Black Holes: The universe’s greatest secret

Image Credit: European Southern Observatory/Event Horizon Telescope, used under the Creative Commons Attribution 4.0 International License

Laura Kenny explores the vast topic of Blackholes—what they are and what we know about them.

NASA joins ‘Black Friday’ celebrations and encourages us to focus our attention on ‘Blackhole Friday’. The rebrand of this retail holiday is an exciting invitation to step back from the spend-happy activity and to get immersed in the world of these wondrous cosmic objects. 

Black holes are widely popularised by the media and science fiction, but the science of black holes remains generally elusive. A black hole is an astronomical object in outer space with a strong gravitational pull that nothing can escape. It uses its force to devour light, matter, and radiation without ever allowing these particles to exit. The surface of the black hole, the event horizon, is where this moment occurs. Here, the velocity needed to resist the gravitational pull exceeds the speed limit of the cosmos and draws in anything within its force field. Dr Barry Wardell, PhD candidate Josh Mathews and Dr Sarp Akcay of the School of Physics at UCD explained to the University Observer how black holes are not so many actual holes but rather heavy and dense objects in space. Dr Barry Wardell explained that their ‘mass is compressed into a single dense point or ring’ and ‘their gravitational field is extremely strong in their vicinity.’

There are two types of black holes; stellar size black holes, created when massive stars die and supermassive black holes, which are millions of times larger than our Sun. Black holes are also, quite simply, elusive cosmic beasts that continue to perplex and fascinate humankind. That fascination stems from our curiosity about the destination of various solar particles when they disappear past the event horizon and how the gravitational force of a black hole manages to devour everything in its path. These cosmic wonders are spread throughout the Milky Way galaxy, with more prominent black holes situated at the centre of more giant galaxies. They are subject to growth by accreting matter that is drawn into them, including other black holes.

New research frontiers have achieved more than simply changing our understanding of black holes. They have reminded us of our progress in building and drawing upon science and technology to discover the extra-terrestrial world.

Black holes have captivated scientists for decades. It is an important field of research because new results provide illuminating insights into their role in the evolution of galaxies and the universe at large. British scientists John Mitchell, Albert Einstein and German astronomer Karl Schwarzschild have contributed extensively to this domain. Einstein’s general theory of relativity was particularly influential in describing how mass bends spacetime, producing gravity. This theory was seminal, leading to the discovery that matter could be packed into regions of space from which there is no escape - what we know today as a black hole. Scientific research in this field is also essential to challenge past theories and see if they require iteration or revision. 

In conversation with the University Observer, Dr Antonio Martin-Carrillo, Assistant Professor of the School of Physics at UCD shed more light on the value of studying black holes due to their ability to act as ‘the most extreme laboratories in the Universe’ that are ‘impossible to replicate on earth’. They can be used to study ‘different areas of Physics: relativity, plasma physics, magnetism, particle acceleration…and many others.’ The value of learning about such extreme conditions is that it can offer ‘ways of testing theories and possibly even pushing the boundaries of these theories.’

The first-ever photograph of a black hole was captured in 2019 using the Event Horizon Telescope, marking a breakthrough moment in our understanding of this mysterious cosmic occurrence. The image appears as a dark circle surrounded by an orbiting disk of hot, glowing matter. The project results from two decades of work done by more than two hundred scientists as one huge international endeavour. Due to the large distance from Earth, the EHT team had to formulate a creative solution to capture the black hole. Their answer to the scientific dilemma was to link up radio telescopes worldwide to form an earth-sized lens. The black hole captured is located at the centre of a galaxy called M87, situated about fifty-five million light-years away. Even still, the black hole remains enigmatic because it is impossible to photograph its interior due to the fact that light cannot escape it. All media documentation before this image had taken the form of illustrations and simulations. The first-ever photograph of a black hole thus overcame a herculean task by capturing what was believed to be unseeable. 

Dr Martin-Carrillo offered further insight into the implications of the first image, speaking about how ‘we are still learning things from this image, and new results are still coming up.’ One exciting implication is that, with the use of a method called polarimetry, we have seen ‘how the magnetic field affects the photons generated in the accretion disk around the supermassive black hole.’ With this discovery, we are going to be able to study, like never before, ‘how magnetic fields behave around black holes and how activity in this very compact region of space can drive the powerful relativistic jets.’ Dr Barry Wardell describes how the image was also ‘the first detection of the event horizon of a black hole, and it came as a relief to everyone that it was there.’

New research frontiers have achieved more than simply changing our understanding of black holes. They have reminded us of our progress in building and drawing upon science and technology to discover the extra-terrestrial world. Beyond this, the most significant result of capturing this image is probably the exciting possibilities for further research and exploration. The photograph gives body to what has always existed in a conceptual and, therefore, somewhat mystical realm. It invites an entire range of possibilities relating to other discoveries and domains in Outer space and has energised scientific researchers worldwide. 

It is certainly an exciting time for the black hole and Blackhole Friday offers the perfect opportunity to revel in this excitement and to celebrate these cosmic beasts.