In this article, I will attempt to gather as much information as possible about black holes. Let's start by understanding what we're talking about when we refer to them...
Firstly, in science, when we use terms like "black" or "dark," it's to indicate our ignorance about the subject. Black holes were named as such due to the ignorance we have about what happens inside them, and because they indeed appear black in relation to the dark space around them, making them very difficult to detect.
So how do we detect them if they are black and exist in a black void? Through the distortion of spacetime. What do I mean by that? Firstly, it requires luck to have a specific geometry. So, in our line of sight, there needs to be a star, and between us and the distant star, there must be a black hole. Then something incredible happens! The star will appear twice (at least) in a photograph. This occurs because its light travels towards us and across spacetime, which bends around the black hole. Thus, the star's light appears at various points around the black hole. It's like having the same star multiple times in the sky, as seen in the photograph below, where two stars are visible to our eye (let's overlook the fact that they are quasars and not stars), but in reality, it's the same star appearing twice because its light is distorted by an intervening galaxy.
This phenomenon is called "gravitational lensing" and is predicted by the theory of relativity. Another beautiful photograph of the phenomenon follows.
So, in order to see a black hole, either this phenomenon must occur, or a neighboring star must "plunge" into it. In that case, the resulting photograph would look something like the following:
We've explained how we detect black holes and why they ultimately have this color.
It's quite simple: In order to see an object, it either needs to emit light or reflect light. In both cases, for light to reach our eyes, it must be able to travel to us. Black holes, being black objects, absorb all wavelengths of visible light (which is why they appear black), and they have such strong gravity that the speed at which light travels (300,000,000 m/s) isn't sufficient to escape the gravitational pull of the black hole. So, to escape, one would need a much greater speed, but within the universe (note the word "within"), nothing can travel faster than light.
Therefore, black holes appear black.
But what were they before they became black holes?
The theory states that black holes were once massive stars, so massive that at some point (for unknown reasons, perhaps because as they move through space, they attract many bodies that don't enter orbit around them but fall onto their surface, thereby increasing their mass), gravity pulls all the mass of the star into its core, into a very small point. This point, the theory says, has no dimensions; it's smaller than a grain of sand, perhaps even smaller than an atom! This point is called the "singularity." All matter and energy are concentrated in a dimensionless point in space with infinite gravity and zero volume. So, it's a star that emits no light...
How far does the region of influence of the black hole extend?
Good question. If we were close to a black hole, there would be a spherical region around this singularity where nothing would have sufficient escape velocity and would inevitably fall into the gravitational field. However, this region, as expected, cannot extend to infinity; it will have a finite radius. Beyond this radius, light will have the speed needed to escape the attraction. This radius is called the "event horizon." Within it, various strange things happen, like the emission of Hawking radiation, which we saw, and another strange phenomenon we'll see.
It's important to understand that spacetime itself is pushed toward the center of the black hole. So, whatever is on it is forced to go there because the space and time vectors point in that direction. Outside the event horizon, there is a significant, but certainly milder, distortion. Therefore, the speed of light initially, and subsequently lower speeds, are sufficient to escape the attraction. If, for example, the treadmill of a gym (spacetime) pushes us toward the black hole and we climb onto the treadmill with a Bugatti Chiron, it won't pull us back but we'll ultimately escape quite easily.
But what paradoxes occur at the event horizon and beyond?
This is where the fun begins. Stay with me!
Let's say you have a friend, Leyla. You're both wearing your suits and traveling toward the nearby black hole. As it's customary, ladies go first, so you've let her walk ahead while occasionally shining a flashlight backward to make sure you don't stumble anywhere in the (empty) space.
At some point, Leyla crosses the critical surface called, as we mentioned, the event horizon. How will you notice? At that point, time for Leyla moves agonizingly slowly, almost stopping, so you'll see her forever frozen in that exact spot, flashing the light at you. Forever. The only possible differentiation would be if she starts to rotate around the black hole, but you'll never see her move inward. Why? Because for you to see her move inward means that light would have to escape the black hole's gravity to reach your eye, and this can't happen if someone enters and crosses the event horizon. Now, this "forever" we mentioned has a problem. Leyla's photons and the photons from her flashlight are matter and have energy. Since Leyla isn't there but has crossed into the hole, we'd constantly see something producing light, i.e., producing energy and matter for eternity, violating the principle that matter and energy neither destroy nor create but change forms. Here, we'd have the creation of matter (light), which seems quite paradoxical.
What will Leyla see?
When Leyla passes into the hole, her clock will stop, gravity will become infinite, and time will no longer progress. With infinite gravity but no time, I imagine she won't ever reach the center, as to reach it, time would need to progress for movement to occur, another paradox of this theory. (I'll think more about this and write some ideas in the future, as all physical laws collapse inside a black hole; on the other hand, if the black hole belongs to the universe, it must be governed by the same laws).
Looking back, what will Leyla see?
She'll see everything. Remember, for her, time doesn't pass, but for the rest of the universe, it progresses normally. Also, whatever has entered the black hole hasn't escaped. So, she'll simultaneously see the past and the future. The entire universe.
If she had the ability to travel faster than the speed of light, where do you think she would end up? Anywhere she wanted, and as soon as she left the hole, her now, her present, would continue normally from where she had stopped. However, it could simultaneously be her past or her future.
The simultaneous time we were talking about...
Food for thought...
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