# LIGO: Laser Interferometer Gravitational Observatory

.. 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]

Noise Sources ~~~~~~~~~~~~~~~~~~~~~~

For initial LIGO, seismic noise dominates below about 60Hz, suspension thermal noise between 60 and 180Hz, and radiation pressure shot noise above 180Hz.

Sensitivity ~~~~~~~~~~~~~~~~~~~~~

Advanced LIGO will use 40kg sapphire test masses with sensitivity of about 10e-19 meters: 1/10000 of an atomic nucleus, 10e-13 of a wavelength, and half of the entire mirror’s wave function.

LISA ~~~~~~~~~~~~~

5e6 km separations between three spacecraft, 1 (astronomical unit, ~1.5e8 from the sun. 1 watt lasers. The heterodyne detection is of the beat frequencies at each spacecraft of the two incoming beams. Doppler shifts of spacecraft must be taken into account, due not only to sun radiation pressure etc, but varying gravitational fields from planetary orbits.

The test masses inside LISA should be free falling and have relative separations stable to 10e-9 cm (10e-5 wavelength of light).

LISA’s sensitivity is in the milihertz regime.

.. note: (insert LISA noise curve?)

Data Analysis ~~~~~~~~~~~~~~~~~~~~

Using matched filtering (eg, take cross correlation between two waveforms, integrating their product), frequency sensitivity will be around the inverse of the number of cycles of waveform (for LIGO, around 20,000 cycles, for LISA around 200,000 cycles).

This technique requires known, theoretically derived waveforms (within phase/amplitude). There are other methods when we don’t have good guesses about the waveform we are looking for…

## References

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