First I am sorry if this question is repeated
Is there a way to send a private message to a user?
There is no private message facility and the team has regularly and robustly turned down all such suggestions.
You can get another user's attention by tagging them in the comments (though they need to have participated in the discussion or editing of a post for that to work), and you can (with sufficient rep) open a chat room for holding a conversation. That last suggestion isn't private in the sense that others are excluded but you probably won't have a lot of people poking their noses in.
The mantra is "This is not a social network!".
However, some Physics SE user find this to be insufficiently friendly and have responded with the meta.physics most "Opt-in to be always pingable to answer questions" where you are explicitly invited to ping them in order to direct their attention to other posts.