Gravitational Waves - The Sound of the Dark Universe

INTERNATIONAL
SPACE
SCIENCE
INSTITUTE
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Published by the Association Pro ISSI No. 39, May 2017

Editorial
Today, dear reader, we invite you
to an experience you have never
had before. Let us visit a concert
hall whose stage is not just a few
planks, but rather the entire Universe
and the audience, to whom
we humans have just belonged
since autumn 2015, is a multitude
of unknown beings scattered right
throughout the Universe (if there
are any others …). The principle
performers are such prominent artists
as neutron stars, black holes,
etc. Similar, however, to earthly
entertainers, their programme is
devoted to the saga of birth and
death, the two very vital themes so
mercilessly ruling our lives and
that of the cosmic artists too.
One hundred years ago, the idea of
gravitational waves saw its initial
flimsy dawn. Discarded again by
the most prominent luminaries at
the time, it returned with clearer
contours briefly, just to fall into
oblivion again. Some decades later,
its elusive shadow reappeared suddenly
when astronomers observed
a pair of stars, which obviously
were emitting energy in the form
of gravitational waves. That earned
the lucky discoverers the Nobel
Prize. Again, however, a lot of
time passed without an iota of a
trace on the scene where in the
meantime an armada of engineers
and scientists had gathered to get
hold of gravitational waves. They
made it finally on 14 September
2015 when signals allowed them to
listen to the first sounds of the dark
Universe. The piece was a threnody
of two merging black holes,
not quite on front stage, no, rather,
an exciting 1.3 billion light years
away. Only a few months later, a
second performance enthused the
audience as an unexpected bonus
suggesting that the artists might offer
such presentations frequently
and accessibly to all those prepared
to listen.
To take full advantage of the new
opportunity, a connoisseur is
needed who helps interpret the
melodies. Fortunately, we have a
renowned expert amongst us: Professor
Karsten Danzmann of the
Max Planck Institute for Gravitation
Physics and the Institute for
Gravitation Physics of the Leibniz
Universität, Hannover. In his enthralling
Pro ISSI talk on 12 October
2016, he portrayed gravitational
waves, their cosmic sources,
the marvellous technologies required
to grasp them and the
messages they convey. One thing
became very clear in his talk: tremendous
progress in a variety of
technological and scientific fields
has been required to enable humankind
to attend the universal
concertos. In fact: a new concert
season has begun.
We are thankful to Prof. Danzmann
for his most valuable support
in publishing the current issue of
Spatium and wish our readers
some memorable moments in the
company of the cosmic philharmonic
orchestra.
Hansjörg Schlaepfer
Brissago, May 2017
Impressum
ISSN 2297–5888 (Print)
ISSN 2297–590X (Online)
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SPATIUM 39 2

Gravitational Waves:
the Sound of the Dark Universe 1
by Prof. Karsten Danzmann, Max Planck Institute for Gravitation Physics and
Institute for Gravitation Physics of the Leibniz Universität, Hannover
Prologue
Albert Einstein Institute, Hannover
14 September 2015
Physicist Marco Drago sits calmly
in front of his computer screen: he
cannot know that this day will
change his and all his colleagues’
lives. Just before leaving for lunch,
at 11:50, Marco notices an alert on
his screen. This is not very exciting
though: such alerts appear routinely
at irregular intervals. On the
other hand, Marco also knows that
an alarm might indicate a dramatic
event billions of light years away.
This is the stuff he and hundreds
of colleagues all over the world
have been chasing after for many
years.
In order to facilitate the physicists’
job, a variety of software packages
monitors the incoming signals.
Each of these programmes can
generate an alarm urging the
scientist on duty to have a closer
look at the event. One of those
packages aims at recognizing patterns
possibly coming from two
merging black holes. Another system
screens the signal in search of
orbiting neutron stars and still another
systems aims at characterizing
the statistic properties of the
detector noise.
No wonder, therefore, that Marco
at first interprets the alarm as a system-generated
check alert as he
had seen so many before. Yet, what
if this very alert indicated a real signal?
Marco’s interest is aroused. He
decides to inspect the waveform on
the screen (Fig. 2) in more detail.
No doubt, it is a marvellous signal,
too exciting to be artificial. Hence,
he calls his colleague Andrew
Lundgren from across the hall. Andrew
gets excited at once. They
decide to alarm their colleagues at
the detector operations centre in
the United States from where the
signal came from. Yet, it is the
middle of the night there with only
the night watch on duty, who, of
Fig. 1: Albert Einstein on the summit of his career, 1921. In the frame of his
theory of general relativity, he had predicted the existence of gravitational waves
five years before. The science community then had to wait a hundred years to see
Einstein’s prediction verified experimentally in 2016. (Image credit: Ferdinand
Schmutzer).
1
The text reports on a talk by Prof. Karsten Danzmann for the Pro ISSI audience on 12 October 2016.
It was prepared by Dr. Hansjörg Schlaepfer and reviewed by Prof. Danzmann.
SPATIUM 39 3

