In his Eaton Hall laboratory, Associate Professor of Physics Steven Penn welds a
coating sample for a future LIGO detector.


How an HWS Professor’s Research Aided the Historic Discovery of Gravitational Waves

by Steven Bodnar

Einstein was right.

One hundred years after publishing his general theory of relativity, which predicts the existence of gravitational waves, an international team of scientists, including Associate Professor of Physics Steven Penn, confirmed Einstein’s postulation in a breakthrough discovery.

On Sept. 14, 2015, gravitational waves – ripples in the fabric of space-time – were observed by scientists at the Laser Interferometer Gravitational-wave Observatory (LIGO), two synchronized monitoring facilities comprised of two 4-km long interferometers that are arguably the most precise instruments ever built. The detection, which is the result of decades of painstaking research, has opened a new window to the cosmos.

“I have waited a long time for this day,” said Penn, an MIT-trained physicist who has devoted nearly two decades of research with the LIGO Scientific Collaboration (LSC), the organization that directs LIGO research. The historic news became public on Feb. 11, 2016, when the National Science Foundation (NSF) and scientists from Caltech, MIT and others from the LSC, convened at the National Press Club to make the first announcement about the detection.

The headlines that ensued were epic. CNN declared “The Holy Grail of modern physics;” The Washington Post reported a “cosmic breakthrough,” and Scientific American called it “a new era of astrophysics.” A discovery so important it marks a new frontier of astronomy and physics – a new beginning, said Penn, a co-author on the Physical Review Letters article demonstrating the discovery.

Following the detection and after careful analysis, physicists concluded that the gravitational waves were produced some 1.3 billion years ago when two black holes about 29 and 36 times more massive than the sun collided at one-half the speed of light to form a single black hole. In less than a second, about three times the sun’s mass was converted into gravitational waves, with a peak-power output about 50 times that of the entire visible universe.

“This detection is the beginning of a new era: The field of gravitational wave astronomy is now a reality,” said Gabriela González, LSC spokesperson and professor of physics and astronomy at Louisiana State University.

Since that first historic discovery, LIGO scientists recently made a second landmark detection of gravitational waves, also produced by a pair of colliding black holes some 1.4 billion years ago. Findings from the new observation show the second set of black holes were far less massive than the first detected pair. The discovery puts scientists on a promising path forward to mapping out black holes in the universe.

The HWS Connection

Through his work, Penn has significantly contributed to the mirror substrate and coating design for the LIGO detectors, which Caltech’s David H. Reitze, executive director of the LIGO Laboratory, describes as the most precise measuring device in the world. Penn discovered how to significantly reduce the thermal noise in the material fused silica, which led to the selection of fused silica for the Advanced LIGO mirror substrates and suspensions. With Advanced LIGO, the newly completed upgrade to the Initial LIGO detectors, scientists will be able to increase the amount of the universe that can be probed by a thousandfold.

“Our first observation of gravitational waves has been incredibly exciting, and it justifies all our hard work on Advanced LIGO,” Penn said. “We would never have seen this event with Initial LIGO.”

Conceived, built and operated by Caltech and MIT, the LIGO Observatories are funded by the NSF, along with significant support from the Max Planck Society, Science and Technology Facilities Council and Australian Research Council.

Each LIGO observatory consists of a two-and-a-half-mile (4-km) long L-shaped interferometer in which laser light is split into two beams that travel back and forth down the arms (four-foot diameter tubes kept under a very low vacuum). The beams are used to monitor the distance between mirrors precisely positioned at the end of each arm. The distance between the mirrors, according to Einstein’s theory, will change by an infinitesimal amount when a gravitational wave passes by the detector.

“LIGO observes gravitational waves by reflecting light off the interferometer mirrors to precisely monitor their location,” Penn said. “The dynamics of that reflection are incredibly important if you want to know the mirror position to within one-ten-thousandth the diameter of a proton (10-19 meter). That fundamental interaction – of the light with the mirror coating – is the focus of our research at HWS. It is one of the places where the rubber meets the road in this experiment because coating noise is a primary limit to Advanced LIGO’s design sensitivity.”

In 1998, astrophysicist Yuri Levin showed that thermal noise from the mirror coatings would limit the sensitivity of Advanced LIGO. His work launched LIGO’s trailblazing research in coating thermal noise. Penn was part of the team that originally determined the source of the coating noise and that later developed the coating that would be used for Advanced LIGO.

“The Advanced LIGO detectors are a tour de force of science and technology, made possible by a truly exceptional international team of technicians, engineers and scientists,” said David Shoemaker of MIT, the project leader for Advanced LIGO.

The LIGO detectors operate at room temperature, or 300 degrees K above absolute zero. Thus the mirrors, and everything else, are hot — the mirror molecules are moving vigorously and causing the mirror to vibrate like a cylinder of jelly with a tiny amplitude. Nevertheless, the amplitude of the mirror’s thermal excitations is 10 million times larger than Advanced LIGO’s length sensitivity. That is equivalent to measuring sea-level to the accuracy of a hair width during a tsunami — a seemingly impossible challenge.

