Hard SciFi’s not hard
Mike says “I worry that the classification ‘Hard SF’ is off-putting to many readers. It makes it sound like any story labeled as such will be hard to understand, which is not necessarily true. ‘Hard’ is really just a term to refer to the fact that we can be quantitative about some scientific topics.”
SciFi or Fantasy
I’ve seen other definitions that contrast hard science fiction with fantasies: Hard science fiction says “this could really happen.” That definition may impose some limitations on authors, but what constitutes “technology so advanced it seems to be magic” and what constitutes fantasy varies from book to book. How much the author chooses to explain the science versus how much is simply presented varies, too.
Check out Mike’s list for some good reading.