30 | NewScientist | 3 November 2018detector hardware. Noise is a huge problem
in gravitational wave detections.
Hence why there are detectors in different
places. We know that gravitational waves
travel at the speed of light, so any signal is only
legitimate if it appears in all the detectors at
the right time interval. Subtract that common
signal, and what is left is residual noise unique
to each detector at any moment, because its
seismic vibrations and so on constantly vary.
This is LIGO’s main ploy for extracting a
gravitational wave signal from the noise. But
when Jackson and his team looked at the data
from the first detection, their doubts grew. At
first, Jackson printed out graphs of the two raw
signals and held them to a window, one on top
of the other. He thought there was some
correlation between the two. He and his team
later got hold of the underlying data the LIGO
researchers had published and did a calculation.
They checked and checked again. But still they
found that the residual noise in the Hanford
and Livingston detectors had characteristics
in common. “We came to a conclusion that
was very disturbing,” says Jackson. “They
didn’t separate signal from noise.”
The Danish team wrote up their research
and posted it online. After receiving no
response from the LIGO collaboration, they
submitted it to the Journal of Cosmology and
Astroparticle Physics. The journal’s editor,
Viatcheslav Mukhanov of the Ludwig
Maximilian University in Munich, Germany,
is a world-renowned cosmologist. The
editorial and advisory boards include top
physicists such as Martin Rees from the
University of Cambridge, Joanna Dunkley
at the University of Oxford and Andrei LindeThe gravitational wave observed in September 2015 is obscured by noise in the raw signal
from LIGO’s Livingston detector. It is only visible in the cleaned-up plot, where irrelevant
frequencies have been removed and the scale is magniied 100 times
Time (seconds)Amplitude0 5 10 15 20 25 30SOURCE: arXiv 1706.04191In 2014, the operators of the
BICEP2 telescope made an
announcement so momentous there
was talk of a Nobel prize. A year later
however, far from making their way
to Stockholm for the award ceremony,
they were forced to admit they had been
fooled by an embarrassing noise.
Situated at the South Pole, BICEP2
had been scanning the cosmic
microwave background, the pattern
of radiation left on the sky from light
emitted soon after the big bang. The big
announcement was that it had found
that gravitational waves had affected
the pattern in such a way that proved a
core theory of cosmology. The theory in
question was inflation, which says the
universe went through a period of
superfast growth right after the big
bang. For almost four decades it had
been unproven. Now, suddenly,
inflation’s supporters were vindicated.
Except awkward warnings emerged
within weeks, suggesting that cosmic
dust clouds had scattered the radiation
in a way that fooled the BICEP2
researchers. In the end, the team’s
estimate of the amount of dust present
and the analysis of the kind of noise the
dust would produce both proved to be
flawed. Noise can hoodwink even the
smartest. That is why, despite LIGO being
a highly respected collaboration, there is
good reason to take questions about its
noise analysis seriously (see main story).EMBARRASSING
NOISESof Stanford University in California.
Mukhanov sent the paper for review
by suitably qualified experts. Reviewers’
identities are routinely kept secret so they
can comment freely on manuscripts, but these
were people with a “high reputation”, says
Mukhanov. “Nobody was able to point out a
concrete mistake in the Danish analysis,” he
says. “There is no mistake.”
A storm in a teacup, still? General relativity
is one of our most well-verified theories,
after all, so there is every reason to think its
prediction of gravitational waves is correct.
We know LIGO should be sensitive enough to
detect them. The instruments are finding the
waves at exactly the right rate predicted by
theory. So why worry about this noise?Seek and ye shall find
There’s a simple answer to that question.
Physicists have made mistakes before,
mistakes that have been exposed only by
paying close attention to experimental noise
(see “Embarrassing noises”, left).
The first step to resolving the gravitational
wave dispute is to ask how LIGO’s researchers
know what to look for. The way they excavate
signal from noise is to calculate what a signal
should look like, then subtract it from the
detected data. If the result looks like pure,
residual noise, they mark it as a detection.
Working out what a signal should look like
involves solving Einstein’s equations of
general relativity, which tell us how
gravitational forces deform space-time. Or at
least it would if we could do the maths. “We are
unable to solve Einstein’s equations exactly
for the case of two black holes merging,” says
Neil Cornish at Montana State University, a
senior figure among LIGO’s data analysts.
Instead, the analysts use several methods to
approximate the signals they expect to see.
The first, known as the numerical method,
involves cutting up space-time into chunks.
Instead of solving the equations for a
continuous blob of space, you solve them for
a limited number of pieces. This is easier but
still requires huge computing power, meaning
it can’t be done for every possible source of
gravitational waves.
A more general approach, known as the
analytic method, uses an approximation of
Einstein’s equations to produce templates
for gravitational wave signals that would be
created by various sources, such as black holes
with different masses. These take a fraction of
a second to compute, but aren’t accurate
enough to model the final merger of two blackCUTTING THROUGH THE NOISE