In March 2023, the Advanced Laser Interferometer Gravitational-Wave Observatory (LIGO) is scheduled to begin its fourth year-long observing period. Scientists on the ground in Hanford, Washington, and Livingston, Louisiana, have spent the last two years upgrading hardware and software to increase the sensitivity of the detectors and enable them to detect “weaker” gravitational waves to recognize more events than ever before.
At the same time, members of the Advanced LIGO team are continuously working on refinements for future observation periods ahead. Stefan W. Ballmer, Professor of Physics at the College of Arts and Sciences, was a member of the team involved in designing and building the LIGO detectors.
To continue this work, Ballmer recently received a $555,000 grant from the National Science Foundation (NSF) to develop optical cavity mismatch detection technologies and suspension actuators for the next-generation detector, a renewal of funding for detector technology for gravitational-wave astrophysics.
The award also provides support for Ballmer’s graduate students, including Elena Capote, who is currently on site in Hanford, helping to fine-tune detector alignment and control systems to ensure the detector performs as designed.
“These detectors are complicated machines with thousands of control loops that keep four main mirrors and another 30 pendant mirrors aligned and steered in length to keep the light resonant,” says Ballmer. “Every time you make a change, it really becomes a new detector that needs retuning.”
This is 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 – tubes that run perpendicular to each other. A powerful laser beam is split in 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, it should take the light exactly the same amount of time to travel to and from the mirror at the end of each tunnel. However, when a gravitational wave penetrates the Earth, it changes the distance between the mirrors, causing the light rays to return at different times.
By comparing both beams, LIGO is able to measure the stretching of spacetime caused by gravitational waves, a landmark observation first made in 2015 with the first physical confirmation of a gravitational wave emitted by two colliding black holes in nearly 1.3 billion light-years away.
According to Ballmer, the higher the laser power in the 2.5-mile arms, the more accurately scientists can determine the movement of the arm. However, the usable laser power is currently limited by deficiencies in the optical system of the detectors. “The laser’s optical phase front returning from the detector can be distorted by thermal effects in the mirrors,” he says.
Ballmer is working on a diagnostic camera that records thermal distortions in the detector and allows scientists to determine their cause and effect. While a prototype camera was developed as part of a previous award, “this continued support is dedicated to making this camera available and miniaturizing it, making it easier to use on the job site,” he says.
The award also supports joint research with scientists at MIT to redesign the test mass suspensions for the current detectors to use heavier masses. “Randomly arriving photons push the test masses around, so the heavier the test masses are, the less they move when accidentally hit by a photon,” Ballmer explains. “Moving to heavier test masses is a way to increase sensitivity at low frequencies.”
Previous research has focused on new coatings for the mirrors. As part of the current grant, Ballmer is also conducting research and development to integrate these coatings onto the detector. “The new coatings have much lower thermal noise, but they don’t work with some auxiliary laser frequencies in the detector. Changing the mirror coatings therefore requires different changes to the detector, and so the R&D for this award consists of prototyping the new detector systems that are compatible with the new coating types,” he says.
Ballmer says these developments will not only be used to upgrade the LIGO detectors in the fifth or sixth observing cycle, but can also serve as the basis for the next generation of detectors.
Ballmer was a principal investigator on the Cosmic Horizon Explorer Study, a project design for the third generation of detectors that will have 10 times the sensitivity of Advanced LIGO. The Cosmic Explorer will extend the detection range of black hole-neutron star mergers to cosmic distances. “We will see mergers in the very first stars to form in the universe,” he says.
The 100-page study will inform next steps in NSF funding decisions for the project, which Ballmer said will likely focus on proposing the site and developing the conceptual design for the detector. “We’ve all just seen these beautiful images from the James Webb Telescope showing the most distant and earliest galaxies of light. So with Cosmic Explorer, we would see black hole mergers in these early galaxies,” he says.
About Stefan W. Ballmer
Ballmer joined Syracuse University in 2010. Leading up to his contributions to LIGO’s Nobel Prize-winning work, he received a 2013 NSF CAREER Award for Aiding Detector Technology in the Age of Gravitational-Wave Astrophysics and provided $860,000 in research funding over five years.
In October 2021, Ballmer was named a Fellow of the American Physical Society (APS) for his instrumental role in the construction and commissioning of the Advanced LIGO detectors and the scientific interpretation of their observations, his leadership in the development of gravitational-wave detectors and mentoring of the next Generation of gravitational wave experimenters.
Born in Switzerland, Ballmer held an adjunct visiting professorship 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’s degree from ETH Zurich in Switzerland. As an avid passenger, Ballmer enjoys flying in his free time, is an instrument flight instructor and holds a commercial pilot’s license.