Does anyone, any entity own copyright on the contents of the Physic's Stack Exchange Q&A that exist?
Yes, when you post any content to any Stack Exchange site you retain the copyright to that content, but you also grant irrevocable rights to the public, including Stack Exchange, as to how they can use that content.
More precisely, whenever you post any question, answer, or comment to the site, you are effectively doing three things:
- You're certifying that you're the creator of all the content that you post (or that any other content, such as images, is covered under fair use);
- You retain the copyright to that work; and
- You provide the work to the public, and as a subset of that to Stack Exchange, under a Creative Commons Attribution Sharealike (CC BY-SA) license.
This license enables Stack Exchange (along with everybody else) to distribute the work in any form they want to, and even charge for it if they so desire, so long as (i) they attribute the work to you, and (ii) they distribute it under the CC BY-SA license, or some compatible set of conditions.
This license is irrevocable: once you post here and the license is granted, you cannot demand that Stack Exchange delete the post, and indeed other people can put your content on other sites without you being able to stop them, so long as they follow the conditions of the license. If this makes you uncomfortable, you should not post here.
That said, you retain all other rights to the work not covered by the original license. Thus, you're able to e.g. sell the copyright to a publisher, or sue people who post it elsewhere and don't attribute it correctly, or indeed re-license it to the public under any other license (or combination of licenses) that you want to.
If you do find your content posted on other sites and you are unhappy with how it's presented (which does happen often enough) then the mother-meta post A site (or scraper) is copying content from Stack Exchange. What do I do? has good help on what to do.
Finally, it's important to note that the above only applies to user-provided content. The actual site design, logos, and other SE-built assets are owned by Stack Exchange, and cannot be reused (particularly for commercial purposes) without their authorization.
At the bottom of every page is a link labeled "Legal".
Among the things found there is the copyright policy. Like all sites in the Stack Exchange network submitting to Physics grants Stack Exchange a license to use your content under a Creative Commons license. See the legal page for details.
That means that the copyright has not been transferred, but that certain rights have been granted to Stack Exchange and to the public—the copyright holder can't prevent use of the content that is consistent with the license.