Gravitational Wave

Arezu DehghafnarPhysics Department

SUT

Overview

# What is Gravitational Wave

# The Sources

# Detecting GWs

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 system.

Allow us to study astronomical entities that we either know little about, or have yet to discover.

Why are they important?

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.

This picture represents Gravity Waves produced by a pair of rotating neutron stars.

This picture represents ripples in a pond disturbed by a rock.

http://focus.aps.org/story/v8/st3

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, 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

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 of GWs.

Outer layers of star blown away.

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?

The answer is:Very Carefully

Mmm,

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

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.

Laser Interferometer

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 designPresent 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