The Guide

The mys­ter­ies of black holes.

Sky at Night Magazine - - CONTENTS - With Ben Skuse

Ablack hole is a re­gion of space where mat­ter has be­come so densely packed that its grav­ity will let noth­ing es­cape – in­clud­ing light. Time comes to a stop at its edge, and its cen­tre may hide a point of in­fin­itely small vol­ume and in­fi­nite den­sity, where all laws of physics break down.

Pos­sess­ing the ter­ri­fy­ing power to de­vour any­thing un­lucky enough to pass within their grasp, it is lit­tle sur­prise that these cos­mic chasms have be­come a sta­ple of sci-fi since the term ‘black hole’ was coined in 1964.

Al­most 50 years be­fore this dis­cov­ery, Karl Sch­warzchild’s so­lu­tions to Ein­stein’s the­ory of gen­eral rel­a­tiv­ity had pre­dicted how a black hole could form. In 1916, Sch­warzchild pre­dicted that if mat­ter could be drawn tightly enough to­gether, it could suf­fer a cat­a­clysmic col­lapse to an in­fin­i­tes­i­mal point called a sin­gu­lar­ity, a bot­tom­less pit in the fab­ric of space-time.

He also pro­vided a use­ful dis­tance from the cen­tre of the black hole you would not want to go be­yond, the Sch­warzchild ra­dius. This marks the bound­ary where the speed re­quired to es­cape the grav­i­ta­tional pull of a black hole is equal to the speed of light. Be­yond this point, mat­ter and light are for­ever trapped. This is why we are not able to ac­tu­ally see in­side a black hole.

Given that they emit noth­ing, not even light, how do we know that black holes even ex­ist? The an­swer is that we have ob­served their ef­fects on other ce­les­tial bod­ies and ma­te­rial that we can see.

The light fan­tas­tic

One of the brightest clues is the ac­cre­tion disc; a flat­tened band of gas, dust and other de­bris from a star that has come close to the black hole but not quite fallen in. The par­ti­cles within the ac­cre­tion disc are ac­cel­er­ated to tremen­dous speeds by the black hole’s grav­ity, in the process re­leas­ing heat, X-rays and gamma rays, which can be seen by ded­i­cated ob­ser­va­to­ries. By also mea­sur­ing the or­bit of the star around the hid­den ob­ject, sci­en­tists can in­fer the lat­ter’s mass and size, and thereby con­firm it is in­deed a black hole. Us­ing this tech­nique, tens of stel­lar-mass black holes have been found. Sim­i­larly, many hun­dreds of su­per­mas­sive black holes, which sit at the heart of most gal­ax­ies, have been dis­cov­ered by ob­serv­ing the fast or­bits of stars and gas, and see­ing stars be­ing lit­er­ally torn apart by the strong grav­i­ta­tional field.

Black holes are so dense that they suck up any ma­te­rial that passes too close; even light can’t es­cape their clutches

Ra­di­a­tion em­a­nat­ing from ac­cre­tion discs is one way we can in­fer that a black hole ex­ists

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