Gravitational Wave Sources: Changing Perspectives

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Gravitational Wave Sources Changing Perspectives
The KipFest
2 June 2000
California Institute of Technology
A Symposium on Kip Thornes 60th Birthday

• Bernard Schutz
• (PhD Kip 71)
• Albert Einstein Institute Max Planck Institute
  for Gravitational Physics, Golm, Germany
• and
• Department of Physics and Astronomy
• Cardiff University, Cardiff, UK

http//www.aei-potsdam.mpg.de schutz_at_aei-potsdam.
mpg.de
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Gravitational Wave Theory Ghosts and Illusions
for 40 Years

• Gravitational radiation posed fundamental
  problems to theorists in GR for a remarkably long
  time. Einstein repeatedly wavered about their
  reality.
• Early workers probably defeated themselves by
  expecting too much a universal definition of the
  energy in a wave, a proof of energy conservation,
  a clean separation between physical and
  coordinate effects.
• These expectations, based on Maxwells equations,
  could not be fulfilled. The equivalence principle
  made localization impossible, and the
  nonlinearity of GR made a strict separation of
  waves from the background impossible too.

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Getting it Right in the 50s and 60s

• Real progress came when physicists accepted that
  gravitational waves could only be cleanly defined
  in certain asymptotic limits. Bondi demonstrated
  energy balance far away, Penrose made this
  visually accessible by defining null infinity,
  Arnowitt-Deser-Misner showed how waves contribute
  to mass measured far away, Isaacson showed how to
  localize their energy gauge-invariantly within a
  region the size of a wavelength (but not
  smaller).
• Gravitational radiation is no longer a ghost, but
  it is not quite concrete either. It is the
  dynamical essence of GR. Through nonlinearity,
  gravitational waves can create mass,
  singularities, even black holes. Most
  interestingly, they carry information about what
  Kip Thorne calls the dark side of the Universe.

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Physical Gravitational Waves

• Kip started his career just as the fog was
  lifting, and gravitational radiation physics has
  been a consistent interest. His PhD (1965) thesis
  studied gauge-invariant conservation laws for
  cylindrically-symmetric gravitational waves (the
  C-energy). By 1969 he was calculating GW
  amplitudes from pulsating neutron stars (Thorne
  Campolattaro with algebraic computing!!) and
  working with Burke on deriving the quadrupole
  radiation reaction formula. He suggested I look
  into pulsations of rotating stars.
• There were still many open questions, but not
  ones of reality. The most important was to
  establish the correct way to calculate radiation
  from self-gravitating systems, dealing with the
  nonlinearity of GR satisfactorily. This was not
  finally settled until the early 1980s by
  Chandrasekhar, Damour, and many others. Extending
  this post-Newtonian approximation to high order
  is still an important area of work.

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The Importance of the Field h

• For gravitational wave detection, all one needs
  is a calculation of h and of back-reaction in the
  source. This can be done without worrying about
  energy!
• Direct approaches to calculating h had been
  followed from the beginning, but were hampered by
  the lack of a consistent post-Newtonian
  approximation.
• In the first decades of GR, nobody thought
  gravitational waves (or black holes,
  gravitational lensing, ) could be observed, so
  most physicists ignored GR entirely. For example,
  even Chandrasekhar decided not to go into
  relativity in the early 50s because he feared it
  would be the death of his career.
• Reciprocally, what experimenter would take up the
  challenge of building such a complex apparatus to
  observe a radiation that many specialists doubted
  was even real?

(Answer Weber!)
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The First Experiments

• Weber ignored the doubts of some theorists
  because he had done the field/source calculations
  himself, at least in linear theory, and he
  believed the results.
• He also knew his first detector would be far from
  the required sensitivity, but he wanted to start.
  He began building the first bar detector in 1960.
• In retrospect, when he started he was probably
  too optimistic about the technology of his day.
  After investing a decade of effort, he allowed
  himself to believe that he was seeing
  gravitational waves at an amplitude and rate far
  higher than he or anyone else had predicted. The
  sad episode ended when other experiments failed
  to confirm him, leaving Weber isolated.
• But his claims stimulated many groups to build
  detectors to check his results. Almost all of
  todays big projects have grown from those
  efforts to prove or disprove Weber.

