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To pursue a PhD in theoretical high-energy physics, one needs to be half retarded and half advanced -- half-advanced for wanting to capture the excitement of the past, and half retarded for seeing a future in it.


2d
comment What is meant by the phrase “this operator does not renormalize this other operator”, and how can understand it using diagrammatic arguments?
BTW, this recent paper might be of interest: arxiv.org/abs/1505.01844
May
16
comment Can bosons have anti-particles?
Taylor's point seems to be centered on the following idea "Bosons operate under different laws and can be created singly. This is a crucial distinction and is in nature of being either matter particles or force carriers." (which I simply don't understand)
May
16
comment Can bosons have anti-particles?
By definition charge conjugation $C$ is that operator which swaps particles and anti-particles. And I have to agree with @innisfree.
May
16
comment Can bosons have anti-particles?
Would you call the Higgs (doublet; not just the radial mode) an elementary scalar that is electrically carged?
Mar
11
comment Why are $SU(N)$ gauge theories easier to handle for $N\rightarrow \infty$?
It's an expansion parameter, like any other. In the large-N limit, many diagrams become sub-dominant. So it's enough to deal with a dominant diagrams, which are easier to deal with.
Mar
3
comment Notion of distance in a Conformal Field Theory
To add to my comment above, if distances really did not matter, then you are wrecking serious havoc with the notion of locality/causality! (You most definitely don't do that in a conformal quantum field theory)
Mar
1
comment Notion of distance in a Conformal Field Theory
"In a conformal field theory, due to the scale invariance only angles - and not distances - matter." That's not quite true! All that scale invariance guarantees you is that things transform nicely as you zoom in and zoom out. You don't see something weird happening as you zoom past a special scale. So one can pick a scale unit for calculation, and then ensure that all physical answers are non-dimensionalized by the same unit, so that they respect scale invariance.
Feb
18
comment How does the notion of topological order relate to the Landau-Ginzburg theory of phase transitions?
Are both 1 & 2 necessaary, or are they only heuristic indicators? Does entanglement structure undergo any change across a spontaneous symmetry breaking transition, which could have been described in the LG paradigm? Or would the possible existence of local order parameters preclude you from calling these quantum phase transitions?
Feb
15
comment How to compute this loop integral?
Ah, those slides start with the Feynman rules for scalar+gauge theory but then switch to QED. The way to write the numerators is to start at a particular vertex or propagator, and then go around the loop. Since this diagram has only bosons, the order in which you multiply contributions from the vertices and propagators will not matter. If you had QED, for example, the $\gamma$-matrices on the fermion-fermion-photon vertices would not commute and you would have to keep careful track of the ordering.
Jan
24
comment Is there a simple layman way to explain the incompatibilities between quantum mechanics and (general) relativity to high school students?
Read the first paragraph here: en.wikipedia.org/wiki/D-brane#Theoretical_background
Dec
31
comment Branch cuts in two-point function
Read on below eqn (7.20) in P&S. The threshold is given by the sum of masses of the two particles of the two-particle state, and those (pole) masses are well-defined and scheme independent.
Dec
24
comment Are all fermions massless at high temperatures?
For comments on finite temperature field theory in general, there are tons of reviews or lecture notes or even a few textbooks. I think some of them should mention this point paranthetically.
Dec
24
comment Are all fermions massless at high temperatures?
Take a look at physics.stackexchange.com/questions/131197/…
Dec
24
comment Lorentz transformations for scalar fields in QFT — Peskin and Schroder
@StephenBlake: That's a physical input. If the state called "vacuum" wasn't invariant under Poincare transformations (among them Lorentz transformations), then we'd notice all kinds of wacky effects like non-conservation of momentum and/or angular momentum. To avoid such behaviour, the vacuum state better be homogeneous, isotropic and boost-invariant.
Nov
21
comment What are orbifolds and why are they useful and interesting for physics?
Is there any intuition for why wavefunctions tend to concentrate near the singular points?
Nov
17
comment Is it possible to derive the effective potential of a given theory by only using the RGE equations?
If you only have a (first-order) differential equation, then will you not need some "initial/boundary condition" to fix a solution? Also, I don't see how this relates to extended Higgs sectors.
Nov
14
comment Are Matsubara states pure states?
One more thing realized: We use Wick rotation all over the place, to calculate loop integrals in zero temperature QFT. At zero temperature, one better not have any decoherence -- that should be a coherent sum.
Nov
13
comment Are Matsubara states pure states?
If you think of the "in-out" S-matrix (density-matrix like object) then once you compactify and impose boundary conditions for going around that circle, you're saying that the "initial" and "final" states must be related in a particular way. It sort of removes the off-diagonal elements of the matrix. I interpret that as decohering.
Nov
13
comment Are Matsubara states pure states?
That confuses me too -- I thought it might still be some linear combination of pure states (though any semblance of normalization is now lost) -- which tempts me to believe that the decohering might depend on the compactification. I'm not sure if one can define unitary basis vectors after compactification (without Wick rotation). There's no unique way to get from one time to another (you could move into the past or the future) :-?
Nov
13
comment Are terms with spinors analogous to $ ( \partial_\mu \Phi )(\partial^\mu \Phi)$ forbidden in the Lagrangian?
To me, the crux of the question is why can we not take ${(\partial \psi)}^2$ as the kinetic term instead of the usual one, and have $[\psi] = 1$. With regards to that, it seems that due to the extra derivative, this term is always going to have a larger mass scaling dimension than the usual kinetic term, so might not survive in the IR. I wonder if such a term is common in EFTs closer to the cutoff.