From ab­surd ideas to black holes

A ship­bound stu­dent with time to kill was the first to fig­ure out the fate of dy­ing stars.

Cosmos - - Contents -

IN 1930 A 20-year-old In­dian stu­dent named Subrah­manyan Chan­drasekhar was sail­ing from Madras to Eng­land to pur­sue his stud­ies in as­tro­physics. Dur­ing the voy­age he toyed with equa­tions de­scrib­ing the sta­bil­ity of stars. And from a few lines of this math­e­mat­ics, a mo­men­tous dis­cov­ery emerged.

As­tronomers of the day had only a sketchy understanding of what makes stars tick. They knew that a star is a ball of hot gas en­gag­ing in a cosmic bal­anc­ing act. The gas tries to ex­pand out into the vac­uum of the sur­round­ing space but grav­ity holds it back. In stars like the sun, an equi­lib­rium is achieved, but only as long as the gas burns fuel to gen­er­ate heat, which we now know is pro­duced by nu­clear re­ac­tions in the core.

How­ever, un­cer­tainty sur­rounded the ques­tion of what hap­pens when the fuel runs out. It seemed that grav­ity would in­evitably gain the up­per hand, caus­ing the star to con­tract, and the smaller the ra­dius, the fiercer the grav­i­ta­tional force would be­come at the sur­face. As­tronomers had long been fa­mil­iar with tiny stars known as white dwarfs, which con­tain a mass com­pa­ra­ble to the sun but squashed into a vol­ume roughly the size of the Earth. Th­ese burned-out stel­lar rem­nants are so dense that their atoms are pressed cheek by jowl. Fur­ther com­pres­sion would mean the atoms them­selves would be crushed, which was ini­tially as­sumed to be im­pos­si­ble due to the laws of quan­tum physics.

From his nau­ti­cal cal­cu­la­tions Chan­drasekhar dis­cov­ered oth­er­wise. The equa­tions sug­gested that if a star has a big enough mass, the crush­ing ef­fect of its im­mense grav­ity would cause the atomic elec­trons to ap­proach the speed of light, ren­der­ing the stel­lar ma­te­rial more squishy and herald­ing the fur­ther grav­i­ta­tional col­lapse of the star. In the ab­sence of any other fac­tor, the ball of mat­ter would im­plode to­tally and van­ish down its own grav­i­ta­tional well, form­ing an ob­ject that today we call a black hole. But in the early 1930s such an ob­ject was con­sid­ered too out­landish to take se­ri­ously.

Chan­drasekhar was able to cal­cu­late the crit­i­cal mass above which this grav­i­ta­tional in­sta­bil­ity would set in. The an­swer he ob­tained was 1.44 so­lar masses, now known as the Chan­drasekhar limit. On reach­ing Eng­land, he an­nounced his re­sult, only to find it was ig­nored or dis­missed as non­sense from a young up­start. The most dis­tin­guished as­tronomer of the day, Sir Arthur Ed­ding­ton, pub­licly ridiculed Chan­drasekhar in an in­fa­mous en­counter at the Royal Astro­nom­i­cal So­ci­ety in 1935, declar­ing that there should be a law of na­ture “to pre­vent a star from be­hav­ing in this ab­surd way!”

Yet his­tory proved Ed­ding­ton wrong. If a burned-out star has a mass ex­ceed­ing Chan­drasekhar’s limit, it does in­deed col­lapse. One pos­si­ble fate is to form a so-called neu­tron star, in which the atoms are crushed into neu­trons and the ob­ject sta­bilises at a ra­dius about the size of Syd­ney. Neu­tron stars were dis­cov­ered in the 1960s and today form an im­por­tant branch of astron­omy. Most of them have masses not far from the Chan­drasekhar limit. More mas­sive stars end their days by to­tally col­laps­ing. When they shrink to a few kilo­me­tres across, their grav­ity is so great that even light can­not es­cape, and a black hole re­sults.

Al­though it took decades for the con­cept of a black hole to be fully un­der­stood and ac­cepted, the ba­sic idea was hid­ing in plain sight since just af­ter Al­bert Ein­stein first pub­lished his gen­eral the­ory of rel­a­tiv­ity in 1915. Chan­drasekhar ac­knowl­edged this in his 1983 No­bel Prize ad­dress, where he wrote: “This im­por­tant re­sult is im­plicit in a fun­da­men­tal pa­per by Karl Sch­warzschild pub­lished in 1916”. Al­though the the­o­ret­i­cal pos­si­bil­ity of a black hole was in­her­ent all along in Ein­stein’s the­ory, it took the youth­ful ge­nius of Chan­drasekhar to prove that such an ob­ject could re­sult from the trans­for­ma­tion of a dy­ing star.

By the time of the Prize, the ex­is­tence of black holes had be­come firmly es­tab­lished, and Subrah­manyan Chan­drasekhar’s cal­cu­la­tions fully vin­di­cated. Yet he was so stung by Ed­ding­ton’s de­ri­sion, he de­cided to leave the UK in 1937 and set­tle in the US, where he fol­lowed a dis­tin­guished ca­reer un­til his death in 1995.

Chan­drasekhar died leav­ing open a fas­ci­nat­ing ques­tion. Might there ex­ist an in­ter­me­di­ate state be­tween a neu­tron star and a black hole? This would be an ob­ject above 1.44 so­lar masses, too heavy to form a neu­tron star, but pre­vented from to­tal col­lapse by an ex­otic form of ul­tra-dense mat­ter such as a soup of quarks – the con­stituents of pro­tons and neu­trons. To date no­body has dis­cov­ered a quark star, but the no­tion re­mains a the­o­ret­i­cal pos­si­bil­ity, per­haps await­ing the at­ten­tion of an­other stu­dent ge­nius with the in­sight to set­tle the mat­ter.

In the 1930s, a ‘ black hole’ was too out­landish to take se­ri­ously.

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