24 | New Scientist | 7 September 2019
E
ARLY one Saturday when I
was 10, my mother dragged
me across Los Angeles to
see, of all things, a documentary.
It was Errol Morris’s A Brief History
of Time, about Stephen Hawking’s
life and work. As Hawking
discussed the “singularities”
at the centre of black holes,
I was stunned to learn that there
was something Albert Einstein
had been unable to resolve. That
black hole captured me for life.
Black holes gain form in
Einstein’s general theory of
relativity, which proposes that
space and time are unified into
a space-time curved by the
presence of massive objects.
General relativity encourages us
to move away from thinking about
a mysterious force called gravity.
Instead, it says that a body such as
the sun is so massive that it bends
space-time. Planets orbit the sun
because the straightest line they
can take in space-time near it is
an elliptical path around it.
Take Einstein’s picture to its
logical conclusion and you end
up asking yourself what happens
when an object is so massive that
it effectively folds space-time in
on itself. This is a black hole.
Black holes are often described
as being akin to a deep well: if you
fall in, you can’t get out. But what
happens inside its boundary, or
“event horizon”, is even more
fascinating. The normal properties
of space and time seem to switch
places. In everyday life, we can
only move forward in time, but
inside a black hole, things can only
move forward in space – like being
on an irreversible conveyor belt.
What is that conveyor belt
moving towards? Mathematically,
space-time curvature becomes
infinite at a black hole’s central
singularity, whatever that means.
But we don’t know whether
that reflects physical realityor just the theory breaking down.
As a university student, I
wanted to get involved in the
latest research on black holes.
I learned the hard way that this
isn’t straightforward. There is the
way black holes are discussed in
popular literature and the type
of research portrayed in Morris’s
documentary, focusing on the
quantum properties and space-
time properties of black holes.
Then, there are astrophysical
black holes: real objects that
seem to act like black holes and
form, we think, when massive
objects such as stars collapse.It is hard to see black holes,
so black hole astrophysics
focuses on observations of what
happens near them. As exotic
as black hole theory seems, we
apparently have one supermassive
black hole relatively close to us.
Sagittarius A* is a bright radio
source at the Milky Way’s centre,
and we are fairly certain that it is
a black hole with the mass of a
few million suns.
Some of the most intriguing
questions about black holes come
from studying objects known as
active galactic nuclei (AGN):
extremely bright, compact
galactic centres believed to have
black holes of even greater mass
at their cores. These come in a
variety of classes. Quasars giveout a lot of radio waves – they
are “radio-loud”. But maybe my
favourite AGNs are blazars, which
are not only very bright and radio-
loud, but also spew out jets made
of particles travelling close to light
speed. Now there’s a thing: if black
holes suck in and hide everything,
why is it that some of them have
particles flying away at high
speeds? And why do only
some do this, and not others?
One possibility is that all AGNs
have jets, but we can only see some
of them. This would hardly lessen
the core mystery, however, and
that is before you get to the fact
that blazar jets and their host
galaxies have a variety of different
colours that change over decades.
The exciting thing is that we are
making progress on a lot of fronts.
A recent paper from a team led by
Jedidah Isler at Dartmouth College
in New Hampshire showed that
we might be able to find a unifying
model of blazars that explains
how their colour changes in time.
Meanwhile, the big development
earlier this year was when the
Event Horizon Telescope imaged
the region immediately at a black
hole boundary. Over the past
two years, gravitational wave
experiments have also found
multiple small, star-mass black
holes – most recently seeing
one eating a neutron star.
After an undergraduate thesis
focused on astrophysical black
holes and doctoral work that
included considering how black
holes would work if gravity were
slightly different from how
Einstein proposed, I have moved
on to thinking about neutron
stars, which are thought to form
when objects not massive
enough to make black holes
collapse. But for me, as for many
others, black holes will remain
my first gateway to the wonders
EHT COLLABORATION of the universe. ❚This column appears
monthly. Up next week:
Graham Lawton“ There is the way
black holes are
discussed in popular
literature and
then there are
black holes
that really
exist”When is a black hole not a black hole? When it’s an astrophysical
black hole, of course. These massive, mysterious objects encapsulate
the majesty of the cosmos for Chanda Prescod-WeinsteinField notes from space-time
What I’m reading
Kaiama L. Glover’s
translation of Dance on
the Volcano, a novel by
the late Haitian writer
Marie Vieux-Chauvet.What I’m watching
I really liked the
documentary Hail
Satan? about grass-roots
political activism and
religious freedom.What I’m working on
My lecture notes for the
course I am teaching this
semester, Introduction to
astrophysics.Chanda’s week
Chanda Prescod-Weinstein
is an assistant professor of
physics and astronomy, and
a core faculty member in
women’s studies at the
University of New Hampshire.
Her research in theoretical
physics focuses on cosmology,
neutron stars and particles
beyond the standard modelViews Columnist