I'm a PhD student in theoretical Physics. Recently I was watching a lecture and the professor told a story that as a PhD student he used to engage in side projects with postdocs in order to get more experience, learn something and try to publish as well, all of which worked very nicely for him. This would work for both sides because he would help and at the same time learn and gain experience.
Now, there is one particular topic I'm very interested for a long time. This topic is being activelly investigated by some of the most renowed researchers around the world and they say that there is a lot to be done, what makes me consider it very worthwhile to be able to work in this field. To be honest, that is the topic I want to work with after I finish my PhD! I have studied some of the main papers on the subject but still didn't have any idea of something to do that could end up in a publication. My guess is that this happens because I lack experience.
In that case the story my professor told inspired me in doing the same. The issue is that in my department (and as far as I know in my country, unfortunatelly) I can't find people working on the subject. In particular there is no postdoc working on it in my department.
That said, I know about one postdoc from another country which actively works on the field and already published very relevant papers on the subject. I've watched some of his recorded seminars on YouTube and he seems quite engaged in really getting people to understand the subject.
In that case would it be possible to do what my professor did remotely? Could I send a message to this guy and ask if I could participate and help with something in his work in order for me to get started in the field? Or would this be frowned upon and considered unacceptable? If the answer is yes, how should I approach this?