All of you budding string theorists out there might want to check out An Introduction to String Theory and D-Brane Dynamics by Richard Szabo. At about 140 pages, this is a compact book (sorry for the pun) that gives you a quick and dirty introduction to d-branes.
A basic picture of d-branes and the universe is that (leaving out time for the purposes of this discussion) we live in a three dimensional world that is embedded in a higher dimensional space. The world we live in is a brane while the remaining extra dimensions form the "bulk". The term brane is derived from membrane, think of a sheet of rubber, its a flat 2-D surface in our 3-D world. Add a dimension and you have a 3-D "brane" that sits in a higher dimensional space.
Apparently string theory tells us that the fields of the standard model are trapped on the brane (electromagnetism, i.e. light for example), but gravity can "leak off" the brane and is free to travel into the higher dimensional bulk. This is because of the boundary conditions on the strings that transmit the known forces, the string state representing the photon has its ends stuck on the brane, but the graviton is a closed string that can go where it wants to, so alot of them end up traveling into the bulk which we can't perceive directly with our senses or instruments. So gravity appears to be a much weaker force.
So to the book. I have to say that Szabo's book is one of the most clearly written physics texts I have come across. Its a five star book in my opinion. I am not sure how it would do as a first time exposure to string theory, I read it after having slogged through a lot of other books. But its a lot clearer and more to the point than all of them. Plus the small size makes it a nice way to tackle string theory and get the main ideas quickly.
The book does require some background in field theory, so you should know how to work with Lagrangians, the action, and getting the equations of motion for example. But its highly recommended for those with the requisite physics and math background.
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