A community ad caught my eye, and I visited http://www.physicsoverflow.org. What are the links between Physics SE and Physics Overflow and how are they different from each other?
Compared to the former theoretical physics site, we have slightly lowered the bar to ask questions to graduate-level upward and broadened the scope to include experimental physics and phenomenology.
Apart from the high-level Q&A, PhysicsOverflow offers also a Reviews section, dedicated to discuss and peer review (mostly ArXiv but other sources can be considered too) papers publicly and "in real time".
PhysicsOverflow was built outside the SE network to not have to fullfill any externally prescribed activity criteria for graduation (Area51 statistics). As Danu hinted at, this also allows us to avoid having to obey the "SE model" or SE rules, policies, and guidelines where they would clash with the goals of a high-level academic community (MathOverflow negociated a special agreement with SE before joining the network).
Upon forceful deletion of my previous answer, by some users I will be reviewing once again some differences between Physics overflow and stack exchange. I will be brief in my answer, factual and less passionate.
1) Physics Stack Exchange imposes,(Sometimes even year long) suspensions of accounts. The details of the which are kept hidden from the users. It is only the moderators, and members of the stack exchange family who make the decisions, and it has happened despite protest from the larger userbase.
Physics overflow does not allow for this kind of closed door policies, and as yet there have been no instances of banning users in the site.
2) Physics overflow, operates with the policy that a User's content will be be protected in the form that the user wishes to preserve. It makes exceptions for spam and other severe or legal abuses. Users or moderators cannot remove any disagreeable content, and Bad content is subject to review through discussion and not through unilateral deletion.
Physics stack exchange allows for moderators to close questions directly, If they find the content to be disagreeable. Its relies on the trust its places on the moderators to be fair with their judgements. Any disagreeable content(an example being my previous answer) is removed, without allowing for the natural process of peer review, down-voting etc, if the moderators(or other high reputation users) feel it to be appropriate.
3) Physics Overflow Implements Rotating of moderator positions(currently 1 yr cycle), to facilitate the process of understanding the position of both the moderators and users from a practical point of view.
Physics Stack Exchange implements Life time moderatorship. Some reasons being listed here.
These are some of the differences between the sites, apart from obvious ones such as their intended audience.
preamble:- I don't really have any hard feelings against SE any longer; I honestly do understand their point of view and way of functioning. It's just that the goals of PO are very different from those of StackExchange. I also figured out that a lot of my complaints when active here were unjustified (I'm talking about the "wrong answers should be deleted" nonsense that I liked to spread).
There are fundamental differences between PhysicsOverflow and Physics Stack Exchange, besides just the scope and level. Dilaton has already mentioned the difference in scope and level, but received a lot of flak for not mentioning what I initially thought were minor differences in policy, but upon further thought are actually important to point out, as mentioned by Emilio Pisanty and others.
The main goal of PO is to support academic communities. SE's main goal is to support a library of interesting content (as their unofficial motto says, to "make the internet better". I think both points of view are perfectly reasonable.
This fundamental difference is from where a lot of the policy differences come from. For instance, PhysicsOverflow has a user rights document, the moderator manual is made public and prominent from the PO homepage, and so on. PO's reviews section is a completely original idea, although there's also ArXiV-Analytics, but there are subtle differences which means the two aren't competitors either.
Comments are viewed as first-class citizens on PO, as they are important for an open discussion forum. Stack Exchange views them as somewhat inferior forms of content, again, because they do hinder the goal of forming neat Q&A libraries.
It's also true that PO does not have strict rules against rudeness. We have, however, recently adopted some guidelines that encourage politeness, although don't mandate them. It's a surprising kind of propaganda spread in a number of places that we somehow "encourage" rudeness. We absolutely don't.
The comments about "what if sexism/racism arises? will that be removed?" seem to be a sort of appeal to emotion, but anyway - such content rarely arises on a professional physics site, and is illegal in a number of countries, including the European Union where PO's servers are hosted, and where PO's registrant lives, and therefore does become a natural exception to the user rights.
