The Gravity Wave Hunt

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The Gravity Wave Hunt
LIGO and LISA
KATIE WOODS NICHOLAS ELLENS
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WHAT IS GRAVITATIONAL RADIATION?

• According to Einsteins theory of general
  relativity, the force of gravity is due to
  curvature of space-time itself. Such curvature is
  caused by the presence of mass in our universe.
• In a similar way as accelerating electric charges
  produce electromagnetic waves, massive objects
  accelerating in space-time produce gravitational
  waves. For gravitational waves, the massive
  objects must accelerate in a manner that is not
  spherically or cylindrically symmetric. When this
  occurs, ripples in space-time spread outward,
  just like ripples in a puddle.
• Gravitational radiation is the energy carried by
  these waves.

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MAGNITUDE

• The magnitude of most gravitational waves is
  incredibly small
• For instance, the gravitational waves produced by
  the earth orbiting the sun are on the order of
• Considering that objects of interest are located
  light years away (large r), the magnitude
  decreases to 10-26m for systems like that of the
  earth and the sun
• Systems of binary neutron stars, binary black
  holes, and supernovae (assumed to be asymmetric),
  should produce radiation of 1020 times greater
  than that of the earth/sun. However, these tend
  to be far away.
• The largest magnitude waves expected near earth
  should be on the order of 10-21m

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INDIRECT EVIDENCE ORBITAL DECAY

• Although gravitational radiation has not yet been
  directly observed, it has been indirectly shown
  to exist through the Hulse-Taylor observations of
  the binary pulsar system PSR191316

• The system has been observed since its discovery
  in 1974, and the evolution of its orbit is in
  complete agreement with the loss of energy due to
  gravitational waves.

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EFFECTS OF A PASSING GRAVITATIONAL WAVE

• Consider a perfectly flat region in space, with a
  few motionless particles lying in a plane. As a
  gravitational wave passes through, perpendicular
  to the plane, the particles will oscillate in a
  manner depicted by the diagrams.
  

• The area enclosed by the test particles remains
  the same, and there is no motion along the
  direction of propagation.

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DETECTING GRAVITATIONAL WAVES

• The most simple type of gravitational wave
  detector is called a Weber bar

• Should a gravitational wave pass through this
  bar, it is possible it would hit the bar at its
  resonance frequency, effectively amplifying the
  wave.

• It consists of a solid piece of metal, equipped
  with electronics designed to detect any
  vibrations

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LIGOLaser Interferometer Gravitational Wave
Observatory

• A more sensitive apparatus designed to detect
  gravitational waves is the laser interferometer,
  involving separate masses placed several
  kilometers apart, acting as two ends of a bar.
• Several laser interferometers exist, but LIGO is
  probably the most prominent at this time
• LIGO is a joint project run by MIT and Caltech.

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LIGO

• There are three branches of LIGO, one in
  Livingston, Louisiana, and two near Richland,
  Washington.

• The distance between the two sites is about 3000
  km, which is important because it allows for the
  use of triangulation to determine where the waves
  are coming from.

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LIGO contd

• The basic setup of the three observatories
  consists of two vacuum tubes, 2 to 4 km in
  length. The two arms are at 90 degree angles to
  each other.
• The primary interferometer is composed of mirrors
  suspended at each of the corners of the L, a
  laser emits a 10 watt beam, and hits a beam
  splitter at the vertex of the L.

• The two beam paths run down each arm of the L,
  and are kept out of resonance. so the light waves
  interfere destructively as they travel through
  the cavity, and no light hits the photodiode

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• As was mentioned before, when a gravitational
  wave passes by, the space-time in the local area
  is altered.
• Depending on the polarization of the wave, and
  where it is coming from, this can result in the
  length of the cavities changing.
• The change in length of the cavity will knock it
  out of resonance, and the light in the cavity
  will be out of phase with the incoming light.
• When a gravitational wave passes through the
  interferometer, the distances along the arms of
  the interferometer are changed, and the beams
  become less out of phase, and light will hit the
  photodiode, creating a current that can is
  recorded as a signal.

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NOISE

• Noise sources include seismic waves, cars and
  trains passing by the detector, falling logs and
  even waves crashing on the shore hundreds of
  miles away
• These all cause similar effects to real
  gravitational wave signals, and one of the pains
  of this type of setup is trying to reduce the
  motions of the mirrors due to noise.
• One other limitation on all detectors is shot
  noise, which occurs because the laser being used
  is composed of photons, and there are random
  fluctuations associated with the intensity of the
  beam (the number of photons arriving in a given
  time interval)

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LISALaser Interferometer Space Antenna

• LISA is a work in progress
• It will consist of three space-based satellites
  which will act as vertices of a triangular
  interferometer
• Due for launch in 2015, it should provide data
  for 5 years

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• The main difference between LISA and ground-based
  interferometers is that LISA should observe
  gravitational waves in the 10-4 to 10-1 Hz bands,
  as opposed to ground-based interferometers which,
  when performing optimally, will observer in 101
  to 103 Hz frequency bands
• LISA is thus designed to monitor binary stellar
  objects (such as neutron stars or black holes)
  over long periods as they orbit each other (on
  the order of months or years).
• LIGO (and other ground-based observatories) and
  LISA complement each other well as LISA can see
  long-duration radiation, possibly even predicting
  a short burst of radiation that could be recorded
  by LIGO

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• LISAs design presents many technical challenges
• The three spacecraft are held in an equilateral,
  triangular formation, each separated by 5 million
  kilometres. Once stabilised, the separation
  between each will vary at most by 10km, with
  2mm/s variation in velocity
• This is accomplished using precise, delicate
  booster control

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• Each spacecraft contains a small, highly-polished
  test mass which is free-floating in gravity. The
  spacecraft tracks this mass with a precision of a
  few nanometres.
• The test masses are highly polished to also play
  a part in reflecting the laser where appropriate
• Just like LIGO, LISA employs laser
  interferometery to detect slight changes in
  distance between the satellites

• Each satellite acts as the base of an
  interferometer so the system actually consists of
  several interferometers. The results of each
  system can be combined using linear algebra to
  provide better precision

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• The spacecraft will trail the earth in its solar
  orbit by about 20 degrees, or about 50 million
  kilometres
• This distance allows for a minimization of
  radiation and orbit distortions at a minimum,
  while still retaining an acceptable communication
  rate

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LIMITATIONS

• At low frequencies, the resolution of LISA is
  dominated by temperature fluctuations.
• At optimal frequencies, the resolution is highest
  but is ultimately limited by an affect called
  shot noise
• At higher frequencies, the resolution is
  dominated by the antenna transfer noise, which is
  proportional to the frequency.

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• The detection of gravitational waves would be a
  milestone in the history of physics, but is it
  likely to happen soon? In 2004, it was estimated
  that the chances of a definite detection by 2010
  was 1 in 6, using ground-based observations.
• If all goes well with LISA, it will definitely
  be able to detect gravitational waves, if they
  are as theorized. However, it is not clear how
  well how well LISA will be able to pick up
  individual signals as LISA does not offer any
  directional observation tools

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References

• http//lisa.nasa.gov/
• http//www.ligo.caltech.edu/
• http//www.ligo.org/results/pdf/riles_aps2004.pdf
• http//www.ligo.org/results/pdf/riles_aps2004.pdf
• http//online.kitp.ucsb.edu/online/plecture/thorne
  
• LISA mission overview - A. Hammesfahr
• LISA technology-concept, status, prospects
  Karsten Danzmann
• http//www.lisa-science.org/resources/talks-articl
  es/mission