5.2.6 The Great Eclipse of 1919
Unequivocal? Or Confirmation Bias?
Evaluating the deflection of light near the sun during a solar eclipse calls for delicate measurements with sensitive equipment hastily transported and set up in the path of the eclipse. Changes in temperature may alter the focal length and focus of the instrument. A comparison shot of the star field without the sun is essential to identify subtle variations in the optical path that may mimic the effect sought. And the best prepared team may still fail due to war or weather [[i]]. Several teams tried to observe the eclipses of 1912 and 1914 and had failed [[ii]]. The failures of past expeditions were a blessing in disguise. One of Einstein’s biographers notes:
Suppose the war had not come, and Finlay-Freundlich had been able to observe the 1914 eclipse, finding a deflection of 1.7 seconds of arc at a time when Einstein was predicting a deflection of only 0.83 seconds of arc. Or that in America Hale and his astronomer friends, without having to wait for an eclipse, had quickly been able to find that the deflection was double this predicted value. Imagine how tame Einstein’s 1915 calculation of 1.7 would then have seemed. He would have been belatedly changing the value after the event, having first been shown to have been wrong. People would have felt that he had made an arbitrary ad hoc adjustment—which in fact he had not—and the deflection of light would have lost the tremendous impact that it had as a prediction. But the war had occurred, and the predicted deflection had been confirmed under circumstances of high drama, at a time when nations were war-weary and heartsick. The bent rays of starlight had illumined a world in shadow, revealing a unity of man that transcended war [[iii]].
In 1918, the Lick Observatory team (who we last saw getting rained out trying to observe an eclipse in Russia in 1914) had the home court advantage as the June 8, 1918 eclipse passed over the continental United States [[iv]]. With their equipment still stranded in Russia however, they had to improvise and make do with borrowed instruments. They succeeded in capturing stellar images near the darkened sun, but after a long, arduous, and challenging analysis. American astronomer William Wallace Campbell (1862–1938) reported (without much confidence) that their results “preclude the larger Einstein effect, but not the small amount expected according to the original Einstein hypothesis” [[v], [vi]]. In other words, Soldner’s long-forgotten prediction and Einstein’s original 1911 prediction appeared consistent with the Lick Observatory result. The Lick team believed their measurement falsified the larger deflection prediction of the 1915 General Relativity Theory.
The stage was set for Arthur Stanley Eddington (1882–1944) to try. A Quaker and lifelong bachelor, he already had the experience of a rained-out eclipse expedition to Brazil in 1912 [[vii], [viii]]. As a pacifist, he refused to serve in World War I. In lieu of being sent to an internment camp, Cambridge University successfully secured a scientific deferment for him on the condition that he participate in the solar eclipse expedition of 1919 [[ix], [x], [xi]]. England’s Astronomer Royal, Frank Watson Dyson (1868–1939), dispatched a team from the Royal Observatory at Greenwich to Sobral, Brazil to view the eclipse from there, while Eddington led a team from the Cambridge Observatory to Príncipe, an island off the coast of West Africa [[xii]].
Years later, Eddington confided to his student, the Indian-American astronomer Subrahmanyan Chandrasekhar (1910–1995), that left to himself, he would not have planned the expeditions since he was fully convinced of the truth of the general theory of relativity! [[xiv]] Nevertheless, both Dyson’s expedition to Sobral, Brazil and Eddington’s expedition to Príncipe Island were well equipped.
Both parties took identical 13-inch astrographic lenses of 11 feet 3 inches focal length. That taken to Sobral came from the Astrographic Telescope at Greenwich. That taken to Principe came from the Astrographic Telescope at Oxford. The object glasses were attached on arrival to hollow steel tubes (made by Harvey & Co.), with the lens at one end and the photographic plate holder at the other and then mounted horizontally so that the sunlight could be reflected into them by a 16 inch diameter flat mirror.
The flat mirror or coelostat, was slowly rotated to follow the Sun, using a clock driven by a heavy weight, that ran for about half an hour before the weight had to be rewound. Both astrographic lenses were stopped down to 8 inches diameter to improve their image quality.
The Greenwich Sobral group, also took a 4 inch diameter lens with a focal length of 19 feet, loaned to them by Father Cortie. This was fed by an 8 inch diameter coelostat loaned by the Royal Irish Academy. The Greenwich group also took a 3½ inch Detached Telescope (R.O. No. 3) and the Eros Micrometer [[xv]].
