Skip to main content
SpringerLink
Log in

Louis Essen and the Velocity of Light: From Wartime Radar to Unit of Length

  • Published:
Physics in Perspective Aims and scope Submit manuscript

Abstract

Louis Essen (1908–1997), working at the National Physical Laboratory in Teddington, England, was the first scientist to realize that the value for the velocity of light used widely during World War II was incorrect. In 1947 he published his first determination of it, which was 16 kilometers per second higher than the accepted value, causing a great deal of controversy in the scientific community. His new value was not accepted for several years, until it was shown that it improved the precision of range-finding by radar. Essen’s result has remained as the internationally accepted value despite a number of attempts to improve on it. I discuss Essen’s work and also examine other optical and nonoptical determinations that were made in the United States, and their limits of accuracy. I also identify the reasons why it took so long for Essen’s new value to be accepted, and how it led to changes in the definition of the units of length and time.

This is a preview of subscription content, log in via an institution to check access.

Access this article

Log in via an institution

Price excludes VAT (USA)
Tax calculation will be finalised during checkout.

Instant access to the full article PDF.

Institutional subscriptions

Fig. 1
Fig. 2
Fig. 3
Fig. 4
Fig. 5

Similar content being viewed by others

Notes

  1. The English physicist James Chadwick (1891–1974) and future English physicist Charles D. Ellis (1895–1980) also were interned in Ruhleben for the duration of the Great War.

  2. To calculate distances the travel times of radio signals were multiplied by the velocity of light.

  3. Although it is now customary to refer to the speed of light rather than the velocity of light, Essen always used the latter term in his papers criticizing Einstein’s theory of relativity, which I discuss later.

  4. In November 1919, Britain’s Imperial Communications Committee of the Cabinet recommended the establishment of a research board in wireless telegraphy under the Department of Scientific and Industrial Research (DSIR), and a Radio Research Board was set up in January 1920. It was constituted to “direct any research of a fundamental nature that may be required, and any investigation having a civilian as well as a military interest.” See G. Gardiner, “Radio Research at Ditton Park–Some Historical Notes,” Radio Research Organisation Newsletter No. 9 (January 15, 1962).

  5. Gee, the first hyperbolic navigation system, was proposed in 1937 although its development did not begin until 1940. TRE subsequently developed the Gee-H and Oboe navigation systems to satisfy the need for precision targeting or “blind bombing” during World War II.

  6. The Lecher wire system, which can be used to measure the frequency of standing waves, was developed by the Austrian physicist Ernst Lecher (1856–1926) in Vienna in 1888.

  7. Michelson took part in the planning of this experiment, but Francis G. Pease and Frederick Pearson carried out the measurements in 1931 and 1932. At the time of Michelson’s death on May 9, 1931, only 36 of the 233 series of observations had been made.

  8. Essen doubted that Hansen’s approach would produce a satisfactory value for the velocity of light. His main concerns were that errors could be introduced in the metrological measurements and when calculating the skin effect; see his interview by Paul Forman, Smithsonian Institution, Washington, D.C., September 28, 1983.

  9. Hansen died in Palo Alto, California, on May 23, 1949, 4 days before his 40th birthday.

  10. 1 MHz = 106 Hz = 106 cycles per second.

  11. The NPL presented the two original cavity resonators Essen had used to measure the velocity of light to the Science Museum, London, and are now in the Museum’s storehouse in West London.

  12. In 1934 Robert A. Watson-Watt, later Sir Robert (1892–1973), briefly investigated the velocity of propagation of radio-frequency waves in the atmosphere at NPL’s Radio Research outstation at Slough as part of a study of “Atmospherics”; see Radio Research Board, Confidential Quarterly Progress Report, June 1, 1934. In late 1942, when the Oboe system was under development, it became apparent that not enough was known about the speed of electromagnetic-wave propagation in the atmosphere, so TRE set up a special mobile calibration unit (MCU), which was a transportable unit used to establish the actual variation in the refractive index of the medium and the necessary operational corrections. The MCU was disbanded shortly after VE Day; see Nigel Ellis-Robinson, OBE, “Calibration of Blind-Bombing Systems,” in Radar: A Wartime Miracle, recalled by Colin Latham and Anne Stobbs and by men and women who played their part in it for the RAF (Stroud: Alan Sutton Publishing Limited, 1996), pp. 106–108.

