The research-level tag applies to questions that arise in graduate and post-secondary work. These questions often require domain-specific knowledge and could not be answered from a general source or may be beyond the level typically covered by Wikipedia and other popular sources. research-level ...
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What does it mean for a Hamiltonian to be gapped?
I've read some papers recently that talk about gapped Hamiltonians or gapless systems, but what does it mean?
Edit: Is an XX spin chain in a magnetic field gapped? Why or why not?
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1answer
216 views
Covariant derivatives
I need correctly define covariant derivatives on the coset space $G/H$, where a group $G \equiv \{X_i, Y_a\}$ ($X$ and $Y$ are generators) have a subrgroup $H \equiv \{X_i\}$
Lie algebra of $G$ has ...
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2answers
130 views
Literature on fractal properties of quasicrystals
At the seminar where the talk was about quasicrystals, I mentioned that some results on their properties remind the fractals. The person who gave the talk was not too fluent in a rigor mathematics ...
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1answer
50 views
Limitations in using FLEX as a DMFT solver
When using the fluctuating exchange approximation (FLEX) as a dynamical mean field theory (DMFT) solver, Kotliar, et al. (p. 898) suggest that it is only reliable for when the interaction strength, ...
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1answer
205 views
Lagrangian for Euler Equations in general relativity
The stress energy tensor for relativistic dust
$$
T_{\mu\nu} = \rho v_\mu v_\nu
$$
follows from the action
$$
S_M = -\int \rho c \sqrt{v_\mu v^\mu} \sqrt{ -g } d^4 x
= -\int c \sqrt{p_\mu ...
11
votes
1answer
66 views
Are possible gauge fields in a Lagrangian theory always determined by the structure of the charged degrees of freedom?
An elementary example to explain what I mean. Consider introducing a classical point particle with a Lagrangian $L(\mathbf{q} ,\dot{\mathbf{q}}, t)$. The most general gauge transformation is $L ...
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1answer
46 views
Relationship between Weak Cosmic Censorship and Topological Censorship
The weak cosmic censorship states that any singularity cannot be in the causual past of null infinity (reference).
The topological censorship hypothesis states that in a globally hyperbolic, ...
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votes
1answer
102 views
Holographic Renormalization in non-AdS/non-CFT
In AdS/CFT, the story of renormalization has an elegant gravity dual. Regularizing the theory is done by putting a cutoff near the conformal boundary of AdS space, and renormalization is done by ...
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2answers
142 views
Gauge invariance for electromagnetic potential observables in test function form
This is a reference request for a relationship in quantum field theory between the electromagnetic potential and the electromagnetic field when they are presented in test function form. $U(1)$ gauge ...
11
votes
1answer
51 views
Stabilizer formalism for symmetric spin-states?
This question developed out of conversation between myself and Joe Fitzsimons. Is there a succinct stabilizer representation for symmetric states, on systems of n spin-1/2 or (more generally) n higher ...
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votes
1answer
49 views
Principle behind fidelity balance in quantum cloning
If we do optimal state estimation on an unknown qubit, we can recreate a state with fidelity $F_c=2/3$ with respect to the original. Let us define the "quantum information content" $I_q=1-2/3=1/3$ as ...
11
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1answer
113 views
Majorana-like representation for mixed symmetric states?
Is there a generalization of the Majorana representation of pure symmetric $n$-qubit states to mixed states (made of pure symmetric $n$-qubit)?
By Majorana representation I mean the decomposition of ...
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votes
3answers
133 views
POVMs that do not require enlargement of the Hilbert space
The usual justification for regarding POVMs as fundamental measurements is via Neumark's theorem, i.e., by showing that they can always be realized by a projective measurement in a larger Hilbert ...
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votes
2answers
70 views
Discussions of the axioms of AQFT
The most recent discussion of what axioms one might drop from the Wightman axioms to allow the construction of realistic models that I'm aware of is Streater, Rep. Prog. Phys. 1975 38 771-846, ...
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votes
2answers
416 views
Is there a method for differentiating fractional quantum Hall states aside from finding Chern numbers?
The ground state for a quantum Hall system on a torus with fractional filling factor can be classified by the Chern number, which is why the Hall conductance is quantized. Is there another method or ...
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1answer
79 views
CHSH violation and entanglement of quantum states
How is the violation of the usual CHSH inequality by a quantum state related to the entanglement of that quantum state?
Say we know that exist Hermitian and unitary operators $A_{0}$, $A_{1}$, ...
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votes
1answer
87 views
Higgs Field - Is its discovery truly “around the corner”?
Rather surprised I haven't seen many questions or discussion regarding the rumored confirmation of the Higgs field. As I understand it, the energies where they saw things were actually quite a bit ...
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1answer
82 views
Metric interpretation of self-adjoint extensions?
I am wondering if beyond physical interpretation, the one dimensional contact interactions (self-adjoint extensions of the the free Hamiltonian when defined everywhere except at the origin) have a ...
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1answer
186 views
Can the CPT theorem be valid if Lorentz invariance is only spontaneously broken?
Earlier, I asked here whether one can have spontaneous breaking of the Lorentz symmetry and was shown a Lorentz invariant term that can drive the vacuum to not be Lorentz invariant. How relaxed are ...
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1answer
81 views
Some questions on a version of the O'Raifeartaigh model
This form is taken from a talk by Seiberg to which I was listening to,
Take the Kahler potential ($K$) and the supersymmetric potential ($W$) as,
$K = \vert X\vert ^2 + \vert \phi _1 \vert ^2 + ...
10
votes
3answers
108 views
Physical interpretation to the category of CFTs
This question comes from reading Andre's question where I wandered whether that question even makes sense physically. In mathematics, VOAs form a category, does this category as a whole have a ...
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votes
2answers
29 views
Extensions of DHR superselection theory to long range forces
For Haag-Kastler nets $M(O)$ of von-Neumann algebras $M$ indexed by open bounded subsets $O$ of the Minkowski space in AQFT (algebraic quantum field theory) the DHR (Doplicher-Haag-Roberts) ...
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1answer
46 views
Physical interpretation of superstrings
The scalar fields $X^\mu$ in bosonic string theory have a clear physical interpretation - they describe the embedding of the string in spacetime.
Adding fermionic fields on the worldsheet is a ...
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1answer
132 views
Normalization of the Chern-Simons level in $SO(N)$ gauge theory
In a 3d SU(N) gauge theory with action $\frac{k}{4\pi} \int \mathrm{Tr} (A \wedge dA + \frac{2}{3} A \wedge A \wedge A)$, where the generators are normalized to $\mathrm{Tr}(T^a ...
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2answers
825 views
Quantum memories: What are they?
Searching the literature for the term "quantum memory" seems to bring up results from two different communities.
On the one hand there are quantum opticians, who see a quantum memory as something ...
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1answer
190 views
Technical naturalness of Yukawa couplings
Naturalness in the sense of 't Hooft tell us that a small parameter is a signal of a symmetry such that the parameter will be zero when the symmetry is exact. I am puzzled about how this principle is ...
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2answers
271 views
How can we define BF theory on a general 4-manifold?
(I have rewritten the question some, with new understanding)
4d BF theory is classically presented as the TFT arising from the Lagrangian
$B\wedge F$,
where $B$ is an abelian 2-connection (locally ...
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4answers
544 views
Is there an intuitive description of vacuum entanglement?
People often refer to the fact that the vacuum is an entangled state (It's even described as a maximally entangled state).
I was trying to get a feeling for what that really means. The problem is ...
10
votes
1answer
57 views
N=2 SSM without a Higgs
In arXiv:1012.5099, section III, the authors describe a supersymmetric extension to the standard model in which there is no Higgs sector at all, in the conventional sense. The up-type Higgs is a ...
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2answers
44 views
Examples of heterotic CFTs
I'm trying to get a global idea of the world of conformal field theories.
Many authors restrict attention to CFTs where the algebras of left and right movers agree. I'd like to increase my intuition ...
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1answer
452 views
Entanglement in time
Quantum entanglement links particles through time, according to this study that received some publicity last year:
New Type Of Entanglement Allows 'Teleportation in Time,' Say Physicists at The ...
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0answers
140 views
Hypersingular Boundary Operator in Physics
This has been a question I've been asking myself for quite some time now. Is there a physical Interpretation of the Hypersingular Boundary Operator?
First, let me give some motivation why I think ...
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4answers
300 views
Applications of Geometric Topology to Theoretical Physics
Geometric topology is the study of manifolds, maps between manifolds, and embeddings of manifolds in one another. Included in this sub-branch of Pure Mathematics; knot theory, homotopy, manifold ...