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Early Source Calculations

• Webers claims stimulated theoreticians in the
  late 1960s too.
• Supernovae were believed to be the most likely
  source of detectable radiation. Webers bar was
  optimized for them.
• But supernovae could not explain Webers claimed
  observations, so many groups took up the
  challenge. Kips group at Caltech collaborated
  and competed with groups at Princeton, Maryland,
  Chicago, and Austin, among others. Many groups in
  Europe also began source calculations. It was an
  exciting time!
• Out of this excitement came some fundamental
  advances (black hole dynamics/ thermodynamics/uniq
  ueness, neutron star stability, and (later)
  measurement below the quantum limit see talks
  by Caves and Braginsky), some seminal
  technologies (numerical relativity, the Teukolsky
  equation, radiation reaction), and some false
  hopes (gravitational synchrotron radiation).

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Early Developments at Low Frequencies

• In 1967 Braginsky and Gertsenshtein proposed
  Doppler tracking of spacecraft for low-frequency
  detection. Anderson (1971) did the first analysis
  of tracking data.
• The idea for interferometers in space came later,
  from Weiss. (See later slide).
• In the first application of GWs in astrophysics,
  Faulkner (1971) proposed that GWs regulate orbits
  of some cataclysmic variables. More and more
  short-period binaries were discovered by X-ray
  and UV observations. The number of known sources
  was growing.
• The identification of Cyg X-1 as a strong
  black-hole candidate in the X-ray data of Uhuru
  (launched 1970) made black holes more real,
  making it easier for theorists to postulate that
  QSOs were powered by massive black holes. (See
  later slide.)

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The Gravitational Wave Sky in MTW (1973)

• Intense theoretical work had produced many
  significant results, but no new realistic
  sources! The conclusion that Weber was not seeing
  gravitational waves vindicated this failure and
  most people turned away from further source
  calculations.
• Supernovae were still regarded as the most
  promising, but realistic limits on their
  radiation amplitudes were discouraging.
• Spinning neutron stars pulsars were also a
  possibility, as unpredictable then as they remain
  today.
• The early Universe was recognized as a possible
  source, but a good mechanism was not there
  inflation and cosmic defects were in the future.
• Bar detectors were the only practical technology.
  MTW does not treat interferometers for
  gravitational radiation. Source strength is
  always measured in terms of energy, not
  amplitude. Detectors are regarded as bolometers.

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We Have a Data Point!

• The discovery in 1974 of the Hulse-Taylor pulsar
  PSR191316, and its subsequent intensive study by
  Taylor, had important ramifications.
• It stimulated theorists to clear up their
  remaining confusions over the post-Newtonian
  approximation.
• It reassured us that GR is quantitatively correct
  for gravitational waves, at least for this kind
  of system. This was an important consideration
  for approval of funding for large detectors 15-20
  years later.
• It led, within a few years, to the understanding
  that similar systems coalescing in remote
  galaxies are among the strongest predictable
  sources observable from the ground. Clark and
  Eardley provided the first key insights around
  1978, but it was not until the later development
  of interferometers that the importance of these
  systems became clear.

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The Rise of Interferometers

• First proposed independently by Gertsenshtein
  Pustovoit, Weber, and Weiss, the first was built
  by Forward in 1972 at Hughes Aircraft.
• By the end of the 1970s it was clear that the
  quantum limit for bars was a serious one. Kip did
  fundamental work on QND (see talks by Caves,
  Braginsky), and this continues to be one of his
  important research interests. But bars had an
  obvious problem.
• At the same time new technology had made laser
  interferometers promising. Their broad bandwidth
  and potentially large size made them attractive.
  Better mirrors (developed for military air
  navigation systems) allowed systems to reach
  required power densities by using the
  Drever-Schilling idea of power recycling.
• Suddenly one could design 10-21 and 10-22
  detectors that one believed one could actually
  build. Groups at MIT, Glasgow, and Munich started
  on this path.

