2019-09-14_New_Scientist

8 | New Scientist | 14 September 2019

THE Nobel prizewinning LIGO

collaboration has published a

paper describing in more detail

than ever before how it analyses

gravitational wave signals, partly

in response to an investigation

by New Scientist. But some

physicists still say LIGO’s work

contains errors.

Almost no one doubts that

gravitational waves exist. They are

a prediction of general relativity,

a highly successful physics theory.

When the Laser Interferometer

Gravitational-Wave Observatory

(LIGO) first announced it had

detected one in 2016 it was cause

for celebration but not surprise.

But a team of researchers based

at the Niels Bohr Institute in

Copenhagen, Denmark, has since

questioned whether LIGO’s signal

analysis is reliable.

LIGO’s detectors aim to spot

space itself being rhythmically

squeezed and compressed. They

do this by firing lasers along tubes

roughly 4 kilometres long and

checking how the distance they

travel changes. However, these

changes are minuscule and the

detectors pick up random noise,

such as weak seismic tremors,

as well as gravitational waves.

To minimise the risk of noise

creating false alarms, LIGO

initially used two detectors

situated 3000 kilometres apart.

The noise experienced by each

should be entirely different.

However, the Danish group

claimed to have found similarities

in the noise seen by both detectors

when they observed that first

gravitational wave. This suggested

LIGO’s signal processing hadn’t

been done properly, the team said.

A New Scientist investigation

reported on all this and exposed

more irregularities in the

presentation of LIGO’s data

(3 November 2018, p 28).

This prompted the LIGO

collaboration to promise a full

justification of its techniques,

which has now been published

(arxiv.org/abs/1908.11170).

The LIGO collaboration, which

has been augmented by a third

detector in Italy, is confident in its

methods, the paper says. It says

that “there are no anomalous or

unexpected correlations to be

seen”. It also suggests that the

Danish analysis is flawed.

“The Danish group neglected

to implement basic steps in the

analysis,” says Patrick Brady at

the University of Wisconsin-

Milwaukee, a LIGO spokesperson.

That includes “windowing”

the data, which isolates particular

wave frequencies for analysis.

However, the Danish researchers

still insist that windowing is a

mistake as it skews the signal

and renders further analysis

unreliable. “The data windowing

techniques that LIGO has adopted

are known to distort phases,” says

Andrew Jackson, spokesperson for

the Danish group.

Jackson and his colleagues also

object to LIGO’s use of “whitened”

data. This practice involves

reducing the level of prominent

frequencies in the signal, which

arise because of the vibration

of wires that suspend the laser-

guiding mirrors in the detector.

The Danish group believes this

also creates distortions.

It can’t prove this, the group

says, because LIGO hasn’t released

enough raw data. However,

LIGO’s new paper points out that

four independent groups have

performed analyses of the

available data, and their outcomes

support LIGO’s conclusions.

Among those external scientists

are John Moffatt and Martin Green

at the Perimeter Institute for

Theoretical Physics in Canada.

They disagree with the Danish

group’s analysis. “I remain

convinced that their analysis

and conclusions are not correct,”

says Green. ❚

“ ‘The data windowing

techniques LIGO has

adopted are known

to distort signals“

Astrophysics

Michael Brooks

News

Ancient humans

Neanderthals may

have been taller

than we thought

THE biggest collection of

Neanderthal footprints yet found

hints we may have underestimated

our ancient cousins’ height. 

More than 250 fossil footprints

were found in the bed of a coastal

creek in Le Rozel, northern

France. They were made around

80,000 years ago and preserved

in sandy mud. 

“The discovery of so many

Neanderthal footprints at one

site is extraordinary,” says Isabelle

De Groote at Liverpool John Moores

University, UK, who wasn’t involved

with the study.

Before this discovery, only

nine Neanderthal footprints were

known, from four different sites,

says Jérémy Duveau of the French

National Museum of Natural

History in Paris.

Although Duveau and his team

can’t be certain who made the

footprints, Neanderthals were the

only known hominins in Europe at

that time – Homo sapiens arrived

35,000 years later.

Based on the size of the

footprints, the team estimated that

more than 90 per cent of them were

made by children and adolescents,

with the youngest being around

2 years old (PNAS, doi.org/dbbg). 

Evidence from skeletons has

previously suggested that adult

Neanderthals were smaller than

adult modern humans, usually

reaching between 150 centimetres

and 160 centimetres tall. But some

of the Le Rozel footprints seem

to have been made by someone

with a height of 175 centimetres.

This is the average height of a

man in the US today. ❚

A Neanderthal

footprint

discovered

at Le Rozel in

northern France

DOMINIQUE CLIQUET

Have we seen gravitational

waves? Some still doubt it

LIGO’s detectors must

screen out noise from tiny

seismic tremors

CALTECH/MIT/LIGO LAB

Alison George