Seeing Black Holes: from the Computer to the Telescope

universe

Review Seeing Black Holes: From the Computer to the Telescope

Jean-Pierre Luminet 1,2,3

1 Aix Marseille University, CNRS, LAM, 13013 Marseille, France; [email protected] or [email protected] 2 Aix Marseille University, CNRS, CPT, 13009 Marseille, France 3 Observatoire de Paris, CNRS, LUTH, 92195 Meudon, France Received: 7 July 2018; Accepted: 6 August 2018; Published: 9 August 2018

Abstract: Astronomical observations are about to deliver the very first telescopic image of the massive black hole lurking at the Galactic Center. The mass of data collected in one night by the Event Horizon Telescope network, exceeding everything that has ever been done in any scientific field, should provide a recomposed image in 2018. All this, forty years after the first numerical simulations performed by the present author.

Keywords: black hole; numerical simulation; observation; general relativity

1. Introduction According to the laws of general relativity (for recent overviews at the turn of its centennial, see, e.g., [1,2]), black holes are, by definition, invisible. Contrary to uncollapsed stars, their surface is neither a solid nor a gas; it is an intangible frontier known as the event horizon. Beyond this horizon, gravity is so strong that nothing escapes, not even light. Seen projected onto the background of the sky, the event horizon would probably resemble a perfectly black disk if the black hole is static (Schwarzschild black hole) or a slightly flattened disk if it is rotating (Kerr black hole). A black hole however, be it small and of stellar mass or giant and supermassive, is rarely “bare”; in typical astrophysical conditions it is usually surrounded by gaseous matter. It forms an accretion disk in which the spinning gas is accelerated to large speeds by the huge gravity, releasing heat and high energy electromagnetic radiation. A giant black hole, as can be found in the centre of most galaxies, may also be surrounded by a cluster of stars, the orbital dynamics of which is strongly influenced by it. In essence, a black hole remains invisible, but in its own special way, it lights up the matter it attracts. Logically, scientists have wondered what a black hole lit up by its surrounding matter would look like. Educational or artistic representations can be seen in popular science magazines in the form of a sphere seeming to float in a whirlpool of glowing gas. These images, although forceful, fail to convey the astrophysical reality. A black hole can be described correctly using computer simulations that take account of the complex distortions made by the gravitational field on space-time and on the paths of light rays that follow its fabric. These were performed for the first time in 1978 by the author of this article [3]. Today, progress in astronomical observation is about to deliver the first telescopic image of the shadow of a giant black hole, thanks to the ambitious Event Horizon Telescope (EHT) programme (for a popular account, e.g., [4]).

Universe 2018, 4, 86; doi:10.3390/universe4080086 www.mdpi.com/journal/universe Universe 2018, 4, 86 2 of 12 Universe 2018, 4, x FOR PEER REVIEW 2 of 11

2. Black Black Holes Holes Simulated The notion of the blackblack holehole shadowshadow waswas introduced introduced for for the the first first time time in in 1972 1972 by by James James Bardeen Bardeen at ata Summera Summer school school in Les in Houches,Les Houches, France France [5]. He [5]. initiated He initiated research research on gravitational on gravitational lensing bylensing spinning by spinningblack holes black by computing holes by computing how the black how hole’s the rotationblack hole’s would rotation affect thewould shape affect of the the shadow shape that of the shadowevent horizon that the casts event on horizon light from casts a background on light from radiating a background screen. radiating screen. Next, Cunningham Cunningham and and Bardeen Bardeen [6] [6 ]ca calculatedlculated the the optical optical appearance appearance of ofa star a star in incircular circular orbit orbit in thein the equatorial equatorial plane plane of ofan an extreme extreme Kerr Kerr black black ho hole,le, taking taking account account of of th thee Doppler Doppler effect effect due due to relativistic motion motion of of the the star, star, and and pointed pointed out out the the corresponding corresponding amplification amplification of of the the star’s star’s luminosity. luminosity. The calculation of the black hole shadow can be generalized to the more complex situation when the radiating source is an accretion disk, each emitti emittingng point of the disk being equivalent to a point-like source in circular orbit. To To create the most realis realistictic possible images of a black hole surrounded by an accretion disk, notnot onlyonly dodo wewe havehave toto calculate calculate the the propagation propagation of of light light rays rays emitted emitted by by the the matter matter in inthe the disk disk through through the the curved curved space-time space-time geometry geometry generated generated by by the the blackblack hole,hole, butbut wewe also have to know the physical properties of the accretion disk it itself,self, in order to know the intrinsic flux flux emitted in its various regions. In In 1978, 1978, I I was was a a young young scient scientistist at at the the Paris-Meudon Paris-Meudon Observatory Observatory and and performed performed the firstfirst accurateaccurate numerical numerical simulation simulation of of the the “photographic” “photographic” appearance appearance of a of Schwarzschild a Schwarzschild black black hole holesurrounded surrounded by a thinby accretiona thin accretion disk. To disk. do so, To I used do theso, IBMI used 7040 the mainframe IBM 7040 of mainframe the Paris-Meudon of the Paris-MeudonObservatory, an Observatory, early transistor an computer early transistor with punch computer card inputs. with Withoutpunch card a computer inputs. visualisation Without a computertool, I had visualisation to create the finaltool, imageI had to by create hand fromthe fina thel digitalimage data.by hand For from this I the drew digital directly data. on For negative this I drewimage directly paper with on negative black India image ink, placingpaper with dots blac morek India densely ink, where placing the dots simulation more densely showed morewhere light. the simulationNext, I took showed the negative more oflight. my Next, negative I took to getthe thenega positive,tive of my the negative black points to get becoming the positive, white the and black the pointswhite backgroundbecoming white becoming and the black. white background becoming black. This image (Figure(Figure1 )1) appeared appeared first first in in the the November November issue issue of aof French a French popular popular magazine magazine [ 7] and [7] andconcluded concluded a 1979 a 1979 article article in a specializedin a specialized journal, journal, with with all equations all equations and technicaland technical details details [3]. [3]. The top of the disk remains visible regardless of the viewing angle—in contrast to the typical views of Saturn’s rings. Indeed, Indeed, the the gravitational gravitational fi fieldeld curves curves the the light light rays rays near the black hole so much thatthat thethe rearrear part part of of the the disk disk is “revealed”.is “revealed”. Even Even if the if blackthe black hole hideshole hides what fallswhat into falls it, into it cannot it, it cannotmask what mask is what behind is behind it. it.

