D-brane facts for kids

D-branes are special objects in string theory, which is a way to describe the smallest parts of our universe. Imagine tiny, tiny strings that vibrate and make up everything around us.

In string theory, these strings can be either "closed" (like a rubber band) or "open" (like a piece of string with two ends). D-branes are like surfaces or "membranes" where these open strings can attach their ends. Think of them as sticky walls in the universe where the ends of these tiny strings are stuck.

The "D" in D-brane stands for "Dirichlet," which is a math term about how things behave at a boundary or edge. So, D-branes are named because open strings end on them in a specific way called "Dirichlet boundary conditions."

D-branes were first thought of by scientists Dai, Leigh, and Polchinski, and also by Hořava, back in 1989. Later, in 1995, Joseph Polchinski made a big discovery. He realized that D-branes were the same as certain types of black hole-like objects in a theory called supergravity. This discovery was very important and helped kick off a new era in string theory research. It also led to ideas like the holographic principle and M-theory, which try to explain how our universe works at its most basic level.

D-branes are usually described by how many spatial dimensions they have. This is shown by a number after the "D." For example, a D0-brane is like a point, a D1-brane is like a line, a D2-brane is like a flat surface, and so on.

What are D-branes?

D-branes are special surfaces or "membranes" in string theory. They are places where tiny, open strings can end. Imagine a very thin, flat sheet floating in space. If you have a tiny string, its ends can be stuck to this sheet. That's a simple way to think about a D-brane.

These objects are very important because they help us understand how different parts of string theory connect. They also play a role in explaining how gravity works at a very small scale.

How D-branes work

In string theory, everything is made of tiny vibrating strings. Some strings are closed loops, like a rubber band. Others are open, meaning they have two ends. D-branes are the "walls" or "surfaces" where these open strings must end.

When an open string ends on a D-brane, it follows special rules called "Dirichlet boundary conditions." These rules basically say that the ends of the string can't move off the D-brane. They are stuck there.

The number after the "D" tells us how many dimensions the D-brane has.

• A D0-brane is like a single point.
• A D1-brane is like a line.
• A D2-brane is like a flat sheet.
• A D3-brane is like our familiar 3D space.
• Scientists study D-branes with up to nine spatial dimensions!

Discovery and Importance

D-branes were first discovered in 1989 by scientists Dai, Robert Leigh, and Joseph Polchinski. Petr Hořava also found them independently around the same time. At first, they were just interesting mathematical ideas in string theory.

Polchinski's Big Idea

In 1995, Joseph Polchinski made a huge breakthrough. He realized that D-branes were not just abstract ideas. He showed they were actually the same as certain types of black hole-like objects predicted by a theory called supergravity. Supergravity combines general relativity (Einstein's theory of gravity) with supersymmetry (a theory about particles).

This discovery was a game-changer for string theory. It helped scientists connect different versions of string theory and led to new ideas like:

• The Holographic Principle: This idea suggests that our 3D universe might be like a hologram. All the information about our universe could be stored on a 2D surface, much like a D-brane.
• M-theory: This is a bigger theory that tries to unite all five different string theories into one single framework. D-branes are key parts of M-theory.

Polchinski's work on D-branes helped start what is known as the "Second Superstring Revolution." It opened up many new ways to understand the universe and how gravity works at its most fundamental level.