The Centennial of Eddington’s Proof

This photograph from the May 29, 1919 total solar eclipse shows one of the stars used to confirm Albert Einstein's general theory of relativity. The red dot shows where the star would have been without the sun's interference. (Image: © Royal Observatory, Greenwich)

This photograph from the May 29, 1919 total solar eclipse shows one of the stars used to confirm Albert Einstein’s general theory of relativity. The red dot shows where the star would have been without the sun’s interference. (Image: © Royal Observatory, Greenwich)

To those of a geeky persuasion – a group that certainly includes WC – there is an important anniversary approaching.

May 29 will be the centennial of Eddington’s confirmation of general relativity.

To understand why this is important we have to go back to 1915 or so. Albert Einstein had published his four scientific papers on general relativity and, among the physics science community at the time, they provoked outrage. Einstein was saying Sir Issac Newton had gotten it wrong. Newton had taken the physical world to be a constant, static, inert. Einstein, in those seminal papers, had described space-time, incorporating the fourth dimension of time. In Einstein’s relativistic universe, space is combined with time which creates the universewide “fabric” called space-time. Objects travel through this fabric, which can be warped, bent and twisted by the masses and motions of objects within space-time.

WC will spare you the math, which he only half understands, but Einstein’s theory of general relativity predicted that the distortion of space-time by the solar mass would be considerably greater than Newton’s simple theory of gravitation. The path of photons, in the presence of the gravity of a star, for example, would show greater apparent deflection under Einsteinian than under Newtonian physics.

Sir Frank Watson Dyson, Astronomer Royal of Britain, had conceived an experiment that would plot the positions of background stars close to the sun’s limb during an eclipse — an experiment that Sir Arthur Eddington would lead two years later, during a 1919 total eclipse.1 If the positions of the stars could be precisely measured during the 1919 eclipse and then compared with their normal positions in the sky, the effects of warped space-time could be observed — beyond what Newton’s classical mechanics would predict. If the position of the stars were altered in exactly the way that Einstein’s theory predicted they should be, then general relativity would have confirmation in the real universe.

Sir Arthur chose some of the stars in the Hyades cluster that were usefully located near the solar limb at the time. And, sure enough, the warped space-time deflected the starlight by a minuscule amount (invisible to the naked eye). The observations from Brazil and Principe were analyzed by Eddington and the deflection matched, within the limits of measurement, the predictions of general relativity. The warping of space-time by the sun’s mass was real and Newton’s inert space had been superseded by a new theory.

Eddington became famous.2 Since then, general relativity has been confirmed many times, in many different ways, but it was Dyson’s idea, confirmed by Eddington’s execution of the experiment, that changed our understanding of the universe. Which is why May 29, 2019 will be a geeky anniversary.3


  1. It’s ironic that fame went to Eddington, not Dyson, who conceived of the experiment. Eddington brought it off, but the idea was Dyson’s. Sic transit famia
  2. But not modest. Interviewed by the New York Times, Eddington was asked if it was really true that only three people in the world understood general relativity, he said nothing. “Don’t be so modest, Eddington!” his questioner said. “On the contrary,” Eddington is supposed to have replied. “I’m just wondering who the third might be.” 
  3. Want more geekiness? For a very nice insight into the way Albert Einstein’s mind worked, read Steven Strogatz’s fine essay, “Einstein’s First Proof” in New Yorker

One thought on “The Centennial of Eddington’s Proof

  1. The story reminds me of a limerick from my college (geeky) days:

    There once was a woman so bright
    She could travel much faster than light
    She took off one day, in a relative way
    And returned the preceding night

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