Related post on SE blog here.
The primary reason we are still in private beta seems to be the low amount of content (by SE standards). On the other hand, people have noted that the site seems to be slowing down, see for instance Has the site stalled?. It's unlikely that someone could ask more than a couple serious questions every week, and so with only a few dedicated users, it might be a month or more before we can generate enough content (and by that time, I suspect many would lose interest).
Inviting experts is one way to generate more content, and is probably the best method we have at the moment. On the other hand, it's not necessarily easy to do so. I know several people who would participate in a public site, but wouldn't want to participate in a private site for various reasons. Currently a lot of the content is in the form of soft questions, which I fear put people away if they are too frequent.
SE's primary fear is the so-called empty restaurant syndrome, where people are reluctant to participate in a site which doesn't seem well-established. The linked post in question discusses "seeding" the site, i.e. asking questions primarily for the sake of generating content. I can't say I'm a fan of the idea, but it does seem that it would alleviate SE's fears.
Should we begin seeding Theoretical Physics? What I would suggest is asking questions for which you know the answer, but for which the answer may not be well-known in that particular field (definitely if you're seeding, the quality of the question should be high). I don't know if any of the posts already were for that purpose, but I've deliberately held off from doing so until now. Of course, it is necessary that someone else can answer it, but this shouldn't be a terrible problem. In the worst case, one could fudge it by giving the question or answer to another user to post.
I realize what I'm suggesting is deceptive and in the long run counter-productive, but I do think it would help the current situation. I'm not convinced either way yet, though.
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