# Are branes as fundamental as strings or not?

Are D-branes in string theory as fundamental as one-dimensional strings, or are strings more fundamental and multi-dimensional branes are "woven" from one-dimensional strings?

In string theory and related theories such as supergravity theories, a brane is a physical object that generalizes the notion of a point particle to higher dimensions

They are called generically string theories, but if a theory is based on branes, the concept of one dimensional string representing elementary point particles has developed into two dimensional branes representing elementary particles.

• There are theories in which the three-dimensional brane is our entire universe. It's strange to think of the universe as a particle. Jan 1 at 9:54
• can you give a link? maybe they are not trying to describe the standard model particles. Jan 1 at 12:19
• en.m.wikipedia.org/wiki/Brane_cosmology Jan 1 at 12:57
• I do not think the link says what you claim. It is cosmology in these models because the extra dimensions are assumed for the universe, but the branes are the usual SM particles Jan 1 at 13:29

$$D$$-branes are as "fundamental" as fundamental strings, at least in the two following precise ways:

1. String dualities (the symmetries of string theory as a whole) exchange fundamental strings with D-branes. As an example, the $$SL(2,\mathbb{Z})$$ symmetry of type $$IIB$$ string theory mix fundamental strings and 1-branes (see An $$SL(2,\mathbb{Z})$$ Multiplet of Type IIB Superstrings ).
2. Fundamental strings descend from $$M$$-branes in the similar ways as $$D$$-branes descend from $$M$$-branes. The prototypical examples are type $$IIA$$ fundamental strings at finite coupling as $$M2$$-branes wrapped in the eleven dimension of $$M$$-theory, and $$D2$$-branes as unwrapped $$M2$$-branes. They are the same object, just with different geometry (which obviously has important dynamical imprints). See the beautiful $$P$$-brane democracy and P-branes from M-branes for further details.
• Thanks. Do strings in string theory interact with each other constantly and all the time, or do they stay free for a while and interact for a while? Jan 1 at 18:43
• @АрманГаспарян It was a real pleasure. The answer to your question depends on the value of the string coupling constant. If it is zero, strings do not interact, if it is positive strings interact; and the more bigger the coupling is, the rate of interaction among strings is equally bigger; as in any other quantum field theory. The difference with string theory is that the string coupling constant the expectation value of a field with its own dynamics. Jan 1 at 18:51
• @АрманГаспарян A question you may ask is the following: How is the dilaton potential determined in string theory? First, string theory is not incomplete in the sense that there is some new dynamics is needed to explain the shape of the dilaton potential. The dynamics of string theory is "circular/boostrapy", the dilaton detemine the rate of interaction of all the fields in the string and all the fields of the string dynamically generate the potential of the dilaton. Jan 1 at 18:55
• @АрманГаспарян Yes, the coupling can be tuned to zero. The latter is equivalent to say that the diaton is able to have a flat potential Jan 1 at 18:58
• @АрманГаспарян Yes, exactly zero coupling its an idealization. The existence of strings forces the apereance of gravity, and gravity is a long range force that couple anything that has energy momentum (in this case the strings). So, at least two strings separated finite distance feel the gravitational potential, and if they scatter with some CM-energy then an effective interaction takes place. However, as in any quantum field theory. You can ideally work computing something by ignoring gravity and supposing that the dilaton has no VeV. Jan 1 at 19:31