Got Crack?: The Nature of the Event Horizon

	The event horizon around a black hole is what is described as a mathematically-defined 
boundary, but what exactly is that?  This is what leads to some confusion and a failure to 
understand the general ludicrousness of a the infamous crack in the event horizon from the Voyager 
episode "Parallax."
	Boundary is self-evident; it is a place that cannot be crossed for some reason.  Usually 
boundaries are physical, such as a wall, and we often tend to think in boundaries in those terms.  
And obviously, if faced with such a boundary, you have two choices: find a way around it, or try to 
break through it.  Hence the crack in the boundary that is the event horizon.  However, the problem 
is that the event horizon is NOT a physical boundary, and as such CAN'T be cracked.  You cannot 
crack something which has no substance.
	We instead describe the boundary as being mathematically-defined, and by that we mean 
that it is a point, line, or surface that is determined using mathematics rather than physical objects.  
The fifty yard-line on a football field is defined as the point located fifty yards from both endzones.  
A line is painted down that spot, but it is not the line that determines the boundary, it merely marks 
where the boundary is.  What's more, it's obvious that there's nothing stopping anyone from 
crossing this boundary; it's just a line.
	Let's start with a simple example to better understand the event horizon.  Let's pretend a 
spike is driven into the ground in a large back yard, and attached to that is a rope with length x.  
The rope is tied to the collar of a dog.  The dog is now unable to move wherever he wants; he is 
limited by the length of the rope.  What is that limit?  It would be a circle with a radius close to x, 
because the dog cannot travel any farther from the spike then the maximum length of the rope.  We 
could then draw a circle with this radius of x centered on the spike and we'll have marked off the 
dog's territory.  It is impossible for the dog to leave that circle.  The circle is a mathematically-
defined boundary, calculated using the length of the rope and the location of the spike.  But what 
needs to be made clear is that it's not the circle that is stopping the dog; it just defines the limits.  
The dog cannot suddenly escape the circle if you crack the circle at some point; you have to deal 
with what's holding the dog back: the rope.
	Now that we understand that, let's take it a step further.  Imagine you have a rocket sled 
(basically a board with runners and an engine on the back) in a large, perfectly uniform field.  You 
mark a spot with a big X and put the sled on it.  You light it up and it travels a certain distance, let's 
say y.  You put a mark on the ground, pull it back to where it starts, and fire it again in a different 
direction.  It stops, and you mark it, and again it travelled distance y.  You repeat this until finally 
you have a perfect circle of radius y centered on the big X.  This circle is also a mathematically-
defined boundary: your sled cannot cross that line when you start on the big X.  Why can't it?  
Several laws of physics: the energy produced in the rocket, the weight of you and the sled, the 
coefficient of friction with the ground, air resistance, and several other factors.  No matter what 
direction you point the sled, it CAN'T cross the line.  Now remember, the line itself is not stopping 
you, it's all those other factors.  You could maybe go a little farther by redesigning your runners or 
making it more streamlined, but unless you make those changes to the rules you can't cross the line.  
The line is the boundary, determined mathematically by the combination of all those forces acting 
against each other; the line only says where the boundary is, it is not what stops you.
	The event horizon in a black hole is the same thing.  The event horizon is determined by 
many characteristics of the black hole, but it is a boundary that is just like the previous two 
examples.  The event horizon is not what stops you from escaping a black hole, it's the laws of 
physics that define where the event horizon is.  To illustrate the absurdity of putting a crack in the 
event horizon, let's apply the same thinking to our previous examples.  The dog in the first example 
is tied to a rope which is attached to a spike, and therefore can’t leave the circle.  If you put a crack 
in the circle, the dog is now free to run as far as it likes regardless of how short the rope is.  Or 
imagine you crack the line around your rocket sled.  Now you can travel as far as you like, zipping 
and sliding across creation because the boundary is now gone.  Hopefully you’ll see why both of 
these ideas are ludicrous, and this is exactly why the crack in the event horizon is ludicrous too.  It 
is not a physical boundary, but rather a spot that you cannot pass without changing the rules.