2019-04-20_New_Scientist

6 | NewScientist | 20 April 2019

THIS WEEK

HUMANITY has had its very first

look at a black hole. Last week,

the Event Horizon Telescope

(EHT), a global collaboration that

uses radio telescopes around the

world to make one Earth-sized

observatory, unveiled its pictures

of the black hole at the centre of

the distant M87 galaxy, the first

direct images of one ever taken.

Now, the even harder work begins:

figuring out what it all means.

The images are the first proof

that the event horizon – the line

at which a black hole’s gravity is

so strong that nothing, not even

light, can escape – is real. They

show light from matter right next

to the black hole bending around

it to create a sort of bright halo,

as predicted by Albert Einstein.

“There are bizarre effects up

close to a black hole,” says Priya

Natarajan at Yale University.

“We are starting to test Einstein’s

theory of general relativity in

a regime that we’ve never done

before.” Even after the first image,

though, many questions remain.

The biggest is how general

relativity, which governs gravity

and the very large, and quantum

mechanics, which concerns the

very small, fit together. The

event horizon is such an extreme

distortion of space-time that

both theories should be in play.

In fact, a black hole may be the

only place in the universe where

we have a chance of figuring out

how they work together – but not

with these pictures.

“Can we really understand the

hard stuff, like what’s going on

at the quantum level at the event

horizon? That’s not likely with

this data,” says Janna Levin at

Barnard College in New York.

“Most astrophysical black holes

are too big to really be exploring

their quantum side.”

What we can learn are the

specifics of supermassive black

holes. For example, matter

surrounding the M87 black hole is

being spewed out as an enormous

jet of plasma and energy. We know

that this must be powered by the

black hole spinning, but we don’t

yet know how quickly it rotates.

Studying the asymmetry of the

light around the black hole should

give us the answer.

Source unknown

We also don’t know where exactly

the light in the images, against

which we can see the black hole’s

silhouette, comes from. Strictly

speaking, the orange glow is

actually a tinted representation

of colourless radio frequency

photons, rather than visible light.

“You’re seeing photons that

are just zipping around the black

hole and they must come from

very nearby, but their source is

unclear,” says Kristin Madsen

at the California Institute of

Technology. They could be

emitted by the disc of material

circling the black hole, doomed

to be swallowed, or they could

come from the base of the jet.

The EHT is also trying to image

Sagittarius A*, the black hole at

the centre of our galaxy. Unlike

the one in M87, it doesn’t seem

to have a jet, but it is interesting

in other ways, particularly because

we may be able to see it changing

over time.

Gaze into the abyss

Amazing images still leave a black hole of knowledge, says Leah Crane

AN

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The first image of a black hole was

unveiled at events across the world

Your black hole questions answered

READERS of New Scientist were

thrilled to see a black hole for the first

time, but the image of the M87 black

hole left many people puzzled. We

gathered questions on our Twitter

account @ newscientist and have

answered some of the best below. For

the full Q&A, visit bit. ly/black-hole-qs

DON’T BLACK HOLES SUCK

EVERYTHING IN, INCLUDING

LIGHT? HOW CAN WE SEE ONE?

The picture is of the black hole’s

silhouette against the bright

material circling it. Nothing we can

see is coming out of the black hole.

WHY IS THE IMAGE BRIGHTER

ON ONE SIDE?

The black hole is rotating. The light

coming towards us appears brighter

and that moving away seems dimmer.

WHERE IS THE EVENT HORIZON?

The event horizon, from beyond

which even light cannot escape, is in

the central black area – the shadow

of the black hole against the material

circling it, and eventually falling in.

HOW BIG IS THE BLACK HOLE?

It is about 7 billion times the mass

of the sun, and about 100 billion

kilometres wide. This is about 22

times the average distance between

Neptune and the sun, so it would

easily swallow the solar system.

DOES EVERY SPIRAL GALAXY

HAVE ONE AT ITS CENTRE?

We think that pretty much all galaxies

have supermassive black holes at

their centres. They may be important

for the formation of big galaxies.

WEREN’T TINY BLACK HOLES

MADE HERE ON EARTH WITH

THE LARGE HADRON COLLIDER?

No. Some people were concerned

the particle accelerator would

generate mini black holes, but it

didn’t. It would require much more

energy than the collider can produce

to do that.

HAVE WE PROVEN HAWKING

RADIATION OR IS IT JUST A

THEORY?

Hawking radiation is meant to

be released by a black hole as it

evaporates. It is very difficult to prove

experimentally or observationally,

and has never been observed. For

now, it is still a purely theoretical

quantum effect.

“ You’re seeing photons that
are just zipping around the
black hole and they must
come from very nearby”