Olivier LooseDecember 20201Catch Me If You Can, Said the Black Hole.About one of the most enigmatic objects in our Universe.It goes without saying that it is hard to observe something that we cannot perceive with ourown eyes. Luckily, science and technology have come a long way in helping us to detect theunseeable.Think of optical and atomic force microscopes showing how the Shewanella oneidensisbacteriumextractsoxygen from toxic metals, mirrors and optical cavities that single out arubidium atom, or spectrographs and telescopes revealing the presence of, among otherchemical elements, neon in the hot gas cloudOmega Nebula, 52 million billion kilometres awayfrom Earth.Pricklier still is to discern something that does not send out any detectable information at all: ablack hole.What is a black hole, and how do we know it is out there if we cannot detect it?A Star Is BornThe idea of a black hole originated in the 18thcentury, when John Michell and Pierre-SimonLaplace coined the conjecturedphenomenon ‘dark star’.In the latter half of the 20thcentury, divided global politics led independently todifferent namesfor the same object: physicists to the east of the Iron Curtain referred to ‘frozen stars,’ whilethose to the west spoke of ‘collapsed stars’.Notwithstanding the political schism, it was John Wheeler–on thesuggestionof an audiencemember attending one of his lectures–who finally managed to popularize the term‘black hole’among academia in 1968, even thoughRobert DickeandAnn Ewingused the phrase alreadyin 1960 and 1964, respectively.
Olivier LooseDecember 20202Risen from the EquationsMathematically, the concept of a black hole arisesfrom Albert Einstein’s equations within histheory of general relativity, in spite ofEinstein’sreluctanceto acknowledge its existence.General relativitymaintainsthat gravity is the result of the bending of spacetime–which inturn is the consequence of the presence of large matter or energy distributions–and that mattermoves along a curved path in spacetime.It was Karl Schwarzschild who in 1916 worked out a solution toEinstein’s formulasand foundthat within a certain distance from a non-rotating, uncharged, spherically symmetricstar’sinnercentre (theSchwarzschild radius) spacetime starts to curve gradually stronger towards itscentre, at which point the curvature becomes apparently infinite–technically called asingularity.Fig. 1. Spacetime configuration of a black hole. (Source:Unnikrishnan MenonandMartin Silvertant).A black hole is then defined as an object with a radius equal to or less than the Schwarzschildradius of its original star. For instance, with a radius of 695,700km, theSunwould turn into ablack hole if we could shrink her down to an object with a radius of not more than 2.95km–the Sun’s Schwarzschild radius. In the case of the Earth, that would come down to 0.89cm.
Olivier LooseDecember 20203Other general relativistic solutions includeReissner-Nordströmblack holes (charged,