SEPTEMBER/OCTOBER 2019 •SKYNEWS 29
passing close enough to the black hole
ends up on a path so curved, it eventually
intersects the event horizon and is lost to
observers.
The dark area provides one of the
key quantitative results from the EHT
project. Its size is determined almost
entirely by the equations of general rela -
tivity—Albert Einstein’s famous theory
of gravity that predicted the existence of
black holes. Using the radius of the dark
centre, astronomers can calculate a black
hole’s mass. The answer from the EHT for
M87’s black hole is a whopping 6.5 billion
solar masses (6.5 billion times our Sun’s
mass). That, in turn, tells us the hole’s
diameter is approximately 38 billion kilo-
metres, which is more than six times the
size of Pluto’s orbit.
LUMPS AND JETS
As we move outward from the dark centre,
the situation becomes increasingly uncer-
tain. The bright orange areas signify radio
emission by ionized gas in the immediate
vicinity of the black hole. The environment
is complex and dynamic, and the EHT’s
view is highly warped by the gravity of the
black hole itself. “We’re not talking about
some smooth medium around the black
hole with small fluctuations on it—it’s just
a turbulent mess,” explains Charles Gam-
mie, an astrophysicist at the University of
Illinois at Urbana-Champaign who coordi-
nates the EHT’s theory working group.
To complicate matters further, the EHT
doesn’t record a photo directly, as a camera
would. Instead, it reconstructs an image by
painstakingly comparing radio signals arriv-ing separately at each of the EHT observato-
ries. The method is guided by carefully de-
rived theoretical models. Even so, random
noise in the signal or in the receivers them-
selves can generate features that are not real.
Broderick notes that the faint glow which
extends toward the upper left and lower
right corners of the image is likely spurious.
Another suspicious area is the apparent
lumpiness in the lower part of the dough-
nut, where some areas look brighter than
others. Again, Broderick is skeptical that
there is genuine physical meaning behind
these differences, though Gammie suspects
the lumps are at least partly due to real
phenomena.
What is more certain is the overall
brightening at the bottom of the ring. The
extra gleam can be attributed to material
moving at velocities approaching the speed
of light along a line of sight directed toward
the telescope. Such conditions produce an
effect known as relativistic beaming, which
causes the emission from a fast-moving
source to brighten in the direction of mo-
tion, like a car headlight.
Broderick assigns much of this bright-
ening to the jet of particles being driven
away from the black hole by powerful mag-
netic fields—a feature apparent in wide-
angle shots of M87. He envisions the jet as
coiling around the rotation axis of the black
hole in a counterclockwise fashion so that
closer in, the most prominent part appears
at the bottom of the doughnut.MOTION PICTURES
Gammie suggests some of the brightening
could be due to the rotation of a disc of gas
encircling the black hole. Although the disc
would be seen roughly face-on from Earth,
it’s tilted enough that one half of the disc
is partly moving toward us, and the other
half is partly moving away. The infalling gas
would be orbiting the black hole at an in-
credible rate and on the very cusp of obliv-
ion, prompting one collaborator, Heino
Falcke of Radboud University in the Nether -
lands, to compare it to looking at “the gates
of Hell.”
EHT team members say the best way
to understand their initial glimpse of the
black hole is to acquire more images and
compare results. This would shed light on
where and how the material producing the
radio waves is moving. The news-making
first picture of a black hole was created
from data gathered in the spring of 2017,VARIATIONS ON A THEMEFour successive images of the central region of M87 from the Event
Horizon Telescope’s 2017 observing campaign show small differences that may (or may not) be due
to physical processes in hot gas moving around a black hole. The timescale of the variations, over
a period of days, is what would be expected from a dynamic region that is several times the size of
our solar system. COURTESY EVENT HORIZON TELESCOPE COLLABORATION
GALACTIC GIANT Containing upwards of one trillion stars, M87 is one of the nearest and
largest active galaxies known. The glowing streak on the galaxy’s right side is a jet of material
being driven away from the black hole by powerful magnetic fields. COURTESY ESOApril 5 April 6 April 10 April 11