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Thursday, 5 May 2011

y11 mod 10 Eddington and the 1919 Eclispe

Relativity and the 1919 eclipse

One of the most famous cases of predictive power was the confirmation of Albert Einstein's theory of general relativity. The theory predicted that if photographs taken of the stars near the edge of the Sun during a solar eclipse were compared to photographs of the same stars when they were not near the Sun, a "bending" of their light by the Sun's gravitational field would be observed. The existing Newtonian theory of physics also -- for different reasons -- predicted some bending, but to a lesser degree. The prediction was made by Einstein in 1915 as a logical outcome of his theory, but it could not be tested until a solar eclipse on May 29, 1919, when observations were made by the astrophysicist Arthur Eddington which seemingly confirmed Einstein's predictions. The news was heralded throughout the world as a revolution in physics, as the Newtonian theory had been conclusively disproved.
The historical reality of the eclipse experiment is more complicated than the textbook account, and highlights some of the problems involved in retrospectively applying a notion such as predictive power. The results of the eclipse observations were far from clear – they were taken at two remote locations (Sobral in Brazil and the Atlantic island of Príncipe) and thus they were forced to use telescopes that sacrificed accuracy in favour of portability. In addition, there were mitigating factors such as the Earth's slight rotation during the eclipse, and the temperature differences between day (when the eclipse pictures were taken) and night (when the control pictures were taken), which caused optical anomalies. From the start, the experiments were far from clear-cut, and relied on a series of assumptions and human judgements.
Moreover, the data from the observations were not as conclusive as was professed. Two telescopes were used at Sobral; one produced 8 photographic plates which recorded a mean deviation from the norm of 1.98″ of arc (1 = 1/3600th of a degree), and the other 18 plates with a mean deviation of 0.86″; the two plates from a single telescope at Principe, though of a poor quality, suggested a mean of 1.62″. Einstein's theory suggested a deviation of approximately 1.75″, while Newton's suggested 0.8″. If all the data had been included, the results would have been inconclusive at best, but Eddington discounted the results obtained from the second Sobral telescope, claiming "systematic error", and gave extra weight to the results from Principe (which he had personally recorded), with little justification or supporting evidence. The Astronomer Royal, Sir Frank Dyson, and the president of the Royal Society, J. J. Thomson, sided with Eddington, and on November 6 declared the evidence was decisively in favour of Einstein's theory; much of the scientific community fell in line and agreed. Nevertheless, there were many scientists at the time who felt there were good reasons to doubt whether the prediction had been accurately fulfilled, or whether the results did not have an alternative interpretation. Subsequent eclipse observations in the 1920s and 1930s failed to provide confirmation, although many other different experiments have since provided much stronger (but less dramatic) proof of relativity.
The 1919 eclipse is one example used in science studies as a demonstration that scientific facts are in various ways constructed, or at least influenced, through a variety of assumptions, institutional forces, and interpersonal relations, and are rarely without expert dispute in their day. The philosopher and historian Thomas Kuhn has famously pointed out that "textbook" histories of science tell the story of the current theory as a linear set of triumphs, when in reality the historical record is much more complicated.

Title to include Einstein and date

Begin with a brief introduction of the theory and Einstein

State the relevant prediction, include a diagram.

Explain the experiment

Explain the difficulties experienced.

Discuss whether the results were clear cut and the conclusion arrived at.

Have further experiments shown the theory to be wrong?