course, wonders about the excited
nocturnal call. Upon returning to
their office, all the scientists plunge
into an examination of the mysterious
signal concluding that it
could indicate a real event indeed.
At that moment, however, everyone
remembers the Big Dog event
when a similar signal caused so
much turmoil. That waveform
looked exactly as if it were coming
from a pair of coalescing black
holes in the constellation of the Big
Dog: the seminal victory seemed
within their grasp. In no time, an
urgent Physical Review Letters paper
is written and then a secret envelope
opened stating that the enigmatic
Big Dog signal was an
artificial injection. The huge common
disappointment now prevents
hundreds of people from talking to
the media thereby granting a handful
of senior specialists the time to
analyse the event so rigorously as
to exclude any false inter preta tion.
Half a year later and exactly one
hundred years after Albert Einstein’s
publication of the general
theory of relativity, the historic
first direct detection of a gravitational
wave is presented at a press
conference at Washington DC on
11 February 2016. It was the last
Fig. 2: The epoch making first signal of a gravitational wave as observed by the
Livingstone detector of the US Laser Interferometer Gravitational Observatory
(LIGO) on 14 September 2015. The horizontal axis represents time, while the vertical
axis shows the strain on the detector exerted by the gravitational wave in arbitrary
units. The source of the waves is a pair of in-spiralling and then merging
black holes. Before merging at t=0.22 s, their orbit rate steadily increases producing
a detector signal with steadily increasing frequency and amplitude. Upon merging,
both, the frequency and the amplitude decay rapidly reflecting the remaining
oscillations of the unified black hole. (Credit: Max-Planck-Institut für
Gravitationsphysik)
and final outcry of a pair of orbiting
black holes when they yelled
just before merging into one single
very massive black hole. Scientists
rated their individual masses
at 29 and 36 times the mass of the
Sun, while the resulting black hole
holds 62 solar masses. Within a
fraction of a second, the remaining
three solar mass equivalents were
converted into energy 2 . With the
speed of light, gravitational waves
then carried the energy through
the Universe, where, 1.3 billion
years later on Earth, they squeezed
the detectors’ 4-km long arms by
a few 1/10,000 of the width of a
proton. So dwarfish might be the
vestiges of a great event!
Marco Drago’s and his colleagues’
observation marks a turning point
in science history comparable only
with the moment when Galileo
Galilei directed the first telescope
to the sky in 1610. While Galilei
used electromagnetic waves to observe
the stars, the new carriers are
gravitational waves that possess so
radically different properties that
their detection inaugurates a new
epoch in astronomy.
The current issue of Spatium is devoted
to gravitational waves, their
sources and the marvellous technologies
required for observing
them. Over and above, this issue
renders homage to Albert Einstein 3
and the scientists of his time, who
laid the cornerstone for our understanding
of gravitational waves.
2
According to Einstein’s most famous equation E = mc² stating that energy E equals mass m times the speed of light c to the
square.
3
Albert Einstein, 1879, Ulm, Germany – 1955, Princeton, USA, theoretical physicist, Nobel Prize Laureate in physics,
1922.
SPATIUM 39 4