Eddington’s Príncipe team suffered cloudy weather and only the brightest stars were visible right at the end of totality [[xvi]]. The Príncipe investigators packed up and headed home, having taken their comparison plates before leaving England [[xvii]]. Their final result for deflection at the edge of the sun was 1.61” ± 0.30.” This aligned with the 1.75” prediction of General Relativity for deflection, but with a wide margin of error [[xviii]].
The Sobral team was blessed with an opening in the clouds that gave them a clear view of the eclipse. They returned to Sobral a month later to take comparison plates [[xix]].
When analyzed, the Sobral achieved a result of 1.98” ± 0.12” with the four-inch telescope, a bit higher. The astrographic telescope at Sobral yielded a result of 0.93” much closer to the Newtonian prediction. This latter result they decided should be discarded as the images appeared somewhat out of focus [[xxi]].
These observations would be said to confirm the overthrow of classical physics with “no shadow of doubt.” It might be worth double checking so important a data set and analysis. That’s not possible, however. Irish astrophysicist and science historian Daniel Kennefick (1965– ), author of No Shadow of a Doubt The 1919 Expedition That Confirmed Einstein’s Theory of Relativity explains:
It is probable that Eddington alone handled the data analysis of the Principe expedition. We cannot determine how this was done because none of the data analysis sheets or photographic plates have survived.... If he kept any notes or records, they were later lost. Even the plates he took have long since disappeared [[xxiv]].
Later historians noted how the eclipse expeditions and their work fell short of the highest standards of science.
Now the eclipse expeditions confirmed the theory only if part of the observations were thrown out and the discrepancies in the remainder ignored; Dyson and Eddington, who presented the results to the scientific world, threw out a good part of the data and ignored the discrepancies.
This curious sequence of reasons might be cause enough for despair on the part of those who see in science a model of objectivity and rationality. That mood should be lightened by the reflection that the theory in which Eddington placed his faith because he thought it beautiful and profound—and, possibly, because he thought that it would be best for the world if it were true—this theory, so far as we know, still holds the truth about space, time and gravity [[xxv]].
While Eddington’s Príncipe plates and analysis are long gone, in 1979, a team equipped with more modern instruments reanalyzed the plates from Sobral [[xxvi]].
The result from the four-inch plate was similar to the original 1919 analysis result. However, the astrographic plate whose result was originally 0.93”, now yielded 1.55” ± 0.34”, a result for the deflection at the edge of the sun much closer to the General Relativity prediction of 1.75” [[xxvii]].
In a more recent 2019 interview, however, Irish astrophysicist and science historian Daniel Kennefick (1965– ), author of No Shadow of a Doubt The 1919 Expedition That Confirmed Einstein’s Theory of Relativity explained:
…no data from [the Sobal] expedition has survived. The photographic plates were lost… the Sobral plates were moved and no one could tell me exactly where they are [[xxviii]].
While no original plates are extant, a copy of an original Sobral photographic plate went to a German observatory, where it miraculously survived.
The team at Landessternwarte Heidelberg-Königstuhl applied “modern image processing techniques — including image restoration, noise reduction, and removal of artifacts recently scanned theirs as part of the Heidelberg Digitized Astronomical Plates (HDAP) project. Unfortunately, without the comparison plates to identify, measure, and remove the imperfections of the particular optical system, this individual plate can’t be used to make a modern reanalysis. As historian of science, John Plaice (1962 – ), notes at his Fiat Lux Substack:
One of the key ideas in modern science is the repeatability of experiments. For direct observations of ongoing phenomena in Nature, repeatability is not possible. The heavens will never be exactly as they were on May 29th, 1919. So if we wish to reëxamine the observations made that day and the interpretations that were made therefrom, it is the raw data that is of most importance [[xxix]].
Despite all these issues and limitations, modern scholars confidently assert that Dyson’s and Eddington’s 1919 results were robust with “no shadow of a doubt” [[xxx], [xxxi]]. Dyson himself wrote in December 1919 that his Greenwich Observatory team was “very satisfied with the eclipse results, as they seemed definitive” [[xxxii]].
Their confidence is difficult to justify when we realize that Dyson soon had significant doubts. He “said that he would not be in the least surprised if the [imminent] 1922 [eclipse] photographs did not confirm the Einstein effect. He thought that possibly they in England had stressed Einstein a little too much” [[xxxiii]]. “The results of the 1919 eclipse were in accordance with Einstein, but it may be that your results of 1922 will not confirm this,” Dyson confided to William Wallace Campbell (1862–1938), the American astronomer leading up a 1922 eclipse expedition [[xxxiv]]. Only after receiving Campbell’s confirmation of the consistent 1922 results [[xxxv], [xxxvi]] did Dyson reply, “I don’t think there is ‘any possible probable shadow of doubt’ about the correctness of Einstein’s prediction of the deflection of light, whatever difficulties may be found with the rest of his theory” [[xxxvii]].
