20 April 2019 | NewScientist | 7For new stories every day, visit newscientist.com/news
The observations of the M 87
black hole, taken over the course
of four days, apparently show
a small amount of variation.
However, it isn’t significant
enough to know whether this
is real or an artefact of the
imaging process. That is part of
the reason why the first images
were of the black hole in M 87
instead of Sagittarius A*: it is
bigger and calmer, so changes
much more slowly.
“Changes that take a week
on M87 take a few minutes on
Sagittarius A*,” says EHT team
member Michael Johnson at
the Harvard-Smithsonian
Center for Astrophysics in
Massachusetts. “It’s like trying
to take a picture of people, but
they’re all running around.”
The activity doesn’t prevent
us from imaging Sagittarius A*,
it just makes it harder to turn
the data from all the telescopes
into one picture. Nevertheless,
the collaboration will probably
release an image of our own
galaxy’s black hole within the
next year or so, says Johnson.
“We know that there are
probably tens of thousandsLIGO is back at it. Having just
restarted on 1 April after months of
upgrades, the Laser Interferometer
Gravitational-Wave Observatory
has already spotted another two
black-hole collisions.
Gravitational waves are ripples
in space-time that occur whenever
massive objects move, like the wake
behind a boat travelling across a
lake. LIGO announced the first-ever
observations of gravitational waves
in 2016 and has now spotted a total of
13 gravitational signatures of pairs of
enormous objects smashing together.
Following the upgrade to the twin
detectors near Livingston in Louisiana
and Hanford in Washington, we
expect to see about one gravitational
wave per week. And, just a few weeks
after the detectors were turned
back on, that expectation is already
becoming reality. The first collision
was announced on 8 April, with the
detection of gravitational waves from
a pair of merging black holes almost
5 billion light years away. LIGO made
the find along with the Virgo
gravitational-wave observatory in Italy.
“This event is very exciting because
it is the first rapid, public alert of a
gravitational-wave detection,” saysLIGO spokesperson Patrick Brady.
“Further analysis is ongoing.”
LIGO and Virgo haven’t released
any official information on the sizes
of the two black holes. However,
based on the strength of the signal
from so far away, they are probably
relatively large like several of the
earlier detections, perhaps around
30 times the mass of the sun, says
astronomer Derek Fox at Pennsylvania
State University. “Just being big and
loud and right at the beginning of the
run is exciting,” he says.
And, in a busy week for black holes,
a second merger was announced
on 12 April, at a distance from us of
around 2.6 billion light years. In fact,
these marvels of general relativity
have become almost commonplace –
you can sign up to be alerted to each
new detection via an iPhone app
called Gravitational Wave Events. The
alerts are intended to let astronomers
around the world know where to look
for follow-up observations, says Fox.
That will come in handy during
this new set of observations, which
is expected to last a year or more.
“This observing run is supposed to
bring a real harvest of black holes
and also probably a few neutron
stars,” says Fox. “Some of us have
been looking forward to this new
era for a long time.” Leah Crane ■of smaller black holes around our
galaxy’s supermassive black hole,
and the little guys can fall into
the big guy,” says Levin. “There’s
all kinds of interesting things we
can learn from looking at how the
supermassive black hole changes.”
We don’t actually know how
matter falls into a black hole or
how it is connected to its jet, and
watching a black hole over time
could help us figure it out.
In the past year, the EHT has
added telescopes that will make
the next sets of images twice as
sharp, says Johnson. Improving
the resolution much more
than that would require major
upgrades to all the telescopes
in the network, or expanding it
to include telescopes in space.
Eventually, years of
observations will let us see
how both the M87 black hole
and Sagittarius A* change over
time. One day, we may even
be able to make a movie of real
black holes – one that will really
suck you in. ■For even more black hole unknowns,
see “Help! I’ve fallen into a black hole”
on page 30Black holes are colliding
across the universe
CHRISTIAN^ OFF
ENBE
RG
/ALAMY^ STOCK
PHOTOOne of LIGO’s huge detectors
is near Livingston, Louisiana