# What is U-duality? and Why U-duality is important?

In string theory, we know three dualities.

S-duality: Extension of Electric-magnetic duality, Duality between strongly coupled qft and weakly coupled qft. (One of the typical example is Seiberg duality in sqft)

T-duality: In short $R\leftrightarrow \frac{1}{R}$

U-duality: In M theory perspective, U-duality usually means T+S duality.(???)

But I am wondering about the last part, U-duality.

What is U-duality and why it is important in string theory?

• are you aware of the mandelstam variables in simple QFT? en.wikipedia.org/wiki/Mandelstam_variables . Seems to me this will be an extension of the concept in the string theoretical formulation (other wise why confuse the issue using u as variable) – anna v Aug 10 '18 at 17:50
• @anna v Actually there's no connection. String theory has S-duality for weak coupling to strong coupling, and T-duality for small radius to large radius, and U-duality was Ashoke Sen's name for dualities combining S and T dualities. – Mitchell Porter Aug 11 '18 at 0:50
• @MitchellPorter thanks for clarifying. Very naughty and unimaginative of those who christened these dualities. – anna v Aug 11 '18 at 4:27
• @anna v it’s not entirely unrelated: string theory has worldsheet duality (which generalises the result that, e.g. s+u mandelstam channels contain t channel to arbitrary amplitudes, associated to ope associativity and modular invariance, or equivalently the ability to cut open the path integral across different cycles without changing the result), and STU dualities are analogous relations in target space, where the corresponding path integral one cuts open is the unknown target space path integral associated to non-perturbative string theory – Wakabaloola Aug 11 '18 at 8:58
• @Wakabaloola thanks for the clarification, ( there is a reason in the madness :) ) – anna v Aug 11 '18 at 12:08

U-dualities are cool. For example, they can be mixures of $$T$$ and $$S$$ dualities, that means that is sometimes possible to stablish dictionaries not just between space parameters like $$R \rightarrow \frac{1}{R}$$ or between couplings as $$e^{\phi}\rightarrow e^{-\phi}$$ but to perform transformations that exchange spacetime moduli with couplings or viceversa.
Now notice that by definition there must exist $$U$$-dualities that are not products of $$T$$ and $$S$$ dualities (otherwise the $$U$$ duality group could be written as a product). New, wonderful and strongly stringy physics comes into play by considering those transformations. I strongly encourage anyone interested in string theory to read the following fabolous papers: Exotic Branes in String Theory, Exotic Branes in M-Theory and the blog post Exotic Branes, U-branes and U-folds.
Other consequences: The jungle of three dimensional supersymmetric field theories is wide wild (see Cordova's talk What's new with Q?). Why are three dimensional supersymmetric theories vastly more difficult that its higher dimensional counterparts? At least for those ones derived from string theory the answer is that truly exceptional $$U$$-duality groups arise after a dimensional reduction at (and below) three dimensions.