Sky & Telescope - May 2019 - 30

Testing Einstein

u PATH TO SUCCESS The path of totality
for the 1919 solar eclipse reached from
South America across the Atlantic
Ocean to Africa. The eclipse expeditions organized by Dyson and
Eddington traveled to determine
what effect, if any, was produced
by the Sun's gravity on the path
of a star's light.

30

M AY 2 0 1 9 * S K Y & T E L E S C O P E

ECLIPSE PATH: G REGG DINDER M A N / S&T;
LIGHT BENDING: LE AH TISCIONE / S&T

M
Ecli
ax
pse
im
end
um
s at
e
clip
sun
Ec
set
se a
lip
t sun
se
set
beg
ins
at sun
set

e
e
is
ris
un
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at s
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at
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Maxi
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end
Eclipse

Though an impressive result, few
were convinced by it. "It was what
you might call a retrodiction
instead of a prediction," says
clipse
e
l
a
i
Daniel Kennefick (Univert
ar
of p
mit
i
l
sity of Arkansas), a leading
ern
rth
o
historian of 20th-century
N
relativity and astronomy.
"I think it suffered in
L
C
people's minds because
world. In them, he added
I
P
SE
AL E
TOT
F
most people at that time
to his special theory of
O
TH
Príncipe
PA
didn't understand the
relativity (which revealed
Sobral
theory very well and
how space and time are
felt Einstein may have
not fi xed and instead
trimmed his calculation
part of the same thing -
to fit the result because he
spacetime) by showing that
knew the result."
spacetime is connected to
A more pleasing and seemmatter. More massive objects
it of partial ecl
ips
n lim
r
e
e
ingly
unbiased test was actually
bend spacetime more and so
uth
So
suggested by Einstein years before,
have a stronger gravitational pull.
in 1911: the deflection of light by the
No longer a Newtonian force, in EinSun's gravitational field. As early as 1801,
stein's world gravity was just the curvaGerman astronomer Johann Georg von Soldture of spacetime.
ner worked out that light grazing the Sun's disk is bent by
To put his mind-bending theory on firm footing, in one
gravitational acceleration. The figure he arrived at was 0.84
of the 1915 papers Einstein solved a mysterious flaw in
arcsecond (a miniscule 0.00023 degree). Without knowledge
Newton's perfect clockwork universe that had long puzzled
of this previous calculation, Einstein did the same sums in the
astronomers. Like all planets, Mercury moves in an ellipse
context of his special theory of relativity and arrived at 0.83
around the Sun, and this ellipse slowly rotates, or precesses,
arcsecond. Later, more accurate calculations showed that both
in space over time. Newton's theory underestimated this
von Soldner and Einstein should have calculated the angle as
precession by a tiny amount, and despite numerous attempts
0.875 arcsecond. But other interpretations such as a competnothing could make the observed orbit fit the theory. But
ing relativistic theory by Finnish theoretical physicist Gunnar
when Einstein calculated Mercury's precession using general
Nordström suggested light would not be bent by the Sun at all.
relativity, the match was perfect.
To discount these alternative theories, Einstein said that
all the astronomical community needed was an opportunity
q MIND-BENDING THEORY If Einstein's theory of relativity was correct,
any light grazing the Sun's disk would be bent by gravitational lensing.
to blot out the Sun to see the starlight grazing its disk: an
By comparing the positions of stars during a total eclipse - when the
eclipse. If light did bend, stars positioned on the sky close to
Sun's light would be blocked by the Moon, making background stars
the Sun would appear to be in a slightly different position to
visible during the day - with where they would normally appear at night,
where they would normally appear, and that slight differEddington and Dyson could measure how much starlight was bent as it
ence could be measured.
passed the Sun, bringing them one step closer to confirming or disproving Einstein's theory.
Bold attempts were made to measure light bending during
eclipses in Brazil (1912) and Russia (1914). But blighted by
Star's
weather and war, they were ultimately unsuccessful. And it
apparent position
was lucky for Einstein that they were. In 1915, while putting
Newton's angle = 0.875 arcsecond
the finishing touches on general relativity, he realized that his
Einstein's angle = 1.75 arcseconds
calculation of light bending neglected the effect of the Sun
Star's
true
distorting spacetime itself. Rerunning the numbers, he found
position
the deflection would be double his first estimate - 1.75 arcseconds. Had any of these early expeditions been successful
and shown a large bending effect before general relativity had
been formulated, Einstein's position in the annals of history
Earth
might have been very different.
Sun
There was now a clear difference between Einstein's and
Newton's light-bending predictions, making the eclipse test
the best chance of settling the issue at the time. But with



Sky & Telescope - May 2019

Table of Contents for the Digital Edition of Sky & Telescope - May 2019

Contents
Sky & Telescope - May 2019 - Cover1
Sky & Telescope - May 2019 - Cover2
Sky & Telescope - May 2019 - 1
Sky & Telescope - May 2019 - Contents
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