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of spacetime, a geodesic can be
curved. So, starlight shining very
close to the edge of the sun passes
into the warp created by the star’s
mass and follows a bending path.
Eddington photographed the stars
revealed by the absences of the
solar glare. These images showed
that the apparent position of the
stars had indeed been shifted due
to the warping of space, an effect
now known as gravitational lensing.
Einstein was proved right.
Einstein’s general theory of
relativity allows astronomers to
make sense of what they observe,
everywhere from the very edge of
the visible universe to the event
horizon of a black hole. Today,
the time dilations of relativity
are taken into account in GPS
technology, while the wavelike
contractions of space predicted
by relativity have recently been
discovered in the LIGO experiment.
Other ideas from relativity are
also being used in the search for
possible answers to the mystery
of dark energy. ■ATOMS, STARS, AND GALAXIES
Mass creates a gravity well that causes an effect called gravitational
lensing, first observed in 1919 by Arthur Eddington. The observed position
of a star is changed by the effect of the sun’s gravity, which causes light
from the star to travel past the sun along a curved path.
Real Observed
The twin paradox
A result known as the “twin
paradox” is illustrated using
a pair of newborn twins. One
stays on Earth, while another
is taken on a rocket on a
journey to a star 4 light-years
away. The rocket travels at
an average velocity of 0.8c,
meaning that it returns from
its 8-light-year journey on the
10th birthday of the twin who
stayed on Earth. However,
to the clock on the rocket, it
is only the other twin’s 6th
birthday. The clock has been
in a moving time frame, so
has been ticking more slowly.
Relativity insists that the
twin on the rocket is also
entitled to consider herself at
rest, which seems to lead to
a paradox—from her point of
view, the twin on Earth had
been the one moving. The
paradox is resolved by the fact
that only the twin in the rocket
has undergone acceleration,
with its consequent time
dilation, both on the way out
and to change direction and
come back. The twin on Earth
has remained in one frame of
reference, while the twin on
the rocket has been in two—
one on the way out and
another on the way back. Thus,
the twins’ situations are not
symmetrical, and the twin who
stayed at home really is now
four years older than her sister.
The twin paradox has been
a popular theme in science
fiction. In the film The Planet
of the Apes, astronauts return
to Earth to find that thousands
of years have elapsed, and the
planet is now ruled by apes.
In the film Interstellar, physics
consultants were employed to
ensure that the time elapsed
for each character was correct
according to relativity.Time is an illusion.
Albert Einstein