I don't understand how to flag the question so it can migrate in other S.E. sites. It seems that when I flag it and I choose "This question belongs on another site in the Stack Exchange network" can be only be migrated to Chemistry meta. I think physics, biology, mathematics should be included.
We don't currently set up migration paths until after a site leaves the beta phase. The notion is that while in beta, a site's on- and off-topic lists will likely be in flux a bit. We don't want to set up migration paths and have content start flying around that was decided is off-topic, then maybe the community decides it was on-topic after all, etc. Additionally, migrations should be fairly rare, anyway; you should not only be sure that a particular question is off-topic for your site: you should be equally sure that it is on-topic for the destination site.
It doesn't hurt to think about which sites might someday be useful to have as migration paths from Chemistry.SE, but they won't be set up quite yet.