The Wave Hunters

Do Einstein's Gravitational Waves Shake the Universe?

When Albert Einstein predicted the existence of gravitational waves in 1916 as part of the general theory of relativity, he was thoroughly convinced that it would never be possible to measure these minute changes in space-time. Even today, direct proof of gravitational waves remains one of the most important open questions in modern science. Their direct observation will open the era of gravitational wave astronomy and thereby make possible completely new insights into the universe—up to its point of creation.

Gravitational waves are measured with light

Over the past decades, laser interferometry has emerged as a suitable method for observing the tiny compressions and expansions of space-time that occur as a gravitational wave passes through. An interferometer thereby exploits the wave properties of light: a shift of two partial waves or partial beams relative to one another is converted by the interferometer into easily measured fluctuations in brightness.

Even today, first-generation gravitational wave detectors, i.e. the German-British GEO600 detector in Ruthe near Hanover, can precisely measure changes in length of 10-19 m on a measurement path—this value is equivalent to 1/10,000th of the diameter of a proton. With such measurement accuracy, inspiralling black hole binaries can be observed as far away as 15 million light years from Earth. The part of the universe accessible in this way includes several dozen galaxies.

Noise from light quanta—why it interferes with our work and how we suppress it

In gravitational wave detectors, laser light is used to measure the tiny expansions and compressions of space that occur upon passage of a gravitational wave. If a gravitational wave passes through the detector, it changes the length of the laser beam and, thus, the intensity of the laser light at the output of the detector. In step with the gravitational wave, a corresponding photocurrent is then generated by means of the photoelectric effect. The resulting data can be stored and processed.

The original light should, of course, ideally exhibit no fluctuations in intensity in order to be able to measure the smallest signals. During tests to create light with no fluctuations in intensity, it was, however, determined that the quantum properties of all objects in nature create interfering noise. This is because light is portioned into photons that are statistically distributed in the beam. These statistics result in fluctuations in the measured brightness on the same time scale as gravitational waves, and, thus, in noise that limits the measurement sensitivity. This quantum noise is also called "shot noise", analogous to the uncorrelated shots of a shell fired from a shotgun.

But the latest research results from the scientists at the Albert Einstein Institute in Hanover show: it is possible to produce light with improved quantum noise properties and to use it in gravitational wave detectors! To do this, photons must be correlated with one another, e.g. by making the temporal spacing of the photons more uniform. In this case, one speaks of light with "squeezed" quantum noise.

The project

This is precisely the focus of Dr. Henning Vahlbruch and Alexander (Sascha) Khalaidovski's project.  These two are young researchers in the working group of Prof. Dr. Roman Schnabel at the Albert Einstein Institute in Hanover and in the QUEST cluster of excellence. With their work, they are providing another component for further increasing the sensitivity of the large GEO600 gravitational wave detector. The group at the AEI is preparing for the installation of a light source with improved quantum noise, a world first. This quantum-optical experiment produces squeezed light that both covers the entire frequency range necessary for gravitational wave detectors and also exhibits a high "degree of squeezing". In the next step, which is documented in the video diary, this light source is to be installed in the detector. The use of this technology in third-generation gravitational wave detectors would enable the detection of all mergers of small black holes in nearly the entire universe. The smallest, i.e. quantum physics, would, for the first time, then be used to measure the largest, i.e. the universe.

GEO600

A central research project with which the QUEST cluster of excellence in Hanover is involved is the German-British GEO600 gravitational wave observatory in Ruthe near Hanover. Together with the American LIGO detectors and French-Italian VIRGO project, scientists are, for the first time, working here to directly measure the gravitational waves predicted by Albert Einstein in order to see or hear previously inaccessible parts of our universe and gain a better understanding of space and time.

Because extremely powerful and precise measurement technologies are necessary in order to observe the weak signals, the measurement of gravitational waves is an international undertaking. The Hanover researchers who are working on the QUEST and GEO600 projects are global leaders in this field. For example, they have developed new methods in laser cooling and atomic interferometry that enable the quantum nature of light and material particles to be used as tools.

The GEO600 is considered a "think tank" among the gravitational wave detectors. While partner projects are currently preparing the next series of measurements, in which the world's most modern lasers—developed in Hanover—are used, the GEO600 is already one step ahead: In the coming months, work with squeezed light and squeezed vacuum will, for the first time, be performed here—technologies that are to be utilised in the next generation of gravitational wave observatories.

Albert Einstein Institute (AEI) Hanover

At the Albert Einstein Institute in Hanover, the Max Planck Society and the Leibniz University of Hanover perform joint experimental gravitational wave research. This includes both basic research as well as applied research in the areas of laser physics, vacuum technology, vibration isolation, and classic and quantum optics. Other core research areas include the development and realisation of algorithms for data analysis for different source types of gravitational wave emission. Together with the theoretical section of the Max Planck Institute for Gravitational Physics in Potsdam, the Albert Einstein Institute is a globally unique centre for gravitational physics that covers all aspects of the subject area.

The AEI Hanover operates the GEO600 gravitational wave detector in Ruthe near Hanover in cooperation with British research insititutions. Researchers from the institute also play a leading role in LISA (Laser Interferometer Space Antenna), the planned space-based gravitational wave detector. The joint project between NASA and ESA is to begin measuring gravitational waves in space beginning in 2021, enabling it to "listen" deeper into the universe than ever before.

Furthermore, the AEI researchers are heavily involved in the development of the Einstein Telescope (ET), an international, third-generation gravitational wave observatory.

The QUEST cluster of excellence, Hanover

Within the scope of the QUEST cluster of excellence, six institutes of the Leibniz University of Hanover and five other research insititutions from Lower Saxony and Bremen are performing unique research at the quantum limit. The goal of the scientific work is to answer fundamental questions of physics on topics such as the structure and the basic forces of our universe. With unprecedented precision, QUEST researchers will use their new measurement technologies to examine not-yet-understood physical phenomena. Their research topics include individual atoms, atomic interferometers, atomic quantum sensors, lasers and atomic clocks and, in the area of astronomy, gravitational waves as well as Earth observation and geodesy.


Scientific institutions participating in QUEST:
Leibniz University of Hanover: Institute for Quantum Optics (IQ), Institute for Gravitational Physics (IGP), Institute for Theoretical Physics (ITP), Institute for Solid State Physics (IFKP), Institute of Geodesy (IFE), Institute of Applied Mathematics (IFAM); Max Planck Institute for Gravitational Physics (Albert Einstein Institute/AEI), Hanover; Laser Zentrum Hannover e.V. (LZH), Hanover; GEO600 gravitational wave detector, Ruthe; Physikalisch-Technische Bundesanstalt (PTB), Braunschweig; Center of Applied Space Technology and Microgravity (ZARM), Bremen





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