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A Brief History of Gravitational Waves

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A Brief History of Gravitational Waves Review A Brief History of Gravitational Waves Jorge L. Cervantes-Cota 1, Salvador Galindo-Uribarri 1 and George F. Smoot 2,3,4,* 1 Department of Physics, National Institute for Nuclear Research, Km 36.5 Carretera Mexico-Toluca, Ocoyoacac, Mexico State C.P.52750, Mexico; [email protected] (J.L.C.-C.); [email protected] (S.G.-U.) 2 Helmut and Ana Pao Sohmen Professor at Large, Institute for Advanced Study, Hong Kong University of Science and Technology, Clear Water Bay, 999077 Kowloon, Hong Kong, China. 3 Université Sorbonne Paris Cité, Laboratoire APC-PCCP, Université Paris Diderot, 10 rue Alice Domon et Leonie Duquet 75205 Paris Cedex 13, France. 4 Department of Physics and LBNL, University of California; MS Bldg 50-5505 LBNL, 1 Cyclotron Road Berkeley, CA 94720, USA. * Correspondence: [email protected]; Tel.:+1-510-486-5505 Abstract: This review describes the discovery of gravitational waves. We recount the journey of predicting and finding those waves, since its beginning in the early twentieth century, their prediction by Einstein in 1916, theoretical and experimental blunders, efforts towards their detection, and finally the subsequent successful discovery. Keywords: gravitational waves; General Relativity; LIGO; Einstein; strong-field gravity; binary black holes 1. Introduction Einstein’s General Theory of Relativity, published in November 1915, led to the prediction of the existence of gravitational waves that would be so faint and their interaction with matter so weak that Einstein himself wondered if they could ever be discovered. Even if they were detectable, Einstein also wondered if they would ever be useful enough for use in science. However, exactly 100 years after his theory was born, on 14 September 2015, these waves were finally detected and are going to provide scientific results. In fact at 11:50:45 am CET on 14 September 2015 Marco Drago—a postdoc —was seated in front of a computer monitor at the Max Planck Institute for Gravitational Physics in Hanover, Germany, when he received an e-mail, automatically generated three minutes before from the monitors of LIGO (for its acronym Laser Interferometer Gravitational wave Observatory). Marco opened the e-mail, which contained two links. He opened both links and each contained a graph of a signal similar to that recorded by ornithologists to register the songs of birds. One graph came from a LIGO station located at Hanford, in Washington State, and the other from Livingston Station in the state of Louisiana [1]. (a) (b) (c) Figure 1. (a): Waveforms from LIGO sites [2] and their location and sonograms. Figure from htt s:!!losc.ligo.org!events!GW150914/. 'he gravitational()ave event GW150914 o*served *+ the LIGO ,anford (H1, left column anels) and Livingston (L1, right column anels) detectors. 'imes are sho)n relative to 14 .e tem*er 2015 at 09:#$:45 /'0. For visualization- all time series are 2ltered )ith a 35–350 ,1 *and- ass 2lter to su ress large 5uctuations outside the detectors6 most sensitive fre7uency *and, and *and-re8ect 2lters to remove the strong instrumental s ectral lines. (b): 0hir ing . arro) (S izella asserina) song: fre7uency versus time (in seconds), sho)ing a song made u of a series of chir s. (c): 9 :i istrelle *at call for echolocation. Bats use ultrasound for “seeing” and for social calls. 'his s ectrogram is a graphic re resentation of fre7uencies against time. 'he color re resents the loudness of each fre7uency. 'his s ectrogram sho)s a falling call- )hich *ecomes a steady note. 'he +ello) and green *lotches are noise. 'his is nearl+ a time reverse of *lack hole merger sonograms. ?arco is a mem*er of a team of 3$ h+sicists )or>ing in ,anover- anal+zing data from ,anford and Livingston. ?arco6s dut+ is to *e a)are of and anal+ze the occurrence of an “event= that records the assage of a gravitational )ave, in one of the four lines that automaticall+ trac> the signals from the detectors in the t)o LIGO o*servatories on the other side of the Atlantic. ?arco noticed that the t)o graphs )ere almost identical- des ite having *een registered independentl+ in sites separated *+ "%$$ >m (see Figure 1a)@ for com arison )e include sonograms from animals (see Figures "* and 1c). 'he time that elapsed *et)een the t)o signals diAered *+ about 7 milliseconds. 'hese almost simultaneous records of signals coming from sites far a)a+ from each other- the similarit+ of their shapes and their large size, could not *e an+thing *ut- either: a ossi*le record of a gravitational )ave traveling at the s eed of light or- a “signal= arti2ciall+ “in8ected= to the detectors *+ one of the four mem*ers of the LIGO rogram )ho are allo)ed to “in8ect= dumm+ signals. 