Is there a nonabelian topological group operation on the reals?

I believe L.E.J. Brouwer showed (in his thesis "Over de Grondslagen der Wiskunde", in Dutch (1907), in a relatively unknown chapter not published elsewhere, AFAIK) that Hilbert's problem on "continuous groups" (i.e. topological groups) being Lie groups was true for the one-dimensional case, the real line. It is believed that Brouwer added this to add some "gravitas" to the thesis (which also contained some more controversial stuff on the foundations of maths).

This answer provides a argument in the same vein. It is pretty elementary, but leaves some details to check.

There's a wonderful link to an elementary disproof in the comments. Here's a completely absurd approach.

1) Gleason-Montgomery-Zippin: every topological group whose underlying space is a manifold is actually a Lie group (with respect to some smooth structure on the topological space). Herein lies the complete absurdity - this is an incredibly hard theorem with more than a decade of work going into it.

2) Invoke the Lie group / Lie algebra correspondence - simply connected Lie groups are in canonical bijection with Lie algebras of the same dimension. There is only one 1-dimensional Lie algebra, hence only one Lie group structure on $\Bbb R$.

Now that one has this, one has the ability to prove a variety of very strong theorems about group structures on compact manifolds (invoking the classification of compact Lie groups); one thing that's particularly nice is that, for instance, $T^n$ admits one and only one group structure, and I think of someone was more careful than I want to be right now they could probably get similar results for $SO(n)$, $U(n)$, etc.

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Note that this is already false in dimension 2: $\Bbb R^2$ can be given the group structure of $\text{Aff}(1)$, the affine transformations of the line, which is not abelian.