How Does String Theory Work ?

twistor59 · #1 by **twistor59** » Aug 03, 2010 7:02 pm

Just a rough idea would be good - I'm still struggling to get an idea how LQG works at the moment so no time to read about strings, but any little insights would be useful and probably of interest to lots of people.

I know there are many variants of string/brane/M theory, but I was just trying to understand how vibration modes of a string could possibly give rise to the multiplicity of particles we see. I had a look at this and it contains the statement

The frequency of the string determines what type of particle it is.

This is what I don't get - particles have all sorts of properties - mass, charge, hypercharge, isospin.... Where do these come from in the sting picture ? Normally particles are described in terms of gauge theories, with the fermions described in terms of representations of the gauge group and the force carrying bosons as connections in bundles built from the gauge group. But, where does the gauge group fit in in string theory ? Spacetime has lots of dimensions, some of which are compactified - is the gauge group somewhere in this compactification or what ?

newolder · #2 by **newolder** » Aug 03, 2010 7:23 pm

Hmmm. “There was an error processing a page. There was a problem reading this document (135) OK” Many,many clicks later... perhaps this other Ed Witten note might help? :dunno:

http://www.sns.ias.edu/~witten/papers/mmm.pdf

MrGray · #3 by **MrGray** » Aug 03, 2010 7:26 pm

Bookmarked.

PhiloKGB · #4 by **PhiloKGB** » Aug 03, 2010 7:32 pm

In the interest of preserving a current internet meme, I would like to ask moderation to alter the thread title to read, "String theory. How the @$%# does it work?"

twistor59 · #5 by **twistor59** » Aug 03, 2010 8:00 pm

newolder wrote:Hmmm. “There was an error processing a page. There was a problem reading this document (135) OK” Many,many clicks later... perhaps this other Ed Witten note might help? http://www.sns.ias.edu/~witten/papers/mmm.pdf

Thanks.

So is it your understanding that our favourite gauge groups, like SU3xSU2XU1, are embedded in E8xE8 (taking that example) and that the E8xE8 is somehow something to do with the CY space ?

hackenslash · #6 by **hackenslash** » Aug 03, 2010 10:07 pm

It's a bit more involved than just the frequency. There are several possible variables. There are open and closed (loops) strings of varying stiffness. Different vibrational patterns, amplitudes and frequencies , on strings of differing stiffness, have different energies, and these give rise to particle masses via E=mc2.

That's it in a nutshell. If I have time tomorrow, I'll post something a bit more substantial about it, as I have some well-read reference sources to hand that explain it in fairly simple terms with some good references for further reading and some peer-reviewed refs.

twistor59 · #7 by **twistor59** » Aug 04, 2010 6:55 am

hackenslash wrote:It's a bit more involved than just the frequency. There are several possible variables. There are open and closed (loops) strings of varying stiffness. Different vibrational patterns, amplitudes and frequencies , on strings of differing stiffness, have different energies, and these give rise to particle masses via E=mc2.

That's it in a nutshell. If I have time tomorrow, I'll post something a bit more substantial about it, as I have some well-read reference sources to hand that explain it in fairly simple terms with some good references for further reading and some peer-reviewed refs.

Thanks yeah that would be great.

String theory is something that mostly happened after I left physics, and for the last twenty odd years I haven't bothered keeping up, only now am I feeling the urge to catch up ...

newolder · #8 by **newolder** » Aug 04, 2010 8:06 am

twistor59 wrote:...
So is it your understanding that our favourite gauge groups, like SU3xSU2XU1, are embedded in E8xE8 (taking that example) and that the E8xE8 is somehow something to do with the CY space ?

My understanding is that all (mathematical) groups are embedded in E8 – but I couldn't prove this in an exam.

Products (associative, non-commutative, wedge & smash) of various groups give rise to descriptions of phenomena we observe (like spin, scattering amplitudes and Dirac's anti-matter). My understanding of Calabi-Yau is that the manifold structure (e.g. the number of holes it has) depends on the dimension count and any 'holes' are mapped to particles. I could easily be way off the mark here tho'. :dunno:

a 3D cross section of the quintic 6D Calabi-Yau Manifold proposed for String Theory

From Paul Nylander's archives.

I look forward to reading Hacker's account in due course... :coffee:

twistor59 · #9 by **twistor59** » Aug 04, 2010 8:19 am

From here:

It is worthwhile to note that the E8 x E8 Heterotic string has historically been considered to be the most promising string theory for describing the physics beyond the Standard Model. It was discovered in 1987 by Gross, Harvey, Martinec, and Rohm and for a long time it was thought to be the only string theory relevant for describing our universe. This is because the SU(3) x SU(2) x U(1) gauge group of the standard model can fit quite nicely within one of the E8 gauge groups. The matter under the other E8 would not interact except through gravity, and might provide a answer to the Dark Matter problem in astrophysics. Due to our lack of a full understanding of string theory, answers to questions such as how is supersymmetry broken and why are there only 3 generations of particles in the Standard Model have remained unanswered. Most of these questions are related to the issue of compactification (discussed on the next page). What we have learned is that string theory contains all the essential elements to be a successful unified theory of particle interactions, and it is virtually the only candidate which does so. However, we don't yet know how these elements specifically come together to describe the physics that we currently observe.

So yes, our gauge group is inside E8 (in that variant of the theory).
For the open string variants the gauge "charge" is carried "at the ends of the strings".

hackenslash · **#10** by **hackenslash** » Aug 04, 2010 8:50 pm

Sorry, guys, I never got near this today, as my attention was taken by other things. I'll try to get to it this week, but Brian Greene's The Elegant Universe is a brilliant précis.

