At 9:50 UTC on the morning of Sep 14, 2015, the two detectors of the Laser Interferometer Gravitational Observatory (LIGO), one in Louisiana and the other in Oregon, simultaneously observed the distinct signal of a possible gravitational wave. The signal swept upward in frequency from 35 Hz to peak at 250 Hz, exactly matching physicists’ predictions for the inspiral, collision, merger and ringdown of a pair of black holes
“We were aware of it the following morning,” Dr. Brennan Hughey said, speaking for himself as well as Drs. Andri Gretarsson and Michele Zanolin. “I was among many people working to validate the signal… only LIGO and VIRGO members knew about it beforehand.
“To be clear, we’ve had lots of events,” explained Dr. Andrei Gretarsson, “but that doesn’t mean they’re gravitational waves.”
“We constantly analyze the data. If there is an interesting event, we want to be sure,” said Dr. Zanolin, pointing out how, the previous year, a similar project called BICEP II announced a detection only to retract it shortly thereafter. “This can be embarrassing,” Zanolin said, “and we don’t want it to happen again.”
After carefully reviewing the data, LIGO researchers concluded that each black hole had roughly 30 times the mass of the Sun, and the collision, which converted some three solar masses to pure energy, occurred about 1.3 billion light-years away. Having survived the most meticulous confirmation process, the discovery was publically announced on the morning of Thursday, Feb 11.
Drs. Hughey, Zanolin and Gretarsson are all part of the Embry-Riddle Aeronautical University LIGO group, which is continuously supported by the National Science Foundation. “I was part of the optical design, of the coatings,” said Gretarsson. “I helped build it; I moved the mirrors many times.” (Each LIGO detector consists of two 4-km-long, perpendicular arms through which laser beams are reflected. When a gravitational wave periodically lengthens and compresses space-time along one arm, the lasers are shifted out of phase. Sub-micron precision is absolutely necessary throughout the system.)
“I was part of the team that was responsible for vetting the event,” said Dr. Hughey. “There was a detection checklist that we ran through.” After ruling out every known or suspected cause of a false alarm, Hughey’s team concluded that the anticipated false alarm rate was less than 1 in 203,000 years.
Dr. Zanolin described himself as “one of the developers that saw it for the first time,” explaining how computers are used to sort out gravitational wave candidates from among thousands of useless signals. [But] “only Brennan [Hughey] was a leader.”
While the signal observed in September and announced on Feb. 11 was emitted by a pair of inspiralling black holes in the moments before their merger, black hole binaries may not remain the only sources of gravitational waves. Dr. Zanolin currently chairs a LIGO group attempting to detect gravitational waves from core-collapse supernovae, cataclysmic events in which the cores of massive stars collapses into pulsars, black holes or other compact remnants, violently expelling the stars’ outer layers in the process. Gravitational wave astronomy gives researchers a window in the interiors of these stars, allowing them to investigate the otherwise invisible mechanics of the collapsing core.
Furthermore, in what Dr. Zanolin described as an “example of the wonderful opportunities students can have in doing research with professors at Embry-Riddle”, two Embry-Riddle students, Jasmine Gill and Marek Szczepańczyk, were also recognized as coauthors of this paper documenting this historic discovery.
“This is the beginning of a new era of physics and astronomy,” Dr. Zanolin concluded. “There will be a lot of opportunity for research in this new field.”