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Detecting Gravitational Waves From Space

• Weiss proposed a space interferometer in 1976.
• Gravity-gradient noise makes the Earth an
  impossible platform for observing below 1 Hz.
  Fluctuations in the local tidal gravitational
  field are larger than expected GW amplitudes.
  They cannot be screened out, but they fall off as
  1/r3 as one goes far away.
• Bender Faller developed a feasible design for a
  space interferometer, supported by NASA funding.
  But it proved difficult to interest NASA (ie
  space physicists) in a mission.
• Why? Possibly because the field simply had no
  profile. The development and funding of ground
  interferometers and the general acceptance that
  galaxies contain massive black holes would
  eventually lift this blight.

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The Perspective at Batelle in 1978

• In 1978 Smarr organized a meeting on
  gravitational radiation that contains the seeds
  of the big changes that were about to occur
  (Smarr 1979).
• Weiss described interferometers and estimated
  their sensitivity, remarkably close to present
  ground and space targets.
• Blandford clearly described the case for black
  holes in galaxies and their possibilities for
  gravitational radiation, but he pointed out that
  there was much skepticism to overcome. He was
  10-15 years ahead of his time!
• Clark, in another far-sighted paper, made a
  strong case for coalescing NS binaries, saying
  that they were now the best sources, and that the
  fact that they emit at low frequencies called
  into question the design of current detectors
  (bars). He only used energy considerations in
  this argument.

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Matched Filtering Changing Data Analysis

• Interferometers were ideal for detecting
  coalescing neutron-star binaries, whose spectrum
  was broad and extended to low frequencies,
  inaccessible to bars. But sensitivity estimates
  at first were mainly based on the expected energy
  in the waves. Bar groups had been doing a form of
  filtering, but this was to remove the detector
  transfer function nothing was known about the
  source waveform apart from its estimated energy.
• Some time around 1983/4, Kip seems to have been
  the first in our field to grasp that matched
  filtering would greatly improve the sensitivity
  of interferometers to such sources as well as
  lead to the extraction of much more information
  from the signal.
• I believe that this was the key theoretical
  insight that led to todays large interferometer
  projects. At last there was a nearly guaranteed
  source that could be seen with a practical
  detector. Funding agencies just had to give the
  money!!

(Well, almost.)
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The Gravitational Wave Sky in 1987

• Kips definitive article in 300 Years of
  Gravitation (Hawking Israel) and the 1987 NATO
  meeting on data analysis (Schutz 1989) show the
  consolidation of the transformation that was
  already underway at Batelle.
• Interferometers (first and second-generation,
  ground and space-based) dominate the discussion.
• Matched filtering is the basis of sensitivity
  estimates and the extraction of information from
  signals.
• Supernovae are still at the 10-21 level
  (optimistic assumptions).
• Coalescing binaries are secure sources for
  advanced detectors.
• Massive black hole coalescences have high S/N
  from space.
• Wagoners (1984) mechanism for driving emission
  from CFS-unstable modes of accreting neutron
  stars is included.
• Cosmological radiation is treated seriously and
  extensively.

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After 1990 An Upsurge of Interest in GW Science
Publications per 2-year period
Search of SCI on keywords gravitational wave,
gravitational waves, gravitational-wave,
and gravitational radiation.
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Large Interferometers the 1st Generation
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The Joy of Starting

• The decade 1985-95 saw the founding of the first
  big interferometer projects, with some false
  starts.
• Kip attracted Drever to Caltech from Glasgow,
  then joined with Weiss to develop LIGO as a
  Caltech-MIT collaboration. Kip (happily) stepped
  aside when the management was centralized under
  Vogt. Barish took over in 1994, and LIGO funding
  began.
• Glasgow, having failed to fund an all-British 1
  km instrument in 1986, joined with the Garching
  group and won approval in 1989 for GEO, a 3 km
  instrument in Germany. Recession and
  re-unification killed the project when funding
  vanished a year later. Regrouping, GEO won
  funding for its 600 m high-risk low-cost detector
  in 1994.
• VIRGO began in parallel with a joint proposal to
  France and Italy. It has grown into a substantial
  collaboration among research institutes of INFN
  and CNRS.