  13. The Mekometer was used in experiments with a radio-wave Michelson interferometer in Germany during World War II, and after the war the NPL decided to explore its possibilities for the measurement of lengths greater than can be dealt with by optical interferometry. Initial work was carried out by the TRE but was transferred to the NPL under K.D. Froome and Robert H. Bradsell (b. ca. 1940).

  14. Essen believed that it would have been beneficial to express the experimental limits of accuracy in three parts: the standard deviation of the individual observations, the systematic error owing to known causes, and an estimate of other possible systematic errors owing to causes not fully understood. Presented in this way, the values that Erik Bergstrand and Carl I. Aslakson obtained (see below), and those obtained by the cavity-resonator methods, would have had much greater weight in the assessment of any of the results prior to World War II. See Essen, “velocity of light” (ref. 17), pp. 69–70.

  15. In 1950 Edward Crisp Bullard succeeded Charles Galton Darwin as Director of the NPL; see Pyatt, National Physical Laboratory (ref. 6), p. 143.

  16. 1 GHz (gigahertz) = 109 Hz = 109 cycles per second.

  17. Bullard was in and out of NPL laboratories at all times; see Pyatt, National Physical Laboratory (ref. 6), p. 143. He made frequent visits, both alone and with visitors, to see progress.

  18. The future King George V (1865–1936) had been created Prince of Wales on the accession of his father, King Edward VII, to the throne in 1901; he ascended to the throne on the death of his father on May 6, 1910, and was crowned King George V on June 22, 1911.

References

  1. L. Essen, Time for Reflection, website http://www.btinternet.com/~time.lord/TheEarlyYears.html, p. 2; also quoted in part in Sir Alan Cook, “Louis Essen, O.B.E. (September 6, 1908–August 24, 1997),” Biographical Memoirs of Fellows of the Royal Society 44 (1998), 141–158; on 143.

  2. Essen, Time for Reflection (ref. 1), website …/TheEarlyYears.html, p. 2.

  3. Ibid., p. 4.

  4. John Jenkin, “Brose, Henry Herman Leopold Adolph (1880–1965),” Australian Dictionary of Biography—Online Edition, website http://www.adb.online.anu.edu.au/biogs/A130304b.htm, p. 1.

  5. Ibid., p. 5.

  6. Edward Pyatt, The National Physical Laboratory: A History (Bristol: Adam Hilger Ltd, 1983), p. 33.

  7. Essen, Time for Reflection (ref. 1), website …/TheEarlyYears.html, p. 5.

  8. Essen, Time for Reflection (ref. 1), website …/QuartzRings.html, p. 1.

  9. Essen, Time for Reflection (ref. 1), website …/TheEarlyYears.html, p. 4.

  10. Cook, “Essen” (ref. 1), p. 144.

  11. Joseph F. Mulligan, “Some Recent Determinations of the Velocity of Light,” American Journal of Physics 20 (1952), 165–172; on 165.

  12. L. Essen, “Velocity of Electromagnetic Waves,” Nature 159 (1947), 611-612; L. Essen and A.C. Gordon-Smith, “The velocity of propagation of electromagnetic waves derived from the resonant frequencies of a cylindrical cavity resonator,” Proceedings of the Royal Society of London [A] 194 (1948), 348–361.

    Google Scholar 

  13. Raymond T. Birge, “The General Physical Constants: as of August 1941 with Details on the Velocity of Light Only,” Reports on Progress in Physics 8 (1941), 90–134; on 99.

  14. L. Essen, “The design, calibration and performance of resonance wavemeters for frequencies between 1,000 and 25,000 Mc/s,” Journal of the Institution of Electrical Engineers 93 Part IIIA, No. 9 (1946), 1413–1425; on 1424.

  15. Ibid.

  16. L. Essen, “13th Charles Vernon Boys Prize: Measurement of the velocity of light,” Year Book of the Physical Society (1957), 74–76; on 74.

  17. L. Essen, “The velocity of light,” Science Progress: A Quarterly Review of Scientific Thought, Work & Affairs 157 (1952), 54–70.

    Google Scholar 

  18. K.D. Froome and L. Essen, The Velocity of Light and Radio Waves (London and New York: Academic Press, 1969), pp. 3–5; N. Ernest Dorsey, “The Velocity of Light,” Transactions of the American Philosophical Society 34, Part I (1944), 1–110; on 13–14.

