PROFESSOR GUIDO MUELLER
Listening to the cosmos
Professor Guido Mueller is committed to exploiting gravitational
waves to better understand the Universe around us. Here, he talks
in detail about his latest research endeavour and scientific goals
To start, could you briefly describe
gravitational waves and where they
come from?
Gravitational waves are distortions in spacetime
generated by accelerated masses. Everybody
knows these depictions of curved spacetime
around a black hole or a star that look like a
bowling ball resting on a rubber sheet. Imagine
two of those orbiting around each other; the
dents in the rubber sheet will move with the
bowling balls and send ripples or waves across
the rubber sheet. These waves are Nature’s
way of telling us that the location of the two
balls is constantly changing. Newtonian gravity
was not able to describe these waves. Laplace
and others tried to solve this puzzle within
Newtonian gravity but failed; it was Einstein’s
theory of general relativity (GR) which provided
the solution. GR allows wave solutions and was
able to put a speed limit – the speed of light –
on changes in gravity. The bowling ball picture
also tells us that the most effective sources are
binary systems where two compact objects
orbit around each other.
What is the general principle behind
measuring these waves?
Gravitational waves are quadrupole waves
which stretch and squeeze spacetime, changing
the distances between freefalling masses in
opposite ways along orthogonal directions.
Imagine a circle that a wave turns into a
horizontal ellipse and then into a vertical ellipse
back into a horizontal ellipse and so on. The
best way to measure these waves is to use a
Michelson interferometer where the beam
splitter is located at the centre of the circle
and the two mirrors are on the circumference
cutting out a 90° segment. The resulting
interference fringes strongly depend on
differential changes in the distances between
12
INTERNATIONAL INNOVATION
the beam splitter and the two mirrors, exactly
what gravitational waves change.
As simple as this sounds, the tricky part is
that gravitational wave strain amplitudes
are not expected to exceed 10-21. In other
words, they change the distance to the
Sun by the diameter of a hydrogen atom.
Measuring them is non-trivial. However, we
expect to make the first direct detection of
gravitational waves within the next five years
with Laser Interferometer Gravitational-Wave
Observatory (LIGO) and VIRGO.
Why are you so confident that the
Advanced LIGO will be able to detect
gravitational waves?
I am leading one of the large subsystems –
Input Optics – for Advanced LIGO, and I see the
progress and the problems first hand. None of
the issues will preclude us from reaching our
target sensitivity between 30 Hz and a few
hundred Hz; below 30 Hz is new territory for
us, and we will have to see how this evolves.
Because of data from NASA’s Swift satellite
observatory, we now have a better handle on
the neutron star merger rate; we should detect
gravitational waves during the first extended
science run. Once we improve by a factor of
10, in order to probe a thousand times larger
volume, we will hear these mergers on a
regular basis.
As Chair of NASAs Gravitational Wave
Science Analysis Group (GW-SAG), could
you outline the main aims and activities of
the group?
The topical SAGs are providing a way for
scientists working in a specific area to organise
themselves; to provide input to NASA on
how they see their field evolving and possible
ways forward; and to engage in and focus the
scientific discussions. The SAGs are also a way
for NASA to obtain outside expertise and to
release information to the scientific community
about what will be possible in the future and
PROFESSOR GUIDO MUELLER
PREHISTORIC BLACK HOLE
© NASA/JPL-CALTECH
Einstein’s messenger
The groundbreaking international evolved Laser Interferometer Space
Antenna mission will measure gravitational waves, ripples in spacetime,
caused by black holes and other fascinating astrophysical objects
THE LASER INTERFEROMETER SPACE
ANTENNA (LISA), an eagerly anticipated joint
venture between NASA and the European Space
Agency (ESA), was brought to an end in 2011
following budget problems at NASA. The project
had been designed to develop and operate a
space-based gravitational wave detector which
would have been sensitive at frequencies of
between 0.03 mHz and 1 Hz.
LISA would have monitored changes in the
separation between fiducial masses from three
spacecraft approximately 5 million km apart.
These gravitational wave-induced changes
or strains in spacetime are caused by highly
accelerated masses such as massive black holes
orbiting around each other. The mission would
have been well positioned to offer a unique insight
into a number of extraordinary astrophysical
objects and, together with electromagnetic
observations, would have led to a significant
furthering of current knowledge of outer space.
SHOOTS OF RECOVERY
how NASA plans to move forward. It is really
a group that organises interaction between
gravitational wave scientists and NASA’s
Physics of the Cosmos programme. Right now,
we focus on eLISA.
