Starting from 80-ies the ideas either coming from physics, or by physicists themselves (e.g. Witten) are shaping many directions in mathematics. It is tempting to paraphrase E. Wigner, saying about "The Unreasonable Effectiveness of Physics in Mathematics".
What can be the reasons for it ?
Do physicists have some tools/ideas/techniques which allow them to make insights, which
are not seen for mathematicians? Or it is just because Witten&K are very … very smart ?
If, yes, what are these tools/ideas ? How to learn/absorb/(put into math. framework) them?
What can be the further applications of these ideas ?
Being a mathematician, but working in physicists surrounding for many years,
I have thought on this questions for quite a while. The recent MO question
Mathematician trying to learn string theory prompts me to ask it here.
I would think, that yes, there are such "ideas". But from some outstanding mathematicians I've heard an opposite opinion.
My vague feeling is that quantum field theory and string theory it is something like
an analysis/differential geometry on infinite-dimensional manifolds. But these manifolds are not abstract, say Banach modeled manifolds, which theory is not so rich, but kind of maps from one finite-dim. manifold to another, which has certain specific structures which are not fully revealed by mathematicians. E.g. vertex operator algebras, arise from maps of circle to manifold, if we map not circle but something higher dimensional there should be something more complicated. Another issue is about Feynman integral, which
allows physicist to use integration techniques in geometric problems, it is not well-defined mathematically, but it might be it cannot be defined in very general form of infinite-dimensional integrals, but again physicist have an intuition where it can be defined, where cannot, and proper mathematical theory should first clarify the setup
where it exists, rather than trying to build general theory which might not exist.
These words are probably very vague, so might be answers help to me clarify.
I think everybody knows the influence of physics happened from 80-ies, but
for completeness let me mention just a few.
Donaldson used instanton moduli spaces in his study of 4-folds.
Faddeev, Drinfeld et. al. created quantum groups
Representation theory of infinite-dimensional algebras have been large influenced by conformal field theory developments.
Witten's contributions are numerous his Fields Medal says more than I can say.
Mirror Symmetry, quantum cohomology etc…
The works of Fields Medalist Kontsevich and Okounkov are largely influenced by physics.
So on an so forth…
Best Answer
My opinion is that physicists transferred from study of "individual objects" to that of "large systems" where the order arises from limit probability laws rather than from simple deterministic formulae and from the study of something "readily observable" to something that is, essentially, "a purely mathematical object" invisible to a direct experiment. This brought them to the realm traditionally reserved for pure mathematicians. And, of course, with their eagerness to use whatever tools they have available in any way that is short of total lunacy, they went on to make predictions, many of which could be confirmed experimentally, leaving a long trail of successes and failures in their wake for mathematicians to explain.
I do not know the situation with the string theory and low dimensional topology but I have some idea about what's going on in random matrices (thanks to Mark Rudelson and his brilliant series of lectures) and in percolation/random zeroes (thanks to Stas Smirnov and Misha Sodin). The thing that saves physicists from making crude mistakes there is various "universality laws".
Here is a typical physicist's argument (Bogomolny and Schmidt). You want to study the nodal domains of a random Gaussian wave $F$ (the Fourier transform of the white noise on the unit sphere times the surface measure). Let's say, we are in dimension 2 and want just to know the typical number of nodal lines (components of the set $\{F=0\}$) per unit area. The stationary random function $F$ has only a power decay of correlations. However, we ignore that and model it with a square lattice that has the same length per unit area as $F$ (this is a computable quantity if you use some standard integral geometry tricks). Now, at each intersection of lattice lines, we choose one of the two natural ways to separate them (think of the intersection as of a saddle point with the crossing lines being the level lines at the saddle level). Then, we get a question (still unresolved on the mathematical level, by the way) about a pure percolation type model. Thinking by analogy once more, we get a numerical prediction.
From the viewpoint of a mathematician, this all is patented gibberish. There is no way to reduce one process to another (or, at least, no one has the slightest idea how this could be done as of the moment of this writing). Still, the Nature is kind enough to make the answers the same or about the same for all such processes and Mathematics is powerful enough to provide an answer (or a part of an answer) for some models, so the physicists run a simulation, and, voila, everything is as they predicted and we are left with 20 years or so worth of work to figure out what is really going on there.
I'm not complaining here, quite the opposite: this story is really quite exciting and the work mentioned is both real and fascinating. We are essentially back to the days when Newton tried to explain the nature of gravity looking at Kepler's laws trying various options and separating what works from what doesn't. I'm only saying that the famous "physicists' intuition", which is so overrated, is actually just the benevolence of Nature. Why should the Nature be so benevolent to us remains a mystery and I know neither a physicist, nor a mathematician, who could shed any light on that. The best explanation so far is contained in Einstein's words "God is subtle, but not malicious", or, in a slightly less enigmatic form, "Nature conceals her mystery by means of her essential grandeur, not by her cunning".