Solar eclipse helped prove general relativity - May 29, 1919

On this day in 1919, a total solar eclipse experiment helped prove Einstein's recently proposed general theory of relativity. 

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Episode Transcript

Speaker 1 (00:00):
This Day in History Class is a production of I
Heart Radio. Hi, I'm Eves and Welcome to This Day
in History Class, a show that uncovers history one day
at a time. Today is May ninth, nineteen. The day

(00:25):
was May twenty ninth, nineteen nineteen. A total solar eclipse
lasted for more than six minutes, casting darkness from South
America to Africa. German physicist Albert Einstein had recently published
his general theory of relativity, but it was not yet proven.
During the eclipse, British astronomer Arthur Eddington led an experiment

(00:49):
to test the deflection of light by a gravitational field.
Eddington's research teams found that their eclipse measurements confirmed Einstein's
predictions about general relativity. Einstein, a professor in Berlin, had
yet to achieve worldwide fame, but after the eclipse experiment,
Einstein rose to prominence. In nineteen fifteen, Einstein published four

(01:12):
papers describing his general theory of relativity. At the time,
Isaac Newton's model of classical mechanics and law of universal
gravitation ruled, but Einstein's work was at odds with Newton's theory.
According to Newton, space was inert and gravity was a
four step pulled objects together. But Einstein's special theory of relativity,

(01:35):
proposed in nineteen o five, said that space and time
were relative informed of four dimensional continuum called space time,
and his general theory of relativity posed gravity as a
distortion in the fabric of space caused by the presence
of massive objects. Einstein and Newton's views on how gravity

(01:56):
influences light also differed. Classical dynamics says that the gravity
of a star can deflect the path of a photon
or a quantum of electromagnetic radiation, but Einstein predicted that
light curves while traveling through spacetime near the warp induced
by an objects gravitational field. So both scientists predicted that

(02:18):
light would bend due to gravity, though newt end predicted
it would do so by only half as much as
Einstein's theory did, but the difference in measurements was small,
and the two theories made similar predictions for tests of
gravity and light. Figuring out which theory was more accurate
seemed pointless at a time when it seemed like science

(02:39):
did not have the capability to measure such a tiny curvature.
But the more massive an object is, the bigger the warp,
and the more its gravity can bend light. Einstein realized
that the Sun was massive enough to measure such a
subtle effect. As the Sun moved toward a background star,
it would bend the stars light and star would appear

(03:01):
to move. This kind of observation would have to be
made when the Sun was out, but its light was blocked,
so stars were visible, and eclipse was the perfect time
to carry out tests of Einstein's theory. German astronomer Erwin
Finlay Fronleech attempted to prove Einstein's theory, but war broke

(03:21):
out and he was arrested as a spy as he
was preparing to experiment during an eclipse in nineteen fourteen.
Astronomers from California's Lick Observatory also attempted to photograph the
eclipse from near Kiev, but clouds got in the way.
Around this time, Einstein realized his calculations about how much
the stars would move was incorrect, and he published his

(03:44):
completed General Theory of Relativity in nineteen fifteen. World War
One kept Einstein's work isolated, and people in the science
community pushed back against the new theory, but Sir Arthur Eddington,
director of the Cambridge Observatory support Or, did Einstein's theory
and wanted to lead an experiment to test it. The

(04:04):
nineteen nineteen total solar eclipse presented an opportunity to carry
out tests. Britain's astronomer, Royal Sir Frank W. Dyson, proposed
an expedition to experiment during the solar eclipse, set to
occur on May twenty ninth, nineteen nineteen. If scientists measured
the position of the stars during the eclipse and compared

(04:26):
them to their normal positions, the tests could confirm the
general theory of relativity. Eddington would lead the expedition. He
took measurements of the stars in the Hyades cluster in
January and February of nineteen nineteen, since the sun would
cross that cluster during the eclipse, and the Royal Society
in Royal Astronomical Society organized expeditions to Principal, An Island

(04:50):
off the coast of West Africa, and to sober Al, Brazil.
The May twenty nine eclipse was one of the longest
in centuries, and despite at whether in Principe for a period,
astronomers had a considerable amount of time to take photos
of the location of the stars and the Hyades cluster.
Months later, the team had measured the precise position of

(05:12):
the stars that were visible on the photographic plates. They
found that their measurements were consistent with Einstein's predictions. The
expedition's results became front page news in Einstein became famous
for his theory. On November six, nineteen nineteen, Eddington and
Dyson presented their findings at a joint meeting of the

(05:34):
Royal Society and the Royal Astronomical Society in England. Not
everyone accepted or understood Einstein's theory, and Einstein himself had detractors,
but later eclipses corroborated Einstein's predictions. By the nineteen sixties,
most physicists had accepted the validity of the general theory
of relativity. The deflection of light passing near a massive

(05:57):
object is now called gravitational len thing. Gravitational lensing has
been used to map out dark matter and discover distant galaxies.
I'm Eves Deacote and hopefully you know a little more
about history today than you did yesterday, and if you'd
like to follow us on social media. You can find

(06:17):
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and Facebook. Thanks for showing up. We'll meet here again tomorrow.
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