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Hi, I am a string theorist and a publicist.


Dec
13
answered What's the Standard Model width of a 125 GeV Higgs?
Dec
13
comment Convergence of quantum effective action to finite loop order
Dear @Squark, I don't think it's possible to treat these two limits differently in any real situation. The real expansion parameter measuring "how much quantum" the theory is - how important the loops are - is always the same. For the electromagnetic interaction, it's the fine-structure constant $e^2/(4\pi \epsilon_0)\hbar c$. This is small, $1/137$, and that's why the loops - or additional external legs - bring suppression. Your unusual asymmetric limit seems to depend on an arbitrary renormalization of the fields by a power of $\hbar$ which is unphysical.
Dec
12
comment Status of the little hierarchy problem
The newest paper with "little hierarchy problem" in the title is one from May 2011 by Feldman, Kane, Kuflik, Lu: arxiv.org/abs/arXiv:1105.3765 - those gravitinos and moduli around 30 TeV do solve it, aside from other problems. Another 2011 paper: arxiv.org/abs/arXiv:1104.3171
Dec
12
comment Theoretical proof forbidding Loschmidt reversal?
Quantum mechanics really forbids the kind of detailed microscopic control that would be needed for the Loschmidt reversal. While in classical physics one could say that for most initial states, the entropy will increase, while for a carefully selected special initial state, it may decrease, quantum mechanics says something else. It always predicts the probabilities only and the probabilities of a big decrease in entropy is small for every initial state, much like if one averages over initial states in classical physics.
Dec
12
comment Hidden observers in Double Slit experiments - Do they matter?
Dear @Peter, you seem to refer to Bohr's quantitative "complementarity" which allows a "partly wave-like" and "partly particle-like" behavior of a quantum of the field. That's OK and you're right but that's not really what the original question was about. The original question talked about the situation when some apparata are completely able to measure the which-slit information (so the action change is very large, in your language), but this measurement isn't accessible to "us". So your answer is arguably off-topic.
Dec
12
comment Are there irreducible tensors of half integral degree in quantum mechanics?
Dear @Friedrich, $J_\pm$ is the same thing as $J_x\pm i J_y$, up to the sign of $i$ and the overall normalization, so the difference between $\{J_x,J_y,J_z\}$ and $\{J_+,J_-, J_z\}$ is just a convention about the basis - which linear combinations of the three quantities are being written as the standardized ones. The 3D space of linear combinations of these 3 tensor operators is exactly the same in both cases. That's why we say that the $j=1$ irreducible representation of the group is unique. The same hold for any allowed $j$, too.
Dec
12
answered Interplay between the cosmological constant and “microscopic” properties of string vacua
Dec
12
comment Convergence of quantum effective action to finite loop order
In other words, your question assumes that you isolate some infinite subset of diagrams and pretend that they're "of the same order in the loop expansion". But they're not. You're using an inconsistent rule for counting the loops. If you have a generic nonlinear effective action of the fields, its curved/nonlinear character does arise from summing multiloop diagrams (if you can derive it from something else at all). Any truncation of the full action must be "with respect to a well-defined limit", e.g. $g\to 0$ (my answer) or $E_\gamma\to 0$ (IR debates). You don't seem to have any new limit.
Dec
12
comment Convergence of quantum effective action to finite loop order
Dear @Squark, I didn't mean just one term, like one diagram. I meant all terms with a particular choice of the external legs. That's what you seem to be computing. Then this is equivalent to calculating a Green's functions (up to some universal factors) and be assured that a very high power of the coupling constant requires a very high number of loops, just like in the calculation of the scattering amplitude. If you want to instantly include arbitrary powers in $\phi$, well, then you are computing the full effective action and you must include all loop corrections, too.
Dec
12
answered Matrix geometry for F-strings
Dec
11
comment Virasoro constraints in quantization of the Polyakov action
Similarly, the old-covariant "pure gauge states" $L_{-n}\phi$ may be produced as BRST "pure gauge" states $Q \lambda$ because the required $\lambda$ may be constructed as something of the sort $b_{-n}\phi$.
Dec
11
comment Virasoro constraints in quantization of the Polyakov action
Let me just mention where the last sentence comes from. The BRST charge is morally $\sum c_{-n} L_n + ccb$ terms. Ignore the latter which vanishes if you eliminate positive $b,c$ excitations. You're left with the first term. It has to annihilate BRST physical states. The terms with negative $n$ do so because $c_{-n}$ annihilates them (no positive $c$ excitations after the choice); the terms with non-negative $n$ do so because $L_n$ has to annihilate them, and you reduce the BRST closedness to invariance under positive (and zero) $L_n-\delta_{n0}$.