The History of
Gravitational
Waves 4
The idea of gravitational waves is
a child of the early 20th century’s
physics: the first documented mention
dates back to Henri Poincaré 5 .
He suggested in early 1905 that –
in analogy to an accelerating electrical
charge producing electromagnetic
waves – accelerated
masses in a relativistic field theory
of gravity should produce gravitational
waves.
In February 1916, Albert Einstein
took up the idea in the framework
of an exchange of letters with Karl
Schwarzschild 6 . In these letters, he
expressed profound scepticism
about the existence of the waves
even although gravitational waves
are a direct result of his general
theory of relativity. As a result, he
submitted a first paper to the Prussian
Academy of Sciences on
22 June 1916. Later in June, he
published a follow-up text in which
he predicted the existence of gravitational
waves travelling with the
speed of light, in analogy to electromagnetic
radiation. This paper,
though, contained a significant error,
which he corrected in 1918,
when he derived a new formula for
the emission of gravitational waves
that, apart from a factor of two, is
still considered the correct one. His
calculations also showed that these
waves have incredibly low amplitude
eluding any observation with
the technology available then.
Over time, and despite his earlier
calculations, Einstein himself again
came to doubt the existence of
gravitational waves to such an extent
that, in 1936, he and his collaborator
Nathan Rosen wrote a
paper presenting this opinion. After
a referee had spotted a mistake in
their argument, Einstein withdrew
it and published the corrected text
in another journal, yet with a completely
different conclusion …
From the 1920s to the early 1950s,
gravitational waves were hardly a
hot topic. Still, in the mid-1950s,
general relativity saw a renaissance
furthered by considerable funding
available to theoretical physics and
by the increasing ability of scientists
to cross international borders
to exchange their ideas. Among the
most pressing questions now pursued
were the existence and properties
of gravitational waves.
Heated discussions took place at
the first international conference
entirely dedicated to general relativity
in Bern in 1955. Yet, based
on work by Hermann Bondi 7 and
Richard Feynman 8 , a broad consensus
emerged that gravitational
waves were a physical reality.
Joseph Weber 9 of the University of
Maryland undertook the first attempts
to observe them. Weber began
experimenting around 1960
and after nearly a decade, he announced
he has gained convincing
evidence of their existence. Weber’s
work had a significant impact
on the scientific community,
sparking a series of experiments
designed to test his results. Even
although none of these confirmed
Weber’s findings, new techniques
and methodologies materialized
that constitute the basis of today’s
observatories.
Indirect proof for the existence of
gravitational waves discovered by
chance in 1974 spurred the race
further. Joseph Taylor 10 and Russell
Hulse 11 observed two closely
orbiting stars, and found that, in
contrast to classical Newtonian
physics, the stars’ orbit period was
steadily declining: the two bodies
were rotating faster and faster
about each other on increasingly
tight orbits. The change was very
small, some 75 µs/year, and was coherent
with Einstein’s prediction
that such a system would lose energy
by the emission of gravitational
waves. In fact, the observation
complies fully with the general
4
After the Press Release by the Max-Planck-Institut für Wissenschaftsgeschichte: One Hundred Years of Gravitational
Waves: the long road from prediction to observation, 11 February 2016.
5
Jules Henri Poincaré, 1854, Nancy, Meurthe-et-Moselle, France – 1912, Paris, French mathematician, theoretical
physicist and philosopher of science.
6
Karl Schwarzschild, 1873, Frankfurt am Main – 1916, Potsdam, German physicist and astronomer.
7
Sir Hermann Bondi, 1919, Vienna – 2005, Vienna, Anglo-Austrian mathematician and cosmologist.
8
Richard Phillips Feynman, 1918, Queens, New York – 1988, Los Angeles, American theoretical physicist.
9
Joseph Weber, 1919, Paterson, New Jersey – 2000, Pittsburgh, Pennsylvania, US-American physicist.
10
Joseph Hooton Taylor, Jr., 1941, Philadelphia, US-American astrophysicist, Nobel Prize Laureate in physics, 1993.
11
Russell Alan Hulse, 1950, New York, US-American physicist, Nobel Prize laureate in physics, 1993.
SPATIUM 39 5

theory of relativity and, as this was
the first, albeit indirect, proof, it
earned them the Nobel Prize in
1993. Many other binary systems
have been found since all fitting in
with theoretical predictions.
The first direct detection by the
LIGO observatory in 2015 marks
the next chapter in the history of
gravitational waves. This became
possible not only as a result of tremendous
technological progress
but also thanks to extensive numerical
simulations for all kinds of
events that produce gravitational
waves. It paved the way to understanding
the observed signals by assembling
a catalogue of cosmic
events and the associated signal
patterns they produce. Simulations
of this sort required advancements
in computing technology as well
as theoretical physics giving way to
the birth of an entirely new field
of science: numerical relativity.
The Secrets of
Gravitational
Waves
In the classical view of Isaac Newton,
space and time are independent
and invariable entities, and the
speed of light is a universal constant.
These concepts were pivotal
to theoretical physics for some 200
years. Yet, when scientists began
pushing the limits further out in
the late 19th century, contradictions
became apparent: for an observer
moving with a significant
fraction of the speed of light, the
velocity of electromagnetic waves
would change under Newton’s
paradigm. Several scientists had already
attempted to overcome this
hurdle when Albert Einstein began
addressing the problem in
1905. In the special theory of relativity,
he saw the speed of light as
the unique universal constant
while time dilates according to an
observer’s velocity. In the subsequent
general theory of relativity,
he introduced the impact of masses
causing space to warp. Time that
expands and space that bends were
both ideas hard to accept 12 : no
wonder the concept of gravitational
waves was way out of the
grasp of the science community.
We are going now to elaborate first
on the nature of gravitational
waves, their characteristics and
later look at the various sources
that generate them.
Fig. 3: The basics of gravitational
waves. In the sketch, a sinusoidal gravitational
wave is assumed to pass perpendicular
to the plane of this sheet of
paper. The wave slightly warps the dimensions
of the sheet as indicated by the
extremely exaggerated distortions of the
circles shown on the lower row. At t=t1,
the strength is maximum and the sheet
is squeezed so that the points on the circle
now assume the form of a horizontally
orientated ellipse. At t=t3, the
strength is maximum in the opposite direction
and the sheet is distorted as indicated
by the dots on the vertically oriented
ellipse.
12
It is comforting to know that Newtonian physics are still today the right choice in all cases that exclude very high
velocities and/or very dense concentration of masses.
SPATIUM 39 6