  19. Froome and Essen, Velocity of Light (ref. 18), p. 6.

  20. R. Blondlot, “Détermination expérimentale de la vitesse de propagation des ondes électromagnétiques,” Comptes rendus hebdomadaries des séances de l’Académie des Sciences 113 (1891), 628–631; idem, Journal de Physique Théorique et Appliquée 10 (1891), 549–561.

  21. Essen, Time for reflection (ref. 1).

  22. A.A. Michelson, F.G. Pease, and F. Pearson, “Measurement of the Velocity of Light in a Partial Vacuum,” The Astrophysical Journal 82 (1935), 26–61, Plates I–II.

    Google Scholar 

  23. Essen, “Velocity of Electromagnetic Waves” (ref. 12); Essen and Gordon-Smith, “velocity of propagation” (ref. 12).

  24. Kees Bol, “A Determination of the Speed of Light by the Resonant Cavity Method,” Physical Review 80 (1950), 298.

    Article  ADS  Google Scholar 

  25. L. Essen, “Proposed New Value for the Velocity of Light,” Nature 167 (1951), 258–259; L. Essen and K.D. Froome, “Dielectric Constant and Refractive Index of Air and its Principal Constituents at 24,000 Mc./s.,” ibid., 512–513; idem, “The Refractive Indices and Dielectric Constants of Air and its Principal Constituents at 24,000 Mc./s.,” Proceedings of the Physical Society [B] 64 (1951), 862–875; K.D. Froome, “Determination of the velocity of short electromagnetic waves by interferometry,” ibid. [A] 213 (1952), 123–141; idem, “A New Determination of the Velocity of Electromagnetic Radiation by Microwave Interferometry,” Nature 169 (1952), 107–108.

    Google Scholar 

  26. Birge, “General Physical Constants” (ref. 13), p. 99.

  27. Dorsey, “Velocity of Light” (ref. 18), p. 87.

  28. Jesse W.M. DuMond and E. Richard Cohen, “Our Knowledge of the Atomic Constants F, N, m, and h in 1947, and of Other Constants Derivable Therefrom,” Reviews of Modern Physics 20 (1948), 82–108; especially 97.

  29. L. Essen, “The velocity of propagation of electromagnetic waves derived from the resonant frequencies of a cylindrical cavity resonator,” Proc. Roy. Soc. [A] 204 (1950), 260–277; on 274.

  30. Michelson, Pease, and Pearson, “Measurement” (ref. 22).

  31. Froome and Essen, Velocity of Light and Radio Waves (ref. 18), p. 29.

  32. Michelson, Pease, and Pearson, “Measurement” (ref. 22), pp. 55–59; Wilmer C. Anderson, “A Measurement of the Velocity of Light,” Review of Scientific Instruments 8 (1937), 239–247; especially 245–247.

  33. L. Essen, “The velocity of light,” Endeavour 15, No. 57 (January 1956), 87–91; on 91.

  34. Essen, “13th Charles Vernon Boys Prize” (ref. 16), p. 75.

  35. Ibid.

  36. Essen, “velocity of light” (ref. 17), p. 69.

  37. E. Bergstrand, “Velocity of Light and Measurement of Distances by High-Frequency Signalling,” Nature 163 (1949), 338; Carl I. Aslakson, “Velocity of Electromagnetic Waves,” ibid. 164 (1949), 711–712.

  38. R.S.J. Spilsbury, Electricity Department Progress Report, NPL Executive Report, April 17, 1951.

  39. Ibid.

  40. Third Conference on High Frequency Measurements, January 14–16, 1953, organized by a joint committee of the American Institute of Electrical Engineers, the Institute of Radio Engineers, and the National Bureau of Standards.

  41. L. Essen, “High-frequency electrical measurements,” Nature 172 (1953), 52.

    Article  ADS  Google Scholar 

  42. L. Essen and J.V.L. Parry, “Symposium on the generation, frequency stabilisation and amplification of electromagnetic oscillations by atomic and molecular resonances,” NPL internal report, April 20, 1956. The symposium was organised by the U.S. Signal Corps Engineering Laboratories, Fort Monmouth, New Jersey, and held on February 29-March 1, 1956.

