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 Jan 14 awarded Nice Answer Jan 14 awarded Yearling Apr 4 awarded Enlightened Apr 4 awarded Nice Answer Jan 14 awarded Yearling Dec 1 reviewed Approve Does the mass of an electron change with its “energy state”? Sep 24 awarded Autobiographer Jan 21 awarded Nice Answer Jan 20 reviewed Approve 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.