Nobel Prize in Physics goes to Gravitational Waves Researchers

Leibniz Universität Hannover and the Max Planck Institute for Gravitational Physics (Albert-Einstein-Institut/AEI) in Hannover and Potsdam have congratulated Rainer Weiss, Barry C. Barish and Kip S. Thorne on being awarded the 2017 Nobel Prize in Physics: "Warmest congratulations to our colleagues; we are thrilled that this award is going to three pioneers in gravitational-wave research. They have never lost sight of their goal and have inspired generations of young scientists," declare Prof. Bruce Allen (Professor at Leibniz Universität), Prof. Alessandra Buonanno and Prof. Karsten Danzmann (Professor at Leibniz Universität), the joint Directors of AEI, and Bernard F. Schutz, Director Emeritus of AEI. "We are proud to be part of the international collaboration that detected the first gravitational wave two years ago when it passed through the earth. This was a turning point for astronomical and astrophysical research. We now have a new tool for observing the universe. The president of Leibniz Universität Hannover is equally enthusiastic. "I am delighted that this year''s Nobel Prize is going to this groundbreaking scientific proof of the existence of gravitational waves," says Prof. Dr. Volker Epping. "I am all the more pleased since scientists in the group led by our Professor Karsten Danzmann in Hannover were substantially involved in the development of the technology for measuring gravitational waves, and are members of the Ligo collaboration."

Scientists at Leibniz Universität Hannover and the Max Planck Institute, in collaboration with British researchers, developed many of the key laser technologies which contributed to the unprecedented sensitivity of aLIGO and tested it in the gravitational-wave detector GEO600 in Ruthe near Hannover. GEO600 serves as an ideas factory and testing facility for advanced detector technologies. A large part of the data analysis also takes place in Hannover. Most of the measurement data from the observatories in the USA land in the Hannover cluster Atlas, the largest computer cluster worldwide for analysing the data of gravitational waves.

Since the 1960s, gravitational wave research has been conducted by an international collaboration of scientists who worked closely together despite the challenges of the cold war and funding shortages in many countries. Max Planck scientists were involved right from the beginning and have made many key contributions. Today the field has grown into a global network of more than 1000 scientists.

Max Planck Society and Leibniz Universität scientists make crucial contributions

Researchers at the Max Planck Institute for Gravitational Physics (Albert Einstein Institute; AEI) in Hannover and Potsdam, Germany, and from the Institute for Gravitational Physics at Leibniz Universität Hannover have made crucial contributions to the discoveries in several key areas:

• development and operation of highly sensitive detectors pushed to the limits of physics,
• efficient data analysis methods running on powerful computer clusters, and
• construction of accurate waveform models for the detection and interpretation of signals.

Pioneering gravitational-wave research in the Max Planck Society

The first direct detection of gravitational waves on 14th September 2015 was the culmination of decades of research in gravitational wave detection in the Max Planck Society going back to the very beginning of the field in the 1960s. A Max Planck group at the MPI for Physics and Astrophysics in Munich led by Heinz Billing († 4th January 2017) conducted coincidence experiments between resonant mass detectors to disprove the early claims of gravitational wave detection in the 1960s. At the beginning of the 1970s, Billing''s group began - at that time as the only people in the world - to work with laser interferometry. The group built prototypes and pushed technology development ahead.

The Max Planck Society consistently supported this group after Billing retired, passing the baton to Gerd Leuchs in 1986 and in 1989 to Karsten Danzmann. With British partners at the Universities of Glasgow and Cardiff, they were the first to design and propose a large-scale interferometric detector with 3 kilometre long arms, but funding for such an instrument was not available in Germany.

In 1995 the Max Planck Society brought Bernard Schutz over from Cardiff to Germany to help found the AEI, first in Potsdam and in 2002 in Hannover, with the explicit mission of becoming a world centre for gravitational-wave research. Leibniz Universität Hannover and the Volkswagen Foundation had come on board just before, and cooperation with Glasgow and Cardiff was intensified. In 1994 was the starting point for GEO600, a low-cost German-British gravitational wave observatory, which - in parallel to observation runs with the LIGO and Virgo instruments - has been serving as a think tank for detector development ever since. The high-end technology created here now forms the heart of all large gravitational wave observatories, including Advanced LIGO.

Predicting gravitational wave signals with novel methods to solve Einstein equations

While experimentalists were constructing ever more sensitive instruments, theorists were developing precise ideas of what the expected gravitational wave signals and their sources would be like. Soon it became clear that complex data analysis methods would be needed to detect the faint signals. Bernard Schutz had pioneered these methods with data from the Munich and Glasgow small detector prototypes, and the AEI became a world centre for the development of sophisticated analysis methods. Schutz also established what was then the world''s largest group for supercomputer simulations of black-hole mergers; such simulations were an integral part of the detection and interpretation of Advanced LIGO''s observations.

Simulated gravitational waveforms are important but not sufficient. Since data-analysis algorithms use several hundred thousand templates and it may take weeks to produce one single simulation, it is crucial to develop approximate but fast methods to solve the Einstein equations, so that waveforms can be quickly generated.

In the late 1990s Alessandra Buonanno, since 2014 director at the Max Planck Institute for Gravitational Physics in Potsdam and College Park Professor at the University of Maryland, and Thibault Damour (IHES, Paris) developed a novel approach to the binary-orbit problem that combines several approximate methods for constructing waveforms from coalescing binary black holes. Over the last 15 years this formalism has been developed into a highly accurate method that also includes results from numerical-relativity simulations, and extends to binary neutron stars. Scientists at the AEI in Potsdam, and earlier at the University of Maryland, have been building accurate waveform models combining the best tools to solve Einstein equations and have been using them to detect gravitational waves in the Advanced LIGO observation runs. AEI Potsdam researchers also employ those waveform models to infer astrophysical and fundamental physics properties of the binary systems, and to test General Relativity.

Finding gravitational waves: Data analysis on high-performance supercomputers

Researchers at the Max Planck Institute for Gravitational Physics in Hannover, led by Bruce Allen, use these templates to analyse the detector data on high-performance supercomputers. Once the signals have been found, the templates are used to infer astrophysical information upon the detection: where exactly is the source? What is its nature? Black holes and/or neutron stars? What are their masses and spins?

For the first detections of gravitational waves, the AEI researchers carried out most of the production data analysis. In addition, the majority of the computational resources for the discovery and analysis of the Advanced LIGO data were provided by Atlas, the most powerful computer cluster in the world designed for gravitational-wave data analysis, operated by the AEI in Hannover. Atlas has provided about 160 million CPU core hours for the analysis of Advanced LIGO data, almost half of the global LIGO computing efforts.

Illuminating the dark side of the Universe

This close interplay of experiment, simulations, analytical calculations, and data analysis ultimately allows scientists to bring light into the dark and invisible side of the Universe. Today''s Nobel Prize announcement honours the founding fathers of this field whose pioneering work rendered the dawn of a new era of astronomy possible.