A black hole is an area of space-time dimensions where gravity has won over mass. Black holes are the evolutionary end point of a relatively big star (by ‘big’ I mean a star whose mass 10 to 15 times the mass of the Sun). When a star that massive dies, or in other words, runs out of fuel and undergoes a supernova explosion, it leaves behind burnt-out stellar remnant. These relics would later collapse on themselves due to gravitational forces, creating what’s mathematically known as a singularity, that’s briefly, a point of zero volume and infinite density. To give you a better idea, if Earth were a black hole, the blue planet would be the size of a marble. For a really cool animation to introduce black holes, check this link from an award winning astronomy website.
|Black Holes are where God divided by zero.|
Keep in mind, that these black holes are not, contrary to popular belief, cosmic vacuum cleaners. For example, if the Sun were to be replaced by a black hole of equal mass, the Earth’s orbit would remain unchangeable.
The gravity of a black hole is so strong that its escape velocity exceeds the speed of light (quick definition of escape velocity: the lowest velocity that a body must have in order to escape the gravitational attraction of a particular object), since we don't know of anything that moves faster than the speed of light, then nothing can escape it, ergo the appellation "black hole".
For a quick classification of black holes:
The anatomy of a black hole consists of: (for a cool animation click here)
1. Event horizon (at the heart of the black hole): a fictional area, which is perfectly spherical if a black hole is static. According to Einstein’s theory of general relativity, the planets in our Solar System follow trajectories that are bent due to a deformation of space-time that's created by the Sun's mass. The same goes for a black hole, at the event horizon, where this deformation is so powerful that everything is forced to go in circles around it. Also, the closer you get to a black hole, the more time slows down due to gravitational time dilation, consequently, if an object were to fall into a black hole, it would take it an infinite of time to reach its center. Remember, it's one of these places in the universe where all physics and math breaks down. An object falling into a black hole appears to slow down as it approaches the event horizon, taking an infinite time to reach it. More information on the matter is given later on. Eventually, at a point just before it reaches the event horizon, the falling object becomes so dim that it can no longer be seen.
|Anatomy of a black hole|
2. The Singularity: is defined to be one that contains geodesics (quick definition: a straight line that's curved due to space-time distortions, according to the general theory of relativity) that cannot be extended in a smooth manner. At the center of a black hole lies the singularity, where matter is crushed to infinite density, the pull of gravity is infinitely strong, and space-time has infinite curvature. Here it's no longer meaningful to speak of space and time, much less space-time. Makes you wonder of the reality of the physics around you.
|Conical singularity at the center of a black hole|
3. Accretion Disk (also called ergosphere): that’s a portion of space-time that’s brighter than the black hole as some photons and other particles can still escape, it's an area that's being dragged by the event horizon in the direction of the rotation. This process, predicted by the theory of general relativity is known as frame dragging. It behaves much like a star in some cases because there are nuclear reactions going on there, trying to condense all the material that’s being dragged.
4. Jet Streams: Due to conservation of angular momentum, material falling into the gravitational well created by a massive object will typically form a disc-like structure around the object. Friction within the disc causes angular momentum to be transported outward, allowing matter to fall further inward, releasing potential energy and increasing the temperature of the gas.
Your guide to getting sucked into a black hole, the ultimate adventure:
Step 1: Find the black hole.
Black holes don’t emit any detectable kind of light, so astronomers stylishly came up with equations calculating all types of light (visible light, X-rays, radio waves, etc.) and electromagnetic radiation emitted by neighboring material, because when any type of gas orbits a black hole, it gets heated by friction (remember I mentioned frame dragging?). The most practical way is using infrared light detectors, for example, NASA’s Spitzer Space Telescope. Electromagnetic radiation with wavelengths between those of radio waves and visible light is called infrared light.
|NASA's Spitzer Space Telescope|
Step 2: Approach the black hole.
Now that you have a black hole in sight, you have to cross inside the Schwarzschild’s radius. This radius is by definition, calculated using the equation of escape velocity as follows:
|How to calculate Schwarzschild's radius|
According to Roger Penrose's Principle of Cosmic Censorship, the fact that the laws of physics break down inside the event horizon has no impact on the physics outside the black hole. To make things more palpable, if the Sun were to be replaced with a black hole (again) with the same mass as the Sun, the Schwarzschild radius would be 3 Km (1,86 miles), so you’ll have to get really close and intimate with black hole to get ‘vacuumed’.
|Schema of a black hole|
Step 3: Fall in. Try Not To Panic.
Here’s where everything gets interesting. The closer you get to the center of a black hole, the more time slows down, and the more the dimensions of space and time get stretched, so as you fall in, you’ll get stretched (what theoretical physicist Michio Kaku calls 'getting spaghettified') for an infinite amount of time then eventually you’ll turn into a particle so small it's invisible. Let me explain, to make things more fathomable, let’s do a little experiment:
On a warm sunny day, you and I, build a spaceship, then take it for a spin near a black hole. Once we cross inside the Schwarzschild’s radius, I kick you out of the shuttle only for scientific purposes. As you get sucked into the black hole, you don’t notice anything abnormal with the ticking of your watch, however, I can see that the watch in your hand is moving progressively slower than the one in my hand. Remember that this is Einstein’s theory of general relativity, where everything is relative, even time; your time is going slower than mine is. If we were to jump in together, our clocks would still be synchronized. The dimensions of space and time are distorted in a way unfamiliar to us, so think in different dimensions to understand this. From my spaceship, I would still be in familiar dimensions and I would just see your motions slow down until you’re frozen (totally unmoving) as you get near the event horizon, and it would take you forever (and I mean infinity) to reach the center of a black hole. Theoretically, what you'd experience is different, your body first falls in, like falling into nothingness, no bumps no disturbances, then at a certain distance, your body will get stretched the closer you get to the center of the black hole, if you were falling head first inside, the gravitational pull on your head would be much stronger than that on your feet, you'll get torn apart. Nevertheless, once you reach the singularity point, after an infinite amount of time, you’ll get squished into a point of infinite density. Kind of like this:
|Falling into a black hole|
How could this be our conception of reality when theoretical physics gives us two stories? One in which you’re frozen in time, and an other in which you’re, well, dead. String theory, a theory that's still being developed, might offer a logic explanation.
For a more vivid explanation and a quick recapitulation, check this video out:
Reference and credits: