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 ...
41
votes
4answers
3k views
Gauge symmetry is not a symmetry?
I have read before in one of Seiberg's articles something like, that gauge symmetry is not a symmetry but a redundancy in our description, by introducing fake degrees of freedom to facilitate ...
15
votes
3answers
891 views
Are elementary particles actually more elementary than quasiparticles?
Quarks and leptons are considered elementary particles, while phonons, holes, and solitons are quasiparticles.
In light of emergent phenomena, such as fractionally charged particles in fractional ...
4
votes
1answer
586 views
What is the relationship between string net theory and string / M-theory?
I've just learned from this one of Prof. Wen's answers that there exists a theory called string net theory. Since I've never heard about this before it picks my curiosity, so I`d like to ask some ...
13
votes
1answer
181 views
Sympletic structure of General Relativity
Inspired by physics.SE: http://physics.stackexchange.com/questions/15571/does-the-dimensionality-of-phase-space-go-up-as-the-universe-expands/15613
It made me wonder about symplectic structures in ...
7
votes
0answers
132 views
Magnetic monopole and electromagnetic field quantization procedure
From the Maxwell's equations point of view, existence of magnetic monopole leads to unsuitability of the introduction of vector potential as $\vec B = \operatorname{rot}\vec A$. As a result, it was ...
15
votes
5answers
1k views
Simple models that exhibit topological phase transitions
There are a number of physical systems with phases described by topologically protected invariants (fractional quantum Hall, topological insulators) but what are the simplest mathematical models that ...
11
votes
5answers
2k views
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?
7
votes
8answers
970 views
What is the name of the principle saying it is meaningless to talk/ask questions that can not be measured/tested?
Watching quantum mechanics lectures and it was mentioned that it is pointless/meaningless to try to talk/question things that can not be tested/measured.
Is this a principle? And if so what is it's ...
13
votes
6answers
252 views
Is there a theorem that says that QFT reduces to QM in a suitable limit? A theorem similar to Ehrenfest's theorem?
Is there a theorem that says that QFT reduces to QM in a suitable limit?
Of course, it should be, as QFT is relativisitc quantum mechanics.
But, is there a more manifest one? such as Ehrenfest's ...
12
votes
2answers
181 views
Renormalization in string theory
I'm taking a course in string theory and have encountered renormalization for the first time (and I suspect it isn't the last).
Specifically, while quantizing the bosonic and spinning strings, an ...
17
votes
10answers
1k views
What is spontaneous symmetry breaking in QUANTUM systems?
Most descriptions of spontaneous symmetry breaking, even for spontaneous symmetry breaking in quantum systems, actually only give a classical picture.
According to the classical picture, spontaneous ...
12
votes
4answers
2k views
A pedestrian explanation of conformal blocks
I would be very happy if someone could take a stab at conveying what conformal blocks are and how they are used in conformal field theory (CFT). I'm finally getting the glimmerings of understanding ...
13
votes
2answers
432 views
Topological Charge. What is it Physically?
I have seen the term topological charge defined in an abstract mathematical way as a essentially a labeling scheme for particles which follows certain rules. However I am left guessing when trying to ...
19
votes
5answers
1k views
Could gravity be an emergent property of nature?
Sorry if this question is naive. It is just a curiosity that I have.
Are there theoretical or experimental reasons why gravity should not be an emergent property of nature?
Assume a standard model ...
12
votes
2answers
1k views
What is a resonating valence bond (RVB) state?
There's something known as a "resonating valence bond" (RVB) state, which plays a role in at least some attempts to understand physics of high-$T_c$ superconductors. This, roughly, involves a state ...
11
votes
3answers
973 views
How Non-abelian anyons arise in solid-state systems?
Recently it has been studied non-abelian anyons in some solid-state systems. These states are being studied for the creation and manipulation of qubits in quantum computing.
But, how these ...
2
votes
2answers
161 views
Why is fractional statistics and non-Abelian common for fractional charges?
Why non integer spins obey Fermi statistics?
Why is fractional statistics and non-Abelian common for fractional charges?
7
votes
4answers
207 views
Different kinds of S-matrices?
It seems to me that the notion of an "S-matrix" refers to several different objects
One construction you can find in the literature is allowing the coupling constant to adiabatically approach 0 in ...
4
votes
1answer
318 views
A physical understanding of fractionalization
all! Is there a physical understanding of fractionalization in condensed matter physics? The textbook approach is theoretical, not physical. I'm thinking of spin-charge separation for electrons, the ...
10
votes
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 ...
16
votes
3answers
139 views
Regularization of the Casimir effect
For starters, let me say that although the Casimir effect is standard textbook stuff, the only QFT textbook I have in reach is Weinberg and he doesn't discuss it. So the only source I currently have ...
7
votes
0answers
283 views
Measurement of Tangential Momentum Accomodation?
(this question is a crosspost from theoretical physics.)
I am using atomic force microscopy (AFM) for characterizing
pores of the size of nanometers for application in gas flow. For
this, knowing ...
5
votes
1answer
61 views
Choice and identification of vacuums in AdS/CFT
I know how we define a vacuum in flat space QFT and also in a curved space QFT. But, can somebody tell me how do the choice of vacuum state in (say) the CFT side of AdS/CFT changes the choice of ...
3
votes
1answer
39 views
Spectrum of Free Strings
As far as I understand, both in bosonic and superstring theory one considers initially a free string propagating through D-dimensional Minkowskispace. Regardless of what quantization one uses, at the ...
36
votes
6answers
775 views
The Role of Rigor
The purpose of this question is to ask about the role of mathematical rigor in physics. In order to formulate a question that can be answered, and not just discussed, I divided this large issue into ...
