# Branes strings lqg toe and other fancy shortcuts

1. Jun 14, 2004

### Tom McCurdy

Alright I swear that I read in scientific american that Loop Quantum Gravity is trying to put fouth a new toe to compete with string theory. However in discussion with Marcus he told me that they are not trying to have any sort of theory that would lead to toe.

Part 2
I was reading in Pop Sci-- Journey to the 10th dim from March 2004 and it discusses a theory that needs 10 dim (similar to old string theory) needing Calabi-Yau manifolds. However as far as I know that only theory that needed 10 was string theory but since its transformation thanks to ed witten to M-Theory (Magic Mystery or Matrix... lol) they said it needs 11 dimensions. Then they discuss brane theory. Now I thought from elegant Universe brane theory was something that string theory predicted. However the article discribes it as something that string theory kinds of steals and uses because it fits nicely. They also discribe SuperSymetry in the same way, as something string theory just adapted.

I thought that it was something that it was predicted from string theory as well again thanks to elegant universe.

Part 3
Besides String Theory anyone name any other theories that are supose to lead to toe...

2. Jun 14, 2004

Staff Emeritus
To understand the relation of string theory to M-theory you have to have a feel for perturbative physics. A lot of physics calculation is done perturbatively: they assume the interactions are very weak, almost not there at all, just a "small perturbation" away from zero. So then the "coupling" is a small number, and they do their calculations with infinite series using powers of the coupling. Say we let the coupling be k. Then many physical quanitities will be like $$a_0 + a_1k + a_2k^2 + ...$$, and because k is small, the terms get small fast, and you can get pretty accurate with just the first few terms. But if k is too big, the series won't converge at all, and your quantities will be meaningless.

So in essence they are sneaking around the edge of the action, colonizing the rim where it drops off close to zero. This is what they mean by "perturbative physics". And what they dearly wish is that they could do "non-perturbative physics" and get at the action in the middle where it's strong.

So now I can explain that all of superstring theory, which is defined in 10 dimensions, is the perturbative edge of non-perturbative M-theory, which has an extra dimension. There were actually five different pertubative superstring theories, until they discovered some non-perturbative dualities that showed they were just five different sectors on the weak action edge of M-theory, and you could get from one to the other by "going across the middle".

And M-theory after several years is still not well understood, precisely because it is non-perturbative and the well-known math tricks don't work. Only a very few non-perturbative methods are known.

The trouble with magazine popularization is that they are so narrowly focussed they can fail to give you the important context.

Last edited: Jun 14, 2004
3. Jun 14, 2004

### Tom McCurdy

how do you know all this? lol

ty again.... pretty soon i am going to owe you my first born

4. Jun 14, 2004

### jeff

First born? Don't be ridiculous! (Uhm...have any puppies? )

5. Jun 14, 2004

### sol2

ignoring Jeff's post.......keep it(knowledge) in the family....?

6. Jun 14, 2004

### marcus

sol2 we know from traditional sources that this is the customary
payment for someone who shows you how to spin straw into gold

(see Grimm, the Rmplstlskn chapter)

and the guy must be literate
because he has to give a talk on string to some Brits this summer
and he had a lot of straw
and selfAdj just showed him an elegant way to turn it into expository gold
bravo to both

Last edited: Jun 14, 2004
7. Jun 14, 2004

Staff Emeritus
AS Jeff might have been implying, I am a student, although an old one, but not a teacher. I await correction, just as you, Sol, always note that you do.

And no, I am not going to jump up and down if you guess my name!

8. Jun 14, 2004

### sol2

Marcus I assure you my intentions are pure, and I recognize Tom's efforts as I understood what he is faced with being so young. Bravo to the aspiring for they can become something of a "mover," ahead in time:)

But to self adjoint namesake, no similarity in last name, I know.

You have been a fine teacher. You just didn't know it:)

As a student you remained open to listen and correct. So you have taken it to heart, for the younger generations. We "lead" by example? You must have had some fine teachers

Last edited: Jun 14, 2004