44 DISCOVERMAGAZINE.COMA HUNDRED YEARS AGO,
Albert Einstein wasn’t a
household name. He was a
professor in Berlin, known
to scientists, intellectuals,
his divorced wife and the
first cousin who would
soon become his second
wife — but not to the world.
His rise to superstardom began on May 29, 1919,
when the moon and sun lined up just right for a solar
eclipse. Photos of the astronomical event showed
something strange: A few of the stars visible during
the blackout were in the wrong place.
Einstein had foreseen this. Using his theory of
general relativity, he made the seemingly crazy bet
that the stars’ positions in the sky would shift during
an eclipse, and even calculated by how much.
“Stars moved by exactly the amount general
relativity predicted,” says Mark Hurn, a departmental
librarian at the University of Cambridge’s Institute of
Astronomy. “It was the first experimental evidence for
general relativity being on the right track.”
Not since Edmond Halley prophesied the appear-
ance of his namesake comet had a scientific prediction
come true so spectacularly. But whereas Halley vin-
dicated Isaac Newton’s view of the universe, Einstein
sought to topple it. And in a historical twist, the
astronomers who made the German-born physicist
a superstar came from England: Newton’s birthplace
and Germany’s enemy during World War I, which
had ended just before the eclipse. Their scientific
quest to test Einstein’s theory would be celebrated
for moving beyond the horrors of war.“The romantic nature of this business of postwar
reconciliation caught the public’s imagination,”
says Daniel Kennefick, a historian of physics at the
University of Arkansas. It all added up to sudden fame
for the physicist behind the prediction. “The public
became fixated on Einstein because of this eclipse.”STARING AT THE SUN
General relativity abandoned Newton’s idea that grav-
ity is a force pulling objects together. (See “It’s All
Relative,” page 48.) It reimagined gravity as a warping
of time and space — a distortion in the fabric of the
universe. According to the mathematics of relativity,
light traveling through this distortion will change its
path, accommodating the universe’s warps and wefts.
The more massive an object, the bigger the distortion,
and the more its gravity can bend light.
During the decade Einstein spent developing his
theory, he realized that the sun was massive enough
to make this effect noticeable. As the sun moves in the
sky toward a background star, he said, it should bend
the star’s light. The star will appear to move.
Of course, testing this prediction wasn’t easy
because stars aren’t visible during the day; they’re
washed out by the sun’s light. And at night, when
stars do appear, the sun isn’t around to bend their
light. Only when the sun is out but its light blocked
could Einstein’s work be checked. That’s why, while
working out the kinks in his theory in 1911, he asked
astronomers to start looking to the heavens during
eclipses.
The first to answer that call was the young German
astronomer Erwin Finlay-Freundlich, a tragic figure
who dedicated much of his life to proving Einstein’s
light-bending right and never succeeded. He started
by analyzing photographs of historical eclipses,
but the stars weren’t clear enough to test Einstein’s
idea. So Freundlich raised money — nearly borrow-
ing a sum from Einstein himself — for a voyage to
present-day Ukraine, where an eclipse was expectedA
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MAP:^ GSFC/NASAEinstein’s general theory of relativity suggests
that the sun’s gravity bends the path of light
from distant stars. It’s a testable prediction, but
only during a total solar eclipse. The eclipse of
1919 (globe, right) fit the bill nicely.Apparent
location
of starActual
location
of starPath of starlightSunEarth