The 1919 Eclipse and Relativity

1. The 1919 Eclipse and Relativity A story with many aspects Dennis Miller Presented to the Arbeitskreis Philosophie Kelkheim. Originally in German as Naturwissenschaften und Gesellschaft: Sonnenfinsternis 1919 und die Relativitätstheorie. 25.11.2019

2. The 1919 Solar Eclipse Expedition Aspects of the story Theoretical physics space, time, light, gravity Astronomical measurements very difficult: small effects, many uncertainties Science & society research landscape, political & international aspects reaction of media and general public Scientific method nature of scientific theories and how are they confirmed ? data analysis Physics Philosophy, socienty Or as an adventure story: 100 years ago, immediately after the first world war, it was very difficult to undertake such an expedition. There were many risks - logistics, problems with apparatus, weather. Considering how many things could go wrong success seems unlikely. Nevertheless, the heros finally triumph with a spectacular result.

3. Relativity Theory - Overview Special Relativity Theory - Speed of light in free space is always the same. Does not depend on speed of source or observer. - The laws of physics are the same for all observers moving with constant speed. → At very high speeds length and time different for different observers: surprising consequences that contradict everyday experience! → Mathematical combinations of space and time that are the same for all observers (invariants). Equivalence of gravity and acceleration - This is not a new idea, but Einstein brought it into focus and discussed the consequences General Relativity Theory: - Extension of previous theory to include acceleration - Gravity: geometrical phenomenon in curved space-time - Difficult mathematics

4. Three tests of general relativity Einstein's suggestion ● Light deflection by sun's gravitational field ● Solar red shift Spectral lines shifted to longer wavelenths by sun's gravity ● Perihelion of mercury The orbit of mercury shows small deviations from predictions of Newton's theory (unexplained precession 43″ per century). General relativity gives quantitative explanation.

5. Light deflection by gravity Apparent position True position Sun Earth The deflection is very small and decreases with the distance of the light ray from the sun. Stars near the sun cannot normally be seen except during a total eclipse. Einstein wrote to George Hale, a leading American astronomer, to ask if there might be some other method to observe the deflection. His answer was no: the only possibility would be to take accurate photographs during an eclipse.

6. Timeline Relativity Theory a = 1.75" a = 0.87" Theory Public interest in Relativity Einstein becomes famous Special Relativity General Relativity Results announced Eclipse WWI a is the deflection angle for a star near the edge of the sun. There are two theoretical values: in 1915 Einstein saw that General Relativity indicates the angle should be twice as large as his original prediction made in 1911. The predictions for a are the ones quoted by Eddington. Einstein originally gave a slightly lower value, 0.83", for the simple theory.

7. Light deflection: three theoretical possibilities Value of a 1.75″ Einstein. General Relativity. 0.87″ Newton. This is Einstein's first prediction. It is compatible with Newton's theory together with the gravity / acceleration equivalence principle. This value was derived at the beginning of the 19th century, but the work had been forgotten and Einstein was not aware of it until about 1922. 0″ No deflection. At the time most most scientists did not expect gravity to deflect light rays. Theoretical physicists, however, increasingly saw problems with this simple idea.

8. Einstein and Eddington both were both pacifists. They belonged to minority religious groups (Eintstein Jewish, Eddington Quaker). Eddington and Dyson knew eachother well. Both had been very successful in mathematics as Cambridge undergraduates. Dyson would have heard about Relativity from Eddington. Photos: Wikipedia

9. Solar Eclipse 29 May 1919 A good opportunity because: - Bright stars near the sun - Long totality (about 5 min at observation stations) Stations selected - Sobral, Brazil - Principe, a volcanic island, in 1919 Portuguese colony Sobral Principe

10. Previous attempts to measure light deflection during an eclipse - all unsuccessful - 1912: Argentinian expedition to Brazil. Rain – no observations 1914: German expedition: Eclipse in south Russla Outbreak of WWI – no observations The astronomers were interned but returned to Germany as part of a prisoner exchange after a few weeks. 1918: Eclipse in USA. Problems with equipment In spite of considerable effort, the data evaluation did not give a reliable conclusion. No publication in a scientific journal.

11. Planning the eclipse expedition * Meetings of Joint Permanent Eclipse Committee Britain had the advantage of a permanent committee to organise eclipse expeditions. In other countries observatories decided individually whether to observe eclipses. The committee decided to start planning for the 1919 eclipse in spite of the war. In Nov. 1917 nobody knew how long the war would last or how it would end. Planning in wartime was difficult, in particular as many of the technical staff were serving in the armed forces. The expedition was considered a matter of national importance: a counterweight to Germany's strong position in theoretical physics. During the war there was no direct communication between British and German scientists. However, both Einstein and Eddington had contacts to colleagues in neutral Holland.

