MARGINALIA
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.
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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