What Is A Black Hole?
When we venture out on a camping trip and gaze up at the starry night sky, we marvel at the sight of stars and perhaps even glimpse a planet. Yet, in the darkness, there is something else lurking. Much like a great white shark stalking the ocean's depths, this unseen entity is the ultimate predator of the universe. These enigmatic phenomena are entirely invisible, leaving us unaware of their presence when observing with visible light. While stars illuminate the heavens, these mysterious entities consume their light, shrouded in secrecy and known as black holes. Join us as we delve into mankind's most compelling theories and explanations of these extraordinary cosmic objects.
Escape Velocity
Light is the fastest thing in the universe, traveling at 300,000 km/s (186,000 mi/s). According to Einstein's theory of relativity, this is the universe’s speed limit. A black hole, however, has such intense gravity that the escape velocity exceeds the speed of light. This is why not even light can escape from it. Every celestial body in our universe has an escape velocity, which is the minimum speed needed to break free from its gravitational pull. For example, the escape velocity on Earth is 11.19 km/s. Escape velocity is directly proportional to a celestial body's mass, so the more massive an object, the higher its escape velocity. A black hole has so much mass concentrated into a single point called a singularity that the escape velocity exceeds the speed of light.
This image shows how if you accelerate something fast enough, it will escape Earth’s surface and either, enter a stable orbit around the Earth like a satellite or completely leave the Earth like space probes.
Image by John M Jennings - https://johnmjennings.com/escape-velocity/
Equations Describe Everything
Einstein's theory of general relativity, published in 1915, predicted the existence of black holes. It describes the properties of the universe, space, and time. General relativity is a set of equations with multiple solutions, one of which describes a black hole. Karl Schwarzschild, a German physicist, provided a solution to Einstein's equations, describing how matter compressed to a critical point would cause time to stop near this object. Schwarzschild later developed this idea into the Schwarzschild radius, which defines the size of a non-rotating black hole by just figuring out the mass of the black hole.
How A Black Hole Forms
Black holes form from the remnants of massive stars that have ended their life cycle. When a high-mass star exhausts its nuclear fuel, it can no longer counterbalance gravity, leading to a collapse. This collapse results in a supernova explosion, leaving behind either a neutron star or a black hole. The star's matter is compressed into a singularity during the black hole formation. Although theoretically, any mass can be turned into a black hole, in practice, only massive stars undergo this transformation.
The Anatomy Of A Black Hole
A black hole can be divided into several parts. The Innermost Stable Circular Orbit (ISCO) is where objects can orbit the black hole without falling in. Closer to the black hole is the accretion disk, where temperatures can reach millions of degrees due to friction and collisions, emitting a broad range of electromagnetic radiation. The photon sphere is the closest region where light can orbit the black hole, but this orbit is highly unstable. Another part of a black hole is the Event Horizon, the point of no return. It is an invisible boundary, and when crossed, nothing can escape the black hole, not even light(but if you want to return, you have to travel faster than the speed of light, which is scientifically impossible). The most mind-puzzling part of a black hole is the singularity. It is not a place inside a black hole, but rather a location in time, in the future because inside a black hole, time and space reverse roles meaning that inside a black hole, you have limitless space, but time is finite. So when time inside a black hole runs out, you reach the singularity.
The Anatomy of A Black Hole
Credit: ESO, ESA/Hubble, M. Kornmesser/N. Bartmann
Entering A Black Hole
A black hole drastically distorts space-time, causing time to slow down near it. This phenomenon, known as time dilation, is well-represented in the film Interstellar. According to Schwarzschild’s equations, an observer watching someone fall into a black hole would see them slow down as they approach the event horizon. Inside a supermassive black hole, tidal forces would be less extreme, potentially allowing survival for a while. The concept of a "singularity" instead of a singularity remains speculative and part of ongoing theoretical research. You see, a singularity is a point in space that has x,y, and z coordinates in space and a location in time as well, and the black hole is spinning, how can the singularity also spin? Because it is only a point in space and points in space can’t have angular momentum. So physicists, like always, came up with another to explain this. Instead of a singularity, why not have a ring? This ring might act as a way to continue traveling through the black hole and out the other side.
Interstellar film’s black hole scene
Credit: Image from CBC, How Does Interstellar Measure Up Against The First Photo of an Actual Black Hole
How Do We Know That They Exist?
Einstein’s theory predicted black holes, and later observations have provided evidence for their existence. We observe the gravitational influence of black holes on nearby stars and gas. For instance, the stars orbiting an invisible massive object at the center of our galaxy indicate the presence of Sagittarius A*, our supermassive black hole. The Event Horizon Telescope captured an image of a black hole’s shadow, surrounded by the bright accretion disk, providing visual evidence of their existence. We took this even further and took a picture of our black hole.
The first image of Sagittarius A*( Our local supermassive galaxy)
Wait a second, how can you take a picture of a black hole? Since supermassive black holes are massive, they attract gas and anything around them. Most people think they would just fall into the black hole, which is somewhat true, but some stuff might just orbit the black hole, forming a bright luminous ring around the event horizon called an accretion disk, the bright orange-colored ring we see in the pictures. Sometimes, the accretion disk is so bright, that it outshines an entire galaxy. This is known as a quasar, the brightest object in the universe. So currently black holes hold two titles, the most mysterious objects and the brightest objects in the universe.
To fully understand the inner mechanisms of black holes, we have to leave the pen and paper at some point and send something inside it. This is the thing about physics, there are just some things you can’t prove with your chalkboard in your room. If black holes seem mind-twisting for you, then (you might want to strap on here), you should also know that black holes have their opposite twins, a white hole (white holes exist in pen and paper, but it’s just a matter of time before we find one). So the next time you look at the sky, just remember a black hole may be looking back at you without you even knowing about it…
