F O C U S Invisible U NIV E R S E

Theoreticians at the M A X P L A N C K IN S TIT U T E F O R G R A VIT A TIO N A L P H Y SIC S Collisions ( A L B E RT EIN S T EIN IN S TIT U T E) in Golm near Potsdam have succeeded in calculating the shape and intensity of gravitational waves emitted by two black holes coalescing with one another. Their results are crucial for the success of the German-British that Make gravitational wave detector GEO600, located in Hanover, which recently passed its first test. Physicists hope that it will enable them for the very first time to detect Waves in Space the existence of gravitational waves as predicted by Einstein.

GEO600, located in Ruthe near Hanover. t the end of October in the small A rather inconspicuous hut is all that marks A town of Ruthe near Hanover, the presence of a high-tech facility. the scientists working with Karsten Danzmann from the Max Planck In- stitute for Quantum Optics felt their hearts give a leap. The researchers sent a laser beam into their gravita- tional wave detector for the first time. Travelling at the speed of light, the beam first hit an optical element that split it into two and redirected it in two vertically parallel directions. The two smaller beams were then / GEO600 ENGER – B HYSICS P RAVITATIONAL G FOR /MPI NFORMATIONSTECHNIK I ENTRUM FÜR -Z USE -Z ONRAD : K ICTURES

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reflected in a series of mirrors and However, these are decidedly modest luminates evenly. However, if a brought together to a single point. amounts in comparison to the real gravitational wave sweeps over it, That was it. The experiment ran un- giants in the universe. For example, the space in each arm of the interfer- changed for four days, without any- two compact neutron stars orbiting ometer is compressed and stretched a thing out of the ordinary happening. each other at a distance of one hun- short interval of time apart. At that In fact, that is exactly what the team dred kilometres over a period of one moment, the two laser beams are no had hoped would be the case, be- hundredth of a second generate a longer travelling through space that cause it showed that the GEO600 power equivalent to 1045 Watts. The is flat but through space that is bent works. This may not sound very power emitted in the form of gravi- – much as a ship in a storm rides up spectacular, but it represents the out- tational waves from the explosion of over high waves. As a result, each come of years of research, new de- a massive star, a supernova, is of a beam travels a little further or a little velopments, and sophisticated exper- similar magnitude. These are the re- less far than it did before. And this iments. The goal of these efforts is to sults the gravitational physicists are manifests as a brief flicker in the in- measure for the first time the gravita- on the look-out for. terference pattern. tional waves predicted by Einstein. Whilst this may sound simple in WAITING FOR According to the General Theory principle, in actuality it is only just A BRIEF FLICKER of Relativity, the gravity of a collec- within the boundaries of what is tion of matter, such as a star or a The proof works according to the technically feasible. If the laser beam planet, can be described as a warp- following principle: gravitational travels a distance of one kilometre, a ing of space. This can be illustrated waves spread out in space at the gravitational wave alters this path- with the example of a billiard ball speed of light. Wherever they appear, way by as little as a billionth of the sitting on a rubber cloth and making space is compressed and stretched diameter of an atom, which itself an indentation in it. If another body for a fraction of a second before tak- measures a mere ten millionth of a finds its way into this hollow, it ap- ing on its original form again. It is millimetre. An incredible undertak- pears to be attracted by the ball and this temporary distortion of space ing. But now the researchers want to rolls towards it. If a body in space that the researchers hope to measure get started. However, one gravita- accelerates in such a way, it emits using laser interferometers such as tional wave detector on its own gravitational waves: warped space the GEO600. At the heart of this ap- would not be sufficient to obtain re- spreads out in every direction in paratus is a very stable, high-perfor- liable results. For one thing, it would waves. In the billiard ball analogy, mance laser. Using a semi-transpar- be unable to localise a source in the waves would fan out from the loca- ent mirror its beam is split into two sky. In addition, a flicker in the in- tion of the ball on the rubber cloth. beams that run away from one an- terference pattern would not consti- Einstein himself did not believe other to left and right. A mirror is tute proof of a gravitational wave One of the 600-metre-long pipes. They are suspended in such a way that that anyone would ever be able to situated at the end of each pathway, passing through. It might be caused they will not vibrate when exposed to external tremors. prove the existence of “his” gravita- reflecting the light back. Both beams by some other disturbance. tional waves. Today, however, that return to the semi-transparent mir- For this reason, other instruments dream has come close to realization, ror, which redirects them in such a of this kind are currently being set up even if only for the most extreme way that they come together in a around the world in addition to the processes in the universe. The fol- common point and overlap. The GEO600. The American LIGO, with lowing example may serve to eluci- beams generate a so-called interfer- two facilities in Hanford and Liv- date this. On its orbit around the sun ence pattern at this point, whose ingston (USA) respectively, is due to the Earth emits gravitational waves brightness is measured by a special undergo its first test run shortly. Each that have a power of 200 Watts, instrument. of these interferometers has arms while Jupiter manages to achieve the As long as this arrangement is not that are four kilometres long. VIRGO, considerable sum of 5300 Watts. disturbed, the interference pattern il- with three kilometre-long arms, is

Computer simulations at the Max Planck Institute for Gravitational Physics show the oscillation mode of a gravitational wave that arises when two black holes weighing ten solar masses merge.

