A personal view of GW150914 + links to released data and relevant papers

A LIGO (Livingston?) prototype drawn by David

The whole world has been rejoicing the finding of the first gravitational waves. This is 40some years after the 1972 MIT paper that outlined the basic design of LIGO and estimated the principal noise sources in a kilometer scale detector. The site construction began 1994, and by the time I started college in 2001 both the LIGO sites and GEO600 were operating, and started taking data. I visited the Livingston detector in 2003. My brother, Mihai Bondarescu, and I were organizing the first gravitational wave course at Louisiana State University. It was sponsored by Edward Seidel and Gabrielle Allen, but it could not take place in the regular semester because we were students ourselves and had free time only in vacation. So, this course happened in the winter break. Our students were so enthusiastic that they showed up to class on Xmas eve, and New Year Day. They were all part of the LIGO collaboration, and proudly showed us the detector. They had come to our course on vacation because they were hoping that LIGO would detect gravitational waves soon, and wanted to be prepared. 

Sensitivity: Today's Advanced LIGO vs. Last Run of Initial LIGO - 2010

It is amazing that the community persisted for so long, and succeeded. It is not unexpected. Most scientists thought that advanced LIGO would find gravitational waves, but just not in one of the Engineering runs. I am particularly humbled by this in part because it happened the first day Andrew Lundgren was acting as LIGO's detector characterization chair. So, 15 years after sitting in Kip Thorne's group meeting, and watching him lecture on gravitational waves, I had close to a front row seat to their detection.

When Gabriela Gonzales and Michael Landry confirmed that it was not a blind injection, Andy stared shaking. He regained his composure enough to continue running the first conference call after the detection, and although it ran overtime, they went through the agenda as planned.  The last time he shook like that was when he drove the space shuttle at the age of 10. No, it was not the real space shuttle, it was just a very realistic simulation built by NASA. But the gravitational wave was real.

So, what does the detection mean? This detection proved that black holes in the 20+ solar mass range exist in the nearby universe, and that they coalesce just as we predicted from numerical relativity simulations! This was the first detection of such objects and it was done by measuring the emitted gravitational waves. We were able to measure space-time shakes caused by a binary black hole coalescence. These waves traveled unobstructed to Earth at the speed of light and reached us 1.3 billion years later. They then stretched and squeezed the arms of the two LIGO detectors, which are 4 kilometres long, by about 1/4000 of the size of an electron. This stretching and squeezing was measured with incredible confidence! The 5.1 sigma detection entails a false alarm rate of 1 in a few million, i.e., the experiment would have to be repeated more than 3 million times to expect the same conclusion by accident.

The data surrounding the event is publicly available for download.

The first 16 days of coincident data ("coincident" means both the Hanford and Livingston LIGO detectors were on and had good quality data) has been thoroughly analyzed. The data around the GW150914 event is released. Note that you have click on the "Gravitational Wave Strain Data" link to get it. They released 4096 seconds, which is a bit more than an hour around the GW150914 event. This is processed data, e.g., the lowest and highest frequency are damped through filters to make the signal more clear.

Two Black Hole Binaries? FAR is the False Alarm Rate, which is very low. A 5.1 sigma detection is very confident.

They found one event at 5.1-sigma (29 and 36 solar masses) and another weaker candidate (13 and 23 solar masses) at 2-sigma in 16 days of data. There are more than two months of data from Observing Run 1, which started on September 12 and ended in January 12, that have not been analyzed. So, stay tuned for more details from the black hole world!

Papers: The LIGO collaboration (~ 1000 people) wrote 12 technical articles that report on this event and on the detailed investigation that followed. I originally wanted to write more about each them, but I am letting Andy do it because he understands them better. What I want to re-emphasize is that there are two potential events. The second one is described in the "First results from the search for binary black hole coalescence paper with Advanced LIGO" paper, which is where I have the table from.

LISA Pathfinder: Another success. As pointed by Leo Stein in the previous post, LISA Pathfinder is at the Lagrange point L1. The pull of the Earth and the Sun are equal there. It reached its destination on January 22, and just yesterday (February 17) the second test mass has been successfully released. The purpose of the Pathfinder mission was to test LISA technology, which will be our first human-built, space-based gravitational wave detector. LISA will target a lower frequency band in the gravitational wave sky where supermassive black holes from the centers of galaxies merge. It will also see white dwarfs and neutron star binaries. It is planned to be launched by the European Space Agency in 2034. We hope that the US will re-join the mission and bring back its third arm, which was lost when descoping to fit in a smaller budget. My son, Edward, drew LISA with all three arms in his and David's most recent book. They were so excited by this detection that they decided to introduce the topic of gravitational waves to children (and parents) everywhere.