# Tag Info

3

I figure that string theory is a new breed of QFT which looks at fields in terms of a network of strings and also incorporates gravity into its module, You should read some reviews of what string theory is . String theory rejects the idea of a point particle as the fundamental constituent of the theory, which is the central concept in quantum field ...

1

For a quick (and somewhat dirty) way of deriving the bound, do the following: Recall, that in Poincaré coordinates, the metric of $AdS_{d+1}$ is $$ds=\frac{1}{z^2}\left(dz^2+\eta^{\mu\nu}dx^\mu dx^\nu\right).$$ The equation of motion for a scalar is $$(\Box-m^2)\,\phi(z,x)=0,$$ where $\Box=\frac{1}{\sqrt{-g}}\partial_a \sqrt{-g}g^{ab}\partial_b$. ...

1

An informal answer until you get one from an expert: the extra spatial dimensions are spatial dimensions conceptually similar to the ones in our 3D space, and the objects exists in this higher dimensional space. The reason you do not notice them macroscopically (such as why an object cannot change direction into the fourth spatial dimension and disappear ...

-1

That's right, there is the following equation: $$X^{\mu}=Z^{\mu}(\sigma)) (dX^{\mu}=\partial_{a}Z^{\mu}(\sigma)d\sigma^{a})$$ for 2d surface embedded into, let say, a flat finite-dimensional target space inside the Polyakov and Nambu-Goto action. The main/only "reason" why people (mostly QFTs theorists who would like to call themselves and be called by ...

3

If you have another model, then use it to generate a set of predictions, and let's compare those to the predictions we get in the world we see. If you think there's some ether filling the universe, then write down some properties it has, and make some predictions with those properties. Talking in vague generalities will only lead you down a rabbit hole of ...

0

The normal ordering makes the expression of $b_{-r}\cdot b_{r+m}$ symmetric around the point $r=-\frac{m}{2}$, and these symmetric parts differ by a sign. This sign difference due to normal ordering makes $m/2$ redundancy.

1

There are two things that define a particle physics model (at low energies). The first one is the gauge group G we want the model to be symmetric under. For the Standard Model (SM) we set this to $G=SU(3)\times SU(2)\times U(1)$ (for good experimental reasons!). This will uniquely determine the number of gauge bosons needed to make the model consistent. The ...

2

A theory with N=2 supersymmetry, where particles have two superpartners, has mirror symmetry built in. Nir Polonsky wrote some papers about an N=2 extension of the standard model (e.g.). The main problem for such a theory are the chiral Yukawa interactions between fermions and the Higgs field, which give fermions their mass in the SM. The mirror symmetry of ...

0

Can the D-brane and p-brane wrapped as compactification of the dimensions? String theories are consistent in 26 (bosonic string theory), 10 (superstring theories) and 11 (M-theory) dimensions. To get our world (4D) one needs to compactify the extra-dimensions. In general $D$ branes are extended $p+1$ dimensional objects with $p$ spatial and one time ...

0

The answer is yes, branes (both $D$ and $p$) can be wrapped around compactified dimensions. There is little difference between the two types of branes with regard to compactification.

2

The number of gauge bosons is restricted by symmetry: a given theory with a certain gauge invariance admits as many gauge bosons as there are generators of the corresponding gauge group. For example, there is one generator for $\mathrm{U}(1)$, resulting in the existence of a photon. $\mathrm{SU}(3)$ admits eight generators, which yield eight gluons. This is ...

1

Two-dimensional surfaces are the Feynman diagrams of string theory. In quantum field theory one sums over one dimensional objects in order to calculate quantities like scattering cross sections or decay rates. This is due to the fact that in this framework, particles are represented as zero dimensional objects, i.e. their world lines are points. In ...

2

Analyzing the spectrum of the strings, one finds that it contains $N^2$ massless vector states, which is precisely the number of gauge fields corresponding to a $U(N)$ group. Note that this is only true for massless oriented open strings; the unoriented case yields $SO(N)$ or $Sp(N)$. As is described in the same chapter of the book, open string states can ...

4

Right now, when people talk of string theory, strings are the fundamental objects -- they aren't made up of anything. So they have an intrinsic property of energy, and energy per unit length corresponds to the string's tension.

0

This terminology is related with spontaneous broken of gauge theory. Higgs mechanics as we are familiar with mass mechanism for gauge boson is related with Higgs phase. and in Coulomb phase gauge boson still exists in massless case and form $\frac{1}{r}$. For $U(1)$ gauge theory it is reduced to coulombic interaction as in electrodynamics.

1

I will not go into explaining what dimensions actually are, but as can be found out for example by reading the respective Wikipedia article, the number of dimensions of a space(-time) coincides with the minimal number of coordinates needed to specify a point. The directions you refer to do not coincide with dimensions as they are generally understood. To ...

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