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This computer simulation shows gravitational waves speeding towards the Earth after the collision of two black holes. The waves are generated close to the centre of the collision and spread out like ripples on a pond.

currently under construction near the LIGO and GEO600 are part of the Einstein Institute, AEI) in Golm, near process. Gravitational waves consti- range of the GEO600 and the other galaxy such as the Milky Way, about Italian town of Pisa. It will be com- Ligo Science Collaboration group, Potsdam, has already made an out- tute the only evidence. Thus simula- detectors. two or three “heavy” stars explode pleted in two to three years’ time. Fi- which means that researchers from standing contribution in this regard. tions are essential in order to estab- Collisions between two black holes every century. However, if as scien- nally, a 300 metre-long interferome- both facilities can access the entire The Lazarus team, as these scientists lish the nature of the sources from are extremely rare. Experts assume tists hope, such events can be ob- ter named TAMA is already up and set of data at any time. VIRGO is to call themselves, has calculated the the signal observed. Previous simu- that such an event happens only served at distances as great as tens running in Mitaka, Japan. The latter join in the collaborative enterprise at shape and intensity of gravitational lations of two colliding black holes once every 100,000 years in a galaxy of millions of light years, then it is not sensitive enough, however, to a later date, and all four locations waves that arise when two black were unable to show the entire such as our own Milky Way. should be possible to register several provide evidence of genuine cosmic will be treated as a single global holes merge into one another. An process of coalescence. The basic Nonetheless, it literally makes waves events of this sort every year. Merg- sources and serves principally to test gravitational wave observatory. Only occurrence of this kind is preceded problem was how to model the wave that are so big that the evidence can ing neutron stars may be an even new technical components. if these machines register a signal at by a lengthy series of events. The emissions and the inside of the black be detected at distances of several more attractive prospect. These are GEO600 has two arms, each of about the same time is it likely that a process begins with two massive hole simultaneously. The Potsdam hundred million light years. Since very compact residues of stellar ma- which is 600 metres in length, its gravitational wave is the cause. Four stars orbiting each other. When they team achieved the crucial break- there is a very large number of terial with diameters of about 20 pipes are fully evacuated, and ulti- instruments have the added advan- have used up all their fuel, they eject through by combining two separate galaxies within this radius around kilometres, whose matter is so tight- mately it will be equipped with the tage that the direction from which their outer crust into space and be- methods: full numerical simulation the Earth, astrophysicists calculate ly compressed that a teaspoonful of best optical components in the the wave came can be approximately come supernovas. Their cores, how- of the collision and an approxima- that they should be able to register a it would weigh a billion metric tons world. Together with LIGO it will be established. This method is similar to ever, collapse and become black tion method taken from perturbation collision of two black holes once on Earth. When such bodies orbit doing pioneering work. “We want to that used in land surveying, where a holes. These orbit one another as be- theory for calculating the radiation every two years. one another and merge in much the try the first coincidence test with bearing of a particular point is taken fore, but since they emit gravitation- given off from the emerging black “The Golm-Hanover connection is same way as black holes, very strong LIGO from December 28th onwards”, from different locations. After the al waves, they lose momentum and hole. working extremely well”, continues gravitational waves are emitted. It says Karsten Danzmann. The devices trial run early in 2002, the data will gradually draw closer to each other. The results obtained by the Golm Danzmann. And on the 1st of Janu- might be possible to register one of are to run for about two weeks. In be evaluated and the temporary opti- Eventually they collide and merge in researchers are good news for their ary 2002 the experimentalists and these events every year. the first instance the scientists want cal elements in GEO600 replaced a fraction of a second. Around three colleagues working with the the theoreticians were brought to- The discovery of gravitational to study how they react to environ- with permanent ones. The German per cent of the mass of black holes is GEO600. This is because they indi- gether administratively as well. The waves would enable astrophysicists mental influences, such as sun and American facilities will initiate transformed into gravitational wave cate that the gravitational waves are AEI now has two official sites, one in to open up a whole new observa- storms or particles of cosmic radia- regular observational operations to- energy when this cosmic collision much stronger than had thus far Golm and the other in Hanover; up tional window on the universe. “And tion. Whilst these do not generate wards the end of 2002. occurs. been supposed. The frequency range until then, the Hanover site had offi- detecting them would also be a land- gravitational waves, they may dis- The cosmic interpretation of the is also more favourable. “Two black cially been part of the Max Planck mark for Einstein’s General Theory BLACK HOLES turb the sensitive electronics in both measured signals they hope to ob- holes, which together are 18 times Institute for Quantum Optics in of Relativity”, says Bernard Schutz, ON THE COMPUTER instruments and give rise to what tain will depend crucially on the heavier than the Sun, emit gravita- Garching, near Munich (cf. page 109 Director of the AEI in Golm. This is seems to be a gravitational signal. In quality of theoretical models. A Although this is the most cata- tional waves with a frequency be- of this edition). because gravitational waves are addition, the physicists want to test group of young physicists (Baker, clysmic event in the universe imag- tween 600 and 900 Hertz”, explains In addition to merging black holes, among the few phenomena predicted a range of software packages that Brügmann, Campanelli, Lousto and inable, it remains completely invisi- Bernd Brügmann, a member of the the researchers hope to detect two by the theory of relativity, for which search for different cosmic gravita- Takahashi) at the Max Planck Insti- ble, as neither light nor radio waves Lazarus team. This frequency band is other kinds of events in particular. no evidence has yet been found. tional signals. tute for Gravitational Physics (Albert nor X-rays are emitted in the located exactly within the measuring One of these are supernovae. In a THOMAS BÜHRKE

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