The fascinating phenomenon that was previously considered purely theoretical but is now providing ground-breaking data in many fields of physics
Appearing in a variety of science fiction stories and usually surrounded by an aura of mystery, myths and surprising theories – black holes have engaged the imagination since the idea of their existence arose in the 18th century. Here we take a look at what actually creates a black hole, what effects are created near black holes, what types of black holes exist in the universe, and what other rumors, theories and myths revolve around them.
What is a black hole?
A black hole is an area in space where there is mass of stars concentrated in a radius of a few kilometers. Since the mass of a black hole is so large and its gravity so strong, anything that comes close enough to it falls into it, even light. All the concentrated mass of the black hole falls into its center, called its gravitational singularity; at this point there is infinite density. A few kilometers from the singularity exists the “event horizon”, a mathematical boundary that marks the “point of no return”, i.e. the point at which the speed needed to escape the gravitational field of the black hole is greater than the speed of light. This point is also the fundamental boundary; from here it is impossible to remove material, light, or any kind of “information”. What falls through is lost forever, and we will never know its fate. The event horizon represents the “surface” of the black hole, which is not actually physical or tangible. From the outside, the horizon appears as a completely dark ball.
Even light is sucked into it. Simulation of a black hole | Source: Science Photo Library
The history of studying black holes
Using mechanics and Newtonian theory of gravitation, an “escape velocity” can be calculated for each star with a known mass and radius, which is the lowest speed that a planet must have in order to escape the gravitational attraction of another planet.
At the end of the 18th century, philosopher Reverend John Michell and mathematician Pierre-Simon Laplace at the same time, but separately proposed the idea of a body so massive and dense that its escape velocity is greater than the speed of light. At that time the notion was dismissed, because in Newton’s Laws there was no explanation for why light would be affected at all by the gravity of a star.
At the beginning of the 20th century, Einstein formulated his general theory of relativity, which comprehensively explained the phenomenon of gravity compared to that given by Newton in the 17th century. The theory of relativity produced the same results as Newton's theory of gravity referring to a weak gravity, and bodies moving relatively slowly compared to the speed of light. The differences begin to appear only at high speeds and in very strong gravitational fields.
Already in 1917 (two years after Einstein’s publication) physicist and astronomer Karl Schwarzschild published a solution to the equations of relativity near a massive and dense (compact) body. In his equations, there was a clearly visible boundary beyond which light could not escape. According to the theory of relativity, light and matter behave the same way in a gravitational field, which supplied a more natural explanation for the existence of a black hole that was capable of attracting light in the first place. The black hole was a good theoretical demonstration of the situation where the theory of relativity behaves quite differently from Newtonian gravity due to the strong gravitational fields that exist near the event horizon.
Since then, theories have advanced to include rotating and charged black holes and now even include a number of fascinating phenomena that occur in the proximity of the black hole.
The effects of gravity
When you're far away from the black hole, there is no difference between its gravity and that of a normal star. So if we would replace our sun with a black hole of the same mass, the Earth would continue in its orbit without interference (the lack of sunlight would bother us of course as living creatures who like light and heat, and actually depend on them). The formidable gravitational force of a black hole stems from its small size. The gravitational pull increases, as you get closer to the concentrated mass in the center. Because of this and due to the fact that the black hole can be the mass of a star, but with the radius of only a few kilometers (instead of millions of kilometers), allows matter to get closer to the black hole and be pulled by the very strong gravitational field.
The radius of the event horizon, called “Schwarzschild radius”, represents the physical size of the black hole. But within a few Schwarzschild radii the strong gravitational pull can already be felt, and a huge force is required to prevent falling inside it. For a particle or a spacecraft that orbits the black hole, already at a distance of just three Schwarzschild radii it will be impossible to find a stable path to stop them from falling into the black hole. At a distance of 1.5 Schwarzschild radii, for example, the gravitational pull is so strong that light rays move in a circular path, so if you were watching a black hole from this distance you would be able see your head on the other side.
According to the theory of relativity, a massive body distorts the space and time around it. A black hole is an extreme example. To an external observer, light emitted proximal to the horizon appears redder (its frequency is decreased), and radio pulses emitted at regular intervals will appear to be emitted in increasing intervals as their source is closer to the event horizon. To an external observer, a clock thrown into the black hole will show time slowing as the clock approaches the horizon, and in fact from the outside it would look like the clock takes an infinite amount of time to pass the event horizon. However, as it approaches the horizon, light emitted from it will fade and appear redder, until we can’t measure it, even at the lowest frequency of radio waves.
Light that comes from a source behind the black hole can circumvent it, because gravity distorts light rays and diverts them. The black hole can serve as a lens that concentrates the light of background stars. In this sense, it is not only possible to see stars that are from the opposite side of the black hole, it is also possible in certain situations to see the same planet reflected several times around the hole, and even a full ring of light that is coming from one source where the light rays surround all sides of the black hole.
In science fiction we often encounter black holes that are the key to a different universe or gateway for traveling back in time. Beyond the strange phenomena already mentioned, there are other ideas that are based on the theory of relativity for the possibility of “wormholes” in such black holes. Of course these are only theories that don’t really have any scientific basis, and anyway there seems to be no way we could actually check if they are correct or not.
