Given that astronomy dates back to early written records from the Babylonians around 1600 BC if not earlier, black holes are a relatively new addition to human knowledge of the cosmos. Black holes cannot be seen therefore it took a long time for the first one to be found.
Composed of matter so compressed that not even light can escape them, they are invisible yet their presence exerts a powerful gravitational force on nearby objects such as stars and gas. Any surrounding material is pulled in by gravity, spiralling inwards to fall onto an accretion disc. Even though the black hole itself cannot be seen, the accretion disc can be very luminous due to the energy released when material falls onto it. Some of the gas will then fall further to the event horizon and be lost forever to the strong gravitational pull of the black hole, whilst other material will escape and be ejected as intense jets of energy, detectable at intergalactic distances.
Speculations about superdense celestial bodies began in the 1700s, but the modern concept of a black hole was first described in 1916 by Albert Einstein. Black holes were predicted by his theory of general relativity which re-interpreted gravity was not as a force, as predicted by Isaac Newton, but rather of as a distortion or warping of space and time. This doesn’t mean that Newton was wrong as his formulations are still valid for gravitational fields that are not too strong. However Einstein laid the ground work for the current view of a black hole as regions of space and time that are distorted in such way that nothing can escape that falls into them, not even light. The term ‘black hole’ was popularised by an American theoretical physicist, John Wheeler (1911-2008), in 1967, who summarised general relativity in the phrase
‘…matter tells space-time how to curve, and space-time tells matter how to move.’
Despite Einstein’s prediction, the first prospective black hole to be detected was not discovered until 1971 in the form of the X-ray source Cygnus X-1. As black holes don’t emit any light, they cannot be found in the traditional way of looking with telescopes. Even though no light is emitted, black holes can be still be detected in other indirect ways, inferring their presence by the effects they have on their surrounding. Astronomers can find black holes by measuring the visible light, radio waves and x-rays emitted by the material in the accretion disc of a black hole. This is exactly how Cygnus X-1 was found. Alternatively, if a star is orbiting a black hole, astronomers can measure the light the star is emitting and consequently the speed through the doppler effect on the wavelength of the radiation. Once this is known, the laws of gravity can be used to infer whether the star is in fact orbiting a black hole or something else by determining its mass.
Black holes are common in the Universe but they come in a variety of sizes. Current observations suggest that black holes come in three main types: supermassive black holes, stellar black holes and intermediate black holes. Supermassive black holes are as domineering as their name, millions of times heavier than the Sun yet having a radius similar to that of the Sun. They are most commonly thought to call the centre of galaxies their home, there is one even lying at the centre of our own galaxy, the Milky Way and another at the centre of the neighbouring galaxy M31 in Andromeda. It isn’t known exactly how supermassive black holes form, but one theory is that they are the result of many smaller black holes merging together. Gas clouds which collapsed together in the distant past could be responsible while another theory is that globular clusters have accumulated and fallen together. Once a supermassive black hole is formed it draws in any surrounding matter from dust and gas around them, increasing in size.
Stellar black holes are the consequence of a large star collapsing. Smaller stars up to at least three times the size of the Sun will burn through the last of their fuel and become a white dwarf or a neutron star. However when a large star collapses it can form a stellar black hole, which is tiny in terms of radius, just a few kilometres across, yet possessing incredible density resulting in a large gravitational pull. Our own galaxy likely contains a few hundred million stellar black holes.
Intermediate black holes are a new discovery as they could be the product of stars colliding in a chain reaction. As a result several black holes will be created in the same region of space, as they start to draw in material they could collide and eventually form a supermassive black hole. Intermediate black holes are hard to find, one was discovered in the arm of a spiral galaxy in 2014, however Tim Roberts of the University of Durham explained Intermediate black holes are …’acting like a long-lost relative that isn’t interested in being found.’
Previously I had said that there were three types of black holes, however NASA’s Hubble Telescope has recently discovered a galaxy called Markarian 231 powered by a quasar, a large object that emits energy and appears like a star. At the centre this galaxy contains two black holes spinning around each other, almost like a double black hole. Such systems might in fact be common in the centre of galaxies, though difficult to find. The primary black hole in this case around 150 million times the mass of the Sun, whereas the partner black hole is 4 million times its mass. As mentioned before black holes have accretion discs of material around them, however this one caused by the double black hole is brighter the rest of the entire galaxy. At some point in the future it is expected that the two black holes will collide, generating an intense burst of gravitational waves.
A black hole is lurking at the centre of our home galaxy, the Milky Way. Could this harm us on Earth? Well the Earth lies about 25,000 light years from the centre of the Milky Way and it takes the Sun around 250 million years to complete one rotation around its centre. The black hole there is far to far away to cause us any harm. While it is easy to imagine a black hole as a giant invisible Pac-man munching on any and all space material, this isn’t in fact the case. The black hole is still tiny compared to the vastness of the Galaxy. It is a bit like trying to drain an entire ocean through a bath plug, it will take a very long time! Even if the Sun was to be replaced by a black hole of the same mass, the planets would also continue to orbit the black hole just as they do the Sun, as it would possess the same gravity. However, when the Sun dies it will eventually end up as a white dwarf star, and not a black hole. It simply does not have enough mass for gravity to take over and cause it to collapse all the way to become a black hole.
Black holes are topics not just subjects of study in astronomy, they feature on the screen, for example in episodes of Futurama, the Simpsons and the award winning Interstellar movie. From stealing sandwiches and sinking the parodied ‘Titanic’ ship in Futurama to taking Matthew McConaughey into another dimension in Interstellar, it is clear that there are different perceptions of black holes and their capabilities. Read about one such perspective by the AOP Director, Michael Burton at this link.
Interstellar is a unique piece of work as the visual effects team took a lot of advice from Kip Thorne, the American theoretical physicist who has worked on gravitational physics and astrophysics. Most of the action in Interstellar is centred on a giant black hole known as ‘Gargantua.’ However on screen it had to be depicted just right, Thorne said himself ‘this is the first time the depiction began with Einstein’s general relativity equations.’ On screen the black hole was depicted as a disc of dust and gas which was further exaggerated by gravitational lensing, an astronomical phenomenon caused by gravity warping the light emitted by stars and other bodies further away. When Matthew McConaughey enters the black hole he finds himself in another dimension, but what would actually happen if someone did enter a black hole?
Well in the instant you cross the event horizon, reality would split into two. In one part you would be incinerated due to Hawking radiation, which is hot particles that are emitted as the event horizon is full of energy. In another part you would continue into the black hole. Inside a black hole the laws of physics break down. Going deeper into it, space warps until it reaches a point when space and time become infinitely curved; this is known as the singularity. At this point, when space and time are meaningless, the laws of physics no longer are relevant. However, when all this happens, where would you end up? Stephen Hawking, the award winning and renowned physicist suggested that any humans entering a black hole would not be able to return home but instead be able to escape somewhere else, to another universe. ‘The existence of alternative histories with black holes suggests this might be possible’, Hawking said. ‘The hole would need to be large and if it was rotating it might have passage to another universe. But you couldn’t come back to our universe. So, although I’m keen on space flight, I’m not going to try that.’ Hawking’s theory could be the answer to a mind-puzzling problem for many physicists but until then we can continue visiting black holes through the movie and TV perceptions that we all love.
Article by Samantha Rotherham, Education Support Officer