Transcript Gravity Waves - Department of Physics

Gravitational Wave
Arezu Dehghafnar
Physics Department
SUT
Overview
# What is Gravitational Wave
# The Sources
# Detecting GWs
Why are they important?
Gravity Waves would give us a new way to observe the universe. Like
a new sense, they would bring a new dimension to astronomy.
They would:
Verify general relativity's prediction that gravity
waves
exist.
Test that they travel at the speed of light.
Test that the graviton has zero rest mass.
Study black holes, and a binary black hole
Allow us to study astronomical entities that we
know little about, or have yet to discover.
system.
either
According to Einstein's theory of
gravity, an accelerating mass
causes the fabric of space-time to
ripple like a pond disturbed by a
rock. These ripples are Gravity
Waves.
http://focus.aps.org/story/v8/st3
This picture represents Gravity Waves
produced by a pair of rotating neutron
stars.
This picture represents ripples in a pond
disturbed by a rock.
www.jointsolutions.co.uk/
docs/pages/leftnav.htm
Electromagnetic Waves
Electromagnetic Waves
oscillate perpendicular to their
motion.
They oscillate in the X and Y
directions and the wave moves
in the Z direction.
Gravity Waves
Gravity waves have 2
polarizations like
Electromagnetic Waves. The
only difference is that Gravity
Wave polarization lies in a
horizontal-vertical “+” shape
and 45 degrees to that in a “x”
shape.
90' Polarisation
45' Polarisation
Gravitational Radiation
Gravitational Radiation, for example, occurs in a
binary system with two massive objects circling
one another. The large accelerations due to their
gravitational attraction would release gravitational
radiation. The noticeable affect of the expelled
radiation is the loss of mechanical energy of the
system, the two circling
objects would draw closer
to one another.
Gravitational Radiation
This was proved in the 1970s when
Russell Hulse and Joseph Taylor
observed that the binary pulsar system,
which consists of two super-massive
stars in close proximity, radiates energy
such that it’s period decreases 75
milliseconds every year. This proves the
existence of Gravity Waves.
binary pulsar
Sources
# supernova
# Stars (When they're still alive!)
# Gravitational Wave Background
# Curiosities (!)
Supernova
Death of a massive
star (10s of solar
masses).
Core collapses into a
neutron star or
black hole.
Non-symmetric
collapse cause burst
Stars
# Stars and their
Planets
# Neutron Stars
# Binaries
# Pulsars
Binary Stars
T ~ an hour
f ~ e-3
Neutron Stars
Pulsars
Pulsars are neutron
stars that emit an
electromagnetic
signal (mainly
observed in radio)
that appears pulsed
from Earth,
analogous to a
lighthouse.
GW Background
hmmmm....
What about
Dark
Matter??!!
How do we detect Gravity
Waves?
Mmm,
The answer is:Very Carefully
Locations of detectors
LIGO: Laser Interferometer
Gravity-wave Observatory
# Consists of two laser interferometers, located far from
one another to eliminate any local disturbances.
# The project is funded by the National Science
Foundation through a cooperative agreement with the
Caltech and MIT.
# The State of Louisiana, through LSU, provided the land
that one LIGO facility was built on. The other is located
in Washington State.
# LIGO’s interferometers are tuned to 100Hz, which is the
frequency of rotating neutron stars.
LIGO: Livingston Observatory
The antennae of the LIGO observatory extend over 4 kilometers (approximately
2.5 miles) in each direction from the central corner station.
LIGO: Laser Interferometer
Gravitational Wave Observatory
LIGO: Hanford Observatory
Laser Interferometer
A laser is split into two beams and aimed down either arm.
The beams reflect off a mirror at the end, return to the middle, bounce
back to the end, and back to the middle for a total of 50 times. This
makes the distance the light travels longer, and increases the sensitivity
of the detector.
Laser Interferometer
Because Gravity Waves alternatively stretch and
compress matter, if there is a phase difference between
the electromagnetic waves of the laser then one arm has
been stretched while the other has been compressed.
Note that any stretch or squeeze would be insanely small,
this is why we have yet to detect it.
But if we do detect some change in length, then that
means there are gravity waves!
AIGO: Australian International Gravitational
Observatory
First laser interferometer detector in the southern hemisphere.
VIRGO
LISA: Laser Interferometer
Space Antenna
This is much like the laser
interferometers on earth, only in
space. The arms could be 5
million kilometers long (1/30 AU).
Possible launch date is around
2010.
spaceplace.jpl.nasa.gov/ lisa_fact2.htm
LISA has 3 million km arms.
Will be able to look at low freqs > mHz.
New design
Present laser interferometer detectors can only measure
gravity waves at around 100Hz. A new detector has
been proposed to measure other frequencies. It would
consist of two approximately 4 meter concentric
metallic spheres cooled to less than 4 degrees Kelvin
and suspended 1 cm apart. The two would be 180
degrees out of phase, so when a Gravity Wave passed
through them one would shrink while the other
expanded. The change in the gap between them would
then have to be measured (on the order of 10^-19 cm).
Thank you