Sunday, June 30, 2013

Methods of Detecting Gravitational Waves I: Resonant-Mass Detectors

As the name of this blog suggests, I use LIGO and similar gravitational-wave detectors (like Virgo and GEO).  These detectors are all interferometric detectors meaning that they use the interference of light to measure gravitational waves.  But interferometers were not the first means used to look for gravitational waves...


In 1966, Joseph Weber of the University of Maryland constructed a gravitational-wave detector that consisted of a very precisely machined cylinder of aluminum 2 meters long and 1 meter in diameter.  The idea was that when a gravitational wave passed over the bar at a specific frequency, the bar would start to ring like a bell.  This "ringing" frequency, also called the resonant frequency, for Weber's bars was 1660 Hz (cycles per second).

Weber working on one of his bars at the University of Maryland, c. 1965.

The way these bars are be used to detect gravitational waves involves the phenomenon of sympathetic resonance.  This is when the vibration of something external to an object matches its resonant or "ringing" frequency and causes it to begin vibrating.  Even after the external vibration stops, the now vibrating object will continue to ring like a bell (and eventually stop ringing just like a bell as well).

Now, when a gravitational wave at or very near to 1660 Hz passes by one of Weber's bars, it will stretch space in one direction and compress it in the perpendicular direction, much like illustrated in the animation below:

This stretching and compressing is the vibration that makes the bar 'ring'.

One property of gravitational waves is that even the strongest of them that we can hope to detect on Earth are exceedingly weak.  Because of that, any ringing of the bar will be too small to hear or even to detect using normal ways of measuring vibration.  Instead, crystals that produce an electric voltage when stretched or compressed (called a piezoelectric crystal) were mounted around the bar.  Measuring the voltage from these crystals is also measuring the motions of the bar that could be from it ringing.

But just like there are many things other than gravitational waves that can cause noise in the data that LIGO collects, there were many other things that could cause the motion of Weber's bars.  Even the vibration of the aluminum atoms in the bar due to their temperature (Weber's bars were kept in a vacuum at room temperature) created significant noise and limited how small of a gravitational wave they could detect.  Ultimately, they were limited to a strain (which is defined to be the change in length divided by the original length of an object) of about 10-16.  To give this scale some reference, a "large" gravitational wave to LIGO would produce a strain of about 10-21 and we think we can expect a gravitational wave this large about once every 10 years!  But, it is still possible that Weber's bars could have detected gravitational waves...

By 1969 Weber thought that he may have detected gravitational waves with his bar detectors and continued to make several claims over the years but none were regarded as significant enough to declare that the first elusive gravitational wave had truly been directly observed.  These claims were ultimately not accepted for many reasons including that other groups were not able to reproduce his rate of detections which Weber was claiming to be up to several a day.  Weber lost the financial support of the National Science Foundation (which now funds LIGO) after a disputed claim of the detection of gravitational waves from a supernova observed in February 1987 (SN1987A).

One of Weber's original bars on display at the LIGO Hanford Observatory in August 2004.

There are many more interesting details surrounding Weber's career that I will write a post on later.  But while the controversy surrounding his claims of detection do not seem to cast him in a favorable light, it was through his work that others became inspired to look for gravitational waves in different ways, including using an interferometer like LIGO does.  Joseph Weber is truly the father of the search for gravitational waves!


Since Weber, there have been many advancements in using resonant-mass bars to detect gravitational waves.  Most bars today are made from new aluminum alloys, are cryogenically cooled to reduce the noise from the bar's thermal vibrations, have mechanical means to amplify the vibration, and piezoelectric crystals have been replaced with even more sensitive motion sensors.  Different shapes (like spheres) have also been used to increase sensitivity to gravitational waves coming from different directions because bars are most sensitive to gravitational wave directly above or below the bar.


Resonant-mass gravitational-wave detectors like Weber's bars and interferometric detectors like LIGO look for the same effect: the stretching and compressing of space caused by the gravitational wave.  Bars are far less expensive than detectors like LIGO but are only sensitive to narrow ranges of gravitational wave frequencies.  Bars are also only sensitive to small portions of the sky at once where detectors like LIGO are sensitive to most of the sky at once (including the sky on the other side of the planet).  But because of how each of these detectors look for gravitational waves, resonant-mass detectors are likely to only be sensitive to the strongest gravitational waves.


If you are interested in the operational state of gravitational-wave detectors (including resonant-mass detectors) click hereAURIGA and NAUTILUS are both bar detectors while GEO 600, LIGO, and Virgo are interferometers.