Whether the 1919 eclipse results were immediately unequivocal or a result of confirmation bias [[xxxviii]] is almost moot, however. Multiple subsequent observations confirmed Dyson’s and Eddington’s conclusion that light was deflected in approximately the amount predicted by Einstein’s 1915 General Relativity theory.
After multiple observations of the 1922 eclipse similarly agreed with the General Relativity prediction, and spectral line displacements aligned with gravitational red shift predictions [[xl]], Einstein’s theory achieved widespread acceptance. Writing in 1929, English physicist, mathematician, and astronomer, James Hopwood Jeans (1877 – 1946) declared:
The general theory of relativity has long passed the stage of being considered an interesting speculation… Indeed, the theory has by now qualified as one of the ordinary working tools of astronomy [[xli]].
Scientists largely gave up on trying to make eclipse measurements after 1973. Successive experiments were not much more precise than Dyson’s and Eddington’s 1919 results, and emerging radio astronomy measurements were setting much tighter bounds. Radar measurements of Mercury and Venus as their apparent positions neared the sun allowed for more accurate measurements [[xlii]]. Two quasars, designated 3C 273 and 3C 279, lie near the sun’s path across the sky, and by using interferometry to compare the signals, astronomers confirmed the predicted angular deflection to 1% accuracy by 1975 [[xliii]].
Recently, amateur astronomer Donald G. Bruns revisited the challenging problem of measuring stellar deflection during an eclipse. He imaged the 2017 solar eclipse, verifying Einstein’s prediction to 3% uncertainty [[xliv]]. Whatever irregularities there may have been in Dyson’s and Eddington’s observations and analysis, the announcement of their results in November 1919, around the one-year anniversary of the end of World War I would have dramatic scientific and cultural impact.
Next time: 5.2.7 A Scientific Superstar is Born
Acknowledgement: I have benefited greatly from discussions with John Plaice of Fiat Lux and other members of the Fields & Energy Telegram discussion group. Check out his Substack, here:
And you’ll find his excellent post on the 1919 eclipse results, here:
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References
[[i]] Collins, Harry and Trevor Pinch, The Golem: What Everyone Should Know About Science, Cambridge: University Press, 1993, pp. 46-47. See: https://amzn.to/4h1CCgw
[[ii]] Córdoba Estelar, “Attempts to prove Einstein`s Theory of Relativity,” Historia del Observatorio Nacional Argentino, S. Paolantonio – E. R. Minniti Morgan Observatorio Astronómico de la Universidad Nacional de Córdoba, 2013. See: http://sion.frm.utn.edu.ar/WDEAIII/wp-content/uploads/2018/09/Relatividad-Eng-Final2.pdf
[[iii]] Hoffmann, Banesh, with Helen Dukas, Albert Einstein: Creator & Rebel, New York: Viking Press, 1972, p. 132-133. See: https://amzn.to/3DK3Nhy
[[iv]] Rothman, Tony, Everything’s Relative: and Other Fables from Science and Technology, New York: John Wiley & Sons, 2003, pp. 83-84.
[[v]] Rothman, Tony, Everything’s Relative: and Other Fables from Science and Technology, New York: John Wiley & Sons, 2003, p. 83.
[[vi]] Earman, John and Clark Glymour, “Relativity and Eclipses: The British Eclipse Expeditions of 1919 and Their Predecessors,” Historical Studies in the Physical Sciences, Vol. 11, No. 1, 1980, pp. 49-85. See pp. 50-51.
[[vii]] Overbye, Dennis, Einstein in Love: A Scientific Romance, New York: Viking, 2000, p. 352.
[[viii]] Kennefick, Daniel, No Shadow of a Doubt, Princeton, NJ: Princeton University Press, 2019, p. 21. See: https://amzn.to/3ZZpkKI
[[ix]] Overbye, Dennis, Einstein in Love: A Scientific Romance, New York: Viking, 2000, pp. 353-354.
[[x]] Ohanian, Hans, Einstein’s Mistakes: The Human Failings of Genius, New York: W.W. Norton & Company, 2008, p. 4.
