Sky and Telescope - December 2015 - 18

1OO YEARS OF
GENERAL RELATIVITY:

Einstein's Coming Golden Age

Astronomy
& the Future
of General
Relativity
Albert Einstein published his general theory of relativity
100 years ago this month. Astronomy took half of that
time to catch up. Now it's running the show.

PEDRO G. FERREIRA The summer of 1963 was eventful by any
standards. In June President Kennedy proclaimed "Ich
bin ein Berliner" to the beleaguered citizens of Berlin. In
July the spy Kim Philby defected to Moscow. In August
Martin Luther King, Jr., declared "I have a dream" to
a crowd of a quarter million. And quietly, around the
swimming pool of a suburban house in Dallas, Texas,
three European emigrés drank strong martinis and
planned to resurrect general relativity.
Einstein's general theory of relativity is our modern
theory of gravity. It took him eight hard years to figure
out, but its essential formula fits on one line. Published
on December 2, 1915, it completely transformed our
understanding of what gravity is.
Einstein abolished the gravitational "force" proposed
by Isaac Newton, which acts at a distance to pull bodies
together according to their masses. Instead, space and
time gain a life of their own. They form an interlocked
unity, in which time is just another distance dimension
except that it's multiplied by the speed of light. This
unified, 4D spacetime becomes warped by the presence
of mass. The warp alters the path that a body takes as
it naturally tries (in the absence of any outside force)
to continue along a straight line. This works because
an object's passage through time is part of its motion
18

December 2015 sky & telescope

NOT SO SIMPLE This snapshot from a high-precision
simulation shows shreds of a neutron star 0.02 second
after much of it spiraled onto a more massive neutron
star, collapsing it into a black hole. The scene is about
200 kilometers (120 miles) wide. Such a violent event
should throw off complex gravitational waves. We're on
the verge of detecting them. Under such extreme conditions, will general relativity work the way we expect?

through this 4D matrix (see the box on page 20.)
In other words, gravity is just geometry.
General relativity explains, with fantastic precision,
the dynamics of everything from pebbles to planets to
galaxies. It has passed every exquisite test that has ever
been thrown at it.
Moreover, as Einstein immediately realized, it
predicted slightly different gravitational motions than
Newton's theory did. The eureka moment for Einstein
came in November 1915, when he found that his spacetime equations perfectly explained the slight drift of
Mercury's perihelion point, something that had baffled
astronomers.
Most famously, general relativity differed from Newtonian gravity in how much it predicted starlight would



Sky and Telescope - December 2015

Table of Contents for the Digital Edition of Sky and Telescope - December 2015

Contents
Sky and Telescope - December 2015 - Cover1
Sky and Telescope - December 2015 - Cover2
Sky and Telescope - December 2015 - 1
Sky and Telescope - December 2015 - Contents
Sky and Telescope - December 2015 - 3
Sky and Telescope - December 2015 - A
Sky and Telescope - December 2015 - B
Sky and Telescope - December 2015 - 4
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Sky and Telescope - December 2015 - Cover3
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