Proposal: Theoretical Physics
Is Theoretical Physics different enough that it deserves its own site separate from the Physics group? I would suggest just merging them.
The Physics SE site has it's own community, and one which is significantly different to the one that we are building here. Note that at present only 18% of committers here participate in the other site. Further, if you look at the bios, you'll see that the overwhelming majority are professional physicists (unlike the other site). I think that speaks volumes about the potential of this proposal, and its distinctness from the Physics SE. I think it would be an appalling waste if this proposal were scuppered by a merger with Physics. I am extremely opposed to any attempt to merge the two, and I will do anything within my power to ensure that this proposal does not become part of the other site.
Lastly, I would ask that people who have no stake in either site (i.e. not being a user or committer to either) allow us space to sink or swim on our own, rather than forcing a merge.
The case is for Theoretical Physics to exist as an alternative proposal to Physics by analogy to Math Overflow and the Mathematics proposal.
Math Overflow vs Mathamtics and the success of Theoretical Computer Science shows that having parallel sistes in this mould can work. We should wait and see whether this proposal can gather support in the Commitment phase.
The level on physics.SE is constantly rising through the enormous effort done by some like Luboš Motl. However, as Joe points out, a lot of the questions are still at best on highschool level and could be answered by pointing to the corresponding wikipedia article.
Lets see it the other way round - Physics.SE created a community which is stable and healthy by now and creating nice content. Hence it can be very useful for TP.SE as a "preliminary step".
As I have seen and discuss here the problem is not exactly for merging but discuss the viability of Physics as a good Q&A site. I can't say much about the quality met in P.SE because it's not my domain but I see some ugly answers. It seems that P.SE is not making the internet a better place.
See also this answer and comments
To me it just doesn't seem like Physics or Theoretical Physics have a large enough audience yet to survive separately.
Theoretical physics is a direct subset of Physics, therefore it does not need a separate site, and it would simply drain interest from physics. I have to vote to close, sorry.
I think the idea of making basic site and advanced site is just bad, because it provokes the first to be junkyard and the second to be discussion forum instead of Q&A.
Everyone mentions MSE and MO, but the opposite construction works very well for instance on CrossValidated.