Black holes: from absurd idea to fact of nature
A shipbound student with time to kill was the first to figure out the fate of dying stars. Paul Davies explains.
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.