In March 2023, the Advanced Laser Interferometer Gravitational-Wave Observatory (LIGO) is set to begin its fourth yearlong observational period. Scientists on site in Hanford, Washington, and Livingston, Louisiana, have spent the last two years on hardware and software upgrades to increase the sensitivity of the detectors, making them capable of sensing “fainter” gravitational waves to detect more events than ever before.

At the same time, members of the Advanced LIGO team are continuously working on refinements for future observation periods years ahead. , professor of physics in the College of Arts and Sciences, was a member of the team that helped design and build the LIGO detectors.

To continue that work, Ballmer was recently awarded a $555,000 to develop technology for sensing optical cavity mismatches and actuators for suspensions for the next generation detector, a renewal of funding for detector technology for gravitational-wave astrophysics.

The award also provides support for Ballmer鈥檚 doctoral students, including Elenna Capote, who is currently on site in Hanford, helping tune the detector alignment and control systems to make sure the detector performs as designed.

鈥淭hese detectors are complicated machines with thousands of control loops keeping four main mirrors and an additional 30 suspended mirrors aligned and controlled in length to keep the light resonant,鈥� Ballmer says. 鈥淓very time you make a change, it really becomes a new detector that has to be re-tuned.鈥�

How LIGO Works

LIGO uses a pair of giant laser detectors called interferometers, located 1,900 miles apart in Hanford, Washington, and Livingston, Louisiana. Each detector contains two 2.5-mile-long vacuum arms鈥攖ubes that run perpendicular to one another. A powerful laser beam is split into two and sent down the arms. Mirrors at the end reflect the light back to where the laser beam was split. Since the arms are the same length, the light should take exactly the same time to travel to the mirror at the end of each tunnel and back. But if a gravitational wave passes through Earth, it changes the distance between the mirrors, causing the light beams to return at different times.

By comparing both beams, LIGO is able to measure the stretching of spacetime caused by gravitational waves, a seminal observation first made in 2015 with the first physical confirmation of a gravitational wave generated by two colliding black holes, nearly 1.3 billion light years away.

According to Ballmer, the higher the laser power in the 2.5-mile-long arms, the more accurately scientists can determine the motion of the arm. But the amount of laser power that can be used is currently limited by imperfections in the detectors鈥� optical system. 鈥淭he optical phase front of the laser coming back from the detector can get distorted by thermal effects in the mirrors,鈥� he says.

Innovating LIGO

Ballmer is working on a diagnostic camera that records thermal distortions in the detector, allowing scientists to determine their cause and effect. While a prototype camera was developed under a previous award, 鈥渢his continued support is for deploying that camera and miniaturizing it, making it easier to use on the site,鈥� he says.

The award also supports collaborative research with scientists at MIT to redesign the test mass suspensions for the current detectors to use heavier masses. 鈥淩andom arrival photons push the test masses around, so the heavier the test masses are, the less they move when they get randomly hit by a photon,鈥� Ballmer explains. 鈥淕oing to heavier test masses is a way to increase low frequency sensitivity.鈥�

Previous research has focused on new coatings for the mirrors. Under the current grant, Ballmer is also exploring research and development to integrate these coatings on the detector. 鈥淭he new coatings have much lower thermal noise, but do not work with some auxiliary laser frequencies in the detector. Changing the mirror coatings thus requires other changes in the detector, and so the R&D that’s going under this award is to prototype the new detector systems compatible with the new types of coatings,鈥� he says.

In addition to being used to upgrade the LIGO detectors in its fifth or sixth observation cycle, Ballmer says these developments can be used as a baseline for the next generation of detectors.

Ballmer was a principal investigator on the Cosmic Horizon Explorer Study, a project planning for the third generation of detectors, which will have 10 times the sensitivity of Advanced LIGO. The Cosmic Explorer will push the detection range of black hole and neutron star mergers out into cosmic distances. 鈥淲e will actually see mergers happening from the very first stars that formed in the universe,鈥� he says.

The 100-page study will inform next steps in NSF funding decisions on the project, which Ballmer says will likely focus on the site proposal and development of the conceptual design for the detector. 鈥淲e鈥檝e all just seen these beautiful images from the James Webb telescope showing the furthest and earliest galaxies of lights. So, with the Cosmic Explorer, if there are black hole mergers in those early galaxies, we would see them,鈥� he says.

性视界 Stefan W. Ballmer

Ballmer joined 性视界 University in 2010. Leading up to his contributions to LIGO鈥檚 Nobel Prize-winning work, he received an NSF CAREER Award in 2013 to support detector technology in the era of gravitational wave astrophysics, providing $860,000 of research funding over five years.

In October 2021, Ballmer was named a (APS), for his critical role in the design and commissioning of the Advanced LIGO detectors and the scientific interpretation of their observations, leadership in the development of third-generation gravitational-wave detectors and mentoring of the next generation of gravitational-wave experimenters.

