
#1
Jul212, 11:57 AM

P: 2,900

I was reading the brief wikipedia entry on string and there it says that strings are onedimensional "unlike an elementary particle which is zerodimensional, or pointlike. Quarks and electrons are thought to be made of strings."
The part about zerodimensional entities being made of onedimensional ones is not easy to understand for me, could someone clarify this? 



#2
Jul212, 12:09 PM

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#3
Jul212, 12:17 PM

P: 10





#4
Jul212, 01:40 PM

P: 2,900

strings and elementary particlesAnother question, also from that page, a string in motion sweeps a surface called worldsheet. Analogous to the worldlines of classical field theory. Is the high number of dimensions in string theory related to the fact that it deals with worldsheets instead of worldlines? 



#5
Jul212, 02:36 PM

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#6
Jul212, 02:51 PM

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#7
Jul312, 07:36 AM

P: 2,900

I'm trying to get a very basic understanding of string theory, and have some more questions. Please excuse my ignorance
Apparently in some string theories like Mtheory something called Mbranes ¿replace?/¿complement? strings as the structure of elementary particles. So for instance we have 2branes, 3branes etc depending on their dimensionality, and I guess analogously to strings and 2D world sheets, 2branes in motion generate 3D world volumes. And so on, so that the maximum Mbrane has M=D1 with D referring to the dimensionality of the string theory space. To bring it closer to a more comfortable knowledge zone for me, I would think of a world tube from relativity as a 3brane in 4spacetime, would this be right? Also what is the difference between the diffferent types of branes, like pbranes, Dbranes, Mbranes etc? 



#8
Jul312, 09:05 AM

P: 5,634

As Powell mentions, the spacetime dimensions provide degrees of freedom that are required to give strings the characteristics we observe in particles. Numbers and shapes of dimensions determine the vibrational modes.
I think it was Ed Witten who realized that string theory, via his "M theory" includes ingredients besides one dimensional strings. Somebody coined 'p branes' to describe multidimensional strings with 'p' a whole number from 0 to 10...pbranes turn out to be heavier than strings. In the SteinhardtTurok cyclic universe, it is the collision of branes that causes 'bangs'....they propose that branes are not smooth multidimensional objects but rather that they ripple..so collisions occur at different points and different times giving rise to quantum irregularities that are believed to precipitate particles. A graviton, for example, is among the lowest energy string vibrations and it's strength is proportional to the length of the string. The Braneworld scenario is one where our untire universe lies within a three brane. pbranes are 'sticky' and the ends of most strings stay attached; we say such strings are open and their freedom is restricted. Gravitons are un attached loops and may drift off into higher dimensions...hence gravity is very weak. Last night on Science Channel I see Lisa Randall has a theory that gravitons actually leak from the multiverse into our universe. apparently this solves a number of interesting issues. Anybody know?? Check out Wikipedia for more on BRANES or membranes. There are some distinctions between Polchinski's pbranes and Dirichelt [D] branes.....I've forgotten... edit: Brian Greene's FABRIC OF THE COSMOS has chapter upon chapter about strings and branes...a used copy is cheap...no math.... 



#9
Jul312, 10:08 AM

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P: 1,570

The RS2 model deals with only a single brane situated transverse to an infinite 5th dimension. The significance of this model was to illustrate that GR can be recovered from a theory with extra dimensions even when the extra dimension is infinite. The key again is a strong warping of the extra dimension to that gravity remains more or less localized on the Planck brane: any leakage is exponentially suppressed by the warping. 



#10
Jul312, 11:56 AM

P: 2,900

About the braneworld, how can it model our universe as a 3brane, is it also a string theory? I thought all string theories had at least 10 dimensions. EDIT:Ok, After reading the article in wikipedia I think I got this one, according to this brane cosmology, our entire universe can be a brane but there are more branes(kind of parallel universes?) and therefore more dimensions,right? Also can anyone confirm my analogy above about the world tube as a 3brane evolving in time? 



#11
Jul312, 02:01 PM

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#12
Jul312, 03:14 PM

P: 2,900





#13
Jul1612, 08:54 PM

P: 1

Regarding elementary particles, the search for the origin of matter means the understanding of elementary particles. And with the advent of holism, the understanding of elementary particles requires an understanding of not only their characteristics, but how they interact and relate to other particles and forces of Nature, the field of physics called particle physics.




#14
Jul2012, 01:40 PM

P: 445

1) Is any electron a 2dimensional charged and closed surface (in extenso a small ball) orbiting around, example given, a proton at some quantized distance? or 2) Is the electron a 2dimensional surface in a sort of equilibrium with different forms (depending on the available quantized energy) centered on the proton with total charge e and mass at rest m_{e}? For any of these two proposed alternatives, where does a model involving a string come into the play, thus justifying the discussion here: "strings and elementary particles"? 



#15
Jul2012, 02:30 PM

P: 49

OK, electrons, quarks etc.. in string theory.
I'll start off by stating that nobody really knows how an electron manifests itself in string theory, unless they know and haven't told anyone ;) Lets first think about a weakcoupling scenario (where strings don't like to interact strongly, so that we can still talk about what individual strings actually do..) and then we'll move on to a strongcoupling scenario (where strings like to interact so strongly that their stringy nature becomes less and less clear..). One idea that I like is that an electron corresponds to an open string (in its lowest energy level) that is stretched between two stacks of intersecting Dbranes. The number of Dbranes in each stack is important. Each endpoint of the open string can attach to any of the branes in a given stack. Furthermore, there is a rotational symmetry, or U(1) symmetry, associated to every brane. If one of the two stacks has 2 branes, then the symmetry becomes U(2), 3 branes lead to U(3), etc. Therefore, if we have two stacks of branes, the first stack having 1 brane and the second having 2 branes, the resulting symmetry is U(1)xU(2), which is almost U(1)xSU(2), the symmetry group (one could say one of the key defining properties) of electrons. How to get SU(2) from U(2) is tricky, and one usually ends up with extra U(1)'s. How to get the right masses is even trickier ... Nevertheless, continuing along these lines, the Standard Model spectrum has symmetry group U(1)xSU(2)xSU(3), so you can imagine having 3 stacks of intersecting Dbranes, each stack having respectively 1, 2 and 3 branes. (Here you're suppose to imagine a triangle, each side of which corresponds to one of the three stacks.) Then, all the ways you can imagine open string attached to these three stacks of branes correspond to all the particles of the standard models, and more ... One common example is that these Dbranes are in fact D6branes, so that there are 4 large dimensions (corresponding to the observed world), and 6 extra dimensions associated to these stacks of D6branes, which have open string attached to them. The total dimensionality is 10, as required by anomaly cancellation conditions. OK, that's a weak coupling manifestation of the standard model. However, there are indications that in fact our world lives in the strongcoupling limit of string theory ... (and that is unfortunate for the poor people like myself who like to be able to compute things!) In the strong coupling limit an extra dimension becomes visible, large, and the total dimensionality is now 11: this is the realm of Mtheory. In fact, one can go quite far along these lines ... Acharya, Kane and Kumar have been voicing for years that Mtheory (on a G2 manifold, don't ask what this is .. ) gives rise to the standard model spectrum with a Higgs of mass 125 Gev (!), long before the LHC result was out (see http://arXiv.org/pdf/1204.2795.pdf and references therein), claiming in fact that this is a GENERIC PREDICTION! Hope that helps! Wakabaloola 


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