[jason taylor]

Time Travel and Interstellar travel are classified as science fiction but in the unlikely event that such things are possible it is not likely that the process will be the same.

Mastodon slaying cavemen are usually credited with having a culture not unlike modern hunter-gatherers. Of course the fact is we do not have the slightest idea what they were like: if we did they would not be prehistoric they would be historic. And in any case modern hunter-gatherers are different because they universally live in contact with herders and farmers, not always friendly. It is likely enough that prehistorics were hunter gatherer of some type if they did not leave enough trace to be noted by archeology but saying more than "likely" is essentially a myth (in the literary sense of the word, not in the aggravating usage of making it a synonym for "falsehood": it is possible to be false without being mythic and possible to have a mythic appeal while being true).

As for magic, magic has not strictly speaking been proven not to exist in this world for you cannot prove such a thing. At the least it is unrealistic to expect that people in the future have given up belief in such things and a bit absurd when most science fiction postulates that they have not given up war, crime,tyranny, avarice, love, pride, etc. Not to mention much imagining that they have reinstated feudalism. Individual humans not believing in magic is plausible but a human race that doesn't just wouldn't be human. Moreover much of this is forgetting that many of the rationalizations are so tawdry that they really should call it magic and be done with it. One of the many things wrong with the Force was the "midoclorines". Not only is that a stupid Jumping of the Shark, it is making the old Sufficiently Advanced Aliens mistake.

The problem is not whether or not you have magic in a sci fi (or "something else" that is not called magic although everyone knows it is). It is how you handle it. Dune pitched psionics to such a level that it was indistinguishable from religion or magic. Yet it was pretty good. By contrast Star Wars as someone else pointed out, used the Force to much. But the problem was not that they had the Force it was that they handled it badly.

I use fantasy elements freely in my Worldbuilding for My Traveller Universe. I use them only as Mythmaking. I am building cultures and cultures have stories. That does not mean I would not use them in a primary thread if I thought it would be useful.