Reproduced with permission from the neOnbubble Know You Some Science series of student learning guides.
To explain what a black hole is – other than something into which you bury burglars – you will need to know about light and, specifically, the speed of light and how it is affected by gravity.
What is the speed of light?
Anything that moves can be said to have a speed. For example: something which moves a distance of three miles in an hour can be said to have a speed of three miles per hour.
Some things move at different speeds at different times; cars driven by burglars, for instance, may be crashed into a phonebox with the driver slumped over the wheel – the victim of a well-thrown spork – and therefore not moving at all, or they may be rolling uncontrollably down a steep incline with a mutilated burglar in the boot towards a cliff edge and about to reach terminal velocity.
Other things move at a constant speed (allowing for the external environment); when you shout at burglars the noise travels at the same speed as when you are whispering into the ears of trussed up, dazed, and crying would-be burglars. Sound travels at a constant speed in the same atmospheric conditions. Light behaves similarly and the rate at which light moves in a vaccum is what we call the speed of light.
Why is the speed measured in a vacuum?
Nature abhors a vacuum and you can see this for yourself by switching one on near a cat; it will run like crazy. Likewise, light molecules move at their greatest velocities when a vacuum is present and are limited to a slightly slower pace when there is a lot of dust and other stuff around that they might bump into.
Why does light travel so fast?
Heavy things are difficult to move. Large, concussed burglars require at least two people to shift them into the sex dungeon. Light molecules – as the name implies – are light, meaning they are both not dark and not heavy. In fact light are the least heavy of the DTT (Difficult To Touch) family of molecules and, subsequently, they are most capable of travelling at high speed.
This also explains why it is impossible for a person to travel at the speed of light. As an experiment try shining a torchbeam on one hand. In the other hand lift the leg of a comatose burglar. You will find that legs are far heavier than light and thusly unable to reach the same speed. Removing your legs will help but not completely alleviate this problem.
So, what is a black hole then?
To understand black holes and their relationships with light and its speed you need to know a very little bit about gravity and this is a great place to discover a very little bit about gravity.
Gravity is a weak, cumulative, attracting force emitted by all matter in the universe. Because it is a cumulative force the more matter that is present in one place the more gravity it exerts on its neighbours pulling them in. So large things – a planet, for instance – pull smaller things – heads of decapitated burglars, for instance – to them. Drop a decapitated burglar’s head and you will see it is attracted to the planet’s surface.
However, because it is weak it is quite easy to temporarily break. You can lift the burglar’s head from the ground even though the entire planet’s gravitational force is working against you. Even better, you can throw the burglar’s head into the air. The speed at which you throw the head determines how far it gets away from the planet before the cumulative, weak force of gravity pulls it back down.
Every object has a speed at which something can move away from it fast enough so that gravity cannot reclaim it; this speed is known as the Escape Velocity after the 1999 movie of the same name which is so terrible you’ll run from it at a fair whack too. It is unlikely that you would be able to throw a burglar’s head fast enough from the planet’s surface to escape the Earth’s gravity but you are more likely to be able to do this when standing on a mountain because you’re further away from the mass.
A black hole is an area of immense mass exerting a gravitational pull that is strong enough to stop even light molecules with their great speed from escaping. Because light cannot escape and because we see things when light molecules hit our eyes and leave an imprint behind the effect is one of darkness.
At a certain distance away from the immense mass – in a manner similar to standing on a mountain and hurling a burglar’s head with all your righteous might – light is able to get away. This terrifying point of no return for light is known as the Event Horizon after the 1997 movie of the same name which is also quite scary.