The Nature of Gravitational
Waves
In Fig. 3, a hypothetical gravitational
wave penetrates this sheet of
paper. Let its strength over time be
sinusoidal, as depicted in the upper
part of the panel. This wave will
slightly distort the paper as indicated
on the lower row: the points
on the circle will form ellipses in
vertical and horizontal directions
alternatively.
As with other waves, a number of
parameters characterize gravitational
waves:
Amplitude: This is the size of the
wave, more precisely, the fraction
of stretching or squeezing of the
circles on the lower row. The amplitude
here is about 0.5, while real
gravitational waves would result in
values many orders of magnitude
less. It is this unimaginably small
effect that makes their direct detection
a fantastic challenge.
Velocity: This is the speed at which
a specific point on the wave, for
example a peak, travels through
space. For the sake of simplicity,
this is equal to the speed of light.
Frequency: This is the number of
peaks and valleys passing per second.
Gravitational waves are expected
to exist in an extremely
wide frequency spectrum ranging
from practically zero up to some
100 Hz.
The Sources of Gravitational
Waves
In order to familiarize ourselves
with the mechanics producing
gravitational waves we are now
looking at various scenarios and investigating
whether the system will
give off energy in the form of gravitational
waves. The reader is encouraged
to consult Spatium 37 13 to
get acquainted with the cosmic objects
that create gravitational
waves.
The following objects do
not radiate gravitational
waves:
– An isolated non-spinning
solid object
moving at a constant
velocity does not radiate
gravitational waves.
– A perfectly symmetric
spinning disk will not
radiate waves. This is
in accordance with the
conservation principle
of angular momentum
since gravitational
waves extract energy
from the system.
The following (non-exhaustive)
list names
examples of objects that
radiate gravitational
waves:
– Two objects orbiting
each other: A planet orbiting
the Sun radiates
gravitational waves
albeit at extremely low
intensities.
– A spinning nonaxisym
metric planetoid,
perhaps with a
large bump or dimple
on the equator, radiates
very small amounts of
gravitational waves.
– A supernova radiates
gravitational waves if
the explosion is not
perfectly symmetrical.
– Two orbiting black
holes always radiate
gravitational waves.
13
Spatium No. 37: The Violent Universe by Thierry Courvoisier, May 2016.
SPATIUM 39 7

From these lists, we can deduce in
more general terms that gravitational
waves are radiated by objects
whose motion involves acceleration,
provided that the motion is
not perfectly spherically symmetric
(like an expanding or contracting
sphere) or rotation, provided
that the object is not rotationally
symmetric (like a spinning disk or
sphere).
Let us now look at some specific
sources of gravitational waves:
Binaries
A binary (system) is a pair of cosmic
objects orbiting each other.
The Hulse-Taylor binary mentioned
above is a typical example
thereof. It contains a pair of neutron
stars of which one is a pulsar
emitting electromagnetic radiation
while the other does not. Each one
has about 1.5 solar mass equivalents
and they orbit each other within a
period of a mere 7.75 hours. They
are 21,000 light years from us. The
system generates gravitational
waves that withdraw energy from
the binary causing the orbits to
come closer and closer eventually
leading to a merger in some
300 million years.
A great number of white dwarf 14
binaries exists in the Universe, circling
each other on extremely tight
orbits. They are typical sources of
gravitational waves. Upon reaching
orbital distances in the order of
10,000 km, they will merge and
explode in a supernova thereby
ending the emission of gravitational
waves.
Another important category are
pairs of black holes. They emit
gravitational waves during their
in-spiral, merger, and ringdown
phases. The largest amplitude of
emission occurs during the merger
phase. In fact, it was this type of
event that made the first observation
of gravitational waves in 2015
possible, see Fig. 2.
Supernovae
A supernova is one of the last stellar
evolutionary stages of a massive
star’s life, whose dramatic destruction
occurs during a final titanic
explosion. This explosion can happen
in one of many ways, but in all
of them, a significant proportion
of the star’s matter is blown away
into surrounding space at extremely
high velocities. Unless
there is perfect spherical symmetry
in these explosions (i.e., unless
matter is jettisoned out evenly in
all directions), the supernova will
radiate energy strongly for a certain
time by emitting gravitational
waves.
Rotating Neutron Stars
Single rotating neutron stars are a
further source of gravitational
waves. In general, however, a spinning
neutron star will not emit a
lot of gravitational radiation because
these extremely dense objects
have such a strong gravitational
field that their shape is almost perfectly
spherical. In some cases,
there might be slight deformities
on the surface: perhaps bumps extending
no more than 0.1 m above
the surface. This makes the neutron
star slightly asymmetric giving
rise to the emission of gravitational
waves as long as the
deformities continue to exist.
The Early Universe
Many models addressing the earliest
phases of the Universe postulate
an inflationary epoch when
space expanded rapidly in a very
short time. The quantum fluctuations
in this expansion may have
emitted gravitational radiation that
should still be detectable today as
a gravitational wave background.
Even though this background signal
is too weak for any currently
operational detector, there are
many other scenarios for what
might have happened in the early
Universe, producing pronounced
signals that may be detectable with
LISA and future ground-based detectors.
Such observations could
provide unprecedented insights
into the earliest phases of the
emerging Universe.
14
A white dwarf is an old very dense star with a mass comparable to that of the Sun, while its volume is comparable to that
of Earth. It has collapsed to that small size when all the fusion processes had ended.
SPATIUM 39 8