[[xi]] Kennefick, Daniel, No Shadow of a Doubt, Princeton, NJ: Princeton University Press, 2019, pp. 77-81. See: https://amzn.to/3ZZpkKI
[[xii]] Kennefick, Daniel, No Shadow of a Doubt, Princeton, NJ: Princeton University Press, 2019, pp. 14-15, 24. See: https://amzn.to/3ZZpkKI
[[xiii]] Dyson and Eddington stand outdoors. Personal subjects: Dyson, Frank Watson, 1868-1939; Eddington, Arthur Stanley, Sir, 1882-1944. Credit Line: AIP Emilio Segrè Visual Archives, W. F. Meggers Collection. Catalog ID: Dyson Frank C3. Copyright: American Institute of Physics. Collection info:fedora/nbla:287968. See: https://repository.aip.org/islandora/object/nbla%3A291610/datastream/OBJ/view
[[xiv]] Chandrasekhar, S., “Einstein and General Relativity: Historical Perspectives,” American Journal of Physics, vol. 47, no. 3, March 1979, pp. 212-217.
[[xv]] Catchpole, Robin and Graham Dolan, “General Relativity and the 1919 Solar Eclipse,” The Royal Observatory Greenwich, no date. See: http://www.royalobservatorygreenwich.org/articles.php?article=1283.
[[xvi]] Kennefick, Daniel, No Shadow of a Doubt, Princeton, NJ: Princeton University Press, 2019, p. 2. See: https://amzn.to/3ZZpkKI
[[xvii]] Kennefick, Daniel, No Shadow of a Doubt, Princeton, NJ: Princeton University Press, 2019, p. 170. See: https://amzn.to/3ZZpkKI
[[xviii]] Dyson Frank Watson, Arthur Stanley Eddington, and C. Davidson. “IX. A determination of the deflection of light by the sun's gravitational field, from observations made at the total eclipse of May 29, 1919,” Philosophical Transactions of the Royal Society of London, Series A, Containing Papers of a Mathematical or Physical Character,” vol. 220, 01 January, 1920, pp. 291–333. Doi: 10.1098/rsta.1920.0009. See: https://royalsocietypublishing.org/doi/pdf/10.1098/rsta.1920.0009.
[[xix]] Kennefick, Daniel, No Shadow of a Doubt, Princeton, NJ: Princeton University Press, 2019, p. 170. See: https://amzn.to/3ZZpkKI
[[xx]] Taken from the 22 November, 1919 edition of the Illustrated London News. See: https://astro.dur.ac.uk/~rjm/Principe/1919eclipse.php
[[xxi]] Dyson Frank Watson, Arthur Stanley Eddington, and C. Davidson. “IX. A determination of the deflection of light by the sun's gravitational field, from observations made at the total eclipse of May 29, 1919,” Philosophical Transactions of the Royal Society of London, Series A, Containing Papers of a Mathematical or Physical Character,” vol. 220, 01 January, 1920, pp. 291–333. Doi: 10.1098/rsta.1920.0009. See: https://royalsocietypublishing.org/doi/pdf/10.1098/rsta.1920.0009.
[[xxii]] “The image shown here is arguably the highest resolution image of the 1919 eclipse, and is the result of applying modern image processing techniques — including image restoration, noise reduction, and removal of artifacts — to that plate copy (un-annotated version here). It unveils stunning details in the solar corona, a giant prominence emerging from the upper right part of the Sun, and stars in the constellation of Taurus (The Bull) that were used to confirm general relativity’s predictions.” Credit: ESO/Landessternwarte Heidelberg-Königstuhl/F. W. Dyson, A. S. Eddington, & C. Davidson. See: https://www.eso.org/public/images/potw1926a/
[[xxiii]] Harvey, Geoffrey M., “Gravitational deflection of light: A re-examination of the observations of the solar eclipse of 1919,” The Observatory, vol. 99, pp. 195–98.
[[xxiv]] Kennefick, Daniel, No Shadow of a Doubt, Princeton, NJ: Princeton University Press, 2019, p. 15, 220.
[[xxv]] Earman, John, and Clark Glymour, “Relativity and eclipses: the British Eclipse expeditions of 1919 and their predecessors,” Historical Studies in the Physical Sciences, vol. 11, no. 1, pp. 49–85. doi:10.2307/27757471
[[xxvi]] Harvey, Geoffrey M., “Gravitational deflection of light: A re-examination of the observations of the solar eclipse of 1919,” The Observatory, vol. 99, pp. 195–98.
