You are absolutely right, but do check out Greg Egan's "Diaspora" for a hard scifi that tries to get around that constraint. More seriously, and slightly less

You are absolutely right, but do check out Greg Egan's "Diaspora" for a hard scifi that tries to get around that constraint. More seriously, and slightly less scifi, you can have more than 3 spacial dimensions and still have stable gravity/electromagnetism if the topology of these dimensions is a bit weirder (very small periodic dimensions for instance).