Figure 1. Simulated photograph of a spherical black hole with thin accretion disk. The system is Figure 1. Simulated photograph of a spherical black hole with thin accretion disk. The system is seen seen from a great distance by an observer at 10° above the disk’s plane, in a frame at rest with the from a great distance by an observer at 10◦ above the disk’s plane, in a frame at rest with the black hole. black hole. © J.-P. Luminet, from [3]. © J.-P. Luminet, from [3].

The curving ofof thethe lightlight rays rays also also generates generates a secondarya secondary image image that that allows allows us tous see to thesee otherthe other side sideof the of accretion the accretion disk, ondisk, the on opposing the opposing side of side the blackof the hole black from hole the from observer. the observer. Very deformed Very deformed optically, optically,the rear part the looksrear part like looks a thin like halo a ofthin light halo around of light the around dark shadow the dark of shadow the black of hole, the black which hole, represents which represents the event horizon enlarged by a factor of 3√3/2 ≈ 2.6 due to the gravitational lens effect. Indeed the gravitational lensing generates an infinity of images of the disk, because the light rays can

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√ the event horizon enlarged by a factor of 3 3/2 ≈ 2.6 due to the gravitational lens effect. Indeed the gravitational lensing generates an infinity of images of the disk, because the light rays can travel any number of times around the black hole before escaping from its gravitational field and being observed by a distant astronomer. The primary image shows the upper side, the secondary image shows the lower side, the third image shows the upper side again, and so on. However, multiple images of order higher than 2 are not relevant for observational purposes because they are stuck to the edge of the black hole shadow. The main feature of this view of the black hole is the significant difference in luminosity between the various regions of the disk. On the one hand, the light shines maximally in the areas closest to the horizon, as the gas is hottest there since it moves more rapidly. On the other hand, the light received by a distant observer is considerably different from the light emitted, due to the combination of the Einstein and Doppler effects; the first caused by the gravity field, the latter by the rapid rotation of the accretion disk. For a distant observer, the light received is considerably amplified on the side of the image where the gas approaches the observer and is weaker on the side it is moving away from. The virtual photo of the black hole was calculated “bolometrically”, i.e., displaying integrated light on the whole electromagnetic spectrum, from the radio to the gamma wave range. It is independent of the mass of the black hole and the flow of gas swallowed, on the condition that the accretion rate remains moderate and the disk is thin (in other situations, the structure of the accretion disk may be thick, take the form of a torus, etc.). This image may therefore describe a stellar black hole 10 km in radius, attracting the gas from an accompanying star, or a giant black hole lying at the centre of a galaxy and sucking in the interstellar gas in thin disk configurations. This initial digital imaging work on black holes was then developed by numerous scientists, who benefited from the rapid progress in computer performances. Colours were added to the images (according to a specific coding dictated by variations in temperature) and background skies, to make the reconstitution as realistic as possible. Moreover, the observer was no longer assumed to be stationary and very distant from the black hole, but moving with it, which introduced a new distortion of the images by the Doppler effect due to the movement of the observer. Finally, the black hole can be rotating, such as in the Kerr solution, which is the most realistic astrophysical situation. However, although this rotation generates an additional asymmetry—the black hole event horizon is no longer strictly spherical—it remains small, even if rotating rapidly. Of the numerous visualisations created, some of which can be found on the Internet [8,9], those made at the start of the 1990s by my colleague Jean-Alain Marck at the Paris-Meudon Observatory, in colour and animated, are the most remarkable (for a technical description, see [10]). We made a film [11] which shows the spectacle that would be seen from the window of a spacecraft falling freely towards the black hole on various trajectories (Figure2). In the autumn of 2014, the world’s media waxed lyrical on the representations of the supermassive black hole, “Gargantua”, imagined by the filmmaker, Christopher Nolan, for his film Interstellar. The American astrophysicist Kip Thorne, a renowned specialist in relativistic astrophysics (he received the Nobel Prize for physics in 2017 for his work on gravitational waves), was technical advisor to a team of 200 graphic animation experts, using the most sophisticated calculation and visualisation tools to model the appearance of a giant black hole measuring one hundred million solar masses, rotating rapidly and surrounded by an accretion disk. The international press headlined the scientific realism of the calculated images, which resulted from a “simulation of unprecedented accuracy”. The most captivating view in the film is calculated for an observer located in the plane of the accretion disk in a frame at rest with the black hole (Figure3). While it correctly describes the primary and secondary images distorted by the gravitational field, it mistakenly shows uniform luminosity of the disk. In other words, it neglects all the physical effects due to its radiating structure and its rapid rotation. As Kip Thorne explained to me in a private email, the filmmaker decided that light asymmetry in the image would have been incomprehensible to the spectators. However, it is precisely this strong Universe 2018, 4, 86 4 of 12 asymmetry of apparent luminosity that is the main signature of a black hole, the only celestial object able to give the internal regions of an accretion disk a speed of rotation close to the speed of light and toUniverse induce 2018 a, very4, x FOR strong PEER DopplerREVIEW effect. 4 of 11

Figure 2. Colour simulations of a black hole accretion disk taking account of the Doppler and Figure 2. Colour simulations of a black hole accretion disk taking account of the Doppler and gravitational shifts. The images are calculated at positions 1 to 8 of the plunging trajectory gravitational shifts. The images are calculated at positions 1 to 8 of the plunging trajectory schematized schematized above. The last picture is taken from inside the black hole, the observer having rotated above. The last picture is taken from inside the black hole, the observer having rotated by 180◦ to watch by 180° to watch the outside. © J.A Marck and J.-P. Luminet, from [12]. the outside. © J.A Marck and J.-P. Luminet, from [12].

Slightly embarrassed by this bending of the scientiﬁcscientific truth,truth, Thorne and his colleaguescolleagues subsequently published an image in a technical journal, taking account of the effects of thethe spectralspectral shifts (Figure 4), but still based on an artistic view of the accretion disk rather than on a physical model [13]. Indeed, from the computer simulations by Marck performed 20 years earlier, Image 4 from Figure 2 is much closer to the astrophysical reality, as it was also calculated as seen by an observer in the equatorial plane, with all the shift effects and the Page and Thorne [14] physical model of a thin accretion disk. For a more detailed criticism of Interstellar science, see [15].

Universe 2018, 4, 86 5 of 12 shifts (Figure4), but still based on an artistic view of the accretion disk rather than on a physical model [13]. Indeed, from the computer simulations by Marck performed 20 years earlier, Image 4 from Figure2 is much closer to the astrophysical reality, as it was also calculated as seen by an observer in the equatorial plane, with all the shift effects and the Page and Thorne [14] physical model of a thin accretionUniverse 2018 disk., 4, x FOR For aPEER more REVIEW detailed criticism of Interstellar science, see [15]. 5 of 11 Universe 2018, 4, x FOR PEER REVIEW 5 of 11

Figure 3. Simulation of an accretion disk around a Kerr black hole as seen by an observer in the Figure 3. Simulation of an accretion disk around a Kerr black hole as seen by an observer in Figureequatorial 3. Simulationplane, shown of anin theaccretion movie disk Interstellar around. ©a DoubleKerr black Negative hole asartists/DNGR/ seen by an TMobserver & © Warner in the the equatorial plane, shown in the movie Interstellar. © Double Negative artists/DNGR/TM & © equatorialBros. plane, shown in the movie Interstellar. © Double Negative artists/DNGR/TM & © Warner Warner Bros. Bros.

Figure 4. An « anemic » accretion disk around a Kerr black hole with spin a/M = 0.6, false colours, FigureDoppler 4. and An gravitational« anemic » accretion shifts (from disk [13]). arou nd a Kerr black hole with spin a/M = 0.6, false colours, Figure 4. An « anemic » accretion disk around a Kerr black hole with spin a/M = 0.6, false colours, Doppler and gravitational shifts (from [13]). Doppler and gravitational shifts (from [13]). As is well-known, the Kerr spacetime metric depends on two parameters, the black hole mass M andAs its is normalized well-known,well-known, angular thethe KerrKerr momentum spacetime spacetime metrica ,metric but there depends dep isends a critical on on two two angular parameters, parameters, momentum the the black black given hole hole by mass amass = M M and its normalized angular momentum a, but there is a critical angular momentum given by a = M and(in units its normalized where G = angularc = 1) above momentum which thea, butevent there horizon is a critical vanishes, angular leaving momentum a naked singularity, given by a =alsoM (in units where G = c = 1) above which the event horizon vanishes, leaving a naked singularity, also (incalled units a Kerr where superspinar. G = c = 1) Although above which such configuratio the event horizonns are generally vanishes, considered leaving a as naked unrealistic, singularity, they called a Kerr superspinar. Although such configurations are generally considered as unrealistic, they alsocan be called studied a Kerr theoretically superspinar. as Althoughtheir external such field configurations is governed are by generally general rela consideredtivity. The as unrealistic,case of the can be studied theoretically as their external field is governed by general relativity. The case of the theyappearance can be studiedof Keplerian theoretically accretion as disks their externalorbiting fielda Kerr is governedsuperspinar by generalhas been relativity. extensively The studied case of appearance of Keplerian accretion disks orbiting a Kerr superspinar has been extensively studied thein [16], appearance which found of Keplerian that it differs accretion significantly disks orbiting from athose Kerr related superspinar to Kerr has black been holes. extensively studied in [16], which found that it differs significantly from those related to Kerr black holes. in [16Another], which way found of that visualising it differs a significantly “bare” black from hole those (without related an toaccretion Kerr black disk), holes. is to calculate the Another way of visualising a “bare” black hole (without an accretion disk), is to calculate the gravitationalAnother mirage way of visualisingit causes on a “bare”the background black hole of (without stars. The an accretionmost spectacular disk), is tointerpretations, calculate the gravitational mirage it causes on the background of stars. The most spectacular interpretations, gravitationalcombining scientific mirage accuracy it causes and on the aesthetics, background were of obtained stars. The in 2006 most by spectacular Alain Riazuelo interpretations, [17]. He combiningcalculated thescientific gravitational accuracy mirage and causedaesthetics, by a wereblack obtainedhole passing in 2006 in front by ofAlain a background Riazuelo [17].of stars, He calculatedthe disk of theour gravitational own galaxy ormirage Magellanic caused clouds by a black (Figure hole 5). passing In the insame front line, of aref. background [18] calculated of stars, the theappearance disk of our of aown cosmic galaxy microwave or Magellanic background clouds to (Figure observers 5). In orbiting the same in line,close ref. vicinity [18] calculated of Kerr black the appearanceholes or superspinars. of a cosmic microwave background to observers orbiting in close vicinity of Kerr black holes or superspinars.

Universe 2018, 4, 86 6 of 12 combining scientiﬁc accuracy and aesthetics, were obtained in 2006 by Alain Riazuelo [17]. He calculated the gravitational mirage caused by a black hole passing in front of a background of stars, the disk of our own galaxy or Magellanic clouds (Figure5). In the same line, ref. [ 18] calculated the appearance of a cosmic microwave background to observers orbiting in close vicinity of Kerr black holesUniverse or 2018 superspinars., 4, x FOR PEER REVIEW 6 of 11

Figure 5. Gravitational lensing produced by a black hole in a direction almost centered on the Large Magellanic Cloud. Above it one easily notices the southernmost part of the Milky Way with, from left to right, Alpha and Beta Centauri, the Southern Cross.Cross. The brightest star close to the LMC is Canopus (seen twice).twice). TheThe second second brightest brightest star star is Achernar,is Achern alsoar, also seen seen twice. twice. The two The large two arcs large are arcs the primaryare the andprimary secondary and secondary images ofimages the LMC, of the the LMC, two the smaller two smaller ones are ones those are of those the Small of the Magellanic Small Magellanic Cloud. CourtesyCloud. Courtesy Alain Riazuelo, Alain Riazuelo, CNRS/IAP. CNRS/IAP.

