A Definition

It is quite difficult to find a good definition about black holes because there are many misconceptions about them. To many, giant cosmic vacuum cleaners would be a better name for these mysterious objects that are thought to roam the universe sucking everything in its path. That is completely wrong, but I'll get back to that later.

Yet, through all of these errors, one fact remains constant and that is that the escape velocity of a black hole is higher than the speed of light.  This means that you need to be traveling faster than the speed of light to escape a black hole once you are past the event horizon. The event horizon is an imaginary line that separates the black hole from the rest of space, it is also known as the point of no escape, which I will be talking about later.

So in a nutshell, a black hole is a region of space that has an escape velocity higher than the speed of light. I know this may seem like very little information but I will give you a LOT more later on, for now lets go over some common misconceptions on black holes.

 

Misconceptions

 

• Cosmic Vacuum Cleaners

Firstly, no, black holes cannot and will not suck in the whole universe. Although it is true that they have a very strong gravitational field, that is only at a very close range. You could even orbit a black hole as normally as you would any other star! IF the sun became a black hole ( which it can't ) the Earth's orbit would pretty much be the same.

 

• Sun --> Black Hole

The sun cannot become a black hole because it is not massive enough. The minimum size of a star that could become a black hole has to be AT LEAST 2.8 x the suns mass. So don't worry.

 

Escape Velocity

 

The signature aspect of a black hole is the fact that its escape velocity is higher than the speed of light, but what is escape velocity??

Escape velocity is the speed at which you need to be traveling to escape a body's gravitational force. For example, you need to be traveling at 11.2 km per second to escape the earths gravity. If you were going any slower then you would fall back down to Earth. The escape velocity ( EV ) of an object depends on its gravity and its gravity depends on its mass, therefore the EV of an object depends on its mass. The sun (whose mass is 333,000 times the earth's) has an EV of 618. km/sec.

 

CALCULATING ESCAPE VELOCITY

We use this equation to calculate escape velocity.

 

 

 

 

 

 

 

 

V --> Velocity

esc --> Escape

G --> Gravitational constant ( 6.67 x 10 to the power of -11 )

M --> Mass

r --> Radius

 

So when we translate this equation, we get ...

 

 

 

But how did we get this formula?

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Well we start off with the simple conservation of energy idea and the energy equation.  (KE = Kinetic Energy and PE = Potential Energy)

 

KE ( initial ) + PE ( initial ) = KE ( final ) + PE ( final )

 

We know, according to Newtons 1st law of motion that once you apply force to an object it will remain at  constant velocity unless an opposing force is applied to it.  In our case, once we have applied force to the rocket and it has escaped the earths gravitational pull, then ( assuming it doesn't meet any obstacles ) it will continue to travel until it reaches infinity. Theoretically is impossible. At this point, the KE ( final ) would be 0 making the PE ( final ), 0.

 

KE ( initial ) + PE ( initial ) = 0

 

Now lets look at this a bit closer.

 

PE =  ( Gravitational constant x mass of the rocket x Mass of the planet/star)/ radius ( of the planet or star) 

KE = 1/2 mass (velocity) squared

 

In our case, we are replacing velocity with escape velocity.  When we calculate gravitational potential energy in astronomy, which is what we are doing now, then we use the furthest point ( which is infinity ) as 0 and everything before that point is negative. This means that the PE in our equation will be negative. So ...

 

 

 

 

 

 

We can move the PE formula to the other side of the equal sign, then cancel out both the smaller m ( which represents the mass of the rocket ) leaving us with ...

 

 

 

 

 

 

 

From here we multiply each side by 2 o make ' escape velocity squared' whole, then we square root both sides giving us ...

 

 

 

 

 

 

 

 

 

ET VOILA! :)

 

Here is  a simple video on Escape Velocity.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

And here is a much more complex one

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

This one is about escape velocity and how it relates of black holes

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Just in case this wasn't made clear enough before, the formula I showed you earlier is for most objects but NOT for black holes.

Black Holes have such strong gravitational fields that this formula simply would not work. But what are gravitational fields and how do they relate to black holes??

 

Gravity

 

The idea of gravity was first proposed by Sir Isaac Newton in 1687 although Galileo Galilei had experimented with the idea 100 year before. You all know the famous story of Isaac and the apple, and thanks to that apple, Newton came up with his theory of universal gravitation which states ...

 

the gravitational force between two bodies is proportional to the product of their masses, and inversely proportional to the square of the distance between them.

 

According to Isaac Newton, Gravity was force, an attraction between 2 objects. The bigger the mass the stronger the attraction. This idea was also written as formula:

 

 

 

 

 

 

 

G= Gravitational Constant

m= Mass of the 1st object

M= Mass of the 2nd object

D= Distance between the centers of gravity of the 2 objects

 

We still use Newtons laws of gravity today, 400 years later. And back then, when Newton had just published his findings, he knew that if his findings did not match those of Johannes Kepler that were found to be true 100 years previous , then they would be wrong. Fortunately when he applied his ideas with Kepler's, they matched perfectly, in fact they even supported the idea and was evidence that what Johannes Kepler was saying is true. Watch this video and go to this link for more on Newton and Kepler.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Einstein, of course, had another idea. He believed that gravity was not a force but instead a warp in the fabric of spacetime. Hold on, what the heck is spacetime??

You see Einstein had this idea,he believed that the 3D universe of space was interconnected with the 1D material of time, creating the 4D fabric of SpaceTime. Einstein was right, he was right a lot. But this doesn't mean that Newton was wrong, no, in fact we still use Newton's equations today, Einstein simply took his idea and made it better. Objects still have  gravitational attraction to each other, and it is this gravity that warps the fabric of space ti

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

You can imagine spacetime the fabric of a trampoline of a mattress.  Objects such as our sun, act like bowling balls on the mattress bending it. Now imagine that were is a golf ball next to the  bowling ball, the golf ball would circle towards the bowling ball right? The golf ball represents a planet.

Object with more mass have stronger gravity, meaning that they make a deeper warp in space time