Like an Olympic athlete, the gen- eral theory of relativity has passed many tests in its century-long ca- reer. Its string of successes began in 1915, when Albert Einstein’s pic- ture of gravity as curved spacetime neatly explained shifts in the orbit of Mercury that had vexed astrono- mers for more than half a century.
In recent decades it has faced more exotic
and extreme tests, such as explaining why
pairs of superdense neutron stars whirling around each other appear to be gradually spiraling toward collision. Here, too,
general relativity triumphed: The stars are
losing energy at exactly the rate expected
if, as the theory predicts, they emit gravitational waves (see p. 1097).
Yet physicists remain unsatisfied. The
tests so far have been too easy, they say.
The gravitational fields involved have been
fairly weak, coming from single stars and
bending or slowing light only very slightly.
If the theory is going to show cracks, it will
be under more extreme, high-field conditions. That matters because—on paper, at
least—general relativity isn’t the only game
in town. Theorists have put forward alternative models for gravity, but in low fields
they look identical to Einstein’s theory. In
strong fields, they begin to change.
Now, searching for a tougher test, re-
searchers are looking toward the center of
our galaxy. There, shrouded in dust, lurks a
bright, compact source of radio waves known
as Sagittarius A* (Sgr A*) for its position in
the sky, near the edge of the constellation
Sagittarius. Because of the way stars move
in its vicinity, astronomers think that Sgr A*
marks the dark heart of the Milky Way: a
supermassive black hole weighing as much
as 4 million suns but crammed into a space
smaller than the distance between the sun
and Mercury. That black hole produces the
most intense gravitational field in our gal-
axy and so provides a unique laboratory for
testing the predictions of general relativity.
Over the next few years, using a range of
new instruments tuned to infrared light
and radio waves—radiation capable of pen-
etrating the clouds of dust and gas around
the galaxy’s core—astronomers are hoping
to see whether Sgr A* is bending relativity
beyond the breaking point.
Two teams of astronomers—one led by
Andrea Ghez of the University of California, Los Angeles (UCLA), and the other by
Reinhard Genzel of the Max Planck Institute for Extraterrestrial Physics (MPE) in
Garching, Germany—are staring at the center of the galaxy more intently than anyone
before them. They are tracking a handful of
6 MARCH 2015 • VOL 347 ISSUE 6226 1089 SCIENCE sciencemag.org
A black hole distorts the image of a disk of dust and
gas around it, courtesy of the special effects team for
the film Interstellar.
THE DARK LAB
To put general relativity to the acid test, researchers are looking inward—
toward the supermassive black hole at the center of the Milky Way
By Daniel Clery