by Frank Kehren
Black holes are among the strangest objects in the Universe. Hard to find, hard to explain, hard to understand and hard to imagine they defy the known laws of physics at their centre and provide mind boggling results at their edges. A black hole is impossible to see but there has been enough evidence to certify their existence. They are an important part of the solar life cycle and perhaps an even more important factor in galaxy formation.
In 1785 a man named Peirre Simon Laplace concluded that if enough mass were packed into a sufficiently small space that the gravity would be so intense that not even light would be able to escape. The idea is based on escape velocities being proportional to the mass of an object. For example, the energy needed to escape the gravitational pull of Earth is 19,700 mph greater than what is needed to break free of the Moon.
Laplace theorised that when gravity reaches a massively high value that the escape velocity would be faster than the speed of light, trapping light particles and rendering the object invisible. During the early 20th century physicists such as Oppenheimer, Volkoff, Snyder and notably Karl Schwarzschild would refine the theory and provide mathematical formulae to back it up.
The only known way for a black hole to form is from the death of a massive star, this is the only way that such a huge mass can be packed into such a small volume. During the main phase of their life stars are trying to explode through nuclear fusion but at the same time are held together by their own gravity. These two forces balance the volume of the star and will define its size.
When the star runs out of fuel for nuclear fusion then there is no outward force and so gravity causes it collapse in on itself. The collapse heats up the core to a massive degree and will cause a supernova explosion. After the supernova all that is left of the star is a highly compressed core, continually collapsing under its own gravity. The gravity becomes so strong that light cannot escape and so a black hole is formed.
Black holes effect spacetime in strangely different ways to other bodies. They cannot be seen by visible light, but have a number of interesting properties. The defining feature of black holes is the event horizon, a real point of no return. The event horizon is the point at which the gravity becomes so intense the the escape velocity reaches light speed. The horizon surrounds the singularity at the centre of the black hole and effectively forms the mouth of the black hole. From the outside, nothing inside the event horizon can be seen.
Outside the event horizon objects can orbit the black hole like they would any other large body of mass. The escape velocity will increase the closer you get to the horizon, but at a large enough distance the orbit will be no different to orbiting a star or planet
Crossing the event horizon will prove fatal to anyone that tried. In theory if one were able to cross an event horizon in a spacecraft it would seem initially unremarkable. Far away objects would appear distorted due to gravitational lensing but you would still be able to see out of the black hole into space. As your craft flies further from the horizon it would experience tidal forces so strong that eventually it would torn apart, right down to the atomic level. The time frame for this happening varies depending on the size of the singularity at the centre but is typically just a few seconds.
What can be confusing is thinking about what a distant observer would see as your doomed craft flew into the black hole. As the approach to the event horizon is made the light will take longer and longer to leave the craft because of the intense gravitational pull. This would make it seem as if the craft is slowing down in time until it reaches a point where the light takes an infinitely long time to reach the observer. Time would appear to stop for an outsider, and it is a property that gave black holes their original name of frozen stars.
There is also the fact that the gravity will cause a massive amount of time dilation to occur, meaning that time actually will pass more slowly for objects close to a black hole than those far away. Both theories are correct and it is one of the most intriguing properties of black holes. In reality an observer would see the craft slow right down and appear more red as the light is red-shifted.
Black holes cannot be seen from Earth but can be detected in a number of ways. One is through gravitational lensing. According to Einstein’s theory of general relativity gravity distorts both space and time. A star or a planet will cause an effect similar to a heavy ball on a rubber sheet, and it is this effect which causes the orbits we can see today. Gravity can also bend light and an massively intense gravitational pull will bend light much more and distort the light from objects behind it. Black holes have been detected from Earth by observing the effects on the light travelling from distant stars such as duplicate images or intense brightening.
Black holes are thought to exist at the centre of most galaxies. They are the only known objects that can reach a sufficient mass within the volumes observed. The mass of supermassive black holes believed to be at the centre of galaxies is measured by observing the orbit speeds of the objects surrounding it. It is a technique that is used to similar effect to work out the mass of bodies in the Solar System such as the Sun and Jupiter.
In mid-2008 there were some fears that the Large Hadron Collider would create a micro black hole that would destroy the Earth. In reality these black holes would be far to small to do this and would evaporate almost instantly. Despite the fears being ill-founded the thought of the planet being swallowed by a black hole showed how much these enigmatic objects have captured the public imagination. They’re often used as plot devices in science fiction, and the mysterious nature of the event horizon further adds to their intrigue. Black holes are certainly one of the most interesting phenomena in the Universe, and let’s hope that we never get too close to one.
Explore spectacular advances in cosmology, relativistic astrophysics, gravitational wave science, mathematics, computational scien…
General Relativity for Babies by Chris Ferrie is a simple introduction to Einstein’s crowning achievement. Baby will learn how mas…
The aim of this groundbreaking new book is to bring general relativity into the undergraduate curriculum and make this fundamental…
Relativity: The Special and General Theory Albert EINSTEIN (1879 – 1955), Translated by Robert W. Lawson (1890 – 1960) This is an…
This comprehensive student manual has been designed to accompany the leading textbook by Bernard Schutz, A First Course in General…