  43. Froome and Essen, Velocity of Light (ref. 18), pp. 137, 139.

  44. Earth Measurer: The Autobiography of Carl Ingman Aslakson. Sect. 7. The Aftermath of War: Rockets, Reconnaissance, and the Velocity of Light, U.S. National Oceanic and Atmospheric Administration, website http://www.history.noaa.gov/stories_tales/ak7.html, p. 20.

  45. Paul Forman, “Atomichron®: The Atomic Clock from Concept to Commercial Product,” Proceedings of the IEEE 73, No. 7 (July 1985), 1181–1204; especially 1188–1189.

  46. Essen, Time for Reflection (ref. 1), website …/TheAtomicClock.html, p. 3.

  47. Ibid.

  48. Essen, Time for Reflection (ref. 1), website …/Relativity.html, p. 1.

  49. Ibid.

  50. Ibid.

  51. L. Essen, “Proposal for a New Æther-Drift Experiment,” Nature 173 (1954), 734; see also idem, “A New Æther-Drift Experiment,” ibid. 175 (1955), 793–794.

  52. L. Essen, “The Clock Paradox of Relativity,” Nature 180 (1957), 1061–1062; on 1061; see also idem, “Bearing of Recent Experiments on the Special and General Theories of Relativity,” ibid. 202 (1964), 787; ibid. 203 (1964), 396; idem, “The Error in the Special Theory of Relativity,” ibid. 217 (1968), 19.

  53. L. Essen, “The clock paradox,” Journal of the Institution of Electrical Engineers 9 (1963), 389–390; on 390.

    Google Scholar 

  54. L. Essen, “Evaluation of some aspects of relativity,” Proceedings of the Institution of Electrical Engineers 115, No. 7 (July 1968), 1853–1856; on 1854.

  55. L. Essen, The Special Theory of Relativity: A Critical Analysis (Oxford: Clarendon Press, 1971).

    Google Scholar 

  56. Essen, Time for Reflection (ref. 1), website …/Relativity.html, p. 2.

  57. Cook, “Essen” (ref. 1), p. 154.

  58. Essen, Time for Reflection (ref. 1), website …/Relativity.html, p. 2.

  59. Essen, Time for Reflection (ref. 1), website …/Afterthoughts.html, p. 1.

  60. Cook, “Essen” (ref. 1), p. 156.

  61. “Resolution 1 of the 17th Conférence Générale des Poids et Mesures (1983): Definition of the metre,” Comptes Rendus de la 17 e CGPM (1983), 1984, 97; website http://www.bipm.org/en/CGPM/db/17/1/.

  62. Cook, “Essen” (ref. 1), p. 143.

  63. Essen, Time for Reflection (ref. 1), website…/Afterthoughts.html, pp. 2 and 1.

  64. John Langdon-Davies, NPL: Jubilee Book of the National Physical Laboratory (London: HMSO, 1951), p. 10.

Download references

Acknowledgments

I give special thanks to Joan Essen for making available to me Louis Essen’s personal papers and for permission to quote from his Time for Reflection. I am indebted to the library staffs of the Department of Manuscripts and University Archives of the University of Cambridge, The Royal Society, and the National Physical Laboratory. I thank Keith Sherwin and William Campbell for helpful discussions and comments. Finally, I thank Roger H. Stuewer for his thoughtful and careful editorial work on my paper.

Author information

Authors and Affiliations

  1. 46 Thames Crescent, Maidenhead, Berkshire, SL6 8EY, UK

    Ray Essen

Authors
  1. Ray Essen
    View author publications

    You can also search for this author in PubMed Google Scholar

Corresponding author

Correspondence to Ray Essen.

Additional information

Ray Essen, who received a degree in physics from the University of Nottingham, is a writer specializing in books about science for the general public. His current book, Revolutions in Time, describes the birth of atomic time.

Rights and permissions

Reprints and permissions

About this article

Cite this article

Essen, R. Louis Essen and the Velocity of Light: From Wartime Radar to Unit of Length. Phys. Perspect. 12, 51–73 (2010). https://doi.org/10.1007/s00016-009-0004-y

Download citation

  • Published:

  • Issue Date:

  • DOI: https://doi.org/10.1007/s00016-009-0004-y

Keywords

Search

Navigation