The Advanced LIGO Project, a major upgrade that will increase the sensitivity of the Laser Interferometer Gravitational-wave Observatories instruments by a factor of 10 and provide a 1,000-fold increase in the number of astrophysical candidates for gravitational wave signals, was officially dedicated in a ceremony on May 19 at the LIGO Hanford facility in Richland, Washington.
This upgrade was based on several improvements including improved test mass mirrors, says Associate Professor of Physics Steven Penn, who played a major role in the design of the mirror substrate and coating.
“Mirrors may sound mundane, but for an interferometer that is trying to sense relative length distortions on the order of 10^(-20) m, the mirror is critical to our observations in the central region of our sensitivity,” Penn says. “Our research remains focused on this mirror research because it continues to be a limiting factor for future detectors that are currently being designed.”
A member of the LIGO Science Collaboration Council, Penn has served on the APS Topical Group on Gravity, the National Science Foundation Grant Review Panel for Experimental Gravity, and the LIGO Science Collaboration Publication & Presentation Committee.
LIGO was designed and is operated by Caltech and MIT, with funding from the National Science Foundation (NSF). Advanced LIGO, funded by the NSF with important contributions from the UK Science and Technology Facilities Council (STFC), the Max Planck Society of Germany, and the Australian Research Council (ARC), is now being brought online, with the first searches for gravitational waves planned for the fall of 2015.
The dedication ceremony featured remarks from speakers including Caltech president Thomas F. Rosenbaum, the Sonja and William Davidow Presidential Chair and professor of physics; Professor of Physics B. Thomas Soifer, the Kent and Joyce Kresa Leadership Chair of Caltech’s Division of Physics, Mathematics and Astronomy; Kirk Kolenbrander, MIT vice president; and France Córdova, director of the National Science Foundation.
“We’ve spent the past seven years putting together the most sensitive gravitational-wave detector ever built. Commissioning the detectors has gone extremely well thus far, and we are looking forward to our first science run with Advanced LIGO beginning later in 2015. This is a very exciting time for the field,” says Caltech’s David H. Reitze, executive director of the LIGO Project.
“Advanced LIGO represents a critically important step forward in our continuing effort to understand the extraordinary mysteries of our universe,” says NSF director Córdova. “It gives scientists a highly sophisticated instrument for detecting gravitational waves, which we believe carry with them information about their dynamic origins and about the nature of gravity that cannot be obtained by conventional astronomical tools.”
Predicted by Albert Einstein in 1916 as a consequence of his general theory of relativity, gravitational waves are ripples in the fabric of space and time produced by violent events in the distant universe-for example, by the collision of two black holes or by the cores of supernova explosions. Gravitational waves are emitted by accelerating masses much in the same way as radio waves are produced by accelerating charges, such as electrons in antennas. As they travel to Earth, these ripples in the space-time fabric bring with them information about their violent origins and about the nature of gravity that cannot be obtained by other astronomical tools.
Although they have not yet been detected directly, the influence of gravitational waves on a binary pulsar system (two neutron stars orbiting each other) has been measured accurately and is in excellent agreement with the predictions. Scientists therefore have great confidence that gravitational waves exist. But a direct detection will confirm Einstein’s vision of the waves and allow a fascinating new window into cataclysms in the cosmos.
LIGO was originally proposed in the 1990s as a means of detecting these gravitational waves. Each of the 4-km-long L-shaped LIGO interferometers (one each at LIGO Hanford and at the LIGO observatory in Livingston, Louisiana) use a laser split into two beams that travel back and forth down long arms (which are beam tubes from which the air has been evacuated). The beams are used to monitor the distance between precisely configured mirrors. According to Einstein’s theory, the relative distance between the mirrors will change very slightly when a gravitational wave passes by. The original configuration of LIGO was sensitive enough to detect a change in the lengths of the 4-km arms by a distance one-thousandth the size of a proton; Advanced LIGO, which will utilize the infrastructure of LIGO, will be 10 times more sensitive.
“To achieve this improvement, we took many lessons learned from initial LIGO, put them together with the results of worldwide R&D, and made a complete redesign and replacement of the detectors,” says David Shoemaker of MIT, the project leader for Advanced LIGO.
Included in the upgrade were changes in the lasers (180-watt highly stabilized systems), optics (40-kg fused-silica “test mass” mirrors suspended by fused-silica fibers), seismic isolation systems (using inertial sensing and feedback), and in how the microscopic motion (less than one billionth of one billionth of a meter) of the test masses is detected.
The change of more than a factor of 10 in sensitivity also comes with a significant increase in the sensitive frequency range and the ability to tune the instrument for specific astrophysical sources. This will allow Advanced LIGO to look at the last minutes of the life of pairs of massive black holes as they spiral closer, coalesce into one larger black hole, and then vibrate much like two soap bubbles becoming one. It will also allow the instrument to pinpoint periodic signals from the many known pulsars that radiate in the range from 500 to 1,000 Hertz (frequencies that correspond to high notes on an organ).
Advanced LIGO will also be used to search for the gravitational cosmic background-allowing tests of theories about the development of the universe only 10-35 seconds after the Big Bang.
LIGO research is carried out by the LIGO Scientific Collaboration (LSC), a group of some 950 scientists at universities around the United States and in 15 other countries. The LSC network includes the LIGO interferometers and the GEO600 interferometer, a think thank and test bed for advanced detector techniques. GEO600 is located near Hannover, Germany, and designed and operated by scientists from the Max Planck Institute for Gravitational Physics and Leibniz Universität Hannover, along with partners in the United Kingdom funded by the Science and Technology Facilities Council (STFC). The LSC works jointly with the Virgo Collaboration-which designed and constructed the 3-km-long Virgo interferometer located in Cascina, Italy-to analyze data from the LIGO, GEO, and Virgo interferometers.
Penn earned his B.S. and Ph.D. from the Massachusetts Institute of Technology before joining the HWS faculty in 2002. He has taught previously at the University of Washington and Syracuse University, where he is currently an adjunct research professor. In 2005, he was elected to the Executive Board of the APS Topical Group on Gravitation and, in 1992, received the Karl Taylor Compton Award for Overall Excellence. He has been invited to give lectures around the world and has published in venerable science journals, including Classical and Quantum Gravity, Astrophysical Journal, Physical Review Letters and Review of Scientific Instruments.
The top photo features Associate Professor of Physics Steven Penn working in his lab at the beginning of the LIGO research project in 2008, other photos feature him teaching on campus in the past several years.