Suppose ,I have asked a question in physics.stackexchange.Lets say that the question remains unanswered for a long period of time or say I don't obtain any satisfactory answers to that question.In such a case if I know few users who, I believe or hope can answer the question then how can I invite them to answer that question.
As you can't put a bounty on your questions yet, you can just ping the required user in chat (if he's available), or just use comments under any of his posts1.
1 I've done this methodology once (a change, for thanking a guy). I don't think this is a nice way for getting answers. So before you're gonna do this, keep in mind that
- You shouldn't go around disturbing him, by commenting below all of his posts (which would lead to moderation). I commented below his post. The next day, I visited his profile. I noted the
seen"xx hours ago". I was quite sure that he had visited the site after I had commented. Though there's no guarantee of him seeing it, there's a larger chance that he'd have seen it. Because, SE provides you with an inbox by which you'll definitely get notified, which is quite helpful that you don't have to go to your post to see a comment. Again, you shouldn't disturb that particular user. You can't force someone when he's not interested into, or unable to provide an answer for your question.
- Comments should be useful to a particular post. As you're violating it, make sure that you delete your comment after visiting his profile.
And, don't go around jumping from user after user, for answering your post. It's likely to work for 1 or maybe 2 (max.), but I would never suggest you to disturb everyone like this...
There is no tool to bring a specific question to the attention to specific users, and all suggestions that there should be have been roundly and firmly rejected on the mother-meta. Stack Exchange is not a social network and there is no way to push content at individual users.
You can, if you have enough rep, offer a bounty to (hopefully, there are no guarantees) draw more attention to your question in a general way. This, too is not guaranteed. The last bounty I offered expired without eliciting a single additional answer.