12. Expedition timeline Eclipse Arr. Principe Dept. Principe Arr. England Madeira Principe Team Sobral Team Arr. England Dept. Liverpool Arr. Madeira Arr. Brazil Comparison Plates Dept. Brazil Mar. Apr. May Jun. Aug. Feb. Jul. 1919

13. Results: timeline 12.09.1919 Bournemouth meeting of British Association for Advancement of Science: Eddington and Cottingham show photo with protuberance. Brief, preliminary report: light deflection has been measured. Sept 1919 Einstein receives news via H. A. Lorentz (NL) that results confirm general relativity. 06.11.1919 Official presentation of results at meeting of Joint Eclipse Committee. Jan 1920 The November presentation published as a scientific paper.

14. How large is the light deflection ? There are many pictures like this in the internet. The angle is drawn larger so that it can been seen properly. We're not talking about 10 or 20 times larger: the angle is about 30.000 times smaller than shown! Apparent position True position Sun Earth

15. Evaluation Angular units 1° = 60‘ = 3600“ Apparent sizes: Moon and Sun approx. 30’ Stars approx. 3-4” (due to atmospheric turbulence) Displacement smaller than diameter of star on the photo --> Very small displacements: various small effects must be considered - Stellar aberration: seasonal effect due to movement of the Earth - Differential refraction: refraction by Earth’s atmosphere depends on height above the horizon. - Small changes of magnification (e.g. from temperature changes) Photo of stars during eclipse Photo of stars at night Precise measurement of positions Relativity: Deflection decreases with distance from Sun Scale: Increase in magnification gives deflection that increases with distance from the middle of the plate. Mathematical Evaluation - Relativity effects - Scale effects Additionally small effects due to inexact alignment of eclipse and comparison plates are also be taken into account.

16. Results of eclipse expedition: Nov. 1919 presentation Deflection angle at sun's rim Theory: 3 possibilities Sobral: 1.98″ ± 0.12 1.75″ Einstein 0.87″ “Newton” Principe: 1.61″ ± 0.30 0″ no deflection The presentation was published in Philosophical Transactions of the Royal Society a few weeks later.

17. Public reaction The confirmation of Einstein's theory was a sensation - not just for the scientific world but for the general public too. Numerous articles appeared in newspapers and magazines. New York Times 10.11.1919 “Lights all Askew in the Heavens” “Einstein Theory Triumphs” USA - Sensationalist headlines “Revolution in Science” “Newtonian Ideas Overthrown” The Times 07.11.1919 Britain - Einstein versus Newton “A patriot fiddler-composer of Luton Wrote a funeral march which he played with the mute on, To record, as he said, that a Jewish-Swiss-Teuton Had partially scrapped the Principia of Newton.” Punch 19.11.1919 Vossische Zeitung 18.11.1919 “Einstein und Newton” Germany - Factual reports. E. Freundlich complains of too little public interest. Vossische Zeitung 30.11.1919 “Zum Siege der Relativitätstheorie” (by Erwin Freundlich)

18. Why did Einstein and the theory of relativity became so famous outside the arcane world of theoretical physics? - Theoretical physics is difficult to understand. - Little practical application. Quantum theory, developed about the same time, was technologically more important, but aroused less public interest. Surprising at first sight - No conflict with religion (unlike theory of evolution). - Confirmation of the theory as dramatic event (most scientific theories are confirmed by the accumulation of results). - Aftermath of war: the public wanted something that captured the imagination. - New ideas on space and time fitted revolutionary developments in the arts, such as cubism. Possible explanations - Popular accounts of the theory did not use technical terms but strange and intriguing combinations of familiar words: “curved space”, “fourth dimension”. - Surprisingly, the difficult mathematics of general relativity. According to American newspapers only twelve men understood the theory – an exotic elite group that aroused public interest.

19. Did Dyson and Eddington really show Einstein was right? The positive reaction in the press was based on the conclusions as presented at the November 1919 meeting. But how certain were the results really? Many modern accounts are critical. Kragh, for example, writes: This was, in fact, a too-optimistic conclusion that could be obtained only by a treatment of the available data that came close to manipulation, including rejection of data that did not agree with Einstein’s prediction.* The next slides show the background to this critical view. * In Quantum Generations, his history of 20th century physics

20. Results for the three telescopes Include in final result ? Poor quality measurements (focus problems). Result uncertain but no quantitative error bar possible. Sobral Astrographic No Sobral 4 Inch Satisfactory. Yes Principe AstrographicLess data than hoped for (some cloud during eclipse). Evaluation procedure adjusted. Yes

21. Criticism of the evaluation and conclusions In spite of popular acclamation, not all of Eddington's contempories were convinced. This criticism was revived decades later when the eclipse expedition was studied from a historical and philosophical perspective. Earman and Glymour's 1980 paper is considered a key contribution to this discussion. Criticism of Eddington and Dyson's conclusions - The evaluation was insufficiently objective; a method that gave the desired result was preferred (before the expedition Eddington was convinced that General Relativity was correct). Rejection of data not supporting this conclusion (Sobral astrographic) is questionable. - The result was accepted as a definite proof of Einstein's theory because of a good publicity campaign. Objectively, this conclusion was uncertain. - Eddington was considered an expert on general relativity. As a result his conclusions were accepted even when supported by weak arguments. Kennefick defends Eddington und Dyson - Objective reasons for mistrusting the results of the Sobral astrographic telescope. - Dyson led the evaluation of the Sobral results. Unlike Eddington, he was not a firm supporter of General Relativity before the expedition.  Nevertheless, the confirmation of Einstein's theory was less certain than claimed at the time.

