1 June 2019 | New Scientist | 47“ We think
about particles
being tiny but,
theoretically,
they can be
as big as a
galaxy”
operate in 10 dimensions. The six beyond
those we know must be scrunched up in
elaborate fashion to fit into unimaginably
small spaces, or else we would have seen
them. And it is this rich, extra-dimensional
structure, according to Arvanitaki, that gives
rise to all manner of ultralight axions – what
she calls a string axiverse.
Which brings us back to planet-sized
particles. When Arvanitaki was writing up the
string axiverse idea, a visiting colleague asked
if she had heard of black hole superradiance.
She hadn’t. And yet once she had spent a year
wrapping her head around the idea, she came
to realise it could give us a unique opportunity
to spot axions, if indeed they exist.
Superradiance is a well-established process
used in certain types of laser to multiply
photons. It works on astrophysical scales
too. Basically, if you have a particle of light –
simultaneously a wave, as per the quirks of
quantum mechanics – and you fire it at aspinning black hole, it will extract energy
and angular momentum from the black
hole. In other words, the black hole will give
the photon a kick. Now, if you do the same
thing, but swap the massless photon for a
force-carrying particle with mass, such as an
axion, gravity would confine it to the vicinity
of the black hole. In this case, it would be
almost as if these axion particles are stuck
between the black hole and the surface of a
perfectly spherical mirror.
“So now the [axion] waves scatter from
the spinning black hole, but then keep
bouncing back and forth, and eventually the
amplification becomes exponential,” says
Arvanitaki. In this version of superradiance, a
cloud of gazillions of axions would be created,
which would arrange themselves in an orderly
fashion, she adds, “a lot like those pictures of
atomic orbitals, only on a massive scale”.
The problem is, to make these “black-hole
atoms”, the axion wavelength must be as
long as the black hole is wide. Except that isn’t
a problem here, as wavelength is inversely
proportional to mass, and with axions we
are talking about extremely light particles.
“We tend to think about particles as being
tiny but, theoretically, there is no reason they
can’t be as big as a galaxy,” says Arvanitaki.
All of which was known, at least to a few
people. What Arvanitaki and her colleagues
have recently figured out is that these
axion clouds could reveal themselves in
gravitational waves, the faint ripples in
space-time first picked up by the Laser
Interferometer Gravitational-Wave
Observatory (LIGO) in 2015. And in this
case, you don’t need black holes smashing
together. Axions colliding in the cloud
should annihilate one another to produce
gravitons, the particles thought to comprise
gravitational waves. Essentially then, axions
and black holes combine to dramatic effect,
to produce what Arvanitaki describes as
“gravitational beacons” that shine out in
every direction. Arvanitaki has been working
with researchers from LIGO to prepare for
the detector’s third run, which began in April
and right away detected gravitational waves.
It is expected to continue to find them every
couple of weeks.
Arvanitaki is one of a new generation
of particle physicists seeking to make their
mark not by inventing more elaborate
theories, but by figuring out where – and
how – to look if you want to find something
new. “When nature tells us it doesn’t work
the way we think it should,” she says, “we have
to look in other directions.” ❚Daniel Cossins is a features writer
for New Scientist, specialising in
the physical sciences