3.2 LIGO’s Success
LIGO’s sensitivity has improved significantly since 2009
- During LIGO’s runs from 2002 to 2009 it made no detections. This was considered a good thing because, at the sensitivity levels it could achieve at the time, it was very unlikely to detect gravitational waves.
- From 2009 to 2015 upgrades to LIGO improved its sensitivity greatly, especially in the low-frequency range.
- One of the biggest contributors to the increase in sensitivity was the installation of active isolation systems. These systems measure local seismic motion and use feedback to control motors that work to counteract that motion. The principle is similar to that employed by noise-cancelling headphones.
Confirming a real detection
- One of the most basic criteria for a real detection is that the observed signal must be present at both of LIGO’s sites.
- The two signals cannot be recorded more than 10ms apart. This is because a gravitational wave traveling at the speed of light from Livingston to Hanford would take 10ms to make the trip if it was moving in a straight line through the two locations.
- Both sites also have numerous environmental sensors that measure local seismic activity, atmospheric activity, and electrical activity. Any detected signals should not look like an environmental signal.
- Detector diagnostics should be completely clean during any suspected detection event, otherwise the signal cannot be trusted.
The first detection of a gravitational wave
- The signal detected on September 14, 2015 met all of the above criteria and was determined to be a real gravitational wave.
- The signal was detected in Louisiana first, followed by Washington 7ms later. This indicated generally that it came from the south and traveled very close to (if not at) the speed of light.
- LIGO uses a number of filters to clean up incoming data, but the first detected signal was so strong that it was clearly visible even with minimal filtering.
- The source of the signal was reconstructed both analytically as well as with numerical relativity. Numerical relativity is a field of physics that used supercomputers to evolve the Einstein field equations.
- Different waveforms—each corresponding to a specific set of source parameters—were created using numerical relativity and compared to the detected signal to look for similarities, a technique called matched filtering.
- This led to the conclusion that the source was a pair of black holes, each about 30 solar masses, rapidly orbiting each other and finally merging into a single new black hole. The relative velocities of the black holes were nearly 0.6c just before merging.
Searching for more gravitational waves and concurrent signals
- After the successful detection, astronomers using other types of tools, like EM telescopes and neutrino detectors, checked the regions of sky that the gravitational wave may have come from for other concurrent signals.
- Almost all of the follow-up searches found nothing, and the one possible gamma-ray detection is suspect. However, future gravitational wave detections could be followed up on more effectively, especially as more detectors are built and source localization improves.
- A number of gravitational wave sources, like merging neutron star binaries or supernovae, are expected to produce EM, neutrino, or cosmic ray signals as well.