Black Holes I: The Fatal Attraction of Stars

Ninety-nine years ago, as Europe threw itself sanguinely towards a catastrophic mutual massacre, a thirty-six-year-old Albert Einstein was sending to a scientific journal the article containing the final equations of general relativity. He could hardly have imagined how many and what kind of extraordinary unknown phenomena those equations would reveal.

The equations were complicated, and Einstein did not expect to be able to find exact solutions. And yet just a few weeks later, in January 1916, he received a letter from a lieutenant of the German artillery, who wrote: ‘As you will see, the war has been sufficiently accommodating to allow, despite the machine-gun fire, an excursion into the territory of your ideas.’ The letter was from Karl Schwarzschild, announcing that he had found an exact solution for Einstein’s equations. Four months later, Karl Schwarzschild was dead from an illness developed on the Russian front.

The solution he proposed describes the space surrounding a spherical mass such as the Earth or a star. If the mass is sufficiently extended, it exercises an attraction that is exactly the force of gravity described by Newton three centuries previously, and that we all studied at school. If the mass is concentrated, the force described by the equations of Einstein is more intense than Newton’s force, and one of its effects is to slow down clocks. But there is something strange in the solution found by Schwarzschild: if the mass is extremely concentrated, this solution predicts a spherical surface where all clocks would stop. Where time would stop passing.

What does this mean?

Einstein makes one of his numerous errors by maintaining that this surface, today known as the Schwarzschild surface, or horizon, could never be reached. He writes an article claiming that there could be no such thing as the objects described by the Schwarzschild solution. The article is wrong. Other theorists join in, and a lot of confusion follows. To understand what actually happens on the Schwarzschild surface we have to wait until the sixties, when mathematicians and physicists begin to untangle the threads and understand that the surface is not an impassable limit. In fact, it can be crossed without difficulty.

It is, instead, the limit of the region where gravity is so strong that nothing, not even light, can escape.

John Wheeler, with his gift for words, finds an appropriate name for this phenomenon: black hole. A black hole is a region where there is a mass that is so compact, so collapsed in on itself, that nothing can escape from its tremendous gravitational pull, not even light. A ray of light on the Schwarzschild surface remains stuck there, without moving, without being able to escape, frozen. Nothing escapes from a black hole; everything can enter into it.

The matter seemed more academic than scientific, because for this ‘Schwarzschild surface’ to exist requires an incredible degree of compression. The entire mass of our planet, for example, before it could become a black hole would have to be contained inside a marble with a diameter of just one centimetre.

Surely entities as compressed as this could not actually exist in the universe. Surely it was absurd to expect to squash the Earth into something smaller than a ping-pong ball! Or so it seemed at the time.

Still, when I was studying general relativity at university in the late seventies, my textbook chapter on black holes claimed that they were nothing more than a mathematical curiosity, and that ‘There is nothing like them in our real world.’

It was wrong, as is frequently the case with textbooks.

Already in 1972, an extremely compact and dark object in the Cygnus constellation had aroused the curiosity of astronomers. It would become known as Cygnus X-1. There is another star rotating around it at great speed. A black hole, John Wheeler will write, is like a man dressed in black who waltzes in a barely lit room with a woman dressed in white. We know that it is there only because we can see a bright star whirling around it.

The astronomers concentrated their efforts on Cygnus X-1. They managed to observe the light emanating from the matter that ignites as it spirals around it, drawing ever closer before disappearing, swallowed by the void. Soon afterwards, other, similar objects were identified and studied. All alternative explanations of the phenomenon were gradually eliminated, until the conclusion was inevitable: the heavens are full of black holes. Today, it is estimated that in our galaxy alone there are tens of millions of black holes similar to Cygnus X-1.

But there is more. Ever since the early thirties, it was known that transatlantic communications were interfered with by a strange source of radio waves. In 1974 scientists realized that the source of these waves was beyond the Earth, and that they are emitted in the Sagittarius constellation where the centre of our galaxy is. The observations concentrated on this source, called Sagittarius A,* and very gradually something astonishing became apparent: at the centre of our galaxy there is an immense black hole. Its mass is millions of times greater than that of the sun. There are numerous stars rotating around it. Every so often, one of these stars gets too close to this monstrous galactic Polyphemus and is swallowed like a small fish by a whale.

Today, astronomers are putting in place a network of huge radio antennae, extending from the Arctic to the Antarctic by way of the Rocky Mountains and the Andes, which should be able to ‘see’ the boiling-hot region surrounding the monster, where stars flock uncontrollably together with dust and detritus of every kind, forming a furious vortex in infernal tumult before plummeting into the black well.fn1

Similarly colossal black holes have been observed at the centre of almost all known galaxies. Some of these are voracious, ceaselessly devouring enormous quantities of stars and interstellar gas. The matter that plummets into them boils violently, reaching temperatures of millions of degrees, producing gigantic rays of energy that light up intergalactic space.

The most violent events that we can observe in the universe, such as the intense and mysterious signals that in the past were known as quasars, are produced by these titans, sometimes as luminous as an entire galaxy of 100 billion stars. Can you imagine a galactic storm being unleashed by a monster one billion times greater than the sun?