Observing Gravitational
Waves
In Einstein’s times, gravitational
waves were way out of the reach of
any available technology. However,
in the meantime, progress in
various fields has made direct detection
of these waves feasible. In
fact, several ground-based systems
are currently either operational or
are planned to enter service soon.
Technologies for space-based detector
systems have become available
too. Unfortunately, our planet
is not well suited to serve as the basis
for such experiments, at least in
certain frequency bands. The activity
of the tectonic plates, Earthquakes
as well as man-made sources
of tiny vibrations, such as cars or
trains passing by, lead to detector
signals which are orders of magnitude
larger than gravitational
waves. Hence, the silver bullet for
getting hold of gravitational waves
in their entire spectrum are spaceborne
detectors.
All detectors build on the effect
that gravitational waves exert on
free-floating test masses. This effect
consists of tiny changes in the
distances between the test cubes.
Still, these changes are extremely
small; say in the order of 10 –18 m
for ground-based detectors and
10 –11 m for space-based detectors.
To detect such small dimensions interferometric
systems in their most
sophisticated forms come into play.
Let us look therefore briefly at the
main principles of interferometry
but also at the progress required to
interpret the resulting signals.
Key Technologies
Interferometry
Interferometry is a generic term for
techniques in which the wave nature
of electromagnetic radiation
plays the key role. When two wave
fronts with the same frequency
combine, the resulting intensity
pattern is determined by the phase
difference between the two waves:
waves that are in phase will undergo
constructive interference
while waves of opposite phase will
incur destructive interference (see
Fig. 4). Waves, which are neither
completely in phase nor completely
in the opposite phase, will produce
intermediate intensity patterns allowing
for determination of their
relative phase difference.
The principle of interferometry
saw the light of the day in the late
19th century. Albert Michelson 15
invented the concept when he tried
to prove the existence of a universal
ether as the carrier of light, an
attempt, which of course was
bound to fail. Yet, his instrument
was a seminal invention, which
during time saw continuous improvements
by later authors. Fig. 5
shows the principle of the Michel-
Fig. 4: The basic principles of the interference of waves. When two waves of
the same frequency meet, they interfere either constructively (if they are in phase),
thereby adding their amplitudes, or destructively (if they are in opposite phase)
thereby subtracting their amplitude. (Credit: www.explainthatstuff.com)
15
Albert Abraham Michelson, 1852, Strelno, Poland – 1931, Pasadena, USA, US-American physicist with German roots
and Nobel Prize Laureate in physics 1907.
SPATIUM 39 9

son interferometer. For a gravitational
wave detector, the mirrors
are the test masses, so that any
change of the distance between
them due to a gravitational wave
changes the length of the light
path, which in turn is measured by
interference with the other beam.
For instance, as the wavelength of
red light is in the order of 0.6 µm,
displacements of less than say 0.1
µm can be resolved.
Yet, this value is still a long way off
from resolving the tiny shifts caused
by gravitational waves. In order to
enhance a Michelson interferometer’s
capabilities, interferometers
nowadays make use of an invention
by Charles Fabry 16 and Alfred
Perot 17 who added further optical
elements constituting Fabry-Perot
cavities (not shown in Fig. 5). The
idea is to allow the two beams between
the beam splitter and the respective
mirrors to pass several hundred
times back and forth instead
of just once. This multiplies the effective
displacement of the mirrors
by the same factor thereby greatly
enhancing the sensitivity of the
system.
Yet, with increasing sensitivity,
spurious noise tends to appear that
can mask the signal caused by gravitational
waves. For instance, the
light produced by a laser consists
of a flow of photons. However,
they come at random, just like the
droplets of a rainfall. This leads to
high-frequency noise in the output
of the detector. In addition, for
sufficiently high laser power, the
photons reflected by the mirrors
Fig 5: The basic configuration of a Michelson Interferometer. A light source
(today mostly lasers) emits light in the direction of the beam splitter. This element
produces two beams, of which one travels ahead in the same direction as before,
while the second is reflected by 90°. Both beams travel towards the mirrors, which
reflect them back towards the beam splitter, which acts on the beams again. Finally,
both beams travel towards the detector where they interfere. The test masses’ surfaces
act as mirrors, so that any displacement of the test cubes causes a change in
the path lengths of the beams. The difference between the two optical paths makes
the interference pattern change and the detector deliver an output.
on the test masses transfer a random
momentum, which tends to
disguise a signal at lower frequencies.
External effects, such as seismic
turbulences and other forms of
environmental vibrations acting on
the test masses, are sources of detector
noise. In order to suppress
this type of noise, two (or more)
widely spaced sites are used. The
stochastic environmental noise
will be different at each site so that
appropriate algorithms are able to
suppress it to some extent. On the
other hand, any gravitational wave
signal will be the same at all sites
(aside from an eventual time shift).
All these – and many other – effects
must be taken into account
before an observatory becomes
able to monitor gravitational
waves. As an engineer said: The
16
Maurice Paul Auguste Charles Fabry, 1867, Marseille – 1945, Paris, French physicist.
17
Jean-Baptiste Alfred Perot, 1863, Metz, France – 1925, Paris, French physicist.
SPATIUM 39 10