Absorbing its neighbors in. A black hole sucking the gas from a nearby star | Simulation: Science Photo Library
How are black holes formed?
How are black holes formed created and what causes a planet's entire mass to be compressed into such a small radius? The answer lies in the gravity of massive stars that manage to continually compress all the gas within the core of the star, until the mass collapses to the size of the dot.
Stars produce a lot of heat and radiation during their lives, and the internal pressure created at the core of the star prevents them from collapsing under their own weight. When the nuclear fuel at the core of a star is depleted, the gas collapses inwards in a violent process releasing matter and energy outwards in vast quantities. If the star is considered sufficiently massive, this exploding process is called a supernova. The gas remains from the previous star, yet there is no longer a source of energy to help push against its own gravity, and so the star collapses to a minimal size. If the mass remains large enough, the debris will collapse into a neutron star, where all the electrons are united with the protons and a core of neutrons is formed. If the mass of this core is too big even for the repulsion between the neutrons, the star will collapse inwards. At this stage no force known to science can resist the weight of the gas. The mass is concentrated in one point, and a black hole is created.
When we think about the space between stars we imagine it as almost completely empty. But in the world of quantum laws the vacuum is filled with pairs of particles and anti-particles that appear and disappear at such minute intervals that cannot be measured. If such a pair is created exactly at the event horizon, one particle might fall into the hole and release the other particle, taking a small amount of energy from the black hole. This phenomenon is called Hawking radiation named after a physicist who first described it in the 70s of the 20th century. This radiation is very weak for “ordinary” black holes with a mass of a star, but a very small black hole may lose a large amount of energy as result of it.
Another point explained by quantum mechanics is the singularity at the center of the black hole. According to the theory of relativity, no force can deal with the collapse of matter under its own weight into such a small radius, and so there is no choice but to accept the existence of a single point of almost size zero, in which lies a huge mass with infinite density. Future success in connecting quantum mechanics with relativity (aka quantum gravity) should give better answers to the nature of this problematic point. Since these theories have not yet been developed, no one really knows what is happening today at the center of black holes.
Types of black holes
Apart from stellar black holes that have a mass equivalent to a few times that of our sun, there are other types of black holes that can be important for our understanding of the universe. A striking example are supermassive black holes, with a mass of millions of suns that exist at the center of most galaxies. These black holes feed on nearby gas and stars at the centers of galaxies, and sometimes cause emission of very bright radio waves from the matter being gradually compressed as it falls into the black hole. It is not known how these massive black holes were created, even given the large amount of gas and stars around them, since matter falling into a black hole tends to circle around it, like water flowing down the drain, and therefore it would take a long time for this amount of material to accumulate into one black hole. The measurements that were made show a link between the size of the supermassive black hole at the center of the galaxy and the number of stars within it, indicating a close relationship between the formation of galaxies and the formation of black holes at their center.
Another type of black hole that may exist is the micro black hole. High-energy collisions between particles may lead to a sufficient density for creating a tiny black hole. In theory there can also be black holes with a slightly larger mass, similar to that of our moon, at the size of just a little under a millimeter. These minuscule black holes would emit Hawking radiation, which would cause them to lose=energy (and mass) and ultimately completely disappear. Meanwhile, it is unknown whether such entities exist, how they are created, or what is their impact on the formation of the universe. All of these are open questions in current research.
Not clear if they exist. Simulation of creating a micro black hole in a particle accelerator | Photograph: Science Photo Library
Observations of black holes
Black holes are exotic bodies that are naturally very difficult to see in space. They do not emit light, which means the discovery of black holes and spotting them is very difficult, and rely on indirect measurements.
In the 90s of last century, scientists from the University of California in Los Angeles measured the movement of 90 stars at the center of our galaxy and found they orbited a mass four million times greater than our Sun, with a radius of around a thousandth of a light year. It is hard to imagine such a large amount of mass in an area so small that is not centralized in a supermassive black hole. In that same area in space there is also a strong source of radio waves, known as Sagittarius A. Today it is assumed that radio waves are emitted from the gas surrounding the black hole that heats up as it falls inwards.
Stellar black holes (with a mass that is equivalent to several sun masses) have been discovered indirectly through X-ray observations. In binary systems where a black hole or a neutron star and a normal star are moving around each other, the heavier body vacuums the gas of the partner. Gas surrounds the black hole, heating from friction as it turns around it and falls within it, reaching millions of degrees and emitting strong X-radiation. Observations on the properties of such systems may suggest whether the compact body is a neutron star or a black hole.
In the past year a LIGO experiment detected gravitational waves that apparently were emitted due to a collision between two black holes, each about 300 times heavier than our sun. The discovery itself is important mainly because it was the first time these elusive gravitational waves were discovered, but it also suggests (indirectly) the existence of black holes in the universe.
These discoveries have advanced the status of black holes from theoretically interesting phenomena to astrophysical reality. Today, black holes are integral to our understanding of the evolution of galaxies and the stellar life cycle. They provide a platform for new theories of gravity and quantum mechanics and the variety of phenomena surrounding them are promoting our knowledge of physics in a variety of fields.