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=======================
Gravitational Waves
=======================
.. warning:: This is a rough work in progress!! Likely to be factual errors, poor grammar, etc.
.. note:: Most of this content is based on a 2002 Caltech course taught by
Kip Thorn [PH237]_
Raw Info

Rank 4 Riemann tensors, will cover different gauge.
Waves are double integrals of curvature tensor...
Gravitons as Quantum Particles

Invariance angles: (Spin of quantum particle) = :latex:`$2 pi$` / (invariance angle)
Graviton has :latex:`$\pi$` invariance angle, so it is spin 2; photons have unique :latex:`$\arrow{E}$` vector, so invariance angle is :latex:`$2\pi$`, spin 1
Also describes spin by the group of Lorentz transformations which effect propagation.
Two polarizations: cross and plus, corresponding to spin of particles aligning with or against propagation? (Ref: Eugene Vickner? reviews of modern physics)
Waves' multipole order :latex:`$\geq$` spin of quantum = 2 for graviton ((??))
Waves don't propagate like E, because mass monopoles don't oscillate like charges.
:latex:`$ h \approx \frac{G}{c^2} \frac{M_0}{r} + \frac{G}{c^3} \frac{M'_1}{r} + \frac{G}{c^4} \frac{M''_2}{r} + \frac{G}{c^4} \frac{S'_1}{r} + \frac{G}{c^5} \frac{S''_1}{r}$`
First term: mass can't oscillate
Second term: momentum can't oscillate
Third term: mass quadrupole moment dominates
Fourth term: angular momentum can't oscillate
Fifth term: current quadrupole
Energy

Quick calculation: for a source with mass M, size L, period P, the quadrupole
moment :m:`$M_2 \approx M L^2$`, :m:`$h \approx 1/c^2` (Newtonian potential
energy) ????
h on the order of :m:`$10^{22}$`
Propagation

When wavelength much less than curvature of universe (background), then gravitational waves propagate like light waves: undergo red shifts, gravitational lensing, inflationary red shift, etc.
Sources

Inspirals of bodies into supermassive black holes
Eg, white dwarfs, neutron stars, small black holes.
Supermassive black holes are expected near the centers of galaxies.
Low frequencies (LISA); waveforms could hold data about spacetime curvature
local to the black hole.
Waveforms could be very difficult to predict.
Binary black hole mergers
Broadband signals depending on masses.
Neutron Star/Black hole mergers
Stellar mass objects existing in the main bodies of galaxies.
Higher frequencies (LIGO and AdvLIGO).
Neutron Star/Neutron Star mergers
Have actual examples in our galaxy of these events; but final inspiral rate
is so low that we have must listen in other galaxies. Merger waves will
probably be lost in higher frequency noise, so can't probe local
gravitational curvature.
May observe "tails" of waves: scattering off of high curvature around the
binary.
Pulsars (spinning neutron stars)
Known to exist in our galaxy.
Spectrum

High Frequency: Above 1 Hz, LIGO (10 Hz to 1kHz), resonant bars
Small black holes (2 to 1k suns), neutron stars, supernovas
Low frequency: 1Hz and lower, LISA (10^4 Hz to 0.1 Hz), Doppler tracking of spacecraft
Massive black holes (300 to 30 million suns), binary stars
Very Low Frequency: 10^8 Hz, Pulsar timing (our clocks shifted by gwaves, average of distance pulsars are not over long periods)
Extreme Low Frequency: 10^16 Hz, Cosmic Microwave Background anisotropy
Detectors

:m:`$\Delta L = h L ~ \leq 4 \times 10^{16} \text{cm}$`
LIGO (10 Hz to 1kHz)
Also GEO, VIRGO, TAMA (?), AIGO
LISA (10e4 Hz to 0.1 Hz)
Resonant Bars
~~~~~~~~~~~~~~~
First by Webber.
Currently in Louisiana State University (Allegro), University of West Australia (Niobe), CERN (Explorer), University of Padova (Auriga), and University of Rome (Nautilus)
References

.. [PH237] `Gravitational Waves`:title: (aka ph237), a course taught by Kip Thorne at Caltech in 2002. See http://elmer.tapir.caltech.edu/ph237/ for notes and lecture videos.
