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Groups in QFTby gentsagree
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#1
Oct2813, 08:24 AM

P: 47

Qhy does SO(n) have the same number of dimensions of O(n), whereas SU(n) reduces the dimensions of U(n)? Isn't the constraint the same for both cases, i.e. detM=1?



#2
Oct2813, 08:58 AM

P: 128

If I haven't misunderstood something myself:
The constraint is the same, sure, but the other is complex and the other real: If U∈U(n), det(U)=1 (it preserves the norm) and det(U) is in ℂ, so it's of the form e^{iθ} for some θ∈ℝ  there is one free parameter "in the determinant". Further requiring U∈SU(n) means that det(U) is exactly one, so one "degree of freedom" (one free parameter) is lost and θ=0 exactly. So dim(SU(n))=dim(U(n))1. If O∈O(n), det(O)=1 still, but since det(O) is now in ℝ, there is a discrete set of two possible values instead of a free parameter: Either det(O)=1 or det(O)=1. So, requiring O∈SO(n) just picks one of these two options, that det(O)=1, but it doesn't remove any actual free parameters, so dim(SO(n))=dim(O(n)). (That's obviously not a proof, of course, but merely something that made me understand the reasoning behind it.) 


#3
Oct2813, 09:07 AM

P: 47

Thank you, I had just gotten to a similar conclusion myself. Also, I thought about it topologically: in the orthogonal case, O(n) is formed of two disjoint sets, (i) detR=1 and (ii) detR=1, of which only one is a subgroup {i.e. SO(n), with detR=1, as it contains the identity}. Therefore removing the bit (ii) detR=1 doesn't "get rid" of any subgroup, but only of a coset in O(n). In U(n) nothing of this happens, as it is compact and connected (which I think reflects on the fact that we have a continuous set of possible values, as opposed to the discrete one in O(n)).
I don't enough topology or group theory to be sure about this, but this is the way I thought about it. Can somebody tell me if I'm wrong? 


#4
Oct2913, 08:55 AM

Sci Advisor
P: 303

Groups in QFT
It seems right to me. ##U(n)## is a connected ##n^2  1##dimensional manifold, you can always create a coordinate system where one of the coordinates is ##\theta## the logarithm of the determinant. Hence, ##SU(n)## is simply the ##\theta = 0## submanifold, and so has one dimension less.
In the ##O(n)## case we simply have two manifolds of the same dimension, one indexed with ##+1## and the other indexed with ##1##. ##SO(n)## is simply one of these manifolds. 


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