In order to overcome this problem, Penn and his colleagues develop mirrors that are made from materials that are almost perfectly elastic. A perfectly elastic material will oscillate and never dissipate any of its energy. More importantly a perfectly elastic mirror will move from thermal excitations, but all its motion will be confined to a very narrow resonant frequency.

“Like a crystal wine glass, the ideal mirror rings at a single pure note,” said Penn. “If the mirrors move only at their resonant frequency, then everywhere else, they are quiet, and that’s where we look for gravitational waves.”

The direct observation of gravitational waves validates the research that has already begun on the third generation detectors. “Coating thermal noise has been an intractable problem, but one that we must solve for the next generation of detectors in which we anticipate frequent detections,” Penn said.

The Colleges became one of the first small liberal arts institution to participate in LIGO when Penn joined the HWS faculty in 2002. Since then, he has conducted much of his part of the monumental project from his Eaton Hall laboratory where HWS student researchers have been welcomed over the years.

Currently, Penn chairs the LSC’s Coating Working Group, a subcommittee of the Optics Working Group, which is developing coatings for future detectors. In addition to HWS, the Coating Group is composed of research groups from the University of Glasgow, Stanford University, American University, University of Florida, University of the West of Scotland, Caltech, University of Sannio, Whitman College, Cal State Fullerton, National Tsing Hua University and Embry-Riddle Aeronautical University.

For their incredible work, LSC scientists have received $3 million from the Breakthrough Prize in Fundamental Physics to further pursue the exploration of gravitational waves. In addition, the prestigious Kavli Prize in Astrophysics was awarded to Rainer Weiss, professor of physics, emeritus, from MIT; Kip Thorne, Caltech’s Richard P. Feynman Professor of Theoretical Physics, emeritus; and Ronald Drever, professor of physics, emeritus, also from Caltech, who originally proposed LIGO as a means of detecting gravitational waves in 1980s.

“Our observation of gravitational waves accomplishes an ambitious goal set out over five decades ago to directly detect this elusive phenomenon and better understand the universe, and, fittingly, fulfills Einstein’s legacy on the 100th anniversary of his general theory of relativity,” Reitze said at the time of the February 2016 announcement.

The Pathway to Discovery

In seeking out gravitational waves, scientists during the early 1990s took their research to a united front forming the LSC.

“In 1992, when LIGO’s initial funding was approved, it represented the biggest investment the NSF had ever made,” said France Córdova, NSF director. “It was a big risk. But the National Science Foundation is the agency that takes these kinds of risks. We support fundamental science and engineering at a point in the road to discovery where that path is anything but clear. We fund trailblazers. It’s why the U.S. continues to be a global leader in advancing knowledge.”

More than 90 universities and research institutes in the LSC develop detector technology and analyze data; approximately 250 students are strong contributing members of the collaboration. The LSC detector network includes the LIGO interferometers and the GEO600 detector. The GEO team includes scientists at the Max Planck Institute for Gravitational Physics and Leibniz Universität Hannover, along with partners at the University of Glasgow and Cardiff University, among others.

Virgo research is carried out by the Virgo Collaboration, consisting of more than 250 physicists and engineers belonging to 19 different European research groups.

Independent and widely separated observatories are necessary to determine the direction of the event causing the gravitational waves, and also to verify that the signals are coming from space and are not from some other local phenomenon.

Toward this end, the LIGO Laboratory is working closely with scientists in India to establish a third Advanced LIGO detector on the Indian subcontinent. And the LSC works closely with the Kagra collaboration that is building the first underground and cryogenic gravitational wave interferometer in the Kamioka mine in Japan.

Looking toward the future, the LIGO and Virgo collaborations hope to make more and more frequent observations of black hole and neutron star binary inspirals as Advanced LIGO improves to full sensitivity and Advanced Virgo come on line next year. Penn and his colleagues are already performing research on the design for the next two generations of detectors. “Ultimately we hope to see these inspiral events throughout the entire observable universe,” he said. “Understanding the black hole and neutron star populations will teach us a great deal about stellar evolution.”

The detection of gravitational waves has been described as one of the ‘Holy Grails’ of modern physics and astronomy.

“Within that search for the ‘Holy Grail,’ there is another Holy Grail. At the beginning of the universe – at the earliest moments of the Big Bang – there was a lot of mass-energy creation and a copious production of gravitational waves,” said Penn during the Colleges’ public presentation coinciding with the international announcement. “We should – if we build detectors sensitive enough – be able to see that background of gravitational waves, and thus, have a picture of the early universe at a moment when it was less that 10-10 seconds old. With electromagnetic telescopes – detectors that observe various forms of light – we can only see back to a few hundred thousand years after the Big Bang. With a future detector we may gain a glimpse back to the very moment of our universe’s creation.”



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