Laser Interferometer Gravitational-wave Observatory
By: Don Mock firstname.lastname@example.org
According to Einstein’s theory, from back in 1915, gravitational waves should be out there, everywhere, pervading our existence, giving us a gentle stretch and squish as they constantly pass through us from every direction, traveling at the speed of light. The problem with verifying their presence is that they are so incredibly subtle that they verge on the imperceptible. Even Einstein despaired of ever detecting them. By his calculations, even relatively large gravitational waves would distort the space around us by only one part in 1,000,000,000,000,000,000,000 (10^21). That’s like detecting a fluctuation in the distance between Seattle and San Francisco equivalent to the width of a proton. How in the world could one do that?
Enter LIGO, the Laser Interferometer Gravitational-wave Observatory, a decades-long, NSF-funded effort by a consortium of 83 institutions and over 1000 researchers to measure the seemingly immeasurable, and in the process prove Einstein right. An interferometer works by splitting a beam of laser light and sending it down two perpendicular arms. Each beam then bounces off an end mirror and returns to the beginning where the two beams can be compared. If the lengths of the two perpendicular arms are equal, then the light waves – a train of peaks and troughs – will be exactly in sync.
The trick to LIGO is that the perpendicular arms are 4 km long and the laser beam is made to bounce down the arms and back 280 times before the comparison is made. That’s equivalent to the arms being over a million meters long. The wavelength of the light used is 0.000001064 meters, and the detector that compares the two laser beams can see differences between them down to one billionth of a wavelength. The resultant sensitivity is the desired one part in 10^21. However, vibrations from various local sources, including minor earthquakes or trucks rumbling by, can contaminate the results. So LIGO compares the detector outputs from two interferometers separated by 3000 km, a distance which takes 10 milliseconds to travel at the speed of light. Thus, only signals that show up in both detectors within 10 milliseconds of each other are candidates for being gravitational waves.
For thirteen years, LIGO searched for gravitational waves without success. Then on September 14, 2015, after an upgrade that improved its signal to noise ratio, LIGO unmistakably detected the far distant collision of two black holes, each ~30 times as massive as our own Sun. From the signal pattern it was determined that the two black holes had gone into a death spiral before coalescing into each other, making a single, heavier black hole. The time delay between detection at the two interferometers pointed to a source some 1.3 billion light years away, from a direction located above the southern hemisphere. Einstein was right!
The University of Florida has been a proud contributor to LIGO’s success, as a charter member of the effort, along with MIT and Caltech. UF scientists designed the critical input optics for the interferometers – everything between the laser and the mirrors of the core interferometer. The input optics modulate and condition the laser light to provide an ultra pure beam, with a minimum of frequency, intensity, and geometric fluctuations. UF also supplied the data analysis algorithm that made the initial discovery of the September 14th event. Within the UF physics department, Drs. Hai-Ping Cheng, Sergey Klimenko, Guenakh Mitselmakher, Guido Mueller, David Reitze, David Tanner, and Bernard Whiting, along with many of their students and postdocs over the last 20 years, have been involved in the LIGO project. Several of the faculty members have also held key leadership positions as part of LIGO, most significantly Dr. Reitze, who is currently on leave from UF while he serves as the project’s executive director at Caltech.
Next up for LIGO is to add a third interferometer in India, which will help pinpoint the source of observed events more precisely. In the longer term, a space-based version of LIGO, in solar orbit, is in the works. Since gravitational waves travel through all types of matter, it is hoped that we will eventually be able to observe and map stellar objects in, say, the galactic core, that are currently hidden from more traditional imaging techniques which rely upon the electromagnetic spectrum. Thus, observing gravitational waves is becoming a fundamentally new way of “seeing” the universe. Thank you, Einstein!