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The groups $U(N)$ and $SU(N)$ are the most important Lie groups in quantum field theory. The most popular are the $U(1),SU(2),SU(3)$ groups (these gauge groups form the Standard model). But is there mentioned a $SU(\infty)$ gauge theory in physics literature?

An example of such a theory could be the following: May be $g \in SU(\infty)$ smooth and for a function $f(x,y)$ with spacetime coordinate $x$ and the new $SU(\infty)$ degree of freedom $y$ it holds $gf(x,y) = \int d^4y (g(x,y,y')f(x,y'))$. Now it is straighforward to define a gauge connection and the gauge field strength.

In more non-theoretical words: Some quantum states have degeneracies and these degeneracies are based on a special symmetry (operator) that exists in a quantum system. If now the degeneracy symmetry operator is unitary and local symmetry one can define a gauge theory. Was this concept used in quantum mechanics or does such a concept makes sense?

Another interesting case is this: One can perform the following switch of coordinates $g(x,y,y') = g(x,x-y,x-y')$ and hence the generators $T_a(y,y')$ defined by $g(x,y,y') = \sum_a g_a(x)T_a(y,y')$ become dependent on the spacetime coordinate. Another question: Is it possible to define spacetime dependent generators of a Lie algebra?

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    $\begingroup$ How do you propose to define $SU(\infty)$? $\endgroup$ – Danu Aug 28 '15 at 21:02
  • $\begingroup$ Here, the internal variables $y \in \mathbb R^4$ represent an infinite-dimensional vector space; this vector space is a Hilbert space. $\endgroup$ – kryomaxim Aug 28 '15 at 21:12
  • $\begingroup$ $SU(N)$ is not even a vector space. $\endgroup$ – Danu Aug 28 '15 at 21:35
  • $\begingroup$ Also, your sentence doesn't make much sense to me in general. Could you try to rephrase it? $\endgroup$ – Danu Aug 28 '15 at 21:57
  • $\begingroup$ I am assuming that the $g$ is an unitary operator acting on the internal degree of freedom $y$. Because linear operators can be considered as infinite dimensional matrices while vectors are functions I talked about the matrix representations of $SU(\infty)$. $\endgroup$ – kryomaxim Aug 28 '15 at 22:08
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Comments to the question (v2):

  1. The idea to consider the planar large $N_c\to \infty$ limit in $SU(N_c)$ QCD goes back to Ref. 1.

  2. In light-cone membrane theory, pioneered in Ref. 2, the group $SU(\infty)$ is naturally identified with area-preserving diffeomorphisms ${\rm SDiff}_0(T^2)$ on the torus $T^2$ connected to the identity.

  3. Concretely, OP's proposal resembles a Fourier series expansion of an extra (compact) spacetime dimension. Such exercises are customary in string theory.

References:

  1. G. 't Hooft, A planar diagram theory for strong interactions, Nucl. Phys. B72 (1974) 461.

  2. J. Goldstone, unpublished; J. Hoppe, MIT Ph.D. Thesis, 1982.

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There are apparently several thousand references to "SU(\infty)" on arxiv.org, and some of them are definitely talking about gauge fields or Yang-Mills.

I suspect that some of the time, this will just be a way of talking about the large N limit of SU(N), i.e., not referring to a literal SU(∞) field theory, but rather the N→∞ limit of some quantity in SU(N) field theory.

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  • $\begingroup$ Thanks for your answer. I have asked whether it is possible that the generators of the $\lim_{N \mapsto \infty} SU(N)$ group are explicitely spacetime dependent. Generators of Yang-Mills theories like QCD are constant matrices (Gell-Mann matrices), but it is possible for a field theory that generators are explicitely dependent on the spacetime coordinate? $\endgroup$ – kryomaxim Aug 29 '15 at 9:43
  • $\begingroup$ @kryomaxim I think the fact that you choose a basis for a finite group is because it is finite. You would have have consider whatever theory you work with no consideration of basis. If everything works out fine, I suppose it works. I'm making this analogy with the way mathematicians deal infinite dimensional vector spaces. $\endgroup$ – Sai krishna Deep Oct 27 '15 at 4:38
  • $\begingroup$ Even for the Lorentz group, with which I gather you want your generalized SU(N) to commute, the matrices themselves do not depend on coordinates, even though, of course, they represent coordinate-dependent generators. I wonder if your question stems for the fact that infinite N normally demarcates infinite sets of integers which, when Fourier transformed, amount to continuous phase-space type surfaces cf. , as Qmechanic's answer addresses in 2 & 3; but such surfaces are ancillary world-sheet constructs, and not our spacetime. $\endgroup$ – Cosmas Zachos Apr 5 '16 at 14:24

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