Tyrennil · **#11** by **Tyrennil** » Aug 07, 2010 4:21 am

PhiloKGB wrote:In the interest of preserving a current internet meme, I would like to ask moderation to alter the thread title to read, "String theory. How the @$%# does it work?"

Wouldn't it be "Fucking string theory, how does it work?"

newolder · **#12** by **newolder** » Sep 02, 2010 7:55 pm

Interesting development @Imperial

https://fileexchange.imperial.ac.uk/files/6b579a6086/1005.4915v2.pdf wrote:Falsiable predictions in the fields of high-energy physics or cosmology are hard to come by, especially for ambitious attempts, such as string/M-theory, to accommodate all the fundamental interactions. In the field of quantum information theory, however, previous work has shown that the stringy black hole/qubit correspondence can reproduce well-known results in the classification of two and three qubit entanglement. In this paper this correspondence has been taken one step further to predict new results in the less well-understood case of four-qubit entanglement that can in principle be tested in the laboratory.

Also discussed @ http://www.wired.com/wiredscience/2010/ ... y-quantum/

GreyICE · **#13** by **GreyICE** » Sep 02, 2010 8:02 pm

TESTABLE STRING THEORY?

Nah. Not possible. String Theory doesn't produce testable predictions, it produces pretty pictures :p

twistor59 · **#14** by **twistor59** » Sep 02, 2010 8:10 pm

newolder wrote:Interesting development @Imperial
https://fileexchange.imperial.ac.uk/files/6b579a6086/1005.4915v2.pdf wrote:Falsiable predictions in the fields of high-energy physics or cosmology are hard to come by, especially for ambitious attempts, such as string/M-theory, to accommodate all the fundamental interactions. In the field of quantum information theory, however, previous work has shown that the stringy black hole/qubit correspondence can reproduce well-known results in the classification of two and three qubit entanglement. In this paper this correspondence has been taken one step further to predict new results in the less well-understood case of four-qubit entanglement that can in principle be tested in the laboratory.

Also discussed @ http://www.wired.com/wiredscience/2010/ ... y-quantum/

From the article:

Duff emphasized that this is only a test of string theory as it relates to quantum entanglement, not as a description of the fundamental physics of the universe. The battle over string theory as a theory of everything rages on.

Read More http://www.wired.com/wiredscience/2010/ ... z0yP9WW1ss

So as I read it, this is an application of some techniques developed for string theory to some completely different area of physics ? So it's not really a test of string theory as fundamental physics....

Preno · **#15** by **Preno** » Sep 02, 2010 8:58 pm

newolder wrote:My understanding is that all (mathematical) groups are embedded in E8

No.

My understanding of Calabi-Yau is that the manifold structure (e.g. the number of holes it has) depends on the dimension count

I'm not sure what this is supposed to mean, but at any rate dimensionality is a property of any manifold.

newolder · **#16** by **newolder** » Sep 03, 2010 6:56 am

Preno wrote:
newolder wrote:My understanding is that all (mathematical) groups are embedded in E8
No.

I've failed at rigour again. :doh:

I meant the Lie groups.

My understanding of Calabi-Yau is that the manifold structure (e.g. the number of holes it has) depends on the dimension count
I'm not sure what this is supposed to mean,

http://library.thinkquest.org/27930/stringtheory5.htm wrote:... The first step toward such a goal has been taken in the realization that Calabi-Yau spaces contain holes of various numbers of dimensions which can affect a string's vibrational pattern. This goes a long way toward answering one of particle physics' most intriguing questions: why are there three families of elementary particles? why not one, or four, or any other number? The answer proposed by string theory is as follows...

but at any rate dimensionality is a property of any manifold.

I'm sure it is. :cheers:

Mononoke · **#17** by **Mononoke** » Sep 03, 2010 7:02 am

Preno wrote:
newolder wrote:My understanding is that all (mathematical) groups are embedded in E8
No.

He probably meant lie groups. I'm not sure about this as well. Let me bust out my dummit and see what i can find :thumbup:

newolder · **#18** by **newolder** » Sep 03, 2010 7:04 am

twistor59 wrote:...
From the article:

Duff emphasized that this is only a test of string theory as it relates to quantum entanglement, not as a description of the fundamental physics of the universe. The battle over string theory as a theory of everything rages on.

Read More http://www.wired.com/wiredscience/2010/ ... z0yP9WW1ss

So as I read it, this is an application of some techniques developed for string theory to some completely different area of physics ? So it's not really a test of string theory as fundamental physics....

Yep, i read it that way too, but if quantum entanglement isn't related to fundamental physics in some way or other, I'm a cat. :scratch:

tnjrp · **#19** by **tnjrp** » Sep 03, 2010 8:34 am

Bookmarking
:coffee:

twistor59 · **#20** by **twistor59** » Sep 03, 2010 9:24 am

newolder wrote:
twistor59 wrote:...
From the article:

Duff emphasized that this is only a test of string theory as it relates to quantum entanglement, not as a description of the fundamental physics of the universe. The battle over string theory as a theory of everything rages on.

Read More http://www.wired.com/wiredscience/2010/ ... z0yP9WW1ss

So as I read it, this is an application of some techniques developed for string theory to some completely different area of physics ? So it's not really a test of string theory as fundamental physics....

Yep, i read it that way too, but if quantum entanglement isn't related to fundamental physics in some way or other, I'm a cat.

Yes, I agree, quanglement is certainly pretty fundamental.

But I think the application is of some string theory maths to the modelling of some quanglement scenario (maybe a scenario related to the fancy shit they do in the quantum computing experimentation these days), rather than to stuff like "here's a string model of a gravitino"...