fight for detecting gravitational waves is
the battle against the noise.
If a signal now appears in the detector,
the next major issue is to interpret
it. Unfortunately, telescopes
cannot observe the sources of gravitational
waves, whether they are
black holes or neutron stars. This
prevents any visual corroboration
of an observed event and requires
the new scientific field of numeric
relativity to come into play. Let us
therefore elaborate briefly on the
technique of data filtering and data
interpretation.
Matched Filtering
Lacking any visual validation option,
we have to rely solely on
Einstein’s field equations, which –
notwithstanding their complexity
– allow us to calculate the signals
produced by cosmic events. Based
on solving the field equations, a
catalogue of such cosmic events
and their associated signal patterns
is prepared. To interpret a specific
signal, it is compared with the signals
stored in the catalogue and the
template best matching the observed
signal defines the event
observed.
The process of comparing the signal
with the templates is called
matched filtering: many different
filters compare the incoming signal
with the stored templates. As
there are thousands of such filters
operating simultaneously, extremely
powerful data processing
systems are key to success.
The templates themselves are the
achievement of numerical relativity.
Scientists model the cosmic
processes by solving Einstein’s field
Fig. 6: Overview of the worldwide
gravitational wave observatories
network. In Europe, there is the joint
German-British GEO600 programme
and the French-Italian Virgo project. In
the US, there is the Laser Interferometer
Gravitational Wave Observatory
LIGO with its two detector sites, which
are planned to be upgraded by a third
LIGO Observatory in India. In Japan,
there is the Kamioka Gravitational
Wave Detector (KAGRA) project of the
gravitational wave studies group at the
Institute for Cosmic Ray Research of
the University of Tokyo. The planned
integration of all these systems into one
network will greatly enhance the sensitivity
of the individual observatories.
(Credit: Max-Planck-Institut für
Gravitationsphysik)
SPATIUM 39 11

equations and look at the pattern
of the resulting gravitational wave.
For instance, the evolution of a binary
black hole system is simulated
during its in-spiral and merging
phases. The resulting gravitational
wave signal reflects the specific
process, the properties of the black
holes and so on, and finally gives
rise to just one template. Other
combinations of black holes with
different masses cause different signal
patterns and hence produce
other templates. Thus, the interpretation
of a signal from two
merging black holes requires many
different templates and there are
many different events occurring in
the Universe …
A final remark reminds us of a surprising
commonality across the
Universe: the pattern shown in
Fig. 2 is a so-called chirp-signal. It
is characterized by increasing frequency
and amplitude over time
up to a certain point from where
on both frequency and amplitude
decay quickly. Comparison with
the catalogue of templates allowed
it to be associated with the final
stage of two merging black holes:
this is the sound of the dark Universe.
Yet, it is not only the last
squeal of two merging black holes
that takes the shape of such a chirp
signal; birds also use them in their
songs, bats in search for their prey
use them as do whales in their
communications and even humans
in their speech: this is also the
sound of the living Universe!
Fig. 7: The two LIGO stations in the US are separated by 3,002 km. This great
distance helps supress the noise caused by natural sources (tectonic activities) as well
as human activities, such as that caused by trains or traffic. (Credit: Max-Planck-
Institut für Gravitationsphysik)
Operational Systems
Ground-Based Gravitational
Waves Observatories
Several observatories for detecting
gravitational waves are either in an
advanced stage of development or
now operational (Fig. 6). In Europe,
there are the German-British
GEO600 system and the Italian-
French Virgo system. Two further
systems are under construction in
Japan and India. Finally, in the
United States, there is the Laser Interferometer
Gravitational Wave
Observatory (LIGO). These programmes
are collaborating in a
worldwide network to enhance
their capabilities for the suppression
of local detector noise.
As the first gravitational wave appeared
in the LIGO detectors, we
are going now to present a brief
overview of this programme.
LIGO is a huge international endeavour
bringing more than 80 scientific
institutes in 15 countries
with more than 1,000 scientists together.
Both, the German Max-
Planck-Institute for Gravitational
Physics and the Institute for Gravitational
Physics at the Leibniz
Universität Hannover are important
members of the family not
only on the level of key hardware
deliveries but also in the area of
data processing.
LIGO comprises four distinct facilities
across the United States:
two gravitational wave detector
sites (the interferometers proper)
SPATIUM 39 12

