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No Shadow of a Doubt - Daniel Kennefick ***

It's something of a truism that science tends to go through stages, where each new stage can be typified as 'It's more complicated than we thought.' This book demonstrates that this assertion is also true of history of science. It examines the 1919 eclipse expeditions and their conclusions used to bolster Einstein's general theory of relativity, and how those results have been treated.

This is a very tightly focussed subject for a whole book, and there is a distinct danger here of the material of an article being stretched out to book length - it often did feel that Daniel Kennefick was dragging out a handful of conclusions by repeating the same assertions over and over in subtly different ways. However, this isn't entirely fair as he does give exhaustive detail of the two expeditions which wouldn't have fit in an article, covering how their results were produced and how the controversy (if it could really be called that) arose.

Like many physics professors, Kennefick struggles to explain the details of physics in a way that's accessible to the general reader, but this is only a very small part of the book, which is far more about the history and its implications, and here he is significantly more readable. Though the points may be made rather too often, they are indeed fascinating if you are interested in the way experimental support for scientific theories - and the history of science - develops.

Arguably, as Kennefick points out, eclipse science is an oddball field, as it's very difficult to repeat experiments successfully, particularly as there is only a few-minute window in which to undertake them. This is the context in which we see the developing story of the 1919 eclipse expeditions. From their results being announced through to the 1970s they were generally presented as a triumphant demonstration of Einstein's prediction of the amount the mass of the Sun should warp space, causing stars appearing near it in the sky to be shifted in position. From the seventies onwards - and it's largely how I've seen it presented - it was more seen as a bit of a fudge by English astrophysicist Arthur Eddington, taking results which couldn't really demonstrate anything and making them show what he wanted: that Einstein was correct. Kennefick demonstrates at length that this view is also wildly over-simplistic.

One reason for this is that the myth of Eddington's bias omits the fact that he was only responsible for one of the two expeditions - the other was under the aegis of the Astronomer Royal Frank Dyson (apparently no relation to, but an inspiration for Freeman Dyson). Dyson had no axe to grind and was responsible for the decision, usually blamed on Eddington, of ignoring the data that disagreed with Einstein's predictions. Dyson did this not to cherry pick, but because there were technical problems with the device used that produced these photographic plates, making them difficult to interpret. (Apparently Eddington's only influence was to stop Dyson using the dubious data averaged with the other rather overshot data of Dyson's, which would have brought the results closer to the Einstein prediction.)

Interestingly, and again not revealed in the myth, the remaining 1919 plates were re-measured in the 70s and in fact showed that the ignored data, if measured properly, would also have confirmed the general theory's predictions.

Of course it's entirely possible that Eddington was biassed anyway and was over-confident about the way the results were presented - but  after reading this book, this early effort to test Einstein's theory (which would be verified many times over later by far better tests than the always tricky observation of eclipses) does not seem as flawed as it has repeatedly been presented to be.

An interesting book, then - but it does rather labour the point.
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Review by Brian Clegg

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