Gravitational Waves

The Basics

• According to Newton the force of gravity is instantaneous

• Einstein says nothing can travel faster than light

• Remember from General Relativity: gravity is space-time which has been warped by matter– More mass = More warping

• So how does gravity travel ‘through’ space?

Mechanics of These Waves• Like light Gravity travels as a wave• As electro-magnetic waves propagate

they affect the two planes perpendicular to their line of motion

– The electric and magnetic fields• gravity propagates does the same

thing– Compression and expansions

• Moving charge - light• Moving mass - gravity• Gravitational waves also exhibit a

particle nature– Light has the photon– Gravity has the graviton

• Neither particle has mass• Therefore gravitational waves can

travel at the speed of light• They travel the same speed through

any medium

"Electromagnetic waves are oscillations of the electromagnetic field that propagate through spacetime; gravitational waves are oscillations of the 'fabric' of spacetime itself." (Kip Thorne)

Indirect Evidence

• According to General Relativity a binary system of stars should lose energy to gravitational radiation

• This means the stars should slow down• In 1993 the Nobel Prize was awarded to Hulse and

Taylor for proving this• In 1974 they found a neutron star binary• Over twenty years of observations they found a

shift in the in the orbits which was consistent with the predictions of General Relativity

Strength of These Waves

• If every moving mass causes a gravitational wave why aren’t we all stretching and contracting now?

• Compared to the other 3 forces gravity is very weak.

• A black hole binary, each with the mass of 10 suns, 1 billion light years away, would displace the oceans by only 10 times the diameter of an atomic nucleus

• What can we use to detect such a small affect

INTERFEROMETRY• Capable of detecting a path

difference smaller than the diameter of the nucleus of an atom

• Light passed through beam splitter

• Travels down either arm and is reflected back and out for observation

• If both paths are the same there should be constructive interference

• If the path difference changes the interference pattern will shift which can be detected

• This is used to detect abnormalities in a surface, find the refractive index of an optical material, the change in refractive index etc.

Interferometers to Detect Gravitational Waves

• LIGO - Laser Interferometer Gravitational-wave Observatory

• LISA – Laser Interferometer Space Antenna

LIGO

• In operation since 2002• Two active observatories

in Livingston, Louisiana and Richland, Washington

• Have not detected anything significant

• Each arm is 4000m, these are the largest interferometers ever built to detect gravitational waves

• LIGO uses two interferometers to rule out any discrepancies

• LlGO detects movements as small as one thousandth the diameter of a proton

LIGO – more specs

• The vacuum tubes housing the interferometer have a volume of nearly 300,000 cubic feet

• They are evacuated to a pressure one-trillionth of an atmosphere

• The suspended mirrors are even shielded from vibrations occurring due to the random motion of the atoms within the mirrors

• Over 30 control systems are used to hold the lasers and mirrors in place

• VIRGO is another ground based interferometer

LISA• If approved development will begin in 2004• Launch set in 2011 for a duration of five years • Will consist of three unconnected spacecraft

five million km apart, in an equilateral triangle• 50 million kilometers above Earth • Each spacecraft will contain two test masses

floating freely space craft will follow the masses with an accuracy of 10^-9 m

• Noise due to:– Solar pressure– Solar magnetic field– Earths gravity

• Micro-thrusters will attempt to move LISA out of such noise if detected

• Separation between spacecraft will be maintained with an accuracy of 10^-12 m

• Main craft fires laser to second which fires laser in phase to third which fires laser in phase back the main craft

• This goes in both directions• Each can craft calculates the path deviation• By measuring the relative positions of the test

masses• LISA will not be ‘better’ than LIGO or VIRGO but

will be able to detect different frequencies of waves

– LIGO detects high frequencies– LISA will detect low frequencies

Promising Sources

• Binary Systems– Stars– Neutron Stars– Black Holes

• Non-Spherical Rotating Objects – Super nova– Vibrating Black Holes

• Galaxy Formation• Dark Matter• Big Bang ???

Low Frequency waves due to a rotating binary system

High Frequency waves due to a rotating binary system

MOVIE TIME

Types of Signals• Chirps

– Binary Systems

• Burps– Collapse of Supernova– Black Hole eating a star

• Periodic– Non-Spherical Objects rotating

• Stochastic– Big Bang

• Need much improvement i.e. another observatory

Summary

• Gravitational waves warp spacetime as they travel

• Indirect Evidence exists• Direct evidence places its hope mostly on

interferometers• Since they can travel through anything

gravitational waves offer a new and exciting view of the universe– Comparable to the first use of x-ray etc. astronomy

My sources• LIGO Laboratory Homepage

– http://www.ligo.caltech.edu/• Laser Interferometer Space Antenna

– lisa.jpl.nasa.gov/ • Jillian’s Guide to Gravitational Waves

– http://www.gothosenterprises.com/gravitational_waves/• Ripples in Space Time

– http://archive.ncsa.uiuc.edu/Cyberia/NumRel/GravWaves.html• List of Gravitational Wave Detectors

– www.johnstonsarchive.net/relativity/gwdtable.html • Gravitational Wave Sources

– http://faculty.fortlewis.edu/tyler_c/classes/454/w03/andrew/sources_of_gw.htm

• Propagation of Gravity Waves– http://www.physics.uiuc.edu/Research/CTA/movies/Gw-ns/gw/