Despite their name, black holes are not holes at all. Simply, black holes are big objects in space that hold an extreme amount of matter and energy packed into a very small space. Its gravitational pull is so strong that nothing can escape it, not even radiation or matter. This one-way flow of energy and matter means that anything that goes into a black hole cannot come out, including gas, planets, stars and theoretically, people.
There are defined parts to a black hole. First, there is an imaginary line beyond which nothing can escape that is called the “event horizon”. We call it this because if an event happened past this boundary line, an observer from the outside would not be able to see it. This is because matter and energy cannot escape, which also means that information can’t be transferred in or out of a black hole either. In fact, we would not know if this event had even happened at all. In other words, the event horizon is the point of no return. Within the centre of a black hole is what’s called a “singularity”. This is a single point where the mass of the black hole is concentrated and both density and gravity are infinite.
Many wonder what would happen if they were to fall into a black hole. Objects that would fall into one are not sucked in like a vacuum. Instead, they fall into them. If you were to approach a black hole, you would travel faster and faster because the force of gravity that is pulling you in is so strong. Many scientists believe that this would stretch your body into a string like a piece of spaghetti. Other studies have proposed instead that reaching the event horizon would be like hitting a wall of fire. For someone on the outside who is observing this unlucky space traveller falling into the black hole, the person falling would appear to slow down and eventually freeze as they near the event horizon. For the person falling in, they will feel like time is not moving differently at all. This is known as “time dilation” and it is explained largely by Albert Einstein’s “Theory of Relativity”. In basic terms, the theory says that time is affected by how fast you are going, particularly when you are approaching the speed of light. Basically, the faster you move, the slower time passes for you. Because the strength of gravity is so immense as you fall closer into the centre of a black hole, you would be falling at a speed so fast that time would appear to slow down. In other words, this very strong force of gravity makes time run more slowly.
Fortunately, the nearest black hole to us 265 quadrillion kilometres away in the middle of our Milky Way galaxy.
Today, it is generally agreed that there are three types of black holes:
Stellar black holes are the smallest of the three categories. The stars that create them have collapsed in on themselves at the end of their lives to become black holes that are incredibly dense and can compact the mass of the star into the size of an average city.
Supermassive black holes are believed to be at the heart of every galaxy, including our own Milky Way. This black hole is a trillion times more massive than the Earth. Imagine these objects containing a mass millions of times that of our Sun, but only taking up the size of a single Sun. Unfortunately, we are not certain as to how these giants form, perhaps from merging with other black holes, the collapse of multiple stars, or from growing by collecting nearby gas and dust. They may even have formed at the same time as their host galaxies.
Thirdly, an intermediate black hole forms when stars that are in a common cluster collide together one after another. We know very little about medium-sized black holes of this kind because it was once thought that black holes can only be either relatively small or very large.
Since no light is coming out of black holes, we cannot see them. They are invisible to us, so we must watch how they influence objects that are nearby (such as stars). When a star creeps closer to a black hole, a high-energy light is made that can be seen by special telescopes. If this star gets too close to a black hole, it will be torn apart. For example, in 1971 astronomers noticed mysterious x-rays coming from a bright star around 6,000 light years away in our own Milky Way. It was later confirmed that it was circling something so small yet incredibly dense that it could be nothing other than a black hole (this was the first observation of one). The x-rays were coming from the disk of heated material that was quickly spiralling into the black hole (much like what is depicted in the animation above).
Most recently, evidence to support the existence of black holes has come from the detection of gravitational waves. These were first detected in September of 2015 at the Laser Interferometer Gravitational-Wave Observatory (LIGO). The team was awarded the Nobel Prize for Physics, one of the most world’s most highly regarded honours.
In order to detect these waves, a huge and powerful event must happen that causes a “rumble” in space. This rumble creates gravitational waves, whose vibrations can be detected in very, very tiny amounts. Using two detectors (one in Washington State and the other in Louisiana), the LIGO used laser beams that were fired along two tunnels spanning four kilometres. When the light is sent down these tunnels and returns, it can experience a squeeze, stretch, and pull feeling from the waves.
In September of 2015, the waves measured by the team at LIGO were caused by a collision between two black holes, taking 1.3 billion years for the waves to ripple through space at the speed of light and eventually arrive at the detectors. These black holes were 29 and 36 times the mass of our Sun respectively. This was also the first time that we witnessed two black holes colliding together.
Black holes are also associated to the mysterious concepts of white holes and wormholes. White holes are a theoretical concept that is essentially the opposite of black holes. Black holes are a one-way entrance and white holes are a one-way exit because unlike black holes that suck in lots of matter, white holes spit it out. A white hole can be compared to a water faucet that spews water, while the black hole is the drain that consumes it. Black holes do not let matter or light escape from it, but white holes eject this matter and energy. White holes also have their own singularity too, and it is from this point that everything is ejected. It is impossible to enter a this type of hole however, because the matter that is coming from it would destroy anything attempting to do so. As well, the amount of energy needed to enter a white hole makes it impossible, much like going up a hill that gets forever steeper and impossible to climb. Some scientists have even proposed that the Big Bang itself is a white hole, since it was this event that created all the matter of the universe that spread out from a single point and continues to grow. This is unlikely however because we generally believe that the universe is infinite in size and white holes are very much a theoretical concept.
The reason that a white hole has been proposed but remains theoretical is because this concept can be explained by mathematics. Black holes and white holes together are used to explain the possibility of wormholes. These concepts help solve problems in Albert Einstein’s “General Theory of Relativity”. Some scientists have suggested that black holes lead to another location in space and time where the energy and matter that is being collected here is spit out somewhere else in the form of a white hole. In this way, a wormhole acts as connecting bridge between a black hole and white hole. Think of folding a piece of paper and poking a pencil through both pieces. Naturally, this pencil (wormhole) is a much shorter path from one of these holes to the other, compared to travelling across the paper (or space). Of course, these are still theoretical concepts that remain unproven.
Researchers have also suggested that anything trying to travel through a wormhole would cause it to collapse. Despite this unlikelihood, it is intriguing to consider the idea of a wormhole as a way to travel from one place in time and space to another, even possibly to another universe.