3. From the Computer to the Telescope All these illustrations are virtual images obtained using the equations of general relativity and more or less realistic physical models. But could we see a black hole directly? If astronomers had a sufﬁcientlysufficiently powerful telescope, they would be able to directly observe the shadow cast by the event horizon of a black hole andand thethe hothot markmark ofof thethe accretionaccretion diskdisk surroundingsurrounding it.it. However, several technical challenges challenges prevent prevent the the development development of of such such an an instrument. instrument. The The main main one one is isthe the tiny tiny size size of ofblack black holes holes seen seen from from the the Earth. Earth. The The closest closest known known stellar stellar black black hole, hole, located located in in the the binary X-ray source A0620-00 in our Galaxy, 35003500 light-years away, hashas aa diameterdiameter ofof justjust 4040 km.km. So, we have to aim at close, supermassive black holes, knowing that their size is proportional to their mass. The two most promising candidates candidates are are Sagittarius Sagittarius A* A* (Sgr (Sgr A*), A*), located located 26,000 26,000 light-years light-years (8 kpc) (8 kpc) away away in the in thecentre centre of our of our galaxy, galaxy, with with an anestimated estimated mass mass of of 4.3 4.3 million million solar solar masses masses [19], [19], and and the supergiant M87*, a monster of 6 or 7 billion solar masses, lying at the centre of the giant elliptical galaxy of M87, M87, 55 million light-years (16.5 kpc) away [[20].20]. Everything points to the fact that these black holes are surrounded by a rapidly rotating accretionaccretion disk, possibly formed of star debris previously broken up by tidal forces (Figure6 6).). In terms of intrinsic size, the event horizons of Sgr A* and M87* are 25 million and 36 billion kilometres in diameter, respectively. However, as already mentioned in Section 22,, thethe effecteffect ofof thethe gravitational lenslens causedcaused by by the the black black hole hole ampliﬁes amplifies the the apparent apparent size size of the of eventthe event horizon horizon by a factor by a offactor 2.6. of With 2.6. all With calculations all calculations done, it done, appears it appears that the that shadows the shadows cast by Sgrcast A* by and Sgr M87*A* and on M87* the gaseous on the halosgaseous of theirhalos accretion of their accretion disks have disks an apparent have an diameterapparent of diameter about 50 of microarcseconds. about 50 microarcseconds. This is the angle This is the angle under which we would see an apple on the Moon with the naked eye, requiring a resolution 2000 times greater than that of the Hubble space telescope. The resolution of a telescope is proportional to its aperture (the diameter of its lens) and inversely proportional to the wavelength at which it is observing. Doubling the aperture would show details twice as accurately. Thus, we would need a telescope operating in the visible range 2 km in diameter to resolve the images of Sgr A* and M87*, this is not possible, even in the medium-term. Another difficulty is that the neighborhoods of black holes remain hidden from our view in most frequency

Universe 2018, 4, 86 7 of 12 under which we would see an apple on the Moon with the naked eye, requiring a resolution 2000 times greater than that of the Hubble space telescope. The resolution of a telescope is proportional to its aperture (the diameter of its lens) and inversely proportional to the wavelength at which it is observing. Doubling the aperture would show details twice as accurately. Thus, we would need a telescope operating in the visible range 2 km in diameter to resolve the images of Sgr A* and M87*, this is not possible, even in the medium-term. Another difﬁculty Universe 2018, 4, x FOR PEER REVIEW 7 of 11 is that the neighborhoods of black holes remain hidden from our view in most frequency bands of the bandselectromagnetic of the electromagnetic spectrum. The spectrum. galaxy The centres galaxy are ce buriedntres underare buried dense under clouds dense of dustclouds that of blockdust that out blockmost ofout the most radiation. of the radiation. To pierce To through pierce thisthrough fog, thethis wavelength fog, the wavelength needs to needs be increased. to be increased.

(a) (b)

Figure 6. (a) The trajectories of several stars orbiting around the Galactic Center have been plotted Figure 6. (a) The trajectories of several stars orbiting around the Galactic Center have been plotted from infrared observations continuously performed from 1995 to 2012 by the Keck telescopes in from infrared observations continuously performed from 1995 to 2012 by the Keck telescopes in Hawaii. Hawaii. Their dynamical analysis implies the existence of a central massive black hole, Sgr A*, about Their dynamical analysis implies the existence of a central massive black hole, Sgr A*, about 4 million 4 million solar masses; (b) The giant elliptical galaxy Messier 87, located in the Virgo local solar masses; (b) The giant elliptical galaxy Messier 87, located in the Virgo local supercluster, shows a peaksupercluster, of luminosity shows at a its peak very of center, luminosity interpreted at its very as the center, intense interpreted emission of as gas the falling intense into emission supermassive of gas fallingblack hole intoabout supermassive 6 billion solarblack masses.hole about 6 billion solar masses.

