ASKASTR0
Astronomy’s experts from around the globe answer your cosmic questions.
BLACK HOLE
JETS
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.
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
LIGHT-YEARS?
Robert Byerly, Windsor, California
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
LUCAS V. BARBOSA
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
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
44
A ST R O N O M Y • MAY 2018
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