What impact do you expect the
evolved Laser Interferometer Space
Antenna (eLISA) mission to have on our
understanding of the Universe?
The scientific case for LISA fills many pages and
is difficult to summarise in a few sentences.
Everybody has his/her favourite source or
test to carry out with eLISA data. Black hole
evolution is one of the main scientific goals
often mentioned, and the project will also
essentially measure the density of compact
binaries in our galaxy; we can also isolate and
characterise many individual binaries. Also
among LISA highlights are accurate tests of GR,
especially in the strong field limit, large masses
and high velocities. Asking what we expect to
find is like asking ‘what do you expect to hear
when you start listening?’ – we’ll hear!
Despite the fact that NASA and ESA have parted
company on the LISA project, there remains
much hope that the mission’s aims can be
fulfilled by the evolved Laser Interferometer
Space Antenna (eLISA). This is a consortial
project which is being led by central figures
from the previous LISA Science Team, and in
which collaborators from Germany, France, Italy,
the UK, Spain, Switzerland, Denmark and The
Netherlands are all represented. Guido Mueller,
Professor of Physics at the University of Florida
and Chair of NASA’s Gravitational Wave Science
Analysis Group (GW-SAG) – which represents the
US’ scientific community at the consortium – is
confident that the project can break new ground
in furthering our fundamental understanding of
gravitational waves and, in the process, unlock a
number of remaining mysteries about the nature
and behaviour of the cosmos.
USING LISA AS A SPRINGBOARD
It is clear why a continuation of LISA is seen as
desirable, and the research staff within eLISA
are keen to align themselves strongly with the
goals and approaches of the earlier proposed
mission, as Mueller explains: “LISA is a very well
known project, and was consistently ranked as
the mission with the highest science value by
the National Academy of Science during decadal
and other reviews”.
be positioned in an equilateral triangle with a
baseline of 1 million km, and will exploit laser
interferometry in order to assess the changes in
distance between the masses at the picometre
level. The aim of the project is to provide the
first ever measurements of gravitational waves
between 100 µHz and 1 Hz. The hope is that
eLISA will pick up signals from supermassive
black holes as they spiral and collide with one
another with relativistic velocities. “Imagine
two one million solar mass black holes orbiting
around each other with a period of only a few
thousand seconds, or speeds close to the speed
of light; we know that these mergers happen,
and the only way to identify them is through
gravitational waves,” notes Mueller.
A PIONEERING PROJECT
Due to the similarities between gravitational
waves and acoustic waves – both being
generated by the coherent motion of
macroscopic masses which modulate their
surroundings; air pressure in one case,
spacetime in the other – analogies are often
made between the monitoring of gravitational
waves and the idea of ‘listening’ to the Universe.
Gravitational waves are created by the
accelerated motion of objects, as long as the
motion is not perfectly spherically symmetric.
Because of their penetrative qualities,
gravitational waves enable researchers to chart
the motion of black holes and stars with a high
level of accuracy, even if they are distant or
hidden behind other objects.
The data gathered by eLISA will enable Mueller
and his colleagues to locate massive black
hole mergers out to redshifts of up to 20,
looking beyond the time of reionisation which
electromagnetic radiation cannot penetrate.
Using this novel approach, eLISA is expected
to shed light on the first black holes and their
evolution, and will constitute a fundamentally
different way of studying the Universe. “eLISA
will open the gravitational-wave window and
start listening to the symphony the Universe is
playing for us every day,” notes Mueller.
SINGLE SATELLITE WITH A LASER BEAM
© AEI/MM/EXOZET
eLISA plans to use free falling proof masses inside
three spacecraft – one mother and two daughter
crafts. These space-based observatories will
WWW.RESEARCHMEDIA.EU
13
INTELLIGENCE
A LISA SATELLITE ANTENNA
DIRECTS ITS IMPACT
ON THE EARTH
© AEI/MM/EXOZET
GROUND AND SPACE-BASED
GRAVITATIONAL WAVE OBSERVATORIES:
The Laser Interferometer Space
Antenna (LISA)
The Laser Interferometer GravitationalWave Observatory (LIGO)
OBJECTIVES
To build ‘microphones’ which listen to the
symphony the Universe is playing every day.
The research focuses on technologies for
ground and space-based gravitational wave
detectors. These detectors will listen to the
most energetic events in our Universe and
teach us about the history of black holes and
galaxies.