Thus, at the millimetre wavelengths typical in radioastronomy, galaxy centres become almost transparent. The The problem problem is isthat that when when the the wavelength wavelength is doubled, is doubled, resolution resolution is divided is divided by two, by such two, thatsuch the that size the of size the of telescope the telescope needs needs to be increased to be increased further. further. So, to So,be able to be to able observe to observe the central the central black holeblack in hole our in galaxy our galaxy in the in millimetre the millimetre domain, domain, we wewould would need need a aradiotelescope radiotelescope about about 5000 5000 km km in diameter. Impossible? Not at all, because at these wavelengths, astronomers can use very large base interferometry (VLBI), a technique that combines se severalveral observatories into a single virtual telescope with an aperture as largelarge asas thethe distancedistance thatthat separatesseparates them.them. A terrestrial-sized VLBI network is therefore just sensitive enough to resolve an image as small as 50 ms ofof arcarc angle.angle. This is how the EHT (Event Horizon Telescope) project was designed in the first ﬁrst decade of this century, combining millimetric radiotelescopes across across the the planet planet in in a a network, network, in in the the hope hope of of capturing capturing the the first ﬁrst “real” “real” images of giant black holes (Figure7 7).). The idea of creating a worldwide network to obse observerve the black hole at the centre of the galaxy started in in 1999 1999 at at the the initiative initiative of of the the Dutch Dutch astronomer astronomer Heino Heino Falcke Falcke [21,22], [21,22 who], who works works today today at Nimègueat Nimègue University University in inthe the Netherlands. Netherlands. It Itwa wass further further developed developed by by various radioastronomy groups [23], [23], which merged in 2006 into the EHT consortium [[24].24]. Gradually, the network grew to include severalseveral observatoriesobservatories to to gain gain planetary planetary reach. reach. The The eight eight radioastronomical radioastronomical stations stations currently currently in inthe the network network include include Iram inIram Spain, in theSpain, Large the Millimeter Large Millimeter Telescope (LMT)Telescope in Mexico, (LMT) the in Submillimeter Mexico, the TelescopeSubmillimeter (SMT) Telescope in Arizona, (SMT) the Jamesin Arizona, Clark Maxwell the James Telescope Clark Maxwell (JCMT) and Telescope the Submillimeter (JCMT) and Array the (SMA)Submillimeter in Hawaii, Array the (SMA) South Polein Hawaii, Telescope the (SPT)South in Po Antarctica,le Telescope the (SPT) Atacama in Antarctica, Large Millimeter the Atacama Array Large Millimeter Array (Alma) and the Atacama Pathfinder Experiment (Apex) in Chile. Each of these instruments is located at altitude to reduce the atmospheric absorption of the signals. The whole set creates a virtual observatory with a 5000-km aperture. Two other observatories, the Greenland Telescope Project, in Greenland, and the Iram Noema interferometer, on the Bure plateau in the French Alps, will extend the network further and improve its performance.

Universe 2018, 4, 86 8 of 12

(Alma) and the Atacama Pathﬁnder Experiment (Apex) in Chile. Each of these instruments is located at altitude to reduce the atmospheric absorption of the signals. The whole set creates a virtual observatory with a 5000-km aperture. Two other observatories, the Greenland Telescope Project, in Greenland, and the Iram Noema interferometer, on the Bure plateau in the French Alps, will extend the network Universefurther 2018 and, 4 improve, x FOR PEER its REVIEW performance. 8 of 11

Figure 7. TheThe very very large large base base interferometry interferometry (VLBI (VLBI)) network network of of the the Event Event Horizon Horizon Telescope. Telescope. Courtesy EHT team.

4. A Long Wait Once thethe concept concept was was put put forward, forward, it then it hadthen to behad achieved to be despiteachieved a multitude despite a of multitude observational of observationaland technical and constraints. technical Forconstraints. example, For weather example, conditions weather needconditions to “cooperate” need to “cooperate” so that the so VLBI that thenetwork VLBI cannetwork avail ofcan crystal avail skiesof crystal simultaneously skies simultaneously at eight places at eight on four places continents, on four as continents, observations as observationsare done at the are 1.3 done mm at wavelength, the 1.3 mm at wavelength, which the signals at which are the detected, signals is are also detected, absorbed is and also emitted absorbed by water.and emitted Thus, theby water. main problem Thus, the is themain presence problem of is water the presence vapour in of the water atmosphere. vapour in Another the atmosphere. constraint Anotheris imposed constraint by the useis imposed of the Alma by the telescope use of the in Chile,Alma thetelescope most requestedin Chile, the radio most observatory requested inradio the world.observatory Ultimately, in the theworld. EHT Ultimately, teams only the has EHT a two-week teams only window has a each two-week year in window which to each attempt year the in whichgroup observations.to attempt the Theygroup had observations. to wait until They April had 2017 to towait have until four April full, 2017 clear to nights, have four two forfull, Sgr clear A* nights,and two two for M87*.for Sgr However, A* and theretwo for was M87*. no possibility However, of there seeing was an imageno possibility directly onof aseeing screen. an Building image directlya high-resolution on a screen. image Building by VLBI a high-resolution requires the combination image by VLBI of the requires signals capturedthe combination by the various of the signalsnetwork captured aerials. To by do the so, various atomic clocksnetwork are aerials. used to To measure do so, the atomic arrival clocks time ofare the used signals to measure to one tenth the arrivalof a billionth time of of the a second, signals toto compareone tenth them of a inbillio realnth time of a and second, triangulate to compare with theirthem point in real of time origin and to triangulatereconstitute with an overall their image.point of With origin eight to observatoriesreconstitute an spread overall around image. the globe,With eight including observatories in places withspread poor around Internet the links, globe, the including EHT scientists in places had towith record poor the Internet data separately links, the and EHT store scientists it on hard had disks to recordto combine the data them separately subsequently. and store it on hard disks to combine them subsequently. The mass of data collected exceeded anything that had ever been done in all scientific scientific fields fields together: oneone nightnight of of observation observation collected collected 2 petabytes 2 petabytes of data, of data, as much as asmuch is collected as is collected in one complete in one completeyear of experiments year of experiments at the LHC, at the the LHC, CERN the large-hadron CERN large-hadron collider collider that led that to theled discoveryto the discovery of the ofHiggs-Englert the Higgs-Englert boson boson in 2012, in following2012, following analysis analysis of 4 million of 4 million billion billion proton-proton proton-proton collisions. collisions. The hard disks of data stored in Antarctica then had to wait until December and the end of the long glacialglacial winter,winter, to to be be transported transported in in secure secure flight flight conditions conditions to join to join the severalthe several thousand thousand hard diskshard diskscentralised centralised at the MITat the Haystack MIT Haystack Observatory Observatory in Massachusetts in Massachusetts and the Maxand Planckthe Max Institute Planck forInstitute Radio Astronomyfor Radio Astronomy in Bonn. There, in Bonn. clusters There, of clusters supercomputers of supercomputers started to process started theto process mountain the of mountain data, a task of data, a task that takes several months to obtain just a few pixels of an image of the massive black holes at the centre of the Milky Way and the farthest galaxy, M87. The results will probably not be published until later in 2018. However, as suggested by the digital simulations recently done by the EHT scientists [25], the obtained recomposed image should resemble a brilliant crescent surrounding a black disk, placed on the side where the hot spot of the accretion disk is moving towards the observer (Figure 8). As could already be predicted from our simulations forty years ago (Figure 1), by squinting your eyes to reduce the ocular resolution, the clear outline of a black hole surrounded by its accretion disk is

Universe 2018, 4, 86 9 of 12 that takes several months to obtain just a few pixels of an image of the massive black holes at the centre of the Milky Way and the farthest galaxy, M87. The results will probably not be published until later in 2018. However, as suggested by the digital simulations recently done by the EHT scientists [25], the obtained recomposed image should resemble a brilliant crescent surrounding a black disk, placed on the side where the hot spot of the accretion disk is moving towards the observer (Figure8). As could already be predicted from our simulations forty years ago (Figure1), by squinting your Universe 2018, 4, x FOR PEER REVIEW 9 of 11 eyes to reduce the ocular resolution, the clear outline of a black hole surrounded by its accretion disk is indeed the black silhouettesilhouette of its event horizon surroundedsurrounded by the brilliant spot of thethe diskdisk ampliﬁedamplified by the Doppler effect. A very different, although impr improbableobable result, would mean that general relativity is incorrect in very very strong strong gravitational gravitational fields, ﬁelds, and and new new physics physics would would be be necessary. necessary.

Figure 8. Synthetic image of the time variable black hole Sgr A* that could be recovered with an array Figure 8. Synthetic image of the time variable black hole Sgr A* that could be recovered with an array of 8 radiotelescopes and averaging 8 epochs (from [25]). of 8 radiotelescopes and averaging 8 epochs (from [25]).

5. The Golden Age of Relativistic Astrophysics As well as the hopehope ofof capturingcapturing the firstfirst “photos”“photos” ofof blackblack holes,holes, EHTEHT astrophysicistsastrophysicists hope to garner a lot of informationinformation that will enableenable them to betterbetter understandunderstand the very special physics that operate in the environment close to blackblack holes,holes, namely the giganticgigantic jets ofof particlesparticles andand radiationradiation that some project intointo spacespace at speeds close to that of the speed of light [[26].26]. This is the case for M87*, where thethe jetsjets areare largerlarger thanthan thethe galaxygalaxy itselfitself (Figure(Figure9 9).). IfIf SgrSgr A*A* producesproduces jets,jets, theythey areare tootoo smallsmall oror not brightbright enoughenough to to be be detected detected until until now. now. The The jets playjets anplay important an important role in role theevolution in the evolution of galaxies; of forgalaxies; example, for example, by heating by interstellarheating interstellar space, they space, can they prevent can coolingprevent ofcooling the gas of thatthe gas allows that stars allows to form.stars to The form. most The probable most physicalprobable explanation physical explanation is that they areis that produced they are by theproduced twisted magneticby the twisted fields associatedmagnetic fields with blackassociated holes. with The existenceblack holes. of suchThe magneticexistence fields,of such predicted magnetic by fields, theoretical predicted studies, by involvestheoretical a dynamo-typestudies, involves interaction a dynamo-type between intera a rotatingction black between hole a and rotating the inner black parts hole of and its accretionthe inner disk.parts Jetsof its are accretion expected disk. to be Jets fed energyare expected either byto be an accretionfed energy disk, either or byby conversion an accretion of somedisk, ofor theby rotationalconversion energy of some of theof blackthe rotational holes themselves energy [of27 ,the28]. black Black holeholes magnetic themselves fields [27,28]. are confirmed Black hole by observations,magnetic fields and are EHT confirmed telescopes by are ableobservations, to record theand polarization EHT telescopes signal (Faradayare able rotation) to record related the topolarization the strong signal magnetic (Faraday field ofrotation) the galactic related black to th holee strong Sgr A*. magnetic As the amountfield of the of Faradaygalactic rotationblack hole is proportionalSgr A*. As the to theamount integral of Faraday of the magnetic rotation field is proportional strength along to thethe lineintegral of sight, of the such magnetic observations field allowstrength us along to draw the a line map of of sight, the magnetic such observations field near allow the event us to horizon draw a of map Sgr of A*, the which magnetic could field perhaps near revealthe event the horizon physical of mechanisms Sgr A*, which at thecould origin perhaps of the reve jets.al VLBI the physical observations mechanisms in 2015 at started the origin to provide of the somejets. VLBI clues observations as to the structure in 2015 of st Sgrarted A*’s to magnetic provide some field, hintingclues as atto the the hypothesis structure of of Sgr a rapidly A*’s magnetic rotating blackfield, hinting hole [29 at]. the hypothesis of a rapidly rotating black hole [29]. The use of a setset ofof veryvery differentdifferent instrumentsinstruments andand methodsmethods promisespromises even moremore spectacularspectacular developments inin the the near near future, future, including including an accurate an accurate description description of black of holes black and holes their immediateand their environment.immediate environment. The optical interferometerThe optical interferometer Gravity [30], being Gravity built [30], at the being Very built Large at Telescope the Very (VLT) Large at theTelescope European (VLT) Southern at the ObservatoryEuropean So inuthern Chile, Observatory and the next in generation Chile, and of the optical next telescopesgeneration in of the optical 30 m telescopes in the 30 m diameter class, will be able to follow the stars around Sgr A* orbiting at only a few hundred times the radius of the black hole, and measure the precession of their pericentres to deduce the angular momentum (spin) of the black hole. In particular, it was expected that the follow-up observations of S2 star closely orbiting Sgr A* might allow us to measure the Schwarzschild-like gravitational field of Sgr A* [31]. Indeed, as the present article was just completed, the observation of the high velocity of S2 star at its passage to pericentre in May 2018 and the associated gravitational redshift (Einstein effect) was announced, confirming once again the validity of General Relativity in the regime of strong gravitational field [32]. In the next few years, the radio interferometer SKA (Square Kilometer Array) [33], built in South Africa and Australia, will be able to follow the orbits of pulsars around the galactic black hole, timing them ultra-precisely to

