The Gravity Wave Hunt
LIGO and LISA
WHAT IS GRAVITATIONAL RADIATION?
• According to Einstein’s 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.
• 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 1 1.7 x1010 m
• 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
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 PSR1913+16
• 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
EFFECTS OF A PASSING GRAVITATIONAL
• 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
DETECTING GRAVITATIONAL WAVES
• The most simple type of gravitational wave detector is called a
• Should a
• It consists of a gravitational wave
solid piece of pass through this
metal, equipped bar, it is possible it
with electronics would hit the bar at
designed to its resonance
detect any frequency,
amplifying the wave.
Laser Interferometer Gravitational Wave
• 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.
• 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
it allows for the
the waves are
• 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
• 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
• 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.
• 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)
Laser 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
• 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
• LISA’s 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
• 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
The results of each system
can be combined using
linear algebra to provide
• 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
• At low frequencies, the resolution of LISA is dominated by
• 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.
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
• LISA mission overview - A. Hammesfahr
• LISA technology-concept, status, prospects – Karsten Danzmann