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Worldwide Interferometer Network
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Progress in Sensitivity ?100 in 10-20 years is
possible
Under construction
Planned
Anticipated bounds, in design phase
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The High-Frequency Sky in 2000

• Since 1987 there have been further changes in our
  expectations.
• Coalescing neutron-star binaries may be
  associated with gamma-ray bursts, and their event
  rate may be lower than we thought at first. But
  the binary black-hole coalescence rate may be
  higher.
• The modes underlying Wagoners proposal are
  thought to be stable, but Bildsten (1998) has
  revived the idea using temperature gradients to
  provide the asymmetry. Cyg X-1 is promising.
• r-modes have been found to be unstable by the CFS
  mechanism and may provide another good source for
  advanced detectors. (See talk by Owen.) NS mode
  seismology can determine NS EOS (Andersson
  Kokkotas).
• COBE observations have given a target for the
  cosmological background (very hard to reach) but
  superstring-inspired cosmologies (Veneziano et
  al) may generate more radiation.

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How Will We Recognize GWs?

• Building and operating detectors is the first
  part of the story. After that comes the data
  analysis.
• Detection is always statistical in nature, never
  100 certain. Matched filtering is the key, but
  one must understand and believe the filters.
• The first generation detectors must jointly
  analyze their data to extract maximum
  sensitivity.
• LIGO and GEO are in detailed discussion about an
  agreement for data exchange and joint
  publication.
• The first detection must be iron-clad.

I hope it will soon be time for data-analysts
to put themselves on the line and claim a
discovery!
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LISA
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The Low-Frequency Sky

• Bender brought his studies to Europe in 1993, and
  the result was that ESA adopted LISA as a
  Cornerstone mission in 1995. Interest in the USA
  revived, and we now expect LISA to be a joint
  ESA-NASA mission launched around 2010.
• Massive black holes are now known to exist in
  most external galaxies. Galaxy mergers are
  common, so black hole mergers may be common too.
  Small black holes falling into large ones could
  provide stringent tests of GR.
• There are so many binary star systems in the
  Galaxy that they cannot be resolved at
  frequencies below 1 mHz. Above that, LISA might
  discover hundreds or thousands of new systems.
• LISA will not reach the inflation bound on the
  stochastic background, but a future space mission
  targeted at one of two windows ( lt 1 ?Hz, 0.1-1.0
  Hz) might be the best way to make this most
  fundamental observation.

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More Work for Theorists

• Theorists have as much to do as experimentalists.
• Numerical relativity is needed to provide good
  waveform predictions for BH-BH, BH-NS, NS-NS
  mergers. The Lazarus mixed perturbation-simulation
  approximation may yield first new results next
  year (next slide). Full simulations, proceeding
  in parallel, may also yield first results next
  year.
• To predict supernova radiation it will be
  necessary to do full 3D gravitational collapse
  simulations with neutrino transport.
• Neutron stars could be important sources, and
  theorists are working on r-modes and the Bildsten
  mechanism. Details of NS physics are crucial to
  making credible predictions.
• We have to learn how to calculate
  radiation-reaction on orbits around black holes
  before we can devise filters for signals from
  small black holes falling into large ones.

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Lazarus Project BH-Inspiral Calculations
By Baker, Brügmann, Campanelli, Lousto (AEI)
Misner BH collision as a test problem.
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Gravitational Wave Astronomy

• Gravitational waves are part of astronomy
  already, but there will be more interaction in
  the near future between astronomers and
  gravitational wave observers and theorists.
• Galaxy evolution ? black hole observations
• r-mode observations ? supernova studies, NS
  physics
• GWs from spinning NSs ? radio studies
• LISA advance warning of MBH coalescence ?
  multi-waveband observations of the event
• CMB observations ? stochastic background
• Up to now, most theoretical work on gravitational
  radiation has been done by relativists. Future
  observations will require a new breed of
  phenomenologists, young physicists with a broad
  grasp of astronomy, capable of making models and
  interpreting data.

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Heres One!