A native of Switzerland, Ballmer has held a visiting associate professor position at the University of Tokyo; a postdoctoral fellowship at the National Astronomical Observatory of Japan; and a Robert A. Millikan Fellowship at Caltech. He earned a Ph.D. from MIT and a master鈥檚 degree from ETH Zurich in Switzerland. An aviation enthusiast, Ballmer enjoys flying in his spare time, is an instrument flight instructor and holds a commercial pilot license.

LIGO Opens New Window on the Universe with Observation of Gravitational Waves from Colliding Black Holes

性视界 University integral to detection of gravitational waves by LIGO

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For the first time, scientists have observed ripples in the fabric of spacetime called gravitational waves, arriving at the earth from a cataclysmic event in the distant universe. This confirms a major prediction of Albert Einstein鈥檚 1915 general theory of relativity, and opens an unprecedented new window onto the cosmos.

Gravitational waves carry information about their dramatic origins and about the nature of gravity that cannot otherwise be obtained. Physicists have concluded that the detected gravitational waves were produced during the final fraction of a second of the merger of two black holes to produce a single, more massive spinning black hole. This collision of two black holes had been predicted but never observed.

The gravitational waves were detected on September 14, 2015, at 5:51 a.m. Eastern Daylight Time (9:51 a.m. UTC) by both of the twin Laser Interferometer Gravitational-wave Observatory (LIGO) detectors, located in Livingston, Louisiana, and Hanford, Washington, USA. The LIGO Observatories are funded by the National Science Foundation (NSF), and were conceived, built, and are operated by Caltech and MIT. The discovery, accepted for publication in the journal Physical Review Letters, was made by the LIGO Scientific Collaboration (which includes the GEO Collaboration and the Australian Consortium for Interferometric Gravitational Astronomy) and the Virgo Collaboration using data from the two LIGO detectors.

A team of physicists from 性视界 University鈥檚 College of Arts and Sciences has been instrumental in the discovery. They include Peter Saulson, the Martin A. Pomerantz 鈥�37 Professor of Physics; Duncan Brown, the Charles Brightman Endowed Professor of Physics; Stefan Ballmer, assistant professor of physics; and a group of nearly two dozen students and research scientists.

鈥淓instein theorized that gravity was not a force, but a curvature of spacetime,鈥� Saulson says. 鈥淭hink of the two black holes, which we鈥檝e observed, as two bowling balls, rolling along a trampoline. They orbit each other because their mass produces a deep depression in the surface of the trampoline. As the balls orbit, they jiggle the trampoline鈥檚 surface, sending out energy in the form of ripples called gravitational waves.鈥�

But that鈥檚 where the analogy ends. 鈥淚n spacetime, the two black holes eventually collide with one another to form a single black hole,鈥� Saulson continues. 鈥淭he ripples from this cataclysmic event propagate through spacetime at the speed of light. They鈥檝e traveled through the universe for more than a billion years, before reaching us on September 14.鈥�

Brown is a leader in the search for black holes with LIGO. He says LIGO witnessed the two black holes colliding with one another at nearly half the speed of light. 鈥淎s they collided, some of their mass was converted into energy, according to Einstein鈥檚 formula E=mc²,鈥� says Brown, referencing the famous theory of relativity. 鈥淭he peak power output was about 50 times that of the light emitted by all the stars in the universe. It is these gravitational waves that LIGO has observed.鈥�

Like Brown, Ballmer is highly regarded within the LIGO community. Part of Advanced LIGO鈥檚 design team, he spent considerable time in Hanford, building the detector. 鈥淚 was amazed at how soon into its first observation run that Advanced LIGO made this discovery,鈥� he says. 鈥淚 was in the LIGO control room the night before for the final detector tuning. When I returned the next morning, there was a buzz in the air. I鈥檒l never forget staring at the first plots, getting goose bumps.鈥�

The discovery was made possible by the enhanced capabilities of Advanced LIGO, a major upgrade that increases the sensitivity of the instruments compared to the first generation LIGO detectors, enabling a large increase in the volume of the universe probed鈥攁nd the discovery of gravitational waves during its first observation run. The US National Science Foundation leads in financial support for Advanced LIGO. Funding organizations in Germany (Max Planck Society), the U.K. (Science and Technology Facilities Council, STFC), and Australia (Australian Research Council) also have made significant commitments to the project. Several of the key technologies that made Advanced LIGO so much more sensitive have been developed and tested by the German-UK-GEO collaboration. Significant computer resources have been contributed by the AEI Hannover Atlas Cluster, the LIGO Laboratory, 性视界 University and the University of Wisconsin-Milwaukee. Several universities designed, built and tested key components for Advanced LIGO: The Australian National University, the University of Adelaide, the University of Florida, Stanford University, Columbia University of New York and Louisiana State University.

