In a sense, the introduction of M-theory marks the end of “string theory,” because it ceases to be a theory that contains only fundamental strings. M-theory also contains multidimensional membranes, called branes. Strings are only 1-dimensional objects, and therefore only one of the types of fundamental objects that make up the universe, according to the new M-Theory.
Branes have at least three key traits:
Branes exist in a certain number of dimensions, from zero to nine.
Branes can contain an electrical charge.
Branes have a tension, indicating how resistant they are to influence or interaction.
String theory became more complex with the introduction of multidimensional branes. The first branes, called D-branes, entered string theory in 1989. Another type of brane, called a p-brane, was later introduced. Later work showed that these two types of branes were in fact the same thing.
Branes are objects of multiple dimensions that exist within the full 10-dimensional space required by string theory. In the language of string theorists, this full space is called the bulk. One major reason that string theorists didn’t originally embrace branes was because introducing more elaborate physical objects went against the goal of string theory.
Instead of simplifying the theory and making it more fundamental, branes made the theory more complicated and introduced more types of objects that didn’t appear to be necessary. These were the exact features of the Standard Model that string theorists hoped to avoid.
In 1995, though, Joe Polchinski proved that it wasn’t possible to avoid them. Any consistent version of M-theory had to include higher-dimensional branes.