38
votes
2answers
575 views
Analog Hawking radiation
I am confused by most discussions of analog
Hawking radiation in fluids (see, for example,
the recent experimental result of Weinfurtner et
al. Phys. Rev. Lett. 106, 021302 (2011), ...
11
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 ...
8
votes
3answers
491 views
Why are some solitons formed from bosonic fields fermionic?
Some topological solitons formed from bosonic fields have fermionic statistics. Why?
9
votes
3answers
437 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 ...
4
votes
2answers
112 views
Whis is the difference between charge fractionalization in 1D and 2D?
Both 1D Polyacetelene and 2D fractional quantum Hall state can support fractional excitations.
But as I can see, there are some differences: the ground state of Polyacetelene breaks translational ...
12
votes
2answers
160 views
Generalized Complex Geometry and Theoretical Physics
I have been wondering about some of the different uses of Generalized Complex Geometry (GCG) in Physics. Without going into mathematical detail (see Gualtieri's thesis for reference), a Generalized ...
9
votes
1answer
322 views
False vacuum in axiomatic QFT
There is an elegant way to define the concept of an unstable particle in axiomatic QFT (let's use the Haag-Kastler axioms for definiteness), namely as complex poles in scattering amplitudes. Stable ...
7
votes
1answer
191 views
Canonical quantization in supersymmetric quantum mechanics
Suppose you have a theory of maps
$\phi: {\cal T} \to M$
with $M$ some Riemannian manifold,
Lagrangian
$$L~=~ \frac12 g_{ij}\dot\phi^i\dot\phi^j + \frac{i}{2}g_{ij}(\overline{\psi}^i ...
6
votes
1answer
87 views
Forcing quadrupole moments to vanish for a neutral system
For a system of electric charges $q_i$, at positions $\mathbf{r}_i$, with a nonzero net charge $Q=\sum_i q_i$, one can define a "centre of charge" in the obvious way as
$$
...
6
votes
1answer
426 views
Definition and difference between the R-symmetry and the $U(1)_R$ internal symmetry
For a general ${\cal N}$ the R-symmetry group is $U({\cal N})$ but for the ${\cal N}=2$ case why is it $SU(2)$ ? I guess it is again different for ${\cal N}=4$. How does one understand this?
One ...
23
votes
11answers
830 views
Negative probabilities in quantum physics
Negative probabilities are naturally found in the Wigner function (both the original one and its discrete variants), the Klein paradox (where it is an artifact of using a one-particle theory) and the ...
16
votes
1answer
81 views
Asymptoticity of Pertubative Expansion of QFT
It seems to be lore that the perturbative expansion of quantum field theories is generally asymptotic. I have seen two arguments.
i)There is the Dyson instability argument as in QED, that is showing ...
16
votes
1answer
224 views
Why is there no theta-angle (topological term) for the weak interactions?
Why is there no analog for $\Theta_\text{QCD}$ for the weak interaction? Is this topological term generated? If not, why not? Is this related to the fact that $SU(2)_L$ is broken?
6
votes
2answers
91 views
fitting free QFTs into the Haag-Kastler algebraic formulation
Has the free Klein-Gordon quantum field theory been fitted into the
Haag-Kastler algebraic framework? (Actually, John Baez told me "yes", and he should know.) If so, can you describe the basic
...
5
votes
0answers
112 views
Local explanation of the Aharonov-Bohm effect in terms of force fields
Here is an interesting paper for the Physics SE community: On the role of potentials in the Aharonov-Bohm effect, Lev Vaidman, published in PHYSICAL REVIEW A 86, 040101(R) (2012).
You should check it ...
5
votes
2answers
396 views
Why is GR renormalizable to one loop?
I have read in a few places that GR is renormalizable at one loop. (hep-th/9809169 for example, second sentence, although they don't seem to develop this point at all). Is this do to some hidden ...
30
votes
3answers
317 views
What is the use of a Universal-NOT gate?
The universal-NOT gate in quantum computing is an operation which maps every point on the Bloch sphere to its antipodal point (see Buzek et al, Phys. Rev. A 60, R2626–R2629). In general, a single ...
13
votes
2answers
157 views
A resource theory of quantum discord?
Local Operations and Classical Communication (LOCC) is the classic paradigm for studying entanglement. These are things that are `cheap' and unable to produce entanglement as a resource for a quantum ...
11
votes
2answers
141 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 ...
7
votes
1answer
174 views
What is the generalization, if any, of the weak and dominant energy conditions to SUGRA?
In standard general relativity, we have the null energy condition, the weak energy condition related to stability, and the dominant energy condition related to forbidding superluminal causal ...
6
votes
1answer
159 views
Thermodynamic limit “vs” the method of steepest descent
Let me use this lecture note as the reference.
I would like to know how in the above the expression (14) was obtained from expression (12).
In some sense it makes intuitive sense but I would ...
6
votes
1answer
68 views
Asymptotic Completeness, generalized free fields, and the relationship of thermodynamics with infinity
Asymptotic completeness is a strong constraint on quantum field theories that rules out generalized free fields, which otherwise satisfy the Wightman axioms. If we were to take a limit of a list of ...
5
votes
3answers
99 views
What is the physical difference between states and unital completely positive maps?
Mathematically, completely positive maps on C*-algebras generalize positive linear functionals in that every positive linear functional on a C*-algebra $A$ is a completely positive map of $A$ into ...
4
votes
0answers
164 views
The ${\cal N} = 3$ Chern-Simons matter lagrangian
This question is sort of a continuation of this previous question of mine.
I would like to know of some further details about the Lagrangian discussed in this paper in equation 2.8 (page 7) and in ...
16
votes
3answers
148 views
Quantum computing and quantum control
In 2009, Bernard Chazelle published a famous algorithms paper, "Natural Algorithms," in which he applied computational complexity techniques to a control theory model of bird flocking. Control theory ...
