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Judging Einstein

Before most physicists would believe the claims of relativity, they required proof—which would come in the form of a solar eclipse

J. Donald Fernie

This year we celebrate the centenary of Albert Einstein's special theory of relativity. Indeed, 1905 was the year in which Einstein first gave notice of his astonishing abilities. He was but 26 and had just earned his doctorate, but that year he published four papers on separate topics, each of which marked a major advance in physics. The first of these, on the photo-electric effect (the subject of Roald Hoffmann's Marginalium in the previous issue), would bring him the Nobel Prize, but it was the third, on special relativity, that made him both famous and controversial. A decade after this flurry of papers, in 1915, he unveiled the theory of general relativity, shaking again the foundations of science.

To test predictions of Einstein's theories...Click to Enlarge Image

So different was relativity from the prevailing beliefs that most physicists demanded proof that it could explain phenomena that Isaac Newton's canon could not. Satisfying such demands was difficult, because the difference between the two models could only be apparent under extreme conditions. There seemed little hope that any terrestrial experiment could decide between them, but Einstein later identified three astronomical tests. The first was the proper calculation of the orbit of the planet Mercury—a feat that was beyond Newtonian physics (see "In Pursuit of Vulcan" in the September-October 1994 American Scientist). The second test required the comparison of light emitted from atoms in the Sun with light from similar atoms on Earth—relativity predicted that the Sun's light would have a longer wavelength (an example of the so-called redshift). The third test posited that if relativity was true, then rays of starlight that passed near the Sun would be bent compared to the same rays when the Sun was elsewhere in the sky. In each case, the relativistic effects are caused by gravity from the Sun's huge mass.

Early attempts to perform these tests did not silence Einstein's critics, because some observations supported his theory and others did not. Thus, the general theory of relativity yielded a much better solution to the Mercury problem than did Newtonian models, but another prediction of relativity, the redshift of the solar spectrum, could not be verified. (Eventually, astrophysicists learned that several other factors complicated the observation of this phenomenon.) So with one result in favor and another in doubt, the third test became something of a deciding vote for or against relativity.

Einstein first suggested how this light-bending effect could be measured in 1911. He predicted that those rays of starlight that passed closest to the Sun would be deflected by 0.85 arcseconds (0.00023 degree) because of the Sun's gravitational field. However, stars that appear next to the Sun are only visible during a total solar eclipse. To test Einstein's hypothesis, one would have to take photographs during an eclipse that showed background stars near the Sun's disk and compare them with photos taken months earlier or later, when the same stars rose in the night sky. Did stars appearing on opposite sides of the Sun's disk maintain the same spacing when the Sun was gone, or not?

This prediction seemed easy to check. Many pictures of solar eclipses already existed, as did photos of the night sky. Even so, skepticism about Einstein's theory was so prevalent that few astronomers rushed to their archives. And when they did examine previous photographs of solar eclipses, they found that the pictures were unsuited to proving or disproving Einstein's claim: The telescopes had been set to track the Sun's motion across the sky, not the stellar motions, and the slight differences between these perspectives obscured the small, predicted shifts in star positions. However, as time went by and other experiments gave equivocal results, the solar-eclipse experiment represented the best chance to test the truth of relativity.

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