The enigma of black holes

Black holes are perhaps the most bizarre of all celestial objects in the Universe. Objects so dense that in order to escape their gravity, you would have to travel faster than the speed of light! This being impossible.

Over the past few decades, black holes have gained considerable currency in popular culture and science fiction. It is not uncommon to see black holes as plot devices, most notably in the 2014 film Interstellar. Just the mere mention of black holes sets the imagination of people ablaze. But just what are they? In the 18^th century, clergyman and astronomer John Michell conjectured a type of star so incredibly dense that even light would be unable to escape its gravitational influence. Recall that at the speed of light (300,000 kilometres per second) you can circumnavigate the globe seven times in a second! However, this ‘dark star’ remained a matter of imaginative speculation rather than actual scientific inquiry for many years. Of course, until Einstein came along and introduced his theory of General Relativity. The theory revolutionised the way gravity was thought of since Newton. Instead of gravity being an instantaneous force that inexplicably arose between two objects, it was now, according to Einstein, the effect of matter curving the fabric of spacetime. So the Earth revolves around the Sun because the Sun’s mass bends space-time, which results in the Earth ‘falling’ towards it.

The theory also presented the possibility of an object, infinitely dense at the centre, whose escape speed was equivalent to that of light. This mystery object could now, thanks to general relativity, be mathematically described. The first such description was provided by Karl Schwarzchild, whose solution to Einstein’s field equation is known as the Schwarzchild solution. It mathematically defines a boundary for a black hole, known as the Event Horizon. Once an object crosses the Event Horizon, it is lost to the black hole forever. Thus, as long as you steer clear of the Event Horizon, you will avoid falling into the timeless abyss. To make some sense of this, consider the Sun. Say you wanted to make a black hole out of it. Though the Sun is ordained to eventually shed its outer layers and expose its inner core, we can nonetheless calculate what its hypothetical size aka, Event Horizon would be. This turns out to be about 3 kilometres, about the size of a sector in Islamabad! So if the Sun were to somehow suddenly transform into a black hole, you would have to be within 3 kilometres of it to actually fall in. Repeating the same exercise for the Earth yields an Event Horizon of about a centimeter! Despite what theoretical work was pointing to, significant doubts lingered about the actual existence of these objects.

The only way to determine decisively whether black holes exist or not was to observe one in nature. But therein lay another seemingly insurmountable problem. How does an astronomer even look for an object that does not emit any light? Although an isolated black hole would be impossible to find, a black hole with a companion star should produce observable effects. The black hole would pull the companion star’s gas towards it in a process known as accretion. As the black hole continues to accrete more and more gas, this will heat up the gas up to temperatures of millions of degrees Kelvin, producing X-rays! The black hole’s gorging of gas will be erratic, varying from one instance to the other. A fluttering X-ray source known as Cygnus X-1 was the first black hole to be discovered. From the motion of its companion star, the mass of the object was estimated to be about twenty times that of the Sun. Its size, estimated from the fluctuations in its X-rays, was found to be about the size of Jupiter. So imagine stuffing 20 Suns worth of mass into the physical extent of Jupiter! Such an enormously dense object could only be a black hole.

Since the 1970s, dozens of such stellar mass black holes have been discovered. It is thought that these black holes result from massive stars collapsing on themselves after they exhaust their nuclear fuel. After going supernova, what is left behind is a black hole. Soon after, astronomers stumbled upon a new black hole variant, found lurking in the centre of the Milky Way. The object, known as Sagittarius A*, has a mass of about 4 million Suns, contained in a size that roughly equals the orbit of Mercury! The only satisfactory explanation for such an object is once again a black hole. Such black holes are known as supermassive black holes. We now know that supermassive black holes are ubiquitous in the Universe. Almost every galaxy has one. They are especially important in explaining some extremely bright galaxies known as Active Galactic Nuclei, where the main power source for such galaxies is believed to be the exorbitant accretion of gas onto a supermassive black hole.

Also, as recently as 2017 the supermassive black hole at the centre of the active galaxy M87 was directly “imaged”. That is to say, that the extremely hot gas just outside the black hole’s event horizon was revealed via imaging using the Event Horizon Telescope, which is a network of multiple radio telescopes that acts as a giant Earth sized telescope. The images very clearly show the energetic gas, zooming around the black hole at near the speed of light. It also shows the black hole’s ‘shadow’, which is the region from which no light is expected to be seen.

But you may be tempted to ask what would happen if one were to fall into a black hole? What goes on inside a black hole itself is anyone’s guess, since nothing inside the black hole will ever reach us. However, we do know for sure what would happen as you get closer and closer to the black hole. The gravitational force of the black hole will become so extreme that the atoms in your body will be pulled apart and accelerated to speeds close to the speed of light. These ‘tidal forces’ have been observed to wreak havoc on unsuspecting stars that wander too close to the black hole’s domain. I would therefore strongly caution against jumping into a black hole. Instead, let us study these cosmic enigmas from afar, with our intellect and tools of observation.

Published in The Express Tribune, March 14^th, 2024.

Like Opinion & Editorial on Facebook, follow @ETOpEd on Twitter to receive all updates on all our daily pieces.