The Nobel Prize in Physics was awarded Oct. 3 to three researchers involved in the discovery of gravitational waves: Rainer Weiss, of the Massachusetts Institute of Technology, and Kip Thorne and Barry Barish, both of the California Institute of Technology. The three are founders of the Laser Interface Gravitational-wave Observatory project. The National Center for Supercomputing Applications at the University of Illinois was part of the LIGO collaboration that processed data generated by the project. News Bureau physical sciences editor Lois Yoksoulian spoke about the discovery with Illinois research scientist and NCSA Gravity Group leader Eliu Huerta Escudero.
What are gravitational waves?
Einstein’s theory of general relativity states that astrophysical events - such as the collision of two black holes, neutron stars or white dwarfs - generate fluctuations in the curvature of space-time that propagate at the speed of light. These curvature fluctuations, or gravitational waves, travel long distances without being absorbed or scattered. This means that the astrophysical information contained in gravitational waves - the astrophysical properties of black holes or neutron stars, and the geometry of space-time around these objects - travels throughout space-time with no loss. This property also posed a major challenge for their detection for several decades.
How were these waves discovered?
The collision of two black holes is among the most violent and energetic events in the universe. Since most of these events do not emit light, they remained concealed from our eyes until Sept. 14, 2015. On that date, LIGO detected gravitational waves emitted by the merger of two massive, stellar mass black holes. With a single observation, LIGO showed for the very first time that black holes moving at velocities close to the speed of light emit gravitational waves, marking the beginning of a new era in contemporary astronomy.
How have gravitational waves helped us learn new information about the universe?
Neutron stars and black holes mark the final stage of a star’s life. Therefore, detecting the mergers of black holes and neutron stars provides information about the distribution of masses of stars in the universe. Over the last two years, LIGO has detected four events, all of them consistent with the astrophysical properties of black hole collisions, as predicted by Einstein’s theory of general relativity.
It is expected that the merger of two neutron stars, or black hole-neutron star systems, may be accompanied by electromagnetic radiation and neutrinos. If this hypothesis is true, an accurate localization of gravitational wave sources will be critical to inform follow-up observations across the electromagnetic spectrum with astronomical facilities. On Aug. 1, the European Virgo detector joined LIGO to carry the first three-detector observation of a binary black hole merger, improving the localization of this event by a factor of 20. This is a crucial step to realize multimessenger astrophysics, that is, the study of astrophysical phenomena using complementary windows of observation into the universe: gravitational waves, light and neutrinos.
How is NCSA contributing to the LIGO Project?
NCSA has a long history of contributions toward the goals of the LIGO project. NCSA was founded to rectify the lack of supercomputer centers to address computational grand challenge problems across science domains in the U.S. Larry Smarr, NCSA’s founding director, was a pioneer in the use of supercomputers to study the collisions of black holes and neutron stars, and the gravitational waves emitted by these events. Ed Seidel, who led the relativity group at NCSA during Smarr’s directorship, is a former NCSA director and was part of the National Science Foundation’s Black Hole Grand Challenge Program. Seidel led the development of new supercomputer codes to numerically model the mergers of black holes and neutron stars, and supported the Cactus Framework - an open base code used by physicists to study gravitational wave sources.
As part of our formal contribution to LIGO, the NCSA Gravity Group continues to develop and use the numerical relativity software, the National Science Foundation’s Einstein Toolkit, in conjunction with the Blue Waters and XSEDE supercomputers, to generate numerical relativity waveforms that are used for the detection and characterization of gravitational wave sources. The NCSA Gravity Group recently led the configuration of the Blue Waters supercomputer to be used for the detection of new gravitational wave signals; this computational framework was used in the last LIGO discovery campaign. NCSA also supports the LIGO mission with expertise on identity management, cyber-security and network engineering.
Editor’s note:To reach Eliu Huerta Escudero, call 217-265-4083; elihu [at] illinois (p) edu.