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Theoretical physics phd student.


Jan
21
awarded  Nice Answer
Jan
20
reviewed Approve suggested edit on Conclusions From a Junior School Science Experiment
Jan
14
awarded  Yearling
Jan
10
awarded  Custodian
Jul
30
comment Can I call additional conditions on potentials a Gauge choice?
@rajb245 : It's called residual gauge. They are the class of (residual) gauge transformations that leave your gauge choice invariant. Sort of a gauge freedom within a gauge. In the Lorenz gauge you fix $\partial_\mu A^\mu = 0$. So if $A_mu\rightarrow A_mu = \partial_\mu f$ is a gauge transformation then your residual gauge must satisfy: $\partial_\mu^2 f = 0$. These transformations leave the Lorenz gauge intact. All $f$'s which satisfy this equation (+ boundary conditions) describe your residual gauge.
Jul
22
comment Understanding the argument that local U(1) leads to coupling of EM and matter
If you want, you can call it a postulate. I see it as "model building". If you demand that particle number is conserved, then a U(1) symmetry is pretty much automatic. If you want to the system to couple to a gauge field, then you 'demand' local gauge invariance and introduce a minimal coupling. Again, a form of model building.
Jul
22
comment Understanding the argument that local U(1) leads to coupling of EM and matter
The global U(1) symmetry is association with a conservation law: conservation of charge (or particle number, which is the same in this case). This charge is the (integrated) zeroth component of the Noether current associated with the U(1) symmetry. So the fact that charge is conserved goes hand in hand with the U(1) symmetry -- you demand one, and get the other for free.
Jul
18
comment Do topological superconductors exhibit symmetry-enriched topological order?
This paper came out today: arxiv.org/pdf/1307.4403.pdf . It has a large discussion on the p+ip superconductor on the first few pages. It might be of interest to you.
Jul
11
answered Reconciling topological insulators and topological order
Jan
14
awarded  Yearling
Jan
5
awarded  Nice Answer
Jan
3
comment Symmetries of a Free Massless Scalar in Two Dimensions
@Heidar : Yea, your comments make sense. I think Polchinski tries to emphasize that you can construct multiple types of CFTs from the massless boson through precisely the mechanism you describe. The different EM tensors generate different transformations on the bosonic field, which are also symmetries of the (new) action -- he might be referring to this "extra symmetry". Ah well. This discussion would be a lot simpler if you read the chapter, hehe :P PS I'm not from any Scandanavian country, although my name suggests otherwise!
Jan
2
comment Symmetries of a Free Massless Scalar in Two Dimensions
You say: "But by modifying the energy-momentum tensor, you will change the central charge and actually get a whole new CFT. So I would not call that a symmetry of the free scalar theory." But the chapter starts out precisely with this modification of the EM tensor...! It also mentions the change in the central charge. I really think Polchinski is referring to the Coulomb gas formalism... The section is even called "linear dilaton", see e.g. damtp.cam.ac.uk/user/tong/string/seven.pdf page 181.
Jan
2
answered Symmetries of a Free Massless Scalar in Two Dimensions
Dec
14
awarded  Nice Answer
Dec
13
comment Lagrangian of 2D square lattice of point masses connected by springs
That potential is defined as the series entering the Lagrangian :). That sounds a bit silly, but it really is nothing more than having $L = T-V$ and $V$ some function of all the $q$'s. Assuming $V$ is analytic we might as well use its Taylor series. Keeping only the lowest order terms produces Zee's potential.
Dec
13
comment Lagrangian of 2D square lattice of point masses connected by springs
But then you already work in the harmonic regime by assuming a certain form for the potential energy. The more general case includes all higher order terms as well.
Dec
13
comment Limit of Fermi-Dirac distribution as $T$ goes to zero
Which textbook did you find it in? Your expression is correct by the way.
Dec
13
comment Lagrangian of 2D square lattice of point masses connected by springs
There isn't really anything to be "derived". The first term is the kinetic term of all the point masses. The remaining terms are the most generic interactions among the point masses you can write down.
Dec
12
comment Expectation value of time-dependent Hamiltonian
So what part is it exactly that you do not understand?