by Liam Daly Dooley, 6th year, St. Joseph’s College, Borrisoleigh
Since I have started studying physics in school, I have developed an interest in understanding our universe a little better. Most people have a fascination with Black Holes and so I am going to attempt to explain them in my own words. Stephen Hawking has spent most of his life developing theorems and equations to prove their existence!
So what is a black hole?
Simply, a black hole is a point or region in space. It has so much force called gravity, that not even light, the fastest thing anything can travel can escape its attraction, If light can’t escape it, it appears black, hence the name!
To scientists looking at it from telescopes, it just appears as a circle of blackness. But at the heart of a black hole is something called a singularity. A point of zero size and infinite density, yes you read that correctly, zero size and infinite density! It doesn’t seem possible!
Anything can become a black hole, but don’t worry as it would take an extreme amount of work for something like that to happen, not even the sun would be powerful enough!
A black hole is an object, for which nothing can get a high enough escape velocity to get away from. Think of a cannonball being fired straight up in the air. As it goes up, it will be slowed down by gravity and come falling back. If the speed is high enough, however, like a rocket ship, it will keep going until it escapes earth’s gravitational pull.
If you wanted to get a space shuttle off the earth, you would have to get it to the same speed as if you wanted to get a pebble off the earth. The only difference being, the amount of energy needed to get something as heavy as a space shuttle to the right speed is more than that needed for a light pebble.
If the mass of the body was big enough however, then you could get a velocity value that was faster than the speed of light. According to Einstein, time travel is only possible if we are moving faster than the speed of light and we haven’t managed that yet!
Think of a kitchen sink, the water on the outside of the basin is still but as the water goes towards the plunge hole, it is sucked in and can’t escape. This is a Black Hole.
Naturally occurring black holes form when a star collapses. Stars are massive. We call our nearest star, the Sun! Due to their large mass, they have a huge amount of gravitational force. When the force of gravity from a star becomes bigger than the outwards pressure caused by its temperature, the force starts to make the star collapse. All its mass is pulled inwards to a central point. This point gets smaller and smaller, denser and denser as all of the stars mass is squashed into the singularity. Not all collapsing stars form black holes however. In order for an object to form a black hole it has to be compressed below a radius known as the Schwarzschild radius, after physicist Karl Schwarzschild who discovered it in 1916.
So for example if you wanted to turn the earth into a black hole you would have to compress it down to about the size of a mosquito. If you wanted to turn your car into a black hole you would have to squash it down to the size of a neutrino, which is pretty small!
Once an object has been compressed to Schwarzschild radius it will continue to collapse until it becomes a singularity. Centered on this singularity will be a sphere of Schwarzschild radius called the Event Horizon. The Event Horizon is the last distance from which light can escape the pull of the black hole. Inside the event horizon, everything, including light, must move inward, getting crushed at the center
The event horizon itself isn’t some physical barrier in space; it simply represents the last distance at which it is possible to escape the gravitational pull. A person falling into a black hole through the event horizon wouldn’t notice anything different (although they may be preoccupied with the excruciating pain of being crushed and stretched by all that gravity).
Due to the extreme nature of gravity around the event horizon some very weird things can happen. Someone falling into a black hole wouldn’t notice any changes as he went through the event horizon, however, for someone watching at a safe distance it wouldn’t be that simple. Things moving away from a body are slowed down by the gravitational pull, the bigger the pull the more things get slowed down. Also the closer you are the more you are going to be slowed down. As we sit at a safe distance and watch the unlucky person get closer and closer to the black hole they seem to slow down! This can’t be right can it? Stuff moving away is meant to slow down; stuff moving towards the black hole should speed up!
The way we see the person falling in is through photons (particles of light) being reflected off them and into our eye. As they get closer to the black hole the photons get slower due to the increase of gravitational force, so they take longer to reach the observer.
The photons given off when the person crosses the even horizon will be slowed down to 0 by the gravity and so an observer will never see them disappear.
Current research and studies indicate that our universe might be currently inside a White Hole which is the opposite of a black hole. We have no proof of either as of yet though!!!