This is a question that has been winding around my head for a long time and I have not found a convincing answer. The title says everything, but I am going to enrich my question by little more explanations.


As a layman, I have started searching for expositories/more informal, rather intuitive, also original account of non-commutative geometry to get more sense of it, namely, I have looked through


* The English translation of [*Review of non-commutative geometry* by Alain Connes](http://www.ams.org/notices/199707/jones.pdf),
* [*Surveys in non-commutative geometry,* Clay mathematics proceedings, Volume 6](http://www.claymath.org/library/proceedings/cmip06.pdf),
* [*Non-commutative geometry* by Alain Connes](http://www.alainconnes.org/docs/book94bigpdf.pdf),


Nevertheless, I am not satisfied with them at all. It seems to me, that even understanding a simple example, requires much more knowledge that is gained in grad school. Now for me, this field merely contains a lot of highly developed machineries which are more technical (somehow artificial) than that of other fields.


The following are my questions revolving around the significance of this field in **Mathematics**. Of course, they are absolutely related to my main question.



> 
> 1. How can a grad student be motivated to specialize in this field? and
> 2. What is (are) the well-known *result*(s), found solely by non-commutative geometric techniques that could not be proven without them?
> 
> 
> 



What is the significance of non-commutative geometry in mathematics?
$\DeclareMathOperator\coker{coker}$I think I'm in a pretty good position to answer this question because I am a graduate student working in noncommutative geometry who entered the subject a little bit skeptical about its relevance to the rest of mathematics. To this day I sometimes find it hard to get excited about purely "noncommutative" results, but the subject has its tentacles in so many other areas that I never get bored.


Before saying anything further, I need to say a few words about the Atiyah–Singer index theorem. This theorem asserts that if $D$ is an elliptic differential operator on a manifold $M$ then its Fredholm index $\dim(\ker(D)) - \dim(\coker(D))$ can be computed by integrating certain characteristic classes of $M$. Non-trivial corollaries (obtained by "plugging in" well chosen differential operators) include the generalized Gauss–Bonnet formula, the Hirzebruch signature theorem, and the Hirzebruch–Riemann–Roch formula. It was quickly realized (first by Atiyah, I think) that the proof of the theorem can be viewed as a statement about the Poincaré duality pairing between topological K-theory and its associated homology theory (these days called K-homology).


I wasn't around, but I'm told that people were very excited about Atiyah and Singer's achievement (understandably so!). People quickly began trying to generalize and strengthen the theorem, and my claim is that noncommutative geometry is the area of mathematics that emerged from these attempts. Saying that marginalizes the other important reasons for developing the subject, but I think it was Connes' main motivation and in any event it is a convenient oversimplification for a MO answer. It also helps me answer your first question by playing to my personal biases: when I was choosing an area of research I told my adviser that I was interested in learning more about that Atiyah–Singer index theorem and I was led inexorably toward the tools of noncommutative geometry.


The origin of the relationship between NCG and Atiyah–Singer lies in equivariant index theory. Atiyah and Singer realized from the start that if $M$ admits an action by a compact Lie group $G$ and $D$ is invariant under the group action then it is better to think of the index of $D$ as a virtual representation of $G$ (i.e. an element of the $G$-equivariant K-theory of a point) rather than as an integer. If $G$ is not compact then this doesn't really work, but the noncommutative geometers realized that $D$ does have an index in the K-theory of the *reduced group C$^\ast$-algebra* $C\_r^\ast(G)$. Indeed, to a noncommutative geometer equivariant index theory is all about a map $K\_\ast(M) \to K\_\ast(C\_r^\ast(M)$ where $K\_\ast(M)$ is the K-homology of $M$; in the case where $M$ is the universal classifying space of $G$, Baum and Connes conjectured that this map is an isomorphism. Proving this conjecture for more and more groups and understanding its consequences motivates a great deal of the development of NCG to this day.


The conjecture is interesting in its own right if you already care about index theory, but even if you don't injectivity of the Baum–Connes map implies the Novikov conjecture (see Alain Valette's [answer](https://mathoverflow.net/a/88187)) and surjectivity is related to the Borel conjecture. It has numerous other applications, for example to the theory of positive scalar curvature obstructions in Riemannian geometry or to the Kadison–Kaplansky conjecture in functional analysis (which would follow from surjectivity). Recently there has been a lot of interest in connections between the Baum–Connes conjecture and representation theory; the Baum–Aubert–Plyman conjecture in $p$-adic representation theory has its origins in these sorts of considerations.


Much of the rest of NCG can also be traced back to index theory. Kasparov's KK-theory arose as a way to understand maps and pairings between K-theory and K-homology, motivated in part by index theory. Connes' work on noncommutative measure theory arose from his work on index theory for measurable foliations (with applications to dynamical systems). Cyclic (co)homology was invented in part to gain access in a noncommutative setting to the Chern character map from K-theory to cohomology which translates the K-theoretic formulation of the index theorem into a cohomological formula. Connes' theory of spectral triples and noncommutative Riemannian geometry is based on the theory of Dirac operators which was invented by Atiyah and Singer to prove the index theorem. I guess my point with all of this is that all the esoteric machinery of NCG seems less artificial when viewed through the lens of index theory.


