University of Minnesota
School of Physics & Astronomy


LIGO searches for Einstein's gravitational waves

Jeff Mondloch and Eric Thrane with the prototype pendulum they've created for LIGO
Alex Schumann

Many a student has sat in freshman physics and asked, “when am I ever going to use this in real life?” Eric Thrane, a postdoctoral researcher in Professor Vuk Mandic’s LIGO group would answer that question by introducing you to Jeff Mondloch, an undergraduate working on a prototype pendulum for an interferometer designed to measure gravitational-waves–– minute ripples in the fabric of spacetime.

Using a basic pendulum equation that everyone learns in freshman physics, period is proportional to the square root of length, Mondloch and Thrane are attempting to create a pendulum with a period of fifty seconds. A conventional pendulum with a fifty second period would have to be taller than the Sears Tower, which is not practical for a physics experiment. Thrane and Mondloch are working to create a compact prototype, which is less than a meter long, but uses high-powered magnets to increase the pendulum period. The pendulum will be used to suspend mirrors in an interferometer––a device consisting of a series of mirrors, in which a split beam of light travels in L-shaped paths before being recombined. Physicists compare the light travel times for each path to determine if one path becomes longer than the other due to a passing gravitational wave. “If we just bolted mirrors down,” Thrane says, “they wouldn’t be able to move under the influence of a gravitational wave. By suspending the mirrors with wires, they behave as though they are freely falling, at least above the pendulum’s resonant frequency. To measure gravitational waves at lower frequencies, however, we must lower the pendulum’s resonant frequency. Previous LIGO experiments measured down to 40 Hertz, Advanced LIGO is designed to get down to 10 Hertz, but Jeff and I are shooting for 1 Hertz.”
Jeff Mondloch is a junior double majoring in Physics and Philosophy. Eric Thrane’s primary role on LIGO is data analysis. Thrane says that his work goes beyond developing software tools: “Before we begin coding, we have to think about the astrophysics: what’s out there and how can we analyze the data to learn something about the universe. What physics can we do with LIGO to use it to its full capacity?”
The University of Minnesota gravitational-wave group is a part of the LIGO Scientific Collaboration, which aims to detect gravitational waves with the new Advanced LIGO experiment. There is strong indirect evidence for gravitational waves, and they are predicted by Einstein’s theory of general relativity. Even for pessimistic models, Advanced LIGO expects to detect gravitational waves. “Advanced LIGO is as sure a bet as you can get in this business,” Thrane says, “and it is scheduled to begin taking data in 2014.”