[[xxvii]] Kennefick, Daniel, No Shadow of a Doubt, Princeton, NJ: Princeton University Press, 2019, pp. 242-245. See: https://amzn.to/3ZZpkKI
[[xxviii]] Anon., “Daniel Kennefick: The importance of Sobral,” Revista Pesquisa FAPESP, December 2019. See: https://revistapesquisa.fapesp.br/en/daniel-kennefick-the-importance-of-sobral-2/
[[xxix]] Plaice, John, “Where Is the 1919 Data Proving General Relativity?” Fiat Lux, December 15, 2024. See:
[[xxx]] Gilmore, Gerard and Tausch-Pebody Gudrun, “The 1919 eclipse results that verified general relativity and their later detractors: a story re-told,” Notes Rec.76, 21 October 2021. pp. 155–180. See: https://royalsocietypublishing.org/doi/10.1098/rsnr.2020.0040
[[xxxi]] Kennefick, Daniel, No Shadow of a Doubt, Princeton, NJ: Princeton University Press, 2019. As you might imagine from that title! See: https://amzn.to/3ZZpkKI
[[xxxii]] Crelinsten, Jeffrey, Einstein’s Jury: The Race to Test Relativity, Princeton, NJ: Princeton University Press, 2006, pp. 146-147.
[[xxxiii]] Crelinsten, Jeffrey, Einstein’s Jury: The Race to Test Relativity, Princeton, NJ: Princeton University Press, 2006, p. 205.
[[xxxiv]] Crelinsten, Jeffrey, Einstein’s Jury: The Race to Test Relativity, Princeton, NJ: Princeton University Press, 2006, p. 205.
[[xxxv]] W. W. Campbell and R. Trümpler, “Observations on the deflection of light in passing through the Sun's gravitational field, made during the total solar eclipse of September 21, 1922,” Publ. Astr. Soc. Pacific vol. 35, no. 158, 1923.
[[xxxvi]] W. W. Campbell and R. J. Trümpler, “Observations made with a pair of five-foot cameras on the light-deflections in the Sun's gravitational field at the total solar eclipse of September 21, 1922,” Lick Obs. Bull., vol. 13, no. 130, 1928.
[[xxxvii]] Crelinsten, Jeffrey, Einstein’s Jury: The Race to Test Relativity, Princeton, NJ: Princeton University Press, 2006, p. 213.
[[xxxviii]] Nickerson, Raymond S., “Confirmation bias: a ubiquitous phenomenon in many guises,” Rev. Gen. Psychol. vol. 2, 1998, pp. 175–220. See in particular p. 186: https://www.researchgate.net/publication/280685490_Confirmation_Bias_A_Ubiquitous_Phenomenon_in_Many_Guises
[[xxxix]] St. John, Charles E., “Observational Basis of General Relativity,” Publications of the Astronmical Society of the Pacific, vol. XLIV, no. 261, pp. 277-295. See p. 286. See: https://iopscience.iop.org/article/10.1086/124248/pdf
[[xl]] St. John, Charles E., “Observational Basis of General Relativity,” Publications of the Astronmical Society of the Pacific, vol. XLIV, no. 261, pp. 277-295. See p. 286. See: https://iopscience.iop.org/article/10.1086/124248/pdf
[[xli]] Jeans, James, The Universe Around Us, Cambridge: University Press, 1929, pp. 74-75.See: https://archive.org/details/in.ernet.dli.2015.206154/page/n99/mode/2up?q=working
[[xlii]] Shapiro, I. I., “New Method for the Detection of Light Deflection by Solar Gravity,” Science, vol. 157, no. 3790, August 18, 1967, pp. 806–808. doi:10.1126/science.157.3790.806.
[[xliii]] Will, Clifford, Was Einstein Right? Putting General Relativity to the Test, New York: Basic Books, Inc., 1986, pp. 80-86.
[[xliv]] Bruns, Donald G., “Gravitational Starlight Deflection Measurements during the 21 August 2017 Total Solar Eclipse,” Classical and Quantum Gravity, Volume 35, Number 7, 6 March, 2018. DOI 10.1088/1361-6382/aaaf2a. See: https://arxiv.org/pdf/1802.00343, and https://iopscience.iop.org/article/10.1088/1361-6382/aaaf2a.
[[xlv]] Cropped from a photo of seven (7) astronomers and scientists gathered at Mount Wilson Observatory's Hale Library, Santa Barbara Street offices, Pasadena, California. Standing, left to right, are: Milton L. Humason, Edwin Powell Hubble, Charles St. John, Albert Abraham Michelson, Albert Einstein, William Wallace Campbell, and Walter S. Adams. The men are standing in front of a portrait painting and a blackboard, which is in front of bookshelves with books. All of the men are wearing suits. Image courtesy of the Observatories of the Carnegie Institution for Science Collection at the Huntington Library, San Marino, California. See: https://hdl.huntington.org/digital/collection/p15150coll2/id/804/