Although the Earth is theoretically awash in gravitational waves, detecting them is another matter. 鈥淕ravitational waves stretch space, but their effect is almost imperceptible,鈥� Saulson says. 鈥淚t has taken 21st-century technology, a team of hundreds of experts and decades of effort to detect them.鈥�

Each of the two LIGO detectors is a giant laser interferometer. A laser beam is split into two, and then is sent down a pair of 2.5 mile-long tunnels that are perpendicular to one another. Mirrors at the end of the tunnels reflect the light back to where the laser beam was split.

Since both tunnels are the same length, the light takes exactly the same time to travel to the end of each tunnel and back. But if a gravitational wave passes through Earth, it changes the length of the tunnels, causing the light beams to return at different times. By comparing both beams, LIGO is able to measure the stretching of spacetime caused by gravitational waves.

Brown says that LIGO detectors are so sensitive that even the slightest trace of background noise鈥攖he hum of an air compressor, the rumbling of traffic, the crashing of an ocean wave, hundreds of miles away鈥攃an drown out gravitational-wave signals.

Therefore, LIGO scientists need massive amounts of computing power to find signals in the noise. Brown and his LIGO collaborators use a high throughput computing environment called Orange Grid, along with the Crush supercomputer, housed in the Green Data Center on the University鈥檚 South Campus, to detect black holes.

鈥淲hat we have built is akin to Galileo鈥檚 first telescope,鈥� Ballmer says. 鈥淲e have just taken our first look at the universe in a completely new way. There is so much to learn from gravitational waves in the coming years, and likely many surprises.鈥�

LIGO was originally proposed as a means of detecting these gravitational waves in the 1980s by Rainer Weiss, professor of physics emeritus from MIT; Kip Thorne, Caltech鈥檚 Richard P. Feynman Professor of Theoretical Physics, emeritus; and Ronald Drever, professor of physics emeritus also from Caltech.

A. Alan Middleton, professor and chair of the Department of Physics at 性视界, considers the discovery a 鈥渉istoric advance in physics.鈥� 鈥淥ur Gravitational Wave Group has made central contributions to the opening of a new window onto the universe鈥攁 window that has already revealed some of the most exotic objects and awe-inspiring events in all of existence,鈥� he says. 鈥淲ithout question, this group鈥檚 amazing accomplishments help make the University internationally prominent in research.鈥�

The current generation of 性视界 physicists continues the tradition of leadership in gravity research dating back to Peter Bergmann, one of Einstein鈥檚 research assistants and a professor in 性视界鈥檚 physics department from 1947 to 1982. Other University notables include Emeritus Professor Joshua Goldberg, former research scientist Roy Kerr and LIGO spokesperson Gabriela Gonz脿lez G鈥�95. In addition to carrying out one of the first calculations of the emission of gravitational waves by binary stars, Goldberg organized a 1957 landmark conference, in which the physical effects of gravitational waves were predicted and the first experiments to detect these waves were conceived. Kerr, who was on the research staff from 1958 to 1960, discovered the solution to the Einstein field equation of general relativity, which describes the black holes that LIGO saw. Gonz谩lez is professor of physics and astronomy at LSU, as well as one of Saulson鈥檚 former Ph.D. students.

鈥淭oday is a great day for 性视界 University and for scientific research across the globe,鈥� says Chancellor Kent Syverud. “Thanks to the persistence and insight of our faculty, as well as participating students, we now have a more complete picture of the universe. Their work shines new light on how the universe formed, our place in it and where it鈥檚 headed in the future. This accomplishment exemplifies the University鈥檚 commitment to being a great place for research, and further positions the University as a global leader in discovery and exploration.”

Karin Ruhlandt, dean of the College of Arts and Sciences and Distinguished Professor of Chemistry, agrees: 鈥淭his discovery is a reflection of the tenacious, entrepreneurial spirit of our 性视界 scientists. It is a milestone in a new era of research for the Department of Physics, the College of Arts and Sciences and 性视界 University, cultivating a state of wonder about the limits of thought and our view of the universe.鈥�

LIGO research is carried out by the LIGO Scientific Collaboration (LSC), a group of more than a thousand scientists from universities around the United States and in 14 other countries. 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 (Albert Einstein Institute, AEI), Leibniz Universit盲t Hannover, along with partners at the University of Glasgow, Cardiff University, the University of Birmingham, other universities in the United Kingdom and the University of the Balearic Islands in Spain.

Virgo research is carried out by the Virgo Collaboration, consisting of more than 250 physicists and engineers belonging to 19 different European research groups: six from the Centre National de la Recherche Scientifique (CNRS) in France; eight from the Istituto Nazionale di Fisica Nucleare (INFN) in Italy; two in The Netherlands with Nikhef; the Wigner RCP in Hungary; the POLGRAW group in Poland; and the European Gravitational Observatory (EGO), the laboratory hosting the Virgo detector near Pisa in Italy.

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