Friday, February 17, 2012

Q: Is there anything like LIGO outside of the USA?

I mentioned before that I've noticed in my blog visit statistics that some people find my site by searching questions that they have about gravitational waves.  They show up as the search keywords (don't worry - I have no way of knowing who searched and visited this site).  Many of the questions are great and I am not sure that my blog completely answered their question, at least in a concise way.  Today, I am answering one of these "searched for" questions:

"Is there anything like LIGO outside of the USA?"

Yes!  First, let me establish that there are 2 LIGO observatories in the United States.

Why are there 2 LIGO's?

Detectors like LIGO, gravitational wave interferometers, are sensitive to gravitational waves coming from nearly any place in the sky, including the sky that's above the other side of the planet.  The fact that gravitational waves can travel through matter and come out the other side unchanged is a huge advantage over doing astronomy using different forms of light and allows LIGO to have this amazing sensitivity.  It does have the downside that, given only a single detector, we cannot tell where a detected gravitational wave came from on the sky.  So, we built 2 detectors in the United States to ensure that, even if no other countries built gravitational wave detectors, we would be able to narrow down the location of any detections. 


LIGO Livingston's sensitivity to sources coming directly above locations on Earth.  Red is the highest sensitivity and blue is the lowest.  Since this is a flat map of a spherical object, sizes are distorted.  CLICK TO SEE DETAIL.
The time it will take a gravitational wave traveling at the speed of light to travel between the LIGO Hanford and Livingston observatories in seconds.  The black line is the circle on the sky the would produce a detection at both LIGOs at the same time.  CLICK TO SEE DETAIL.

Note that I said "narrow down".  Given 2 detectors and the detection time at each, we can start to triangulate possible sky locations of a gravitational wave to a circle on the sky that corresponds to the locations that could produce the observed difference in detection time (since we expect gravitational waves to travel at the speed of light, the maximum time it would take to travel between the 2 LIGO detectors is about 0.01 seconds).  With 3 detectors we can narrow the location to 2 points on the sky and with 4 or more we can find the 1 source.  Of course, if there was an optical event like a supernova on the 2-detector sky circle that for a gravitational wave detection at the same time, it would be probable that the optical event was also the source of the gravitational wave.

Another reason to have at least 2 detectors is eliminate the possibility that a local vibration is mistaken for a real gravitational wave.  LIGO is very sensitive to vibrations from our environment.  It is possible for a passing truck or a dropped hammer near the detector to make the mirrors inside vibrate in such a way that it "looks" like a gravitational wave.  In order to avoid making mistakes like this, we do not believe that anything is a gravitational wave unless we see the same signal in both detectors within the time it would take it to travel between detectors.  Therefore, we only consider a candidate detection if we see the same signal within +/- 0.01 seconds of when the signal is seen in one of the detectors.

Now, back to the original question:

Is there anything like LIGO outside of the USA?
There are several other gravitational wave interferometers in other countries.  The Virgo detector is located outside of Pisa, Italy, the GEO600 detector is located in Hannover, Germany, and the TAMA300 and the future KAGRA (formerly known as the LCGT) is located in Japan.  LIGO collaborates extensively with all of these detectors so that any time detectors are collecting data at the same time, that data is shared.  We also share technology so that all of the detectors are as sensitive as they can be.  The more observatories that see the same gravitational wave, the better we can localize it and the more we can know about it.

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