String theory "computation" for math undergrad audience - MathOverflow most recent 30 from http://mathoverflow.net 2013-05-18T23:54:38Z http://mathoverflow.net/feeds/question/47770 http://www.creativecommons.org/licenses/by-nc/2.5/rdf http://mathoverflow.net/questions/47770/string-theory-computation-for-math-undergrad-audience String theory "computation" for math undergrad audience Balazs 2010-11-30T09:06:35Z 2010-11-30T17:51:56Z <p>I am giving a talk on String theory to a math undergraduate audience. I am looking for a nice and suprising mathematical computation, maybe just a surprising series expansion, which is motivated by string theory and which can be motivated and explained relatively easily. Examples of what I have in mind are the results in Dijkgraaf's "Mirror symmetry and the elliptic curve", or the "genus expansion" of the MacMahon function (aka DT/GW for affine three-space), but I am not sure I can fit either into the time I have. Any thoughts? </p> http://mathoverflow.net/questions/47770/string-theory-computation-for-math-undergrad-audience/47786#47786 Answer by Richard Eager for String theory "computation" for math undergrad audience Richard Eager 2010-11-30T11:50:57Z 2010-11-30T11:50:57Z <p>Ginsparg's Applied Conformal Field Theory (hep-th/9108028 section 7.6) has a nice proof of the Jacobi Triple-Product formula and Euler's pentagonal number theorem. The equalities can be interpreted as the equivalence between the partition function of a free chiral boson and the partition function of two chiral fermions on a torus. This is an example of bosonization and plays an important role in string theory.</p> <p>The proof can be explained without any reference to physics, but the crucial difference in statistics (Boson/Fermion) employed in the proof becomes obscured.</p> http://mathoverflow.net/questions/47770/string-theory-computation-for-math-undergrad-audience/47790#47790 Answer by Eric Zaslow for String theory "computation" for math undergrad audience Eric Zaslow 2010-11-30T13:17:29Z 2010-11-30T13:17:29Z <p>Two counting problems -- from my own very biased and personal viewpoint -- that can perhaps be motivated:</p> <ol> <li>Counting triangles on the torus = theta function relation for elliptic curve. (I tried to squeeze this into a public lecture one time.)</li> <li>Counting symmetric polynomials of degree k in 24 variables = partition function of chiral bosonic string => counting curves on K3 by heterotic duality: 24, 24 + 24*25/2 = 324, etc.</li> </ol> <p>But these can't beat calculating an actual partition function (as in Richard Eager's answer), unless you're trying to emphasize mathiness.</p> http://mathoverflow.net/questions/47770/string-theory-computation-for-math-undergrad-audience/47794#47794 Answer by Jeff Harvey for String theory "computation" for math undergrad audience Jeff Harvey 2010-11-30T14:31:39Z 2010-11-30T14:31:39Z <p>I agree that computing partition functions has many pretty applications. My favorite is the use of Jacobi's abstruse identity between theta functions, $\theta_3^4-\theta_4^4=\theta_2^4$, to show the equality between the number of bosons and fermions in open superstring theory as required by supersymmetry. This is explained in sec 4.3 of "Superstring Theory" by Green, Schwarz and Witten.</p> <p>Another short calculation which quickly gets to the heart of the connection between string theory and gravity is the demonstration that bosonic string theory contains a massless spin two excitation. One way to do this requires regularizing a divergent zero point energy via $\sum_{n=1}^\infty n \rightarrow \sum_{n=1}^\infty n^{-s}$ and then analytically continuing to $s=-1$ to obtain $\zeta(-1)=-1/12$. See sec 2.3 of GSW.</p> http://mathoverflow.net/questions/47770/string-theory-computation-for-math-undergrad-audience/47805#47805 Answer by Jim Bryan for String theory "computation" for math undergrad audience Jim Bryan 2010-11-30T15:49:28Z 2010-11-30T15:49:28Z <p>I've given a talk about the equivalence between 1+1 TQFTs and Frobeneus algebras to an undergraduate audience with great success. It has great pictures and a clear, beautiful idea. The "computation" can then be the beautiful formula for the number of degree $d$ covers of a genus $g$ Riemann surface as a sum over irreducible representations of the symmetric group $S_d$</p> <p>$$Z(g)=\sum _{R} \left(\frac{d!}{\dim(R)}\right)^{2g-2}$$</p> <p>That last computation requires that your audience knows some representation theory of finite groups, but that might be true for the Oxford undergrads.</p> http://mathoverflow.net/questions/47770/string-theory-computation-for-math-undergrad-audience/47816#47816 Answer by Willie Wong for String theory "computation" for math undergrad audience Willie Wong 2010-11-30T17:51:56Z 2010-11-30T17:51:56Z <p>Maybe derive the <a href="http://en.wikipedia.org/wiki/Polyakov_formula" rel="nofollow">Polyakov formula</a>? Like KConrad says, whether it can be understood on an undergraduate level depends a lot on your presentation and the level of the undergraduates. But the basic idea behind the formula, if I remember correctly, can all be explained using a little bit of Riemannian geometry/representation theory plus a bit of complex analysis. </p> <p>(I recently saw a talk where it was perfectly understandable and impressive for masters-level students.)</p> <p>(You can also segue into explaining why the universe is 26 dimensional.)</p>