# Tag Info

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One of ideas associated with string theory is the ekpyrotic universe. This starts with brane cosmology i.e. the idea that our universe is a four dimensional brane floating around in the ten dimensional string theory spacetime. There will be many such brane worlds and the ekpyrotic idea is that a collision between two branes would appear just like the Big ...

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QED is non-perturbative because it is not defined by the sum over Feynman diagrams. Standard QFT does not rely on the perturbative expansion to define scattering amplitudes, it only uses it to compute them. String theory through CFT, on the other hand, defines the string scattering amplitude through the sum over worldsheets. This is not a perturbative ...

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Your confusion comes from thinking that going to superstrings simply means adding fermions in the spectrum. The spectrum is instead different. For bosonic string (let's focus on NN boundary conditions and open strings) you have something like: $$\alpha' m^2=N-1$$ where N is the number operator of the transverse vibrational excitations of the bosonic ...

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I think the idea is that there are no open strings in IIA, IIB in perturbation theory around the vacuum. As you say, we know D-branes exist in IIA and IIB, and these are defined as the submanifolds where open strings can end, so if a theory has D-branes it should also have open strings. But to have an open string means that you necessarily have a heavy, ...

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The spin $s$ of a particle characterizes how the rotation generators act on it. In $D$ dimensions, you represent the little group $SO(D-1)$ for massive particles and $SO(D-2)$ for massless ones. In fact, you really need to consider its universal cover $\textrm{Spin}(n)$ which happen to be just its double cover. Now, you can define the spin to be the largest ...

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If there are eleven dimensions as M-Theory asserts Let us suppose it is true would that mean that the majority of what we are made from exists in the seven other dimensions? Define dimension: In physics and mathematics, the dimension of a mathematical space (or object) is informally defined as the minimum number of coordinates needed to ...

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The similarity are strong, but the point is that treating gravity as a quantum field theory doesn't work. At best you can work with an effective field theory of the spin 2 graviton on a fixed background, and within the validity of the effective theory you can even extract predictions, for instance the quantum gravity shift to the Mercury's perihelion. ...

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I cannot quite vouch for exhaustive panoramas, but the crucial point is that GL(N), SU(N) matrices are representable in a nonhermitean basis discovered by Sylvester in 1882, the clock and shift matrices which he called nonions for N=3 (long before the Gell-Mann basis!), sedenions, etc. Their braiding relations, and maximal grading, and hence commutators, ...

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The explanation lies in the fact that you want to consider BPS states, or equivalently, short supersymmetric multiplets, because you are dealing with an extremal black hole. Now you could ask why we want to impose this restriction on the spectrum. The answer is that results based on the BPS spectrum can be generalized from weak coupling to strong coupling. ...

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It's much simpler than you think! The analogy is meant to underline that if you look the wire from far away it appears a 1-d path, while if you zoom you can see that the path is actually taking place in 3-d, because the wire is an extended object rather than really a line, even though two directions are far smaller than the third one. Now the ...

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“With the development of quantum chemistry in the 20th century, however, a vast number of previously inexplicable dimensionless physical constants were successfully computed from theory.” I guess that you ask this question because you read the above sentence. But quantum chemistry is not the fundamental theory, it is based on quantum mechanics. So ...

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It depends on the constant you are talking about. For instance, it is an observational fact that the dimensionless constants $q_1 / q_2$, where $q_1$ and $q_2$ are the electric charges of two arbitrary particles, are rational numbers. It is a longstanding question of theoretical physics to understand why it is so. In some cases, one can explain why ...

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