Space is chocked full of weird and often terrifying objects, but perhaps the most infamous of them all is the “Black Hole”. These shadowy chasms of mystery have incited fear and wonder for decades. Humans intrinsically fear that which cannot be seen or understood. Take horror movies as an example: nowadays they are generally special effects extravaganzas, brimming with a kaleidoscopic of visuals and unnecessary gratuitous gore. But most movie buffs will tell you that the best way to build tension, suspense and fear, is to allow our boundless imagination to do the work for us. The Omen, Alien and The Shining are all seen as classics for a reason!
Our knowledge of space is vast, but black holes remain an enigma. During my stargazing evenings, it is normal to receive a barrage of questions pertaining to these caliginous cosmic curiosities. Where do they come from? What is inside them? How can we find them? The greatest minds on the planet are still trying to unravel these mysteries, so my answers are usually somewhat simple! That being said, as technology evolves, so does our understanding. Here is what we do know:
First, let us dispel one of the most common myths surrounding black holes. They do not suck. Nor do they maraud through space eating everything in their path like some sort of apocalyptic Pacman! There is an overwhelming opinion that black holes drift through space hoovering up everything in their path like a giant vacuum cleaner. This is simply not the case. In fact, if we replaced our Sun with a black hole of the same mass (very important!), absolutely nothing in our solar system would change – except that it would be pitch black, freezing and incapable of harbouring life. The mass of an object, and the way in which its gravity warps space and time, is the key.
So, what then is a black hole? In simple terms, it is an object that has such an enormous amount of mass and gravity, that nothing, not even light, can escape its grasp. The more massive the black hole, the wider its sphere of influence. Anything crossing this plane will never be seen again. This boundary, the point at which gravity permits light to escape, is known as the event horizon. It is worth noting that we tend to think of these objects as 2-dimensional and flat, but they are 3-dimensional, their influence spreading out from its centre like a bubble. Perhaps ‘black sphere’ would be a better term?
But what is at the centre of a black hole? We just do not know. If light cannot escape, nothing can, and that includes information. Beneath the event horizon lies a void that is the very epitome of the unknown; a true heart of darkness. Relativity and quantum physics have theories, but nothing can be tested, let alone proved. Black holes feed on material, and that material falls towards its centre. Akin to passing through a giant oesophagus, it is stretched out as the gravity increases, until it all occupies the same tiny space at the black hole’s core. This core is known as the singularity. The more it feeds, the more the mass increases, and the wider its event horizon spreads.
We do know that there are various types of black holes, based on their size and gravity.
Stellar Mass Black Holes are the smallest category and are formed when the cores of supergiant stars collapse, or when two neutron stars collide and merge. Their mass ranges from a few solar masses to around 50 times the mass of our Sun, with an event horizon radiating only a few 100kms.
Intermediate Sized Black Holes are rare but can be found at the centre of some globular star clusters. They range from 50 – 50 000 times the mass of our Sun and are thought to form when stars in a cluster rapidly collapse in a devastating chain reaction, ultimately merging together.
Super Massive Black Holes can be found at the heart of almost all galaxies, including our own. As their name suggests, they are enormous, containing the mass of billions of Suns, their influence spreading out over billions of kilometres, some even larger than the entirety of our solar system! Their formation is poorly understood but they are possibly the result of multiple stellar mass black holes merging, or even the rapid collapse of giant dust clouds in the early Universe.
You might wonder how we can study these mysterious phenomena at all, bearing mind they are invisible. The idea of black holes was originally theorised by the great Albert Einstein based on his General Theory of Relativity. The first one to be discovered was in 1964, but it took 30 years of scientific study and debate between Stephen Hawking and Kip Thorne before it was officially declared.
Since then, our technology has improved dramatically, and we have identified over 50 stellar mass black holes in the Milky Way alone. We can infer their existence by monitoring their affect on nearby stars. We understand the laws of gravity very well – we slingshot spacecraft around planets, and NASA’s recent ‘DART’ mission even managed to fire a fridge-sized object into a tiny asteroid billions of kilometres away! We can look at the orbital path of stars and work out the size and mass of the object they are orbiting. Something so small and but so massive, can only be a black hole.
Recently, the first photograph of a super massive black hole was taken. It may seem counter-intuitive to photograph an object that does not emit any light, but surrounding the event horizon is a huge melting pot of radiation and energy being created by the objects about to be swallowed, as they whirl around the periphery at almost the speed of light. Their death throws create highly charged particles which can be seen, and captured by a large enough radio telescope. Unfortunately such an instrument does not exist, but sometimes the sum of its parts is greater than the whole.
After years of planning, in 2017, radio telescopes from eight sites around the world trained their dishes at the centre of a galaxy in the constellation of Virgo, known as M87. The combined power of the ‘Event Horizon Telescope’, translated to a resolution more than 4 000 times that which is capable by Hubble. Four days and 5 000 terabytes later, this astronomical amount of data was processed, and the first photograph of a black hole, or more precisely, its silhouette was revealed in 2019. Since then, the data has been reprocessed and in 2023, a clearer photo was produced. To some, it may not look like much, but this achievement should be heralded as a red-letter day in astronomy. It was the day that the unseen became seen, and marked the start of a new era of discovery that could hold the very key to unravelling the fabric of the Universe.
What secrets these incredible objects might reveal remains to be seen. Whether or not they can rip holes in the space-time continuum, forming wormholes between points in space, or even parallel universes, is confined to the realm of science fiction movies – for now at least. Who knows what else we will discover? I for one cannot wait to see where these inky rabbit holes may lead, and what wonderlands might reside in their murky depths.