COLLABORATIONS/PARTNERS:
David Tanner; John Conklin, University of
Florida
Robin Stebbins, Goddard Space Flight Center
Neil Cornish, Montana State University
Scott Hughes, MIT
LIGO Science Collaboration www.lsc.org
LIGO Laboratory www.ligo.caltech.edu
eLISA Consortium http://elisascience.org
COMMUNITY BUILDING/OUTREACH
International Research Experience for
Undergraduate Students in Gravitational
Physics: www.phys.ufl.edu/ireu
Gravitational-wave SAG: http://pcos.gsfc.
nasa.gov/sags/gwsag.php
FUNDING
National Science Foundation
LIGO-Laboratory
NASA
CONTACT
Professor Guido Mueller
Department of Physics
University of Florida
Office: 2370 NPB
2001 Museum Road Gainesville, Florida 32611-8440
USA
DEVELOPING A
HARDWARE-BASED SIMULATOR
The projected launch date for the eLISA
mission is 2028, a timescale which is set by
the available funding in Europe, and not by
the current status of the technology. Given
the amount of preparation and work that went
already into LISA, a launch of eLISA as early
as 2022 appears to be feasible. For example,
Mueller and his colleagues created a simulator
and validated the effectiveness of time delay
interferometry (TDI). The LISA pathfinder will
test the gravitational reference sensor in a
dedicated space mission in 2015. “Any LISA-like
mission requires gravitational reference sensors,
masses in free fall and laser interferometry.
The laser interferometry between the widely
separated spacecraft has to deal with a few
boundary conditions which are unusual for ultraprecision interferometry,” clarifies Mueller.
The conditions created by eLISA’s proposed
setup will be unprecedented, and any traditional
Michelson-type setup would drown in laser
frequency or phase noise. For this reason, the
researchers at eLISA are planning to employ
time delay interferometry in order to exploit the
fact that the phase evolution of each laser field
will be measured a number of times on each of
the three spacecraft. By accurately timing these
measurements and correctly subtracting timeshifted data streams from all spacecraft, Mueller
and his collaborators will be able to cancel up to
10 orders of magnitude of laser frequency noise,
enabling the extraction of gravitational wave
information at design sensitivity.
© ESA
Pioneering an approach which enables the delay
of laser phase information for many seconds
within a laboratory setting, and recreating it
to allow the generation of LISA-like signals,
the researchers have created datastreams
which include, amongst other things, timedependent Doppler shifts, laser frequency
noise and gravitational waves. Using this
approach, they have demonstrated that the
concept underpinning eLISA is fully operational
and practical which, in conjunction with
complementary studies at NASA’s Jet Propulsion
Laboratory, has allayed many of the concerns
surrounding the study’s measurement principle.
UNANSWERED QUESTIONS
Gravitation is the force that drives the
development of the Universe. “Gravitation
is talking to us via gravitational waves and
these waves will tell us when and how the first
black holes formed and how they grew. This is
closely linked to the evolution of galaxies and
of the Universe as a whole,” Mueller elucidates.
”We will also survey compact galactic binaries
formed between neutron stars, white dwarfs
and stellar mass black holes to learn more about
our Milky Way. Be prepared for the unexpected.
We have never before listened to the Universe;
it will tell us unexpected things.”
Mueller and his colleagues – both at the
University of Florida and within the eLISA
Consortium – are optimistic that, through the
use of gravitational waves, light can be shed
on many of the remaining questions regarding
black holes.
© ESA
T +1 352 392 8521 E mueller@phys.ufl.edu
GUIDO MUELLER has been a faculty
member in the Department of Physics at the
University of Florida since 2003. He started
as an Assistant Professor and was promoted
to Associate Professor in 2007 and to full
Professor in 2012. Prior to this, Mueller was
a scientist working at NASA’s Goddard Space
Flight Center on LISA and at the University of
Florida on LIGO.
In 1997, he received a fellowship of the
Japanese Society for the Promotion of
Science and worked at the University of
Electro-Communication in Chofu, Japan.
He received his PhD from the University
of Hannover in 1997 where he worked on
slow light and potential applications in high
resolution interferometry.
14
INTERNATIONAL INNOVATION
The diagram shows the three satellites of the LISA
mission. They lie at the corners of a triangle whose
sides are 5 million km long. If a ripple in space is
caused, for example, by the gravitational waves
of a massive black hole, the distance between the
satellites will change. These minute changes in
distance will be measured using lasers.
This illustration depicts the LISA Technology Package
(LTP), the main technology test package to fly onboard
LISA Pathfinder. The partly transparent view shows the
two proof masses: 46 mm gold/platinum alloy cubes,
housed in individual evacuated enclosures. The cubes
serve both as mirrors for the laser interferometer (red
lines depict the light path) and as inertial references for
the drag-free control system of the spacecraft.