Coming to the relationship between the two sites, PhysicsOverflow has zero issues with PSE. Links to answers on PSE are completely welcome (although it is preferred to link to an imported copy, as those tend too have more content), and PhysicsOverflow very much does want to maintain at least a neutral, if not a cordial relationship with Physics.SE. In fact, we even link to Physics.SE in the email we send to newly registered users, the relevant section is:
Popular physics questions and Undergraduate/K-12 level questions are better suited on Quora or Physics Stack Exchange; Purely mathematical questions (that are not relevant for physicists) will be better received on MathOverflow if research-level, and Mathematics Stack Exchange if not.
Unfortunately, this feeling is not reciprocated from the other side. While I understand the reason for my ban for "promotional content" (although I would have preferred a warning beforehand), I really do not understand the reason for Dilaton's ban for the same stated pretext; Dilaton only posted links to PO threads once the questions were actually answered, yet was suspended for a year for doing so. I wonder if SE would have a similiar reaction, if a user posted links to Wikipedia, or Quora, or Physicsforums, or another Stack Exchange site repeatedly. Is it so bad to post some useful links to help the OP and other people interested in the question? I mean, you guys even made a script to delete all links to PO here! What was that?
I hope to see these issues resolved soon, because unlike say, PF, PO is not intended to be a competitor to PSE at all, rather a completely different platform.
One thing to keep in mind, which was mentioned comments that are now in chat, is that the moderation policies and atmosphere at Physics Overflow do not necessarily match the ones here at Physics Stack Exchange. This can be a good thing or a bad thing, depending of your opinion of the atmosphere thing.
The main difference, in my understanding, is that (as PO sees it), they will not moderate 'frankness' in speech when used to convey physics content. This is something the Physics Overflow organizers and community seem to feel happens here; to quote from their FAQ:
Stack Exchange discourages and censors scientific criticism, and has admitted to have been systematically blocking frankness.
A less charitable view is that PO will let slide a good bit more unpoliteness than you'll see here. It is certainly the case that their stance towards abuse is relatively hazy. The main thing that worries me about PO is well encapsulated by the question Will there be politeness policies on physics overflow?. As Dilaton pointed out, Arnold Neumaier's answer there is quite moderate,
It is not necessary to be rude, and paying a little attention to one's tone is certainly better than just writing from one's gut feelings. A moderate language is one of the many facets of perfection, to be aspired even if not always attained.
and it is the highest-voted answer. However, that answer is hardly conclusive, and it seems that the hard stance is that
There are no rudeness rules, and there will never be.
as commented by a PO moderator.
They do have a section What can I do if I observe bad things? on their FAQ, which I feel is hazy regarding abuse on the site, directing readers to a contact link. Mostly, they really shy away from providing any guidelines on what sort of behaviour is and is not acceptable on the site. Personally, I would be worried that certain types of very cutting ad hominem attacks - of the kind that has been observed and frowned upon on this site before - will be given a lot of free rein on PO.
Now, PO moderators have stated informally that abuse will not be tolerated there, but I'm not sure how that squares with the comment above. At the very least, this is unclear and confusing as a PO user; personally, I would expect that some pretty bad stuff will inevitably slip through the cracks of that lack of clarity. I do not, however, advise people curious about PO to stay away from the site - instead, be aware of these points and make up your own mind whether you want to be part of that community.
For contrast, you can see the "Be Nice" policy at Stack Exchange, and the previous one (active until October 2014). I, for one, do not see anything there that even hints at scientific censorship. It sure feels good to use belittling language when critiquing work that I feel is technically incorrect, but in the end (I think) it only diminishes the quality of my writing, its value to its readers, and the chances that it will be taken seriously. But then, that's just me.
Personally, I would like PO to adopt an approach like arXiv Analytics, which states on its front page, in crisp and clear language, that
We encourage frank, open, and constructive conversation, but frankness doesn't mean lack of civility or lack of respect for others.
It seems, though, that they're still reluctant to put any qualifiers behind 'frankness is allowed'.
PhysicsOverflow was started by a number of users that were not content with the way the SE network works - especially in terms of moderation. They are not affiliated with the SE network. Furthermore, PO is more strict about the level of questions, accepting only questions of graduate level or higher.