Some LIGO Background Information for my Graduate Students
(Quickly penned by M. Holst on evening of February 10, 2016)

As we discussed earlier today, there is a pretty well-confirmed rumor that apparently there has been an unambiguous gravitational wave (GW) detection by the NSF-funded LIGO. The rumor is that both LIGO devices have independently detected a clear binary black hole (or similar) collision and ring-down, and in fact some have said there may have been multiple detections. If true, this is truly astounding in several respects!

Exactly 100 years ago Einstein (and Hilbert) formulated the equations of general relativity as a geometric description of what we experience as gravity, displacing Newton's theory of gravity. Early on it was realized that the equations, which are fundamentally wave equations (with constraint equations along for the ride), predict that gravitationally interacting bodies will emit radiation at the speed of light that will carry energy away from the interaction. Unlike other types of radiation, this radiation is actually a "ripple" in space (and time). Even though the sun and the planets in our solar system are "large" from our perspective, their masses and their relative distances are such that radiation emitted by their gravitational interactions is too small to be detected by any existing technology, or for that matter, any technology we might have in the foreseeable future. However, much more massive interactions that happen at relatively close ranges (such as in-spiraling and colliding black holes) do, at least in theory, emit enough radiation that it could potentially be detected by a device that we could build using late 20th/early 21st century technology. Why would we care to build such a device? First, it would be a completely new test of Einstein's theory of general relativity. Second, it would be the first direct evidence for the existence of gravitational radiation. (The 1993 Hulse/Taylor Nobel Prize was for the discovery of indirect evidence for gravitational radiation, through observations of binary pulsars). Third, it would initiate a complete revolution in astrophysics and astronomy, where gravitational wave detectors become our long-range astronomical telescopes. It would allow us to begin to test a number of hypotheses in cosmology, and open up completely new directions in physics. If you have seen the recent film "Interstellar", my understanding is that the premise for the original version of the screenplay (co-written by Kip Thorne, one of the three LIGO PIs) was a gravitational wave scientist (the hero of that version of the story) detecting a signal in a gravitational wave that was inserted by an alien race as a means of making first contact with us from very far away; sadly, this cool idea did not survive into the final version of the screenplay.

Beginning in the 1960s, physicists began to seriously study the feasibility of building gravitational wave detectors. As early as 1962, papers were being published on various ideas that might be workable; the primary technologies that surfaced were bar detectors (Joseph Weber, 1960s) and laser interferometers (Rainer Weiss, 1970s). In the 1980s and early 1990s, NSF began investing in research on detectors, including the construction of prototypes. In 1992, Caltech (Kip Thorne and Ron Drever) and MIT (Rainer Weiss) formulated the LIGO (Laser Interferometer Gravitational-Wave Observatory) Project, and successfully lobbied for a (very) large NSF Award to build two full-size L-shaped laser interferometer-type detectors at opposite ends of the continental US. This would place the detectors about 1800 miles apart and on different tangent planes to the earth. The detectors are massive, with each arm of the L-shaped device being 4 km long, and with mirrors that effectively extend the path of the internal lasers to thousands of kilometers. One of the detectors (called H1) is located in Hanford, Washington (near PNNL), and the other (called L1) is located in Livingston, Louisiana (not far from LSU).

The LIGO project, initiated in 1992, has become the most ambitious and expensive project that NSF has ever undertaken; to date, the project has exceeded $600M. The engineering requirements for success are truly mind-boggling: to see an event such as the type of black hole or neutron star collision that is likely to occur close enough to the earth to be detected, yet large enough to generate enough radiation to be detected, and still occur frequently enough to be seen in a detection window of weeks or months, requires building a device that can detect distortions in the 4 km spacing between the LIGO mirrors on the order of 10^{-18} meters. This distance is substantially smaller than e.g. the charge radius of a proton.

The two LIGO devices were made operational by 2002, and the first science runs were done from 2002 to 2010. Some "Enhanced LIGO" upgrades made at one point during that first science run, due to substantial technology improvements. However, by 2010 there were still no reported GW detections. In 2010, the detectors were shut down, and a $200M upgrade was made (the initial LIGO NSF Award was about $400M), producing the Advanced LIGO devices. These devices became operational in 2015, and their sensitivity is approximately 4 times the sensitivity of the initial LIGO devices, with much greater range. The rumor is that very soon after the devices went online in Fall 2015, there were unambiguous gravitational wave detections (perhaps not realized until much later when the data was analyzed). This is what the press conference at 10:30am EST Thursday morning will likely discuss. It has probably taken months to sort through all of the data, compare to numerical simulations to nail down what it is that was detected, and to do all of the statistical and error analysis to build a complete scientific case for the detection. Here are some links at NSF and at the LIGO project website about the press conference on Thursday, February 11, 2016: Added Thursday Following the Press Conference: As you all heard, the rumor was confirmed by NSF and the LIGO Scientific Collaboration leadership, and the discovery is pretty much as I outlined above. On 14 September 2015, both detectors nearly simultaenously registered a clear, loud, and violent inspiral, collision, merger, and ringdown of a binary black hole pair, each of which had a solar mass in the range 10-50, with roughly the equivalent of three solar masses in energy released as gravitational radiation. This radiation traveled outward from the collision at the speed of light, reaching the LIGO detectors on earth roughly 1.3 billion years later. What was detected is an incredibly close match to computer simulations of wave emission from this type of binary collision, produced through very detailed numerical simulations of the full Einstein equations. This is exactly what they have been hoping to see; this is truly amazing. Here is a link to the technical article describing the work, which was posted to the journal website when the press conference began.