From imagination to reality : The image of a black hole

drishti-magazine

In the century since Einstein predicted the

existence of black holes in his theory of gravity,

astrophysicists have turned up overwhelming

evidence for the things. They’ve observed the

push and pull of black holes on the orbits of

nearby stars and planets. They’ve heard the

vibrations, or gravitational waves, resonating

from black holes colliding. But they’d never

glimpsed a black hole face to face—until now.

On Wednesday, 10th April 2019 astrophysicists

announced they had captured the first-ever

image of a black hole.

The picture, taken over five days of

observations in April 2017 using eight

telescopes around the world by a collaboration

known as the Event Horizon Telescope, depicts

luminous gas swirling around a supermassive

black hole at the center of M87, a galaxy 54

million light-years away. Past the bright lights,

though, is the black hole’s telltale feature: its

event horizon. The event horizon is the edge of

the spacetime abyss, where gravity is so strong

that no light can escape from it. “It’s the point of

no return,” says FeryalÖzel of the University of

Arizona, who is a member of the EHT

collaboration. In the image, it manifests itself as

the “sudden absence of light,” she says.

Previously, researchers had captured a

blobby jet of light emerging from where the M87

black hole was predicted to be—but they

couldn’t definitively see the black

hole because their instruments

were nowhere near as sharp as

EHT’s. “It’s like going from a

cheap smartphone camera to a

high definition IMAX cinema,”

says astrophysicist Andrew

Strominger of Harvard University,

who was not involved in the work.

south pole telescope

The South Pole Telescope,

one of eight telescopes used to

capture the first black hole image.

This black hole is about 6.5

billion times the mass of the sun.

Still, it’s tiny from a vantage point

on Earth, less than 50

microarcseconds wide in the sky,

which makes it about as hard to

see as a donut placed on the

moon. It took eight different

telescopes to image it. The

telescopes collected observational data that

was synced with the precision of a billionth of a

second.

To see the black hole’s boundary between

light and dark, the astrophysicists captured radio

waves—light 1.3 millimeters in wavelength,

invisible to the human eye—emitted by the gas

swirling around the black hole. The gas emits

light of all different wavelengths, including visible

light, but the researchers chose this particular

wavelength because it can sail through entire

galaxies and even Earth’s own atmosphere

without being absorbed. But they still needed

good weather at all eight of their telescope sitesto see the black hole. Before switching on their

telescopes, they had to monitor the moisture in

the air, says Özel—too much humidity would

ruin their images. To minimize the chance of

rain, they built the telescopes in dry regions,

including the South Pole and the Atacama

Desert in Chile.

M87’s black hole is relatively close to Earth,

as the light coming from it was only emitted 54

million years ago—so we’re seeing it at a more

mature moment in its existence. “At this point

in the age of the universe, black holes have

calmed down,” says Özel. “They’re basically

eating gas trickling in from nearby stars.” M87’s

black hole does emit bright jets of gas, but it’s

still pretty dim compared to younger black holes

that are further away. These younger black holes

accumulate larger amounts of matter, so their

swirls of luminous gas shine brighter.

To capture and interpret the first black hole

image, scientists first created millions of

simulations like this one.

It took two decades of work to capture the

image. Part of that effort was designing, building,

and hauling the hardware to various telescope

sites. But they also had to anticipate what they

might see by nailing down the physics of black

holes as accurately as possible. Özel, who has

been working on photographing a black hole

since her graduate student days in 2000, says

that they’ve created millions of simulations of

black holes, each with different mass, spin

speed, or orientation, among other things.

These simulations helped inform how they

designed their telescopes and where they

pointed them.

But they weren’t just after a pretty picture. In

the zoo of astronomical objects, black holes are

among the most extreme entities to exist. A black

hole, as currently understood, packs an

enormous amount of mass into a single point,

making it—literally—an infinitely dense object.

This density creates a huge gravitational pull

into its center, which no one can peer inside.

“They are the only objects in the universe that

create a region of spacetime inaccessible to the

rest of the universe,” says Özel. Because black

holes are so extreme, researchers want to study

their features to see if they are consistent with

the rest of general relativity. “We all feel we have

an intuitive sense of what space and time are.

But Einstein told us that’s true only in situations

like the ones we’re used to, where the

gravitational field is very weak,” says Strominger.

“When the gravitational field gets strong, there

are all sorts of crazy things that happen.”

Everything they’ve observed so far about

M87—its mass and the size of its event

horizon—is consistent with Einstein’s theory.

But future, more detailed observations could

reveal unexpected features. Strominger wants

to see more detailed images of a fast-spinning

black hole like M87. According to theoretical

calculations, if black holes spin fast enough, they

form a wormhole in spacetime. Future black

hole images could help confirm or refute these

hypotheses. Strominger is anticipating the day

when images are good enough to see a black

hole with its associated wormhole. “This is really,

really weird science fiction stuff, and we’re going

to be seeing it,” he says.

This image is just the beginning, says Özel.

They want to pivot their telescopes toward other

black holes, to amass a whole scrapbook of

black hole images. They also plan to take more,

better-quality pictures of this black hole to

understand it in more detail. Now that they’ve

finally stared into the eyes of the beast, it’s time

to watch how it behaves.