BEYOND THE
W
HEN SCIENTISTS behind the Event Horizon Telescope (EHT) set out to
create a direct image of a black hole, they weren’t expecting the Mona Lisa.
Far from it. However, the now famous orange doughnut they unveiled to
the world last April has at least two things in common with Leonardo da Vinci’s iconic
masterpiece.
First, despite its simplicity, the black hole close-up is a technological tour de force. The
EHT has opened a window into the compact central region of the galaxy Messier 87—a
target occupying an area of sky comparable to a coin lying on the surface of the Moon.
Second, like Mona Lisa’s enigmatic smile, the EHT portrait is a matter of interpretation.
There are tantalizing hints about what’s really going on in the maelstrom of ionized gas
that surrounds M87’s supermassive black hole.
“It was an incredibly difficult image to make,” says Avery Broderick, a physicist at
the University of Waterloo, in Ontario, and a longtime member of the EHT team. “What
fol lows is the equally difficult—if not more difficult—process of trying to determine what
it means.”CIRCULAR REASONING
The EHT is not a single instrument, but many. It relies on radio interfer ometry—a tech-
nique in which multiple widely separated radio dishes are harnessed to produce resolution
far greater than any one receiver could achieve on its own. The EHT network combines
simultaneous measurements from radio observatories scattered across half the planet.
M87, a giant elliptical system containing trillions of stars, is a particularly exciting target
for this global approach.
Located 53 million light-years away, M87 is the source of a dramatic jet of high-speed
particles extending from the galaxy’s centre for hundreds of thousands of light-years into
intergalactic space. Due to its favourable combination of size, distance and power, M87
offers the best opportunity to observe the complex interactions between a central black
hole and the matter swirling around it. How the new image relates to those interactions
and to the bigger question of how supermassive black holes affect the galaxies they inhabit
are mysteries the EHT collaborators are just beginning to tackle.
Ironically, the part of the EHT photo that scientists are most confident they understand
is the part they can’t see—the dark region at the centre. This utterly blank middle is pre-
cisely what would be expected from an object whose gravitational pull is so strong it can
trap light. The point-of-no-return boundary defining the black hole is called the event
horizon. But while the shape of the boundary is approximately spherical, the dark, circular
void is larger than the event horizon’s diameter. This is a consequence of the black hole’s
strong gravity, which bends the path of all light—and radio waves—travelling near it. LightSEPTEMBER/OCTOBER 2019 •SKYNEWS 27
A HOLE IN SPACEHot gas at the centre
of the galaxy M87 forms a ring around a cir -
cular dark region that is occupied by a super-
massive black hole. Computer models suggest
that the lower half of the ring appears brighter
than the top half because it includes material
moving along the telescope’s line of sight at
velocities approaching the speed of light.
COURTESY EVENT HORIZON TELESCOPE COLLABORATION
ENIGMATIC ENGINEIn the artistic rendering
at left, the supermassive black hole at the heart
of the galaxy M87 is surrounded by a disc of
hot gas. High-speed jets of particles move out-
ward along the black hole’s axis of rotation.
While the basic theoretical picture is consistent
with observations by the Event Horizon Tele-
scope, many questions remain about the struc-
ture of the disc and the mechanism that pow-
ers the jets. ILLUSTRATION COURTESY ESO/M. KORNMESSER
Astronomers
now know what a
black hole looks like.
Their next challenge
is understanding
what they’re seeing.
by IVAN SEMENIUKCOSMIC ENIGMA