Tracking down the gentle tremble
Gravitational waves are some of the most spectacular predictions of the 1915 General Theory of Relativity. But fifty years were to pass before physicist Joseph Weber attempted to track them down. At the beginning of the 1970s, Max Planck scientists also began working in this research field and developed second-generation detectors. The groundwork laid by these pioneers meant the waves in space-time ceased to be just figments of the imagination: they were finally ensnared in September 2015.
Text: Helmut Hornung
Albert Einstein had his doubts: it would never be possible to detect gravitational waves, the trembling in space-time was simply too weak! Yet it was he himself who had postulated their existence - they are a product of his General Theory of Relativity which he puts forward in November 1915. A short time later – in 1916 and again in 1918 – he devotes a paper to this phenomenon.
Two decades later, he suddenly has a change of heart: “I have been working with a young colleague [Nathan Rosen] and we have come to the interesting conclusion that gravitational waves do not exist,” Einstein writes to his colleague Max Born. And in 1936 he submits a manuscript to the renowned journal Physical Review – which is returned to the author by a reviewer who considers it to be unsuitable. Albert Einstein is fuming about this embarrassment, but has to accept that his argumentation is indeed flawed. His doubts were unfounded.
And the physics world? It is paying almost no attention to gravitational waves. The same applies to the General Theory of Relativity overall, which has been eking out a rather miserable existence since the 1920s. It is only after Einstein’s death in 1955 that people begin to take an interest in it. Astrophysicists now start to turn their sights towards black holes and quasars: exotic objects which cannot be explained without Einstein’s equations. This renaissance of relativity ultimately benefits gravitational waves, too. They strike a chord with at least one physicist: Joseph Weber.
Born in 1919 in New Jersey, the researcher at the University of Maryland has an idea for a simple experiment: he suspends an incredibly heavy aluminium cylinder – one-and-a-half metres long and 60 centimetres across – in a steel wire loop and attaches piezosensors to its middle to register oscillations. The whole test rig is housed in a vacuum chamber.
But why can gravitational waves cause a solid metal cylinder to tremble? Because they stretch and compress space at right angles to their direction of propagation. Let us imagine that gravitational waves impinge on a spherical balloon: in milliseconds they would first deform it into the shape of an egg and then elongate it to the shape of a sausage.
Weber thinks that gravitational waves originate primarily from cosmic catastrophes within our Milky Way, a supernova, for instance. This is when a star explodes and ejects great amounts of mass into space at the same time as its inner regions are collapsing in on themselves. What remains is a neutron star or a black hole. We now know that these objects can also generate gravitational waves themselves – whenever they are formed in pairs on close orbits and fuse with each other. The waves registered in September 2015 originate from this type of event: from two black holes each with a mass of around 29 and 36 solar masses, which are merged with one another in a galaxy approx. 1.3 billion light years away.
The reach of Weber’s cylinder extends only to a small region within our Milky Way. It is unlikely that a supernova will explode here, especially since there are thought to be only two to four such events per century in the whole galaxy. Nevertheless: Joseph Weber reports success in 1969. His detectors in Maryland and at Argonne National Laboratory 1,000 kilometres away are said to have actually registered gravitational waves, and even several per week!
Other scientists remain sceptical, including those at the Max Planck Institute for Physics and Astrophysics in Munich. In 1970, this becomes the birthplace of gravitational waves research in Germany. “We decided back then to repeat Weber’s experiment as exactly as we could with refined technology and sophisticated data processing,” remembers Walter Winkler, one of the ambitious Max Planck researchers. The group working with Heinz Billing build the world’s most sensitive cylinder detectors in Munich and in Frascati, Italy. They are capable of registering changes in length of 10-15 centimetres. The experiments run from 1972 until 1975. The result: nothing!
The Weber detectors are falling out of fashion – and making room for a new method: interferometry. This idea came originally from the German-American physicist Albert A. Michelson. In 1881 he wanted to use such an instrument in Potsdam to measure the speed of the Earth relative to what was then thought to be the ether. Ninety years later, Philip Chapman, Robert Forward and Rainer Weiss suggest using this type of instrument as a detector for gravitational waves. The light source is to be a laser. But lack of money prevents the three US researchers from getting any further.
Once again, the group from the Max Planck Institute for Physics and Astrophysics comes onto the scene. They are the only people in the world to begin working with this new technology. Its principle is simple: a laser beam impinges on a beam splitter where it is split into two beams; one continues on in a straight line, the other is deflected to the side. At the end of each path is a mirror which reflects the light back to the beam splitter.
This mirror now deflects the beams such that they are superimposed on each other, i.e. they interfere. The light waves arriving at a photodiode oscillate not in phase, but out of phase, however: wave crest meets wave trough, the light waves extinguish each other. If a gravitational wave runs through the system, it compresses and elongates space, thus changing the measurement paths. The light waves are no longer out of phase. The receiver no longer remains dark – a signal appears.
In 1975, the researchers in Munich – working with Heinz Billing are Walter Winkler, Albrecht Rüdiger, Roland Schilling, Lise Schnupp and Karl Maischberger – build a prototype with an arm length of three metres. The light of a 3-watt argon laser is reflected 150 times. But this second-generation detector also has its problems: the frequency of the laser light must be extremely stable, and its basic power is too weak. Moreover, fluctuations in the geometry of the beam lead to unwelcome error signals and allow tremors to shake the mirrors.
To reduce all these unwanted effects, the physicists develop innovative technologies which no modern gravitational wave trap can now do without. From 1983 onwards they continue their pioneering work with a second prototype with 30-metre arm length. This is really short, because in the thousandth of a second that it takes a gravitational wave to traverse the measurement path, the light covers 300 kilometres. The laser beam would therefore have to be en route over the same length in order to observe the wave completely. The researchers avail themselves of a trick which they call delay line and which consists in “folding” the beam path and reflecting the beam backwards and forwards between the mirrors many times, as described above.
Nevertheless: the longer the measurement path, the better. “So in 1985 we submitted an application to build a detector with an arm length of three kilometres,” says Walter Winkler. “But in Germany there was no interest at all in the project and so it was not approved.” A British group suffers the same fate. It had been undertaking similar research since 1977 at the University of Glasgow and had built a detector with 10-metre arm length in 1980. In 1986 the Scottish application for a large interferometer fell on deaf ears.
But similar fates bring people together and so three years later, the two teams decide to collaborate. Only a short time later they submit joint plans for a detector to be built in the Harz mountains in Germany, again without success. The breakthrough finally comes in 1994: the construction of a German-British detector with each arm measuring 600 metres finally becomes reality just outside Hanover. “Herbert Welling from the University of Hanover managed to convince his colleagues to bring the installation to Lower Saxony,” says Winkler.
Karsten Danzmann, then Group Leader in Garching and now Director at the Max Planck Institute for Gravitational Physics, is appointed and given funding by the Federal State of Lower Saxony, the University and the Volkswagen Foundation. With not much money but a lot of hard work – by the British colleagues as well – the researchers get the project off the ground.
The first sod is cut on 4 September 1995. Since 2002 the detector has been operated by the Centre for Gravitational Physics, of which the Max Planck Institute is a member, together with Leibniz Universität Hannover and Glasgow and Cardiff Universities. The main purpose of the installation is as a test laboratory for technologies which are incorporated into other detectors all over the world. And it must not be forgotten that David Shoemaker, Project Leader of Advanced LIGO, was also a member of the Max Planck Group for a while. This American installation with German technology is now the first to have measured gravitational waves and crowned decades of dogged research to track down the gentle tremble from space.