32 | NewScientist | 3 November 2018
says. “But I don’t think they’re quite there yet.”
The Danish group’s independent checks,
published in three peer-reviewed papers,
found there was little evidence for the
presence of gravitational waves in the
September 2015 signal. On a scale from certain
at 1 to definitely not there at 0 , Jackson says
the analysis puts the probability of the first
detection being from an event involving black
holes with the properties claimed by LIGO at
0.000004. That is roughly the same as the odds
that your eventual cause of death will be a
comet or asteroid strike – or, as Jackson puts
it,“consistent with zero”. The probability of
the signal being due to a merger of any sort
of black holes is not huge either. Jackson and
his colleagues calculate it as 0.008.
Simultaneous signal
There is other evidence to suggest that at
least one of the later detections came from
a gravitational wave. On 17 August 2017,
the orbiting Fermi telescope saw a burst of
electromagnetic radiation at the same time
as the LIGO and Virgo detectors picked up a
signal. Analysis of all the evidence suggests
that both signals came from the brutal
collision of two neutron stars.
The double whammy makes LIGO’s
detection seem unequivocal. Even here,
though, the Danish group is dissenting.
They point out that the collaboration initially
wrong,” insists Cornish. “There were very
basic mistakes.” Those “mistakes” boil down
to decisions about how best to analyse the
raw data (see “How to catch a wave”, below).
Not everyone agrees the Danish choices
were wrong. “I think their paper is a good one
and it’s a shame that some of the LIGO team
have been so churlish in response,” says Peter
Coles, a cosmologist at Maynooth University
in Ireland. Mukhanov concurs. “Right now,
this is not the Danish group’s responsibility.
The ball is in LIGO’s court,” he says. “There are
questions that should be answered.”
Brown thinks the Danish group’s analysis is
wrong, but worth engaging with. And Cornish
admits the scrutiny may not be a bad thing.
He and his colleagues plan to put out a paper
describing the detailed properties of the LIGO
noise. “It’s the kind of paper we didn’t really
want to write because it’s boring and we’ve
got more exciting things to do.” But, he adds,
it is important, and increased scrutiny and
criticism may in the end be no bad thing.
“You do have to understand your noise.”
Coles himself doesn’t doubt that we have
detected gravitational waves, but agrees with
Jackson that this cannot be confirmed until
independent scientists can check the raw data
and the analysis tools. “In the spirit of open
science, I think LIGO should release everything
needed to reproduce their results.”
Jackson is unconvinced that explanatory
papers will ever materialise – the collaboration
has promised them before, he says.
“This LIGO episode continues to be the most
shocking professional experience of my
55 years as a physicist,” he says. Not everyone
would agree – but for a discovery of this
magnitude, trust is everything. ■Michael Brooks is a consultant for New ScientistOutput from gravitational
wave detectors is full of
noise. Disentangling the
signal requires decision–
making – and poor ones could
be disastrously misleading.
The best weapon in the
arsenal is known as a Fourier
transform. This splits a signal
into various frequency
components and converts
it into a power spectrum,
which details how much
of the signal’s power is
contained in each of those
components. This can be
done with a window function,
a mathematical tool that
operates on a selected part
of the data. Whether or not
to use one is at the heart of
the disagreement over LIGO’s
results (see main story).
Andrew Jackson’s
dissenting team at the Niels
Bohr Institute in Denmark
chose not to use a window
function, a decision that
LIGO’s Neil Cornish describes
as a “basic mistake”. Jackson
says they didn’t use one
because it subtly alters the
Fourier-transformed data in
a way that can skew the
results of subsequent
processing.
Even with the Fourier
analysis done, judgementsmust be made about the
noise in the detectors. Is it,
for example, distributed in
a predictable pattern
equivalent to the bell-shaped
Gaussian distribution? And
does it vary over time or is it
“stationary”? The appropriate
techniques for processing
the data are different
depending on the answers to
these questions, so reliably
detecting gravitational
waves depends on making
the right assumptions.
Jackson’s group says the
decisions made during the
LIGO analysis are opaque at
best, and probably wrong.HOW TO CATCH
A WAVE
registered the event as a false alarm because
it coincided with what’s known as a “glitch”.
The detectors are plagued by these short,
inexplicable bursts of noise, sometimes
several every hour. They seem to be something
to do with the hardware with which the
interferometers are built, the suspension
wires and seismic isolation devices. Cornish
says that LIGO analysts eventually succeeded
in removing the glitch and revealing the
signal, but Jackson and his collaborators are
again unconvinced by the methods used,
and the fact there is no way to check them.
What are we to make of all this? Nothing,
apparently. “The Danish analysis is justThe first gravitational wave discovery was
announced to the world on 11 February 2016SAUL LOEB/AFP/GETTY IMAGES