22. Scientific Method The criticism of Eddington and Dyson's presentation of their results raises a number of general points: - When should data be rejected? This is generally a difficult question. It must usually be decided on a case-by-case basis, considering details of the data. Rejection of data should be transparent: reports should state which data is rejected and why. Alternative evaluations including this data are also desirable. - How flexible should the evaluation method be? There may be a variety of ways of evaluating the measurements, especially with complex data sets. A flexible approach allows one to choose the most satisfactory method. On the other hand, this could result in choosing the method that gives the best agreement with a preferred conclusion. Modern computer methods are a great advantage in comparing different evaluations. In particular, a critical look at several statistical parameters and graphical presentations helps select an appropriate evaluation.

23. Einstein's 3 tests of general relativity ● Light deflection by sun's gravitational field. Prediction. Confirmation of theory by observation of a single event (eclipse). ● Solar red shift Shift of spectral lines is due to a superposition of two phenomena: - general redshift (relativity). - Doppler effect caused by the movement of gas in the sun. Relativity effects are found only by looking at all the data rather than individual spectral shifts. Confirmation of theory by gradually building up a consistent picture. ● Perihelion of mercury The orbit of mercury showed small deviations from predictions of Newton's theory (first observed 1859). Attempts to explain this with classical mechanics had not been successful. Like the light deflection, this is a very small effect (unexplained precession 43″ per century). General relativity gave a quantitative explanation. Explanation of observed effect, not a prediction.

24. Conclusion The 1919 Eclipse expedition to test Einstein's theory is a very interesting piece of scientific research. It is a well-known topic in the History and Philosophy of Science. This expedition has many aspects: scientific, philosopical and historical. It is a fascinating story, but the drama is not typical for the way science works. However, it provides important insights into scientific method and the interaction of science and society.

25. Epilogue -1: later eclipse work 1979: New Analysis of the Sobral measurements with improved technology Astrographic: a =1.55″ ± 0.34 4 Inch: a = 1.90″ ± 0.11 (Theor. 1.75″) 1973: Last professional solar eclipse expedition to measure light deflection by gravity Univ. Texas; Mauritania a =1.66 ″ ± 0.19 2017: Best results for light deflection Amateur astronomer; USA a =1.752″ ± 0.060 (Theor. 1.751″) Retired physicist D. G. Bruns used modern technology. All the the equipment could be transported in a private car. The setup was programmed to take a large number of photographs during the 2½ minutes of totality. Data analysis indicated a 3.4 % uncertainty, but his result was in fact much closer to the theoretical value.

26. Epilogue -2 In 1913 Einstein asked American astronomer George Hale if there was a trick to measure the light deflection in daylight. His answer was no – it could only be done in a total eclipse. Today the answer is yes – use radio astronomy. - Radio waves, like light, are electromagnetic waves: the same physical laws apply. - The sun is a weak radio source, so strong sources can be seen even when they are nearby. - Positions of radio sources can be measured very accurately. → The agreement with Einstein's theory is better than 99.9%

27. Literature F.W. Dyson, A.S. Eddington, C. Davidson, A Determination of the deflection of light by the Sun’s Gravitational Field from Observations made at the Total Eclipse of May 29, 1919, Phil. Trans. R. Soc. A, 220, 291-332 (1920) J. Earman, C. Glymour, Relativity and Eclipses: The British Eclipse Expeditions of 1919 and Their Predecessors, Historical Studies in the Physical Sciences, 11, 49-85 (1980) M. Longair, Bending space–time: a commentary on Dyson, Eddington and Davidson (1920) ‘A determination of the deflection of light by the Sun’s gravitational field’, Phil. Trans. R. Soc. A, 373: 20140287 (2015) D. Kennefick, No Shadow of a Doubt, Princeton Univ. Press (2019) D. Soares, The 1919 Eddington Eclipse, DOI: 10.13140/RG.2.2.33288.88321 * R. Vaas, Der krumme Einstein-Beweis, Bild der Wissenschaft, 34-41 (Dec. 2019) R. Vaas, Licht auf krummen Touren, Bild der Wissenschaft, 14-29 (Nov. 2019) * Preprint version available on Researchgate. This is a small selection of the numerous publications on the 1919 eclipse expedition. Daniel Kennefick's book was my main source of information.

28. Thank you ! © Dennis Miller 2019