The reality is that science fact and science fiction are dictated based on currently accepted understandings, whether they be true or not. As impossible as something sounds, what we understand as science fact is always bounded by our understanding of our surroundings. As our knowledge expands what was formerly science fact frequently becomes science wrong, sometimes badly wrong. Does that mean that someday perpetual motion will be a reality? Who knows. I am not holding my breath or taking any bets, but there are a lot of highly intelligent people constantly trying to unlock the mysteries of the universe and with so many new discoveries it seems science continues to encroach upon the impossible. Just think about cloaking devices and a transporter a la Star Trek, which are already to some extent realities.