# Question about Superstring and M-theory

1. Jan 3, 2008

### chronie

I just recently started researching and reading about these two theories. I searched the forum and could not find the question I would like to ask, which is: Without getting too difficult is there any obvious differences between the Superstring and M-theory other than that the M-theory contains another dimension.

I know that the Superstring theory is basically an updated String theory but I cannot find the differences for the M-theory.

Much appreciated,

Chronie

2. Jan 3, 2008

### mitchell porter

You may have read that at one time there were five separate 10-dimensional superstring theories, and then in the 1990s they were all shown to be related to each other by something like a change of coordinates. The 11-dimensional "M theory" is also part of that web of relationships, but it's different in several ways. First, it's 11-dimensional - the relationship to the 10-dimensional theories only shows up in limits where one of the 11 dimensions is small. Second, instead of having fundamental strings, it has fundamental "membranes", 2- and 5-dimensional objects (one way to get a string from an M2-brane is to have one of the brane's dimensions extended along the small 11th dimension, so it only has one big dimension left in the 10-dimensional approximation). Third, unlike the superstring theories, the actual equations for M theory are not known very well, although there is an approximation called "Matrix theory", and the low-energy limit (in which strings and branes are treated as particles) is a well-known and much older theory, 11D supergravity.

So the perspective now is that there is only one theory, but in different limits or approximations it looks like M theory or like one of the superstring theories. At one time people referred to the overall united theory as M theory, but "string theory" won out as the overall name (even though there are membranes too), and M theory is now just the name for the 11-dimensional form of "string theory".

3. Jan 3, 2008

### chronie

So basically the M-theory contains a bits of each of the 5 theories, and then has the theory of membranes and not strings. Alrighty thanks for putting that in black and white it was hard for me to find exactly what the differences were because all sources cited back to the original string theory.

4. Jan 5, 2008

### Toastus

When branes collide, do they give off some of their own energy into the new brane being formed? Otherwise that would violate conservation of energy and matter.

5. Jan 6, 2008

### mitchell porter

You definitely have conservation of energy in string theory. But I can't say very much about interactions between branes, there's still way too much I don't understand. The very simple picture is that you have open strings (like a line) and closed strings (like a loop), and open strings have to start and end on a brane (not necessarily the same brane), while closed strings can break away and travel through the space between branes; and open strings give you electromagnetism-like forces (which means weak and strong forces as well), while closed strings give you gravity. So, there are a lot of papers just describing static arrangements of branes, in which the branes are just sitting there, absorbing/radiating strings. Or there are others in which the branes change position with time, because of the inter-brane force created by the strings - there's your transfer of energy. I believe the best-known brane cosmologies fall into this second category - two braneworlds fall towards each other in a fifth dimension, collide and repel, and that's the big bang. About anything more complicated, like one brane emitting another brane, I have no insight at all, except to say that the Matrix-theory formulation of M theory suggests to me that branes of all dimensions can be viewed as aggregates of point-branes held together by string-flux, so it would just amount to a sub-aggregate breaking away. But you might want to ask a professional (many of them blog) for a second opinion.

6. Jan 6, 2008

### chronie

Alright thanks everyone I feel I have a better overall understanding of the two and the overall theory in general.

Share this great discussion with others via Reddit, Google+, Twitter, or Facebook