Pedro Lauridsen Ribeiro

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375 reputation
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bio website cmcc.ufabc.edu.br/…
location Cotia, Brazil
age 35
visits member for 1 year, 4 months
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I'm a professor at the Center of Mathematics, Computation and Cognition of the Federal University of the ABC, Santo André, Brazil.


Mar
7
comment From representations to field theories
In due time: I raised related points in my answer to the following related physics.SE question: physics.stackexchange.com/questions/13488/to-construct-an-action-from-a-given-tw‌​o-point-function/46578
Mar
7
comment From representations to field theories
@user1504 - Sorry if I'm picky, but that would be a(n important!) "non-uniqueness" statement, whereas my counter-example is rather a "non-existence" one, so both statements deal with different issues.
Mar
7
revised From representations to field theories
added a clarifying remark
Mar
7
comment From representations to field theories
@user1504 - Yes, I should have mentioned that. I'll amend my answer accordingly. On the other hand, if you look at the last paragraph of the question, it seemed to me that the OP wanted to add the interaction term at a later stage by minimal coupling, so it seemed reasonable to me to assume that he wanted to get the appropriate "free" part first (please user26143, do correct me if I'm wrong).
Mar
7
answered From representations to field theories
Mar
7
comment From representations to field theories
There is a subfamily of irreps of the Poincaré group - namely, the zero-mass, "continuous-helicity" representations - which does not admit any Lagrangian formulation whatsoever.
Feb
22
comment The shape of speaker cones
More on point (1) of my first comment: the shape of the moving surface has also to do with the interference pattern of the sound wave generated by the spherical part (the dome) with the sound wave generated by the conical part (the cone), so that within the sound aperture angle there are essentially no "dumb" spots. There are modern variations of the standard design where the dome is no longer convex, but I don't know if in that case the dome is still spherical or has a more sophisticated shape.
Feb
22
comment The shape of speaker cones
There are other paradigms for loudspeaker design, though, such as magnetic ribbon loudspeakers, found either as full-range or high-frequency elements in hybrid designs with standard cone low-frequency elements, or electrostatic loudspeaker (ESL) panels. For these designs, the driving elements are extremely thin sheets which are much lighter than cones, allowing for a more faithful sound reproduction (specially for transient sounds, where they definitely outperform standard cone designs). On the other hand, they tend to be more directional and less powerful, unless you work with large panels.
Feb
22
comment The shape of speaker cones
I don't know much about the history of loudspeaker design, but I believe the shape is intended so that (1) the sound is projected at a reasonably open solid angle, and (2) the moving surface is as light and as little deformed when vibrating as possible, considering that the vibration is driven by the motion of the coil glued right behind the junction of the sphere sector and the cone. The latter must be so because either deformation or excessive inertia of the moving surface distort the sound whose waveform is transmitted as an oscillating current through the coil wire.
Feb
21
comment A question about the Bosonization of the Thirring model
Bosonization of Dirac (free) fields in 1+1 dimensions, on its turn, can be found in books on string theory such as Green-Schwarz-Witten or Polchinski.
Feb
21
comment A question about the Bosonization of the Thirring model
Your model is still free in the sense that the field equations are linear, but you no longer have a translation invariant vacuum state if $f$ is not constant. Nevertheless, you still have quasi-free reference states (i.e. such that all truncated $n$-point functions vanish for $n>2$) whose 2-point function have a short distance behavior similar to that of a vacuum state (the so-called Hadamard states). If one works with one of these states, the bosonization procedure shouldn't be that different from the standard one applied to Dirac fields in 1+1 dimensions.
Feb
20
awarded  Critic
Feb
20
comment Generator of local symmetries
@Dan: That is correct (up to some pathological counter-examples), but the first class constraints are not Noether charges, they rather appear in the Noether identities that encode the on-shell vanishing of the corresponding Noether current.
Feb
20
comment A question about the Bosonization of the Thirring model
Sorry, I should have said that my answer refers to your second question (i.e. the Thirring / sine-Gordon bosonization). As for your first question, one must have in mind that bosonization of fermion fields only works in 1+1 dimensions (this is missing from your question). That being said, it doesn't look much different from bosonization of free fermion fields, although your model may get messy depending on the form of $f$. Do you have a specific $f$ in mind?
Feb
20
comment Generator of local symmetries
For a thorough discussion on how Gauss's law is coherent with the second Noether theorem, I recommend the (long) paper by M. Forger and H. Römer, Currents and the Energy-Momentum Tensor in Classical Field Theory: A fresh look at an Old Problem, Annals Phys. 309 (2004) 306-389, arXiv:hep-th/0307199. See also the related physics.SE question physics.stackexchange.com/q/46476
Feb
20
comment Generator of local symmetries
The second Noether states that the Noether current for any infinitesimal local symmetry vanishes on shell. However, this is based on the assumption that local symmetries act as the identity outside a bounded space-time region. Global gauge symmetries are not of this kind, they do not act as the identity as you move to infinity. Just think of Gauss's law: by measuring the electric flux through an arbitrarily large sphere, you can determine the amont of electric charge bounded by it.
Feb
20
revised A question about the Bosonization of the Thirring model
Added reference info and link
Feb
20
answered A question about the Bosonization of the Thirring model
Feb
13
comment Gaussian Probability Distribution?
Once your particles are in a box, the dynamics is no longer free (meaning that you should think of the box as an infinite potential well). Perhaps you should post it as a separate question, referring at the same time to this one (a similar comment applies to your last question, perhaps).
Feb
13
comment Gaussian Probability Distribution?
By the way: please notice that the above form of the amplitude is preserved only by the free dynamics (i.e. the time evolution given by the Schrödinger equation with zero potential).