What the heck is Brane Theory?

Here we go again with the quirky world of science where quantum physics meets cosmology. The problem with trying to understand this branch off of string theory is that it's purely mathematical. Brane is short for membrane, which is a two dimensional dynamic object that can inhabit specetime in a way that only a quantum physicist would love. A brane can have mass, charge and an associated filed just like particles. The difference is that a particle is one dimensional, whereas a brane is a two dimensional object. Physicists often refer to them as a p-brane (Ha,ha!) because they occupy a worldvolume, a p+1 dimensional volume of spacetime.

The reason why this idea was formulated is because of the fact that physicists wanted to reconcile the weak force of gravity with the other three main forces of nature: electromagnetic force, strong and weak nuclear forces. They also needed something to hang a string onto. Strings can be closed or open, and if the string is open one end must be on a D-brane, which is short for Dirichlet boundary condition. This has to do with a mathematical idea involving differential equations used to find the values needed to specify the boundary of a domain. Huh?

Needless to say, this is a very complex concept steeped in crazy mathematics. One place where this has been used is to answer the question; when a gas is sucked into a black hole, would its entropy disappear from the universe or would it add its entropy to the black hole? This argument went on for a long time before D-brane theory solved this argument, an argument that Stephen Hawking lost.

To make a long story short, if one considers that a black hole is made from D-branes then the entropy question is answered, at least mathematically.

So, how could one conceptualize a brane? That depends on what sort of a brane you're talking about. If we consider p-branes, then all one has to do is think of what dimension it has. A quantum particle, a point, is a 0-brane. A string is a 1-brane, and a 2-brane is like the ocean's surface. It's two-dimensional.

If you're talking about a D-brane, you can think of them as two-dimensional fields (think of thin plastic sheets) that have string ends attached to them. One could consider the universe to be a giant D-brane that flaps around in three dimensions. Think of lots (an infinite number) of these D-branes floating around with closed strings floating between them. We can't move at right angles to a D-brane and as a result, can't go to another brane. This is why we're stuck in our universe even though parallel universes are very near. Some cosmologists think that when two of these branes flop too close and make contact, another Big Bang happens, which means that the Big Bang that created our universe is only one of many that have happened and will happen. Now, that's a wild idea!

Thanks for reading.