Author: Aashi Sharma, Class VII F
Space is arguably one of the most fascinating things in the universe. In the wide space, there are things known as black holes, holes that can literally eat you alive! One of the key parts of these bodies is what is referred to as their event horizon. So today, I’m going to elaborate upon them. Enjoy the ride!
I think we all know what the boundary of place is-Let’s take countries as an example. If you enter a country, you are now inside of it, and you can’t exit without some cost, you will probably need some form of identification and need to go by plane. Now the event horizon is a certain boundary of a black hole-except there is no identification to escape it. Once you enter the event horizon, there is no escaping, you are literally freezed in time. (Oh yea, I forgot to add-the event horizon is USUALLY perfectly spherical)
Woah! Let’s understand that last point. What is the meaning of “Freezing in time” So this touches a weird but fascinating topic.
(Keep in mind- Gravitational Time Dilation – The stronger gravity is, the slower time passes relative to someone farther away from that gravity.
Gravity is EXTREMELY intense in a black hole- especially near the event horizon)
Let me explain this from another perspective-Imagine that you’re watching a spacecraft being sucked in by a black hole (let’s say you have a very powerful telescope). The spacecraft is nearing the event horizon at the point when you’re looking at it. Since gravity is so intense at this point, the spacecraft would seem to move slower and slower, until it seems to just stop or freeze in time.
Now that was your perspective from where you observed it.
But if you were in the spaceship, you have actually not frozen, you are moving deeper in the black hole in a matter of seconds, though you can’t send any signals now.
(Shift in perspective-you are now viewing the spacecraft, once it seems to have frozen in time) Now here is something astonishing-when you see the spaceship “freeze in time”, it won’t be too long until you won’t be able to see the aircraft at all.
Why, you ask?
Well this is one of the strangest things about blackholes.
Due to the intense gravity of black holes, it literally stretches light-something known as gravitational redshift happens.
First the rocket will appear to change colour- it will become red, redder- than infrared (invisible to your eyes), then a whole series of others-until you can’t see it anymore, not because the light has disappeared, but because it has stretched so much that your eyes or the telescope cannot detect it anymore.
(Important Notes-
-it is possible to see gravitational redshift, as long as it isn’t too extreme, in this case it is too extreme to see the light.
– if an object has not yet reached the event horizon, it can still potentially escape the black hole by sending signals or using any power it has. However, none of this is possible if the object has entered/crossed the event horizon.
-gravitational redshift and gravitational time dilation are only from the perspective of the viewer (you). Looking at the object’s (in this case-rockets) perspective, all things seem normal. (There will obviously be a huge difference in other aspects of the ship.)
IMPORTANT QUESTION
Why can’t an object escape the event horizon?
Ans. Due to the intense gravitational pull or the black hole at the point, escaping is out of the equation.
-REPEATING AGAIN- if an object enters the event horizon, it can NOT come out. No matter what. It can’t send signals, use power, NOTHING. Remember that.)
WHY IS IT CALLED THE EVENT HORIZON?
I don’t know about you but whenever I come across a term or definition, the first thing I do is find its meaning and why the certain occurrence was given that specific name. So, Let’s break down this term-
Event-Any occurrence in the universe (Example-A flash of light, an asteroid crashing on the earth; heck even moving your pencil!)
Horizon-the limits of your knowledge or experience OR a limit beyond which you cannot see. (Note- Yes, there is another meaning of the term horizon, however this is the one relating to the topic.)
Smashing/correlating these terms together, we get that when something in the universe crosses the event horizon, its events (its future) becomes unknown or invisible to all of the outside world till the end of time.
Therefore, it is a place where no information from the inside can reach the outside. It’s literally the horizon of events.
(Notes-
-The event horizon is a point of no return
-No information from the inside can reach the outside
-It is the horizon of events)
HOW BIG IS THE EVENT HORIZON?
All black holes are not of the same size. Therefore, nor is their event horizon.
For scientific and research purposes, it is necessary to find out the size of event horizons of various black holes.
(I think this is obvious now, but I will state it just in case-
Smaller the black hole, smaller the event horizon,
Bigger the black hole, bigger the event horizon)
So to calculate the size of any event horizon, there is an equation that we use-
rs=2GM/C2
(C2=CxC)
This equation is the Schwarzschild radius, an equation that was left unsolved by Albert Einstein and later solved by Karl Schwarzschild, leading to the name of the equation.
Now this equation is very, very complex. So, what I will do is explain the formulae briefly so you can know the basics, as knowing the depths of the equation is not necessary to understand the concept (It might also be quite boring for some of you)
Let us break down the different meanings of all the jumbled letters in this equation-
G- G is a number that tells us the strength of the gravity between two objects.
DID YOU KNOW?
If G were just a little bit up or down from where it is, we would not have stars and other fascinating things, we could even float off the earth!
In simple words- it tells us the gravitational force of the universe.
M- M is how much mass the black hole contains. Think of it as all the stuff the black hole has swallowed.
C- C is the speed of light- also known as the fastest thing we know!
Now, a question that you may ask yourself is-
“How are we supposed to know the value of G and M? It is not like we can weigh the universe-or black holes.
The answer, I will once again explain in brief, nothing detailed-
Let’s start with G, the strength of gravity of the universe.
We actually measure G from here on Earth, using many, many clever experiments conducted by scientists of course!
As I said earlier, G is the same everywhere in the universe, not only on Earth.
Let’s move on to M- the amount of Mass a black hole contains.
We obviously can’t measure the weight of a black hole, but we can use a bit of physics and data from surrounding objects to get a pretty close estimate.
As we know, black holes are invisible.
So if stars and/or other celestial bodies are rapidly revolving around something invisible-Its most probably a black hole!
(Another question you may ask is “Why do celestial bodies need to revolve around something invisible rapidly or fast for it to be a black hole?
The answer is that if the celestial bodies don’t move fast enough, they too will be sucked in by the black hole.
This is derived from 2-3 physics concepts (there is no official concept) that basically says
The stronger the gravity, the faster something has to move to stay in orbit.)
So, by watching how fast the celestial bodies move around the black hole, scientists can pretty closely calculate the gravity and thus find out the weight of the black hole.
It’s kind of how you could try to calculate the weight of an elephant by watching the way grass shakes whenever it moves.
Now, you may ask-how the heck do scientists do that?
The answers are in use of..PHYSICS!
This may seem a bit confusing but if you apply certain complex formulas you will eventually be able to calculate M.
So, once scientists have all three variables, they find out the size of the event horizon using the given formulae. Here is the general idea-
The bigger the mass of the black hole is, the bigger its event horizon.
Light speed and gravity just decide how it all comes together!
Possible Question-
Why do celestial bodies even orbit black holes in the first place?
Imagine you’re a star. You spot a black hole, and you can’t just run away. The not-so amazing gravity doesn’t allow that (BOO HOO)
Would you rather orbit it and save your life-though you will have to be very careful about how close you orbit, because you could get consumed by the hole if you’re too close, (In other words-still be scared for life about accidentally falling in) – or get consumed by it? (PS-Ignore what I said in the bracket, it isn’t meant to change your answer.)
I’ll let you decide the answer to that question.
And that’s how to calculate the size of the event horizon! Insane, right?
Well, that’s all for now, so I’m signing out. If you like this essay of mine, I’ll try to do the same with another essential part of black holes, their singularity. Until then, Adios my friends!