Universe 2018, 4, 86 10 of 12 diameter class, will be able to follow the stars around Sgr A* orbiting at only a few hundred times the radius of the black hole, and measure the precession of their pericentres to deduce the angular momentum (spin) of the black hole. In particular, it was expected that the follow-up observations of S2 star closely orbiting Sgr A* might allow us to measure the Schwarzschild-like gravitational field of Sgr A* [31]. Indeed, as the present article was just completed, the observation of the high velocity of S2 star at its passage to pericentre in May 2018 and the associated gravitational redshift (Einstein effect) was announced, confirming once again the validity of General Relativity in the regime of strong gravitational field [32]. In the next few years, the radio interferometer SKA (Square Kilometer Array) [33], built in South Africa and Australia, will be able to follow the orbits of pulsars around the Universe 2018, 4, x FOR PEER REVIEW 10 of 11 galactic black hole, timing them ultra-precisely to test their properties. Eventually, the Lisa spatial testinterferometer their properties. (Laser InterferometerEventually, the Space Lisa Antenna) spatial [interferometer34], once in orbit, (Laser will beInterferometer used to capture Space the Antenna)gravitational [34], waves once emittedin orbit, whenwill be small used compact to capture objects the turngravitational around supermassivewaves emitted black when holes small in compactnearby galaxies. objects turn around supermassive black holes in nearby galaxies. Relativistic astrophysics, still in its infancy in the 1970s due to to a a lack of experimental resources, resources, is now now entering entering a a golden golden age. age. Already Already with with EHT, EHT, by bycapturing capturing the thesignals signals of what of what is happening is happening very veryclose closeto a supermassive to a supermassive black hole, black astrophysicists hole, astrophysicists will be willable beto abletest Einstein’s to test Einstein’s theory of theory general of relativitygeneral relativity in the most in theextreme most conditions. extreme conditions. These measurements These measurements will complete will historical complete detection historical of gravitationaldetection of gravitationalwaves from waves2015 [35], from produced 2015 [35], producedwhen pairs when of stellar-mass pairs of stellar-mass black holes black collide, holes providingcollide, providing the best the evidence best evidence so far of so the far existence of the existence of black of holes. black EHT holes. data EHT will data be willable be to ablegive tous give the finalus the proof. final proof.

Figure 9. The radio jet of the elliptical galaxy M87 is also visible in the optical range. Detailed Figure 9. The radio jet of the elliptical galaxy M87 is also visible in the optical range. observations from the Hubble Space Telescope show various substructures, perfectly aligned from Detailed observations from the Hubble Space Telescope show various substructures, perfectly aligned the central nucleus up to distances much greater than the size of the galaxy itself (© STSci/NASA). from the central nucleus up to distances much greater than the size of the galaxy itself (© STSci/NASA).

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