104,410 reputation
7142293
bio website motls.blogspot.com
location Czech Republic
age 40
visits member for 3 years, 9 months
seen 3 hours ago

Hi, I am a string theorist and a publicist.


Jan
7
comment reflection, refraction and diffraction occur in radio waves, which one occurs the most?
Dear David, while I agree that the question isn't totally quantitatively well-defined, I am confident that people who actually understand radio waves would have something to say – whether reflections in the forests may be neglected or not etc.
Jan
7
revised Photon spin projection to arbitrary axis
added 679 characters in body
Jan
7
answered Photon spin projection to arbitrary axis
Jan
7
answered Physical meaning of the energy density of an electrostatic field
Jan
7
answered Differentiation of the action functional
Jan
7
comment Uniqueness of the 5 string theories
Let me just say that at the leading order, the partition sum that has to cancel has relevant open-string-related and unoriented contributions from the cylinder (2 boundaries); Klein Bottle (2 crosscaps); Mobius strip (1 boundary 1 crosscap). Those 3 terms may be written as $(b+c)^2$, formally, where $b$ is an insertion of one boundary and $c$ is the insertion of one cross cap. In this way, one may reduce the cancellation to the cancellation of dilaton tadpoles from a cross cap and from colored boundaries themselves, and this calculation says that the number of half-colors has to be $2^5$.
Jan
7
comment Uniqueness of the 5 string theories
Dear Squark, the anomaly that has to cancel is the "dilaton tadpole", a one-point function of the dilaton vertex operator, see e.g. scholar.google.com/… - It's related to the vacuum energy density in spacetime (partition sum with no insertions). A nonzero value would make the perturbative physics inconsistent, to say the least, because the energy density would blow up as $1/g$ or so and one can't $g$-expand around it.
Jan
7
comment String landscape in different dimensions
A good example of the richness of the vacua even in 7 large dimensions and above. See Triples, Fluxes, and Strings: arxiv.org/abs/hep-th/0103170 - various disconnected components of commuting Wilson lines, dualities between these vacua etc. I think it's a great paper by 7 authors, a substantially undervalued one (with 100 cits or so).
Jan
7
answered Is it possible to create a hologram using X-Rays?
Jan
7
comment Advanced topics in string theory
And one more comment. I still feel that you underestimate the perturbative part of the theory. In the mid 1990s, there's been a huge activity that ultimately clarified many features of the non-perturbative behavior of string theory. But it's still true that most of the phenomena are accessible from one perturbative description or another - dualities are just statements of equivalences (sometimes nonperturbative equivalences) between different descriptions that were known perturbatively. But much of the "explicit" quantitative calculation has to use some expansions.
Jan
7
comment Advanced topics in string theory
Matrix string theory was reviewed in various papers e.g. Susskind-Bigatti and Taylor, see the resource letter by Marolf at the bottom of my 2006 page. Little string theory is extremely special. One should read the original papers and their most important followups. This is a domain intensely studied by a few people in the world so it's obviously not terribly efficient to write "textbooks" of it.
Jan
7
comment Advanced topics in string theory
Otherwise I skipped F-theory, Matrix theory, and little string theory. There are specialized reviews of those because they're really special. To understand F-theory including the applications that give it the "juice", one must understand lots of algebraic geometry, bundles, and so on. A great arena for people who love (advanced) geometry. The physical essence is really simple: it's type IIB where the axion-dilaton $\tau$ is interpreted as the complex structure of a $T^2$ fiber of two new dimensions attached to each point.
Jan
7
comment Advanced topics in string theory
Dear @Squark, thanks for your interest. SFT didn't really "fail". It is a totally consistent and from many viewpoints uniquely useful - "explicitly local, off-shell" - formulation of perturbative open string dynamics and the open-string-related solutions such as D-branes solutions (as tachyon condensation from other D-brane starting points). The most explicit and rigorous framework to discuss tachyon condensation etc. It's very manageable in the bosonic string case. People could have expected some other miraculous things from SFT but there have never been "rational justifications" for them.
Jan
7
comment Advanced topics in string theory
This URL was (for years) on the top of my page, too. ;-)
Jan
7
comment Derivation of the basic equation for Witten diagrams
I've re-edited backslash-box as backslash-Box (capitalized). Note that backslash-square is also possible. Also, there was a mistake in the big equation. The braces are not just braces - which are given a special role in TeX. They are written as backslash-braces (both left and right).
Jan
7
answered Uniqueness of the 5 string theories
Jan
7
comment String landscape in different dimensions
Dear @Squark, but even if the number of noncompact dimensions is well-defined, it's still sensible to subclassify the options according to the number of supercharges. For low number of noncompact dimensions, the number of supercharges may be very small and not too constraining. So only for high dimensions such as $D=9$, the classification is potentially manageable. In $D=9$, one gets various M-theories on Klein Bottle, Mobius strip, and similar funny extra choices aside from the circular compactifications. There are also papers on the $D=6$ landscape etc.
Jan
7
answered Advanced topics in string theory
Jan
6
awarded  mathematical-physics
Jan
5
answered Is it a total or an explicite time derivative in the Schrödinger equation?