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Gravitational Waves and LISA January 11, 2009

Posted by jcconwell in Astronomy, Black Holes, General Relativity.
Tags: , , ,

The AAS meeting in Long Beach this week had many nifty displays. My favorite, since I’m biased toward  general relativity, is the LISA display. LISA stands for Laser Interferometer Space Antenna. Here I am in front of the full scale model of one of three proposed LISA satellites.

The author and a scale model of a LISA satelite

The author and a scale model of a LISA satellite

Now you may wonder why you want an orbiting gravitational wave satellite, especially since we have LIGO (Laser Interferometer Gravitational-Wave Observatory) already taking data. The answer is in the sensitivity diagram below

lisa-ligo_noise_spec1In order to make gravitational radiation you need to have an accelerated mass. The biggest masses with the largest accelerations are colliding black holes and neutron stars. Since most actual collisions are thought to be between orbiting bodies, the frequency of the radiation is related to the orbital frequency = orbits/second.

Now black holes seem to come in two classes. First, stellar mass black holes, created in massive core collapse supernovae. These black holes are around 10 solar masses and have a radius of 30 kilometers (18 miles). The greatest amount of radiation comes just as the two black holes are touching, or merging. The orbital velocities are about the speed of light. and the time to complete one orbit is

(orbital circumference) / velocity = .0006 second

or a frequency of 1600 orbits/second. This about the peak frequency for the radiation from this type of collision. In the diagram above, this frequency band is where LIGO was designed to be the most sensitive.

But there is a second class of black holes, the supermassive holes. These giants are from a million to several billion times the mass of the sun. They seem to form the core in most galaxies, and so when galaxies collide and merge, two orbiting monster black holes will release copious amount of energy. The good news is you can detect this from much further away than the merger of the smaller black holes. The bad news is the frequency.

A two million solar mass hole has a radius of 60 million kilometer and a circumference of about 380 million kilometers. In this case the period for the holes to orbit around each other is much longer

(orbital circumference) / velocity = 126 seconds

or a frequency of .008 orbits/second. A very low frequency, too low to detect on Earth, due earthquakes and seismic activities. This is where the frequency band where  LISA comes in and why you need it in space rather than on Earth.



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