4.2 The First Detection

summary

LIGO made a successful detection on September 14, 2015

Both Advanced LIGO detectors were online and each registered a signal that turned out to be from a binary black hole collision 1.3 billion light-years away.
The wave hit the Louisiana observatory first and reached Washington 7ms later, providing a rough idea of where the wave came from.
The light reaching the photodetector oscillated between bright and dim as the two path lengths changed. Essentially the peaks and troughs of each laser beam came into and out of alignment, causing the intensity of the combined beam to fluctuate.
>The signals at each detector line up perfectly, clearly indicating that the detection was not an anomaly. The maximum strain detected was 10^-21, resulting in path length changes of about 4×10^-18m.
The signal was a chirp, growing in both frequency and amplitude over time. This is because the black holes orbited each other with increasing speed and frequency as they started to collide.

Reconstructing the source from the signal

The changes in frequency over time allowed LIGO scientists to determine the masses of the two black holes. Each was about 30 times as massive as the Sun.
The change in amplitude over time revealed the distance between the two black holes, as well as orientation of their orbit with respect to Earth.
The decay of the signal after the peak of the “chirp” informed the LIGO team about the mass of the final black hole, as well as its rotation speed.
The newly formed black hole was missing 3 solar masses compared to the original pair. This mass was released as energy in the gravitational wave. In the brief instant that the black holes collided, the power output of the gravitational wave was greater than that of all the stars shining in the universe.

The start of a new era in astronomy

LIGO’s detection was the first ever direct observation of gravitational waves. It was also the first direct observation of a binary black hole system.
The detection was also the first test of general relativity in a very strong gravitational field.
Most importantly, this achievement marks the opening of a new window onto the universe. Every time astronomers have turned on a new method of observation they have discovered new, sometimes revolutionary things about the universe.

The future of LIGO

The immediate goal of LIGO is to complete all of the Advanced LIGO upgrades. Once that is finished there are plans to push LIGO’s capabilities even further with more improvements.
Partner observatories will also start to come online, increasing the ability of astronomers to watch the skies with this new technology.
Other ways to measure gravitational waves are on the horizon, including pulsar timing arrays and space-based interferometers, which will allow astronomers to search for gravitational waves in frequency ranges that LIGO can’t reach.