Astronomy

44 ASTRONOMY • MAY 2018

A: You are absolutely right that

not even light can escape from

a black hole, and yet they

launch huge, energetic jets that

we can see. The key here is

while nothing can escape a

black hole once it has fallen past

a certain point — called the

event horizon — the jets origi-

nate from outside that region.

Black holes themselves are

invisible. But outside the event

horizon, infalling matter col-

lects into an accretion disk,

which grows hot due to friction

and emits all sorts of radiation,

from optical through X-rays.

When astronomers observe a

black hole, they’re actually

observing the radiation from

this disk, which ends at the

event horizon.

It is from the accretion disk

that the jets are formed and

launched, again well outside

the event horizon. Astronomers

believe these jets arise through

interactions between the black

hole’s magnetic field and elec-

trically charged plasma par-

ticles in the accretion disk.

When the black hole’s mag-

netic field accelerates them,

they gain enough energy to

glow, allowing us to spot them.

For stellar-mass black holes,

the jets “turn on” and become

visible at a distance of tens of

thousands of miles from the

black hole. For supermassive

black holes, this distance is

millions of times greater.

Alison Klesman

Associate Editor

Astronomy’s experts from around the globe answer your cosmic questions.

BLACK HOLE

JETS

Q: IN THE NOVEMBER 2017

ISSUE’S GRAVITATIONAL

WAVES STORY, ROBERT

NAEYE WROTE, “IF GRAVITA-

TIONAL WAVES DISPERSE,

SLOWER FREQUENCIES WILL

ARRIVE AT THE DETECTORS

AFTER THE FASTER ONES.”

BUT HOW ARE THERE DIF-

FERENT VELOCITIES FOR

DIFFERENT FREQUENCIES?

IT SEEMS ILLOGICAL TO ME.

Aaron Morris

Smyrna, Georgia

A: Gravitational waves are dis-

turbances in the fabric of space-

time that radiate outward from

the rapid movement of massive

objects. And in fact, you are

right: Basic human intuition

says that a high-frequency wave

should travel at the same speed

as a low-frequency wave.

A familiar example of dis-

persion occurs when white light

passes through a prism, which

separates the light into its con-

stituent colors. This occurs

because different wavelengths

(frequencies) of light take dif-

ferent paths through the prism.

As light passes through the

prism, it is absorbed and re-

emitted by the atoms of the

prism differently, depending on

its frequency. Violet light is

slowed down by this absorption

and re-emission process more

than red light, and thus the two

travel very different paths,

spreading out by the time they

exit the prism.

According to Einstein’s the-

ory of general relativity, gravi-

tational waves do not

experience dispersion in this

way. In other words, provided

that general relativity is an

accurate description of nature,

high-frequency gravitational

waves should travel at exactly

the same speed (the speed of

light) as low-frequency gravita-

tional waves. There should be

no preferential absorption or

re-emission of gravitational

waves as they travel through

space-time, regardless of their

frequency. Many alternative

theories, however, predict that

gravitational waves could expe-

rience dispersion as they travel

through space-time; these the-

ories treat gravity and

ASKASTR0

Q: IF NOT EVEN LIGHT CAN ESCAPE

FROM A BLACK HOLE, HOW CAN YOU

EXPLAIN JETS THAT LAUNCH AT NEAR

LIGHT SPEED EXTENDING FOR NUMEROUS

LIGHTYEARS? Robert Byerly, Windsor, California

Black holes themselves are invisible, but they can form huge accretion disks and launch energetic jets. The

accretion disk (orange-red and blue) and jet (purple) occur outside the black hole’s event horizon (the dark sphere)

and are thus visible to astronomers. NASA/JPL-CALTECH

Light waves

disperse as they

pass through a

prism, spreading

into a rainbow.

Gravitational

waves, however,

do not disperse,

meaning both

high- and

low-frequency

gravitational

waves travel

at the same speed

through every

medium.

LUCAS V. BARBOSA