30 ASTRONOMY • JANUARY 2020

Black holes are ubiquitous in our

universe. We have seen them gravitation-

ally act on objects that come too close, and

we have charted the swirls of matter that

fall into them. But, of course, black holes

are black. And anything that gets too close

to the black hole — beyond a point of no

return called the event horizon — is gone

forever. Not even light can escape.

But a black hole does cast a shadow on

the bright material around it. And on

April 10, the Event Horizon Telescope

(EHT) Collaboration released the first-

ever image of the shadow cast by a black

hole, made possible by a network of eight

radio telescopes working in sync from

around the globe. Their work was pub-

lished the same day in six papers in The

Astrophysical Journal Letters, and it rep-

resents the efforts of hundreds of col-

laborators and decades of work.

The photo features the 6.5 billion-

solar-mass supermassive black hole at

the heart of the galaxy M87, which sits

55 million light-years away in the Virgo

Cluster. The black hole itself is about the

size of our solar system, but at the dis-

tance of the Virgo Cluster, it is too

impossibly small for any one telescope

to image. EHT accomplished the feat

using a technique called very-long-base-

line interferometry, or VLBI, to create a

virtual radio telescope with a receiver as

big as Earth. By observing the same

object with eight telescopes at the same

time and carefully combining the results,

EHT could make out structures as small

as 20 micro-arcseconds across — the

visual acuity needed to read a newspaper

in New York from Paris. The shadow

they saw is only about 2.5 times larger

than the black hole’s event horizon,

which stretches about 25 billion miles

(40 billion km) across, or about three

times the diameter of Pluto’s orbit.

The team faced many challenges along

the way. “We were doing something

entirely new. I think the greatest chal-

lenge overall was building up from noth-

ing to an analysis that I’m pretty happy

with, where we simulate everything we

observe and we test our methods and we

make sure that we’re not falsely creating a

doughnut in the sky where none exists,”

says Dan Marrone, an associate professor

at the University of Arizona and member

of the EHT science council and data

analysis working group coordinator.

Other challenges included simply han-

dling the vast amounts of data produced,

which were stored on 960 hard drives

and physically shipped to the Max Planck

Institute in Bonn, Germany, and the MIT

Haystack Observatory in Westford,

Massachusetts, to be matched and com-

bined. That process alone took months,

says Feryal Özel, another EHT collabora-

tor at the University of Arizona.

The resulting image is more than just a

stunning picture, or even proof of concept.

It’s a unique and powerful way to test our

understanding of physics in a laboratory

provided by nature itself. Simply creating

the image confirmed that our theories

appear correct, as the picture matches

1

Astronomers shine

light on a black hole

The Event Horizon

Telescope Collaboration

imaged the black hole at

the heart of M87 (left) using

eight radio telescopes

observing at a wavelength

of 1.3 millimeters. Gravity

warps the bright light of the

hot gas and dust swirling

around the black hole into

a ring, against which the

shadow of the black hole

is visible. At right is a

computer-generated

visualization of a black hole

based on Einstein’s theory

of general relativity,

showing the misshapen

disk in greater detail. The

disk appears brighter in

some regions than others

because light from material

in the disk moving toward

us appears brighter than

light from the material

moving away. EVENT HORIZON

TELESCOPE COLLABORATION; NASA’S

GODDARD SPACE FLIGHT CENTER/JEREMY

SCHNITTMAN

“It’s been my belief for a lot of years that we could

technically do this, but I had no idea what we would see.”