**The measurement of gravitational waves – the beginning of a new era in astrophysics**

December 13th 1888 was an historic day which we barely find in any history book. On this Thursday more than 125 years ago the physicist Heinrich Hertz submitted to the Berlin Academy of Sciences his research report entitled „On the emissions of electrical force“ (original German title: “ÜberStrahlenelektrischer Kraft”) in which he described how he had succeeded in an experiment to transmit electromagnetic waves from a transmitter to a receiver. Hertz’s results provided the basis for many technologies that shaped the 20th century, such as wireless telegraphy or radio and television. His experiments constitute a great moment of glory for experimental and applied physics – as well as for theoretical physics: Almost 25 years before, James Clark Maxwell had published his seminal work „A Dynamical Theory of the Electromagnetic Field“ in which he formulated the equations that govern the universal behavior of electric and magnetic fields and which bear his name today. Maxwell immediately recognized that certain solutions of his equations predicted the existence of oscillating electric and magnetic fields that move through empty space – the very wavesHertz was to measure.

The second Thursday of February 2016 is a day that in importance can be considered equal to the second Thursday in December 1888^{th}. On this February 11 researchers of the international LIGO-Virgo collaboration announced that they succeeded in directly measuring gravitational waves for the first time. The significance of this discovery is comparable to that of the Higgs boson in July 2012 (and has probably removed the suspension around the announcement of the Nobel Prize in Physics due in autumn 2016). The gravitational waves had also been predicted by theoretical physics, by a theory that recently celebrated its 100th anniversary: Albert Einstein’s theory of general relativity. Thus February 11, 2016 marks a moment of greatest glory in the history of both, experimental and theoretical physics. Once again we recognize the impressive ability of science to describe the world in which we live with means of mathematics and experimental creativity and to capture the natural laws on which it is based. And as the discovery of electromagnetic waves initiated a new era in physics, so will the detection of gravitational waves likely mark the beginning of a new era in astronomy.

But let us take a 100 years step back: In his general theory of relativity Einstein formulated that the effects of gravity are no longer given by a time-independent, spatially-acting force, but by an influence on the structure of space-time itself. In other words, gravity is now a consequence of masses causing changes in the geometric structure of four-dimensional space-time. Einstein thus combined what was sharply separated in classical physics: space and force, geometry and gravity. Masses no longer act on each other by forces, but they change the structure of space-time by tweaking or bending it, which in turn „gravitationally“ influences the masses again. The classic flat, so-called „Euclidean geometry“ of space ceases to be valid in Einstein’s theory and is replaced by a locally “bent” geometry, the curvature of which depends on the present mass distribution. In mathematical terms, Einstein’s equations provide a direct link between the mass distribution (represented by a mathematical object called “stress-energy tensor” on one side of the equation) and the geometric characteristics, the so-called “metric”, of space-time (represented by the “curvature tensor” on the other side of the equation). In physical terms: Space is not a container of the physical world, and the time is not an internal parameters of the movement, but both are an integrated object of physics with its own dynamics. Or as the physicist John Archibald Wheeler once summarized in simple terms: “Matter tells space how to curve. Space tells matter how to move“. A direct consequence of Einstein’s theory was that any system of accelerated masses, such as bodies rotating around each other like a double star system or a planet circulating around the sun, generate wavelike distortions of space-time, quite similar to accelerated electrical charges emitting electromagnetic waves. These so-called „gravitational waves“ are thus waves of space-time itself propagating at the speed of light. They cause distances of space (and time), through which they pass to get temporarily compressed and stretched.

Now the gravitational waves produced for example by the earth in its orbit around the sun are immeasurably weak (their total radiated power is a mere 300 W). So for the detection of gravitational waves we need much more powerful sources: violent supernova explosions or neutron stars,or at best black holes, orbiting each other at a small distance to colliding into each other. But because of the great distance of such events from us the effect of these waves once they reach us on Earth is very small (we should probably consider ourselves lucky, because were we close to these events, we would not be in a position to measure anything any longer). Now the LIGO-Virgo Observatory managed to capture a huge event this kind: two black holes with 29, respectively 36 solar masses, which were about to orbit each other for only another dozen times before eventually merging into a more massive black hole with about 60 solar masses. During these last few revolutions before the crash the vibrations of space-time associated with their motion were so strong that we are able to measure them now at a distance of 1.3 billion light years. The signal, however, only lasted for about half a second, because at this time the black holes orbited each other already at nearly the speed of light (and in this short period emitted 50 times more energy than all the stars of the universe together).

Nevertheless, the measurement of theses waves required the highest level of experimental and technical skills. Over a length of the measuring tube (called an interferometer) of four kilometers the physicists had to be able to measure a change in length of the size of a small fraction of an atomic diameter. And to exclude measurement errors, these measurements had to be performed at two different sites, situated in 3000 km distance to each other, in a precisely defined time interval. And apparently this has now been successfully performed. And the results match corresponding theoretical models very well. The physicists will not deny that they had been somewhat lucky: the gravitational wave came with precisely the very frequencies (which aredetermined by the masses of the black holes), which the detectors were about ten times more sensitive to than they had been until recently before a recalibration.

It could be the beginning of a new era, as now the physicists prepare to refine the accuracy of the detectors for more extended frequency ranges in order to capture regular signals from double systems of stellar black holes at different distances and thus from different eras. Thus the physicist now possess a tool, which allows them to examine the laws of gravity under the extreme conditions of black holes. In addition, they now have access to those 99 percent of the universe which cannot be captured by observations of the electromagnetic spectrum (light, radio X-rays, gamma rays). Whether this will give rise to a „gravitational technology“ analogous to the „electrical technology“ is currently difficult to imagine. But was Hertz able to imagine in 1888, what electrical engineering would develop into over the next 100 years?