and two University research centres,
namely the California Institute
of Technology in Pasadena CA
and the Massachusetts Institute of
Technology (MIT), in Cambridge
MA. The two interferometers are
located in Washington (LIGO
Hanford, WA) and on the 3,002
km distant Louisiana site (LIGO
Livingston, LA), Fig. 7. LIGO
builds around two giant Michelson
interferometers with two arms
of 4 km length each. Fabry-Perot
cavities in the optical path allow
the light beams to travel the arms
back and forth 280 times. This
leads to an effective optical path
length of 2,240 km.
The detector signals are not only
sent to the academic centres in the
US, but also to the Albert Einstein
Institute in Hannover where the
world’s largest dedicated computer
cluster ATLAS for gravitational
wave data analyses is available. This
site receives data also from the
French-Italian Virgo observatory
in Pisa. The cluster is amongst the
most powerful computers worldwide:
it features more than 5 × 10 15
byte hard disk capacity and reaches
an incredible speed of about 4 × 10 14
flops per second. It is this facility
that produced the first hot alert noticed
by Marco Drago.
Space-Based Gravitational
Waves Observatories
The shaky Earth’s surface is a challenging
support for gravitational
wave observatories. Space offers an
attractive yet no less demanding alternative:
the size of a space-borne
interferometer is virtually unlimited
and the platforms suffer much
Fig. 8: The LISA Pathfinder concept builds on the layout of ESA’s evolved Laser
Interferometer Space Antenna (eLISA). In fact, it concentrates on one single platform
all the functions required for implementing the final eLISA observatory. In
the eLISA programme, the two test masses that, together with mirrors and beam
splitters, constitute one Michelson interferometer fly on different spacecraft several
million kilometres apart. In contrast, the Pathfinder concept concentrates them on
one single spacecraft where the test cubes are a mere 38 cm apart. This approach is
feasible, as the principle of interferometry does not depend on the length of its arms.
(Image courtesy: Stefano Vitale, Max-Planck-Institut für Gravitationsphysik)
less disturbances. This assessment
as well as bilateral efforts towards
ground-based observatories in Europe
prompted the scientific community
to propose to the European
Space Agency ESA the planning of
a dedicated space-based observatory.
In 1995, the Agency selected
the Laser Interferometer Space Antenna
(LISA) proposal as one of
their major forthcoming science
projects. Initially, it was meant as
a co-operative undertaking with
NASA, but upon their withdrawal
from the programme in 2011, LISA
was studied as an ESA-only mission
for a number of years. This effort
led to the evolved Laser Interferometer
Space Antenna (eLISA).
Recently, NASA has reentered the
LISA programme and the LISA
mission is now being developed for
the L3 large mission flight opportunity
with a launch in the early
2030s. To cope with the demanding
technological challenges, ESA
resolved to adopt a phased approach
by first implementing a
proof-of-concept mission, the
LISA Pathfinder (LPF), before embarking
on the LISA programme
proper.
SPATIUM 39 13

LISA Pathfinder
LISA Pathfinder is a downscaled
version of the LISA concept (Fig. 8):
it contains only two (instead of six)
free-floating test masses in one single
(instead of three separate)
spacecraft several million kilometres
apart. The test cubes are made
of a gold-platinum alloy (Fig. 9). As
with LISA, an interferometer monitors
their relative distance with
extreme precision.
The LISA Pathfinder spacecraft
launched on 3 December 2015; the
scientific phase started on 1 March
2016 and on 7 June 2016, ESA presented
the first scientific results. In
short, it was found that the two test
cubes were indeed falling freely
through space unperturbed by any
external spurious effect to a degree
far beyond expectations. This result
confirmed the feasibility of the
LISA concept as well as the readiness
of the required technologies.
Extensive testing during the subsequent
months allowed scientists
to gain further experimental experience
to optimize the system’s sensitivity.
In December 2016, the
mission was granted a six month
extension during which scientists
and engineers exploited the platform
as a sophisticated physics laboratory
and pushed the experiment
to the limit in preparation for ESA’s
future space observatory of gravitational
waves.
Fig. 9: This artist’s impression illustrates
the LISA Technology Package
on the LISA Pathfinder spacecraft.
Each of the two yellowish containers
houses a gold-platinum test cube of
which one is visible in the cutaway on
the right. The optical bench holding the
interferometer is shown in between the
containers. It is made of extremely stable
20 × 20 cm zerodur ceramic glass
holding 22 mirrors and beam splitters
directly bonded to its surface. The laser
beams measure the cubes’ motion, position,
and orientation without any
physical contact. Various procedures
and controls create an environment free
of any external effect except for gravity.
The LISA Pathfinder mission featured
the first high-precision interferometric
tracking of orbiting bodies in
space. (Credit: ESA)
SPATIUM 39 14

