Sky and Telescope - December 2015 - 22

1OO Years of General Relativity: Einstein's Coming Golden Age
ent to millimeter waves, and so is the dense maelstrom
around Sagittarius A* itself. So we have an unimpeded
view right down to the hottest material immediately
around the event horizon.
The EHT's dishes, carefully linked and exquisitely
phase-matched to work together as an interferometer,
resolve sources almost as well as a planet-size radio
dish would (S&T: Feb. 2012, p. 20). The long baselines,
combined with the very short wavelength used, should
enable the EHT to resolve Sgr A* down to a resolution
of some 15 microarcseconds. That should be enough to
image the black hole against the background of hot gas
immediately surrounding it.
What will the EHT see? The spacetime around the
event horizon should be so warped that light rays will
wrap partly around it, creating an eerie, distorted apparition: a thin crown of light surrounding a silhouetted
dark orb, which should be distorted and magniﬁed by
gravitational lensing to appear about ﬁve times larger
than the actual event horizon.
The picture the Event Horizon Telescope will take
won't be sharp; the hole's magniﬁed silhouette should
appear just 10 or 15 pixels across. But for the ﬁrst time,
we should truly be able to see a black hole's surface
pushing up through our limits of observation.
The shape of the silhouette, and how the bright
crown wraps around it, will tell us whether what's there
matches what GR predicts. For the ﬁrst time we'll be
able to explore the limits of GR directly in a place where

SHEPERD DOELEMAN / CHI-KWAN CHANG (2)

B star 6,000 light-years away, behaved just like matter
spiraling down to the event horizon of a stellar-mass
black hole. Within a few more years, the explanation of
quasars as hot matter jamming into much more massive black holes became well established. My theoretical
colleagues now believe that the supermassive black holes
at the centers of most galaxies play a crucial role in the
development of galaxies themselves. The powerful winds
from such a hole's immediate surroundings can clear
the gas right out of a galaxy and halt its star formation.
Yet we have never directly seen a black hole. Two
things work against us. First of all, it is "black": its
boundary, the event horizon, by deﬁ nition emits no
observable radiation. Second, black holes are very, very
small. A 1-solar-mass black hole has a radius of only
2.95 kilometers. Even the 4-million-solar-mass black
hole at the center of the Milky Way, which drives the
radio source known as Sagittarius A* ("A-star"), has an
event horizon less than a tenth of an astronomical unit
in radius - 20% as large as Mercury's orbit. Trying to
image it from Earth would be like trying to resolve the
width of a poppy seed in Los Angeles from the distance
of New York.
The Event Horizon Telescope should soon do it. The
EHT is a consortium of telescopes in Chile, Europe,
North America, Hawaii, and Antarctica observing at
wavelengths of about 1 mm: in the part of the electromagnetic spectrum between the far infrared and short
microwaves. The Milky Way's dust clouds are transpar-

MONSTER TO EMERGE Close to the Milky Way's central black hole, accreting material should appear very distorted in particular
ways predicted by general relativity. This frame (wide view and close-up) is from a simulation just after the hole has swallowed a "hot
spot" that spiraled in through a weak accretion disk. The hole is assumed to be rapidly spinning. Left: Red is radio emission, green
is infrared, and blue is X-ray. Some of the infalling blob was ejected in jets conﬁned by magnetic ﬂux tubes. The top tube is aimed
more toward us; its base is seen partly superposed on the silhouette of the hole. Right: A closeup with brightnesses adjusted and
recolored to show ﬁner detail. The inner edge of the accretion disk is blueshifted and brightened on the side approaching us, and
dimmed almost to invisibility on the side moving away. Light from behind is warped into view to ring the silhouette completely.

December 2015 sky & telescope

Sky and Telescope - December 2015

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