'he reason )h+ arti2cial signals are in8ected to the s+stem is to the test )hether the o eration of the detectors is correct and if the dut+ o*servers are able to identif+ a real signal. Follo)ing the re-established rotocols- ?arco tried to verif+ )hether the signals )ere real or the “event= was 8ust a dumm+ in8ected signal. .ince aLIGO was still in engineering mode, there was no )a+ to in8ect fake signals- i.e., hard)are in8ections. 'herefore, ever+one )as nearl+ "$$C certain that this was a detection. ,o)ever- it )as necessar+ to go through the protocols of ma>ing sure that this was the case. ?arco as>ed 9ndre) Lundgren- another ostdoc at ,anover- to 2nd out if the latter )as the case. 9ndre) found no evidence of a “dumm+ in8ection.= On the other hand- the t)o signals detected )ere so clear- the+ did not need to *e 2ltered to remove *ac>ground noise. 'he+ )ere o*vious. ?arco and 9ndre) immediatel+ honed the control rooms at Livingston and ,anford. It )as earl+ morning in the /nited .tates and onl+ someone from Livingston res onded. 'here )as nothing unusual to report. Finall+- one hour after receiving the signal- ?arco sent an e-mail to all collaborators of LIGO as>ing if an+one )as a)are of something that might cause a s urious signal. Do one ans)ered the e-mail. Ea+s later- LIGO leaders sent a report stating that there had not *een an+ “arti2cial in8ection.= ;+ then the news had alread+ *een lea>ed to some other mem*ers of the )orld communit+ of astro h+sicists. Finall+ after several months- the oFcial ne)s of the detection of gravitational )aves )as given at a ress conference on "" Februar+ 2016, after the team had ascertained that the signals )ere not the result of some eH erimental failure, or an+ signal locall+ roduced- earthquake, or electromagnetic 5uctuation. 'his announcement is the most im ortant scienti2c ne)s so far this centur+. Gravitational )aves )ere detected after G$ +ears of searching and "$$ +ears since the rediction of their eHistence. 'he scienti2c aper )as u*lished in Physical Review Letters [2]. 'his discover+ not onl+ con2rmed one of the most *asic redictions of General Ielativit+ *ut also o ened a ne) )indo) of o*servation of the universe, and )e affirm )ithout eHaggeration that a ne) era in astronom+ has been born. In )hat follo)s )e shall narrate the 8ourne+ ex erienced in search of gravitational )aves- including their conception in the earl+ t)entieth centur+- their rediction *+ Jinstein in "%"G- the theoretical controvers+- efforts to)ards detection, and the recent discover+. 2. Lost and Found Gravitational Waves On # Kul+ "%$# the Comptes Rendus of the French Academy of Sciences u*lished an article )ritten *+ ,enri :oincare entitled <.ur la d+namique d6 l6électron.= 'his )or> summarized his theor+ of relativit+ [3]. The wor> ro osed that gravit+ )as transmitted through a )ave that :oincaré called a gravitational wave (onde gravifique). It )ould ta>e some +ears for 9l*ert Jinstein to ostulate in "%"# in 2nal form the 'heor+ of General Ielativit+ [&]. ,is theor+ can *e seen as an eHtension of the . ecial 'heor+ of Ielativit+ ostulated *+ him "$ +ears earlier in "%$# [5]. 'he General 'heor+ eH lains the henomenon of gravit+. In this theor+- gravit+ is not a force—a diAerence from De)ton6s La)M*ut a manifestation of the curvature of s ace–time, this curvature *eing caused *+ the resence of mass (and also energ+ and momentum of an o*8ect). In other )ords- Jinstein6s equations match- on the one side, the curvature of local s ace–time )ith- on the other side of the equation- local energ+ and momentum within that s ace–time. Jinstein6s equations are too com licated to *e solved in full generalit+ and onl+ a fe) ver+ s eci2c solutions that descri*e s ace–times with ver+ restrictive conditions of s+mmetr+ are kno)n. Onl+ )ith such restrictions it is ossi*le to sim lif+ Jinstein6s equations and so 2nd eHact anal+tical solutions. For other cases one must make some sim li2cations or ap roHimations that allo) a solution- or there are cases )here equations can *e solved numericall+ using com uters- )ith advanced techniques in the field called Numerical Relativit+. .hortl+ after having 2nished his theor+ Jinstein con8ectured- 8ust as :oincarL had done, that there could *e gravitational )aves similar to electromagnetic )aves. 'he latter are roduced *+ accelerations of electric charges. In the electromagnetic case, )hat is commonl+ found is di olar radiation roduced b+ s)inging an electric di ole. An electric di ole is formed b+ t)o ( ositive and negative) charges that are separated *+ some distance. Oscillations of the di ole separation generate electromagnetic )aves.
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