The ''grim anniversary'' in the history of Physics SE that Emilio alluded to is the shutting down of TheoreticalPhysics SE in April 2012 by the Community Manager of Stack Exchange. Between this announcement and the actual closing act, there was quite some discussion on Physics SE and on the blogs (some of which are no longer accessible; the links are dead) of some participants. In one of these one can read:
The reason that TP.SE was formed was largely due to wanting somewhere
for research level stuff without all of the noise.
As a member of TheoreticalPhysics SE wrote:
We have succeeded in maintaining high quality site which has become a
reliable resource of correct and useful information, which is
sometimes not that easy to obtain. We had top people coming to the
site and contributing high quality content. Quality, rather than
quantity, is of course something that is not easily measurable, but I
think it has value nonetheless. Traffic was an issue, but for a highly
technical site the pattern of traffic, including how it changes with
time, should be evaluated differently.
But although the officials had said in a more general context:
As long as your site shows steady progress and continues to make the
Internet a better place to get expert answers to your questions, it
will march on.
and this is quoted in the shut down notice, ''making the Internet a better place'' is interpreted from the point of view of the SE marketing perspective only: Commercial viability requires high enough traffic rates and these were not maintained by TheoreticalPhysics SE during its beta existence.
Thus the reasons for shutting down the site have more to do with the standards of SE for a site to be tolerable in a commercial enterprise than with the quality of the contributions to the site. In the thread ''Why did Theoretical Physics fail?'' the SE officials didn't question the fact mentioned in the thread opening that
the quality of questions (and answers) was actually good.
But SE required growth and high traffic. (Typical argument: ''Physics [SE] was already a bigger and more established site when it launched.'') In the words of one of the active members of Theoretical Physics SE,
SE closed a Jazz club for not being the same as a Madonna concert.
The actual closing message on TheoreticalPhysics SE meta mentioned that
the questions and answers posted here will be made available for
download and re-use by anyone who wants them.
There was an immediate desire to migrate the site outside the SE framework.
See ''Where do we go from here?'', but for practical reasons it took nearly two years to restart the site as PhysicsOverflow. All questions and answers from TheoreticalPhysics SE and its meta site were migrated to PhysicsOverflow, and the site continues under improved rules. In particular, PhysicsOverflow treats comments on the same level as questions and answers (any length, anytime editing, fully disclosed history, bookmarking). PhysicsOverflow also imports upon request research level posts from Physics SE and MathOverflow; new answers of questions from there are usually reported back on the original site.
PhysicsOverflow hasn't become a Madonna concert with all its noise but it preserves the level, quality, and noise-free enjoyment the Jazz club once called TheoreticalPhysics SE offered to the physics research community. Reflecting the high level, the number of questions, answers, and comments per month by registered users is highly fluctuating, with long term averages around 25, 25, and 75, respectively. These numbers are comparable to those of Theoretical Physics SE at the time of its shut down and do not include the posts of anonymous users or unregistered users imported from elsewhere, which add about 30% to the other numbers.
In view of Emilio Pisanty's comment
There is also PhysicsOverflow, and I think I will leave the telling of
this story to its organizers.
let me add that a timeline of PhysicsOverflow can be found here.
Due to unrelated events, the relations with Physics SE have been characterized by some tensions, as reflected by the very unusual ratio of upvotes and downvotes in the promotion ad (visible to high rep users only). Being quite active on both sites I'd be happy if these tensions would give way to a friendly relationship of the same sort as it exists between Mathematics SE and MathOverflow.