Comments aren't a good medium for back-and-forth; that's that chat is for. There's a Chat FAQ which is useful to re-read once a year or so. The privileges of talking in chat and creating chat rooms come pretty early in the privilege hierarchy.
If you leave a comment with the text
Let us continue this discussion in [chat]
then the special comment markdown will make
[chat] a link to the main Physics chat room. Some guidance.
If you have enough comment back-and-forth on a particular post, the comment box develops a highlight that suggests, with a link, moving your discussion to a chat room. That chat room starts with a link to the parent post and is populated with the previous comments. Users who don’t have the “talk in chat” privilege yet can participate in the conversation anyway, but apparently moderator intervention is required; raise a custom flag on the post and explain what’s happening.
The diamond moderators can create such a post-associated room any time, but we can only migrate the comments once. This happens most frequently when we get an automated flag that a post has accumulated lots of comments in a short time. Sometimes a user will manually flag a comment thread that needs attention before.
Note that, while chat is a better tool for real-time back-and-forth than comments, you can use chat pings to make chat asynchronous as well.