. The observations were performed in 1919 by Arthur Eddington, Frank Watson Dyson, and their collaborators during the total solar eclipse on May 29.
The solar eclipse allowed the stars near the Sun to be observed. Observations were made simultaneously in the cities of Sobral, Ceara, Brazil and in São Tomé and Príncipe on the west coast of Africa. The observations demonstrated that the light from stars passing close to the Sun was slightly bent, so that stars appeared slightly out of position.
Fig 4.1 Arthur Eddington’s deflection of light experiment
4.2 EDDINGTON’S EXPERIMENT ON TOTAL SOLAR ECLIPSE ON MAY 29, 1919
During a total solar eclipse, Sir Arthur Eddington performs the first experimental test of Albert Einstein’s general theory of relativity. The findings made Einstein a celebrity overnight, and precipitated the eventual triumph of general relativity over classical Newtonian physics.
In 1919, Newton’s law of universal gravity still dominated scientific discourse, as it provided extremely accurate explanations of physical observations. But Einstein had a major issue with Newton’s theory: It wasn’t consistent with his own special theory of relativity, which predicted that space and time were relative, forming a four-dimensional continuum called space-time. He conceived a general theory of relativity, in which gravitational fields would cause warps in space-time, thus weaving gravity into the continuum.
One prediction of general relativity was that light should not travel in a perfectly straight line. While traveling through space-time and nearing the warp induced by an object’s gravitational field, light should curve — but not by much. A ray of light nicking the edge of the sun, for example, would bend a minuscule 1.75 arc seconds — the angle made by a right triangle 1 inch high and 1.9 miles long. Newtonian physics also predicted light would bend due to gravity, but only by half as much as Einstein’s theory predicted.
Fig 4.2 Graphical representation of actual and apparent position of the star due to sun’s gravitational field
Such a tiny difference seemed impossible to measure by earthly experiments. In fact, the two theories, though fundamentally opposed, made highly similar predictions for almost all tests of gravity and light. As a result, it was futile to try to understand which one provided a more accurate description of the fundamental laws of the universe.
Sir Frank Watson Dyson, Astronomer Royal of Britain, conceived in 1917 the perfect experiment to resolve the issue. A total solar eclipse on May 29, 1919, would occur just as the sun was crossing the bright Hyades star cluster. Dyson realized that the light from the stars would have to pass through the sun’s gravitational field on its way to Earth, yet would be visible due to the darkness of the eclipse. This would allow accurate measurements of the stars’ gravity-shifted positions in the sky.
Eddington, who led the experiment, first measured the “true” positions of the stars during January and February 1919. Then in May he went to the remote island of Príncipe (in the Gulf of Guinea off the west coast of Africa) to measure the stars’ positions during the eclipse, as viewed through the sun’s gravitational lens.
Eddington also sent a group of astronomers to take measurements from Sobral, Brazil, in case the eclipse was blocked by clouds over Príncipe. Outfitting and transporting the dual expeditions were no small feats in the days before transoceanic airplanes and instantaneous global communication.
Both locations had clear skies, and the astronomers took several pictures during the six minutes of total eclipse. When Eddington returned to England, his data from Príncipe confirmed Einstein’s predictions. Eddington announced his findings on Nov. 6, 1919. The next morning, Einstein, until then a relatively obscure newcomer in theoretical physics, was on the front page of major newspapers around the world.
The bending of light around massive objects is now known as gravitational lensing, and has become an important tool in astrophysics. Physicists now use gravitational lensing to try to understand dark matter and the expansion of the universe.
5. RESULTS OF THE EXPERIMENT
1919 Eclipse, Arthur Eddington went to Island of Principe, off West Africa to observe a total eclipse. Another team went to Sobral, Brazil.
- 2 plates in Principe successful; measured δ = 1.61 ± 0.30 arc sec
- One telescope in Sobral gave δ = 0.93 arc sec
- Another gave δ = 1.90 ± 0.11 arc sec
Eddington only first verified experimentally and published the two values in grievance with GR prediction.
- Actual position of the star
- Apparent position of the star
On 12 September Eddington attended the British Association for the Advancement of Science meeting in Bournemouth, where he showed the eclipse plates with the spectacular prominence and mentioned that measurements made to date indicated a deflection somewhere between the predictions of Newton and Einstein (Observatory Magazine, 1919, 42, 361).On 27 September Einstein (collected papers) wrote to his mother; ‘Today some happy news. H. A. Lorentz telegraphed me that the English expeditions have really verified the deflection of light by the Sun’. Before going on to ask after his mother’s health.
The results of combined observations from Cambridge and Greenwich were finally announced to the world at a joint meeting of the Royal Society and Royal Astronomical Society on 6 November 1919 (Observatory Magazine. 1919, 42, 389). They were subsequently published in Dyson, Eddington & Davidson (1920). In this paper Diagram 2 (above), based on the Sobral results with the 4 inch Cortie lens, convincingly shows that the variation of displacement with distance from the solar limb closely follows Einstein’s predictions and differs significantly from Newton’s prediction. The enhanced version of diagram 2 from Dyson et al. 1920 shows the observed shifts (blue dots) of stars measured on the 4 inch Cortie telescope plates, compared to the predictions of Einstein and Newton as a function of the distance of the stars from the center of the Sun.
One of the most remarkable aspects of the results of the expedition was the reaction of the general public to the discovery. The deflection of light by the Sun was trumpeted as a triumph for the General Theory of Relativity, which overturned many people’s intuitive understanding of the nature of space and time. The day after the announcement of the results at the joint meeting of the Royal Society and the Royal Astronomical Society, the headline in The Times of London read ‘Revolution in Science. New Theory of Universe. Newtonian Ideas Overthrown’. Its 100years since this experiment has been performed successfully which brought a new revolution in Astronomy and space sciences. The eclipse expeditions of 1919 certainly led to the eventual acceptance of Einstein’s general theory of relativity in the scientific community. This theory is now an important part of the training of any physicist and is regarded as the best we have for describing the various phenomena attributable to the action of gravity. The events of 1919 also established Einstein, rightly, as one of the century’s greatest intellects.
8. TO READ MORE ON GRAVITATIONAL LENSING
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