Outlook
During the past few centuries, progress
in technology has gradually
revolutionized astronomy 18 . Galileo
Galilei pioneered the use of telescopes
to enhance visual observations
some 400 years ago. Visible
light, however, is only a small portion
of the electromagnetic spec-
Gravitational waves are a marvellous
carrier of information thanks
to their unique properties: they
pass through any intervening matter
without being scattered significantly
(which, by the way, makes
them difficult to detect). Whereas
light from distant stars may be
blocked out by interstellar dust,
gravitational waves pass through
essentially unimpeded. This feature
allows gravitational waves to
carry information about astronomical
phenomena that have never
trum, and not all objects in the
Universe shine strongly in this particular
band. The emerging use of
radio waves in the 20th century
made the observation of pulsars,
quasars, and other extreme objects
possible thereby strongly pushing
the limits of astrophysics. Observations
in the microwave band have
further opened our eyes to the faint
imprints of the Big Bang. The upcoming
use of gamma rays, x-rays,
ultraviolet light, and infrared light
offered science further dramatic
new insights. Now, the window of
gravitational waves is opening and
astronomers have no doubt that this
again will permit science to make
major leaps forward in our understanding
of the Universe.
Fig. 10: The beauty of a gravitational
wave signal in an artist’s interpretation.
been observed before. A second interesting
property is that a gravitational
wave observatory observes
the whole sky simultaneously,
whereas telescopes always point to
a narrow field of view.
For many of us, the ultimate hope
of gravitational wave astronomy is
to probe the background signals
that emanated from the very Big
Bang, 14 billion years ago. As the
Universe was dark until an age of
300,000 years, electromagnetic
waves cannot tell us anything
about that early phase. This is
where gravitational waves enter
the stage allowing us to witness the
very birth of our Universe.
18
See also Spatium no. 19: 4440: A Secret Number in Astronomy by Giovanni Fabrizio Bignami, August 2007.
SPATIUM 39 15

SPATIUM
The Author
Karsten Danzmann earned his Diploma
in Physics in 1977 and three
years later a PhD degree at the
University of Hannover. In 1982,
he worked as a Visiting Scientist at
Stanford University, USA. One
year later, he received a call
from the Physikalisch-Technische
Bundesanstalt, the German Centre
for Metrology, in Berlin and,
upon an employment by the Stanford
University as an Assistant
Profes sor of Physics, Karsten
Danzmann returned to the USA.
Back again in Germany, he received
an appointment as Project
Leader Gravitational Waves at the
Max Planck Institute for Quantum
Optics at Garching.
In 1993, the Max Planck Institute
for Quantum Optics at Garching
named Professor Danzmann Head
of the Remote Branch Hannover
while the Leibniz Universität Hannover
appointed him simultaneously
as Professor and Director
of the Institute for Gravitational
Physics, two positions he still holds
today. In 2002, he received a further
assignment as the Director of
the Max Planck Institute for Gravitational
Physics (Albert Einstein
Institute) in Hannover.
In 1993 Karsten Danzmann started
his scientific activities as the
Principal Investigator of the
ground-based laser interferometric
gravitational wave detector
GEO600 located near Hannover.
This observatory is designed and
operated by scientists from both
the Max Planck Institute for Gravitational
Physics and the Leibniz
Universität Hannover, along with
partners in the United Kingdom.
Two detectors were construct ed in
the USA (LIGO), and one each in
Italy (Virgo) and Japan (KAGRA).
Scientists from GEO600 and LIGO
collaborate within the LIGO Scientific
Collaboration.
For the European Space Agency,
Prof. Danzmann served in many
different roles, for example as the
speaker of the Laser Interfero meter
Space Antenna (LISA) Study Team,
as a member of ESA’s Fundamental
Physics Advisory Group or as
Co-Principal Investigator for the
ESA LISA Pathfinder mission. On
national grounds, Danzmann has
also served in a variety of positions
thereby shaping the German science
policy in physics. He was for
instance a member of the extraterrestrial
research programme committee
of the German National
Aeronautics and Space Research
Centre (DLR). In the German-US
American bilateral programme
Gravity Recovery And Climate
Experiment (GRACE) follow-on
mission, Prof. Danzmann acted as
the Co-Proposer and Board
Member.
As a distinguished communicator,
Professor Danzmann places great
emphasis on science marketing to
decision-makers and general public
outreach while in the academic
setting, he continues teaching at all
levels from large introductory
physics classes to specialized classes
for Master and PhD curricula carrying
a full academic teaching
load.
A Fellow of the American Physical
Society, Professor Danzmann
has received many honours on national
and international grounds
for his exceptional accomplishments
in science and science policy,
recently for instance the science
prize of the German Land
Niedersachsen.