Dec
11
comment Virasoro constraints in quantization of the Polyakov action
@Squark: otherwise the physical states obtained from the old covariant quantization; BRST quantization; or light-cone quantization can be proved to be equivalent. The BRST-oldcovariant equivalence boils down to the elimination of 2+2 unphysical modes (b, c, two X's) for each $n$ on the world sheet. The BRST symmetry equivalence may be used to eliminate 2 fields (b and one X), and the BRST closedness then eliminates the others (c and one X), and you may get an oldcovariant state as a special BRST stae if you ignore b,c. Then the Q-closedness reduces to the positive-n Virasoro constraints etc.
Dec
11
comment Virasoro constraints in quantization of the Polyakov action
Dear @Squark, right, string theory leads to a world sheet theory with gravity and diff+Weyl (or conformal) symmetry, moduli spaces, etc. But these rules are exactly what you would construct for a QFT/CFT with a dynamical metric and these extra gauge symmetries. It doesn't matter that it's also a "string theory" which is (in spacetime) "more than just a QFT". The world sheet rules are those of a QFT with some gauge symmetries including diff and Weyl, and with the dynamical metric.
Dec
11
comment Convergence of quantum effective action to finite loop order
Sorry, @Squark, -1 for the question because you if you mean neither the sum of the perturbation series, nor the sum over soft photons related to IR divergences, there can't possibly exist anything else that makes sense and that you could meant. You may only add a large number of vertices to a diagram if 1) you also add a high number of loops (and you have uniform external legs) or 2) you are computing an inclusive cross section (you sum over different final states). The former leads to my answer; the second leads to the discussion of IR divergences. No other potentially divergent sum exists.
Dec
11
comment Convergence of quantum effective action to finite loop order
Dear @Squark, as I said in the previous sentence, it's not possible. If you compute any particular amplitude (Green's function, scattering amplitude, or a term in the effective action), a large number of vertices may only be achieved by adding a high number of loops. You can't add vertices without adding loops, unless you are adding applies and oranges which you shouldn't. As some other people mentioned, you could also compute inclusive cross sections with may include a very high number of soft photons (without loops), but you said it's not what you meant, either.
Dec
10
comment Higgs Field - Is its discovery truly “around the corner”?
Dear @Matt, nice mini-answer, +1. One could also say that if it is "perfectly compatible with the SM", the mass is lighter than one would expect for a "typical" non-SUSY Standard Model (by 15 orders of magnitude, in fact, to add a funny twist to it). Incidentally, there will be evidence and "candidates" but the press release won't contain the word "hint". Do you want to make a bet? ;-)
Dec
10
comment Virasoro constraints in quantization of the Polyakov action
A $Q$-cohomology is the class of all vectors $|\psi\rangle$ that satisfy $Q|\psi\rangle=0$ and that are identified by the equivalence $|\psi\rangle \sim |\psi\rangle + Q|\phi\rangle$ for arbitrary vectors $|\phi\rangle$. In the cohomology, you may find some vectors with no "positive" excitations by the $b,c$-ghosts and a certain proper ghost number. If you take these states and ignore all the $b,c$ ghosts in them, you get states of the old covariant quantization. Also, I wanted to stress that the fact that the Hilbert space is interpreted in a stringy way plays no role: it's still a QFT.
Dec
10
comment Virasoro constraints in quantization of the Polyakov action
Dear @Squark, when we're happy to know the BRST formalism, it's a good idea to start with it (there is no central term in it, so no obstruction to impose the annihilation by all generators!), and view the old covariant quantization as a particular convention to choose the representatives from the BRST cohomologies. Because the ghosts-$bc$-free Virasoro generators commute to the central extension, one would need to talk about a complicated representation theory of these "extension" groups and it's not necessarily an insightful approach.
Dec
10
comment Convergence of quantum effective action to finite loop order
OK, then sorry, @Squark, but in that case, I have no idea what you could be possibly asking. You are talking about a convergence of a "sum". The only sum that is at risk of being divergent (and it, indeed, is divergent) is the loop order expansion. There is no "infinite sum" at a finite order. At a finite order, there is always a finite number of diagrams/terms and the sum of a finite number of finite terms is always convergent. It seems that you think that some methods to "complicate" the diagrams (ext. legs?) don't add any $e$; but all of them do.