Translated and annotated by Bertram Schwarzschild
Einstein writes to Hale (1868–1938), director of the Mount Wilson Observatory near Los Angeles, seeking advice about the observability of the gravitational bending of light he had recently deduced from the equivalence principle. 2 Einstein’s 1913 prediction is only half the deflection predicted by the full general theory of relativity, completed two years later.
Zurich, 14 October 1913
Highly honored colleague,
A simple theoretical consideration makes it plausible to assume that light rays in a gravitational field experience bending.
At the edge of the Sun, the total deviation should be 0.84 arcseconds, and it should fall off like 1/R (R being the ray’s [closest] distance from the Sun’s center).
It would therefore be of the greatest interest to know how close to the Sun fixed stars could be seen in daylight with the strongest magnification.
On the advice of my colleague, Professor [Julius] Maurer, I therefore ask you to let me know what you—with your rich experience in these things—take to be achievable with the best modern instruments.
Yours very respectfully,
A. Einstein
Technische Hochschule Zürich
Hale responded that “there is no possibility of detecting the effect in full sunlight.” But he did pronounce the alternative of exploiting a solar eclipse “very promising.” 3
The rest of the story has become Einstein lore: A German team set out to measure the effect in Russia during an upcoming 1914 eclipse. But the outbreak of war intervened. In a sense, that was fortunate, because the team would have been comparing the measurement with Einstein’s first, incorrect prediction. By the time Arthur Eddington’s eclipse expedition set out in 1919, the predicted effect had doubled and the war was over. Eddington’s confirmation of the general-relativistic bending of light, albeit with a large observational uncertainty, made Einstein instantaneously famous.