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3answers
368 views
Rigorous proof of Bohr-Sommerfeld quantization
Bohr-Sommerfeld quantization provides an approximate recipe for recovering the spectrum of a quantum integrable system. Is there a mathematically rigorous explanation why this recipe works? In ...
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2answers
78 views
Nuclear physics from perturbative QFT
Is there a renormalizable QFT that can produce a reasonably accurate description of nuclear physics in perturbation theory? Obviously the Standard Model cannot since QCD is strongly coupled at nuclear ...
9
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1answer
72 views
Conformal QFTs for D > 2
Which conformal QFTs do we know for spacetime dimension d > 2?
I know that for D = 4 we have N = 4 SYM and some N = 2 supersymmetric Yang-Mills + matter models.
What is the complete list of such ...
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1answer
30 views
Dual Pairs in Four Dimensions
Following the conversation here, I am wondering if anyone knows of an example of dual pair with 4-dimensional N=1 SUSY which relates a non-Abelian gauge theory on one side to a theory with a ...
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3answers
199 views
On-shell symmetry from a path integral point of view
Normally supersymmetric quantum field theories have Lagrangians which are supersymmetric only on-shell, i.e. with the field equations imposed. In many cases this can be solved by introducing auxilary ...
9
votes
1answer
39 views
Renyi fractal dimension $D_q$ for non-trivial $q$
For a probability distribution $P$, Renyi fractal dimension is defined as
$$D_q = \lim_{\epsilon\rightarrow 0} \frac{R_q(P_\epsilon)}{\log(1/\epsilon)},$$
where $R_q$ is Renyi entropy of order $q$ ...
9
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1answer
89 views
Is there a background independent closed string field theory?
Analogous to the background independent open string field theory by Witten. If there isn't, what are the main stumbling blocks preventing its construction?
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1answer
64 views
Global symmetry in string theory
It is often stated that in quantum gravity only charges coupled to gauge fields can be conserved. This is because of the no hair theorem. If a charge is coupled to a gauge field then when it falls ...
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votes
1answer
45 views
Accurate quantum state estimation via “Keeping the experimentalist honest”
Bob has a black-box, with the label "V-Wade", which he has been promised prepares a qubit which he would like to know the state of. He asks Alice, who happens also to be an experimental physicist, to ...
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votes
3answers
439 views
Is there any quantum-gravity theory that has flat space-time and gravitons?
Many quantum-gravity theories are strongly interacting. It is not clear
if they produce the gravity as we know it at low energies. So I wonder, is there
any quantum-gravity theory that
a) is a well ...
9
votes
4answers
322 views
The Schwinger model
The Schwinger model is the 2d QED with massless fermions. An important result about it (which I would like to understand) is that this is a gauge invariant theory which contains a free massive vector ...
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votes
3answers
1k views
Wormholes & Time Machines - for *experts* in GR/maths
EDIT: Further clarification in the context of answers/comments received to 20 Jan has been appended
EDIT: 21 Jan - Response to the Lubos Expansion appended [in progress, not yet complete]
EDIT: 23 ...
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3answers
135 views
Hilbert-Schmidt basis for many qubits - reference
Every density matrix of $n$ qubits can be written in the following way
$$\hat{\rho}=\frac{1}{2^n}\sum_{i_1,i_2,\ldots,i_n=0}^3 t_{i_1i_2\ldots i_n} ...
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1answer
277 views
Boundary conditions / uniqueness of the propagators / Green's functions
My question(s) concern the interpretation and uniqueness of the propagators / Green's functions for both classical and quantum fields.
It is well known that the Green's function for the Laplace ...
9
votes
1answer
74 views
How can one build a multi-scale physics model of fluid flow phenomena?
I am working on a problem in Computational Fluid Dynamics, modeling multi-phase fluid flow through porous media. Though there are continuum equations to describe macroscopic flow (darcy's law, ...
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1answer
168 views
Derivation of the effective potential between a quark and an anti-quark
Typically in particle physics books (not in QFT books!) I have often seen this statement that the potential between a heavy quark and its anti-quark can be "empirically" represented as $V(r) = ...
9
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1answer
61 views
Quasiparticles in Bohmian mechanics
My questions are about de Broglie-Bohm "pilot wave" interpretation of quantum mechanics (a.k.a. Bohmian mechanics).
Do quasiparticles have any meaning in Bohmian mechanics, or not? Specifically, is ...
