On February 11, 2016, LIGO (Laser Interferometer Gravitational-Wave Observatory) announced that for the first time "scientists have observed ripples in the fabric of space time called gravitational waves, arriving at the earth from a cataclysmic event in the distant universe." The announcement continued with a note. This observation "confirms a major prediction of Albert Einstein's 1915 general theory of relativity and opens an unprecedented new window into the cosmos. (LIGO-Caltech, 2017)
The general theory of relativity "describes space-time as a kind of sagging mattress where matter and energy, like a heavy sleeper, distort the geometry of the cosmos to produce the effect we call gravity, obliging light beams as well as marbles and falling apples to follow curved paths through space." (Overbye, 2015) The distortion of the fabric of space-time generates waves that ripple through space-time at the speed of light, like the ripples on a pond. As the ripples pass through matter, they cause the distance between its atoms to increase vertically and stretch horizontally in a rhythmic dance. (Max-Planck-Institute, 2017)
Einstein himself was highly skeptical of whether gravity waves could ever be detected because the displacement of atoms would be very tiny (on the order of 10^(-18) meter) even if caused by the acceleration of huge masses. But when on August 14, 2015, at 5:15 a.m. the LIGO Instruments registered a gravity wave and the scientists heard the "chirp" caused by the collapse of two black holes 1.3 billion years ago, a half-century of scientific and engineering work had paid its dividend. (LIGO_Chirp, 2015)
The quest to build a device to detect gravity waves was initiated in 1972 by Rainer 'Rai' Weiss who was an associate professor teacher courses in physics at MIT when he had the insight that led to the creation of LIGO. Rai Weiss recalled that the project began as he tried to solve a teaching problem:
"I said to myself, 'What's the simplest thing I can think of to show these students that you could detect the influence of a gravitational wave?...The obvious thing to me was, let's take freely floating masses in space and measure the time it takes light to travel between them. The presence of the gravitational wave would change that time. [Later] knowing what you could do with lasers, I worked it out: Could you actually detect gravity waves this way? And I came to the conclusion that yes, you could..." (LIGO-Caltech, 2017)
Weiss shared this insight with Kit Thorne, a highly respected theoretical physicist from Caltech. Thorne saw that the benefits of the project more than balanced its considerable risk that it really couldn't be done and became an advocate for solving the detection problems to others in the scientific community.
In 1989 Weiss and Thorne submitted a proposal to the National Science Foundation that included the concept as well as the engineering and technological innovations that had developed since 1972. The NSF provided funding.
Barry Barish from Cal tech now joined Weiss and Thorne to become the LIGO Principal Investigator. He brought the project management and engineering knowledge necessary to bring about the actual construction of LIGO detectors as well as building the complex collaboration of scientists and engineers from around the world. (LIGO-Caltech, 2017)
Construction began in 1994 on the twin LIGO detectors, one in Livingston, Louisiana, and one in Hanford, Washington.
The technology used by LIGO is called an interferometer because it works by merging two or more sources of light to create an interference pattern and is capable of making very small measurements such as those needed to detect gravity waves. The LIGO interferometers are capable measurements as small as 1/10,000 of a proton!
How it works. A laser beam is split into two identical beams by the beam splitter. Each beam travels down one of the two arms, strikes the mirror and is reflected back to the splitter where the reflected beams are merged back to a single beam and sent to the detector.
When the beams were split, each beam was identical with the other; that is, the light waves matched perfectly. If the distance or either mirror alters by even a very small distance, the returning beams will no longer match and the amount of movement can be calculated from the amount of displacement of the merged beams.
The picture shows LIGO Livingston site. The building at the center contains the laser source and the beam splitter that sends laser light up each of the two 4 km long tunnels. At each end of the tunnels are precisely positioned mirrors and at the other is building at the vertex where the laser source, the beam splitter and the detector are located.
As a gravitational wave moves through the earth, “it will actually stretch and shrink it, thereby moving the mirrors ever so slightly and changing the distance the laser beam travels— by only 1/1,000th of the diameter of a proton. Even this very slight movement will create a different interference pattern in the light beam when it reaches the detector, and signaling that a gravitational wave has passed. Using that minute change, scientists are further able to identify the wave’s source and very broadly where in the universe it originated.” (CalTech, 2016)
Humans have observed the skies for thousands of years. The ongoing development of the technology of the telescope allowed us to gather ever-more data from the light collected by its lenses.
The LIGO scientists argue that the successful detection of gravity waves provides a new way to collect information about the universe because gravitational waves also carry information about the motions of objects in the universe. Unlike light energy which can be blocked by objects, distorted and absorbed by matter (such as dust in space or in earth’s atmosphere), the universe is transparent to gravitational information. “Gravitational waves will usher in a new era in astronomy...Humans will be able to observe astrophysical objects that would have otherwise been obscured, as well as the inner mechanisms of phenomena that do not produce light.” (LIGO-Caltech, 2017)
2017 Nobel Prize in Physics:
For the development of the scientific knowledge used to build the LIGO, Rainer Weiss, Kip Thorne and Barry Barish received the 2017 Nobel Prize in Physics. The $1 million prize money split between the three, with Rainer Weiss awarded half the prize money, and Thorne and Barish each received one-quarter of the money.
CalTech. (2016). Fact Sheet NSF and the Laser Interferometer Gravitational-Wave Observatory. Retrieved from https://www.ligo.caltech.edu/system/media_files/binaries/300/original/ligo-fact-sheet.pdf
LIGO_Chirp. (2015). Retrieved from https://www.youtube.com/watch?v=egfBaUdnAyQ&feature=youtu.be
LIGO-Caltech. (2017). LIGO Scientific Collaborative. Retrieved from http://www.ligo.org
Max-Planck-Institute. (2017). The rhythm of geometry. Retrieved from http://www.einstein-online.info/elementary/gravWav/rhythm
Overbye, D. (2015c). A Century Ago, Einstein’s Theory of Relativity Changed Everything. The New York Times. Retrieved from https://www.nytimes.com/2015/11/24/science/a-century-ago-einsteins-theory-of-relativity-changed-everything.html?action=click&contentCollection=Science&module=RelatedCoverage®ion=Marginalia&pgtype=article
Gravity illustration: University of Pittsburg, HPS 0410, Einstein for Everyone, http://www.pitt.edu/~jdnorton/teaching/HPS_0410/chapters/general_relativity_massive/
Rainer Weiss: Masschusetts Institute of Technology, http://web.mit.edu/physics/people/faculty/weiss_rainer.html
Kip Thorne: California Institute of Technology, https://blogs.chapman.edu/wilkinson/2016/05/17/recapping-kip-thornes-lecture-at-chapman/
Barry Barish: California Institute of Technology, https://labcit.ligo.caltech.edu/~BCBAct/
Interferometer diagram: https://www.ligo.caltech.edu/page/ligo-gw-interferometer
LIGO at Livingston, Louisiana: https://www.ligo.caltech.edu/LA/page/what-is-ligo
Dr. John Holton
Dr. John Holton joined the S²TEM Centers SC in July of 2013, as a research associate with an emphasis on the STEM literature including state and local STEM plans from around the nation.