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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 Universe 2018, 4, 86 3 of 12 p 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.
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